This patch came by way of the openntpd github. Linux (and possibly others)
will attempt to bind to 0.0.0.0 when binding to '::' and return an error if
it can't, unless IPV6_V6ONLY is set. See
https://github.com/openntpd-portable/openntpd-portable/issues/19
OK as an in-tree patch? OpenBSD seems to adopt a more liberal
interpretation and not return a failure in the same scenario. (The patch
against the 5.7 tree, but you get the idea)
diff -uNr openntpd-5.7p4/src/server.c new/src/server.c
--- openntpd-5.7p4/src/server.c 2015-03-25 04:18:56.000000000 +0300
+++ new/src/server.c 2016-03-05 18:06:07.946888570 +0300
@@ -40,6 +40,7 @@
size_t sa6len = sizeof(struct in6_addr);
u_int new_cnt = 0;
int tos = IPTOS_LOWDELAY;
+ int on = 1;
int rdomain, fd;
@@ -139,6 +140,12 @@
IPPROTO_IP, IP_TOS, &tos, sizeof(tos)) == -1)
log_warn("setsockopt IPTOS_LOWDELAY");
+#ifdef IPV6_V6ONLY
+ if (la->sa.ss_family == AF_INET6 && setsockopt(la->fd,
+ IPPROTO_IPV6, IPV6_V6ONLY, &on, sizeof(on)) == -1)
+ log_warn("setsockopt IPV6_V6ONLY");
+#endif
+
if (la->rtable != -1 &&
setsockopt(la->fd, SOL_SOCKET, SO_RTABLE, &la->rtable,
