DHCP keeps timing out :(

August 14, 2007 9:03 PM

What can I do to fix this DHCP problem with my Linksys router?

It's been going on for a couple weeks, and I'm at a loss as to how to resolve it. Here's the skinny:

Comcast is the ISP, we're on a nice rented linksys router and motorola cable modem. There are five computers connecting to the router, 3 via ethernet and two wirelessly. The router's handling DHCP locally, but it seems to just die periodically.

What'll happen is this: I'll be surfing/downloading/gaming/whtaever, and I'll lose connectivity. Using windows to repair the connection fails, because it can't renew an IP address. Using ipconfig /release and /renew reveals that the DHCP server has timed out, and pinging the router at these times reveals packet loss. After five or ten minutes, everything works again.

This happens seemingly randomly, but frequently (probably once every hour or two, but possibly more often.)

I've tried most of the layman's router tech support stuff; unplug everything, powercycle, restart. Doesn't seem to help. Also ran through most of the tech support stuff on Linksys' website, and reset the MAC address and checked all the DHCP settings in the router control panel. Didn't have any luck there, either.

Anyone know what else could be causing this? I'd like to have it fixed.

posted by Yelling At Nothing to technology (10 comments total)

Are you sure your router is running the latest firmware? Many issues with DOCSIS systems such as Comcast, including ones that cause repeated dropouts, have been addressed in subsequent firmware releases. You need to identify, carefully, your router model and ( very important ) hardware revision version, and then download the appropriate firmware image to a known directory location on the hard drive of a machine connected by wire to the router. You can, of course, first verify your current firmware revision by logging into the router, and then going to the Status page, and looking for the firmware version in the upper right corner of the page. Then, if you need to update the firmware to a later version, go to the Adminstration page, select the "Firmware Upgrade" tab near the top of the page, and browse to the locatioin of the flash firmware you just downloaded. The router will upgrade from that file.

Don't do this from a wireless connected machine! It is best if you do this with the router and the machine you are flashing from plugged into a UPS, so there is no chance of power surge or power loss in mid-flash.
posted by paulsc at 9:17 PM on August 14, 2007

So you're saying the problem is between your client machines and the router (because you can't ping the router) - not the router's DHCP client and the modem / ISP?

(I ask because I'm not at all familiar with the details of how/what Comcast - or any other US ISP - provides their services. So, for the purposes of this answer, I'm gonna assume the problem is between the client machines and the router.)

As I've said before around here, a lot of the small DHCP clients and servers on routers (particularly Linux-based ones) are buggy junk (e.g. dnsmasq). Coupled with the fact that the DHCP spec is a little ambiguous as to what goes where, what is required, and what is optional, and you can get a whole swathe of problems with lease acquisition and renewal.

If the latest firmware doesn't fix it, then can you set your clients up with fixed IPs and just bypass DHCP altogether?
posted by Pinback at 10:00 PM on August 14, 2007

Well, sure enough, there's a firmware version that's a month old that claims to "resolve issue of working with DHCP relay agent."

We'll see if it solves things.
posted by Yelling At Nothing at 10:16 PM on August 14, 2007

I used to have this problem (and wrote an AskMeFi post about it 2 years ago)--it happened when I was using Azureus mostly, but often when using other software as well.

I updated the firmware to a Sveasoft product and had much better luck.

That said, I've been much more pleased with our Netgear router (the one that replaced the Linksys).
posted by yellowcandy at 10:16 PM on August 14, 2007

Pinback: I'm not really familiar with the details, either; I assume the problem is with the router because I can't ping it consistently while the problem's going on.

I haven't tried giving everyone their own static IP yet; we're roommates in a largish house and it can be tough to track everyone down to troubleshoot it, aside from "yeah my internet's been kinda flaky, too."

I suppose if the firmware doesn't solve things, trying folks on static IPs should be the next step.
posted by Yelling At Nothing at 10:20 PM on August 14, 2007

"... I suppose if the firmware doesn't solve things, trying folks on static IPs should be the next step."
posted by Yelling At Nothing at 1:20 AM on August 15 [+] [!]

Actually, if the firmware update doesn't solve the problem, I'd suggest logging into your modem's internal management Web interface (type 192.168.100.1 into your browser address bar, within 45 seconds after power cycling your router and modem together). You'll probably find a log/status report page in the modem management interface pages, that will be recording various aspects of modem performance during its attempts to connect to the cable network, including error messages and retrys from errors. More important, you'll find a page that list the current upstream and downstream signal levels (in -dBm units) that your modem is getting from the cable system. Comcast technical support can read these, direct from your modem, too.

If the signal levels are not within tight limits, your modem is going to be dropping out intermittently, and possibly pretty frequently. There is nothing you can do directly about this, except to report the problem to Comcast, and have a service tech come out and rectify the problem with adjustements to line amplifiers out on the street, or by solving wiring and signal distribution problems on your premises. The only reason to recommend that you log in to the modem yourself, and examine the error logs, is that sometimes, Comcast pushes a software update to it's modems, that the modems don't take, and you can see this in the status log your modem maintains. If that is what's happening, Comcast will need to give you a new modem.

Most often, such problems are caused by people around you adding or changing their cable services, creating more or less loading on your segment of the cable system, or injecting unwanted local signal feedback into the cable network infrastructure, from poor consumer equipment, or unshielded connections.
posted by paulsc at 10:36 PM on August 14, 2007

I had a linksys router fail in a similar way once, but when I improved the ventilation to the box it got better.
posted by BrotherCaine at 1:09 AM on August 15, 2007

I don't understand why you'd be using DHCP Relay to connect a home LAN to a WAN. I would have thought that your router should be the only device visible to your ISP, and that your router should be doing NAT between your ISP and your LAN, and should therefore be handing out LAN IP addresses with its own DHCP server rather than relaying DHCP from the ISP side.

If a firmware update designed to fix a DHCP Relay bug fixes your issue, you might care to check that your router hasn't been configured with its NAT brains turned off.
posted by flabdablet at 2:19 AM on August 15, 2007

Try to use an alternative firmware as Tomato, Linksys firmware is bad
posted by zouhair at 3:42 PM on August 15, 2007

Just to pop back in and update, the new firmware seems to have fixed things. I'll take a look at the router settings when I get a chance, but for now I'm happy I don't have to deal with Comcast replacing it.
posted by Yelling At Nothing at 1:42 AM on August 17, 2007