Hi all, I am trying to get my head around the problem of bringing network interfaces up and down and checking the state. I googled extensively, but I have not found an answer to the conundrum, and I am hoping that someone here may shed some light.
My target platform is Debian wheezy (no, I can't upgrade it), but what I describe below is very similar on an up-to-date Fedora 27 (that, obviously, has a very different set of userspace tools and a very different kernel). What I am trying to achieve is as follows. Assume that the system in question gets a command (imagine an API of sorts) to reconfigure one of its multiple interfaces. Let's say that the control interface is eth0 and the request is to reconfigure eth3. Here, "reconfigure" may mean, e.g., "change the static IP address and maybe some routing rules" (or toggle between static and DHCP or whatever - staying static seems conceptually simpler). So the is an implementation (bash, run as root, etc.) that goes, in essence, * stop interface eth3 * change eth3 configuration * start interface eth3 which looks natural enough. It is also natural that there are stop() and start() functions that may include some subtleties such as flushing IP address, etc. The start() and stop() functions are used in other workflows and, in general, one would like to check whether the interface is up or down, at least to avoid superfluous error messages when one tries to bring up an interface that is already up, etc. (Such messages may pollute things for the API client, and hiding stderr in general is not a good idea because real errors may be lost.) The problem is that whatever I do in stop() (or by hand), e.g., * ifconfig eth3 down * ifdown --force eth3 (and --no-scripts, and whatever - on Debian) * ip link set eth3 down etc., etc., with infinite variations, the UP flag remains set (I also check with ifconfig, ip link show, etc.). Technically, the UP flag is the IFF_UP bit in the ifr_flags field of struct ifreq accessible through a ioctl (cf. man 7 netdevice). I went as far as writing a C program that uses the ioctl(s) (SIOCGIFFLAGS, SIOCSIFFLAGS) directly. To no avail: right after I toggle IFF_UP I check it again and it is *always* up... Again, this is on both Debian wheezy (old, kernel 3.2.0) and Fedora 27 (fully updated, kernel 4.15.17)... It also does not depend on whether the interface is designated as "auto" (on Debian) or anything else I could think of. I straced all kinds of commands and downloaded sources and did all kinds of things trying to understand what was going on, but so far in vain. By the way, I pulled the sources of ifupdown from Debian's git, and I didn't find any ioctl calls there. Could it be because they don't help (see above)? The only way (well, I could implement my own state...) I managed to work around the *original* is-the-interface-up? problem was by doing an equivalent of ifdown --force eth3 && sleep 1 && ifconfig eth3 0.0.0.0 for stop() and then testing for the presence of "inet" (rather than "UP") in the ifconfig's output (on Debian). By the way, the sleep is essential - without it it won't work, for whatever reason. The workaround above may or may not prove sufficient for the immediate purpose, but a) it's ugly, and b) I would like to understand how one can toggle the IFF_UP flag programmatically. I also have a dim recollection that ifdown/ifup and/or ifconfig down/up used to work in distant past... Any ideas? -- Oleg Goldshmidt | p...@goldshmidt.org _______________________________________________ Linux-il mailing list Linux-il@cs.huji.ac.il http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il
