DHCPREQUEST of <null address>

Glenn Satchell Glenn.Satchell at uniq.com.au
Sun May 4 13:07:48 UTC 2008

>Date: Sun, 04 May 2008 13:31:45 +0100
>From: Alan Jenkins <aj504 at cs.york.ac.uk>
>To: dhcp-users at isc.org
>Subject: DHCPREQUEST of <null address>
>Before I start: my network configuration is somewhat awkward.  I'll 
>explain it for completeness so I'm not accused of holding anything back, 
>but I'm only really interested in these strange DHCPREQUESTs (see subject).
>Second point: I'm using Ubuntu's dhclient.  I will retry using the 
>latest version without any potentially malign distro patches.   It'll 
>take a bit longer though and I've just nailed down the issue, so I'm 
>posting here with details as a first step.  Maybe my problem is 
>familiar, or maybe it'll go away if I use the pure version and provide a 
>little more hard evidence that the distro pacakges suck :-).
>OK, I'm in university dorms with ethernet and an authentication 
>mechanism which is handled by wpa_supplicant.  I hacked up some 
>network-manager scripts so it all works nicely.  Admittedly, sometimes 
>it starts up confused, and I'm assigned an "unauthenticated" IP address 
>which starts "172." (authenticated addresses start with "144.") and my 
>net access doesn't work.   But I can just reconnect a couple of times 
>(by kicking network-manager) and it eventually gets an authenticated IP 
>Back to these strange messages, "DHCPREQUEST of <null address>".
>Does this mean that dhclient has run out of valid leases?  This type of 
>DHCPREQUEST is happening very frequently for me - suggesting that 
>there's a problem where dhclient is failing to renew the current lease 
>before it expires.  [For whatever reason.  No criticism of dhclient 
>But it's not even that.  Have a look at this log:
>May  4 11:52:18 alan-eeepc last message repeated 2 times
>May  4 11:52:36 alan-eeepc dhclient: DHCPREQUEST of <null address> on 
>eth1 to port 67
>May  4 11:52:36 alan-eeepc dhclient: DHCPACK of from 
>May  4 11:52:36 alan-eeepc NetworkManager: <info>  DHCP daemon state is 
>now 3 (renew) for interface eth1
>May  4 11:52:36 alan-eeepc dhclient: bound to -- renewal in 
>1579 seconds.
>May  4 12:18:55 alan-eeepc dhclient: DHCPREQUEST of <null address> on 
>eth1 to port 67
>May  4 12:19:19 alan-eeepc last message repeated 3 times
>May  4 12:20:15 alan-eeepc last message repeated 3 times
>May  4 12:21:26 alan-eeepc last message repeated 5 times
>We see that the renewal time is about 25 minutes, and the <null address> 
>requests happen at the renewal time.  So dhclient thinks the lease has 
>expired as soon as it tries to renew it!  But look at the expiry time in 
>the leases file:
>lease {
>  interface "eth1";
>  fixed-address;
>  option subnet-mask;
>  option routers;
>  option dhcp-lease-time 2840;
>  option dhcp-message-type 5;
>  option domain-name-servers,;
>  option dhcp-server-identifier;
>  option domain-name "york.ac.uk";
>  renew 0 2008/5/4 12:08:23;
>  rebind 0 2008/5/4 12:26:41;
>  expire 0 2008/5/4 12:32:36;
>So the lease shouldn't have expired that quickly. 
>I think this is causing a certain amount of difficulty; the DHCP server 
>isn't responding until... well, in this sequence the next DHCPACK is at 
>12:45.  Really.  Which doesn't make any sense, but then neither does my 
>"unauthenticated address" problem.  I do think the DHCP server generally 
>sucks, but that doesn't really explain what dhclient is doing here.

I've got no idea on the <null address>.

Most dhcp clients attempt to renew the lease about 50% of the way
through. This lets them keep the same IP address for a long time, even
if the lease time is short.


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