How to remotely write to a file using multi-hop SSH

Question

Questions like How to remotely write to a file using SSH are about a direct SSH.

But I am reaching the end of a reverse tunnel which ends on intermediateserver. That is, I'm ssh'ing to one machine through another. And I want to write to a file on the distant machine. The following fails; it ends up writing on the intermediateserver!

ssh -A -t  intermediateserver ssh -A -t -p 666 localhost  "echo 30 > tmp"

The fact that there's a reverse ssh tunnel in there probably doesn't make things more complicated, so more generally, I am sitting on homeserver and going through intermediateserver to distantserver.

(In my case here, distant server appears to intermediate server internally on its port 666, due to a preexisting reverse tunnel) .

Answer 1

For each hop, you'll need a layer of quoting. Essentially, ssh server some command foo bar ends up running $SHELL with the arguments -c and some command foo bar, so:

1. The first layer of quotes is stripped by your local shell, where you ran the ssh command, and
2. Each subsequent hop invokes a shell which strips another layer of quoting.

This in turn means that shell operations like variable expansion, redirection, wildcard expansion all need extra quoting to prevent intermediate shells from acting on them.

For example, if I had the files foo-a and foo-? in my home directory:

$ touch foo-a foo-\?
$ echo foo-*
foo-? foo-a
$ ssh Server1 echo foo-*
foo-? foo-a foo-a # foo-* expanded locally to foo-? foo-a, foo-? expanded again remotely
$ ssh Server1 'echo foo-*'
foo-? foo-a       # foo-* expanded remotely to foo-? foo-a
$ ssh Server2 ssh Server1 'echo foo-*'
foo-? foo-a foo-a # foo-* expanded remotely to foo-? foo-a, foo-? expanded again remotely in the next hop
$ ssh Server2 "ssh Server1 'echo foo-*'"
foo-? foo-a       # foo-* expanded in the last hop

It would be simpler if you used a ProxyCommand or ProxyHost. This will establish a tunnel between the two systems, instead of creating a baklava of shells executing commands:

~ ssh -o ProxyCommand='ssh Server2 nc %h %p' Server1 'echo foo-*'
foo-? foo-a

~ ssh -J Server2 Server1 'echo foo-*'
foo-? foo-a