DHCP suddenly not working

[Brenden T.]

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.