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.

include/arpa/nameser.h

@@ -1,6 +1,11 @@
 #ifndef _ARPA_NAMESER_H
 #define _ARPA_NAMESER_H
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include <stddef.h>
 #include <stdint.h>
 
 #define __NAMESER	19991006
@@ -48,6 +53,8 @@ extern const struct _ns_flagdata _ns_flagdata[];
 #define ns_msg_end(handle) ((handle)._eom + 0)
 #define ns_msg_size(handle) ((handle)._eom - (handle)._msg)
 #define ns_msg_count(handle, section) ((handle)._counts[section] + 0)
+#define ns_msg_getflag(handle, flag) \
+	(((handle)._flags & _ns_flagdata[flag].mask) >> _ns_flagdata[flag].shift)
 
 typedef	struct __ns_rr {
 	char		name[NS_MAXDNAME];
@@ -296,43 +303,20 @@ typedef enum __ns_cert_types {
 #define NS_OPT_DNSSEC_OK        0x8000U
 #define NS_OPT_NSID		3
 
-#define NS_GET16(s, cp) do { \
-	register const unsigned char *t_cp = (const unsigned char *)(cp); \
-	(s) = ((uint16_t)t_cp[0] << 8) \
-	    | ((uint16_t)t_cp[1]) \
-	    ; \
-	(cp) += NS_INT16SZ; \
-} while (0)
-
-#define NS_GET32(l, cp) do { \
-	register const unsigned char *t_cp = (const unsigned char *)(cp); \
-	(l) = ((uint32_t)t_cp[0] << 24) \
-	    | ((uint32_t)t_cp[1] << 16) \
-	    | ((uint32_t)t_cp[2] << 8) \
-	    | ((uint32_t)t_cp[3]) \
-	    ; \
-	(cp) += NS_INT32SZ; \
-} while (0)
-
-#define NS_PUT16(s, cp) do { \
-	register uint16_t t_s = (uint16_t)(s); \
-	register unsigned char *t_cp = (unsigned char *)(cp); \
-	*t_cp++ = t_s >> 8; \
-	*t_cp   = t_s; \
-	(cp) += NS_INT16SZ; \
-} while (0)
-
-#define NS_PUT32(l, cp) do { \
-	register uint32_t t_l = (uint32_t)(l); \
-	register unsigned char *t_cp = (unsigned char *)(cp); \
-	*t_cp++ = t_l >> 24; \
-	*t_cp++ = t_l >> 16; \
-	*t_cp++ = t_l >> 8; \
-	*t_cp   = t_l; \
-	(cp) += NS_INT32SZ; \
-} while (0)
+#define NS_GET16(s, cp) (void)((s) = ns_get16(((cp)+=2)-2))
+#define NS_GET32(l, cp) (void)((l) = ns_get32(((cp)+=4)-4))
+#define NS_PUT16(s, cp) ns_put16((s), ((cp)+=2)-2)
+#define NS_PUT32(l, cp) ns_put32((l), ((cp)+=4)-4)
 
+unsigned ns_get16(const unsigned char *);
+unsigned long ns_get32(const unsigned char *);
+void ns_put16(unsigned, unsigned char *);
+void ns_put32(unsigned long, unsigned char *);
 
+int ns_initparse(const unsigned char *, int, ns_msg *);
+int ns_parserr(ns_msg *, ns_sect, int, ns_rr *);
+int ns_skiprr(const unsigned char *, const unsigned char *, ns_sect, int);
+int ns_name_uncompress(const unsigned char *, const unsigned char *, const unsigned char *, char *, size_t);
 
 
 #define	__BIND		19950621
@@ -464,4 +448,8 @@ typedef struct {
 #define	PUTSHORT		NS_PUT16
 #define	PUTLONG			NS_PUT32
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif

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;
+}
+