eventdns: all nameservers have failed

[Scott Bennett]

     On Thu, 16 Jul 2009 09:53:36 -0400 Praedor Atrebates <praedor at yahoo.com>
wrote:
>I get this message now in my tor log.  No other useful info provided.
>
>What is the cause?  I have been cleaning up my system daemons (Mandriva linux) 
>and terminated a few services that I didn't believe I needed.  

     I'm not sure that anyone knows yet.  I get this messages, too, usually
followed within a few tens of milliseconds by another message, saying that
one of the name servers is available again.  Here's an example from this
morning.

Jul 16 07:48:37.512 [warn] eventdns: All nameservers have failed
Jul 16 07:48:37.535 [notice] eventdns: Nameserver 68.87.72.134 is back up

For quite a while, I thought this was a network-connection problem, but I'm
no longer so sure.  My tor node is currently limited to 10% or less of the
Internet connection's transmit rate limit, and not much else is ordinarily
going on over the connection.  It has been a while, but IIRC, Roger wrote me
once that he thought there might be a well hidden bug in libevent that shows
up in this way.  The gap between the two messages in pairs of messages like
the one shown above is only very rarely much over 100 ms in length.  Given
that the default timeout in libresolv is, I think, 3 s (used to be 6 s, but
was probably shortened due to the Internetwide speed increases over the years),
it does seem odd that the gaps are nearly always so short.  If it is due to
a libevent bug, then the libevent maintainers would have to be encouraged to
go after it, I suppose, rather than the tor project folks.
>
>What are the services/ports required for tor to operate properly?  What 
>service being shut down might cause this message?
>
     Besides tor's own ports, only the domain name service (UDP port 53)
should be needed that I can think of offhand.  (I can't swear that TCP port
53 isn't also needed, but I can't think of any reason tor would need it.)


                                  Scott Bennett, Comm. ASMELG, CFIAG
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