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.

Solution 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.

Solution 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

Solution 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"

Solution 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.

Solution 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.

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Comments

[glenn jackman]

Are you asking if you can transfer from a remote host to a remote host, or are you asking how to do it without having to supply a password?

[glenn jackman]

my scp man page says "Copies between two remote hosts are also permitted."

[peterh]

Thanks, it is good to hear. To scp you can give a -P flag (it was written by some BSD people, this because its argument handling is so tragic :-( ), but it seems you can't specify different ports on the remote hosts. I am sorry, but I think, only this workaround lefts (or there are a lot of trickier solutions, using ssh but avoiding scp - for example, sftpfs, but they are not the simplest). I extended my workaround with the port settings.

[modulitos]

Also worth noting, for Google Compute Engine, there is support for adding to your ~/.ssh/config file: cloud.google.com/compute/docs/gcloud-compute but I don't think that AWS has the same support

[Red Bottle]

perez is there a way to achieve scp through two different ports for the two different remote machines from the comman line?

[holmberd]

Since you won't see the output as you normally do, a tip is to enable the verbose mode with -v.

[Vanni]

This is THE answer. Please note that the path starts after the first /, so if you want an absolute path you'll put // after the port (eg. scp://[email protected]:22//etc/*), or it would be interpreted as local to user's home. You can use -v to see what files are being transferred, very useful when using wildcards.

[Fons MA]

This post clearly says: "routing the traffic through the third PC", but I, at least, didn't parse it to mean: "This will be extremely slow". If that's what you need you'll have to set up remote1 with the relevant credentials. It's a one-off thing though, then it'll work.

[Silidrone]

How can I use sshpass here? I need to pass in the password for both servers.

[Chris]

@Vanni, words do not describe how much your simple comment o if you want an absolute path you'll put // after the port (eg. scp://[email protected]:22//etc/*), or it would be interpreted as local to user's home just helped....I've wasted nearly a complete day pulling my hair out because of precisely that. I must have read the rest of the internet about SCP before finding this...Thanks!!

[Vanni]

@Chris I'm glad it helped! :)

[Shautieh]

@FonsMA quite often, it's better to be slower than to give credentials to servers that shouldn't have them.

[Admin]

What is the added value of your answer regarding other answers?

[Admin]

Brevity and simplicity.