The hardening process step by step

Below is a post-installation, step-by-step procedure for hardening a Debian 2.2 GNU/Linux system. This is one possible approach to such a procedure and is oriented toward the hardening of network services. It is included to show the entire process you might use during configuration. Also, see . Install the system, taking into account the information regarding partitioning included earlier in this document. After base installation, go into custom install. Do not select task packages. Using dselect, remove all unneeded but selected packages before doing [I]nstall. Keep the bare minimum of packages for the system. Update all software from the latest packages available at security.debian.org as explained previously in . Implement the suggestions presented in this manual regarding user quotas, login definitions and lilo Make a list of services currently running on your system. Try:

$ ps aux $ netstat -pn -l -A inet # /usr/sbin/lsof -i | grep LISTEN
You will need to install `lsof-2.2` for the third command to work (run it as root). You should be aware that lsof can translate the word LISTEN to your locale settings. In order to remove unnecessary services, first determine what package provides the service and how it is started. This can be accomplished by checking the program that listens in the socket. The following shell script, which uses the programs lsof and dpkg, does just that:
#!/bin/sh # FIXME: this is quick and dirty; replace with a more robust script snippet for i in `sudo lsof -i | grep LISTEN | cut -d " " -f 1 |sort -u` ; do pack=`dpkg -S $i |grep bin |cut -f 1 -d : | uniq` echo "Service $i is installed by $pack"; init=`dpkg -L $pack |grep init.d/ ` if [ ! -z "$init" ]; then echo "and is run by $init" fi done
Once you find any unwanted services, remove the associated package (with dpkg --purge), or disable the service from starting automatically at boot time using update-rc.d (see ). For inetd services (launched by the superdaemon), check which services are enabled in /etc/inetd.conf using:
$ grep -v "^#" /etc/inetd.conf | sort -u
Then disable those services that are not needed by commenting out the line that includes them in /etc/inetd.conf, removing the package, or using update-inetd. If you have wrapped services (those using /usr/sbin/tcpd), check that the files /etc/hosts.allow and /etc/hosts.deny are configured according to your service policy. If the server uses more than one external interface, depending on the service, you may want to limit the service to listen on a specific interface. For example, if you want internal FTP access only, make the FTP daemon listen only on your management interface, not on all interfaces (i.e, 0.0.0.0:21). Re-boot the machine, or switch to single user mode and then back to multiuser using the commands:
# init 1 (....) # init 2
Check the services now available, and, if necessary, repeat the steps above. Now install the needed services, if you have not done so already, and configure them properly. Use the following shell command to determine what user each available service is running as:
# for i in `/usr/sbin/lsof -i |grep LISTEN |cut -d " " -f 1 |sort -u`; \ > do user=`ps ef |grep $i |grep -v grep |cut -f 1 -d " "` ; \ > echo "Service $i is running as user $user"; done
Consider changing these services to a specific user/group and maybe chroot'ing them for increased security. You can do this by changing the /etc/init.d scripts which start the service. Most services in Debian use start-stop-daemon, which has options (`--change-uid` and `--chroot`) for accomplishing this. A word of warning regarding the chroot'ing of services: you may need to put all the files installed by the package (use dpkg -L) providing the service, as well as any packages it depends on, in the chroot'ed environment. Information about setting up a chroot environment for the ssh program can be found in . Repeat the steps above in order to check that only desired services are running and that they are running as the desired user/group combination. Test the installed services in order to see if they work as expected. Check the system using a vulnerability assessment scanner (like `nessus`), in order to determine vulnerabilities in the system (i.e., misconfiguration, old services or unneeded services). Install network and host intrusion measures like `snort` and `logcheck`. Repeat the network scanner step and verify that the intrusion detection systems are working correctly.

For the truly paranoid, also consider the following: Add firewalling capabilities to the system, accepting incoming connections only to offered services and limiting outgoing connections only to those that are authorized. Re-check the installation with a new vulnerability assessment using a network scanner. Using a network scanner, check outbound connections from the system to an outside host and verify that unwanted connections do not find their way out.

FIXME: this procedure considers service hardening but not system hardening at the user level, include information regarding checking user permissions, SETUID files and freezing changes in the system using the ext2 file system.

Configuration checklist

This appendix briefly reiterates points from other sections in this manual in a condensed checklist format. This is intended as a quick summary for someone who has already read the manual. There are other good checklists available, including Kurt Seifried's and .

FIXME: This is based on v1.4 of the manual and might need to be updated. Limit physical access and booting capabilities Enable a password in the BIOS. Disable floppy/cdrom/... booting in the system's BIOS. Set a LILO or GRUB password (/etc/lilo.conf or /boot/grub/menu.lst, respectively); check that the LILO or GRUB configuration file is read-protected. Partitioning Separate user-writable data, non-system data, and rapidly changing run-time data to their own partitions Set `nosuid,noexec,nodev` mount options in /etc/fstab on ext2/3 partitions that should not hold binaries such as /home or /tmp. Password hygiene and login security Set a good root password Install and use PAM Add MD5 support to PAM and make sure that (generally speaking) entries in /etc/pam.d/ files which grant access to the machine have the second field in the pam.d file set to `requisite` or `required`. Tweak /etc/pam.d/login so as to only permit local root logins. Also mark authorized tty:s in /etc/security/access.conf and generally set up this file to limit root logins as much as possible. Add pam_limits.so if you want to set per-user limits Tweak /etc/pam.d/passwd: set minimum length of passwords higher (6 characters maybe) and enable MD5 Add group wheel to /etc/group if desired; add pam_wheel.so group=wheel entry to /etc/pam.d/su For custom per-user controls, use pam_listfile.so entries where appropriate Have an /etc/pam.d/other file and set it up with tight security Set up limits in /etc/security/limits.conf (note that /etc/limits is not used if you are using PAM) Tighten up /etc/login.defs; also, if you enabled MD5 and/or PAM, make sure you make the corresponding changes here, too Tighten up /etc/pam.d/login Disable root ftp access in /etc/ftpusers Disable network root login; use or . (consider installing `sudo`) Use PAM to enforce additional constraints on logins? Other local security issues Kernel tweaks (see ) Kernel patches (see ) Tighten up log file permissions (/var/log/{last,fail}log, Apache logs) Verify that SETUID checking is enabled in /etc/checksecurity.conf Consider making some log files append-only and configuration files immutable using chattr (ext2/3 file systems only) Set up file integrity (see ). Install `debsums` Log everything to a local printer? Burn your configuration on a boot-able CD and boot off that? Disable kernel modules? Limit network access Install and configure ssh (suggest PermitRootLogin No in /etc/ssh/sshd_config, PermitEmptyPasswords No; note other suggestions in text also) Disable or remove in.telnetd, if installed Generally, disable gratuitous services in /etc/inetd.conf using update-inetd --disable (or disable inetd altogether, or use a replacement such as xinetd or rlinetd) Disable other gratuitous network services; ftp, DNS, WWW etc should not be running if you do not need them and monitor them regularly. In most cases mail should be running but configured for local delivery only. For those services which you do need, do not just use the most common programs, look for more secure versions shipped with Debian (or from other sources). Whatever you end up running, make sure you understand the risks. Set up chroot jails for outside users and daemons. Configure firewall and tcpwrappers (i.e. ); note trick for /etc/hosts.deny in text. If you run ftp, set up your ftpd server to always run chroot'ed to the user's home directory If you run X, disable xhost authentication and go with ssh instead; better yet, disable remote X if you can (add -nolisten tcp to the X command line and turn off XDMCP in /etc/X11/xdm/xdm-config by setting the requestPort to 0) Disable remote access to printers Tunnel any IMAP or POP sessions through SSL or ssh; install stunnel if you want to provide this service to remote mail users Set up a log host and configure other machines to send logs to this host (/etc/syslog.conf) Secure BIND, Sendmail, and other complex daemons (run in a chroot jail; run as a non-root pseudo-user) Install tiger or a similar network intrusion detection tool. Install snort or a similar network intrusion detection tool. Do without NIS and RPC if you can (disable portmap). Policy issues Educate users about the whys and hows of your policies. When you have prohibited something which is regularly available on other systems, provide documentation which explains how to accomplish similar results using other, more secure means. Prohibit use of protocols which use clear-text passwords (telnet, rsh and friends; ftp, imap, http, ...). Prohibit programs which use SVGAlib. Use disk quotas. Keep informed about security issues Subscribe to security mailing lists Configure `apt` for security updates -- add to /etc/apt/sources.list an entry (or entries) for http://security.debian.org/ Also remember to periodically run apt-get update ; apt-get upgrade (perhaps install as a cron job?) as explained in .

