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DprssdIsntFn
Premium
join:2004-01-12
Lakeland, FL

1 edit

How to diagnose DHCP response from ISP issue. [SOLVED]

I'm building a linux based router on an old PC. I want to use it to replace my current OTS router.

I've reached the point where I want to test the new router's NAT functionality but I can't even get to first base yet.

When I hook it up to my cable modem, the new router's DHCP client receives an IP address but no other info. No Gateway address for the cable ISP's network nor any DNS addresses.

I know this ISP assigns IP addresses and apparently ties them to the MAC address of the device {computer or router} that you originally connect to the cable modem with.

I do appear to correctly receive a new IP address {new MAC address in my new router} but my /etc/resolv.conf file is updated with null info.

If I connect my new router as a regular PC on my LAN with the old router in place, the new PC's DHCP client corretly picks up a new IP {internal LAN} address complete with Gateway address {the old router's LAN IP address} and the two passed through from the cable modem DNS addresses. This tells me that the new router's DHCP client is functioning correctly.

Can someone tell me what I'm missing or what I can do to more completely diagnose the problem?


Bink
Villains... knock off all that evil

join:2006-05-14
Castle Rock, CO
kudos:4

Re: How to diagnose DHCP responnse from ISP issue.

Fire up tcpdump on your Linux router and see what’s going on.


aguen
Premium
join:2003-07-16
Grants Pass, OR
kudos:2
reply to DprssdIsntFn

This may seem "silly" but are you sure you are using the correct nic for your routers wan port to the modem?



DprssdIsntFn
Premium
join:2004-01-12
Lakeland, FL

Problem solved.

When hooking up a device with a different MAC address, the cable modem requires a forced reset to pick up a new IP address to associate with the new device's MAC address. Apparently, upstream won't respond when it detects a MAC address different from the one that upstream has already associated with an already assigned IP address. There is a timeout period to force release of the old IP address, but it's some unknown length of time.

This is not something I would have ever considered in the course of normal diagnostics.



billaustin
they call me Mr. Bill
Premium,MVM
join:2001-10-13
North Las Vegas, NV
kudos:3
reply to DprssdIsntFn

Did you reset the cable modem after connecting the new router? Some have a reset button, some just need power-cycled. I have only dealt with COX cable, but their modems have to be reset every time you change the connected device.

Some cable providers track the MAC address of the attached device. You either have to call and have it changed, or spoof the MAC address of the old device on the new one.

You discovered the answer while I was typing. This is common for cable, but not for DSL.



DprssdIsntFn
Premium
join:2004-01-12
Lakeland, FL

said by billaustin:

This is common for cable, but not for DSL.

I'm used to working directly with DHCP servers on LANs. So I consider this binding of IP address to MAC address to be .. unnatural.

To force a reset, I had to unplug the power cord for the cable modem AND remove the backup battery because I have the bundled VOIP service.

And this is unnatural!


NormanS
I gave her time to steal my mind away
Premium,MVM
join:2001-02-14
San Jose, CA
kudos:11
Reviews:
·SONIC.NET
·Pacific Bell - SBC

said by DprssdIsntFn:

said by billaustin:

This is common for cable, but not for DSL.

I'm used to working directly with DHCP servers on LANs. So I consider this binding of IP address to MAC address to be .. unnatural.

I find it quite natural ...

Mapping IP Addresses to MAC Addresses.


I'd show you the D-Link DIR-655 mapping, but it is a few miles to the west of my current location. It creates a group of "sticky static" IP addresses.
--
Norman
~Oh Lord, why have you come
~To Konnyu, with the Lion and the Drum


billaustin
they call me Mr. Bill
Premium,MVM
join:2001-10-13
North Las Vegas, NV
kudos:3
reply to DprssdIsntFn

What you are actually seeing is the behavior of the cable modem. It learns the MAC address of the first device connected to it after it is powered on, and won't allow proper upstream communication of any other device that is connected later without being reset. The modems with a battery usually have a small reset button on the back that can be pressed with the end of a paper clip. Sometimes, though, it is easier to just remove the battery and power-cycle it.