[SOLVED] Make systemd change hostname without logging out

JuventusFC · 08-24-2021, 05:01 AM

I have a systemd service that reads some environment values (that may change at startup) and set the hostname accordingly.

The service at the moment is a mix of several tries

Code:

[Unit]
Description=Hostname Setting Service
After= read_variables.service
Requires= read_variables.service network-pre.service
Before= systemd-hostnamed.service  network-pre.service
DefaultDependencies=false

[Service]
ExecStart=/bin/bash -c "echo name$VARIABLE > /etc/hostname; systemctl start systemd-hostnamed"
StandardOutput=journal+console
EnvironmentFile=/folder/environment_output.txt
Type=oneshot

[Install]
WantedBy=multi-user.target

The problem is this one:

Code:

at n-1-th startup, environment variables are set at n-1-th value
reboot 
n-th startup
n-1-th values are read in /etc/hostname and then hostname is set to name@n-1
correct n-th values are read with  read_variables.service and put in environment_output
correct hostname is set in /etc/hostname
But hostname is not modified on the fly so I will have hostname@n-1 instead of hostname@n

So my question is how to run these scripts before systemd prints the line:

systemd[1]: Set hostname to XYZ

wpeckham · 08-24-2021, 08:11 AM

WHY do you want to dynamically reset the hostname? What purpose does this serve?

michaelk · 08-24-2021, 08:44 AM

You can change hostname on the fly just not from editing /etc/hostname.
hostnamectl can change hostname on the fly.

As posted why do you want to change hostnames but I would guess this is for DHCP / DNS.

wpeckham · 08-24-2021, 08:55 AM

At this point I have to point out that on the server the hostname serves several purposes and the host can have only one. On the network the host name can be totally different, or it can even have more than one, and represents the interface it has onto the network. DHCP clients allow you to specify the hostname to be used on the network, and it need not be the same as the name by which the host knows itself.

I would really like to understand the purpose, so we can address the need instead of only the literal question.

JuventusFC · 08-25-2021, 04:25 AM

Thank you, I have resolved using hostnamectl set-hostname script launched on a script referred by a systemd service, and adding some SELinux rules.

The need for such a task was due to a job assignment in order to know on ssh prompt the IP of the macchine connected to, even if It was a bit redundant with the fact that the IP can be known with ifconfig, but I had to do it so...