Comcast Internet Gateway Questions

awells · 2011-10-08 18:29:40

I have been using Arch for several months as a reliable Internet gateway, but I have had some issues since I got a new modem and 50down/10up package.

The most critical issue is that when something goes down, it takes sometimes hours of tinkering to bring it back up. I have dhcpcd & ifplugd running on eth0. There seems to be a consistent issue with retrieving an IP address. I think it's because the modem hands out an IP address in the 192.168.100.x subnet initially and then the real WAN IP address once an Internet connection is established. That's what happens when I connect an XP laptop and then reboot the modem. Does anybody have any configuration recommendations to make this setup more bullet proof? (auto-renew IP address when the modem resets, etc).

Also, I am having bandwidth issues when running through the server. When I connect my laptop to the modem directly, I get the advertised speeds. But - when I connect through the Arch server, I get only ~2-4mb up. I have learned from the previous modem that i need to set the MTU to 576. I do that by putting "/sbin/ifconfig eth0 mtu 576" in /etc/rc.local. I'm not sure if this is contributing to the slow speed or not.

The server is a Supermicro 1U with these two NICs:

02:00.0 Ethernet controller: Intel Corporation 82574L Gigabit Network Connection
03:00.0 Ethernet controller: Intel Corporation 82574L Gigabit Network Connection

Any advise would be appreciated.

pyther · 2011-10-10 12:39:05

There is really no reason why you need to set your MTU so low. You are breaking up a single packet into 3 packets for every transmission you got going on.

1500 or 1492 or the typical values and should work. If they don't you need to get in contact wiht Comcast.

awells · 2011-10-10 17:29:53

I will generally only call Comcast if I can replicate the issue on a WinXP laptop connected directly to the modem. I highly doubt they will support my arch setup.

The connection will drop if I run "/sbin/ifconfig eth0 mtu 1492" or "/sbin/ifconfig eth0 mtu 1500", and I see some weird error messages in dmesg until I manually set it back to 576:

[  124.373946] e1000e 0000:02:00.0: eth0: changing MTU from 576 to 1472
[  126.847708] e1000e: eth0 NIC Link is Up 1000 Mbps Full Duplex, Flow Control: None
[  136.799985] e1000e 0000:02:00.0: eth0: changing MTU from 1472 to 576
[  137.003819] e1000e 0000:02:00.0: eth0: changing MTU from 576 to 1472
[  141.377705] e1000e: eth0 NIC Link is Up 1000 Mbps Full Duplex, Flow Control: None
[  150.426471] e1000e 0000:02:00.0: eth0: changing MTU from 1472 to 576
[  150.520225] e1000e 0000:02:00.0: eth0: Reset adapter
[  150.635366] e1000e 0000:02:00.0: eth0: changing MTU from 576 to 1472
[  153.561040] e1000e: eth0 NIC Link is Up 1000 Mbps Full Duplex, Flow Control: None
[  163.003036] e1000e 0000:02:00.0: eth0: changing MTU from 1472 to 576
[  163.096903] e1000e 0000:02:00.0: eth0: Reset adapter
[  163.221536] e1000e 0000:02:00.0: eth0: changing MTU from 576 to 1472
[  167.727701] e1000e: eth0 NIC Link is Up 1000 Mbps Full Duplex, Flow Control: None
[  175.303059] e1000e 0000:02:00.0: eth0: changing MTU from 1472 to 576
[  178.061056] e1000e: eth0 NIC Link is Up 1000 Mbps Full Duplex, Flow Control: None

So I will change it to the higher setting and then watch the logs scroll until I switch it back. It works if I place it behind a router, but that sort of defeats the of having this as the internet GW.

pyther · 2011-10-10 17:47:48

What type of router? What is the mtu on the WAN interface of the router? Setting the MTU to 576 is just too small for a fast speed connection. From what I've been able to find most comcast uses a MTU size of 1500.

awells · 2011-10-10 17:54:13

It's a Linksys running DD-WRT. I'm pretty sure it was 1500.

I know the Xbox fails to connect because of the MTU setting (requires ~1300 or something), and it works when connected through the router, so it was definitely higher than what I'm getting now.

Not sure if this is related: https://bugs.archlinux.org/task/20543

awells · 2011-10-19 16:01:56

So this is weird. I took a Cisco RV016 router and placed it between the modem and the Arch server. So now there is an extra hop to get out to the web. From my desktop computer, it hops through the Arch server, and then through the Cisco router. The result? 55 down / 11 up, which is as advertised speeds.

Although this works, this router is borrowed to test my theory and it shouldn't be needed anyway. I would like to remove the router from the setup since it's unnecessary.

My brother tried connecting his Arch laptop to the modem last weekend and had the same issue, so it seems to be something in the configuration of the OS.

pyther · 2011-10-19 17:06:20

What happens if you use a live cd? Are you using the same speedtest host each time? If not you can get very different results!

Just be clear right now you just have the modem directly plugged into the arch box (no routing), right?

awells · 2011-10-19 17:12:38

I have not tried a live CD, but I can certainly do that.

Right now, I have that Cisco Router in place as a workaround. When I'm not troubleshooting, I have the arch box connected directly into the modem.

Arch Linux

#1 2011-10-08 18:29:40

Comcast Internet Gateway Questions

#2 2011-10-10 12:39:05

Re: Comcast Internet Gateway Questions

#3 2011-10-10 17:29:53

Re: Comcast Internet Gateway Questions

#4 2011-10-10 17:47:48

Re: Comcast Internet Gateway Questions

#5 2011-10-10 17:54:13

Re: Comcast Internet Gateway Questions

#6 2011-10-19 16:01:56

Re: Comcast Internet Gateway Questions

#7 2011-10-19 17:06:20

Re: Comcast Internet Gateway Questions

#8 2011-10-19 17:12:38

Re: Comcast Internet Gateway Questions

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