Setting up a stand-alone IDS

You can easily set up a dedicated Debian system as a stand-alone Intrusion Detection System using `snort` and a web-based interface to analyse the intrusion detection alerts: Install a base Debian system and select no additional packages. Install one of the Snort versions with database support and configure the IDS to log alerts into the database. Download and install BASE (Basic Analysis and Security Engine), or ACID (Analysis Console for Intrusion Databases). Configure it to use the same database than Snort. Download and install the necessary packages

Typically the needed packages will be installed through the dependencies
.

BASE is currently packaged for Debian in `acidbase` and ACID is packaged as `acidlab`

It can also be downloaded from , or .
. Both provide a graphical WWW interface to Snort's output.

Besides the base installation you will also need a web server (such as `apache`), a PHP interpreter and a relational database (such `postgresql` or `mysql`) where Snort will store its alerts.

This system should be set up with at least two interfaces: one interface connected to a management LAN (for accessing the results and maintaining the system), and one interface with no IP address attached to the network segment being analyzed. You should configure the web server to listen only on the interface connected to the management LAN.

You should configure both interfaces in the standard Debian /etc/network/interfaces configuration file. One (the management LAN) address can be configured as you would normally do. The other interface needs to be configured so that it is started up when the system boots, but with no interface address. You can use the following interface definition:

auto eth0 iface eth0 inet manual up ifconfig $IFACE 0.0.0.0 up up ip link set $IFACE promisc on down ip link set $IFACE promisc off down ifconfig $IFACE down

The above configures an interface to read all the traffic on the network in a stealth-type configuration. This prevents the NIDS system to be a direct target in a hostile network since the sensors have no IP address on the network. Notice, however, that there have been known bugs over time in sensors part of NIDS (for example see related to Snort) and remote buffer overflows might even be triggered by network packet processing.

You might also want to read the and the documentation available at the .

Setting up a bridge firewall

This information was contributed by Francois Bayart in order to help users set up a Linux bridge/firewall with the 2.4.x kernel and `iptables`. Kernel patches are no more needed as the code was made standard part of the Linux kernel distribution.

To configure the kernel with necessary support, run `make menuconfig` or `make xconfig`. In the section Networking options, enable the following options:

[*] Network packet filtering (replaces ipchains) [ ] Network packet filtering debugging (NEW) <*> 802.1d Ethernet Bridging [*] netfilter (firewalling) support (NEW)

Caution: you must disable this if you want to apply some firewalling rules or else iptables will not work:

[ ] Network packet filtering debugging (NEW)

Next, add the correct options in the section IP: Netfilter Configuration. Then, compile and install the kernel. If you want to do it the Debian way, install `kernel-package` and run make-kpkg to create a custom Debian kernel package you can install on your server using dpkg. Once the new kernel is compiled and installed, install the `bridge-utils` package.

Once these steps are complete, you can complete the configuration of your bridge. The next section presents two different possible configurations for the bridge, each with a hypothetical network map and the necessary commands. A bridge providing NAT and firewall capabilities

The first configuration uses the bridge as a firewall with network address translation (NAT) that protects a server and internal LAN clients. A diagram of the network configuration is shown below:

Internet ---- router ( 62.3.3.25 ) ---- bridge (62.3.3.26 gw 62.3.3.25 / 192.168.0.1) | | |---- WWW Server (62.3.3.27 gw 62.3.3.25) | | LAN --- Zipowz (192.168.0.2 gw 192.168.0.1)

The following commands show how this bridge can be configured.

# Create the interface br0 /usr/sbin/brctl addbr br0 # Add the Ethernet interface to use with the bridge /usr/sbin/brctl addif br0 eth0 /usr/sbin/brctl addif br0 eth1 # Start up the Ethernet interface /sbin/ifconfig eth0 0.0.0.0 /sbin/ifconfig eth1 0.0.0.0 # Configure the bridge ethernet # The bridge will be correct and invisible ( transparent firewall ). # It's hidden in a traceroute and you keep your real gateway on the # other computers. Now if you want you can config a gateway on your # bridge and choose it as your new gateway for the other computers. /sbin/ifconfig br0 62.3.3.26 netmask 255.255.255.248 broadcast 62.3.3.31 # I have added this internal IP to create my NAT ip addr add 192.168.0.1/24 dev br0 /sbin/route add default gw 62.3.3.25
A bridge providing firewall capabilities

A second possible configuration is a system that is set up as a transparent firewall for a LAN with a public IP address space.

Internet ---- router (62.3.3.25) ---- bridge (62.3.3.26) | | |---- WWW Server (62.3.3.28 gw 62.3.3.25) | | |---- Mail Server (62.3.3.27 gw 62.3.3.25)

The following commands show how this bridge can be configured.

# Create the interface br0 /usr/sbin/brctl addbr br0 # Add the Ethernet interface to use with the bridge /usr/sbin/brctl addif br0 eth0 /usr/sbin/brctl addif br0 eth1 # Start up the Ethernet interface /sbin/ifconfig eth0 0.0.0.0 /sbin/ifconfig eth1 0.0.0.0 # Configure the bridge Ethernet # The bridge will be correct and invisible ( transparent firewall ). # It's hidden in a traceroute and you keep your real gateway on the # other computers. Now if you want you can config a gateway on your # bridge and choose it as your new gateway for the other computers. /sbin/ifconfig br0 62.3.3.26 netmask 255.255.255.248 broadcast 62.3.3.31

If you traceroute the Linux Mail Server, you won't see the bridge. If you want access to the bridge with ssh, you must have a gateway or you must first connect to another server, such as the "Mail Server", and then connect to the bridge through the internal network card.

Basic IPtables rules

This is an example of the basic rules that could be used for either of these setups.

iptables -F FORWARD iptables -P FORWARD DROP iptables -A FORWARD -s 0.0.0.0/0.0.0.0 -d 0.0.0.0/0.0.0.0 -m state --state INVALID -j DROP iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT # Some funny rules but not in a classic Iptables sorry ... # Limit ICMP # iptables -A FORWARD -p icmp -m limit --limit 4/s -j ACCEPT # Match string, a good simple method to block some VIRUS very quickly # iptables -I FORWARD -j DROP -p tcp -s 0.0.0.0/0 -m string --string "cmd.exe" # Block all MySQL connection just to be sure iptables -A FORWARD -p tcp -s 0/0 -d 62.3.3.0/24 --dport 3306 -j DROP # Linux Mail Server Rules # Allow FTP-DATA (20), FTP (21), SSH (22) iptables -A FORWARD -p tcp -s 0.0.0.0/0 -d 62.3.3.27/32 --dport 20:22 -j ACCEPT # Allow the Mail Server to connect to the outside # Note: This is *not* needed for the previous connections # (remember: stateful filtering) and could be removed. iptables -A FORWARD -p tcp -s 62.3.3.27/32 -d 0/0 -j ACCEPT # WWW Server Rules # Allow HTTP ( 80 ) connections with the WWW server iptables -A FORWARD -p tcp -s 0.0.0.0/0 -d 62.3.3.28/32 --dport 80 -j ACCEPT # Allow HTTPS ( 443 ) connections with the WWW server iptables -A FORWARD -p tcp -s 0.0.0.0/0 -d 62.3.3.28/32 --dport 443 -j ACCEPT # Allow the WWW server to go out # Note: This is *not* needed for the previous connections # (remember: stateful filtering) and could be removed. iptables -A FORWARD -p tcp -s 62.3.3.28/32 -d 0/0 -j ACCEPT
Sample script to change the default Bind installation.

This script automates the procedure for changing the bind version 8 name server's default installation so that it does not run as the superuser. Notice that bind version 9 in Debian already does this by default

Since version 9.2.1-5. That is, since Debian release sarge.
, and you are much better using that version than bind version 8.

This script is here for historical purposes and to show how you can automate this kind of changes system-wide. The script will create the user and groups defined for the name server and will modify both /etc/default/bind and /etc/init.d/bind so that the program will run with that user. Use with extreme care since it has not been tested thoroughly.

You can also create the users manually and use the patch available for the default init.d script attached to .

