resolv: Add beginnings of a libresolv test suite

38 files changed, 2906 insertions, 0 deletions

diff --git a/support/Makefile b/support/Makefile
index 9544a08..7e34fcb 100644
--- a/support/Makefile
+++ b/support/Makefile
@@ -26,21 +26,42 @@ extra-libs-noinstall := $(extra-libs)
 
 libsupport-routines = \
   check \
+  check_addrinfo \
+  check_dns_packet \
+  check_hostent \
+  check_netent \
   delayed_exit \
   ignore_stderr \
   oom_error \
+  resolv_test \
   set_fortify_handler \
   support_become_root \
   support_enter_network_namespace \
+  support_format_address_family \
+  support_format_addrinfo \
+  support_format_dns_packet \
+  support_format_herrno \
+  support_format_hostent \
+  support_format_netent \
   support_record_failure \
+  support_run_diff \
   support_test_main \
   support_test_verify_impl \
   temp_file \
   write_message \
+  xaccept \
   xasprintf \
+  xbind \
   xcalloc \
+  xconnect \
+  xfclose \
+  xfopen \
   xfork \
+  xgetsockname \
+  xlisten \
   xmalloc \
+  xmemstream \
+  xpoll \
   xpthread_barrier_destroy \
   xpthread_barrier_init \
   xpthread_barrier_wait \
@@ -52,12 +73,18 @@ libsupport-routines = \
   xpthread_join \
   xpthread_mutex_lock \
   xpthread_mutex_unlock \
+  xpthread_once \
   xpthread_sigmask \
   xpthread_spin_lock \
   xpthread_spin_unlock \
   xrealloc \
+  xrecvfrom \
+  xsendto \
+  xsetsockopt \
   xsocket \
+  xstrdup \
   xwaitpid \
+  xwrite \
 
 libsupport-static-only-routines := $(libsupport-routines)
 # Only build one variant of the library.
diff --git a/support/check_addrinfo.c b/support/check_addrinfo.c
new file mode 100644
index 0000000..793b34f
--- /dev/null
+++ b/support/check_addrinfo.c
@@ -0,0 +1,42 @@
+/* Compare struct addrinfo values against a formatted string.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/check_nss.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <support/check.h>
+#include <support/format_nss.h>
+#include <support/run_diff.h>
+
+void
+check_addrinfo (const char *query_description, struct addrinfo *ai, int ret,
+                const char *expected)
+{
+  char *formatted = support_format_addrinfo (ai, ret);
+  if (strcmp (formatted, expected) != 0)
+    {
+      support_record_failure ();
+      printf ("error: addrinfo comparison failure\n");
+      if (query_description != NULL)
+        printf ("query: %s\n", query_description);
+      support_run_diff ("expected", expected,
+                        "actual", formatted);
+    }
+  free (formatted);
+}
diff --git a/support/check_dns_packet.c b/support/check_dns_packet.c
new file mode 100644
index 0000000..a2c16b1
--- /dev/null
+++ b/support/check_dns_packet.c
@@ -0,0 +1,42 @@
+/* Check that a DNS packet buffer has the expected contents.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/check_nss.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <support/check.h>
+#include <support/format_nss.h>
+#include <support/run_diff.h>
+
+void
+check_dns_packet (const char *query_description,
+                  const unsigned char *buffer, size_t length,
+                  const char *expected)
+{
+  char *formatted = support_format_dns_packet (buffer, length);
+  if (strcmp (formatted, expected) != 0)
+    {
+      support_record_failure ();
+      printf ("error: packet comparison failure\n");
+      if (query_description != NULL)
+        printf ("query: %s\n", query_description);
+      support_run_diff ("expected", expected, "actual", formatted);
+    }
+  free (formatted);
+}
diff --git a/support/check_hostent.c b/support/check_hostent.c
new file mode 100644
index 0000000..b0661b1
--- /dev/null
+++ b/support/check_hostent.c
@@ -0,0 +1,42 @@
+/* Compare struct hostent values against a formatted string.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/check_nss.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <support/check.h>
+#include <support/format_nss.h>
+#include <support/run_diff.h>
+
+void
+check_hostent (const char *query_description, struct hostent *h,
+               const char *expected)
+{
+  char *formatted = support_format_hostent (h);
+  if (strcmp (formatted, expected) != 0)
+    {
+      support_record_failure ();
+      printf ("error: hostent comparison failure\n");
+      if (query_description != NULL)
+        printf ("query: %s\n", query_description);
+      support_run_diff ("expected", expected,
+                        "actual", formatted);
+    }
+  free (formatted);
+}
diff --git a/support/check_netent.c b/support/check_netent.c
new file mode 100644
index 0000000..f208624
--- /dev/null
+++ b/support/check_netent.c
@@ -0,0 +1,42 @@
+/* Compare struct netent values against a formatted string.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/check_nss.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <support/check.h>
+#include <support/format_nss.h>
+#include <support/run_diff.h>
+
+void
+check_netent (const char *query_description, struct netent *e,
+              const char *expected)
+{
+  char *formatted = support_format_netent (e);
+  if (strcmp (formatted, expected) != 0)
+    {
+      support_record_failure ();
+      printf ("error: netent comparison failure\n");
+      if (query_description != NULL)
+        printf ("query: %s\n", query_description);
+      support_run_diff ("expected", expected,
+                        "actual", formatted);
+    }
+  free (formatted);
+}
diff --git a/support/check_nss.h b/support/check_nss.h
new file mode 100644
index 0000000..6ab2824
--- /dev/null
+++ b/support/check_nss.h
@@ -0,0 +1,42 @@
+/* Test verification functions for NSS- and DNS-related data.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#ifndef SUPPORT_CHECK_NSS_H
+#define SUPPORT_CHECK_NSS_H
+
+#include <netdb.h>
+#include <sys/cdefs.h>
+
+__BEGIN_DECLS
+
+/* Compare the data structures against the expected values (which have
+   to be formatted according to the support_format_* functions in
+   <support/format_nss.h>).  If there is a difference, a delayed test
+   failure is recorded, and a diff is written to standard output.  */
+void check_addrinfo (const char *query_description,
+                     struct addrinfo *, int ret, const char *expected);
+void check_dns_packet (const char *query_description,
+                       const unsigned char *, size_t, const char *expected);
+void check_hostent (const char *query_description,
+                    struct hostent *, const char *expected);
+void check_netent (const char *query_description,
+                   struct netent *, const char *expected);
+
+__END_DECLS
+
+#endif  /* SUPPORT_CHECK_NSS_H */
diff --git a/support/format_nss.h b/support/format_nss.h
new file mode 100644
index 0000000..07e32f5
--- /dev/null
+++ b/support/format_nss.h
@@ -0,0 +1,41 @@
+/* String formatting functions for NSS- and DNS-related data.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#ifndef SUPPORT_FORMAT_NSS_H
+#define SUPPORT_FORMAT_NSS_H
+
+#include <netdb.h>
+#include <sys/cdefs.h>
+
+__BEGIN_DECLS
+
+/* The following functions format their arguments as human-readable
+   strings (which can span multiple lines).  The caller must free the
+   returned buffer.  For NULL pointers or failure status arguments,
+   error variables such as h_errno and errno are included in the
+   result.  */
+char *support_format_address_family (int);
+char *support_format_addrinfo (struct addrinfo *, int ret);
+char *support_format_dns_packet (const unsigned char *buffer, size_t length);
+char *support_format_herrno (int);
+char *support_format_hostent (struct hostent *);
+char *support_format_netent (struct netent *);
+
+__END_DECLS
+
+#endif  /* SUPPORT_FORMAT_NSS_H */
diff --git a/support/resolv_test.c b/support/resolv_test.c
new file mode 100644
index 0000000..80d8c74
--- /dev/null
+++ b/support/resolv_test.c
@@ -0,0 +1,1150 @@
+/* DNS test framework and libresolv redirection.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/resolv_test.h>
+
+#include <arpa/inet.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <nss.h>
+#include <resolv.h>
+#include <search.h>
+#include <stdlib.h>
+#include <string.h>
+#include <support/check.h>
+#include <support/namespace.h>
+#include <support/support.h>
+#include <support/test-driver.h>
+#include <support/xsocket.h>
+#include <support/xthread.h>
+#include <unistd.h>
+
+/* Response builder. */
+
+enum
+  {
+    max_response_length = 65536
+  };
+
+/* List of pointers to be freed.  The hash table implementation
+   (struct hsearch_data) does not provide a way to deallocate all
+   objects, so this approach is used to avoid memory leaks.  */
+struct to_be_freed
+{
+  struct to_be_freed *next;
+  void *ptr;
+};
+
+struct resolv_response_builder
+{
+  const unsigned char *query_buffer;
+  size_t query_length;
+
+  size_t offset;                /* Bytes written so far in buffer.  */
+  ns_sect section;              /* Current section in the DNS packet.  */
+  unsigned int truncate_bytes;  /* Bytes to remove at end of response. */
+  bool drop;                    /* Discard generated response.  */
+  bool close;                   /* Close TCP client connection.  */
+
+  /* Offset of the two-byte RDATA length field in the currently
+     written RDATA sub-structure.  0 if no RDATA is being written.  */
+  size_t current_rdata_offset;
+
+  /* Hash table for locating targets for label compression.  */
+  struct hsearch_data compression_offsets;
+  /* List of pointers which need to be freed.  Used for domain names
+     involved in label compression.  */
+  struct to_be_freed *to_be_freed;
+
+  /* Must be last.  Not zeroed for performance reasons.  */
+  unsigned char buffer[max_response_length];
+};
+
+/* Response builder. */
+
+/* Add a pointer to the list of pointers to be freed when B is
+   deallocated.  */
+static void
+response_push_pointer_to_free (struct resolv_response_builder *b, void *ptr)
+{
+  if (ptr == NULL)
+    return;
+  struct to_be_freed *e = xmalloc (sizeof (*e));
+  *e = (struct to_be_freed) {b->to_be_freed, ptr};
