Linux network diagnostics

Last Updated on April 18, 2017 by Dave Farquhar

I was doing a little research for Gatermann about Linux networking. I didn’t find what I was looking for, but I found something interesting: a pair of tools co-written by Donald Becker called mii-tool and mii-diag.
The source code for it is available at scyld.com but Debian includes a package for it (mii-diag). It allows you to force your network card to re-negotiate its speed with your hub or switch, which is useful if it’s constantly negotiating the wrong speed. In Windows you can usually open the network control panel and force duplex operation and speed. In Linux, that requires playing around with module options, which aren’t always consistent across drivers (because they’re not all written by the same people) or, if you compiled your driver into the kernel, passing boot parameters. Either way, you’re forced to reboot.

Run mii-diag to find out the status of your card (and commentary on the situation from the authors, in some cases). You can run mii-tool -r to force a renegotiation nicely, or run with the -f parameter to force it to a certain speed (if you’re interested in forcing a speed, you’re probably chasing 100 megabit, full duplex).

If your system is mysteriously not connecting, like my Web server was yesterday after I moved it, this tool can be useful in fixing it. I wish I’d known about it yesterday. I eventually solved the problem by rebooting until it worked right. (I don’t think my server’s 3Com NIC likes my Linksys router/switch much.)

So if you want to change your network’s speed for any reason without rebooting, this is the tool to do it (and it doesn’t make you hunt the Web and Usenet for the module parameters).