resolv: Add DNS packet parsing helpers geared towards wire format

The public parser functions around the ns_rr record type produce
textual domain names, but usually, this is not what we need while
parsing DNS packets within glibc.  This commit adds two new helper
functions, __ns_rr_cursor_init and __ns_rr_cursor_next, for writing
packet parsers, and struct ns_rr_cursor, struct ns_rr_wire as
supporting types.

In theory, it is possible to avoid copying the owner name
into the rname field in __ns_rr_cursor_next, but this would need
more functions that work on compressed names.

Eventually, __res_context_send could be enhanced to preserve the
result of the packet parsing that is necessary for matching the
incoming UDP packets, so that this works does not have to be done
twice.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

1 files changed, 92 insertions, 0 deletions

diff --git a/include/arpa/nameser.h b/include/arpa/nameser.h
index 6e4808f..c27e788 100644
--- a/include/arpa/nameser.h
+++ b/include/arpa/nameser.h
@@ -103,5 +103,97 @@ libc_hidden_proto (__libc_ns_samename)
    must point one past the last byte in the packet.  */
 int __ns_name_length_uncompressed (const unsigned char *p,
 				   const unsigned char *eom) attribute_hidden;
+
+/* Iterator over the resource records in a DNS packet.  */
+struct ns_rr_cursor
+{
+  /* These members are not changed after initialization.  */
+  const unsigned char *begin;	/* First byte of packet.  */
+  const unsigned char *end;	/* One past the last byte of the packet.  */
+  const unsigned char *first_rr; /* First resource record (or packet end).  */
+
+  /* Advanced towards the end while reading the packet.  */
+  const unsigned char *current;
+};
+
+/* Returns the RCODE field from the DNS header.  */
+static inline int
+ns_rr_cursor_rcode (const struct ns_rr_cursor *c)
+{
+  return c->begin[3] & 0x0f;	/* Lower 4 bits at offset 3.  */
+}
+
+/* Returns the length of the answer section according to the DNS header.  */
+static inline int
+ns_rr_cursor_ancount (const struct ns_rr_cursor *c)
+{
+  return c->begin[6] * 256 + c->begin[7]; /* 16 bits at offset 6.  */
+}
+
+/* Returns the length of the authority (name server) section according
+   to the DNS header.  */
+static inline int
+ns_rr_cursor_nscount (const struct ns_rr_cursor *c)
+{
+  return c->begin[8] * 256 + c->begin[9]; /* 16 bits at offset 8.  */
+}
+
+/* Returns the length of the additional data section according to the
+   DNS header.  */
+static inline int
+ns_rr_cursor_adcount (const struct ns_rr_cursor *c)
+{
+  return c->begin[10] * 256 + c->begin[11]; /* 16 bits at offset 10.  */
+}
+
+/* Returns a pointer to the uncompressed question name in wire
+   format.  */
+static inline const unsigned char *
+ns_rr_cursor_qname (const struct ns_rr_cursor *c)
+{
+  return c->begin + 12;		/* QNAME starts right after the header.  */
+}
+
+/* Returns the question type of the first and only question.  */
+static inline const int
+ns_rr_cursor_qtype (const struct ns_rr_cursor *c)
+{
+  /* 16 bits 4 bytes back from the first RR header start.  */
+  return c->first_rr[-4] * 256 + c->first_rr[-3];
+}
+
+/* Returns the clss of the first and only question (usally C_IN).  */
+static inline const int
+ns_rr_cursor_qclass (const struct ns_rr_cursor *c)
+{
+  /* 16 bits 2 bytes back from the first RR header start.  */
+  return c->first_rr[-2] * 256 + c->first_rr[-1];
+}
+
+/* Initializes *C to cover the packet [BUF, BUF+LEN).  Returns false
+   if LEN is less than sizeof (*HD), if the packet does not contain a
+   full (uncompressed) question, or if the question count is not 1.  */
+_Bool __ns_rr_cursor_init (struct ns_rr_cursor *c,
+			   const unsigned char *buf, size_t len)
+  attribute_hidden;
+
+/* Like ns_rr, but the record owner name is not decoded into text format.  */
+struct ns_rr_wire
+{
+  unsigned char rname[NS_MAXCDNAME]; /* Owner name of the record.  */
+  uint16_t rtype;		/* Resource record type (T_*).  */
+  uint16_t rclass;		/* Resource record class (C_*).  */
+  uint32_t ttl;			/* Time-to-live field.  */
+  const unsigned char *rdata;	/* Start of resource record data.  */
+  uint16_t rdlength;		/* Length of the data at rdata, in bytes.  */
+};
+
+/* Attempts to parse the record at C into *RR.  On success, return
+   true, and C is advanced past the record, and RR->rdata points to
+   the record data.  On failure, errno is set to EMSGSIZE, and false
+   is returned.  */
+_Bool __ns_rr_cursor_next (struct ns_rr_cursor *c, struct ns_rr_wire *rr)
+  attribute_hidden;
+
 # endif /* !_ISOMAC */
 #endif