+  b->to_be_freed = e;
+}
+
+void
+resolv_response_init (struct resolv_response_builder *b,
+                      struct resolv_response_flags flags)
+{
+  if (b->offset > 0)
+    FAIL_EXIT1 ("response_init: called at offset %zu", b->offset);
+  if (b->query_length < 12)
+    FAIL_EXIT1 ("response_init called for a query of size %zu",
+                b->query_length);
+  if (flags.rcode > 15)
+    FAIL_EXIT1 ("response_init: invalid RCODE %u", flags.rcode);
+
+  /* Copy the transaction ID.  */
+  b->buffer[0] = b->query_buffer[0];
+  b->buffer[1] = b->query_buffer[1];
+
+  /* Initialize the flags.  */
+  b->buffer[2] = 0x80;                       /* Mark as response.   */
+  b->buffer[2] |= b->query_buffer[2] & 0x01; /* Copy the RD bit.  */
+  if (flags.tc)
+    b->buffer[2] |= 0x02;
+  b->buffer[3] = 0x80 | flags.rcode; /* Always set RA.  */
+
+  /* Fill in the initial section count values.  */
+  b->buffer[4] = flags.qdcount >> 8;
+  b->buffer[5] = flags.qdcount;
+  b->buffer[6] = flags.ancount >> 8;
+  b->buffer[7] = flags.ancount;
+  b->buffer[8] = flags.nscount >> 8;
+  b->buffer[9] = flags.nscount;
+  b->buffer[10] = flags.adcount >> 8;
+  b->buffer[11] = flags.adcount;
+
+  b->offset = 12;
+}
+
+void
+resolv_response_section (struct resolv_response_builder *b, ns_sect section)
+{
+  if (b->offset == 0)
+    FAIL_EXIT1 ("resolv_response_section: response_init not called before");
+  if (section < b->section)
+    FAIL_EXIT1 ("resolv_response_section: cannot go back to previous section");
+  b->section = section;
+}
+
+/* Add a single byte to B.  */
+static inline void
+response_add_byte (struct resolv_response_builder *b, unsigned char ch)
+{
+  if (b->offset == max_response_length)
+    FAIL_EXIT1 ("DNS response exceeds 64 KiB limit");
+  b->buffer[b->offset] = ch;
+  ++b->offset;
+}
+
+/* Add a 16-bit word VAL to B, in big-endian format.  */
+static void
+response_add_16 (struct resolv_response_builder *b, uint16_t val)
+{
+  response_add_byte (b, val >> 8);
+  response_add_byte (b, val);
+}
+
+/* Increment the pers-section record counter in the packet header.  */
+static void
+response_count_increment (struct resolv_response_builder *b)
+{
+  unsigned int offset = b->section;
+  offset = 4 + 2 * offset;
+  ++b->buffer[offset + 1];
+  if (b->buffer[offset + 1] == 0)
+    {
+      /* Carry.  */
+      ++b->buffer[offset];
+      if (b->buffer[offset] == 0)
+        /* Overflow.  */
+        FAIL_EXIT1 ("too many records in section");
+    }
+}
+
+void
+resolv_response_add_question (struct resolv_response_builder *b,
+                              const char *name, uint16_t class, uint16_t type)
+{
+  if (b->offset == 0)
+    FAIL_EXIT1 ("resolv_response_add_question: "
+                "resolv_response_init not called");
+  if (b->section != ns_s_qd)
+    FAIL_EXIT1 ("resolv_response_add_question: "
+                "must be called in the question section");
+
+  resolv_response_add_name (b, name);
+  response_add_16 (b, type);
+  response_add_16 (b, class);
+
+  response_count_increment (b);
+}
+
+void
+resolv_response_add_name (struct resolv_response_builder *b,
+                          const char *const origname)
+{
+  /* Normalized name.  */
+  char *name;
+  /* Normalized name with case preserved.  */
+  char *name_case;
+  {
+    size_t namelen = strlen (origname);
+    /* Remove trailing dots.  FIXME: Handle trailing quoted dots.  */
+    while (namelen > 0 && origname[namelen - 1] == '.')
+      --namelen;
+    name = xmalloc (namelen + 1);
+    name_case = xmalloc (namelen + 1);
+    /* Copy and convert to lowercase.  FIXME: This needs to normalize
+       escaping as well.  */
+    for (size_t i = 0; i < namelen; ++i)
+      {
+        char ch = origname[i];
+        name_case[i] = ch;
+        if ('A' <= ch && ch <= 'Z')
+          ch = ch - 'A' + 'a';
+        name[i] = ch;
+      }
+    name[namelen] = 0;
+    name_case[namelen] = 0;
+  }
+  char *name_start = name;
+  char *name_case_start = name_case;
+
+  bool compression = false;
+  while (*name)
+    {
+      /* Search for a previous name we can reference.  */
+      ENTRY new_entry =
+        {
+          .key = name,
+          .data = (void *) (uintptr_t) b->offset,
+        };
+
+      /* If the label can be a compression target because it is at a
+         reachable offset, add it to the hash table.  */
+      ACTION action;
+      if (b->offset < (1 << 12))
+        action = ENTER;
+      else
+        action = FIND;
+
+      /* Search for known compression offsets in the hash table.  */
+      ENTRY *e;
+      if (hsearch_r (new_entry, action, &e, &b->compression_offsets) == 0)
+        {
+          if (action == FIND && errno == ESRCH)
+            /* Fall through.  */
+            e = NULL;
+          else
+            FAIL_EXIT1 ("hsearch_r failure in name compression: %m");
+        }
+
+      /* The name is known.  Reference the previous location.  */
+      if (e != NULL && e->data != new_entry.data)
+        {
+          size_t old_offset = (uintptr_t) e->data;
+          response_add_byte (b, 0xC0 | (old_offset >> 8));
+          response_add_byte (b, old_offset);
+          compression = true;
+          break;
+        }
+
+      /* The name does not exist yet.  Write one label.  First, add
+         room for the label length.  */
+      size_t buffer_label_offset = b->offset;
+      response_add_byte (b, 0);
+
+      /* Copy the label.  */
+      while (true)
+        {
+          char ch = *name_case;
+          if (ch == '\0')
+            break;
+          ++name;
+          ++name_case;
+          if (ch == '.')
+            break;
+          /* FIXME: Handle escaping.  */
+          response_add_byte (b, ch);
+        }
+
+      /* Patch in the label length.  */
+      size_t label_length = b->offset - buffer_label_offset - 1;
+      if (label_length == 0)
+        FAIL_EXIT1 ("empty label in name compression: %s", origname);
+      if (label_length > 63)
+        FAIL_EXIT1 ("label too long in name compression: %s", origname);
+      b->buffer[buffer_label_offset] = label_length;
+
+      /* Continue with the tail of the name and the next label.  */
+    }
+
+  if (compression)
+    {
+      /* If we found an immediate match for the name, we have not put
+         it into the hash table, and can free it immediately.  */
+      if (name == name_start)
+        free (name_start);
+      else
+        response_push_pointer_to_free (b, name_start);
+    }
+  else
+    {
+      /* Terminate the sequence of labels.  With compression, this is
+         implicit in the compression reference.  */
+      response_add_byte (b, 0);
+      response_push_pointer_to_free (b, name_start);
+    }
+
+  free (name_case_start);
+}
+
+void
+resolv_response_open_record (struct resolv_response_builder *b,
+                             const char *name,
+                             uint16_t class, uint16_t type, uint32_t ttl)
+{
+  if (b->section == ns_s_qd)
+    FAIL_EXIT1 ("resolv_response_open_record called in question section");
+  if (b->current_rdata_offset != 0)
+    FAIL_EXIT1 ("resolv_response_open_record called with open record");
+
+  resolv_response_add_name (b, name);
+  response_add_16 (b, type);
+  response_add_16 (b, class);
+  response_add_16 (b, ttl >> 16);
+  response_add_16 (b, ttl);
+
+  b->current_rdata_offset = b->offset;
+  /* Add room for the RDATA length.  */
+  response_add_16 (b, 0);
+}
+
+
+void
+resolv_response_close_record (struct resolv_response_builder *b)
+{
+  size_t rdata_offset = b->current_rdata_offset;
+  if (rdata_offset == 0)
+    FAIL_EXIT1 ("response_close_record called without open record");
+  size_t rdata_length = b->offset - rdata_offset - 2;
+  if (rdata_length > 65535)
+    FAIL_EXIT1 ("RDATA length %zu exceeds limit", rdata_length);
+  b->buffer[rdata_offset] = rdata_length >> 8;
+  b->buffer[rdata_offset + 1] = rdata_length;
+  response_count_increment (b);
+  b->current_rdata_offset = 0;
+}
+
+void
+resolv_response_add_data (struct resolv_response_builder *b,
+                          const void *data, size_t length)
+{
+  size_t remaining = max_response_length - b->offset;
+  if (remaining < length)
+    FAIL_EXIT1 ("resolv_response_add_data: not enough room for %zu bytes",
+                length);
+  memcpy (b->buffer + b->offset, data, length);
+  b->offset += length;
+}
+
+void
+resolv_response_drop (struct resolv_response_builder *b)
+{
+  b->drop = true;
+}
+
+void
+resolv_response_close (struct resolv_response_builder *b)
+{
+  b->close = true;
+}
+
+void
+resolv_response_truncate_data (struct resolv_response_builder *b, size_t count)
+{
+  if (count > 65535)
+    FAIL_EXIT1 ("resolv_response_truncate_data: argument too large: %zu",
+                count);
+  b->truncate_bytes = count;
+}
+
+
+size_t
+resolv_response_length (const struct resolv_response_builder *b)
+{
+  return b->offset;
+}
+
+unsigned char *
+resolv_response_buffer (const struct resolv_response_builder *b)
+{
+  unsigned char *result = xmalloc (b->offset);
+  memcpy (result, b->buffer, b->offset);
+  return result;
+}
+
+static struct resolv_response_builder *
+response_builder_allocate
+  (const unsigned char *query_buffer, size_t query_length)
+{
+  struct resolv_response_builder *b = xmalloc (sizeof (*b));
+  memset (b, 0, offsetof (struct resolv_response_builder, buffer));
+  b->query_buffer = query_buffer;
+  b->query_length = query_length;
+  TEST_VERIFY_EXIT (hcreate_r (10000, &b->compression_offsets) != 0);
+  return b;
+}
+
+static void
+response_builder_free (struct resolv_response_builder *b)
+{
+  struct to_be_freed *current = b->to_be_freed;
+  while (current != NULL)
+    {
+      struct to_be_freed *next = current->next;
+      free (current->ptr);
+      free (current);
+      current = next;
+    }
+  hdestroy_r (&b->compression_offsets);
+  free (b);
+}
+
+/* DNS query processing. */
+
+/* Data extracted from the question section of a DNS packet.  */
