The newest development model and 2.6.14 [LWN.net]

The newest development model and 2.6.14

Posted Nov 5, 2005 0:07 UTC (Sat) by emkey (guest, #144) [Link]

They are all e1000's in this case. And actually we have six total, though only five are in use. Two are built in copper, and the other four are provided by dual fibre E1000 cards. The first two remained the same (eth0, eth1) after the upgrade. The cards swapped though. (IE, eth2, eth3 became eth4, eth5 and eth4, eth5 became eth2, eth3)

A few minutes with tcpdump solved the mystery as to why things weren't working properly after the upgrade.

There may be a way around this, but there really shouldn't need to be.

The newest development model and 2.6.14

Posted Nov 6, 2005 23:47 UTC (Sun) by zblaxell (subscriber, #26385) [Link]

I think the real problem is that there is no guarantee of stability in unit enumeration in most distros. The mechanism certainly exists in the kernel, and has existed since 2.1.somewhere-near-100, but there isn't a user-space implementation installed by default on most distros.

eth0 is "the first detected ethernet card", eth5 is "the sixth detected ethernet card". If the eth0 card's PCI bus controller dies (as mine did a year or two ago), suddenly eth5 becomes eth4, eth4 becomes eth3, etc. This does horrible things if you were enforcing some kind of security on those devices, and the machine manages to come back up after this sort of failure. This can be triggered with just a lightning strike and a reboot--same kernel version, same distribution, but suddenly some or all of the ethernet cards have new names because a low-numbered one got zapped. It's inconvenient, but don't blame the kernel developers for breaking your fragile configuration.

This isn't a new problem that arose in 2.6, it has *always* been there. Use 'ip name ...' or 'nameif' to force your network devices to have specific names that don't match any possible default name. Set up your routing and firewall rules to use the specific names, and firewall everything that has an anonymous "eth0"-style name to the DROP target. Once configured, your interfaces will never be renamed again, although now you'll have to update the MAC addresses table every time you swap out a card or build a new machine.

Distro vendors could help people in your situation (only read half of the manuals, built a broken configuration, got surprised when the interest payment on technical debt became due) by including a user-space tool which assigns dynamic but persistent device names, so "eth0" would become "the first ethernet card *ever* detected in the system", "eth1" would be "the second ethernet card *ever* detected," etc. Single-user systems would only see "eth0", gateway hosts would have "eth0" through "eth5" that behave the way you expect between reboots, machines which replaced a broken NIC would have just an "eth1" since there's no way for the system to know if the card-formerly-known-as-eth0 might come back one day. It might be somewhat inconvenient to replace a card (you'd have to update routing table and firewalls for eth1 instead of eth0), but that's what you get for not reading the manual--and if you did, you'd probably find the state file that defines the persistent mapping and just edit it manually.

The newest development model and 2.6.14

Posted Nov 7, 2005 2:23 UTC (Mon) by emkey (guest, #144) [Link]

In our situation we would never come up with a networking card "missing". We have mechanisms in place to make sure this doesn't happen.

Being able to tie a device to a given MAC address is potentially interesting though.

As for not reading the manuals, I don't read the source code to the kernel either. The environment I work in takes hundreds of pages to document and is changing in small ways on a daily basis. I read LWN and am always on the lookout for new sources of information, but the simple truth of the matter is I do not have the time to be an expert in every single aspect of Linux, networking, etc. I wish I did.

Thanks for the information.