Strange network problem, TCP kills my Uplink

lordy

Dear all,

since today I am experiencing a very weird network problem at my parents house that I hope you can help me with. We recently got a new cable connection (16/1 MBit with Cisco Cable Modem and DHCP) which has been working fine for the last 2 weeks. Before that we were using a very slow (1024/128 kbit) DSL connection with PPPoE. The router in use is a FritzBox 7050 with a modified firmware (FreeTZ).

This afternoon there was a major power outage in the area. Since then I have a very strange problem. The router gets his IP via DHCP from the Cable-Modem and is able to ping hosts on the Internet (e.g. Google DNS and others). Clients on the LAN which have an address out of 192.168.178.0/24 are also able to ping internet hosts. Name resolution (via Google) is also working so you can ping by name.

When I now try to open a website it loads the first few kilobytes and then stalls. Ping immediately stops working from the hosts AND the router. For testing I have disconnected all clients from the LAN (2 PCs, 1 Linux-Box, 1 Printer, 1 Access-Point) and only tested with one client. The results are the same: As soon as you establish your first TCP session the whole connection is gone. I have tried resetting and powercycling the cable-modem and router but that didn't change anything.

I first thought that the router might be broken due to a power spike or something but I have now reconnected the old DSL line and it is working just fine. I am puzzled.

I would be grateful if you have any ideas or pointers.

Regards,
Lordy

tiersten

Sounds like a MTU mismatch.

lordy

Well, I have seen MTU mismatches before but that wouldn't explain why the whole connection is dead.

I need to reset the cable modem and the router to get back online after I tried to open a website.

instant000

If you shark it, what's the output look like? That would be the best clue at this moment.

lordy

I will try to shark it sometime this week. I first need to find a second NIC for my laptop that I can attach via USB or PCMCIA to turn it into a bridge.

lordy

OK, I haven't managed to get a **** of the traffic yet but I have tried re-flashing the box.

Unfortunately to no avail :-/

networker050184

Have you spoke with the provider?

lordy

The thing is, when I connect a PC to the cable-modem everything works fine.

It gets a public IP via DHCP and everything is good while it's connected to exactly the same cable as the router was before. So I am pretty confident that the router is somehow causing the problems.

I recently got a Cisco 2970 from my employer (after shutting down an old site) so I will try to get a traffic **** with that. Today I had a Linux Laptop connected between the router and the cable-modem but although I think I configured the bridge correctly (with brctl) it somehow didn't seem to be transparent so the router didn't get a DHCP reply. I hope it will be a little easier to monitor with the Cisco...

cyberguypr

You are describing the exact same issue I experienced two weeks ago. I thought my router was dead but it ended up being a defective cable modem. Have you tried running the PC directly connected to the modem for an extended period of time? In my case for some reason it would fail within 1-2 minutes when going through the router. When I connected directly to the modem it took 5-10 minutes for it to fail. I called my provider and they sent a joke of a technician who came with no tools or anything. he looked at the PC literally for 90 seconds and said nothing was going on. When I requested another tech the guy brought the fancy meter, checked the quality of the line and found no issue. He then saw the issue while it happened and just replaced the modem. Everything has been fine and dandy since.

lordy

Ok, so today I replaced the router (Fritzbox 7050 -> 7270, just for the record) and now everything works fine.

Very odd, I still assume that either the power outage or a spike somehow damaged the old box. I will give it to a friend of mine who wants to debug it. If he finds anything I will post it here.