Index: src/kern.c
--- src/kern.c.orig
+++ src/kern.c
@@ -114,6 +114,9 @@ void k_set_if(uint32_t ifa) {
 void k_join(struct IfDesc *ifd, uint32_t grp) {
     struct ip_mreq mreq;
 
+    if (config_getinterface(ifd->Name) == NULL)
+        return;
+
     mreq.imr_multiaddr.s_addr = grp;
     mreq.imr_interface.s_addr = ifd->InAdr.s_addr;
 
@@ -135,6 +138,9 @@ void k_join(struct IfDesc *ifd, uint32_t grp) {
 
 void k_leave(struct IfDesc *ifd, uint32_t grp) {
     struct ip_mreq mreq;
+
+    if (config_getinterface(ifd->Name) == NULL)
+        return;
 
     mreq.imr_multiaddr.s_addr = grp;
     mreq.imr_interface.s_addr = ifd->InAdr.s_addr;