I set up shared keys (no passphrase for cron automation) on 2 servers to backup home directories from one to the other.

Server A is the backup storage.

Server B is the sender of of the files to Server A.

My question is:
IF Server B is hacked, what are the security issues to Server A?


Thanks to all in advance.

If server B gets cracked and the intruders find the keys to A, then they can get into A with whatever permissions you have granted the account that uses the keys. So at the minimum it is a good idea to restrict what the keys can do. See "command=" in the manual page for sshd(8) Better, would be to also use a proper passphrase on the key. You can use an agent on B to hold the key and have the cron script access the agent. Then you have to enter the passphrase only when B gets rebooted, at least in theory.

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I have never been able to get rsync to run between "chroot jailed" users. That would help a lot.

chroot with rsync might be quite hard. However, you can lock the keys down quite a bit. If you are doing the exact same rsync command each and every time, rsync's options can be made part of the public key over on the destination (A) . Then if the source machine (B) gets cracked, all the intruders could do would be to make an new transfer.

The rsync command is always the same; /usr/bin/backup

With my limited Linux abilities, I have NO clue how to do what you just said.

the rsync command is this cron jobbed (I hope; it worked manually last night.):

#!/bin/bash
rsync -avz -e ssh /home/* root@00.00.00.00:/home/backup

echo "All done with Backup to `hostname` on `date`" |mailx -s "Backup finished to `hostname`" myemail@address.com &

=======================
Things in place already:

1. user on Server B has one 5 minute window to make the connection per week on Server A.

2. I thought about this but do not know if it is a good or very bad idea. Move the Keys on Server B to a non-shell account once the 5 minute window is up ????

If you have this:

Then you can run it manually with the SSH client in verbose mode:

And you will see a line approximately like this:

Then on the server in the authorized_keys file, you can preface your key with command="..." something approximately like this, but with your own public key:

And since you are using root you should have PermitRootLogin without-password or PermitRootLogin forced-commands-only It would be best if you could do without root on the remote machine and use a normal user there.

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I really wish when Linux programs are done they would update the "man pages" later. Like I saw in a post --time-limit but not anything in the man pages for rsync; as of yet I have not tried it.

I saw the command at "rsync.samba.org" but had no idea how to do the "command" line goo. They lost me in the following discussion.

Thanks I am trying now.

These are HOME directories so either ROOT or a user that is chown user.root; neither of them do I really want to add access on either server but this is a much lesser evil than hoping users will ever back up anything even once in a very long time.

Your distro might have 'timeout' which would be used in front of the program to be timed. It will kill the process with a TERM signal if it is still running when the time runs out.

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We are getting there but NOT there yet.

I put in the "command=" line and it works great... to stop ssh to 00.00.00.00

but if I ssh to hostname.com (server A) I can get right on in.

Yes thank you.

TIMEOUT is there.

Did you set PermitRootLogin forced-commands-only in sshd_config on machine A?
You should then still be able to ssh in as a regular user but not as root unless the command is set inside authorized_keys.

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Yes. It is there but when I saw this:

Are you sure you want to continue connecting (yes/no)? yes

I didn't go the rest of the way to find a black hole.

I am still going to do my 5 minute interval but this "command=" is great!!

Now if someone could update the documentation in at rsync.samba.org.

I will add this for those who have different SSH port settings:

# Remember HOST & PORT setting in /etc/ssh/ssh_config on Server A (host)
# Host 00.00.00.00
# Port = 000