#!/bin/sh # Change the default Debian bind v8 configuration to have it run # with a non-root user and group. # # DO NOT USER this with version 9, use debconf for configure this instead # # WARN: This script has not been tested thoroughly, please # verify the changes made to the INITD script # (c) 2002 Javier Fernández-Sanguino Peña # # This program is free software; you can redistribute it and/or modify # it under the terms of the GNU General Public License as published by # the Free Software Foundation; either version 1, or (at your option) # any later version. # # This program is distributed in the hope that it will be useful, # but WITHOUT ANY WARRANTY; without even the implied warranty of # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the # GNU General Public License for more details. # # Please see the file `COPYING' for the complete copyright notice. # restore() { # Just in case, restore the system if the changes fail echo "WARN: Restoring to the previous setup since I'm unable to properly change it." echo "WARN: Please check the $INITDERR script." mv $INITD $INITDERR cp $INITDBAK $INITD } USER=named GROUP=named INITD=/etc/init.d/bind DEFAULT=/etc/default/bind INITDBAK=$INITD.preuserchange INITDERR=$INITD.changeerror AWKS="awk ' /\/usr\/sbin\/ndc reload/ { print \"stop; sleep 2; start;\"; noprint = 1; } /\\\\$/ { if ( noprint != 0 ) { noprint = noprint + 1;} } /^.*$/ { if ( noprint != 0 ) { noprint = noprint - 1; } else { print \$0; } } '" [ `id -u` -ne 0 ] && { echo "This program must be run by the root user" exit 1 } RUNUSER=`ps eo user,fname |grep named |cut -f 1 -d " "` if [ "$RUNUSER" = "$USER" ] then echo "WARN: The name server running daemon is already running as $USER" echo "ERR: This script will not do any changes to your setup." exit 1 fi if [ ! -f "$INITD" ] then echo "ERR: This system does not have $INITD (which this script tries to change)" RUNNING=`ps eo fname |grep named` [ -z "$RUNNING" ] && \ echo "ERR: In fact the name server daemon is not even running (is it installed?)" echo "ERR: No changes will be made to your system" exit 1 fi # Check if there are options already setup if [ -e "$DEFAULT" ] then if grep -q ^OPTIONS $DEFAULT; then echo "ERR: The $DEFAULT file already has options set." echo "ERR: No changes will be made to your system" fi fi # Check if named group exists if [ -z "`grep $GROUP /etc/group`" ] then echo "Creating group $GROUP:" addgroup $GROUP else echo "WARN: Group $GROUP already exists. Will not create it" fi # Same for the user if [ -z "`grep $USER /etc/passwd`" ] then echo "Creating user $USER:" adduser --system --home /home/$USER \ --no-create-home --ingroup $GROUP \ --disabled-password --disabled-login $USER else echo "WARN: The user $USER already exists. Will not create it" fi # Change the init.d script # First make a backup (check that there is not already # one there first) if [ ! -f $INITDBAK ] then cp $INITD $INITDBAK fi # Then use it to change it cat $INITDBAK | eval $AWKS > $INITD # Now put the options in the /etc/default/bind file: cat >>$DEFAULT <<EOF # Make bind run with the user we defined OPTIONS="-u $USER -g $GROUP" EOF echo "WARN: The script $INITD has been changed, trying to test the changes." echo "Restarting the named daemon (check for errors here)." $INITD restart if [ $? -ne 0 ] then echo "ERR: Failed to restart the daemon." restore exit 1 fi RUNNING=`ps eo fname |grep named` if [ -z "$RUNNING" ] then echo "ERR: Named is not running, probably due to a problem with the changes." restore exit 1 fi # Check if it's running as expected RUNUSER=`ps eo user,fname |grep named |cut -f 1 -d " "` if [ "$RUNUSER" = "$USER" ] then echo "All has gone well, named seems to be running now as $USER." else echo "ERR: The script failed to automatically change the system." echo "ERR: Named is currently running as $RUNUSER." restore exit 1 fi exit 0

The previous script, run on Woody's (Debian 3.0) custom bind (version 8), will modify the initd file after creating the 'named' user and group and will

Security update protected by a firewall

After a standard installation, a system may still have some security vulnerabilities. Unless you can download updates for the vulnerable packages on another system (or you have mirrored security.debian.org for local use), the system will have to be connected to the Internet for the downloads.

However, as soon as you connect to the Internet you are exposing this system. If one of your local services is vulnerable, you might be compromised even before the update is finished! This may seem paranoid but, in fact, analysis from the has shown that systems can be compromised in less than three days, even if the system is not publicly known (i.e., not published in DNS records).

When doing an update on a system not protected by an external system like a firewall, it is possible to properly configure your local firewall to restrict connections involving only the security update itself. The example below shows how to set up such local firewall capabilities, which allow connections from security.debian.org only, logging all others.

The following example can be use to setup a restricted firewall ruleset. Run this commands from a local console (not a remote one) to reduce the chances of locking yourself out of the system.

# iptables -F # iptables -L Chain INPUT (policy ACCEPT) target prot opt source destination Chain FORWARD (policy ACCEPT) target prot opt source destination Chain OUTPUT (policy ACCEPT) target prot opt source destination # iptables -A OUTPUT -d security.debian.org --dport 80 -j ACCEPT # iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT # iptables -A INPUT -p icmp -j ACCEPT # iptables -A INPUT -j LOG # iptables -A OUTPUT -j LOG # iptables -P INPUT DROP # iptables -P FORWARD DROP # iptables -P OUTPUT DROP # iptables -L Chain INPUT (policy DROP) target prot opt source destination ACCEPT all -- 0.0.0.0/0 0.0.0.0/0 state RELATED,ESTABLISHED ACCEPT icmp -- 0.0.0.0/0 0.0.0.0/0 LOG all -- anywhere anywhere LOG level warning Chain FORWARD (policy DROP) target prot opt source destination Chain OUTPUT (policy DROP) target prot opt source destination ACCEPT 80 -- anywhere security.debian.org LOG all -- anywhere anywhere LOG level warning

Note: Using a DROP policy in the INPUT chain is the most correct thing to do, but be very careful when doing this after flushing the chain from a remote connection. When testing firewall rulesets from a remote location it is best if you run a script with the firewall ruleset (instead of introducing the ruleset line by line through the command line) and, as a precaution, keep a backdoor

Such as knockd. Alternatively, you can open a different console and have the system ask for confirmation that there is somebody on the other side, and reset the firewall chain if no confirmation is given. The following test script could be of use:
#!/bin/bash while true; do read -n 1 -p "Are you there? " -t 30 ayt if [ -z "$ayt" ] ; then break fi done # Reset the firewall chain, user is not available echo echo "Resetting firewall chain!" iptables -F iptables -P INPUT ACCEPT iptables -P FORWARD ACCEPT iptables -P OUTPUT ACCEPT exit 1

Of course, you should disable any backdoors before getting the system into production.

configured so that you can re-enable access to the system if you make a mistake. That way there would be no need to go to a remote location to fix a firewall ruleset that blocks you.

FIXME: This needs DNS to be working properly since it is required for security.debian.org to work. You can add security.debian.org to /etc/hosts but now it is a CNAME to several hosts (there is more than one security mirror)

FIXME: this will only work with HTTP URLs since ftp might need the ip_conntrack_ftp module, or use passive mode.

Chroot environment for SSH

Creating a restricted environment for SSH is a tough job due to its dependencies and the fact that, unlike other servers, SSH provides a remote shell to users. Thus, you will also have to consider the applications users will be allowed to use in the environment.

You have two options to setup a restricted remote shell: Chrooting the ssh users, by properly configuring the ssh daemon you can ask it to chroot a user after authentication just before it is provided a shell. Each user can have their own environment. Chrooting the ssh server, since you chroot the ssh application itself all users are chrooted to the defined environment.

The first option has the advantage of making it possible to have both non-chrooted and chrooted users, if you don't introduce any setuid application in the user's chroots it is more difficult to break out of it. However, you might need to setup individual chroots for each user and it is more difficult to setup (as it requires cooperation from the SSH server). The second option is more easy to setup, and protects from an exploitation of the ssh server itself (since it's also in the chroot) but it will have the limitation that all users will share the same chroot environment (you cannot setup a per-user chroot environment). Chrooting the ssh users

You can setup the ssh server so that it will chroot a set of defined users into a shell with a limited set of applications available.

Using `libpam-chroot`

Probably the easiest way is to use the `libpam-chroot` package provided in Debian. Once you install it you need to: Modify /etc/pam.d/ssh to use this PAM module, add as its last line

You can use the debug option to have it send the progress of the module to the authpriv.notice facility

:
session required pam_chroot.so
set a proper chroot environment for the user. You can try using the scripts available at /usr/share/doc/libpam-chroot/examples/, use the `makejail`

You can create a very limited bash environment with the following python definition for makejail, just create the directory /var/chroots/users/foo and a file with the following contents and call it bash.py:

chroot="/var/chroots/users/foo" cleanJailFirst=1 testCommandsInsideJail=["bash ls"]

And then run makejail bash.py to create the user environment at /var/chroots/users/foo. To test the environment run:

# chroot /var/chroots/users/foo/ ls bin dev etc lib proc sbin usr
program or setup a minimum Debian environment with `debootstrap`. Make sure the environment includes the needed devices

In some occasions you might need the /dev/ptmx and /dev/pty* devices and the /dev/pts/ subdirectory. Running MAKEDEV in the /dev directory of the chrooted environment should be sufficient to create them if they do not exist. If you are using kernels (version 2.6) which dynamically create device files you will need to create the /dev/pts/ files yourself and grant them the proper privileges.

