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

[Andrey Yurovsky]

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).

> 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.