+struct query_info
+{
+  char qname[MAXDNAME];
+  uint16_t qclass;
+  uint16_t qtype;
+};
+
+/* Update *INFO from the specified DNS packet.  */
+static void
+parse_query (struct query_info *info,
+             const unsigned char *buffer, size_t length)
+{
+  if (length < 12)
+    FAIL_EXIT1 ("malformed DNS query: too short: %zu bytes", length);
+
+  int ret = dn_expand (buffer, buffer + length, buffer + 12,
+                       info->qname, sizeof (info->qname));
+  if (ret < 0)
+    FAIL_EXIT1 ("malformed DNS query: cannot uncompress QNAME");
+
+  /* Obtain QTYPE and QCLASS.  */
+  size_t remaining = length - (12 + ret);
+  struct
+  {
+    uint16_t qtype;
+    uint16_t qclass;
+  } qtype_qclass;
+  if (remaining < sizeof (qtype_qclass))
+    FAIL_EXIT1 ("malformed DNS query: "
+                "query lacks QCLASS/QTYPE, QNAME: %s", info->qname);
+  memcpy (&qtype_qclass, buffer + 12 + ret, sizeof (qtype_qclass));
+  info->qclass = ntohs (qtype_qclass.qclass);
+  info->qtype = ntohs (qtype_qclass.qtype);
+}
+
+
+/* Main testing framework.  */
+
+/* Per-server information.  One struct is allocated for each test
+   server.  */
+struct resolv_test_server
+{
+  /* Local address of the server.  UDP and TCP use the same port.  */
+  struct sockaddr_in address;
+
+  /* File descriptor of the UDP server, or -1 if this server is
+     disabled.  */
+  int socket_udp;
+
+  /* File descriptor of the TCP server, or -1 if this server is
+     disabled.  */
+  int socket_tcp;
+
+  /* Counter of the number of responses processed so far.  */
+  size_t response_number;
+
+  /* Thread handles for the server threads (if not disabled in the
+     configuration).  */
+  pthread_t thread_udp;
+  pthread_t thread_tcp;
+};
+
+/* Main struct for keeping track of libresolv redirection and
+   testing.  */
+struct resolv_test
+{
+  /* After initialization, any access to the struct must be performed
+     while this lock is acquired.  */
+  pthread_mutex_t lock;
+
+  /* Data for each test server. */
+  struct resolv_test_server servers[resolv_max_test_servers];
+
+  /* Used if config.single_thread_udp is true.  */
+  pthread_t thread_udp_single;
+
+  struct resolv_redirect_config config;
+  bool termination_requested;
+};
+
+/* Function implementing a server thread.  */
+typedef void (*thread_callback) (struct resolv_test *, int server_index);
+
+/* Storage for thread-specific data, for passing to the
+   thread_callback function.  */
+struct thread_closure
+{
+  struct resolv_test *obj;      /* Current test object.  */
+  thread_callback callback;     /* Function to call.  */
+  int server_index;             /* Index of the implemented server.  */
+};
+
+/* Wrap response_callback as a function which can be passed to
+   pthread_create.  */
+static void *
+thread_callback_wrapper (void *arg)
+{
+  struct thread_closure *closure = arg;
+  closure->callback (closure->obj, closure->server_index);
+  free (closure);
+  return NULL;
+}
+
+/* Start a server thread for the specified SERVER_INDEX, implemented
+   by CALLBACK.  */
+static pthread_t
+start_server_thread (struct resolv_test *obj, int server_index,
+                     thread_callback callback)
+{
+  struct thread_closure *closure = xmalloc (sizeof (*closure));
+  *closure = (struct thread_closure)
+    {
+      .obj = obj,
+      .callback = callback,
+      .server_index = server_index,
+    };
+  return xpthread_create (NULL, thread_callback_wrapper, closure);
+}
+
+/* Process one UDP query.  Return false if a termination requested has
+   been detected.  */
+static bool
+server_thread_udp_process_one (struct resolv_test *obj, int server_index)
+{
+  unsigned char query[512];
+  struct sockaddr_storage peer;
+  socklen_t peerlen = sizeof (peer);
+  size_t length = xrecvfrom (obj->servers[server_index].socket_udp,
+                             query, sizeof (query), 0,
+                             (struct sockaddr *) &peer, &peerlen);
+  /* Check for termination.  */
+  {
+    bool termination_requested;
+    xpthread_mutex_lock (&obj->lock);
+    termination_requested = obj->termination_requested;
+    xpthread_mutex_unlock (&obj->lock);
+    if (termination_requested)
+      return false;
+  }
+
+
+  struct query_info qinfo;
+  parse_query (&qinfo, query, length);
+  if (test_verbose > 0)
+    {
+      if (test_verbose > 1)
+        printf ("info: UDP server %d: incoming query:"
+                " %zd bytes, %s/%u/%u, tnxid=0x%02x%02x\n",
+                server_index, length, qinfo.qname, qinfo.qclass, qinfo.qtype,
+                query[0], query[1]);
+      else
+        printf ("info: UDP server %d: incoming query:"
+                " %zd bytes, %s/%u/%u\n",
+                server_index, length, qinfo.qname, qinfo.qclass, qinfo.qtype);
+    }
+
+  struct resolv_response_context ctx =
+    {
+      .query_buffer = query,
+      .query_length = length,
+      .server_index = server_index,
+      .tcp = false,
+    };
+  struct resolv_response_builder *b = response_builder_allocate (query, length);
+  obj->config.response_callback
+    (&ctx, b, qinfo.qname, qinfo.qclass, qinfo.qtype);
+
+  if (b->drop)
+    {
+      if (test_verbose)
+        printf ("info: UDP server %d: dropping response to %s/%u/%u\n",
+                server_index, qinfo.qname, qinfo.qclass, qinfo.qtype);
+    }
+  else
+    {
+      if (test_verbose)
+        {
+          if (b->offset >= 12)
+            printf ("info: UDP server %d: sending response:"
+                    " %zu bytes, RCODE %d (for %s/%u/%u)\n",
+                    server_index, b->offset, b->buffer[3] & 0x0f,
+                    qinfo.qname, qinfo.qclass, qinfo.qtype);
+          else
+            printf ("info: UDP server %d: sending response: %zu bytes"
+                    " (for %s/%u/%u)\n",
+                    server_index, b->offset,
+                    qinfo.qname, qinfo.qclass, qinfo.qtype);
+          if (b->truncate_bytes > 0)
+            printf ("info:    truncated by %u bytes\n", b->truncate_bytes);
+        }
+      size_t to_send = b->offset;
+      if (to_send < b->truncate_bytes)
+        to_send = 0;
+      else
+        to_send -= b->truncate_bytes;
+
+      /* Ignore most errors here because the other end may have closed
+         the socket. */
+      if (sendto (obj->servers[server_index].socket_udp,
+                  b->buffer, to_send, 0,
+                  (struct sockaddr *) &peer, peerlen) < 0)
+        TEST_VERIFY_EXIT (errno != EBADF);
+    }
+  response_builder_free (b);
+  return true;
+}
+
+/* UDP thread_callback function.  Variant for one thread per
+   server.  */
+static void
+server_thread_udp (struct resolv_test *obj, int server_index)
+{
+  while (server_thread_udp_process_one (obj, server_index))
+    ;
+}
+
+/* Single-threaded UDP processing function, for the single_thread_udp
+   case.  */
+static void *
+server_thread_udp_single (void *closure)
+{
+  struct resolv_test *obj = closure;
+
+  struct pollfd fds[resolv_max_test_servers];
+  for (int server_index = 0; server_index < resolv_max_test_servers;
+       ++server_index)
+    if (obj->config.servers[server_index].disable_udp)
+      fds[server_index] = (struct pollfd) {.fd = -1};
+    else
+      {
+        fds[server_index] = (struct pollfd)
+          {
+            .fd = obj->servers[server_index].socket_udp,
+            .events = POLLIN
+          };
+
+        /* Make the socket non-blocking.  */
+        int flags = fcntl (obj->servers[server_index].socket_udp, F_GETFL, 0);
+        if (flags < 0)
+          FAIL_EXIT1 ("fcntl (F_GETFL): %m");
+        flags |= O_NONBLOCK;
+        if (fcntl (obj->servers[server_index].socket_udp, F_SETFL, flags) < 0)
+          FAIL_EXIT1 ("fcntl (F_SETFL): %m");
+      }
+
+  while (true)
+    {
+      xpoll (fds, resolv_max_test_servers, -1);
+      for (int server_index = 0; server_index < resolv_max_test_servers;
+           ++server_index)
+        if (fds[server_index].revents != 0)
+          {
+            if (!server_thread_udp_process_one (obj, server_index))
+              goto out;
+            fds[server_index].revents = 0;
+          }
+    }
+
+ out:
+  return NULL;
+}
+
+/* Start the single UDP handler thread (for the single_thread_udp
+   case).  */
+static void
+start_server_thread_udp_single (struct resolv_test *obj)
+{
+  obj->thread_udp_single
+    = xpthread_create (NULL, server_thread_udp_single, obj);
+}
+
+/* Data describing a TCP client connect.  */
+struct tcp_thread_closure
+{
+  struct resolv_test *obj;
+  int server_index;
+  int client_socket;
+};
+
+/* Read a complete DNS query packet.  If EOF_OK, an immediate
+   end-of-file condition is acceptable.  */
+static bool
+read_fully (int fd, void *buf, size_t len, bool eof_ok)
+{
+  const void *const end = buf + len;
+  while (buf < end)
+    {
+      ssize_t ret = read (fd, buf, end - buf);
+      if (ret == 0)
+        {
+          if (!eof_ok)
+            {
+              support_record_failure ();
+              printf ("error: unexpected EOF on TCP connection\n");
+            }
+          return false;
+        }
+      else if (ret < 0)
+        {
+          if (!eof_ok || errno != ECONNRESET)
+            {
+              support_record_failure ();
+              printf ("error: TCP read: %m\n");
+            }
+          return false;
+        }
+      buf += ret;
+      eof_ok = false;
+    }
+  return true;
+}
+
+/* Write an array of iovecs.  Terminate the process on failure.  */
+static void
+writev_fully (int fd, struct iovec *buffers, size_t count)
+{
+  while (count > 0)
+    {
+      /* Skip zero-length write requests.  */
+      if (buffers->iov_len == 0)
+        {
+          ++buffers;
+          --count;
+          continue;
+        }
+      /* Try to rewrite the remaing buffers.  */
+      ssize_t ret = writev (fd, buffers, count);
+      if (ret < 0)
+        FAIL_EXIT1 ("writev: %m");
+      if (ret == 0)
+        FAIL_EXIT1 ("writev: invalid return value zero");
