DHCP suddenly not working

[Rick Stevens]

Brenden T. wrote:
> Hi all,
> 
> I've got an older Red Hat 7.2 system that I've been using for NAT and 
> firewall for over a year, connected to my upstream ISP.  Suddenly, I 
> can't DHCP from it anymore.  The tech when I called had me switch over 
> to windows on another system, and that works. So they blame my Linux 
> system, said it wasn't on their end even tho things are completely 
> unchanged, and that was that.
> 
> Querry: Does anyone have an idea what might be the problem?
> 
> I tried adding the -r option to dhcpcd from the command line which did 
> not help.  I took a trace w/ tcpdump and it showed very little 
> difference.  The Linux DHCP was simply ignored, no response at all.  I 
> blew away my dhcp cache so it couldn't ask for old ip addresses (windows 
> doesn't).  I switched network cards on the windows PC, it worked fine so 
> I know they are not memorizing MAC addresses.  (And my DSL modem never 
> did care about MAC address before anyway.)
> 
> The only difference I saw in outgoing packets was that Linux fills in 
> the seconds-since-start field and windows doesn't appear to.
> 
> I haven't tried upgrading Linux yet, and I can't rule out a compromised 
> system, but I don't think so.  Upgrading is not a problem, I only use 
> this system for nat, so there nothing on it to lose really, but I'd like 
> to actually discover the problem first rather than just reinstall stuff 
> at random.
> 
> Any help would be much appreciated.

There's several possibilities.  It may be that the DHCP session isn't
being dropped properly by Linux.  Try:

	/etc/rc.d/init.d/network stop
	(reboot your cable or DSL modem)
	/etc/rc.d/init.d/network start

It could also be that your ISP changed their DHCP servers and they
don't handle the Linux client properly anymore.  They should.  DHCP
is DHCP.  If they adhere to the RFC (many don't), it should work.
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- Rick Stevens, Senior Systems Engineer     rstevens at vitalstream.com -
- VitalStream, Inc.                       http://www.vitalstream.com -
-                                                                    -
-   The light at the end of the tunnel is really an oncoming train.  -
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