[ldns-users] pyLDNS Problems

[Willem Toorop]

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Hi Paul and Chris,

I found the culprit!

ldns_resolver_send_pkt in resolver.c first tries normal UDP
transmission. Then it tries UDP transmission with edns0 size 4096.
When fragments are dropped (which was the case with you) a network error
is returned and ldns_resolver_send_pkt then continues trying TCP.
However, because of the network error, the non responsive nameservers in
the resolver (in your case 76.10.157.65) is marked unreachable by
ldns_send. As a result the ldns_resolver has no reachable nameservers
any more and doesn't even try to send a query out.

The reachability of nameservers is in the round trip time information
(_rtt field) of an ldns_resolver struct.
A round trip time of LDNS_RESOLV_RTT_INF means unreachable.

I have fixed the issue by preserving the round trip time information
when edns0 with size 4096 is tried. The fix is in revision r3551 and
will be in the 1.6.11 release.

Thanks for you bug report!

Willem

On 19-09-11 12:31, Willem Toorop wrote:
> On 18-09-11 21:13, Paul Wouters wrote:
>> Does it work 3 times? The test first checks google.com, then xelerance.com,
>> then google.com again. The last two fail. This might be related to EDNS
>> and/or
>> UDP fragments gone missing.
> 
> Paul, Chris, I can finally reproduce the error. Even on my own Ubuntu
> box when I drop fragments from the PREROUTING table with:
> 
> iptables -t mangle -A PREROUTING -f -j DROP
> 
> Hopefully I can follow up with a fix now soon.
> 
> Regards, Willem
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