dhcpcd ver 3.x, mtu, and VMWare (Solved)

[dweigert]

I've had a lovely time trying to troubleshoot an issue with VMWare. I did an emerge --sync, then and emerge -uD --newuse world. All went well, no issues at home with VMWare. Only when I connected with the corporate LAN, the Windows XP session inside of VMWare began throwing fits. No web pages loaded, no access to the Exchange servers, etc.

Funny thing, telnet and the windows remote desktop worked fine. This is clue #1

Nothing worked no matter what I did to get Wndows working again, including creating a fresh install of XP in another VM. With the help of our networking experts, I discovered today that the MTU of eth0 was 576. Clue #2....

A bit of digging and I found out that dhcpcd ver 3.x has a default fallback MTU of 576. Hmmmm.... I downgraded dhcpcd back to the latest ver 2 in portage, and have had no problems since.

[UberLord]

A bit of digging and I found out that dhcpcd ver 3.x has a default fallback MTU of 576. Hmmmm.... I downgraded dhcpcd back to the latest ver 2 in portage, and have had no problems since.

Right. But we only use that fallback if the DHCP server in question requests that we use a lower MTU than 576. The chances are that your DHCP server is faulty.

You can diagnose this yourself. emerge wireshark and watch the UDP packets being sent during dhcpcd starting up. In the first packet we make no MTU request, so you won't see anything in the reply. In the 2nd request we do request an MTU value if the server has one. If it does then it sends it back and we use that MTU.

Maybe you could do this and tell me what MTU the server is trying to set and some info about your DHCP server.

Thanks

[dweigert]

I can tell you that the DHCP servers are windows boxes with a Cisco DHCP Server solution on it. The vintage is uncertain, as are the revisions of the servers.
I wish I had the clout to tell them to fix it...

*Shrug* Not the choice I would have made if I was in charge of the infrastructure...

Since the infrastructure isn't in my control, I can only do things from my end. The biggest issue was the MTU mismatch between Linux and the Windows XP running inside VMWare. Windows set an MTU of 1500. The dhcpcd client set 576. The resulting clash was a really messy thing to figure out.

Just out of curiosity, wouldn't a more sane default for ethernet be an MTU of 1500?

Dan

[UberLord]

Just out of curiosity, wouldn't a more sane default for ethernet be an MTU of 1500?

We don't do anything with the MTU unless the DHCP server requests that we do.
And yes 1500 is a sane default.

[UberLord]

http://dev.gentoo.org/~uberlord/dhcpcd-mtu.patch

This patch may or may not help.

[Dun]

Bumping this thread just to confirm (already done on bugzilla) the problem
with net-misc/dhcpcd-3.0.16-r1 as dhcp client with my ISP (Fastweb IT).

MTU requested 64 (too low) and then set to 576.

Solved using dhclient.