diff --git a/sys/kern/uipc_socket.c b/sys/kern/uipc_socket.c
--- a/sys/kern/uipc_socket.c
+++ b/sys/kern/uipc_socket.c
@@ -3699,6 +3699,19 @@
 
 }
 
+int
+sosetfib(struct socket *so, int fibnum)
+{
+	if (fibnum < 0 || fibnum >= rt_numfibs)
+		return (EINVAL);
+
+	SOCK_LOCK(so);
+	so->so_fibnum = fibnum;
+	SOCK_UNLOCK(so);
+
+	return (0);
+}
+
 #ifdef SOCKET_HHOOK
 /*
  * Wrapper for Socket established helper hook.
@@ -3847,21 +3860,7 @@
 			break;
 
 		case SO_SETFIB:
-			error = sooptcopyin(sopt, &optval, sizeof optval,
-			    sizeof optval);
-			if (error)
-				goto bad;
-
-			if (optval < 0 || optval >= rt_numfibs) {
-				error = EINVAL;
-				goto bad;
-			}
-			if (((so->so_proto->pr_domain->dom_family == PF_INET) ||
-			   (so->so_proto->pr_domain->dom_family == PF_INET6) ||
-			   (so->so_proto->pr_domain->dom_family == PF_ROUTE)))
-				so->so_fibnum = optval;
-			else
-				so->so_fibnum = 0;
+			error = so->so_proto->pr_ctloutput(so, sopt);
 			break;
 
 		case SO_USER_COOKIE:
diff --git a/sys/net/rtsock.c b/sys/net/rtsock.c
--- a/sys/net/rtsock.c
+++ b/sys/net/rtsock.c
@@ -423,6 +423,30 @@
 	return (0);
 }
 
+static int
+rts_ctloutput(struct socket *so, struct sockopt *sopt)
+{
+	int error, optval;
+
+	error = ENOPROTOOPT;
+	if (sopt->sopt_dir == SOPT_SET) {
+		switch (sopt->sopt_level) {
+		case SOL_SOCKET:
+			switch (sopt->sopt_name) {
+			case SO_SETFIB:
+				error = sooptcopyin(sopt, &optval,
+				    sizeof(optval), sizeof(optval));
+				if (error != 0)
+					break;
+				error = sosetfib(so, optval);
+				break;
+			}
+			break;
+		}
+	}
+	return (error);
+}
+
 static void
 rts_detach(struct socket *so)
 {
@@ -2702,6 +2726,7 @@
 	.pr_flags =		PR_ATOMIC|PR_ADDR,
 	.pr_abort =		rts_close,
 	.pr_attach =		rts_attach,
+	.pr_ctloutput =		rts_ctloutput,
 	.pr_detach =		rts_detach,
 	.pr_send =		rts_send,
 	.pr_shutdown =		rts_shutdown,
diff --git a/sys/netinet/ip_output.c b/sys/netinet/ip_output.c
--- a/sys/netinet/ip_output.c
+++ b/sys/netinet/ip_output.c
@@ -1088,10 +1088,22 @@
 		    sopt->sopt_dir == SOPT_SET) {
 			switch (sopt->sopt_name) {
 			case SO_SETFIB:
+				error = sooptcopyin(sopt, &optval,
+				    sizeof(optval), sizeof(optval));
+				if (error != 0)
+					break;
+
 				INP_WLOCK(inp);
-				inp->inp_inc.inc_fibnum = so->so_fibnum;
+				if ((inp->inp_flags & INP_BOUNDFIB) != 0 &&
+				    optval != so->so_fibnum) {
+					INP_WUNLOCK(inp);
+					error = EISCONN;
+					break;
+				}
+				error = sosetfib(inp->inp_socket, optval);
+				if (error == 0)
+					inp->inp_inc.inc_fibnum = optval;
 				INP_WUNLOCK(inp);
-				error = 0;
 				break;
 			case SO_MAX_PACING_RATE:
 #ifdef RATELIMIT
diff --git a/sys/netinet6/ip6_output.c b/sys/netinet6/ip6_output.c
--- a/sys/netinet6/ip6_output.c
+++ b/sys/netinet6/ip6_output.c
@@ -1648,10 +1648,22 @@
 		    sopt->sopt_dir == SOPT_SET) {
 			switch (sopt->sopt_name) {
 			case SO_SETFIB:
+				error = sooptcopyin(sopt, &optval,
+				    sizeof(optval), sizeof(optval));
+				if (error != 0)
+					break;
+
 				INP_WLOCK(inp);
-				inp->inp_inc.inc_fibnum = so->so_fibnum;
+				if ((inp->inp_flags & INP_BOUNDFIB) != 0 &&
+				    optval != so->so_fibnum) {
+					INP_WUNLOCK(inp);
+					error = EISCONN;
+					break;
+				}
+				error = sosetfib(inp->inp_socket, optval);
+				if (error == 0)
+					inp->inp_inc.inc_fibnum = optval;
 				INP_WUNLOCK(inp);
-				error = 0;
 				break;
 			case SO_MAX_PACING_RATE:
 #ifdef RATELIMIT
diff --git a/sys/sys/socketvar.h b/sys/sys/socketvar.h
--- a/sys/sys/socketvar.h
+++ b/sys/sys/socketvar.h
@@ -555,6 +555,7 @@
 int	sosend_generic(struct socket *so, struct sockaddr *addr,
 	    struct uio *uio, struct mbuf *top, struct mbuf *control,
 	    int flags, struct thread *td);
+int	sosetfib(struct socket *so, int fibnum);
 int	soshutdown(struct socket *so, enum shutdown_how);
 void	soupcall_clear(struct socket *, sb_which);
 void	soupcall_set(struct socket *, sb_which, so_upcall_t, void *);