[Pdns-users] UDP Connection Table Exhaustion?

Matthew Walster - Gyron matthew.walster at gyron.net
Thu Jul 2 15:41:22 UTC 2009

Hey there,

I've inherited a DNS recursive resolver infrastructure based on PowerDNS recursor, and there are reports of strange DNS errors occasionally - either NXDOMAIN or just timeouts.

Looking into it, ip_conntrack seems to think there are currently 30000 connections, with a connection table size of 65535. Some users do have high figures, such as 1000 for just one IP, but on the large part, most IPs have low connection quantities.

I've tried changing the UDP timeout to 5 seconds, and this alleviated some of the issues, but didn't wholly fix it. Unfortunately, the software is currently running on CentOS 4.7, PowerDNS recursor version 3.1.4 - so a little older than I'd like, but it is only allowing queries from the CIDR ranges we have allocated to us so it shouldn't be too overloaded. Load averages are in the 0.10-0.25 range, and memory usage is 1GB of 3GB RAM. It's currently running on VMWare, but it will eventually run on a blade or similar.

Does anyone have any tips and/or tricks for running a medium-scale DNS recursive resolver appropriate to my situation? Medium being bigger than "run it off a DSL router" but smaller than "get a server farm to do it"!

Kind regards,

Matthew Walster

Network Architect
Gyron Internet Ltd

mail:   matthew.walster at gyron.net
web:    http://gyron.net/

tel:    0845 888 6900
fax:    0845 888 6910

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