Bug 498658

Created attachment 342132 [details]
Use 60 seconds as minimum for a lease

Description of problem:
When a system running dhclient gets a very short lease, it goes completely crazy and it looks like kind of a DOS.

Version-Release number of selected component (if applicable):
dhcp-3.0.5-18.el5
Note: all ISC dhcp versions seem affected, I have checked dhcp-4.1.1b1 and it seems to do exactly the same.

How reproducible:

Steps to Reproduce:
1. Configure a dhcp server with 'dhcp-lease-time 2'
2. Start 'dhclient' on a client
  
Actual results:
The client tries to renew it's lease in a loop as fast as it can. The network connection becomes almost unusable and the system becomes almost unresponsive. Of course dhclient floods the clients log and may quickly fill the /var filesystem. The system spends much time running dhclient-script for every request. The whole thing looks like a DOS.
As a side effect the same log flooding happens on the server - but he gets what he deserves.

Expected results:
dhclient should add a sanity check to make sure it doesn't bring the system down when getting short leases. The OpenBSD and other BSD folks have done so and they seems to live well with it: http://www.openbsd.org/cgi-bin/cvsweb/src/sbin/dhclient/dhclient.c.diff?r1=1.57;r2=1.58;f=h

Additional info:
AFAIK DHCP doesn't define a minimum lease time. However, it's not good that a badly configured DHCP server can melt down it's clients so easy. I have seen this kind of 2 second leases on mobile broadband networks (GPRS/UMTS/HDSPA). I usually get leases of about 300 seconds but from time to time it is reduced to 2 seconds for whatever reason.
If, in a large corporat network, someone is able to run it's own DHCP server and configures 'dhcp-lease-time 2' on it, he may be able to make a lot of machines unusable very quickly. It just looks too easy to me.
Attached patch derived from OpenBSD uses a minimum of 60 seconds for it's lease which means it starts renewing it every ~27 seconds.