scp between two remote hosts from my (third) pc

Question

I have two remote hosts.
host1-> 10.3.0.1
host2-> 10.3.0.2
Both run an ssh server.

The ssh server listens on port 22 in host1 and on port 6969 in host2. Now, using my local machine, I need to copy something from host1 to host2 without logging into either host1 or host2 via ssh. Something like,

scp [email protected]:/path/to/file [email protected]/path/to/file

How can I do this, please note that the two hosts use different ports for ssh.

Answer 1

In the past, the way in which scp worked, when called (naively) to copy files between remote systems, was very inconvenient: if you wrote, for instance

    scp user1@remote1:/home/user1/file1.txt user2@remote2:/home/user2/file1.txt

scp would first open an ssh session on remote1, and then it would run scp from there to remote2. For this to work, you would have to set up the authorization credentials for remote2 on remote1.

The modern way to do it, instead, ("modern" because it was implemented only a few years ago, and perhaps not everybody has a -3-capable scp) requires two steps.  The first necessary step is to use ~/.ssh/config to set up all options for the connection to both remote1 and remote2, as follows:

    Host remote1.example.org
    Port 2222
    IdentityFile /path/to/host1-id_rsa

    Host remote2.example.org
    Port 6969
    IdentityFile /path/to/host2-id_rsa

This way it becomes possible to pass all necessary options to the command without ambiguities: for instance, if we had said on the CLI use port 2222 without the above configuration, it would have been unclear whether we were referring to remote1 or to remote2, and likewise for the file containing the cryptgraphic keys. This way the CLI remains tidy and simple.

Secondly, use the -3 option, as follows:

    scp -3 user1@remote1:/home/user1/file1.txt user2@remote2:/home/user2/file1.txt

The -3 option instructs scp to route traffic through the PC on which the command is issued, even though it is a 3rd party to the transfer. This way, authorization credentials must reside only on the issuing PC, the third party.

Answer 2

The source and target can be specified as a URI in the form scp://[user@]host[:port][/path]

so you can run:

scp -3 scp://[email protected]:22/path/to/file scp://[email protected]:6969/path/to/file

Answer 3

Last time I tried this, scp wasn't able to do that. Your command line looks okay. This workaround will work:

ssh -p port_on_machine1 user@machine1 "cat /path/to/file/one"|ssh -p port_on_machine2 user@machine2 "cat >/path/to/file/two"

Answer 4

In my case, I was doing a remote to remote copy, withouth the -3 argument. The port given with the '-P' parameter works with the 1st server, but port 22 is used with the 2nd one.

ssh -P 1234 [email protected] [email protected]

The solution is to edit the /etc/ssh/ssh_config file in server1 and add these lines:

Host *.otherdomain.com
   Port  1234

In this way, the port 1234 is used for both of them. It could be different too.

This solution has better throughput than previous solutions, because communitation is direct.

Answer 5

I know, this topic is a few years old, but OpenSSH 8.4 (released 2020-09-27) added agent forwarding to scp and sftp.

Now it's possible to copy a file from one remote to another, without routing through your local machine or provide credentials on the first host, to authenticate against the second host.

scp -A user1@remote1:/home/user1/file1.txt user2@remote2:/home/user2/file1.txt

Warning! Using agent forwarding is a security issue, when the first host is compromised or when you are affected from a mitm attack.

Answer 6

You can also provide the SOURCE and DESTINATION identity files in the CLI options -o IdentityFile rather than setting these up in .ssh/config. This is handy for e.g. a one-off -3 copy

scp -3 -o IdentityFile=~/.ssh/source.key -o IdentityFile=~/.ssh/dest.key [email protected]:/path/to/file [email protected]:/dest/location/

Answer 7

If you have .ssh/config on your local machine properly set up for two remote servers, you can do:

scp -3 server1:~/path_file server2:~/path_file