. Configure /etc/security/chroot.conf so that the users you determine are chrooted to the directory you setup previously. You might want to have independent directories for different users so that they will not be able to see neither the whole system nor each other's. Configure SSH: Depending on your OpenSSH version the chroot environment might work straight of the box or not. Since 3.6.1p2 the do_pam_session() function is called after sshd has dropped privileges, since chroot() needs root priviledges it will not work with Privilege separation on. In newer OpenSSH versions, however, the PAM code has been modified and do_pam_session is called before dropping priviledges so it will work even with Privilege separation is on. If you have to disable it modify /etc/ssh/sshd_config like this:
UsePrivilegeSeparation no

Notice that this will lower the security of your system since the OpenSSH server will then run as root user. This means that if a remote attack is found against OpenSSH an attacker will get root privileges instead of sshd, thus compromising the whole system.

If you are using a kernel that implements Mandatory Access Control (RSBAC/SElinux) you can avoid changing this configuration just by granting the sshd user privileges to make the chroot() system call.

If you don't disable Privilege Separation you will need an /etc/passwd which includes the user's UID inside the chroot for Privilege Separation to work properly.

If you have Privilege Separation set to yes and your OpenSSH version does not behave properly you will need to disable it. If you don't, users that try to connect to your server and would be chrooted by this module will see this:

$ ssh -l user server user@server's password: Connection to server closed by remote host. Connection to server closed.

This is because the ssh daemon, which is running as 'sshd', is not be able to make the chroot() system call. To disable Privilege separation you have to modify the /etc/ssh/sshd_config configuration file as described above.

Notice that if any of the following is missing the users will not be able to logon to the chroot: The /proc filesystem needs to be mounted in the users' chroot. The necessary /dev/pts/ devices need to exist. If the files are generated by your running kernel automatically then you have to manually create them on the chroot's /dev/. The user's home directory has to exist in the chroot, otherwise the ssh daemon will not continue.

You can debug all these issues if you use the debug keyword in the /etc/pam.d/ssh PAM definition. If you encounter issues you might find it useful to enable the debugging mode on the ssh client too.

Note: This information is also available (and maybe more up to date) in /usr/share/doc/libpam-chroot/README.Debian.gz, please review it for updated information before taking the above steps.

Patching the ssh server

Debian's sshd does not allow restriction of a user's movement through the server, since it lacks the chroot function that the commercial program sshd2 includes (using 'ChrootGroups' or 'ChrootUsers', see ). However, there is a patch available to add this functionality available from (requested and available in in Debian). The patch may be included in future releases of the OpenSSH package. Emmanuel Lacour has ssh deb packages for sarge with this feature. They are available at . Notice that those might not be up to date so completing the compilation step is recommended.

After applying the patch, modify /etc/passwd by changing the home path of the users (with the special `/./` token):

joeuser:x:1099:1099:Joe Random User:/home/joe/./:/bin/bash

This will restrict both remote shell access, as well as remote copy through the ssh channel.

Make sure to have all the needed binaries and libraries in the chroot'ed path for users. These files should be owned by root to avoid tampering by the user (so as to exit the chroot'ed jailed). A sample might include:

./bin: total 660 drwxr-xr-x 2 root root 4096 Mar 18 13:36 . drwxr-xr-x 8 guest guest 4096 Mar 15 16:53 .. -r-xr-xr-x 1 root root 531160 Feb 6 22:36 bash -r-xr-xr-x 1 root root 43916 Nov 29 13:19 ls -r-xr-xr-x 1 root root 16684 Nov 29 13:19 mkdir -rwxr-xr-x 1 root root 23960 Mar 18 13:36 more -r-xr-xr-x 1 root root 9916 Jul 26 2001 pwd -r-xr-xr-x 1 root root 24780 Nov 29 13:19 rm lrwxrwxrwx 1 root root 4 Mar 30 16:29 sh -> bash
./etc: total 24 drwxr-xr-x 2 root root 4096 Mar 15 16:13 . drwxr-xr-x 8 guest guest 4096 Mar 15 16:53 .. -rw-r--r-- 1 root root 54 Mar 15 13:23 group -rw-r--r-- 1 root root 428 Mar 15 15:56 hosts -rw-r--r-- 1 root root 44 Mar 15 15:53 passwd -rw-r--r-- 1 root root 52 Mar 15 13:23 shells
./lib: total 1848 drwxr-xr-x 2 root root 4096 Mar 18 13:37 . drwxr-xr-x 8 guest guest 4096 Mar 15 16:53 .. -rwxr-xr-x 1 root root 92511 Mar 15 12:49 ld-linux.so.2 -rwxr-xr-x 1 root root 1170812 Mar 15 12:49 libc.so.6 -rw-r--r-- 1 root root 20900 Mar 15 13:01 libcrypt.so.1 -rw-r--r-- 1 root root 9436 Mar 15 12:49 libdl.so.2 -rw-r--r-- 1 root root 248132 Mar 15 12:48 libncurses.so.5 -rw-r--r-- 1 root root 71332 Mar 15 13:00 libnsl.so.1 -rw-r--r-- 1 root root 34144 Mar 15 16:10 libnss_files.so.2 -rw-r--r-- 1 root root 29420 Mar 15 12:57 libpam.so.0 -rw-r--r-- 1 root root 105498 Mar 15 12:51 libpthread.so.0 -rw-r--r-- 1 root root 25596 Mar 15 12:51 librt.so.1 -rw-r--r-- 1 root root 7760 Mar 15 12:59 libutil.so.1 -rw-r--r-- 1 root root 24328 Mar 15 12:57 libwrap.so.0
./usr: total 16 drwxr-xr-x 4 root root 4096 Mar 15 13:00 . drwxr-xr-x 8 guest guest 4096 Mar 15 16:53 .. drwxr-xr-x 2 root root 4096 Mar 15 15:55 bin drwxr-xr-x 2 root root 4096 Mar 15 15:37 lib
./usr/bin: total 340 drwxr-xr-x 2 root root 4096 Mar 15 15:55 . drwxr-xr-x 4 root root 4096 Mar 15 13:00 .. -rwxr-xr-x 1 root root 10332 Mar 15 15:55 env -rwxr-xr-x 1 root root 13052 Mar 15 13:13 id -r-xr-xr-x 1 root root 25432 Mar 15 12:40 scp -rwxr-xr-x 1 root root 43768 Mar 15 15:15 sftp -r-sr-xr-x 1 root root 218456 Mar 15 12:40 ssh -rwxr-xr-x 1 root root 9692 Mar 15 13:17 tty
./usr/lib: total 852 drwxr-xr-x 2 root root 4096 Mar 15 15:37 . drwxr-xr-x 4 root root 4096 Mar 15 13:00 .. -rw-r--r-- 1 root root 771088 Mar 15 13:01 libcrypto.so.0.9.6 -rw-r--r-- 1 root root 54548 Mar 15 13:00 libz.so.1 -rwxr-xr-x 1 root root 23096 Mar 15 15:37 sftp-server
Chrooting the ssh server

If you create a chroot which includes the SSH server files in, for example /var/chroot/ssh, you would start the ssh server chroot'ed with this command:

# chroot /var/chroot/ssh /sbin/sshd -f /etc/sshd_config

That would make startup the sshd daemon inside the chroot. In order to do that you have to first prepare the contents of the /var/chroot/ssh directory so that it includes both the SSH server and all the utilities that the users connecting to that server might need. If you are doing this you should make certain that OpenSSH uses Privilege Separation (which is the default) having the following line in the configuration file /etc/ssh/sshd_config:

UsePrivilegeSeparation yes

That way the remote daemon will do as few things as possible as the root user so even if there is a bug in it it will not compromise the chroot. Notice that, unlike the case in which you setup a per-user chroot, the ssh daemon is running in the same chroot as the users so there is at least one potential process running as root which could break out of the chroot.

Notice, also, that in order for SSH to work in that location, the partition where the chroot directory resides cannot be mounted with the nodev option. If you use that option, then you will get the following error: PRNG is not seeded, because /dev/urandom does not work in the chroot.

Setup a minimal system (the really easy way)

You can use `debootstrap` to setup a minimal environment that just includes the ssh server. In order to do this you just have to create a chroot as described in the document. This method is bound to work (you will get all the necessary componentes for the chroot) but at the cost of disk space (a minimal installation of Debian will amount to several hundred megabytes). This minimal system might also include setuid files that a user in the chroot could use to break out of the chroot if any of those could be use for a privilege escalation.

Automatically making the environment (the easy way)

You can easily create a restricted environment with the `makejail` package, since it automatically takes care of tracing the server daemon (with strace), and makes it run under the restricted environment.

The advantage of programs that automatically generate chroot environments is that they are capable of copying any package to the chroot environment (even following the package's dependencies and making sure it's complete). Thus, providing user applications is easier.

