How to run a systemd oneshot service whenever the system gets a new IPv6 address

d-busdhcpipv6networkingsystemd

I would like to run a systemd oneshot service whenever my system gets a new IPv6 address assigned to it from the network router (this would be on boot, and periodically throughout its uptime).

Ideally, this event would be only for IPv6 address updates, and not include potential IPv4 address changes.

Is it possible to monitor networkd dbus activity for this kind of event with systemd?

My ultimate goal is to hit a dynamic DNS endpoint whenever this happens, so that this system's IPv6 DNS record avoids going stale.

Best Answer

I wrote something like this many years ago, called (unimaginatively) autoddns.

It doesn't call a systemd service (I don't think systemd existed at the time!) but it does call an external process to perform the DNS update (nsupdate). It does however monitor IP address additions and deletions directly at the netlink level, which means it will pick them up no matter how they happen (manual administrative configuration, SLAAC, DHCP, Network Manager). It reacts instantaneously when changes occur. It does support filters (designed for automatically excluding things like link-local addresses), so you can get it to respect IPv6 address changes and ignore IPv4 ones, as you wish to do.

Perhaps you can use it or modify it to meet your needs. Most of the magic for detecting and reacting to IP address changes is in watchip.c, I think.

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One possible workaround I'm trying is

  [Unit]
    Description=Tags instance and EBS vols
    After=docker.service
    Requires=docker.service

    [Service]
    ExecStartPre=/usr/bin/docker run --rm -v /opt/tag.sh:/tag.sh -v /opt:/ack --entrypoint=/bin/sh alpine /tag.sh
    ExecStartPre=/usr/bin/sh -c '[[ -e /opt/TAG_SUCCESS ]]'
    ExecStart=/usr/bin/rm /opt/TAG_SUCCESS
    Restart=on-failure
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Where the script tag.sh will touch /ack/TAG_SUCCESS at the end of execution. I believe this gets pretty close to the desired behavior because ExecStartPre will sequentially execute, waiting for successful completion before calling ExecStart and the unit is only considered Starting once ExecStart is called by which point we're sure we've completed the startup task.

But this still feels like a giant hack?

Ubuntu – Systemd-networkd: Run script after DHCP client aqcuires new address

I don't know of any systemd solution, but you can monitor changes to interfaces efficiently with ip monitor and pipe the output into a script that looks for the changes that interest you. For example, if I do

$ ip monitor address dev enp3s0

then add a new alias address to interface enp3s0 I get the line

2: enp3s0    inet 192.168.77.77/24 brd 192.168.77.255 scope global enp3s0:1
       valid_lft forever preferred_lft forever

I presume you would see something similar for dhcp changes.

If you prefer, you can write a C program to do the same, see this ipevent.c.

Best Answer

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