+      /* Find the buffers that were successfully written.  */
+      while (ret > 0)
+        {
+          if (count == 0)
+            FAIL_EXIT1 ("internal writev consistency failure");
+          /* Current buffer was partially written.  */
+          if (buffers->iov_len > (size_t) ret)
+            {
+              buffers->iov_base += ret;
+              buffers->iov_len -= ret;
+              ret = 0;
+            }
+          else
+            {
+              ret -= buffers->iov_len;
+              buffers->iov_len = 0;
+              ++buffers;
+              --count;
+            }
+        }
+    }
+}
+
+/* Thread callback for handling a single established TCP connection to
+   a client.  */
+static void *
+server_thread_tcp_client (void *arg)
+{
+  struct tcp_thread_closure *closure = arg;
+
+  while (true)
+    {
+      /* Read packet length.  */
+      uint16_t query_length;
+      if (!read_fully (closure->client_socket,
+                       &query_length, sizeof (query_length), true))
+        break;
+      query_length = ntohs (query_length);
+
+      /* Read the packet.  */
+      unsigned char *query_buffer = xmalloc (query_length);
+      read_fully (closure->client_socket, query_buffer, query_length, false);
+
+      struct query_info qinfo;
+      parse_query (&qinfo, query_buffer, query_length);
+      if (test_verbose > 0)
+        {
+          if (test_verbose > 1)
+            printf ("info: UDP server %d: incoming query:"
+                    " %d bytes, %s/%u/%u, tnxid=0x%02x%02x\n",
+                    closure->server_index, query_length,
+                    qinfo.qname, qinfo.qclass, qinfo.qtype,
+                    query_buffer[0], query_buffer[1]);
+          else
+            printf ("info: TCP server %d: incoming query:"
+                    " %u bytes, %s/%u/%u\n",
+                    closure->server_index, query_length,
+                    qinfo.qname, qinfo.qclass, qinfo.qtype);
+        }
+
+      struct resolv_response_context ctx =
+        {
+          .query_buffer = query_buffer,
+          .query_length = query_length,
+          .server_index = closure->server_index,
+          .tcp = true,
+        };
+      struct resolv_response_builder *b = response_builder_allocate
+        (query_buffer, query_length);
+      closure->obj->config.response_callback
+        (&ctx, b, qinfo.qname, qinfo.qclass, qinfo.qtype);
+
+      if (b->drop)
+        {
+          if (test_verbose)
+            printf ("info: TCP server %d: dropping response to %s/%u/%u\n",
+                    closure->server_index,
+                    qinfo.qname, qinfo.qclass, qinfo.qtype);
+        }
+      else
+        {
+          if (test_verbose)
+            printf ("info: TCP server %d: sending response: %zu bytes"
+                    " (for %s/%u/%u)\n",
+                    closure->server_index, b->offset,
+                    qinfo.qname, qinfo.qclass, qinfo.qtype);
+          uint16_t length = htons (b->offset);
+          size_t to_send = b->offset;
+          if (to_send < b->truncate_bytes)
+            to_send = 0;
+          else
+            to_send -= b->truncate_bytes;
+          struct iovec buffers[2] =
+            {
+              {&length, sizeof (length)},
+              {b->buffer, to_send}
+            };
+          writev_fully (closure->client_socket, buffers, 2);
+        }
+      bool close_flag = b->close;
+      response_builder_free (b);
+      free (query_buffer);
+      if (close_flag)
+        break;
+    }
+
+  close (closure->client_socket);
+  free (closure);
+  return NULL;
+}
+
+/* thread_callback for the TCP case.  Accept connections and create a
+   new thread for each client.  */
+static void
+server_thread_tcp (struct resolv_test *obj, int server_index)
+{
+  while (true)
+    {
+      /* Get the client conenction.  */
+      int client_socket = xaccept
+        (obj->servers[server_index].socket_tcp, NULL, NULL);
+
+      /* Check for termination.  */
+      xpthread_mutex_lock (&obj->lock);
+      if (obj->termination_requested)
+        {
+          xpthread_mutex_unlock (&obj->lock);
+          close (client_socket);
+          break;
+        }
+      xpthread_mutex_unlock (&obj->lock);
+
+      /* Spawn a new thread for handling this connection.  */
+      struct tcp_thread_closure *closure = xmalloc (sizeof (*closure));
+      *closure = (struct tcp_thread_closure)
+        {
+          .obj = obj,
+          .server_index = server_index,
+          .client_socket = client_socket,
+        };
+
+      pthread_t thr
+        = xpthread_create (NULL, server_thread_tcp_client, closure);
+      /* TODO: We should keep track of this thread so that we can
+         block in resolv_test_end until it has exited.  */
+      xpthread_detach (thr);
+    }
+}
+
+/* Create UDP and TCP server sockets.  */
+static void
+make_server_sockets (struct resolv_test_server *server)
+{
+  while (true)
+    {
+      server->socket_udp = xsocket (AF_INET, SOCK_DGRAM, IPPROTO_UDP);
+      server->socket_tcp = xsocket (AF_INET, SOCK_STREAM, IPPROTO_TCP);
+
+      /* Pick the address for the UDP socket.  */
+      server->address = (struct sockaddr_in)
+        {
+          .sin_family = AF_INET,
+          .sin_addr = {.s_addr = htonl (INADDR_LOOPBACK)}
+        };
+      xbind (server->socket_udp,
+             (struct sockaddr *)&server->address, sizeof (server->address));
+
+      /* Retrieve the address. */
+      socklen_t addrlen = sizeof (server->address);
+      xgetsockname (server->socket_udp,
+                    (struct sockaddr *)&server->address, &addrlen);
+
+      /* Bind the TCP socket to the same address.  */
+      {
+        int on = 1;
+        xsetsockopt (server->socket_tcp, SOL_SOCKET, SO_REUSEADDR,
+                     &on, sizeof (on));
+      }
+      if (bind (server->socket_tcp,
+                (struct sockaddr *)&server->address,
+                sizeof (server->address)) != 0)
+        {
+          /* Port collision.  The UDP bind succeeded, but the TCP BIND
+             failed.  We assume here that the kernel will pick the
+             next local UDP address randomly.  */
+          if (errno == EADDRINUSE)
+            {
+              close (server->socket_udp);
+              close (server->socket_tcp);
+              continue;
+            }
+          FAIL_EXIT1 ("TCP bind: %m");
+        }
+      xlisten (server->socket_tcp, 5);
+      break;
+    }
+}
+
+/* One-time initialization of NSS.  */
+static void
+resolv_redirect_once (void)
+{
+  /* Only use nss_dns.  */
+  __nss_configure_lookup ("hosts", "dns");
+  __nss_configure_lookup ("networks", "dns");
+  /* Enter a network namespace for isolation and firewall state
+     cleanup.  The tests will still work if these steps fail, but they
+     may be less reliable.  */
+  support_become_root ();
+  support_enter_network_namespace ();
+}
+pthread_once_t resolv_redirect_once_var = PTHREAD_ONCE_INIT;
+
+void
+resolv_test_init (void)
+{
+  /* Perform one-time initialization of NSS.  */
+  xpthread_once (&resolv_redirect_once_var, resolv_redirect_once);
+}
+
+/* Copy the search path from CONFIG.search to the _res object.  */
+static void
+set_search_path (struct resolv_redirect_config config)
+{
+  memset (_res.defdname, 0, sizeof (_res.defdname));
+  memset (_res.dnsrch, 0, sizeof (_res.dnsrch));
+
+  char *current = _res.defdname;
+  char *end = current + sizeof (_res.defdname);
+
+  for (unsigned int i = 0;
+       i < sizeof (config.search) / sizeof (config.search[0]); ++i)
+    {
+      if (config.search[i] == NULL)
+        continue;
+
+      size_t length = strlen (config.search[i]) + 1;
+      size_t remaining = end - current;
+      TEST_VERIFY_EXIT (length <= remaining);
+      memcpy (current, config.search[i], length);
+      _res.dnsrch[i] = current;
+      current += length;
+    }
+}
+
+struct resolv_test *
+resolv_test_start (struct resolv_redirect_config config)
+{
+  /* Apply configuration defaults.  */
+  if (config.nscount == 0)
+    config.nscount = resolv_max_test_servers;
+
+  struct resolv_test *obj = xmalloc (sizeof (*obj));
+  *obj = (struct resolv_test) {
+    .config = config,
+    .lock = PTHREAD_MUTEX_INITIALIZER,
+  };
+
+  resolv_test_init ();
+
+  /* Create all the servers, to reserve the necessary ports.  */
+  for (int server_index = 0; server_index < config.nscount; ++server_index)
+    make_server_sockets (obj->servers + server_index);
+
+  /* Start server threads.  Disable the server ports, as
+     requested.  */
+  for (int server_index = 0; server_index < config.nscount; ++server_index)
+    {
+      struct resolv_test_server *server = obj->servers + server_index;
+      if (config.servers[server_index].disable_udp)
+        {
+          close (server->socket_udp);
+          server->socket_udp = -1;
+        }
+      else if (!config.single_thread_udp)
+        server->thread_udp = start_server_thread (obj, server_index,
+                                                  server_thread_udp);
+      if (config.servers[server_index].disable_tcp)
+        {
+          close (server->socket_tcp);
+          server->socket_tcp = -1;
+        }
+      else
+        server->thread_tcp = start_server_thread (obj, server_index,
+                                                  server_thread_tcp);
+    }
+  if (config.single_thread_udp)
+    start_server_thread_udp_single (obj);
+
+  int timeout = 1;
+
+  /* Initialize libresolv.  */
+  TEST_VERIFY_EXIT (res_init () == 0);
+
+  /* Disable IPv6 name server addresses.  The code below only
+     overrides the IPv4 addresses.  */
+  __res_iclose (&_res, true);
+  _res._u._ext.nscount = 0;
+
+  /* Redirect queries to the server socket.  */
+  if (test_verbose)
+    {
+      printf ("info: old timeout value: %d\n", _res.retrans);
+      printf ("info: old retry attempt value: %d\n", _res.retry);
+      printf ("info: old _res.options: 0x%lx\n", _res.options);
