sender source IP address on UDP socket bound to INADDR_ANY in golang

[Patrick M. Hausen]

Hi Peter and everyone else following,

> Am 12.05.2021 um 14:06 schrieb Peter Jeremy via freebsd-net <freebsd-net at freebsd.org>:
> As I see it, the possibilities boil down to:
> 1) The Go code isn't enabling IPPROTO_IP.IP_RECVDSTADDR on the socket.
> 2) There's a FreeBSD kernel bug that mean setting IP_RECVDSTADDR
>   isn't being correctly reflected into the recvmsg control message.
> 3) The control message isn't being correctly plumbed through from
>   recvmsg(2) to the Go RecvMsg() return.
> 
> Note that a lot of the relevant Go library code is BSD- or FreeBSD-
> specific so it's also possible that there is a bug in the Go library
> code.

do you have some spare time and would you be so kind to look at our discussion
here: https://github.com/AdguardTeam/AdGuardHome/issues/3015

Andrey from the AdGuard team references this golang issue:
https://github.com/golang/go/issues/8329

Which references this FreeBSD issue:
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=193246

What I as a sysadmin can observe is that the test code Andrey gave me
binds to *.53 on IPv4 and IPv6 although I start it with `-l 0.0.0.0` which is
clearly an IPv4 "any" address.

I am not 100% familiar with the API but as I understand you can treat
IPv4 as IPv6 via the socket interface by using an IPv4-mapped IPv6
address. So far so good.
But then of course you have an AF_INET6 socket and it seems that
FreeBSD does not allow setting IPv4 specific options via setsockopt()
because it's an IPv6 socket. Correct?

Why can you have a single socket on both address families, anyway?
IPv4 and IPv6 are as "related" as IP and IPX - if you go dual stack,
treat them both separately - no?

Any light you can shed on this issue greatly appreciated.

Thanks,
Patrick
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Patrick M. Hausen
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