To set up the environment using makejail's provided examples, just create /var/chroot/sshd and use the command:

# makejail /usr/share/doc/makejail/examples/sshd.py

This will setup the chroot in the /var/chroot/sshd directory. Notice that this chroot will not fully work unless you:

Mount the procfs filesystem in /var/chroot/sshd/proc. Makejail will mount it for you but if the system reboots you need to remount it running:

# mount -t proc proc /var/chroot/sshd/proc

You can also have it be mounted automatically by editing /etc/fstab and including this line:

proc-ssh /var/chroot/sshd/proc proc none 0 0
Have syslog listen to the device /dev/log inside the chroot. In order to do this you have modify /etc/default/syslogd and add -a /var/chroot/sshd/dev/log to the SYSLOGD variable definition.

Read the sample file to see what other changes need to be made to the environment. Some of these changes, such as copying user's home directories, cannot be done automatically. Also, limit the exposure of sensitive information by only copying the data from a given number of users from the files /etc/shadow or /etc/group. Notice that if you are using Privilege Separation the sshd user needs to exist in those files.

The following sample environment has been (slightly) tested in Debian 3.0 and is built with the configuration file provided in the package and includes the `fileutils` package:

. |-- bin | |-- ash | |-- bash | |-- chgrp | |-- chmod | |-- chown | |-- cp | |-- csh -> /etc/alternatives/csh | |-- dd | |-- df | |-- dir | |-- fdflush | |-- ksh | |-- ln | |-- ls | |-- mkdir | |-- mknod | |-- mv | |-- rbash -> bash | |-- rm | |-- rmdir | |-- sh -> bash | |-- sync | |-- tcsh | |-- touch | |-- vdir | |-- zsh -> /etc/alternatives/zsh | `-- zsh4 |-- dev | |-- null | |-- ptmx | |-- pts | |-- ptya0 (...) | |-- tty | |-- tty0 (...) | `-- urandom |-- etc | |-- alternatives | | |-- csh -> /bin/tcsh | | `-- zsh -> /bin/zsh4 | |-- environment | |-- hosts | |-- hosts.allow | |-- hosts.deny | |-- ld.so.conf | |-- localtime -> /usr/share/zoneinfo/Europe/Madrid | |-- motd | |-- nsswitch.conf | |-- pam.conf | |-- pam.d | | |-- other | | `-- ssh | |-- passwd | |-- resolv.conf | |-- security | | |-- access.conf | | |-- chroot.conf | | |-- group.conf | | |-- limits.conf | | |-- pam_env.conf | | `-- time.conf | |-- shadow | |-- shells | `-- ssh | |-- moduli | |-- ssh_host_dsa_key | |-- ssh_host_dsa_key.pub | |-- ssh_host_rsa_key | |-- ssh_host_rsa_key.pub | `-- sshd_config |-- home | `-- userX |-- lib | |-- ld-2.2.5.so | |-- ld-linux.so.2 -> ld-2.2.5.so | |-- libc-2.2.5.so | |-- libc.so.6 -> libc-2.2.5.so | |-- libcap.so.1 -> libcap.so.1.10 | |-- libcap.so.1.10 | |-- libcrypt-2.2.5.so | |-- libcrypt.so.1 -> libcrypt-2.2.5.so | |-- libdl-2.2.5.so | |-- libdl.so.2 -> libdl-2.2.5.so | |-- libm-2.2.5.so | |-- libm.so.6 -> libm-2.2.5.so | |-- libncurses.so.5 -> libncurses.so.5.2 | |-- libncurses.so.5.2 | |-- libnsl-2.2.5.so | |-- libnsl.so.1 -> libnsl-2.2.5.so | |-- libnss_compat-2.2.5.so | |-- libnss_compat.so.2 -> libnss_compat-2.2.5.so | |-- libnss_db-2.2.so | |-- libnss_db.so.2 -> libnss_db-2.2.so | |-- libnss_dns-2.2.5.so | |-- libnss_dns.so.2 -> libnss_dns-2.2.5.so | |-- libnss_files-2.2.5.so | |-- libnss_files.so.2 -> libnss_files-2.2.5.so | |-- libnss_hesiod-2.2.5.so | |-- libnss_hesiod.so.2 -> libnss_hesiod-2.2.5.so | |-- libnss_nis-2.2.5.so | |-- libnss_nis.so.2 -> libnss_nis-2.2.5.so | |-- libnss_nisplus-2.2.5.so | |-- libnss_nisplus.so.2 -> libnss_nisplus-2.2.5.so | |-- libpam.so.0 -> libpam.so.0.72 | |-- libpam.so.0.72 | |-- libpthread-0.9.so | |-- libpthread.so.0 -> libpthread-0.9.so | |-- libresolv-2.2.5.so | |-- libresolv.so.2 -> libresolv-2.2.5.so | |-- librt-2.2.5.so | |-- librt.so.1 -> librt-2.2.5.so | |-- libutil-2.2.5.so | |-- libutil.so.1 -> libutil-2.2.5.so | |-- libwrap.so.0 -> libwrap.so.0.7.6 | |-- libwrap.so.0.7.6 | `-- security | |-- pam_access.so | |-- pam_chroot.so | |-- pam_deny.so | |-- pam_env.so | |-- pam_filter.so | |-- pam_ftp.so | |-- pam_group.so | |-- pam_issue.so | |-- pam_lastlog.so | |-- pam_limits.so | |-- pam_listfile.so | |-- pam_mail.so | |-- pam_mkhomedir.so | |-- pam_motd.so | |-- pam_nologin.so | |-- pam_permit.so | |-- pam_rhosts_auth.so | |-- pam_rootok.so | |-- pam_securetty.so | |-- pam_shells.so | |-- pam_stress.so | |-- pam_tally.so | |-- pam_time.so | |-- pam_unix.so | |-- pam_unix_acct.so -> pam_unix.so | |-- pam_unix_auth.so -> pam_unix.so | |-- pam_unix_passwd.so -> pam_unix.so | |-- pam_unix_session.so -> pam_unix.so | |-- pam_userdb.so | |-- pam_warn.so | `-- pam_wheel.so |-- sbin | `-- start-stop-daemon |-- usr | |-- bin | | |-- dircolors | | |-- du | | |-- install | | |-- link | | |-- mkfifo | | |-- shred | | |-- touch -> /bin/touch | | `-- unlink | |-- lib | | |-- libcrypto.so.0.9.6 | | |-- libdb3.so.3 -> libdb3.so.3.0.2 | | |-- libdb3.so.3.0.2 | | |-- libz.so.1 -> libz.so.1.1.4 | | `-- libz.so.1.1.4 | |-- sbin | | `-- sshd | `-- share | |-- locale | | `-- es | | |-- LC_MESSAGES | | | |-- fileutils.mo | | | |-- libc.mo | | | `-- sh-utils.mo | | `-- LC_TIME -> LC_MESSAGES | `-- zoneinfo | `-- Europe | `-- Madrid `-- var `-- run |-- sshd `-- sshd.pid 27 directories, 733 files

For Debian release 3.1 you have to make sure that the environment includes also the common files for PAM. The following files need to be copied over to the chroot if makejail did not do it for you:

$ ls /etc/pam.d/common-* /etc/pam.d/common-account /etc/pam.d/common-password /etc/pam.d/common-auth /etc/pam.d/common-session

Manually creating the environment (the hard way)

It is possible to create an environment, using a trial-and-error method, by monitoring the sshd server traces and log files in order to determine the necessary files. The following environment, contributed by José Luis Ledesma, is a sample listing of files in a chroot environment for ssh in Debian woody (3.0):

