Question Network routing issue after update (loss of default route source IP / DNS failure on Ubuntu 24.04)

[bigbear67]

Server operating system version

Ubuntu 24.04.4 LTS

Plesk version and microupdate number

Plesk Obsidian v18.0.79_build1800260722.17

Hello everyone,

This is the second time that a system/environment update has broken my server by completely cutting off outgoing Internet access (DNS resolution timing out with Could not resolve host).

The root cause is that the default route loses the src <Server_IP> attribute during network reloads, which blocks outgoing traffic and causes heavy Ajax-dependent modules (like pm_advancedpack) to timeout, spiral out of control, and choke the server.

ip route replace default via <Gateway_IP> dev eth0 src <Server_IP>

How to make it permanent under systemd-networkd (when no netplan config is present):
Create a systemd service file at /etc/systemd/system/custom-route.service:

[Unit]

Description=Set custom default route with source IP

After=network.target



[Service]

Type=oneshot

ExecStart=/usr/sbin/ip route replace default via <Gateway_IP> dev eth0 src <Server_IP>

RemainAfterExit=yes



[Install]

WantedBy=multi-user.target

Then run:

systemctl daemon-reload

systemctl enable custom-route.service

systemctl start custom-route.service

Has anyone else experienced this loss of the source IP on the default route after system updates? It's quite critical in a production environment.