Default to IPv4.

Index: src/iperf_api.c
--- src/iperf_api.c.orig
+++ src/iperf_api.c
@@ -2860,7 +2860,7 @@ iperf_defaults(struct iperf_test *testp)
     testp->stats_interval = testp->reporter_interval = 1;
     testp->num_streams = 1;
 
-    testp->settings->domain = AF_UNSPEC;
+    testp->settings->domain = AF_INET;
     testp->settings->unit_format = 'a';
     testp->settings->socket_bufsize = 0;    /* use autotuning */
     testp->settings->blksize = DEFAULT_TCP_BLKSIZE;