[dns-operations] Using IP_RECVERR/IPV6_RECVERR on resolver client sockets

[Florian Weimer]

* Paul Wouters:

> On Tue, 8 Jan 2019, Florian Weimer wrote:
>
>> Subject: [dns-operations] Using IP_RECVERR/IPV6_RECVERR on resolver client
>>     sockets
>>
>> Someone noticed that the Linux kernel only puts some networking-related
>> errors on the socket error queue for connected UDP sockets:
>>
>>  <https://sourceware.org/bugzilla/show_bug.cgi?id=24047>
>
> Here, let me show you my 20 year old scar:
>
> https://github.com/libreswan/libreswan/blob/master/programs/pluto/server.c#L1120
>
> /* Process any message on the MSG_ERRQUEUE
>  *
>  * This information is generated because of the IP_RECVERR socket option.
>  * The API is sparsely documented, and may be LINUX-only, and only on
>  * fairly recent versions at that (hence the conditional compilation).

For a (stub) resolver, that's much easier: You just try the next server
when poll fails with POLLERR, like before.  IP_RECVERR only reports more
errors.  MSG_ERRQUEUE is entirely optional.

Thanks,
Florian