Building a Chrooted sftp Environment

There was a time, not so very long ago, when we used to enjoy running an ftp server and locking our users into tiny little chrooted jails. While we still enjoy denying users their freedom, we now prefer to do so using a maximum security facility. The sftp file transfer program, which comes with OpenSSH server, gives users an interactive interface like ftp but performs transfers over an encrypted ssh transport. In this day and age, it is not unreasonable to expect users to start using an ssh client, even if they are running Windows. If they don’t have one already, tell them to download Putty. There are also nice commercial clients, and if users are technically adept and so inclined, they can use openssh over cygwin.

Building a chrooted ssh

By design, OpenSSH does not include the capacity to be chrooted, as the developers contend such functionality belongs in the OS. Luckily, a third party patch has been developed. The patch, a pre-patched openssh tarball, and a good document about setting up the chrooted sftp are available at http://chrootssh.sourceforge.net.

Download the tarball for openssh, and the chrootssh patch. Untar the openssh sources, then apply the patch.

[usr-4@srv-3 ssh]$ tar xzf openssh-3.6.1p2.tar.gz
[usr-4@srv-3 ssh]$ cd openssh-3.6.1p2
[usr-4@srv-3 openssh-3.6.1p2]$ patch -p1 < ../osshChroot-3.6.1.diff
patching file session.c

Now build the chroooted OpenSSH.

[usr-4@srv-3 openssh-3.6.1p2]$ ./configure --with-md5-password
[usr-4@srv-3 openssh-3.6.1p2]$ make

Before you make install, you may want to make a copy of your current ssh binaries, if they are installed in /usr/local/bin and /usr/local/sbin, which is where openssh will put them by default. The install will not overwrite your config files or host keys, though if you’re paranoid like us you’ll back them up anyway.

[root@srv-3 openssh-3.6.1p2]# make install

This goes swimmingly on my Red Hat 7.3 workstation. Now, you’ll need to kill the old sshd and start the new one. In my case, I have been running sshd from a different location, /usr/sbin/sshd which is where Red Hat installs it. In order to keep the rc script working, either change the path to sshd in your sshd rc script, (/etc/rc.d/init.d/sshd or something like that) or create a link like so:

[root@srv-3 openssh-3.6.1p2]# mv /usr/sbin/sshd /usr/sbin/sshd.old
[root@srv-3 openssh-3.6.1p2]# ln -s /usr/local/sbin/sshd /usr/sbin/sshd
[root@srv-3 ssh]# service sshd stop
Stopping sshd:                                             [  OK  ]
[root@srv-3 ssh]# service sshd start
Starting sshd:                                             [  OK  ]

Make sure you can ssh to your machine from another box. If sshd is working, we can proceed to my favorite part, setting up the chrooted jail.

Building a Jail

The chrooted environment must contain everything a user needs to copy files back and forth using sftp. This includes utilities used by sftp, libraries, a home directory, and even some device files. This will keep the user safely off the rest of the system. Before you get too excited, keep in mind that chrooted jails can be broken. But not easily. Referring to the document at chrootssh.sourceforge.net, we’ll build our jail. We’re going to call ours alcatraz.

[root@srv-3 u01]# mkdir alcatraz
[root@srv-3 alcatraz]# mkdir bin dev home lib usr
[root@srv-3 alcatraz]# cd bin

Copying the necessary binaries:

[root@srv-3 bin]# cp /bin/bash /bin/cp /bin/ls /bin/mkdir /bin/mv 
/bin/rm /bin/rmdir .
[root@srv-3 bin]# ln -s bash sh

Determining which libraries are needed and copying them into place:

[root@srv-3 bin]# cd ../lib
[root@srv-3 lib]# ldd ../bin/bash
libtermcap.so.2 => /lib/libtermcap.so.2 (0x4002b000)
libdl.so.2 => /lib/libdl.so.2 (0x40030000)
libc.so.6 => /lib/libc.so.6 (0x40033000)
/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)
[root@srv-3 lib]# cp /lib/libtermcap.so.2 .
[root@srv-3 lib]# cp /lib/libdl.so.2 .
[root@srv-3 lib]# cp /lib/libc.so.6 .
[root@srv-3 lib]# cp /lib/ld-linux.so.2 .
[root@srv-3 lib]# ldd ../bin/cp
libc.so.6 => /lib/libc.so.6 (0x4002b000)
/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)
[root@srv-3 lib]# ldd ../bin/ls
libtermcap.so.2 => /lib/libtermcap.so.2 (0x4002b000)
libc.so.6 => /lib/libc.so.6 (0x40030000)
/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)

And so on. We also need sftp itself.

[root@srv-3 alcatraz]# cd usr
[root@srv-3 usr]# mkdir lib
[root@srv-3 usr]# mkdir -p local/libexec
[root@srv-3 usr]# cp /usr/local/libexec/sftp-server local/libexec/
[root@srv-3 usr]# ldd local/libexec/sftp-server
libutil.so.1 => /lib/libutil.so.1 (0x4002b000)
libz.so.1 => /usr/lib/libz.so.1 (0x4002f000)
libnsl.so.1 => /lib/libnsl.so.1 (0x4003d000)
libcrypto.so.2 => /lib/libcrypto.so.2 (0x40051000)
libcrypt.so.1 => /lib/libcrypt.so.1 (0x40117000)
libc.so.6 => /lib/libc.so.6 (0x40144000)
libdl.so.2 => /lib/libdl.so.2 (0x4026b000)
/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)
[root@srv-3 usr]# cp /lib/libutil.so.1 ../lib/
[root@srv-3 usr]# cp /usr/lib/libz.so.1 lib/
[root@srv-3 usr]# cp /lib/libnsl.so.1 ../lib/
[root@srv-3 usr]# cp /lib/libcrypto.so.2 ../lib/
[root@srv-3 usr]# cp /lib/libcrypt.so.1 ../lib/
[root@srv-3 usr]# cp /lib/libc.so.6 ../lib/
cp: overwrite `../lib/libc.so.6'? n
[root@srv-3 usr]# cp /lib/libdl.so.2 ../lib/
cp: overwrite `../lib/libdl.so.2'? n

