Failover communication

Loren M. Lang lorenl at
Sun Mar 18 01:44:38 UTC 2012

On 3/14/2012 7:20 AM, Smith Bill wrote:
> Hi
> Does anyone have any detailed information regarding the communication 
> that happens between two dhcp servers configured for failover -- I 
> have two servers that don't seem to talk to each other and the tcpdump 
> is showing block icmp traffic so I guess I need to know if dhcp 
> communicates with its failover partner via icmp.

The DHCP Failover protocol uses TCP only.  I believe this IETF Internet 
Draftdiscusses a proposed version of it, but as it never became an RFC, 
it may not be 100% accurate.

As for using tcpdump, set a capture filter like "tcp port 9999" and 
replace 9999 with the port line you used in the failover peer block from 
dhcpd.conf.  To analyze the protocol, I recommend using Wireshark.  You 
can use Wireshark to do the capture directly, or you can tell tcpdump to 
save it's capture to a file which Wireshark can open.  Use the tcpdump 
options "-w dhcp-failover.pcap -s 0" to save it to a file.  Wireshark 
can decode a decent amount of the protocol and it shouldn't be too hard 
to figure out what's wrong.  Here's my failover configuration:

failover peer "peer" {
         port 7111;
         peer address;
         peer port 7111;
         max-response-delay 60;
         max-unacked-updates 10;
         mclt 3600;
         load balance max seconds 3;

> Regards
> Bill Smith
> Senior Solutions Architect
> Architecture & Design H&NS North
> Fujitsu Services
> Tel: 07867 821165
> Email:bill.smith at
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Loren M. Lang
lorenl at

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