socket-util: add wrappers for binding socket to ifindex/ifname

socket_bind_to_ifindex() uses the the SO_BINDTOIFINDEX sockopt of kernel
5.0, with a fallback to SO_BINDTODEVICE on older kernels.

socket_bind_to_ifname() is a trivial wrapper around SO_BINDTODEVICE, the
only benefit of using it instead of SO_BINDTODEVICE directly is that it
determines the size of the interface name properly so that it also works
for unbinding. Moreover, it's an attempt to unify our invocations of the
sockopt with a size of strlen(ifname) rather than strlen(ifname)+1...

src/basic/missing_socket.h

@@ -32,6 +32,10 @@ struct sockaddr_vm {
 #define SO_PEERGROUPS 59
 #endif
 
+#ifndef SO_BINDTOIFINDEX
+#define SO_BINDTOIFINDEX 62
+#endif
+
 #ifndef SOL_NETLINK
 #define SOL_NETLINK 270
 #endif

src/basic/socket-util.c

@@ -1354,3 +1354,39 @@ int sockaddr_un_set_path(struct sockaddr_un *ret, const char *path) {
                 return (int) (offsetof(struct sockaddr_un, sun_path) + l + 1); /* include trailing NUL in size */
         }
 }
+
+int socket_bind_to_ifname(int fd, const char *ifname) {
+        assert(fd >= 0);
+
+        /* Call with NULL to drop binding */
+
+        if (setsockopt(fd, SOL_SOCKET, SO_BINDTODEVICE, ifname, strlen_ptr(ifname)) < 0)
+                return -errno;
+
+        return 0;
+}
+
+int socket_bind_to_ifindex(int fd, int ifindex) {
+        char ifname[IFNAMSIZ] = "";
+
+        assert(fd >= 0);
+
+        if (ifindex <= 0) {
+                /* Drop binding */
+                if (setsockopt(fd, SOL_SOCKET, SO_BINDTODEVICE, NULL, 0) < 0)
+                        return -errno;
+
+                return 0;
+        }
+
+        if (setsockopt(fd, SOL_SOCKET, SO_BINDTOIFINDEX, &ifindex, sizeof(ifindex)) >= 0)
+                return 0;
+        if (errno != ENOPROTOOPT)
+                return -errno;
+
+        /* Fall back to SO_BINDTODEVICE on kernels < 5.0 which didn't have SO_BINDTOIFINDEX */
+        if (!if_indextoname(ifindex, ifname))
+                return -errno;
+
+        return socket_bind_to_ifname(fd, ifname);
+}

src/basic/socket-util.h

@@ -198,3 +198,6 @@ static inline int setsockopt_int(int fd, int level, int optname, int value) {
 
         return 0;
 }
+
+int socket_bind_to_ifname(int fd, const char *ifname);
+int socket_bind_to_ifindex(int fd, int ifindex);