On Mon, Jul 31, 2017 at 7:24 AM, Lennart Poettering <lenn...@poettering.net> wrote: > On Fr, 28.07.17 12:08, Andrey Yurovsky (yurov...@gmail.com) wrote: > >> I have an embedded target where the hostname is expected to be a >> string derived in part from the MAC address of an Ethernet interface. >> I've been looking at how to properly set the system's hostname and >> also have systemd-networkd use it in the DHCP request it sends out, >> however there seems to be an order of operations issue. >> >> 1. in systemd/core/main.c the /etc/hostname contents are unconditionally read >> 2. I can add a service that uses the special network-pre.target to >> override /etc/hostname with my generated string and I see that while >> the initial string is pickedup by systemd, the new hostname will in >> fact be used >> 3. I then have a .network file specifying DHCP on that Ethernet interface >> >> But then on initial boot I see that the DHCP Discover coming out has >> option 12 set to the original hostname that systemd picked up in >> main.c, even though the network-pre.target caused my unit to run. I >> can then reboot the system and this time main.c picks up the "new" >> hostname and option 12 is indeed set to this. >> >> One workaround I found was to have my unit write the Hostname= option >> to the .network file but that seems like the wrong approach. >> >> Is there a correct way to replace or otherwise set the hostname and >> have systemd use it from the beginning and ensure that the DHCP client >> specifies it in option 12? > > Hmm, I am not sure I follow. Do you want the hostname to be "sticky"? > i.e. if you boot up once, and your special hostname is not initialized > yet, you initialize from whatever the MAC address is, and then store > it to /etc/hostname, and from that point on and for all future boots > it's supposed to stay fixed? Or do you want it to be fully > dynamic: as soon as an ethernet device shows up, set the hostname, > and when no device has shown up the hostname should remain > uninitialized, and on subsequent boot everything starts from fresh, > with no previous data?
Sorry, what I mean is the hostname is "sticky" but is always the same (there's just the one built-in Ethernet interface, nothing is dynamic), the trick is I always want that generated hostname used and I never want to see a DHCP Discover go out with a default hostname that isn't this generated string (it's a bit off but is meant to maintain expected behavior from this device). > Under the assumption you want the latter: just drop /etc/hostname, so > that no static hostname is managed by systemd/hostnamed. In this case > the system will boot up with the fallback hostname (which is > "localhost" unless your distro overrides that at compile time). Then, > add a udev rule that is run when an interface shows up, and that > changes the hostname as necessary, maybe by invoking the > /usr/bin/hostname binary. I didn't think of that, thank you! That makes a lot of sense. > The DHCP client in networkd will query the hostname the instant it > starts setting up the DHCP session. It will use whatever is set at > that point in time. Hence, if you set the hostname from the udev rule > things should be properly race-free as networkd will only take > possession of any interface after the udev rule ran, and hence will > necessarily initialize its DHCP client at a point in time the hostname > is set to what you want it to be set to. Got it. Thank you for taking the time to answer my weird question, I really appreciate it. _______________________________________________ systemd-devel mailing list systemd-devel@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/systemd-devel