+      printf ("info: old _res.nscount value: %d\n", _res.nscount);
+      printf ("info: old _res.ndots value: %d\n", _res.ndots);
+    }
+  _res.retrans = timeout;
+  _res.retry = 4;
+  _res.nscount = config.nscount;
+  _res.options = RES_INIT | RES_RECURSE | RES_DEFNAMES | RES_DNSRCH;
+  _res.ndots = 1;
+  if (test_verbose)
+    {
+      printf ("info: new timeout value: %d\n", _res.retrans);
+      printf ("info: new retry attempt value: %d\n", _res.retry);
+      printf ("info: new _res.options: 0x%lx\n", _res.options);
+      printf ("info: new _res.nscount value: %d\n", _res.nscount);
+      printf ("info: new _res.ndots value: %d\n", _res.ndots);
+    }
+  for (int server_index = 0; server_index < config.nscount; ++server_index)
+    {
+      _res.nsaddr_list[server_index] = obj->servers[server_index].address;
+      if (test_verbose)
+        {
+          char buf[256];
+          TEST_VERIFY_EXIT
+            (inet_ntop (AF_INET, &obj->servers[server_index].address.sin_addr,
+                        buf, sizeof (buf)) != NULL);
+          printf ("info: server %d: %s/%u\n",
+                  server_index, buf,
+                  htons (obj->servers[server_index].address.sin_port));
+        }
+    }
+
+  set_search_path (config);
+
+  return obj;
+}
+
+void
+resolv_test_end (struct resolv_test *obj)
+{
+  res_close ();
+
+  xpthread_mutex_lock (&obj->lock);
+  obj->termination_requested = true;
+  xpthread_mutex_unlock (&obj->lock);
+
+  /* Send trigger packets to unblock the server threads.  */
+  for (int server_index = 0; server_index < obj->config.nscount;
+       ++server_index)
+    {
+      if (!obj->config.servers[server_index].disable_udp)
+        {
+          int sock = xsocket (AF_INET, SOCK_DGRAM, IPPROTO_UDP);
+          xsendto (sock, "", 1, 0,
+                   (struct sockaddr *) &obj->servers[server_index].address,
+                   sizeof (obj->servers[server_index].address));
+          close (sock);
+        }
+      if (!obj->config.servers[server_index].disable_tcp)
+        {
+          int sock = xsocket (AF_INET, SOCK_STREAM, IPPROTO_TCP);
+          xconnect (sock,
+                    (struct sockaddr *) &obj->servers[server_index].address,
+                    sizeof (obj->servers[server_index].address));
+          close (sock);
+        }
+    }
+
+  if (obj->config.single_thread_udp)
+    xpthread_join (obj->thread_udp_single);
+
+  /* Wait for the server threads to terminate.  */
+  for (int server_index = 0; server_index < obj->config.nscount;
+       ++server_index)
+    {
+      if (!obj->config.servers[server_index].disable_udp)
+        {
+          if (!obj->config.single_thread_udp)
+            xpthread_join (obj->servers[server_index].thread_udp);
+          close (obj->servers[server_index].socket_udp);
+        }
+      if (!obj->config.servers[server_index].disable_tcp)
+        {
+          xpthread_join (obj->servers[server_index].thread_tcp);
+          close (obj->servers[server_index].socket_tcp);
+        }
+    }
+
+  free (obj);
+}
diff --git a/support/resolv_test.h b/support/resolv_test.h
new file mode 100644
index 0000000..39dd625
--- /dev/null
+++ b/support/resolv_test.h
@@ -0,0 +1,169 @@
+/* DNS test framework and libresolv redirection.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#ifndef SUPPORT_RESOLV_TEST_H
+#define SUPPORT_RESOLV_TEST_H
+
+#include <arpa/nameser.h>
+#include <stdbool.h>
+#include <sys/cdefs.h>
+
+__BEGIN_DECLS
+
+/* This struct provides context information when the response callback
+   specified in struct resolv_redirect_config is invoked. */
+struct resolv_response_context
+{
+  const unsigned char *query_buffer;
+  size_t query_length;
+  int server_index;
+  bool tcp;
+};
+
+/* This opaque struct is used to construct responses from within the
+   response callback function.  */
+struct resolv_response_builder;
+
+/* This opaque struct collects information about the resolver testing
+   currently in progress.  */
+struct resolv_test;
+
+enum
+  {
+    /* Maximum number of test servers supported by the framework.  */
+    resolv_max_test_servers = 3,
+  };
+
+/* Configuration settings specific to individual test servers.  */
+struct resolv_redirect_server_config
+{
+  bool disable_tcp;             /* If true, no TCP server is listening.  */
+  bool disable_udp;             /* If true, no UDP server is listening.  */
+};
+
+/* Instructions for setting up the libresolv redirection.  */
+struct resolv_redirect_config
+{
+  /* The response_callback function is called for every incoming DNS
+     packet, over UDP or TCP.  It must be specified, the other
+     configuration settings are optional.  */
+  void (*response_callback) (const struct resolv_response_context *,
+                             struct resolv_response_builder *,
+                             const char *qname,
+                             uint16_t qclass, uint16_t qtype);
+
+  /* Per-server configuration.  */
+  struct resolv_redirect_server_config servers[resolv_max_test_servers];
+
+  /* Search path entries.  The first entry serves as the default
+     domain name as well.  */
+  const char *search[7];
+
+  /* Number of servers to activate in resolv.  0 means the default,
+     resolv_max_test_servers.  */
+  int nscount;
+
+  /* If true, use a single thread to process all UDP queries.  This
+     may results in more predictable ordering of queries and
+     responses.  */
+  bool single_thread_udp;
+};
+
+/* Configure NSS to use, nss_dns only for aplicable databases, and try
+   to put the process into a network namespace for better isolation.
+   This may have to be called before resolv_test_start, before the
+   process creates any threads.  Otherwise, initialization is
+   performed by resolv_test_start implicitly.  */
+void resolv_test_init (void);
+
+/* Initiate resolver testing.  This updates the _res variable as
+   needed.  As a side effect, NSS is reconfigured to use nss_dns only
+   for aplicable databases, and the process may enter a network
+   namespace for better isolation.  */
+struct resolv_test *resolv_test_start (struct resolv_redirect_config);
+
+/* Call this function at the end of resolver testing, to free
+   resources and report pending errors (if any).  */
+void resolv_test_end (struct resolv_test *);
+
+/* The remaining facilities in this file are used for constructing
+   response packets from the response_callback function.  */
+
+/* Special settings for constructing responses from the callback.  */
+struct resolv_response_flags
+{
+  /* 4-bit response code to incorporate into the response. */
+  unsigned char rcode;
+
+  /* If true, the TC (truncation) flag will be set.  */
+  bool tc;
+
+  /* Initial section count values.  Can be used to artificially
+     increase the counts, for malformed packet testing.*/
+  unsigned short qdcount;
+  unsigned short ancount;
+  unsigned short nscount;
+  unsigned short adcount;
+};
+
+/* Begin a new response with the requested flags.  Must be called
+   first.  */
+void resolv_response_init (struct resolv_response_builder *,
+                           struct resolv_response_flags);
+
+/* Switches to the section in the response packet.  Only forward
+   movement is supported.  */
+void resolv_response_section (struct resolv_response_builder *, ns_sect);
+
+/* Add a question record to the question section.  */
+void resolv_response_add_question (struct resolv_response_builder *,
+                                   const char *name, uint16_t class,
+                                   uint16_t type);
+/* Starts a new resource record with the specified owner name, class,
+   type, and TTL.  Data is supplied with resolv_response_add_data or
+   resolv_response_add_name.  */
+void resolv_response_open_record (struct resolv_response_builder *,
+                                  const char *name, uint16_t class,
+                                  uint16_t type, uint32_t ttl);
+
+/* Add unstructed bytes to the RDATA part of a resource record.  */
+void resolv_response_add_data (struct resolv_response_builder *,
+                               const void *, size_t);
+
+/* Add a compressed domain name to the RDATA part of a resource
+   record.  */
+void resolv_response_add_name (struct resolv_response_builder *,
+                               const char *name);
+
+/* Mark the end of the constructed record.  Must be called last.  */
+void resolv_response_close_record (struct resolv_response_builder *);
+
+/* Drop this query packet (that is, do not send a response, not even
+   an empty packet).  */
+void resolv_response_drop (struct resolv_response_builder *);
+
+/* In TCP mode, close the connection after this packet (if a response
+   is sent).  */
+void resolv_response_close (struct resolv_response_builder *);
+
+/* The size of the response packet built so far.  */
+size_t resolv_response_length (const struct resolv_response_builder *);
+
+__END_DECLS
+
+#endif /* SUPPORT_RESOLV_TEST_H */
diff --git a/support/run_diff.h b/support/run_diff.h
new file mode 100644
index 0000000..af340ef
--- /dev/null
+++ b/support/run_diff.h
@@ -0,0 +1,31 @@
+/* Invoke the system diff tool to compare two strings.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#ifndef SUPPORT_RUN_DIFF_H
+#define SUPPORT_RUN_DIFF_H
+
+/* Compare the two NUL-terminated strings LEFT and RIGHT using the
+   diff tool.  Label the sides of the diff with LEFT_LABEL and
+   RIGHT_LABEL, respectively.
+
+   This function assumes that LEFT and RIGHT are different
+   strings.  */
+void support_run_diff (const char *left_label, const char *left,
+                       const char *right_label, const char *right);
+
+#endif /* SUPPORT_RUN_DIFF_H */
diff --git a/support/support.h b/support/support.h
index fc7fba9..136e0ce 100644
--- a/support/support.h
+++ b/support/support.h
@@ -52,6 +52,7 @@ void *xcalloc (size_t n, size_t s) __attribute__ ((malloc));
 void *xrealloc (void *p, size_t n);
 char *xasprintf (const char *format, ...)
