DHCP Client Debug Verbosity

Russell Dekema dekemar at umich.edu
Tue Nov 24 11:26:15 UTC 2015

On Mon, Nov 23, 2015 at 7:27 PM, Russell Dekema <dekemar at umich.edu> wrote:
> Good evening,
> Beyond issuing the -v flag on the dhclient command line, is there a
> way to increase the level or verbosity of debugging output?
> I am trying to troubleshoot a problem in which the ISC DHCP client
> sends a DHCPDISCOVER and I can see what seems like a valid DHCPOFFER
> come back to the client over the wire, but the DHCP client appears to
> be ignoring the offer. Adding the -v flag to the dhclient command line
> does not produce any useful troubleshooting information; it simply
> shows repeated DHCPDISCOVER frames being sent, and nothing else.
> I have read the dhclient man page and searched the web extensively but
> haven't found any way to do this. I am hoping for a middle ground
> between "-v" and running the client in gdb or extensively
> instrumenting the code with printf()s (or the like), as those are both
> a bit outside my normal realm of expertise.
> Thanks in advance for any suggestions.
> Sincerely,
> Rusty Dekema

To answer my own question and perhaps save a future troubleshooter
some time, the answer is to edit includes/site.h and uncomment as many
of the #define DEBUG_[xyz] lines as you see fit.

In my case, it revealed the following:

DHCPDISCOVER on ib0 to port 67 interval 4 (xid=0x31fd9d0e)
DHCPOFFER in wrong transaction.

Off the top of your heads, does anyone know what goes into the
transaction matching logic other than the xid? According to my packet
captures, the xid on the received DHCPOFFER matches the xid on the
transmitted DHCPDISCOVER.


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