Merge pull request #17807 from poettering/bindtodevice

use SO_BINDTOIFINDEX while connect()

src/resolve/resolved-dns-scope.c

@@ -412,9 +412,36 @@ static int dns_scope_socket(
         if (ret_socket_address)
                 *ret_socket_address = sa;
         else {
+                bool bound = false;
+
+                /* Let's temporarily bind the socket to the specified ifindex. The kernel currently takes
+                 * only the SO_BINDTODEVICE/SO_BINDTOINDEX ifindex into account when making routing decisions
+                 * in connect() — and not IP_UNICAST_IF. We don't really want any of the other semantics of
+                 * SO_BINDTODEVICE/SO_BINDTOINDEX, hence we immediately unbind the socket after the fact
+                 * again.
+                 *
+                 * As a special exception we don't do this if we notice that the specified IP address is on
+                 * the local host. SO_BINDTODEVICE in combination with destination addresses on the local
+                 * host result in EHOSTUNREACH, since Linux won't send the packets out of the specified
+                 * interface, but delivers them directly to the local socket. */
+                if (s->link &&
+                    !manager_find_link_address(s->manager, sa.sa.sa_family, sockaddr_in_addr(&sa.sa))) {
+                        r = socket_bind_to_ifindex(fd, ifindex);
+                        if (r < 0)
+                                return r;
+
+                        bound = true;
+                }
+
                 r = connect(fd, &sa.sa, salen);
                 if (r < 0 && errno != EINPROGRESS)
                         return -errno;
+
+                if (bound) {
+                        r = socket_bind_to_ifindex(fd, 0);
+                        if (r < 0)
+                                return r;
+                }
         }
 
         return TAKE_FD(fd);

src/resolve/resolved.c

@@ -58,7 +58,7 @@ static int run(int argc, char *argv[]) {
                 if (r < 0)
                         return log_error_errno(r, "Could not create runtime directory: %m");
 
-                /* Drop privileges, but keep three caps. Note that we drop those too, later on (see below) */
+                /* Drop privileges, but keep three caps. Note that we drop two of those too, later on (see below) */
                 r = drop_privileges(uid, gid,
                                     (UINT64_C(1) << CAP_NET_RAW)|          /* needed for SO_BINDTODEVICE */
                                     (UINT64_C(1) << CAP_NET_BIND_SERVICE)| /* needed to bind on port 53 */
@@ -83,7 +83,7 @@ static int run(int argc, char *argv[]) {
         (void) manager_check_resolv_conf(m);
 
         /* Let's drop the remaining caps now */
-        r = capability_bounding_set_drop(0, true);
+        r = capability_bounding_set_drop((UINT64_C(1) << CAP_NET_RAW), true);
         if (r < 0)
                 return log_error_errno(r, "Failed to drop remaining caps: %m");