add basic dns record parsing functions

based on patch by Timo Teräs, with some corrections to bounds checking
code and other minor changes.

while they are borderline scope creep, the functions added are fairly
small and are roughly the minimum code needed to use the results of
the res_query API without re-implementing error-prone DNS packet
parsing, and they are used in practice by some kerberos related
software and possibly other things. at this time there is no intent to
implement further nameser.h API functions.

1 files changed, 171 insertions, 0 deletions

diff --git a/src/network/ns_parse.c b/src/network/ns_parse.c
new file mode 100644
index 0000000..3ff33a8
--- /dev/null
+++ b/src/network/ns_parse.c
@@ -0,0 +1,171 @@
+#define _BSD_SOURCE
+#include <errno.h>
+#include <stddef.h>
+#include <resolv.h>
+#include <arpa/nameser.h>
+
+const struct _ns_flagdata _ns_flagdata[16] = {
+	{ 0x8000, 15 },
+	{ 0x7800, 11 },
+	{ 0x0400, 10 },
+	{ 0x0200, 9 },
+	{ 0x0100, 8 },
+	{ 0x0080, 7 },
+	{ 0x0040, 6 },
+	{ 0x0020, 5 },
+	{ 0x0010, 4 },
+	{ 0x000f, 0 },
+	{ 0x0000, 0 },
+	{ 0x0000, 0 },
+	{ 0x0000, 0 },
+	{ 0x0000, 0 },
+	{ 0x0000, 0 },
+	{ 0x0000, 0 },
+};
+
+unsigned ns_get16(const unsigned char *cp)
+{
+	return cp[0]<<8 | cp[1];
+}
+
+unsigned long ns_get32(const unsigned char *cp)
+{
+	return (unsigned)cp[0]<<24 | cp[1]<<16 | cp[2]<<8 | cp[3];
+}
+
+void ns_put16(unsigned s, unsigned char *cp)
+{
+	*cp++ = s>>8;
+	*cp++ = s;
+}
+
+void ns_put32(unsigned long l, unsigned char *cp)
+{
+	*cp++ = l>>24;
+	*cp++ = l>>16;
+	*cp++ = l>>8;
+	*cp++ = l;
+}
+
+int ns_initparse(const unsigned char *msg, int msglen, ns_msg *handle)
+{
+	int i, r;
+
+	handle->_msg = msg;
+	handle->_eom = msg + msglen;
+	if (msglen < (2 + ns_s_max) * NS_INT16SZ) goto bad;
+	NS_GET16(handle->_id, msg);
+	NS_GET16(handle->_flags, msg);
+	for (i = 0; i < ns_s_max; i++) NS_GET16(handle->_counts[i], msg);
+	for (i = 0; i < ns_s_max; i++) {
+		if (handle->_counts[i]) {
+			handle->_sections[i] = msg;
+			r = ns_skiprr(msg, handle->_eom, i, handle->_counts[i]);
+			if (r < 0) return -1;
+			msg += r;
+		} else {
+			handle->_sections[i] = NULL;
+		}
+	}
+	if (msg != handle->_eom) goto bad;
+	handle->_sect = ns_s_max;
+	handle->_rrnum = -1;
+	handle->_msg_ptr = NULL;
+	return 0;
+bad:
+	errno = EMSGSIZE;
+	return -1;
+}
+
+int ns_skiprr(const unsigned char *ptr, const unsigned char *eom, ns_sect section, int count)
+{
+	const unsigned char *p = ptr;
+	int r;
+
+	while (count--) {
+		r = dn_skipname(p, eom);
+		if (r < 0) goto bad;
+		if (r + 2 * NS_INT16SZ > eom - p) goto bad;
+		p += r + 2 * NS_INT16SZ;
+		if (section != ns_s_qd) {
+			if (NS_INT32SZ + NS_INT16SZ > eom - p) goto bad;
+			p += NS_INT32SZ;
+			NS_GET16(r, p);
+			if (r > eom - p) goto bad;
+			p += r;
+		}
+	}
+	return ptr - p;
+bad:
+	errno = EMSGSIZE;
+	return -1;
+}
+
+int ns_parserr(ns_msg *handle, ns_sect section, int rrnum, ns_rr *rr)
+{
+	int r;
+
+	if (section < 0 || section >= ns_s_max) goto bad;
+	if (section != handle->_sect) {
+		handle->_sect = section;
+		handle->_rrnum = 0;
+		handle->_msg_ptr = handle->_sections[section];
+	}
+	if (rrnum == -1) rrnum = handle->_rrnum;
+	if (rrnum < 0 || rrnum >= handle->_counts[section]) goto bad;
+	if (rrnum < handle->_rrnum) {
+		handle->_rrnum = 0;
+		handle->_msg_ptr = handle->_sections[section];
+	}
+	if (rrnum > handle->_rrnum) {
+		r = ns_skiprr(handle->_msg_ptr, handle->_eom, section, rrnum - handle->_rrnum);
+		if (r < 0) return -1;
+		handle->_msg_ptr += r;
+		handle->_rrnum = rrnum;
+	}
+	r = ns_name_uncompress(handle->_msg, handle->_eom, handle->_msg_ptr, rr->name, NS_MAXDNAME);
+	if (r < 0) return -1;
+	handle->_msg_ptr += r;
+	if (2 * NS_INT16SZ > handle->_eom - handle->_msg_ptr) goto size;
+	NS_GET16(rr->type, handle->_msg_ptr);
+	NS_GET16(rr->rr_class, handle->_msg_ptr);
+	if (section != ns_s_qd) {
+		if (NS_INT32SZ + NS_INT16SZ > handle->_eom - handle->_msg_ptr) goto size;
+		NS_GET32(rr->ttl, handle->_msg_ptr);
+		NS_GET16(rr->rdlength, handle->_msg_ptr);
+		if (rr->rdlength > handle->_eom - handle->_msg_ptr) goto size;
+		rr->rdata = handle->_msg_ptr;
+		handle->_msg_ptr += rr->rdlength;
+	} else {
+		rr->ttl = 0;
+		rr->rdlength = 0;
+		rr->rdata = NULL;
+	}
+	handle->_rrnum++;
+	if (handle->_rrnum > handle->_counts[section]) {
+		handle->_sect = section + 1;
+		if (handle->_sect == ns_s_max) {
+			handle->_rrnum = -1;
+			handle->_msg_ptr = NULL;
+		} else {
+			handle->_rrnum = 0;
+		}
+	}
+	return 0;
+bad:
+	errno = ENODEV;
+	return -1;
+size:
+	errno = EMSGSIZE;
+	return -1;
+}
+
+int ns_name_uncompress(const unsigned char *msg, const unsigned char *eom,
+                       const unsigned char *src, char *dst, size_t dstsiz)
+{
+	int r;
+	r = dn_expand(msg, eom, src, dst, dstsiz);
+	if (r < 0) errno = EMSGSIZE;
+	return r;
+}
+