# Not getting Gateway and DNS address info from ISP [SOLVED]

## dufeu

Because of recent issues with random disconnects from my ISP, I've decided to replace my Linksys WRT45Gv2 router with a re-purposed PC using Gentoo GNU/Linux as the base. I eventually would like to be able independently diagnose WTF is going on re: ISP tech gabbling.

I've built new router following the Home Router Guide. I've reached the point where I believe I'm ready to at least NAT with new box.

When I hook up the new router to the cable modem, I get an IP address but nothing else. No Gateway address. No DNS addresses. Nada.

This ISP provides a 'dynamic' IP address assignment but ties that address to the MAC of the device with which you connect for the first time to the cable modem. The IP address doesn't change thereafter. My old router has no problem receiving either the current cable network gateway or the current DNS addresses.

My expectation was to receive a new {different first time} IP address, Gateway and 2 DNS addresses when hooking up the new router.

The new router receives what appears to be a valid IP address but my new router's DHCP client doesn't appear to receive any other info.

When I hook up the old router, it receives the previously associated IP address and ancillary addresses. When I now hook up the new router, I receive the apparently newly associated IP address but no ancilary addresses.

When I connect the new router as a normal PC onto my LAN, it gets assigned a LAN IP address as you'd normally expect and gets both my LAN's old router Gateway address and the passed through DNS addresses also just as expected. This tells me that my DHCP client on the new router is properly installed and configured.

I figure if worst comes to worst, I can spoof the old routers MAC address for the cable modem connection but ..

I've never seen this type of behaviour before and would like some edification as to what may be happening. I can't seem to find any docs which can tell me if I'm just totally mistaken or if the ISP is jerkling me around.

Constructive suggestions welcome!!

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## NeddySeagoon

dufeu,

What IP address do you get with your Gentoo router connected to your ISP?

If its in the range 169.254.1.0 through 169.254.254.255, its not coming from your ISP.  Thats the self assigned link local range.

dhcpcd self assigns an IP in this range if contact with a dhcp server fails and it was built with USE=zeroconf.

What do you get out of your cable modem, real ethernet or PPPoE?

The former just works, the latter requires that you set up PPPoE.

ISPs like PPPoE as it works like dialup, so it makes account management simple (for them).

All this stuff will be on your ISPs website, or should be.

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## dufeu

 *NeddySeagoon wrote:*   

> dufeu,
> 
> What IP address do you get with your Gentoo router connected to your ISP?
> 
> If its in the range 169.254.1.0 through 169.254.254.255, its not coming from your ISP.  Thats the self assigned link local range.
> ...

 

You're correct. I had forgotten about the self assign range when dhcp fails. It's been a long while since I built and set up a router.

My ISP 'should just work' as it's 'real ethernet' - a DOCSIS 2.0 cable modem on what is being upgraded to DOCSIS 3.0 network. Service has been {and continues to be} a pain these last few months so it's pretty easy to blame the ISP. I'm still getting random complete disconnects {including the bundled VOIP service} about 6plus times a day.

However, this time the problem really appears to be between seat and keyboard. One thing I found was I forgot to turn off were the iptables rules set I was  playing with when I built the system. I thought I was testing without iptables being turned on.

It's not quite 'real ethernet' since they tie whatever dynamic IP address is initially assigned to the MAC address of the device you hook up to the modem. Fortunately, they're not one of the ISPs which will stop responding if they find a different MAC address at your end. They claim that the next available IP is assigned to the new MAC address.

Sigh ..

Still haven't figured out what I'm doing such that the new PC's dhcp works when booting with the old router but doesn't when booting directly with the cable modem.

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## dufeu

I found the problem and it's really, really, really .. inane.

I needed to force a reset on the cable modem each time I change the device I connect to it.

Go figure.

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