[Resolved] Faulty MTU with Intel e1000 card

strc · Tux's lil' helper Joined: 07 Jul 2002 Posts: 77

Hi,

Recently, I had some network problems with an app (Second Life), and eventually determined that the issue was caused by a "bad" MTU on my Intel e1000 ethernet card. The MTU was somehow set to 576; a simple

Keruskerfuerst · Posted: Wed Apr 18, 2007 6:59 pm Post subject:

Usually the maximum setting of MTU is 1492 and not 1500.
I really don´t know any cause, why MTu is set to 576.

strc · Tux's lil' helper Joined: 07 Jul 2002 Posts: 77

Thanks for the info Keruskerfuerst. Hopefully there's no real difference between 1492 and 1500. =)

I guess I should add that last night I set the MTU to 1500, but by this morning it was 576 again, even though I didn't reboot or restart any system processes. :roll:

Maybe some crappy app is changing the MTU, but I'd imagine it would need root privileges...

think4urs11 · Posted: Wed Apr 18, 2007 7:38 pm Post subject:

strc · Tux's lil' helper Joined: 07 Jul 2002 Posts: 77

As an update, my ethernet card has had its MTU reset to 576 twice in the past 12 hours or so. In the meantime, I haven't rebooted or restarted any network apps run by root. To fix the problem, I've been manually setting the MTU back to 1500 using ifconfig. Any insights into my MTU setting's bad behavior are greatly appreciated, for example if a certain application would mistake my ethernet card for an X.25 device.

UberLord · Posted: Wed Apr 18, 2007 10:56 pm Post subject:

It's an issue with dhcpcd-3.0.16 with some routers.
Upgrading to dhcpcd-3.0.17 masks the issue with the router.
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strc · Tux's lil' helper Joined: 07 Jul 2002 Posts: 77

Oh wow, I guess that's the answer, uberlord. The timestamp on my dhcpcd-3.0.16 binary coincides with the time I started to have these issues. I updated to 3.0.17 and will consider the issue resolved unless the MTU returns to 576. Thanks for your help.

UberLord · Posted: Thu Apr 19, 2007 6:51 am Post subject:

Still, I would bitch to the people that made the router as it's informing the NIC that its MTU should be 64, which is lower than the bare minimum 68 for ethernet and 576 for dhcp. dhcpcd-3.0.16 set the mtu to 576 if lower than 576, 3.0.17 now ignores MTU values lower than 576.

You could argue that either implementation is correct.
You cannot argue that the router is correct as a whireshark trace will prove me right

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strc · Tux's lil' helper Joined: 07 Jul 2002 Posts: 77

Well, 576 was too low for some of my apps. I tried 3.0.17 as you recommended, but it also sets the MTU to 576. Reverting to 2.0.5-r1 keeps the MTU at 1500. I'm not sure if that's broken behavior (maybe the DHCP server is giving an MTU of 576 and 2.0.5 ignores it?), but the end result is more useful to me.

By the way, my ISP's router seems to be a Cisco, from the MAC prefix (00:05:74). I'm not sure if that indicates more bugs or fewer bugs than your average router.

UberLord · Posted: Thu Apr 19, 2007 3:48 pm Post subject:

Heh - some ISP's set the MTU to 576 regardless. There's also the -M option in 3.0.17 which means totally ignore the MTU ....
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Stever · Posted: Thu May 03, 2007 8:52 pm Post subject:

Thanks for the tip on the -M option - wireshark confirms that my Cable ISP also seems to be setting MTU to 576.

Now could there be any good reason for this? MTU of 1500 has been working fine for years, and as far as I can tell with wireshark nobody else is even requesting the MTU in their DHCP requests.

P.S. Compiling wireshark on 64MB RAM is a pretty good way to see if your router still works under heavy swapping

UberLord · Posted: Thu May 03, 2007 10:34 pm Post subject:

Well, we had a request that dhcpcd obey the MTU of DHCP servers. Which is a valid request. See this thread for a valid example of this. https://forums.gentoo.org/viewtopic-t-557082.html

As to why some cable companies set 576 ... well, maybe they think a really low MTU results in more reliable data flow even if it affects overall throughput. I would bitch to your ISP personally. They must have some explanation, no matter how silly

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Cyker · Veteran Joined: 15 Jun 2006 Posts: 1746

Either that or they're using the same DHCP servers to serve broadband and their modem racks - Modem/PPP connections typically use the 576 value for MTU.

Stever · Posted: Fri May 04, 2007 2:29 pm Post subject:

Setting the MTU based on the DHCP server provided value definitely seems like the right behavior, I'm just glad you provided a way to override it. Just because I plug an ethernet cord into a modem doesn't mean I trust the people at the other end to configure my hardware. All I need from them is an IP address and a default route - the rest I can handle myself