I have a webserver running on a local machine, which spontaneously started to send about 400 DHCP requests per second, constantly changing its IP address, until I killed dhclient several hours later. Simply restarting the server did not help, I actually had to kill dhclient in order for it to stop.
The server is configured to use DHCP, and the router is configured to give it a static IP address.
This is the output of grep -i dhc /var/log/syslog.1
(.1 because I restarted it) for the first few seconds (the rate increased dramatically later, but not the overall pattern):
Sep 11 04:36:01 net-server dhclient: DHCPREQUEST on eth0 to 10.0.0.1 port 67
Sep 11 04:36:01 net-server dhclient: DHCPACK from 10.0.0.1
Sep 11 04:36:01 net-server dhclient: DHCPDECLINE on eth0 to 255.255.255.255 port 67
Sep 11 04:36:01 net-server dhclient: DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 4
Sep 11 04:36:04 net-server dhclient: DHCPREQUEST on eth0 to 255.255.255.255 port 67
Sep 11 04:36:04 net-server dhclient: DHCPOFFER from 10.0.0.1
Sep 11 04:36:04 net-server dhclient: DHCPACK from 10.0.0.1
Sep 11 04:36:04 net-server dhclient: DHCPDECLINE on eth0 to 255.255.255.255 port 67
Sep 11 04:36:04 net-server dhclient: DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 3
Sep 11 04:36:04 net-server dhclient: DHCPREQUEST on eth0 to 255.255.255.255 port 67
Sep 11 04:36:04 net-server dhclient: DHCPOFFER from 10.0.0.1
Sep 11 04:36:05 net-server dhclient: DHCPACK from 10.0.0.1
Sep 11 04:36:05 net-server dhclient: DHCPDECLINE on eth0 to 255.255.255.255 port 67
Sep 11 04:36:05 net-server dhclient: DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 8
Later, lines like Sep 11 05:13:01 net-server ifup[436]: /sbin/dhclient-script: 28: .: Can't open /usr/share/sendmail/dynamic
started to appear, but since they did not appear for the better part of an hour, I don't imagine that they are causally related.
Since I guess I didn't actually ask anything yet, my question is: what could possibly cause a Linux server to change its IP this often, and how can I prevent it?
Searching the internet was entirely unproductive, because in most cases "many" requests means every few minutes, not hundreds per second. I can provide more information if requested, but at the moment I don't even know enough about the situation to ask a proper question about it.