For testing IPv6 multicast on Linux use any multicast capable interface.

GNUmakefile

@@ -17,6 +17,10 @@ install:
 	install -c -m 555 -s udpbench -D -t ${DESTDIR}${BINDIR}
 	install -c -m 444 udpbench.1 -D -t ${DESTDIR}${MANDIR}1
 
+# use any multicast capable interface; as -L1 is set, test runs on loopback
+INTERFACE =	$(shell ip link show | \
+    awk '/MULTICAST/{print gensub(":.*","",1,$$2); exit}')
+
 TEST =
 TEST += localhost
 test-localhost:
@@ -70,14 +74,14 @@ test-mcast:
 	grep -q 'sockname: 224.0.0.123 ' out
 	grep -q 'recv: syscalls ' out
 
-# XXX Linux fails to receive on loopback, disable test for now.
-#TEST += mcast6
+# XXX Linux fails to send on loopback, use any ethernet
+TEST += mcast6
 test-mcast6:
 	@echo -e '\n==== $@ ===='
-	./udpbench -Ilo -p0 -t3 recv ff04::123 | tee out & \
+	./udpbench -I$(INTERFACE) -p0 -t3 recv ff04::123 | tee out & \
 	    sleep 1; \
 	    port=`awk '/^sockname:/{print $$3}' out`; \
-	    ./udpbench -L1 -T0 -p$$port -t1 \
+	    ./udpbench -I$(INTERFACE) -L1 -T0 -p$$port -t1 \
 	    send ff04::123 || exit 1; \
 	    wait $$!
 	grep -q 'sockname: ff04::123 ' out
@@ -96,14 +100,14 @@ test-mcast-repeat:
 	grep -q 'sockname: 224.0.0.124 ' out
 	grep -q 'recv: syscalls ' out
 
-# XXX Linux fails to receive on loopback, disable test for now.
-#TEST += mcast6-repeat
+# XXX Linux fails to send on loopback, use any ethernet
+TEST += mcast6-repeat
 test-mcast6-repeat:
 	@echo -e '\n==== $@ ===='
-	./udpbench -N2 -Ilo -p0 -t3 recv ff04::123 | tee out & \
+	./udpbench -N2 -I$(INTERFACE) -p0 -t3 recv ff04::123 | tee out & \
 	    sleep 1; \
 	    port=`awk '/^sockname:/{print $$3}' out | sort -u`; \
-	    ./udpbench -N2 -L1 -T0 -p$$port -t1 \
+	    ./udpbench -N2 -I$(INTERFACE) -L1 -T0 -p$$port -t1 \
 	    send ff04::123 || exit 1; \
 	    wait $$!
 	grep -q 'sockname: ff04::123 ' out