   __attribute__ ((format (printf, 1, 2), malloc));
+char *xstrdup (const char *);
 
 __END_DECLS
 
diff --git a/support/support_format_address_family.c b/support/support_format_address_family.c
new file mode 100644
index 0000000..06c6af7
--- /dev/null
+++ b/support/support_format_address_family.c
@@ -0,0 +1,35 @@
+/* Convert an address family to a string.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/format_nss.h>
+
+#include <support/support.h>
+
+char *
+support_format_address_family (int family)
+{
+  switch (family)
+    {
+    case AF_INET:
+      return xstrdup ("INET");
+    case AF_INET6:
+      return xstrdup ("INET6");
+    default:
+      return xasprintf ("<unknown address family %d>", family);
+    }
+}
diff --git a/support/support_format_addrinfo.c b/support/support_format_addrinfo.c
new file mode 100644
index 0000000..744aad4
--- /dev/null
+++ b/support/support_format_addrinfo.c
@@ -0,0 +1,202 @@
+/* Convert struct addrinfo values to a string.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/format_nss.h>
+
+#include <arpa/inet.h>
+#include <errno.h>
+#include <stdio.h>
+#include <support/support.h>
+#include <support/xmemstream.h>
+
+static size_t
+socket_address_length (int family)
+{
+  switch (family)
+    {
+    case AF_INET:
+      return sizeof (struct sockaddr_in);
+    case AF_INET6:
+      return sizeof (struct sockaddr_in6);
+    default:
+      return -1;
+    }
+}
+
+static void
+format_ai_flags (FILE *out, struct addrinfo *ai, int flag, const char *name,
+                 int * flags_printed)
+{
+  if ((ai->ai_flags & flag) != 0)
+    fprintf (out, " %s", name);
+  *flags_printed |= flag;
+}
+
+static void
+format_ai_one (FILE *out, struct addrinfo *ai, int *flags)
+{
+  /* ai_flags */
+  if (ai->ai_flags != *flags)
+    {
+      fprintf (out, "flags:");
+      int flags_printed = 0;
+#define FLAG(flag) format_ai_flags (out, ai, flag, #flag, &flags_printed)
+      FLAG (AI_PASSIVE);
+      FLAG (AI_CANONNAME);
+      FLAG (AI_NUMERICHOST);
+      FLAG (AI_V4MAPPED);
+      FLAG (AI_ALL);
+      FLAG (AI_ADDRCONFIG);
+      FLAG (AI_IDN);
+      FLAG (AI_CANONIDN);
+      FLAG (AI_IDN_ALLOW_UNASSIGNED);
+      FLAG (AI_IDN_USE_STD3_ASCII_RULES);
+      FLAG (AI_NUMERICSERV);
+#undef FLAG
+      int remaining = ai->ai_flags & ~flags_printed;
+      if (remaining != 0)
+        fprintf (out, " %08x", remaining);
+      fprintf (out, "\n");
+      *flags = ai->ai_flags;
+    }
+
+  {
+    char type_buf[32];
+    const char *type_str;
+    char proto_buf[32];
+    const char *proto_str;
+
+    /* ai_socktype */
+    switch (ai->ai_socktype)
+      {
+      case SOCK_RAW:
+        type_str = "RAW";
+        break;
+      case SOCK_DGRAM:
+        type_str = "DGRAM";
+        break;
+      case SOCK_STREAM:
+        type_str = "STREAM";
+        break;
+      default:
+        snprintf (type_buf, sizeof (type_buf), "%d", ai->ai_socktype);
+        type_str = type_buf;
+      }
+
+    /* ai_protocol */
+    switch (ai->ai_protocol)
+      {
+      case IPPROTO_IP:
+        proto_str = "IP";
+        break;
+      case IPPROTO_UDP:
+        proto_str = "UDP";
+        break;
+      case IPPROTO_TCP:
+        proto_str = "TCP";
+        break;
+      default:
+        snprintf (proto_buf, sizeof (proto_buf), "%d", ai->ai_protocol);
+        proto_str = proto_buf;
+      }
+    fprintf (out, "address: %s/%s", type_str, proto_str);
+  }
+
+  /* ai_addrlen */
+  if (ai->ai_addrlen != socket_address_length (ai->ai_family))
+    {
+      char *family = support_format_address_family (ai->ai_family);
+      fprintf (out, "error: invalid address length %d for %s\n",
+               ai->ai_addrlen, family);
+      free (family);
+    }
+
+  /* ai_addr */
+  {
+    char buf[128];
+    uint16_t port;
+    const char *ret;
+    switch (ai->ai_family)
+      {
+      case AF_INET:
+        {
+          struct sockaddr_in *sin = (struct sockaddr_in *) ai->ai_addr;
+          ret = inet_ntop (AF_INET, &sin->sin_addr, buf, sizeof (buf));
+          port = sin->sin_port;
+        }
+        break;
+      case AF_INET6:
+        {
+          struct sockaddr_in6 *sin = (struct sockaddr_in6 *) ai->ai_addr;
+          ret = inet_ntop (AF_INET6, &sin->sin6_addr, buf, sizeof (buf));
+          port = sin->sin6_port;
+        }
+        break;
+      default:
+        errno = EAFNOSUPPORT;
+        ret = NULL;
+      }
+    if (ret == NULL)
+        fprintf (out, "error: inet_top failed: %m\n");
+    else
+      fprintf (out, " %s %u\n", buf, ntohs (port));
+  }
+
+  /* ai_canonname */
+  if (ai->ai_canonname != NULL)
+    fprintf (out, "canonname: %s\n", ai->ai_canonname);
+}
+
+/* Format all the addresses in one address family.  */
+static void
+format_ai_family (FILE *out, struct addrinfo *ai, int family, int *flags)
+{
+  while (ai)
+    {
+      if (ai->ai_family == family)
+        format_ai_one (out, ai, flags);
+      ai = ai->ai_next;
+    }
+}
+
+char *
+support_format_addrinfo (struct addrinfo *ai, int ret)
+{
+  int errno_copy = errno;
+
+  struct xmemstream mem;
+  xopen_memstream (&mem);
+  if (ret != 0)
+    {
+      fprintf (mem.out, "error: %s\n", gai_strerror (ret));
+      if (ret == EAI_SYSTEM)
+        {
+          errno = errno_copy;
+          fprintf (mem.out, "error: %m\n");
+        }
+    }
+  else
+    {
+      int flags = 0;
+      format_ai_family (mem.out, ai, AF_INET, &flags);
+      format_ai_family (mem.out, ai, AF_INET6, &flags);
+    }
+
+  xfclose_memstream (&mem);
+  return mem.buffer;
+}
diff --git a/support/support_format_dns_packet.c b/support/support_format_dns_packet.c
new file mode 100644
index 0000000..d073ce0
--- /dev/null
+++ b/support/support_format_dns_packet.c
@@ -0,0 +1,215 @@
+/* Convert a DNS packet to a human-readable representation.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/format_nss.h>
+
+#include <arpa/inet.h>
+#include <resolv.h>
+#include <support/check.h>
+#include <support/support.h>
+#include <support/xmemstream.h>
+
+struct in_buffer
+{
+  const unsigned char *data;
+  size_t size;
+};
+
+static inline bool
+extract_8 (struct in_buffer *in, unsigned char *value)
+{
+  if (in->size == 0)
+    return false;
+  *value = in->data[0];
+  ++in->data;
+  --in->size;
+  return true;
+}
+
+static inline bool
+extract_16 (struct in_buffer *in, unsigned short *value)
+{
+  if (in->size < 2)
+    return false;
+  *value = (in->data[0] << 8) | in->data[1];
+  in->data += 2;
+  in->size -= 2;
+  return true;
+}
+
+static inline bool
+extract_32 (struct in_buffer *in, unsigned *value)
+{
+  if (in->size < 4)
+    return false;
+  unsigned a = in->data[0];
+  unsigned b = in->data[1];
+  unsigned c = in->data[2];
+  unsigned d = in->data[3];
+  *value = (a << 24) | (b << 16) | (c << 8) | d;
+  in->data += 4;
+  in->size -= 4;
+  return true;
+}
+
+static inline bool
+extract_bytes (struct in_buffer *in, size_t length, struct in_buffer *value)
+{
+  if (in->size < length)
+    return false;
+  *value = (struct in_buffer) {in->data, length};
+  in->data += length;
+  in->size -= length;
+  return true;
+}
+
+struct dname
+{
+  char name[MAXDNAME + 1];
+};
+
+static bool
+extract_name (struct in_buffer full, struct in_buffer *in, struct dname *value)
+{
+  const unsigned char *full_end = full.data + full.size;
+  /* Sanity checks; these indicate buffer misuse.  */
+  TEST_VERIFY_EXIT
+    (!(in->data < full.data || in->data > full_end
+       || in->size > (size_t) (full_end - in->data)));
+  int ret = dn_expand (full.data, full_end, in->data,
+                       value->name, sizeof (value->name));
+  if (ret < 0)
+    return false;
+  in->data += ret;
+  in->size -= ret;
+  return true;
+}
+
+char *
+support_format_dns_packet (const unsigned char *buffer, size_t length)
+{
+  struct in_buffer full = { buffer, length };
+  struct in_buffer in = full;
+  struct xmemstream mem;
+  xopen_memstream (&mem);
+
+  unsigned short txnid;
+  unsigned short flags;
+  unsigned short qdcount;
+  unsigned short ancount;
+  unsigned short nscount;
+  unsigned short adcount;
+  if (!(extract_16 (&in, &txnid)
+        && extract_16 (&in, &flags)
+        && extract_16 (&in, &qdcount)
+        && extract_16 (&in, &ancount)
+        && extract_16 (&in, &nscount)
+        && extract_16 (&in, &adcount)))
+    {
+      fprintf (mem.out, "error: could not parse DNS header\n");
+      goto out;
+    }
+  if (qdcount != 1)
+    {
+      fprintf (mem.out, "error: question count is %d, not 1\n", qdcount);
+      goto out;
+    }
+  struct dname qname;
+  if (!extract_name (full, &in, &qname))
+    {
+      fprintf (mem.out, "error: malformed QNAME\n");
+      goto out;
+    }
+  unsigned short qtype;
+  unsigned short qclass;
+  if (!(extract_16 (&in, &qtype)
+        && extract_16 (&in, &qclass)))
+    {
+      fprintf (mem.out, "error: malformed question\n");
+      goto out;
+    }
+  if (qtype != T_A && qtype != T_AAAA && qtype != T_PTR)
+    {
+      fprintf (mem.out, "error: unsupported QTYPE %d\n", qtype);
+      goto out;
+    }
+
+  fprintf (mem.out, "name: %s\n", qname.name);
+
+  for (int i = 0; i < ancount; ++i)
+    {
+      struct dname rname;
+      if (!extract_name (full, &in, &rname))
+        {
+          fprintf (mem.out, "error: malformed record name\n");
+          goto out;
+        }
+      unsigned short rtype;
