support mix of IPv4 and v6 nameservers in resolv.conf

a v6 socket will only be used if there is at least one v6 nameserver
address. if the kernel lacks v6 support, the code will fall back to
using a v4 socket and requests to v6 servers will silently fail. when
using a v6 socket, v4 addresses are converted to v4-mapped form and
setsockopt is used to ensure that the v6 socket can accept both v4 and
v6 traffic (this is on-by-default on Linux but the default is
configurable in /proc and so it needs to be set explicitly on the
socket level). this scheme avoids increasing resource usage during
lookups and allows the existing network io loop to be used without
modification.

previously, nameservers whose address family did not match the address
family of the first-listed nameserver were simply ignored. prior to
recent __ipparse fixes, they were not ignored but erroneously parsed.

src/network/__dns.c

@@ -11,6 +11,7 @@
 #include <ctype.h>
 #include <unistd.h>
 #include <pthread.h>
+#include <errno.h>
 #include "__dns.h"
 #include "stdio_impl.h"
 
@@ -35,9 +36,9 @@ int __dns_doqueries(unsigned char *dest, const char *name, int *rr, int rrcnt)
 		struct sockaddr_in sin;
 		struct sockaddr_in6 sin6;
 	} sa = {0}, ns[3] = {{0}};
-	socklen_t sl;
+	socklen_t sl = sizeof sa.sin;
 	int nns = 0;
-	int family = AF_UNSPEC;
+	int family = AF_INET;
 	unsigned char q[280] = "", *r = dest;
 	int ql;
 	int rlen;
@@ -75,10 +76,12 @@ int __dns_doqueries(unsigned char *dest, const char *name, int *rr, int rrcnt)
 		for (s=line+11; isspace(*s); s++);
 		for (z=s; *z && !isspace(*z); z++);
 		*z=0;
-		if (__ipparse(ns+nns, family, s) < 0) continue;
+		if (__ipparse(ns+nns, AF_UNSPEC, s) < 0) continue;
 		ns[nns].sin.sin_port = htons(53);
-		family = ns[nns++].sin.sin_family;
-		sl = family==AF_INET6 ? sizeof sa.sin6 : sizeof sa.sin;
+		if (ns[nns++].sin.sin_family == AF_INET6) {
+			family = AF_INET6;
+			sl = sizeof sa.sin6;
+		}
 	}
 	if (f) __fclose_ca(f);
 	if (!nns) {
@@ -93,6 +96,29 @@ int __dns_doqueries(unsigned char *dest, const char *name, int *rr, int rrcnt)
 	sa.sin.sin_family = family;
 	fd = socket(family, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0);
 
+	/* Handle case where system lacks IPv6 support */
+	if (fd < 0 && errno == EAFNOSUPPORT) {
+		if (family != AF_INET6) return EAI_SYSTEM;
+		fd = socket(AF_INET, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0);
+		family = AF_INET;
+	}
+	if (fd < 0) return EAI_SYSTEM;
+
+	/* Convert any IPv4 addresses in a mixed environment to v4-mapped */
+	if (family == AF_INET6) {
+		setsockopt(fd, IPPROTO_IPV6, IPV6_V6ONLY, &(int){0}, sizeof 0);
+		for (i=0; i<nns; i++) {
+			if (ns[i].sin.sin_family != AF_INET) continue;
+			memcpy(ns[i].sin6.sin6_addr.s6_addr+12,
+				&ns[i].sin.sin_addr, 4);
+			memcpy(ns[i].sin6.sin6_addr.s6_addr,
+				"\0\0\0\0\0\0\0\0\0\0\xff\xff", 12);
+			ns[i].sin6.sin6_family = AF_INET6;
+			ns[i].sin6.sin6_flowinfo = 0;
+			ns[i].sin6.sin6_scope_id = 0;
+		}
+	}
+
 	pthread_cleanup_push(cleanup, (void *)(intptr_t)fd);
 	pthread_setcancelstate(cs, 0);