Linux Restricted Shells: rssh and scponly
Restricted shells like rssh and scponly give sysadmin the possibility to limit the operations that Linux user can do, for example you can create user that will be allowed to copy files via scp but won’t be permitted to login into system’s command line. This is quite important security feature that should be considered by every sysadmin to prevent unauthorized activity by users for example over SSH.
If you have some online storage that is used for uploading backup data over scp or rsync/ssh from remote hosts then it is highly recommended to use restricted shells for those incoming connections and make sure that even if the attacker has got username/password (or key) then he (or she!) won’t be able to break into your system.
scponly is extremely simple restricted shell, user account that has scponly binary as its shell won’t be able to do anything except transfer data from remote host via scp protocol or via rsync/scp. rssh provides little bit more features: you can limit users to use selected protocols like scp, sftp, rsync, cvs or rdist either in chroot environment or not.
I prefer using yum or aptitude to install such kind of software like rssh or scponly so the fastest way is to try one of below commands depending on your needs:
apt-get install rssh
apt-get install scponly
yum install rssh
yum install scponly
If there are problems to find desired restricted shell in your Linux distro’s repository then you should download sources and do some ./configure, make and make install. Here are the links: latest rssh .tar.gz, latest scponly .tgz.
scponly doesn’t need any configuration and works out of the box so you just should set it as a shell for user account. Here are some examples.
Create new user account with scponly as shell:
useradd -s /usr/sbin/scponly user1
Modify user account to set rssh as a shell:
usermod -s /usr/sbin/rssh user2
Where /usr/sbin/scponly is binary executable of scponly.
rssh comes with text configuration file usually stored in /etc/rssh.conf. You can either setup per-user settings there or configure global restrictions for all accounts which are using rssh. Default rssh.conf file is well commented so there shouldn’t be any problems to configure rssh as you needs. At the same time, here are some examples.
If you wish to restrict all users to scp and rsync only then you should uncomment lines in rssh.conf like below:
Now coming to per-user examples. User peter is allowed to use scp protocol only, the following line in rssh.conf will do that:
User ann is allowed to scp, rsync only:
As you can see enabled protocols in per-user setup are specified as 11000 (scp, sftp), 11111 (scp, sftp, cvs, rdist, rsync) or 00000 (no protocols enabled). 022 in above examples specifies umask.
Let’s assume you’ve created user1 and enabled only scp and rsync using rssh. An attempt to access the server via SSH under user1 account will end with the following output:
artiomix$ ssh firstname.lastname@example.org