+      unsigned short rclass;
+      unsigned ttl;
+      unsigned short rdlen;
+      struct in_buffer rdata;
+      if (!(extract_16 (&in, &rtype)
+            && extract_16 (&in, &rclass)
+            && extract_32 (&in, &ttl)
+            && extract_16 (&in, &rdlen)
+            && extract_bytes (&in, rdlen, &rdata)))
+        {
+          fprintf (mem.out, "error: malformed record header\n");
+          goto out;
+        }
+      /* Skip non-matching record types.  */
+      if (rtype != qtype || rclass != qclass)
+        continue;
+      switch (rtype)
+        {
+        case T_A:
+          if (rdlen == 4)
+              fprintf (mem.out, "address: %d.%d.%d.%d\n",
+                       rdata.data[0],
+                       rdata.data[1],
+                       rdata.data[2],
+                       rdata.data[3]);
+          else
+            fprintf (mem.out, "error: A record of size %d: %s\n", rdlen, rname.name);
+          break;
+        case T_AAAA:
+          {
+            char buf[100];
+            if (inet_ntop (AF_INET6, rdata.data, buf, sizeof (buf)) == NULL)
+              fprintf (mem.out, "error: AAAA record decoding failed: %m\n");
+            else
+              fprintf (mem.out, "address: %s\n", buf);
+          }
+          break;
+        case T_CNAME:
+        case T_PTR:
+          {
+            struct dname name;
+            if (extract_name (full, &in, &name))
+              fprintf (mem.out, "name: %s\n", name.name);
+            else
+              fprintf (mem.out, "error: malformed CNAME/PTR record\n");
+          }
+        }
+    }
+
+ out:
+  xfclose_memstream (&mem);
+  return mem.buffer;
+}
diff --git a/support/support_format_herrno.c b/support/support_format_herrno.c
new file mode 100644
index 0000000..42279f6
--- /dev/null
+++ b/support/support_format_herrno.c
@@ -0,0 +1,45 @@
+/* Convert a h_errno error code to a string.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/format_nss.h>
+
+#include <support/support.h>
+
+char *
+support_format_herrno (int code)
+{
+  const char *errstr;
+  switch (code)
+    {
+    case HOST_NOT_FOUND:
+      errstr = "HOST_NOT_FOUND";
+      break;
+    case NO_ADDRESS:
+      errstr = "NO_ADDRESS";
+      break;
+    case NO_RECOVERY:
+      errstr = "NO_RECOVERY";
+      break;
+    case TRY_AGAIN:
+      errstr = "TRY_AGAIN";
+      break;
+    default:
+      return xasprintf ("<invalid h_errno value %d>\n", code);
+    }
+  return xstrdup (errstr);
+}
diff --git a/support/support_format_hostent.c b/support/support_format_hostent.c
new file mode 100644
index 0000000..5622f69
--- /dev/null
+++ b/support/support_format_hostent.c
@@ -0,0 +1,75 @@
+/* Convert a struct hostent object to a string.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/format_nss.h>
+
+#include <arpa/inet.h>
+#include <stdio.h>
+#include <support/support.h>
+#include <support/xmemstream.h>
+
+static int
+address_length (int family)
+{
+  switch (family)
+    {
+    case AF_INET:
+      return 4;
+    case AF_INET6:
+      return 16;
+    }
+  return -1;
+}
+
+char *
+support_format_hostent (struct hostent *h)
+{
+  if (h == NULL)
+    {
+      char *value = support_format_herrno (h_errno);
+      char *result = xasprintf ("error: %s\n", value);
+      free (value);
+      return result;
+    }
+
+  struct xmemstream mem;
+  xopen_memstream (&mem);
+
+  fprintf (mem.out, "name: %s\n", h->h_name);
+  for (char **alias = h->h_aliases; *alias != NULL; ++alias)
+    fprintf (mem.out, "alias: %s\n", *alias);
+  for (unsigned i = 0; h->h_addr_list[i] != NULL; ++i)
+    {
+      char buf[128];
+      if (inet_ntop (h->h_addrtype, h->h_addr_list[i],
+                     buf, sizeof (buf)) == NULL)
+        fprintf (mem.out, "error: inet_ntop failed: %m\n");
+      else
+        fprintf (mem.out, "address: %s\n", buf);
+    }
+  if (h->h_length != address_length (h->h_addrtype))
+    {
+      char *family = support_format_address_family (h->h_addrtype);
+      fprintf (mem.out, "error: invalid address length %d for %s\n",
+               h->h_length, family);
+      free (family);
+    }
+
+  xfclose_memstream (&mem);
+  return mem.buffer;
+}
diff --git a/support/support_format_netent.c b/support/support_format_netent.c
new file mode 100644
index 0000000..51a3a6b
--- /dev/null
+++ b/support/support_format_netent.c
@@ -0,0 +1,50 @@
+/* Convert a struct netent object to a string.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/format_nss.h>
+
+#include <arpa/inet.h>
+#include <stdio.h>
+#include <support/support.h>
+#include <support/xmemstream.h>
+
+char *
+support_format_netent (struct netent *e)
+{
+  if (e == NULL)
+    {
+      char *value = support_format_herrno (h_errno);
+      char *result = xasprintf ("error: %s\n", value);
+      free (value);
+      return result;
+    }
+
+  struct xmemstream mem;
+  xopen_memstream (&mem);
+
+  if (e->n_name != NULL)
+    fprintf (mem.out, "name: %s\n", e->n_name);
+  for (char **ap = e->n_aliases; *ap != NULL; ++ap)
+    fprintf (mem.out, "alias: %s\n", *ap);
+  if (e->n_addrtype != AF_INET)
+    fprintf (mem.out, "addrtype: %d\n", e->n_addrtype);
+  fprintf (mem.out, "net: 0x%08x\n", e->n_net);
+
+  xfclose_memstream (&mem);
+  return mem.buffer;
+}
diff --git a/support/support_run_diff.c b/support/support_run_diff.c
new file mode 100644
index 0000000..a963e01
--- /dev/null
+++ b/support/support_run_diff.c
@@ -0,0 +1,76 @@
+/* Invoke the system diff tool to compare two strings.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/run_diff.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <support/check.h>
+#include <support/support.h>
+#include <support/temp_file.h>
+#include <sys/wait.h>
+#include <xunistd.h>
+
+static char *
+write_to_temp_file (const char *prefix, const char *str)
+{
+  char *template = xasprintf ("run_diff-%s", prefix);
+  char *name = NULL;
+  int fd = create_temp_file (template, &name);
+  TEST_VERIFY_EXIT (fd >= 0);
+  free (template);
+  xwrite (fd, str, strlen (str));
+  TEST_VERIFY_EXIT (close (fd) == 0);
+  return name;
+}
+
+void
+support_run_diff (const char *left_label, const char *left,
+                  const char *right_label, const char *right)
+{
+  /* Ensure that the diff command output is ordered properly with
+     standard output.  */
+  TEST_VERIFY_EXIT (fflush (stdout) == 0);
+
+  char *left_path = write_to_temp_file ("left-diff", left);
+  char *right_path = write_to_temp_file ("right-diff", right);
+
+  pid_t pid = xfork ();
+  if (pid == 0)
+    {
+      execlp ("diff", "diff", "-u",
+              "--label", left_label, "--label", right_label,
+              "--", left_path, right_path,
+              NULL);
+      _exit (17);
+    }
+  else
+    {
+      int status;
+      xwaitpid (pid, &status, 0);
+      if (!WIFEXITED (status) || WEXITSTATUS (status) != 1)
+        printf ("warning: could not run diff, exit status: %d\n"
+                "*** %s ***\n%s\n"
+                "*** %s ***\n%s\n",
+                status, left_label, left, right_label, right);
+    }
+
+  free (right_path);
+  free (left_path);
+}
diff --git a/support/xaccept.c b/support/xaccept.c
new file mode 100644
index 0000000..387fc9f
--- /dev/null
+++ b/support/xaccept.c
@@ -0,0 +1,32 @@
+/* accept with error checking.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/xsocket.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <support/check.h>
+
+int
+xaccept (int fd, struct sockaddr *sa, socklen_t *salen)
+{
+  int clientfd = accept (fd, sa, salen);
+  if (clientfd < 0)
+    FAIL_EXIT1 ("accept (%d): %m", fd);
+  return clientfd;
+}
diff --git a/support/xbind.c b/support/xbind.c
new file mode 100644
index 0000000..59e0c3a
--- /dev/null
+++ b/support/xbind.c
@@ -0,0 +1,30 @@
+/* bind with error checking.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/xsocket.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <support/check.h>
+
+void
+xbind (int fd, const struct sockaddr *sa, socklen_t sa_len)
+{
+  if (bind (fd, sa, sa_len) != 0)
+    FAIL_EXIT1 ("bind (%d), family %d: %m", fd, sa->sa_family);
+}
diff --git a/support/xconnect.c b/support/xconnect.c
new file mode 100644
index 0000000..b4fe94c
--- /dev/null
+++ b/support/xconnect.c
@@ -0,0 +1,30 @@
+/* connect with error checking.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/xsocket.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <support/check.h>
+
+void
+xconnect (int fd, const struct sockaddr *sa, socklen_t sa_len)
+{
+  if (connect (fd, sa, sa_len) != 0)
+    FAIL_EXIT1 ("connect (%d), family %d: %m", fd, sa->sa_family);
+}
diff --git a/support/xfclose.c b/support/xfclose.c
new file mode 100644
index 0000000..45ca08c
--- /dev/null
+++ b/support/xfclose.c
@@ -0,0 +1,33 @@
+/* fclose with error checking.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/xstdio.h>
+
+#include <support/check.h>
+#include <stdlib.h>
+
+void
+xfclose (FILE *fp)
+{
+  if (ferror (fp))
+    FAIL_EXIT1 ("stdio stream closed with pending errors");
+  if (fflush (fp) != 0)
+    FAIL_EXIT1 ("fflush: %m");
+  if (fclose (fp) != 0)
+    FAIL_EXIT1 ("fclose: %m");
+}
diff --git a/support/xfopen.c b/support/xfopen.c
new file mode 100644
index 0000000..436ab55
--- /dev/null
+++ b/support/xfopen.c
@@ -0,0 +1,31 @@
+/* fopen with error checking.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/xstdio.h>
