DHCPREQUEST of <null address>

Alan Jenkins aj504 at cs.york.ac.uk
Sun May 4 12:31:45 UTC 2008

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";
  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.


More information about the dhcp-users mailing list