systemd: start service at boot time after network is really up (for WoL purpose)

Question

I have a computer at work which I sometimes wakeup from home in order to access it but when boots and gets another IP address from our DHCP server, how can I access it?

The situation and my “workflow” is as follows:

• From my home PC I connect to the office's VPN
• ssh to a dedicated server in office's LAN (it has a fixed IP address)
• on that server I call a script that broadcasts a WoL packet with my office PC's MAC address
• my office PC starts up (it really does, for sure!)

Now in theory I'd be able to SSH to my office PC if only I'd knew its IP address. Sometimes it gets the same, sometimes it changes. To circumvent this I had the following idea:

• after booting my office PC does an SSH to the office server and writes its own IP address into some text file on the server
• I examine that file on the server (which I can connect to because of its fixed IP), find the IP address of my office PC, and can then SSH from my home PC to my office PC

All computers run Linux; Ubuntu 14.04 at home, SLES on the office server, OpenSUSE 13.1 on my office PC. This is all not a problem.

For this all to work I simply need a script on my office PC that runs at boot time when the network is up and running.

My script (publish_ip.sh) is like follows:

# get own IP address:
ip=$(ip addr show | awk '$1=="inet" && $2 !~ /127\.0\.0\.1/ { split($2, a, "/"); print a[1]}');

# SSH to the office server (10.64.5.84) and write own IP address to a file there:
ssh -x -e none 10.64.5.84 "echo $(date) $ip >> office_pc_address.txt"

To run this script at boot time I created a systemd service file, publish-ip.service, for my office PC:

[Unit]
Description=publishes own IP address
Wants=network.target
After=network.target

[Service]
Type=oneshot
ExecStartPre=/usr/bin/sleep 30
ExecStart=/home/perldog/bin/publish_ip.sh
User=perldog

[Install]
WantedBy=multi-user.target

But this is what I always get on my office PC:

linux-tz7m:/usr/lib/systemd/system # systemctl status publish-ip.service
publish-ip.service - publishes own IP address
   Loaded: loaded (/usr/lib/systemd/system/publish-ip.service; enabled)
   Active: failed (Result: exit-code) since Mon 2016-02-29 12:17:34 CET; 4 days ago
  Process: 1688 ExecStart=/home/perldog/bin/publish_ip.sh (code=exited, status=255)
  Process: 1016 ExecStartPre=/usr/bin/sleep 30 (code=exited, status=0/SUCCESS)
 Main PID: 1688 (code=exited, status=255)

Feb 29 12:17:34 linux-tz7m publish_ip.sh[1688]: ssh: connect to host 10.64.5.84 port 22: Network is unreachable

Obviously my service starts and also calls my script but the SSH command in that script fails with Network is unreachable.

I tried everything in my service file so that it runs only after the network is up, but I don't get it. I tried Wants=network.target, After=network.target, WantedBy=multi-user.target, and even inserted an ExecStartPre=/usr/bin/sleep 30. Nothing worked. I always get Network is unreachable when my script is called and tries to SSH to the office server.

Question: What settings are required in my service file so that it runs only after the office server is reachable with SSH?

Note: When I'm at office and my office PC is up-and-running, both my script and the service work perfectly, i.e. systemctl start publish-ip.service works without any errors.

Answer 1

You can delay the starting of your job by adding a ping loop as an ExecStartPre script:

[Service]
   ExecStartPre=/bin/sh -c 'until ping -c1 google.com; do sleep 1; done;'

Found here

Answer 2

I tried all these targets, and they all were reached before DHCP got an IP address. Go figure:

• network-online.target
• remote-fs.target
• nfs-client.target
• dbus.service

What did work was enabling these two:

systemctl enable systemd-networkd.service systemd-networkd-wait-online.service

And then setting

After=systemd-networkd-wait-online.service
Wants=systemd-networkd-wait-online.service

Now it got started after DHCP got an IP address. (A mount point in my case, but could have been your service too)

(On debian9/stretch)

Answer 3

Not a full solution, but I set up an SSH tunnel once the computer boots; it restarts every 5 seconds to prevent getting killed for spamming while the network is still down; you could use something like this paired with your WOL, if you can use the fixed IP system as a bastion host:

[Unit]
Description=Keep open a reverse tunnel to my home server
After=network.target

[Service]
ExecStart=/usr/bin/ssh -NT hometunnel
RestartSec=5
Restart=always

[Install]
WantedBy=multi-user.target

Relevant part of root's .ssh/config:

Host                    hometunnel
HostName                <redacted>
IdentityFile    /root/.ssh/<redacted>
User                    <redacted>
RemoteForward   <redacted> localhost:22
ExitOnForwardFailure yes
ServerAliveInterval 60
ServerAliveCountMax 5

Answer 4

There used to be a tool called arpwatch which was keeping a log of all networked devices' IP addresses and MAC addresses, so you could simply grep for your computer's MAC and get the IP.

There's also a tool called arping, you can grep through the output for the MAC address.

Another option would be to either keep a random port open on your computer and scan the whole network with nmap for that specific port, or nmap the whole network for hosts that are up and grep for your MAC address...

nmap -sP 192.168.1.0/24