Re: [Libevent-users] evdns timeout behaviour

[Nick Mathewson <nickm@xxxxxxxxxxxxx>]

On Fri, Jul 6, 2012 at 6:06 PM, Catalin Patulea <catalinp@xxxxxxxxxx> wrote:
> Hi,
>
> I'm interested in what happens when a freshly created evdns with more than
> one nameserver encounters a timeout on the first request.
>
> It seems evdns distinguishes between retransmits and reissues. Retransmits
> are triggered by timeouts and always point to the same ns, chosen at
> first-transmit time. Reissues, on the other hand, are triggered by bad
> responses from nameservers, like NOTIMPL or REFUSED, and do cause a new ns
> to be picked in a round-robin fashion.
>
> The net effect is that on a fresh evdns with 2 nameservers, a single
> evdns_resolve is tried only against one nameserver. A second evdns_resolve
> is needed to talk to the second ns.
>
> This is different from glibc (2.7), where a new nameserver appears to be
> chosen before each retransmit. I'm not sure whether glibc makes any
> distinction between retransmit and reissue.
>
> Can anyone comment on these semantics? What was the rationale? Is there any
> way to get evdns to behave like glibc without significant code changes? (I
> don't see anything obvious in evdns_set_option.)

I'm not aware of any deep reason for the current behavior; if somebody
wanted to do a patch so that retransmission happened to a different
nameserver on timeout too, that would be fine with me.  (Unless
somebody knows a reason why that would be a terrible idea.)

yrs,
-- 
Nick
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