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

[Patrick M. Hausen]

Hi!

> Am 11.05.2021 um 12:38 schrieb Peter Jeremy <peter at rulingia.com>:
> 
> On 2021-May-08 19:05:56 +0200, "Patrick M. Hausen" <hausen at punkt.de> wrote:
>> I am facing a problem that is perfectly explained by the semantics
>> of the socket interface for UDP, if one assumes that the application
>> in question binds to INADDR_ANY and does not specifically set the
>> sender address when sending datagrams.
> ...
>> Their code in question is here:
>> https://github.com/AdguardTeam/dnsproxy/blob/1163404e605c3dfbeab360fc3540fc290f61a321/proxyutil/udp_unix.go#L47
> 
> So, they say that they retrieve "the net interface IP the packet was
> sent to (dst addr) from the socket's OOB data" and I agree that's what
> the referenced code does.  I hadn't heard of that behaviour before and
> went digging...

Thank you. I received some code with internal debugging added from the
AdGuard core team and will try that today or tomorrow. If I read the quote
from the documentation correctly, on possible explanation would be them
calling recvmsg() but forgetting to setsockopt()?

Kind regards,
Patrick
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