DHCP Server not able to handle the Traffic

Simon Hobson dhcp1 at thehobsons.co.uk
Sun Apr 26 08:58:11 UTC 2009

Agarwal Vivek-RNGB36 wrote:

>Iam using ISC-DHCP-3.0.5 installed on RHEL5
>When I take a tcpdump on the DHCP Interface; I saw around 1000 DHCP 
>Discovers at the interface but when I see /var/log/messages I can 
>see only around 500 DHCP DISCOVERs over there.
>Can anyone help me in debugging that where are the discovers being 
>lost and how can I check resolve that

Need more information than that ! Over what period is this - 1000 
messages in one second, or 100 messages over one day, or something in 
between ?

The server does have a limited response rate, and if you exceed that 
then buffers will fill and packets will get discarded. The limit is 
highly dependent on several factors which get discussed fairly 
regularly on this list - usually in response to questions of the form 
"how many clients will the server support" or "will the server 
support X clients".

There are several things you can do to improve performance. The thing 
to remember is that the server is single threaded and so anything 
that slows it down will restrict it's performance.

Logging. It logs stuff quite frequently, so first thing to do is 
configure your logging to be asynchronous - otherwise the DHCP server 
must wait while each log message is written to disk. That's in your 
syslog configuration, and in the 'old fashioned' original syslog it's 
a case of preceding a log file name with a '-'. For newer syslog 
variants, check the manuals.

DNS. Depending on your config, the DNS service could be getting 
updated. Make sure that this is working correctly, and make sure you 
aren't doing excessive updates (eg by setting "update-static-leases 
true" when you have lots of static address assignments.)

Lease file storage. The server is required (as in absolute MUST) to 
write all leases to permanent storage before offering them to 
clients. Make sure the leases file is on a disk with good performance 
and which isn't too busy from other activities. In extreme, some 
people have put this on battery-backed ram disks etc.

And the obvious one, can you reduce the query rate from clients ? Do 
you have short lease times ? If so, then increasing them will reduce 
the query rate.

Simon Hobson

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