+
+#include <support/check.h>
+#include <stdlib.h>
+
+FILE *
+xfopen (const char *path, const char *mode)
+{
+  FILE *fp = fopen (path, mode);
+  if (fp == NULL)
+    FAIL_EXIT1 ("could not open %s (mode \"%s\"): %m", path, mode);
+  return fp;
+}
diff --git a/support/xgetsockname.c b/support/xgetsockname.c
new file mode 100644
index 0000000..c66981e
--- /dev/null
+++ b/support/xgetsockname.c
@@ -0,0 +1,30 @@
+/* getsockname with error checking.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/xsocket.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <support/check.h>
+
+void
+xgetsockname (int fd, struct sockaddr *sa, socklen_t *plen)
+{
+  if (getsockname (fd, sa, plen) != 0)
+    FAIL_EXIT1 ("setsockopt (%d): %m", fd);
+}
diff --git a/support/xlisten.c b/support/xlisten.c
new file mode 100644
index 0000000..e099e5b
--- /dev/null
+++ b/support/xlisten.c
@@ -0,0 +1,30 @@
+/* listen with error checking.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/xsocket.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <support/check.h>
+
+void
+xlisten (int fd, int backlog)
+{
+  if (listen (fd, backlog) != 0)
+    FAIL_EXIT1 ("listen (%d, %d): %m", fd, backlog);
+}
diff --git a/support/xmemstream.c b/support/xmemstream.c
new file mode 100644
index 0000000..697d7c7
--- /dev/null
+++ b/support/xmemstream.c
@@ -0,0 +1,42 @@
+/* Error-checking wrappers for memstream functions.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/xmemstream.h>
+
+#include <errno.h>
+#include <stdlib.h>
+#include <support/check.h>
+#include <support/xstdio.h>
+
+void
+xopen_memstream (struct xmemstream *stream)
+{
+  int old_errno = errno;
+  *stream = (struct xmemstream) {};
+  stream->out = open_memstream (&stream->buffer, &stream->length);
+  if (stream->out == NULL)
+    FAIL_EXIT1 ("open_memstream: %m");
+  errno = old_errno;
+}
+
+void
+xfclose_memstream (struct xmemstream *stream)
+{
+  xfclose (stream->out);
+  stream->out = NULL;
+}
diff --git a/support/xmemstream.h b/support/xmemstream.h
new file mode 100644
index 0000000..bb3394a
--- /dev/null
+++ b/support/xmemstream.h
@@ -0,0 +1,49 @@
+/* Error-checking wrappers for memstream functions.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#ifndef SUPPORT_XMEMSTREAM_H
+#define SUPPORT_XMEMSTREAM_H
+
+#include <stdio.h>
+#include <sys/cdefs.h>
+
+__BEGIN_DECLS
+
+/* Wrappers for other libc functions.  */
+struct xmemstream
+{
+  FILE *out;
+  char *buffer;
+  size_t length;
+};
+
+/* Create a new in-memory stream.  Initializes *STREAM.  After this
+   function returns, STREAM->out is a file descriptor open for
+   writing.  errno is preserved, so that the %m format specifier can
+   be used for writing to STREAM->out.  */
+void xopen_memstream (struct xmemstream *stream);
+
+/* Closes STREAM->OUT.  After this function returns, STREAM->buffer
+   and STREAM->length denote a memory range which contains the bytes
+   written to the output stream.  The caller should free
+   STREAM->buffer.  */
+void xfclose_memstream (struct xmemstream *stream);
+
+__END_DECLS
+
+#endif /* SUPPORT_XMEMSTREAM_H */
diff --git a/support/xpoll.c b/support/xpoll.c
new file mode 100644
index 0000000..3aaecde
--- /dev/null
+++ b/support/xpoll.c
@@ -0,0 +1,32 @@
+/* poll with error checking.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/xsocket.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <support/check.h>
+
+int
+xpoll (struct pollfd *fds, nfds_t nfds, int timeout)
+{
+  int ret = poll (fds, nfds, timeout);
+  if (ret < 0)
+    FAIL_EXIT1 ("poll: %m");
+  return ret;
+}
diff --git a/support/xpthread_once.c b/support/xpthread_once.c
new file mode 100644
index 0000000..6ba0c69
--- /dev/null
+++ b/support/xpthread_once.c
@@ -0,0 +1,25 @@
+/* pthread_once with error checking.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/xthread.h>
+
+void
+xpthread_once (pthread_once_t *guard, void (*func) (void))
+{
+  xpthread_check_return ("pthread_once", pthread_once (guard, func));
+}
diff --git a/support/xrecvfrom.c b/support/xrecvfrom.c
new file mode 100644
index 0000000..2cd473a
--- /dev/null
+++ b/support/xrecvfrom.c
@@ -0,0 +1,33 @@
+/* recvfrom with error checking.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/xsocket.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <support/check.h>
+
+size_t
+xrecvfrom (int fd, void *buf, size_t buflen, int flags,
+           struct sockaddr *sa, socklen_t *salen)
+{
+  ssize_t ret = recvfrom (fd, buf, buflen, flags, sa, salen);
+  if (ret < 0)
+    FAIL_EXIT1 ("error: recvfrom (%d), %zu bytes buffer: %m", fd, buflen);
+  return ret;
+}
diff --git a/support/xsendto.c b/support/xsendto.c
new file mode 100644
index 0000000..6963aa7
--- /dev/null
+++ b/support/xsendto.c
@@ -0,0 +1,35 @@
+/* sendto with error checking.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/xsocket.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <support/check.h>
+
+void
+xsendto (int fd, const void *buf, size_t buflen, int flags,
+         const struct sockaddr *sa, socklen_t salen)
+{
+  ssize_t ret = sendto (fd, buf, buflen, flags, sa, salen);
+  if (ret < 0)
+    FAIL_EXIT1 ("sendto (%d), %zu bytes, family %d: %m",
+                fd, buflen, sa->sa_family);
+  if (ret != buflen)
+    FAIL_EXIT1 ("sendto (%d) sent %zd bytes instead of %zu", fd, ret, buflen);
+}
diff --git a/support/xsetsockopt.c b/support/xsetsockopt.c
new file mode 100644
index 0000000..8996554
--- /dev/null
+++ b/support/xsetsockopt.c
@@ -0,0 +1,31 @@
+/* setsockopt with error checking.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/xsocket.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <support/check.h>
+
+void
+xsetsockopt (int fd, int level, int name, const void *val, socklen_t vallen)
+{
+  if (setsockopt (fd, level, name, val, vallen) != 0)
+    FAIL_EXIT1 ("setsockopt (%d, %d, %d), %zu bytes: %m",
+                fd, level, name, (size_t) vallen);
+}
diff --git a/support/xsocket.h b/support/xsocket.h
index e9ff49e..c376e09 100644
--- a/support/xsocket.h
+++ b/support/xsocket.h
@@ -19,9 +19,20 @@
 #ifndef SUPPORT_XSOCKET_H
 #define SUPPORT_XSOCKET_H
 
+#include <poll.h>
 #include <sys/socket.h>
 #include <sys/types.h>
 
 int xsocket (int, int, int);
+void xsetsockopt (int, int, int, const void *, socklen_t);
+void xgetsockname (int, struct sockaddr *, socklen_t *);
+void xconnect (int, const struct sockaddr *, socklen_t);
+void xbind (int, const struct sockaddr *, socklen_t);
+void xlisten (int, int);
+int xaccept (int, struct sockaddr *, socklen_t *);
+void xsendto (int, const void *, size_t, int,
+              const struct sockaddr *, socklen_t);
+size_t xrecvfrom (int, void *, size_t, int, struct sockaddr *, socklen_t *);
+int xpoll (struct pollfd *, nfds_t, int);
 
 #endif /* SUPPORT_XSOCKET_H */
diff --git a/support/xstdio.h b/support/xstdio.h
new file mode 100644
index 0000000..99601ba
--- /dev/null
+++ b/support/xstdio.h
@@ -0,0 +1,32 @@
+/* Error-checking wrappers for stdio functions.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#ifndef SUPPORT_XSTDIO_H
+#define SUPPORT_XSTDIO_H
+
+#include <stdio.h>
+#include <sys/cdefs.h>
+
+__BEGIN_DECLS
+
+FILE *xfopen (const char *path, const char *mode);
+void xfclose (FILE *);
+
+__END_DECLS
+
+#endif /* SUPPORT_XSTDIO_H */
diff --git a/support/xstrdup.c b/support/xstrdup.c
new file mode 100644
index 0000000..62650fa
--- /dev/null
+++ b/support/xstrdup.c
@@ -0,0 +1,30 @@
+/* strdup with error checking.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/support.h>
+
+#include <string.h>
+
+char *
+xstrdup (const char *s)
+{
+  char *p = strdup (s);
+  if (p == NULL)
+    oom_error ("strdup", strlen (s));
+  return p;
+}
diff --git a/support/xthread.h b/support/xthread.h
index f0dc0fa..375d882 100644
--- a/support/xthread.h
+++ b/support/xthread.h
@@ -51,6 +51,7 @@ pthread_t xpthread_create (pthread_attr_t *attr,
 void xpthread_detach (pthread_t thr);
 void xpthread_cancel (pthread_t thr);
 void *xpthread_join (pthread_t thr);
+void xpthread_once (pthread_once_t *guard, void (*func) (void));
 
 /* This function returns non-zero if pthread_barrier_wait returned
    PTHREAD_BARRIER_SERIAL_THREAD.  */
diff --git a/support/xunistd.h b/support/xunistd.h
index f0c7419..0fe532a8 100644
--- a/support/xunistd.h
+++ b/support/xunistd.h
@@ -30,6 +30,9 @@ __BEGIN_DECLS
 pid_t xfork (void);
 pid_t xwaitpid (pid_t, int *status, int flags);
 
+/* Write the buffer.  Retry on short writes.  */
+void xwrite (int, const void *, size_t);
+
 __END_DECLS
 
 #endif /* SUPPORT_XUNISTD_H */
diff --git a/support/xwrite.c b/support/xwrite.c
new file mode 100644
index 0000000..452359e
--- /dev/null
+++ b/support/xwrite.c
@@ -0,0 +1,39 @@
+/* write with error checking and retries.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/xunistd.h>
+
+#include <support/check.h>
+
+void
+xwrite (int fd, const void *buffer, size_t length)
+{
+  const char *p = buffer;
+  const char *end = p + length;
+  while (p < end)
+    {
+      ssize_t ret = write (fd, p, end - p);
+      if (ret < 0)
+        FAIL_EXIT1 ("write of %zu bytes failed after %zd: %m",
+                    length, p - (const char *) buffer);
+      if (ret == 0)
+        FAIL_EXIT1 ("write return 0 after writing %zd bytes of %zu",
+                    p - (const char *) buffer, length);
+      p += ret;
+    }
+}