Now sftp should work. We just need a couple of device files, /dev/null and /dev/zero:

[root@srv-3 usr]# cd ../dev
[root@srv-3 dev]# ls -l /dev/null /dev/zero
crw-rw-rw-    1 root     root       1,   3 Apr 11  2002 /dev/null
crw-rw-rw-    1 root     root       1,   5 Apr 11  2002 /dev/zero
[root@srv-3 dev]# mknod null c 1 3
[root@srv-3 dev]# mknod zero c 1 5
[root@srv-3 dev]# ls -l
total 0
crw-r--r--    1 root     root       1,   3 Jul 28 15:15 null
crw-r--r--    1 root     root       1,   5 Jul 28 15:15 zero

Let’s see if the chroot works.

[root@srv-3 root]# chroot /u01/alcatraz /bin/sh
[I have no name!@srv-3 /]# pwd
/
[I have no name!@srv-3 /]# ls
bin  dev  home  lib  usr

It works! But as you can see, functionality is limited. If we had an /etc/passwd file, for instance, we’d have a normal root prompt instead of the identity crisis listed above.

Setting up Users, Refining.

Let’s add a user whose home directory is chrooted and test the chroot functionality of ssh.

[root@srv-3 bin]# useradd -d /u01/alcatraz/./home/usr-3 usr-3
[root@srv-3 bin]# passwd usr-3
Changing password for user usr-3.
New password: 
Retype new password: 
passwd: all authentication tokens updated successfully.
[root@srv-3 bin]# ssh srv-4
root@srv-4's password: 
Last login: Mon Jul 28 13:44:49 2003 from srv-3.upthe.com
[root@srv-4 root]# ssh usr-3@srv-3
usr-3@srv-3's password: 
bash-2.05a$ pwd
/home/usr-3
bash-2.05a$ cd ../..
bash-2.05a$ ls
bin  dev  home  lib  usr

Yep, we are definitely in our jail. But lets see what we can do in this jail:

bash-2.05a$ mkdir z
mkdir: cannot create directory `z': Permission denied
bash-2.05a$ cd
bash-2.05a$ pwd
/home/usr-3
bash-2.05a$ mkdir z
bash-2.05a$ ls -l
total 4
drwxr-xr-x    2 548      548          4096 Jul 28 22:39 z
bash-2.05a$ cd ..
bash-2.05a$ rmdir usr-3
rmdir: `usr-3': Permission denied

Looks pretty good! The last thing we have to do is lock down little usr-3’s shell so she can use only sftp. We are not allowing interactive logins, chrooted or no. The easiest way to do this is to use sftp-server as the shell. It’s a little ugly, but it works.

[root@srv-4 root]# ssh usr-3@srv-3
usr-3@srv-3's password: 
Last login: Mon Jul 28 15:36:54 2003 from srv-4.upthe.com
Connection to srv-3 closed.
[root@srv-4 root]# sftp usr-3@srv-3
Connecting to srv-3...
usr-3@srv-3's password: 
sftp> pwd
Remote working directory: /home/usr-3
sftp> put /etc/group
Uploading /etc/group to /home/usr-3/group

The ugly part is that the session just hangs until interrupted when interactive login is attempted. You can always write a wrapper, but remember it must work within your chroot environment. Finally, let’s tighten up our jail a little bit more. Let’s take away usr-3’s write permissions on her own home directory! Why, you ask? I’ll show you.

[usr-4@srv-4 .ssh]$ sftp usr-3@srv-3
Connecting to srv-3...
usr-3@srv-3's password: 
sftp> mkdir .ssh
sftp> lcd .ssh
sftp> cd .ssh
sftp> put id_dsa.pub authorized_keys                        
Uploading id_dsa.pub to /home/usr-3/.ssh/authorized_keys
sftp> exit
[usr-4@srv-4 .ssh]$ sftp usr-3@srv-3
Connecting to srv-3...
sftp> 

This is fine, if you want to allow the user to write keys and circumvent the need for a valid password. But if you want to control access via passwords, lock down the home directory and give them write permissions on a directory below it.

[root@srv-3 usr-3]# mkdir files
[root@srv-3 usr-3]# chown usr-3:usr-3 files
[root@srv-3 usr-3]# chmod 700 files
[root@srv-3 usr-3]# ls -l
total 12
drwx------    2 usr-3     usr-3         4096 Jul 28 16:35 files
[root@srv-3 usr-3]# cd ..
[root@srv-3 home]# chown root:root usr-3
[root@srv-3 home]# ls -l
total 4
drwx------    3 root     root         4096 Jul 28 16:35 usr-3

That will keep usr-3 from playing her naughty tricks. I’m looking forward to imprisoning many users with this system. True, there’s no proof they’ve done anything wrong, but I’m sure they’re just waiting for the opportunity!