I've tried to enable postfix on a machine w/out IPv6 support in the
kernel and it died with "socket: Protocol not supported".

I think there are two issues here

1) we return wrong errno.  To quote
   http://www.fefe.de/libowfat/changes-0.23.txt

    work around freebsd 5.4 brokenness (if you don't have IPv6 in the
    kernel, socket(PF_INET6,SOCK_STREAM,0) returns EPROTONOSUPPORT
    instead of EPFNOSUPPORT, which basically says "yeah, I know IPv6,
    but TCP?  never heard of it")

2) postfix checks for *another* wrong errno: EAFNOSUPPORT instead of
   EPFNOSUPPORT (search for the former in src/util/inet_proto.c)


PS: Please, keep me Cc'ed, I'm not subscribed to tech-net@

SY, Uwe
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