Comment 0 for bug 1877176

[impact]

a systemd with a 64-character hostname (the maximum hostname length for Linux) will cause a dhcp server to reject its dhcp lease due to passing the invalid hostname in the dhcp lease request.

[test case]

$ cat /etc/systemd/network/10-ens3.network
[Match]
Name=ens3

[Network]
DHCP=yes

set hostname to 64-char name, e.g.:

$ sudo hostnamectl set-hostname a123456789b123456789c123456789d123456789e123456789f123456789g123

restart networkd:

$ sudo systemctl restart systemd-networkd

check logs:

root@a123456789b123456789c123456789d123456789e123456789f123456789g123:~# journalctl -b -u systemd-networkd --no-pager | grep 'DHCP error'
May 06 19:01:30 a123456789b123456789c123456789d123456789e123456789f123456789g123 systemd-networkd[737]: ens3: DHCP error: Client failed: Invalid argument

[scope]

this is fixed by commit 9740eae694e93b06658ff3b3045b22b591561e7c which is included in Bionic and later. This is needed only for Xenial.

[other info]

this is a follow on to bug 1862232, which corrected sd-dhcp-client.c to continue networkd dhcp even if the hostname is invalid, however the older code in Xenial doesn't correctly detect the invalid hostname, so this additional patch is needed.