Notice that there are no SETUID files. This makes it more difficult for remote users to escape the chroot environment. However, it also prevents users from changing their passwords, since the passwd program cannot modify the files /etc/passwd or /etc/shadow.
.: total 36 drwxr-xr-x 9 root root 4096 Jun 5 10:05 ./ drwxr-xr-x 11 root root 4096 Jun 3 13:43 ../ drwxr-xr-x 2 root root 4096 Jun 4 12:13 bin/ drwxr-xr-x 2 root root 4096 Jun 4 12:16 dev/ drwxr-xr-x 4 root root 4096 Jun 4 12:35 etc/ drwxr-xr-x 3 root root 4096 Jun 4 12:13 lib/ drwxr-xr-x 2 root root 4096 Jun 4 12:35 sbin/ drwxr-xr-x 2 root root 4096 Jun 4 12:32 tmp/ drwxr-xr-x 2 root root 4096 Jun 4 12:16 usr/ ./bin: total 8368 drwxr-xr-x 2 root root 4096 Jun 4 12:13 ./ drwxr-xr-x 9 root root 4096 Jun 5 10:05 ../ -rwxr-xr-x 1 root root 109855 Jun 3 13:45 a2p* -rwxr-xr-x 1 root root 387764 Jun 3 13:45 bash* -rwxr-xr-x 1 root root 36365 Jun 3 13:45 c2ph* -rwxr-xr-x 1 root root 20629 Jun 3 13:45 dprofpp* -rwxr-xr-x 1 root root 6956 Jun 3 13:46 env* -rwxr-xr-x 1 root root 158116 Jun 3 13:45 fax2ps* -rwxr-xr-x 1 root root 104008 Jun 3 13:45 faxalter* -rwxr-xr-x 1 root root 89340 Jun 3 13:45 faxcover* -rwxr-xr-x 1 root root 441584 Jun 3 13:45 faxmail* -rwxr-xr-x 1 root root 96036 Jun 3 13:45 faxrm* -rwxr-xr-x 1 root root 107000 Jun 3 13:45 faxstat* -rwxr-xr-x 1 root root 77832 Jun 4 11:46 grep* -rwxr-xr-x 1 root root 19597 Jun 3 13:45 h2ph* -rwxr-xr-x 1 root root 46979 Jun 3 13:45 h2xs* -rwxr-xr-x 1 root root 10420 Jun 3 13:46 id* -rwxr-xr-x 1 root root 4528 Jun 3 13:46 ldd* -rwxr-xr-x 1 root root 111386 Jun 4 11:46 less* -r-xr-xr-x 1 root root 26168 Jun 3 13:45 login* -rwxr-xr-x 1 root root 49164 Jun 3 13:45 ls* -rwxr-xr-x 1 root root 11600 Jun 3 13:45 mkdir* -rwxr-xr-x 1 root root 24780 Jun 3 13:45 more* -rwxr-xr-x 1 root root 154980 Jun 3 13:45 pal2rgb* -rwxr-xr-x 1 root root 27920 Jun 3 13:46 passwd* -rwxr-xr-x 1 root root 4241 Jun 3 13:45 pl2pm* -rwxr-xr-x 1 root root 2350 Jun 3 13:45 pod2html* -rwxr-xr-x 1 root root 7875 Jun 3 13:45 pod2latex* -rwxr-xr-x 1 root root 17587 Jun 3 13:45 pod2man* -rwxr-xr-x 1 root root 6877 Jun 3 13:45 pod2text* -rwxr-xr-x 1 root root 3300 Jun 3 13:45 pod2usage* -rwxr-xr-x 1 root root 3341 Jun 3 13:45 podchecker* -rwxr-xr-x 1 root root 2483 Jun 3 13:45 podselect* -r-xr-xr-x 1 root root 82412 Jun 4 11:46 ps* -rwxr-xr-x 1 root root 36365 Jun 3 13:45 pstruct* -rwxr-xr-x 1 root root 7120 Jun 3 13:45 pwd* -rwxr-xr-x 1 root root 179884 Jun 3 13:45 rgb2ycbcr* -rwxr-xr-x 1 root root 20532 Jun 3 13:45 rm* -rwxr-xr-x 1 root root 6720 Jun 4 10:15 rmdir* -rwxr-xr-x 1 root root 14705 Jun 3 13:45 s2p* -rwxr-xr-x 1 root root 28764 Jun 3 13:46 scp* -rwxr-xr-x 1 root root 385000 Jun 3 13:45 sendfax* -rwxr-xr-x 1 root root 67548 Jun 3 13:45 sendpage* -rwxr-xr-x 1 root root 88632 Jun 3 13:46 sftp* -rwxr-xr-x 1 root root 387764 Jun 3 13:45 sh* -rws--x--x 1 root root 744500 Jun 3 13:46 slogin* -rwxr-xr-x 1 root root 14523 Jun 3 13:46 splain* -rws--x--x 1 root root 744500 Jun 3 13:46 ssh* -rwxr-xr-x 1 root root 570960 Jun 3 13:46 ssh-add* -rwxr-xr-x 1 root root 502952 Jun 3 13:46 ssh-agent* -rwxr-xr-x 1 root root 575740 Jun 3 13:46 ssh-keygen* -rwxr-xr-x 1 root root 383480 Jun 3 13:46 ssh-keyscan* -rwxr-xr-x 1 root root 39 Jun 3 13:46 ssh_europa* -rwxr-xr-x 1 root root 107252 Jun 4 10:14 strace* -rwxr-xr-x 1 root root 8323 Jun 4 10:14 strace-graph* -rwxr-xr-x 1 root root 158088 Jun 3 13:46 thumbnail* -rwxr-xr-x 1 root root 6312 Jun 3 13:46 tty* -rwxr-xr-x 1 root root 55904 Jun 4 11:46 useradd* -rwxr-xr-x 1 root root 585656 Jun 4 11:47 vi* -rwxr-xr-x 1 root root 6444 Jun 4 11:45 whoami* ./dev: total 8 drwxr-xr-x 2 root root 4096 Jun 4 12:16 ./ drwxr-xr-x 9 root root 4096 Jun 5 10:05 ../ crw-r--r-- 1 root root 1, 9 Jun 3 13:43 urandom ./etc: total 208 drwxr-xr-x 4 root root 4096 Jun 4 12:35 ./ drwxr-xr-x 9 root root 4096 Jun 5 10:05 ../ -rw------- 1 root root 0 Jun 4 11:46 .pwd.lock -rw-r--r-- 1 root root 653 Jun 3 13:46 group -rw-r--r-- 1 root root 242 Jun 4 11:33 host.conf -rw-r--r-- 1 root root 857 Jun 4 12:04 hosts -rw-r--r-- 1 root root 1050 Jun 4 11:29 ld.so.cache -rw-r--r-- 1 root root 304 Jun 4 11:28 ld.so.conf -rw-r--r-- 1 root root 235 Jun 4 11:27 ld.so.conf~ -rw-r--r-- 1 root root 88039 Jun 3 13:46 moduli -rw-r--r-- 1 root root 1342 Jun 4 11:34 nsswitch.conf drwxr-xr-x 2 root root 4096 Jun 4 12:02 pam.d/ -rw-r--r-- 1 root root 28 Jun 4 12:00 pam_smb.conf -rw-r--r-- 1 root root 2520 Jun 4 11:57 passwd -rw-r--r-- 1 root root 7228 Jun 3 13:48 profile -rw-r--r-- 1 root root 1339 Jun 4 11:33 protocols -rw-r--r-- 1 root root 274 Jun 4 11:44 resolv.conf drwxr-xr-x 2 root root 4096 Jun 3 13:43 security/ -rw-r----- 1 root root 1178 Jun 4 11:51 shadow -rw------- 1 root root 80 Jun 4 11:45 shadow- -rw-r----- 1 root root 1178 Jun 4 11:48 shadow.old -rw-r--r-- 1 root root 161 Jun 3 13:46 shells -rw-r--r-- 1 root root 1144 Jun 3 13:46 ssh_config -rw------- 1 root root 668 Jun 3 13:46 ssh_host_dsa_key -rw-r--r-- 1 root root 602 Jun 3 13:46 ssh_host_dsa_key.pub -rw------- 1 root root 527 Jun 3 13:46 ssh_host_key -rw-r--r-- 1 root root 331 Jun 3 13:46 ssh_host_key.pub -rw------- 1 root root 883 Jun 3 13:46 ssh_host_rsa_key -rw-r--r-- 1 root root 222 Jun 3 13:46 ssh_host_rsa_key.pub -rw-r--r-- 1 root root 2471 Jun 4 12:15 sshd_config ./etc/pam.d: total 24 drwxr-xr-x 2 root root 4096 Jun 4 12:02 ./ drwxr-xr-x 4 root root 4096 Jun 4 12:35 ../ lrwxrwxrwx 1 root root 4 Jun 4 12:02 other -> sshd -rw-r--r-- 1 root root 318 Jun 3 13:46 passwd -rw-r--r-- 1 root root 546 Jun 4 11:36 ssh -rw-r--r-- 1 root root 479 Jun 4 12:02 sshd -rw-r--r-- 1 root root 370 Jun 3 13:46 su ./etc/security: total 32 drwxr-xr-x 2 root root 4096 Jun 3 13:43 ./ drwxr-xr-x 4 root root 4096 Jun 4 12:35 ../ -rw-r--r-- 1 root root 1971 Jun 3 13:46 access.conf -rw-r--r-- 1 root root 184 Jun 3 13:46 chroot.conf -rw-r--r-- 1 root root 2145 Jun 3 13:46 group.conf -rw-r--r-- 1 root root 1356 Jun 3 13:46 limits.conf -rw-r--r-- 1 root root 2858 Jun 3 13:46 pam_env.conf -rw-r--r-- 1 root root 2154 Jun 3 13:46 time.conf ./lib: total 8316 drwxr-xr-x 3 root root 4096 Jun 4 12:13 ./ drwxr-xr-x 9 root root 4096 Jun 5 10:05 ../ -rw-r--r-- 1 root root 1024 Jun 4 11:51 cracklib_dict.hwm -rw-r--r-- 1 root root 214324 Jun 4 11:51 cracklib_dict.pwd -rw-r--r-- 1 root root 11360 Jun 4 11:51 cracklib_dict.pwi -rwxr-xr-x 1 root root 342427 Jun 3 13:46 ld-linux.so.2* -rwxr-xr-x 1 root root 4061504 Jun 3 13:46 libc.so.6* lrwxrwxrwx 1 root root 15 Jun 4 12:11 libcrack.so -> libcrack.so.2.7* lrwxrwxrwx 1 root root 15 Jun 4 12:11 libcrack.so.2 -> libcrack.so.2.7* -rwxr-xr-x 1 root root 33291 Jun 4 11:39 libcrack.so.2.7* -rwxr-xr-x 1 root root 60988 Jun 3 13:46 libcrypt.so.1* -rwxr-xr-x 1 root root 71846 Jun 3 13:46 libdl.so.2* -rwxr-xr-x 1 root root 27762 Jun 3 13:46 libhistory.so.4.0* lrwxrwxrwx 1 root root 17 Jun 4 12:12 libncurses.so.4 -> libncurses.so.4.2* -rwxr-xr-x 1 root root 503903 Jun 3 13:46 libncurses.so.4.2* lrwxrwxrwx 1 root root 17 Jun 4 12:12 libncurses.so.5 -> libncurses.so.5.0* -rwxr-xr-x 1 root root 549429 Jun 3 13:46 libncurses.so.5.0* -rwxr-xr-x 1 root root 369801 Jun 3 13:46 libnsl.so.1* -rwxr-xr-x 1 root root 142563 Jun 4 11:49 libnss_compat.so.1* -rwxr-xr-x 1 root root 215569 Jun 4 11:49 libnss_compat.so.2* -rwxr-xr-x 1 root root 61648 Jun 4 11:34 libnss_dns.so.1* -rwxr-xr-x 1 root root 63453 Jun 4 11:34 libnss_dns.so.2* -rwxr-xr-x 1 root root 63782 Jun 4 11:34 libnss_dns6.so.2* -rwxr-xr-x 1 root root 205715 Jun 3 13:46 libnss_files.so.1* -rwxr-xr-x 1 root root 235932 Jun 3 13:49 libnss_files.so.2* -rwxr-xr-x 1 root root 204383 Jun 4 11:33 libnss_nis.so.1* -rwxr-xr-x 1 root root 254023 Jun 4 11:33 libnss_nis.so.2* -rwxr-xr-x 1 root root 256465 Jun 4 11:33 libnss_nisplus.so.2* lrwxrwxrwx 1 root root 14 Jun 4 12:12 libpam.so.0 -> libpam.so.0.72* -rwxr-xr-x 1 root root 31449 Jun 3 13:46 libpam.so.0.72* lrwxrwxrwx 1 root root 19 Jun 4 12:12 libpam_misc.so.0 -> libpam_misc.so.0.72* -rwxr-xr-x 1 root root 8125 Jun 3 13:46 libpam_misc.so.0.72* lrwxrwxrwx 1 root root 15 Jun 4 12:12 libpamc.so.0 -> libpamc.so.0.72* -rwxr-xr-x 1 root root 10499 Jun 3 13:46 libpamc.so.0.72* -rwxr-xr-x 1 root root 176427 Jun 3 13:46 libreadline.so.4.0* -rwxr-xr-x 1 root root 44729 Jun 3 13:46 libutil.so.1* -rwxr-xr-x 1 root root 70254 Jun 3 13:46 libz.a* lrwxrwxrwx 1 root root 13 Jun 4 12:13 libz.so -> libz.so.1.1.3* lrwxrwxrwx 1 root root 13 Jun 4 12:13 libz.so.1 -> libz.so.1.1.3* -rwxr-xr-x 1 root root 63312 Jun 3 13:46 libz.so.1.1.3* drwxr-xr-x 2 root root 4096 Jun 4 12:00 security/ ./lib/security: total 668 drwxr-xr-x 2 root root 4096 Jun 4 12:00 ./ drwxr-xr-x 3 root root 4096 Jun 4 12:13 ../ -rwxr-xr-x 1 root root 10067 Jun 3 13:46 pam_access.so* -rwxr-xr-x 1 root root 8300 Jun 3 13:46 pam_chroot.so* -rwxr-xr-x 1 root root 14397 Jun 3 13:46 pam_cracklib.so* -rwxr-xr-x 1 root root 5082 Jun 3 13:46 pam_deny.so* -rwxr-xr-x 1 root root 13153 Jun 3 13:46 pam_env.so* -rwxr-xr-x 1 root root 13371 Jun 3 13:46 pam_filter.so* -rwxr-xr-x 1 root root 7957 Jun 3 13:46 pam_ftp.so* -rwxr-xr-x 1 root root 12771 Jun 3 13:46 pam_group.so* -rwxr-xr-x 1 root root 10174 Jun 3 13:46 pam_issue.so* -rwxr-xr-x 1 root root 9774 Jun 3 13:46 pam_lastlog.so* -rwxr-xr-x 1 root root 13591 Jun 3 13:46 pam_limits.so* -rwxr-xr-x 1 root root 11268 Jun 3 13:46 pam_listfile.so* -rwxr-xr-x 1 root root 11182 Jun 3 13:46 pam_mail.so* -rwxr-xr-x 1 root root 5923 Jun 3 13:46 pam_nologin.so* -rwxr-xr-x 1 root root 5460 Jun 3 13:46 pam_permit.so* -rwxr-xr-x 1 root root 18226 Jun 3 13:46 pam_pwcheck.so* -rwxr-xr-x 1 root root 12590 Jun 3 13:46 pam_rhosts_auth.so* -rwxr-xr-x 1 root root 5551 Jun 3 13:46 pam_rootok.so* -rwxr-xr-x 1 root root 7239 Jun 3 13:46 pam_securetty.so* -rwxr-xr-x 1 root root 6551 Jun 3 13:46 pam_shells.so* -rwxr-xr-x 1 root root 55925 Jun 4 12:00 pam_smb_auth.so* -rwxr-xr-x 1 root root 12678 Jun 3 13:46 pam_stress.so* -rwxr-xr-x 1 root root 11170 Jun 3 13:46 pam_tally.so* -rwxr-xr-x 1 root root 11124 Jun 3 13:46 pam_time.so* -rwxr-xr-x 1 root root 45703 Jun 3 13:46 pam_unix.so* -rwxr-xr-x 1 root root 45703 Jun 3 13:46 pam_unix2.so* -rwxr-xr-x 1 root root 45386 Jun 3 13:46 pam_unix_acct.so* -rwxr-xr-x 1 root root 45386 Jun 3 13:46 pam_unix_auth.so* -rwxr-xr-x 1 root root 45386 Jun 3 13:46 pam_unix_passwd.so* -rwxr-xr-x 1 root root 45386 Jun 3 13:46 pam_unix_session.so* -rwxr-xr-x 1 root root 9726 Jun 3 13:46 pam_userdb.so* -rwxr-xr-x 1 root root 6424 Jun 3 13:46 pam_warn.so* -rwxr-xr-x 1 root root 7460 Jun 3 13:46 pam_wheel.so* ./sbin: total 3132 drwxr-xr-x 2 root root 4096 Jun 4 12:35 ./ drwxr-xr-x 9 root root 4096 Jun 5 10:05 ../ -rwxr-xr-x 1 root root 178256 Jun 3 13:46 choptest* -rwxr-xr-x 1 root root 184032 Jun 3 13:46 cqtest* -rwxr-xr-x 1 root root 81096 Jun 3 13:46 dialtest* -rwxr-xr-x 1 root root 1142128 Jun 4 11:28 ldconfig* -rwxr-xr-x 1 root root 2868 Jun 3 13:46 lockname* -rwxr-xr-x 1 root root 3340 Jun 3 13:46 ondelay* -rwxr-xr-x 1 root root 376796 Jun 3 13:46 pagesend* -rwxr-xr-x 1 root root 13950 Jun 3 13:46 probemodem* -rwxr-xr-x 1 root root 9234 Jun 3 13:46 recvstats* -rwxr-xr-x 1 root root 64480 Jun 3 13:46 sftp-server* -rwxr-xr-x 1 root root 744412 Jun 3 13:46 sshd* -rwxr-xr-x 1 root root 30750 Jun 4 11:46 su* -rwxr-xr-x 1 root root 194632 Jun 3 13:46 tagtest* -rwxr-xr-x 1 root root 69892 Jun 3 13:46 tsitest* -rwxr-xr-x 1 root root 43792 Jun 3 13:46 typetest* ./tmp: total 8 drwxr-xr-x 2 root root 4096 Jun 4 12:32 ./ drwxr-xr-x 9 root root 4096 Jun 5 10:05 ../ ./usr: total 8 drwxr-xr-x 2 root root 4096 Jun 4 12:16 ./ drwxr-xr-x 9 root root 4096 Jun 5 10:05 ../ lrwxrwxrwx 1 root root 7 Jun 4 12:14 bin -> ../bin// lrwxrwxrwx 1 root root 7 Jun 4 11:33 lib -> ../lib// lrwxrwxrwx 1 root root 8 Jun 4 12:13 sbin -> ../sbin//
Chroot environment for Apache Introduction

