The kprop client seems to be working well, but kpropd currently gives me
this warning when run in standalone mode:
  kpropd: Protocol not available while unsetting IPV6_V6ONLY option

This appears to happen because getaddrinfo() is returning a list of two
addresses (the first is the IPv4 wildcard address, and the second is the
IPv6 wildcard address), and kpropd is only using the first.

Checking that res->ai_family is AF_INET6 before calling setsockopt() at
line 277 suppresses the warning, but then it's still listening only for
IPv4 connections.

I've found that asking getaddrinfo() for an AF_INET6 address with
AI_ADDRCONFIG added to hints.ai_flags seems to reliably return an error
when IPv6 isn't supported, and falling back from there to asking for an
AF_INET address, while not particularly elegant, seems to do the job.

HTH,

Nalin

Index: src/slave/kpropd.c
===================================================================
--- src/slave/kpropd.c	(revision 24148)
+++ src/slave/kpropd.c	(working copy)
@@ -250,14 +250,22 @@
 retry:
 
     memset(&hints, 0, sizeof(hints));
-    hints.ai_family = AF_UNSPEC;
+    hints.ai_family = AF_INET6;
     hints.ai_socktype = SOCK_STREAM;
-    hints.ai_flags = AI_PASSIVE;
+    hints.ai_flags = AI_PASSIVE | AI_ADDRCONFIG | AI_V4MAPPED;
 
     error = getaddrinfo(NULL, port, &hints, &res);
     if (error != 0) {
-        (void) fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(error));
-        exit(1);
+        memset(&hints, 0, sizeof(hints));
+        hints.ai_family = AF_INET;
+        hints.ai_socktype = SOCK_STREAM;
+        hints.ai_flags = AI_PASSIVE | AI_ADDRCONFIG;
+
+        error = getaddrinfo(NULL, port, &hints, &res);
+        if (error != 0) {
+            (void) fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(error));
+            exit(1);
+        }
     }
 
     finet = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
@@ -274,8 +282,9 @@
     /* Typically, res will be the IPv6 wildcard address.  Some systems, such as
      * the *BSDs, don't accept IPv4 connections on this address by default. */
     val = 0;
-    if (setsockopt(finet, IPPROTO_IPV6, IPV6_V6ONLY, &val, sizeof(val)) < 0)
-        com_err(progname, errno, "while unsetting IPV6_V6ONLY option");
+    if (res->ai_family == AF_INET6)
+        if (setsockopt(finet, IPPROTO_IPV6, IPV6_V6ONLY, &val, sizeof(val)) < 0)
+            com_err(progname, errno, "while unsetting IPV6_V6ONLY option");
 #endif
 
     /*