socket: ignore EADDRINUSE when joining multicast group

If an interface is configured with multiple addresses, llmnrd tries to
join the LLMNR multicast group for each address instead of once per
interface. Ideally llmnrd should track whether the group is already
joined but since the kernel tells us that the group is already joined,
just omit the error message on EADDRINUSE.

This fixes the following error case:

$ ip address add 2001:db8:1::/48 dev eth0
$ ./llmnrd -6
Starting llmnrd on port 5355, hostname foobar
Added IPv4 address 127.0.0.1 on interface lo
Added IPv4 address 10.42.0.42 on interface eth0
Added IPv6 address ::1 on interface lo
Added IPv6 address 2001:db8:1:: on interface eth1
Error: Failed to join IPv6 multicast group on interface eth1: Address already in use
Added IPv6 address fe80::xxxx:xxxx:xxxx:xxxx on interface eth0

Reference: #21
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>

1 files changed, 16 insertions, 6 deletions

diff --git a/socket.c b/socket.c
index 1103779..73713eb 100644
--- a/socket.c
+++ b/socket.c
@@ -209,9 +209,14 @@ int socket_mcast_group_ipv4(int sock, unsigned int ifindex, bool join)
 
 	if (setsockopt(sock, IPPROTO_IP, join ? IP_ADD_MEMBERSHIP : IP_DROP_MEMBERSHIP,
 		       &mreq, sizeof(mreq)) < 0) {
-		log_err("Failed to join IPv4 multicast group on interface %s: %s\n",
-			if_indextoname(ifindex, ifname), strerror(errno));
-		return -1;
+		/* ignore error if we attempt to join a group the interface is
+		 * already part of */
+		if (!join || errno != EADDRINUSE) {
+			log_err("Failed to %s IPv4 multicast group membership on interface %s: %s\n",
+				join ? "add" : "drop", if_indextoname(ifindex, ifname),
+				strerror(errno));
+			return -1;
+		}
 	}
 
 	return 0;
@@ -232,9 +237,14 @@ int socket_mcast_group_ipv6(int sock, unsigned int ifindex, bool join)
 
 	if (setsockopt(sock, IPPROTO_IPV6, join ? IPV6_ADD_MEMBERSHIP : IPV6_DROP_MEMBERSHIP,
 		       &mreq6, sizeof(mreq6)) < 0) {
-		log_err("Failed to join IPv6 multicast group on interface %s: %s\n",
-			if_indextoname(ifindex, ifname), strerror(errno));
-		return -1;
+		/* ignore error if we attempt to join a group the interface is
+		 * already part of */
+		if (!join || errno != EADDRINUSE) {
+			log_err("Failed to %s IPv6 multicast group membership on interface %s: %s\n",
+				join ? "add" : "drop", if_indextoname(ifindex, ifname),
+				strerror(errno));
+			return -1;
+		}
 	}
 
 	return 0;