Librarium Whitehat
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"Inveniam viam aut faciam" : I will either find a way, or I shall make one


Openssh is a wonderful tool, providing encrypted links for a variety of fun connections. Once of the most useful drop-in replacements it can be used for is to replace ftp. Using the sftp subsystem of the openssh suite overcomes all the cleartext problems associated with normal ftp. But lets take it a little further, lets "jail" each user so that they can only see their own little part of the server - literally. This is called chrooting, or "changing the root". What happens is that the user, sees the start of their home folder (or as otherwise specified) as the root (/) folder of the server, making sftp just a bit more secure..

What is needed
Well you will need a linux box (Unix works similarily and BSD's have a different way of doing this) and thats about it. If you already have Openssh on the server you can over-install, although it can be easier to remove it and then install cleanly (either way make sure you keep a copy of the configuration files and host keys). You will then need to get the patched version of the Openssh package. You can either download the main source code package and then apply the patch, or download a fully patched version. We will be using the fully patched version in this example (we will also be using the latest zlib libraries). So get the patched Openssh from here, and the zlib libraries from here.

First we do the zlib stuff..
# gzip -d zlib-1.2.3.tar.gz
# tar -xf zlib-1.2.3.tar
#./configure && make && make install

Now we do the patched openssh (remember if you use any fancy options with your ssh then choose the appropriate options)..
# gzip -d openssh-4.2p1-chroot.tar.gz
# tar -xf openssh-4.2p1-chroot.tar
#./configure && make && make install
# ssh -V
OpenSSH_4.2-chrootsshp1, OpenSSL 0.9.7a Feb 19 2003

Now check to make sure that your ssh service can start up as a service (/etc/init.d/sshd restart), and make any changes you need to get it to startup. Once that is all working, then we can move on to creating the actual chroot jail.

Before we get the that though, lets just do a bit of theory. When you create a chroot jail, you are actually limiting the software to a specific set of folders. This has to do with the actual programming calls the software makes, but that is a bit too much detail for this setup. But once you have the software which uses the chroot system calls (in this case the patched openssh tarball), you need to make sure that the utilities and programs used by it have the libraries moved to the right place. You can see what libaries a certain program needs by using ldd..
# ldd /bin/bash =>  (0xffffe000) => /lib/ (0x005a4000) => /lib/ (0x00468000) => /lib/tls/ (0x0031a000)
        /lib/ (0x00301000)

Here we can see what libraries the binary /bin/bash needs to function, so if we are going to use that in our jail, we need to make sure that the libraries needed are also copied. This is why sometimes setting up the jail for the first time can be a bit a trial and error.

Making the jail
In my example I want each person who has sftp access must be in their own jail, and must only be able to use sftp. First lets make sure that they can only use sftp when they login..
echo "/usr/local/libexec/sftp-server" >> /etc/shells

This allows your sftp-server (check where it is on yor system) binary to be specified as a shell in the /etc/passwd file. This can be a bit ugly as if they try to login with anything else other then sftp, the session just hangs. But thats fine by me. Now we will move onto the full jail setup. I have a script I use for this, it creates the jail, creates the user and copies the binaries I want them to have access to into the jail. As always this script is not a work of art, but it works for me, please feel free to adapt, improve, simplify, etc as you see fit..
#USAGE: setup-chroot bob bob

mkdir -p /data/lockhome/$NAME/$NAME
cd /data/lockhome/$NAME
mkdir etc
mkdir bin
mkdir lib
mkdir usr
mkdir usr/bin
mkdir dev
mkdir -p ./usr/local/bin
mkdir -p ./usr/local/libexec/
mknod dev/null c 1 3
mknod dev/zero c 1 5

cd /data/lockhome/$NAME
for bins in /bin/bash /usr/local/libexec/sftp-server /bin/ls /bin/mkdir /bin/mv /bin/rm /bin/rmdir
  cp -v --reply=yes $bins ./$bins
  ldd $bins > /dev/null
  if [ "$?" = 0 ]
    LIBS=`ldd $bins | awk '{ print $3 }'`
    for l in $LIBS
      mkdir -p ./`dirname $l` > /dev/null 2>&1
      cp -v --reply=yes $l ./$l
    LIBS2=`ldd $bins | grep -v -e "=>" | awk '{ print $1 }'`
    for u in $LIBS2
      mkdir -p ./`dirname $u` > /dev/null 2>&1
      cp -v --reply=yes $u ./$u

cd bin
ln -s ./bash ./sh
cd ..
touch etc/passwd
grep /etc/passwd -e "^root" > etc/passwd

/usr/sbin/useradd -s /usr/local/libexec/sftp-server -m -d /data/lockhome/$NAME/./$NAME/$NAME $NAME
echo $PASS | passwd $NAME --stdin
grep /etc/passwd -e "^$NAME" >> /data/lockhome/$NAME/etc/passwd

Now when you login, you should only be able to use sftp and you should only be able to see what is in your jail.

You may want to do logging on your sftp usage as with a normal ftp server, the bad news this is not default behaviour, the good news is that you can do it. You will need to download a patch from here and follow a couple of simple steps. Lets first do the download and patch (I am assuming you already have openssh installed)..
cd openssh-x.x-chroot
make clean
patch < openssh-x.x.sftplogging-v1.4.patch
make && make install

Now if you have previously created any chroot jails you will need to update the sftp-server binary which they use. Ok, no we need to setup the ssh service to use the sftp logging, so edit your sshd_config file (wherever you have it)..

 #vi /usr/local/etc/sshd_config

--now make sure you have the following section (these settings work for me, tweak them to fit your needs)
LogSftp yes
#SftpLogfacility # Default is AUTH, see /etc/syslog.conf for more info
SftpLogLevel DEBUG3# Default is INFO
#SftpUmask # Set a global umask for sftp
SftpPermitChmod yes # Allow, or disallow chmod
SftpPermitChown yes # Allow, or disallow chown/chgrp

# /etc/init.d/sshd restart

The last step is you need to get the data out of the chroot jail into the normal syslog process. You first need to create a /dev/log file in the /dev folder of the chroot jail...
mksock /<chroot jail>/dev/log

Then update your syslog settings to use the new socket, this can be done by editing the init script or by editing a configuration file if your system uses one. Whichever way your syslog works you need to add "-a /<chroot jail>/dev/log" to your syslog daemon startup options. Once this is done, restart your syslog service. When you're finsihed, your new chroot'ed sftp account will have the sessions logged to your syslog log file.

Final Words
While this is a very useful setup, bear in mind that there are still ways to break out of a chroot jail, nothing is ever really 100%. But it does continue raising the bar on your systems a bit higher, making attackers work harder to get anywhere - thats always a good thing. This really is something you can play around with, and can use in many other instances, so as always, have fun and learn.