how to debug dhcp?

David Timms dtimms at bigpond.net.au
Wed Jul 19 14:55:32 UTC 2006


dragoran wrote:
> dragoran wrote:
>> John Reiser wrote:
>>>> when I try to use dhcp for my network device (eth0 - forcedeth) it does
>>>> not get an IP with the result: failed
>>>>     
>>>
>>>  
>>>> 2) how can I debug it to find out what is causing it?
>>>>     
>>>
>>> Boot single-user, invoke the dhcp program from a console in "non-daemon"
>>> mode, and watch.
>> how do I do this (found nothing in the man page)
>>>   For dhcpd, consider flags -d -tf and perhaps -play.
>>> Also, consider running under strace.
>> I am talking about dhclient not dhcpd
>>>   For lowest-level packet trouble,
>>> use a program such as tcpdump or ethereal to check what goes over the 
>>> wire.
>>>
>>>   
>> used ethreal an have not found anything that looks like an error...
>>
> 
> this is what I get in /var/log/messages:
> Jul 19 09:08:39 chello062178124144 dhclient: DHCPDISCOVER on eth0 to 
> 255.255.255.255 port 67 interval 6
> Jul 19 09:08:45 chello062178124144 dhclient: DHCPDISCOVER on eth0 to 
> 255.255.255.255 port 67 interval 9
> Jul 19 09:08:54 chello062178124144 dhclient: DHCPDISCOVER on eth0 to 
> 255.255.255.255 port 67 interval 10
> Jul 19 09:09:04 chello062178124144 dhclient: DHCPDISCOVER on eth0 to 
> 255.255.255.255 port 67 interval 12
> Jul 19 09:09:16 chello062178124144 dhclient: DHCPDISCOVER on eth0 to 
> 255.255.255.255 port 67 interval 19
> Jul 19 09:09:35 chello062178124144 dhclient: DHCPDISCOVER on eth0 to 
> 255.255.255.255 port 67 interval 5
> Jul 19 09:09:40 chello062178124144 dhclient: No DHCPOFFERS received.
> 
> wtf?
> who is broken here? should I fill a bug?
Can you run ethereal on the dhcp server {or a third machine} to ensure 
the packets get to the server ?  firewall stopping udp port 67 or 68 at 
either end ? Slow negotiating switch or mismatched port speeds ?
Is the DHCP server set to be authoritative; ? Without this I believe win 
{for eg} tries hard to use any previous dhcp it last received, not sure 
about fc-whatever.
At the server should see:
DHCP Discover -> server
client        <- DHCP Offer
DHCP Request  -> server
client        <= DHCP ACK

DaveT.




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