Re: bind() udp behavior

From: Jan Harkes
Date: Thu Dec 16 2004 - 09:54:42 EST

On Thu, Dec 16, 2004 at 09:24:59AM -0500, Adam Denenberg wrote:
> I disagree. The linux server should be using unique Transaction ID's
> in the dns header for each unique dns request. Otherwise there is no
> way to distinguish them (same A record request). Of course the
> firewall is going to drop a reply that it thinks it already saw a reply
> for 30ms ago.
> This appears to be a bug in the way glibc is handling things but i
> cannot be sure. That is the goal of my investigation.

Ok, please stop it and _read_ the email, completely, a possible solution
will be at the end.

This is not a problem with the linux kernel, the application (resolver
library) is simply retransmitting the request and the kernel does
exactly as it is told. So first of all this is completely off-topic for

Second of all, it is -not- a glibc issue either. The reply packet was
probably lost after it has passed through the firewall but before it
reached the DNS client machine. So since it has not yet received the
response it is doing the right thing, retransmitting the request.

The problem is... your firewall, it seems to assume that once it has
seen a UDP reply packet that this packet will reach the destination and
the session is closed. This is an incorrect assumption since UDP is per
definition a lossy protocol (unreliable datagram protocol) and the
packet can and will be dropped by any routers, switches, the network
card in your machine, or even by the kernel if there is too much
traffic or a single bit error which makes the checksum fail.

One solution that might work is to run a local caching DNS daemon on a
machine behind the firewall (bind, dnsmasq or something) and set
/etc/resolv.conf to only send requests to the local machine. The local
caching daemon can then forward those requests from a known fixed local
port to your upstream DNS servers. This allows you to open a hole in the
firewall between local_dns_cache:53 and upstream_dns_server:53.


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