The chroot utility is often used to jail a daemon in a restricted tree. You can use it to insulate services from one another, so that security issues in a software package do not jeopardize the whole server. When using the makejail script, setting up and updating the chrooted tree is much easier.

FIXME: Apache can also be chrooted using which is available in `libapache-mod-security` (for Apache 1.x) and `libapache2-mod-security` (for Apache 2.x).

Licensing

This document is copyright 2002 Alexandre Ratti. It has been dual-licensed and released under the GPL version 2 (GNU General Public License) the GNU-FDL 1.2 (GNU Free Documentation Licence) and is included in this manual with his explicit permission. (from the )

Installing the server

This procedure was tested on Debian GNU/Linux 3.0 (Woody) with makejail 0.0.4-1 (in Debian/testing).

Log in as root and create a new jail directory:

$ mkdir -p /var/chroot/apache

Create a new user and a new group. The chrooted Apache server will run as this user/group, which isn't used for anything else on the system. In this example, both user and group are called chrapach.

$ adduser --home /var/chroot/apache --shell /bin/false \ --no-create-home --system --group chrapach

FIXME: is a new user needed? (Apache already runs as the apache user)

Install Apache as usual on Debian: `apt-get install apache` Set up Apache (e.g. define your subdomains, etc.). In the /etc/apache/httpd.conf configuration file, set the Group and User options to `chrapach`. Restart Apache and make sure the server is working correctly. Now, stop the Apache daemon. Install makejail (available in Debian/testing for now). You should also install wget and lynx as they will be used by makejail to test the chrooted server: `apt-get install makejail wget lynx` Copy the sample configuration file for Apache to the /etc/makejail directory:
# cp /usr/share/doc/makejail/examples/apache.py /etc/makejail/
Edit /etc/makejail/apache.py. You need to change the chroot, users and groups options. To run this version of makejail, you can also add a packages option. See the . A sample is shown here:
chroot="/var/chroot/apache" testCommandsInsideJail=["/usr/sbin/apachectl start"] processNames=["apache"] testCommandsOutsideJail=["wget -r --spider http://localhost/", "lynx --source https://localhost/"] preserve=["/var/www", "/var/log/apache", "/dev/log"] users=["chrapach"] groups=["chrapach"] packages=["apache", "apache-common"] userFiles=["/etc/password", "/etc/shadow"] groupFiles=["/etc/group", "/etc/gshadow"] forceCopy=["/etc/hosts", "/etc/mime.types"]

FIXME: some options do not seem to work properly. For instance, /etc/shadow and /etc/gshadow are not copied, whereas /etc/password and /etc/group are fully copied instead of being filtered.

Create the chroot tree: `makejail /etc/makejail/apache.py` If /etc/password and /etc/group were fully copied, type:
$ grep chrapach /etc/passwd > /var/chroot/apache/etc/passwd $ grep chrapach /etc/group > /var/chroot/apache/etc/group
to replace them with filtered copies. Copy the Web site pages and the logs into the jail. These files are not copied automatically (see the preserve option in makejail's configuration file).
# cp -Rp /var/www /var/chroot/apache/var # cp -Rp /var/log/apache/*.log /var/chroot/apache/var/log/apache
Edit the startup script for the system logging daemon so that it also listen to the /var/chroot/apache/dev/log socket. In /etc/default/syslogd, replace: `SYSLOGD=""` with `SYSLOGD=" -a /var/chroot/apache/dev/log"` and restart the daemon (`/etc/init.d/sysklogd restart`). Edit the Apache startup script (/etc/init.d/apache). You might need to make some changes to the default startup script for it to run properly with a chrooted tree. Such as: set a new CHRDIR variable at the top of the file; edit the start, stop, reload, etc. sections; add a line to mount and unmount the /proc filesystem within the jail.
#! /bin/bash # # apache Start the apache HTTP server. # CHRDIR=/var/chroot/apache NAME=apache PATH=/bin:/usr/bin:/sbin:/usr/sbin DAEMON=/usr/sbin/apache SUEXEC=/usr/lib/apache/suexec PIDFILE=/var/run/$NAME.pid CONF=/etc/apache/httpd.conf APACHECTL=/usr/sbin/apachectl trap "" 1 export LANG=C export PATH test -f $DAEMON || exit 0 test -f $APACHECTL || exit 0 # ensure we don't leak environment vars into apachectl APACHECTL="env -i LANG=${LANG} PATH=${PATH} chroot $CHRDIR $APACHECTL" if egrep -q -i "^[[:space:]]*ServerType[[:space:]]+inet" $CONF then exit 0 fi case "$1" in start) echo -n "Starting web server: $NAME" mount -t proc proc /var/chroot/apache/proc start-stop-daemon --start --pidfile $PIDFILE --exec $DAEMON \ --chroot $CHRDIR ;; stop) echo -n "Stopping web server: $NAME" start-stop-daemon --stop --pidfile "$CHRDIR/$PIDFILE" --oknodo umount /var/chroot/apache/proc ;; reload) echo -n "Reloading $NAME configuration" start-stop-daemon --stop --pidfile "$CHRDIR/$PIDFILE" \ --signal USR1 --startas $DAEMON --chroot $CHRDIR ;; reload-modules) echo -n "Reloading $NAME modules" start-stop-daemon --stop --pidfile "$CHRDIR/$PIDFILE" --oknodo \ --retry 30 start-stop-daemon --start --pidfile $PIDFILE \ --exec $DAEMON --chroot $CHRDIR ;; restart) $0 reload-modules exit $? ;; force-reload) $0 reload-modules exit $? ;; *) echo "Usage: /etc/init.d/$NAME {start|stop|reload|reload-modules|force-reload|restart}" exit 1 ;; esac if [ $? == 0 ]; then echo . exit 0 else echo failed exit 1 fi

FIXME: should the first Apache process be run as another user than root (i.e. add --chuid chrapach:chrapach)? Cons: chrapach will need write access to the logs, which is awkward.

Replace in /etc/logrotate.d/apache `/var/log/apache/*.log` with `/var/chroot/apache/var/log/apache/*.log` Start Apache (/etc/init.d/apache start) and check what is it reported in the jail log (/var/chroot/apache/var/log/apache/error.log). If your setup is more complex, (e.g. if you also use PHP and MySQL), files will probably be missing. if some files are not copied automatically by makejail, you can list them in the forceCopy (to copy files directly) or packages (to copy full packages and their dependencies) option the /etc/makejail/apache.py configuration file.

Type `ps aux | grep apache` to make sure Apache is running. You should see something like:

root 180 0.0 1.1 2936 1436 ? S 04:03 0:00 /usr/sbin/apache chrapach 189 0.0 1.1 2960 1456 ? S 04:03 0:00 /usr/sbin/apache chrapach 190 0.0 1.1 2960 1456 ? S 04:03 0:00 /usr/sbin/apache chrapach 191 0.0 1.1 2960 1456 ? S 04:03 0:00 /usr/sbin/apache chrapach 192 0.0 1.1 2960 1456 ? S 04:03 0:00 /usr/sbin/apache chrapach 193 0.0 1.1 2960 1456 ? S 04:03 0:00 /usr/sbin/apache
Make sure the Apache processes are running chrooted by looking in the /proc filesystem: `ls -la /proc/process_number/root/.` where process_number is one of the PID numbers listed above (2nd column; 189 for instance). The entries for a restricted tree should be listed:
drwxr-sr-x 10 root staff 240 Dec 2 16:06 . drwxrwsr-x 4 root staff 72 Dec 2 08:07 .. drwxr-xr-x 2 root root 144 Dec 2 16:05 bin drwxr-xr-x 2 root root 120 Dec 3 04:03 dev drwxr-xr-x 5 root root 408 Dec 3 04:03 etc drwxr-xr-x 2 root root 800 Dec 2 16:06 lib dr-xr-xr-x 43 root root 0 Dec 3 05:03 proc drwxr-xr-x 2 root root 48 Dec 2 16:06 sbin drwxr-xr-x 6 root root 144 Dec 2 16:04 usr drwxr-xr-x 7 root root 168 Dec 2 16:06 var

To automate this test, you can type:`ls -la /proc/`cat /var/chroot/apache/var/run/apache.pid`/root/.`

FIXME: Add other tests that can be run to make sure the jail is closed?

The reason I like this is because setting up the jail is not very difficult and the server can be updated in just two lines:

apt-get update && apt-get install apache makejail /etc/makejail/apache.py
See also

If you are looking for more information you can consider these sources of information in which the information presented is based: , this program was written by Alain Tesio