Only accept IPv6 in server IPv6 socket

Set IPV6_V6ONLY flag on server socket. Not all operating systems
support mixing v4/v6 in one socket, so separate them all the time.

src/common.c

@@ -172,6 +172,12 @@ get_addr(char *host, int port, int addr_family, int flags, struct sockaddr_stora
 
 int
 open_dns(struct sockaddr_storage *sockaddr, size_t sockaddr_len)
+{
+	return open_dns_opt(sockaddr, sockaddr_len, -1);
+}
+
+int
+open_dns_opt(struct sockaddr_storage *sockaddr, size_t sockaddr_len, int v6only)
 {
 	int flag;
 	int fd;
@@ -190,6 +196,10 @@ open_dns(struct sockaddr_storage *sockaddr, size_t sockaddr_len)
 	fd_set_close_on_exec(fd);
 #endif
 
+	if (sockaddr->ss_family == AF_INET6 && v6only >= 0) {
+		setsockopt(fd, IPPROTO_IPV6, IPV6_V6ONLY, (const void*) &v6only, sizeof(v6only));
+	}
+
 #ifdef IP_OPT_DONT_FRAG
 	/* Set dont-fragment ip header flag */
 	flag = DONT_FRAG_VALUE;

src/common.h

@@ -106,6 +106,7 @@ void check_superuser(void (*usage_fn)(void));
 char *format_addr(struct sockaddr_storage *sockaddr, int sockaddr_len);
 int get_addr(char *, int, int, int, struct sockaddr_storage *);
 int open_dns(struct sockaddr_storage *, size_t);
+int open_dns_opt(struct sockaddr_storage *sockaddr, size_t sockaddr_len, int v6only);
 int open_dns_from_host(char *host, int port, int addr_family, int flags);
 void close_dns(int);
 

src/iodined.c

@@ -2687,7 +2687,9 @@ main(int argc, char **argv)
 			goto cleanup;
 		}
 		if ((addrfamily == AF_UNSPEC || addrfamily == AF_INET6) &&
-			(dns_fds.v6fd = open_dns(&dns6addr, dns6addr_len)) < 0) {
+			/* Set IPv6 socket to V6ONLY */
+			(dns_fds.v6fd = open_dns_opt(&dns6addr, dns6addr_len, 1)) < 0) {
+
 			retval = 1;
 			goto cleanup;
 		}