The third-party Supertars have this capability (/Reviews/supertars.html ) as do some public domain backup utilities.
You can do it yourself, but you need to establish user equivalency and have the remote execution commands enabled ("rcmd" on SCO, "rsh" on Unixware and Linux). Many sites disable these because they are considered bad security risks.
So the first thing to try is something as simple as
rcmd otherbox "/bin/ls /tmp"
unless and until that works, you aren't going to be able to do anything more involved. Don't assume that because it works for "john" that it will work for "pete" and especially for "root"- read the man pages, and be sure your machine names are correct in ~/.rhosts or hosts.equiv- often the problem is that you think of your machine as xyz.someplace.com, but the other machine thinks of you just as "xyz" or vice versa- pings will often tell the story.
If it does work, consider that you can now pipe the output of any command here to the inpupt of any command there. So you could, for example, do:
tar cvf - . | rcmd otherbox dd of=/dev/rStp0
or whatever else you'd like. But please- check your backups, and check them especially well if these are machines with different OS'es- you could find that you cannot easily read back what you wrote!
You could also use ssh:
tar cvf - . | ssh user@otherbox "dd of=/dev/rStp0"
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