Configure SSH to read HostName for a Host from a file

Question

I have the following host definition in my ~/.ssh/config file:

Host testbox
  User abc
  Port 12345
  HostKeyAlias=hello
  HostName 123.123.123.123

The problem is, this host changes IP all the time. The IP is refreshed and stored in this text file ~/ip.txt as it's contents.

Is there a way to configure SSH such that when I run

ssh testbox

I automatically connect to the newest IP read from ~/ip.txt?

(I would prefer not to use dynamic DNS if possible.)

Instead of witing to ip.txt, you could change your /etc/hosts file and ssh by hostname. — stark, Dec 18 '20 at 14:15
@stark Thanks for responding, but would prefer a solution that doesn't require root privileges. — Jim, Dec 18 '20 at 15:11
@Jim: How do you receive new IP address? Instead of storing it in ~/ip.txt you could edit ~/.ssh/config instead. — Arkadiusz Drabczyk, Dec 18 '20 at 18:52

sitaram · Accepted Answer · 2020-12-19 13:45:09Z

6

if you ~/ip.txt can be made to contain

hostname 1.2.3.4

then you can

include ~/ip.txt

at the appropriate place in the ~/.ssh/config.

answered Dec 19 '20 at 13:45

sitaram

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Arkadiusz Drabczyk · Accepted Answer · 2020-12-20 18:45:21Z

3

IIUC, you could use ProxyCommand like that:

Host test
Port 12345
ProxyCommand ssh -p %p "$(cat ~/ip.txt)" nc localhost %p
StrictHostKeyChecking no

answered Dec 18 '20 at 19:16

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This looks promising. But it's not working. I tried: ``` Host testbox User abc Port 12345 ProxyCommand ssh "$(cat ~/ip.txt)" nc localhost %p ``` StrictHostKeyChecking makes no difference in whether it works or not. I tried using the IP directly instead of the $(), in case it was something wrong with the cat part and it doesn't work either. Can you explain the nc localhost %p part? – Jim Dec 20 '20 at 16:04
@Jim: see updated answer. nc is used to forward SSH traffic as explained at en.wikibooks.org/wiki/OpenSSH/Cookbook/Proxies_and_Jump_Hosts. Notice that it has to be installed on the server. – Arkadiusz Drabczyk Dec 20 '20 at 18:45
I needed to add %r to the command as well in order to use the right username. But it's still not working: "Warning: untrusted X11 forwarding setup failed: xauth key data not generated kex_exchange_identification: Connection closed by remote host" This error occurs even if I use "ForwardX11 no" – Jim Dec 21 '20 at 5:28
@Jim: can you try ProxyCommand ssh -l %r -p %p "$(cat ~/ip.txt)" nc localhost 22? – Arkadiusz Drabczyk Dec 21 '20 at 11:00
This works! A couple of questions: 1) I don't think I need StrictHostKeyChecking=no if I use HostKeyAlias? 2) Why port 22 specifically? I also tried a different port and it didn't work. – Jim Dec 22 '20 at 17:23

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