Take the 2-minute tour ×
Server Fault is a question and answer site for system and network administrators. It's 100% free, no registration required.

Using Solaris and Linux servers and OpenSSH, is it possible to prevent users from copying files using "scp" while still allowing shell access with "ssh"?

I realize that 'ssh $server "cat file" ' type file accesses are much harder to prevent, but I need to see about stopping "scp" for starters.

Failing that, is there a way to reliably log all SCP access on the server side through syslog?

share|improve this question
    
If you wanted to close ssh but not scp you could have used this: sublimation.org/scponly/wiki/index.php/Main_Page Too bad you want it the other way around :-\ –  samoz Jun 19 '09 at 18:36
    
I have the same question but for other reason. In my case I like to turn off the SFTPD and SCPD on the server. Reason is that we do allow file transfers but we like the users to do the transfers via our copy node. This is due to how we sepparate the load on our links. So according to this loop it is easy to turn off SFTPD, but if I understand correctly it is pretty much impossibly to turn off SCPD? –  user133866 Aug 29 '12 at 1:06

8 Answers 8

While you could edit your /etc/ssh/sshd_config to look something like this:

ForceCommand           /bin/sh
PermitOpen             0.0.0.0
AllowTcpForwarding     no
PermitTunnel           no
# Subsystem sftp       /usr/lib/openssh/sftp-server
PermitUserEnvironment  no

I would instead determine what the user is likely to use it for. Because if there are only a few commands that you want them to have access to, I would instead remove the ability for them to even invoke a normal ssh shell.

AllowUsers             root
PermitRootLogin        forced-commands-only

PermitUserEnvironment  no

AllowTcpForwarding     no
PermitTunnel           no

# Subsystem sftp       /usr/lib/openssh/sftp-server
Subsystem smb-reload   /usr/bin/smbcontrol smbd reload-config
Subsystem status       /opt/local/bin/status.sh

ssh root@example -s smb-reload

If you find that you really do need to be able to run a normal shell, the most you really can hope for, is to slow them down, and make it more difficult.

share|improve this answer

As others have noted, you can't block scp (well, you could: rm /usr/bin/scp, but that doesn't really get you anywhere).

The best you can do is to change the users' shell to a restricted shell (rbash) and only then to run certain commands.

Remember, if they can read files, they can copy/paste them off the screen. Binary files? xxd/uuencode/mmencode all get around this.

I'd also suggest using process accounting to help you track activity.

share|improve this answer
    
Process accounting helps a bit, but the historical process accounting was really useless (e.g. logging only the basename of the command run). I'd like to hear about any modern successes with process accounting that is actually useful. –  carlito Jun 29 '09 at 6:14
1  
How about using a patched restricted shell that also logs all commands run to a pipe somewhere? A centralized .bash_history kind of idea. –  MikeyB Jun 29 '09 at 13:43
    
Actually on the server side you would have to delete /usr/lib/openssh/sftp-server, but I think sshd has a built in sftp server. –  Brad Gilbert Jul 2 '09 at 5:04
    
@Brad: Any commands specified by the client are still run via the shell; so if sftp-server isn't in the default PATH (which it's not) changing the shell to a restricted one is enough to disable it, you don't have to delete the binary. –  MikeyB Jul 2 '09 at 13:31

Your best bet isn't to lock down scp, but to use a file system with ACLs to prevent read access. You could probably do something with SELinux to prevent certain applications from reading from certain files.

share|improve this answer
    
Or you could do both. –  msanford Jun 20 '09 at 14:11

You gain nothing by stopping "scp" when you're still allowing literally infinite additional mechanisms of transferring files. Disallowing scp but allowing other mechanisms of copying files is a method of lying to auditors. Often auditors ask to be lied to. Usually I see auditors working with managers to make fake fixes, so that they can state something like "the scp file transfer command has been disabled, so that files can not be copied from the server using scp".

Now a reasonable logging mechanism would be nice. Maybe auditd finally works on Linux. Maybe Solaris finally added some mechanism or dtrace could be used safely. It's reasonable to want the OS to log every time a file is accessed. Of course there's no difference between "reading" and "copying". But this can satisfy an auditor and give significant security to the system. Your logs could be so noisy that the data is useless, or even that you're forced to keep a ridiculously short audit trail. (e.g. you can't log every read() - and one application that does something surprising can make logging every open() a disaster).

share|improve this answer

Depending on what SSH is needed for, you may be able to achieve this goal (for non-trivial) files by using IPTables to terminate sessions if the packet size is bigger then, say 1400 bytes. This means that interactive ssh will mostly work, but as soon as something tries to send a 1500 byte packet - like scp should for a file larger then 1499 bytes assuming a standard MTU of 1500, it will terminate the connection.

This will also prevent the "catting" attack you mention.

Unfortunately this means that you may have problems editing some files with a text editor, if the screen needs to draw more then 1400 characters, or if you need to cat a long file or do a long directory listing.

In the simplest case a command to do this might look something like

iptables -I OUTPUT -p tcp --dport 22 -m length --length 1400 -j DROP

We can make this work better by combining the packet length checks with ipt_recent, so that you allow a limited number of packets larger then 1400 bytes within a set timeframe (say 8 packets per 5 seconds)- this would allow packets up to 12k to slip through, but may give you the interactivity you will need for editing files etc. You can, ofcourse tweek the number of packets.

This might look something like

iptables -I OUTPUT -p tcp --dport 22 -m length --length 1400 -m recent --name noscp --set 
iptables -I OUTPUT -m recent --name noscp --update --seconds 5 --hitcount 8 -j DROP

A cluefull hacker can work around these limitations by setting an MTU of less then 1400 on his machine (or force mtu or similar). Also, while you can't limit this to certain users, you can limit it by IP by modifying the iptables lines as appropriate !!

Cheers, David Go

share|improve this answer

No. scp and ssh operate on the same ports and use the same protocol. If you open an ssh session, you can even share your connection with subsequent scp calls using options like ControlMaster.

If you do not want people to copy particular files off of a machine, you should not give them any kind of shell access to the machine.

share|improve this answer
    
Yes, the obvious answer would be to lock up the system and not give out access. In reality however, my company has auditors who say that we need to prevent files from being copied off of the servers and / or log attempts despite the fact that we seriously limit ssh access and have a robust RBAC system in place. –  Jason Jun 19 '09 at 18:40
1  
@Jason: Then you need to log file access. Even if you disabled scp, how would you stop someone from running: ssh server 'cat /path/to/file' > copy ? –  derobert Jun 19 '09 at 19:29

This is not possible actually after a little googling.

Check out this discussion: http://www.mydatabasesupport.com/forums/unix-admin/387261-how-restrict-ssh-users-block-scp-sftp.html

share|improve this answer

There is a way to use 'scponly' as the shell to disable interactive ssh and allow scp, but I am not aware of anything existing that works in the reverse manner.

You may be able to explore hacking the scponly shell to accomplish the reverse.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.