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

[Andrey Yurovsky]

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?

Thanks,

  -Andrey