[systemd-devel] systemd/hostnamed: setting the hostname and using it in the DHCP Discover

[J Decker]

On Mon, Jul 31, 2017 at 7:38 AM, Andrey Yurovsky <yurovsky at gmail.com> wrote:

> On Mon, Jul 31, 2017 at 7:24 AM, Lennart Poettering
> <lennart at poettering.net> wrote:
> > On Fr, 28.07.17 12:08, Andrey Yurovsky (yurovsky at 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).
>
>
There's an option under [dhcp] section
https://www.freedesktop.org/software/systemd/man/systemd.network.html

SendHostname=
When true (the default), the machine's hostname will be sent to the DHCP
server.

UseHostname=
When true (the default), the hostname received from the DHCP server will be
set as the transient hostname of the system

Hostname=
Use this value for the hostname which is sent to the DHCP server, instead
of machine's hostname.


would think if you set your name there it won't matter what /etc/hostname
is?

>
> > 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.
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