The goal of this article is to isolate a small public web server on a simulated demilitarized zone (DMZ) network, and to restrict the local network access in case the server is breached. It is an extra security layer added to an existing home server setup.

The DMZ consists of an internal network 10.10.20.0/24 connected to br0 bridge device. On this network I place a Linux namespaces security sandbox at 10.10.20.10, running a web server. In case an intruder gets control of the web server, he will be running with low privileges as a generic www-data user. The host firewall configuration will not allow him to open connections anywhere outside DMZ network.

To build the sandbox I use Firejail on a Debian 7 computer. For any other Linux distribution the setup steps are similar. All the commands specified below are executed as root.

Step 1: Install Firejail

The download page provides source code (./configure && make && sudo make install), deb (dpkg -i firejail.deb) and rpm (rpm -i firejail.rpm) packages. The project page also lists an Arch Linux package. The software has virtually no dependencies and it will work with any 3.x Linux kernel.

Step 2: Install nginx web server

Install nginx or any other web server you are familiar with. Stop the server and make sure it is not started by default at power up:

# apt-get install nginx # /etc/init.d/nginx stop Stopping nginx: nginx. # insserv -r nginx

On Debian and Ubuntu, nginx serves pages from /usr/share/nginx/www directory. The logs are stored in /var/log/nginx/.

Step 3: Configure 10.10.20.0/24 network

Create and configure the internal 10.10.20.0/24 network. The host interface br0 has the address 10.10.20.1:

# apt-get install bridge-utils # brctl addbr br0 # ifconfig br0 10.10.20.1/24

You should have in this moment the bridge interface up an running:

# ifconfig br0 br0 Link encap:Ethernet HWaddr e6:55:ca:1c:29:4a inet addr:10.10.20.1 Bcast:10.10.20.255 Mask:255.255.255.0 inet6 addr: fe80::e455:caff:fe1c:294a/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:0 errors:0 dropped:0 overruns:0 frame:0 TX packets:6 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:0 (0.0 B) TX bytes:468 (468.0 B)

Step 4: Host firewall configuration

Forward TCP port 80 on the host to TCP port 80 in sandbox, and drop all traffic originated on DMZ network. Also, enable routing on the host. My iptables script is as follows:

#!/bin/bash # netfilter cleanup iptables --flush iptables -t nat -F iptables -X iptables -Z iptables -P INPUT ACCEPT iptables -P OUTPUT ACCEPT iptables -P FORWARD ACCEPT # enable ipv4 forwarding echo "1" > /proc/sys/net/ipv4/ip_forward # forward host tcp port 80 to our sandbox iptables -t nat -A PREROUTING -p tcp --dport 80 -j DNAT --to 10.10.20.10:80 # drop any traffic originated on 10.10.20.0/24 network iptables -A FORWARD -i br0 -m state --state NEW,INVALID -j DROP iptables -A INPUT -i br0 -m state --state NEW,INVALID -j DROP

Step 5: Start the sandbox

Start the server using Firejail:

# firejail --private --seccomp --net=br0 --ip=10.10.20.10 \ "/etc/init.d/rsyslog start; \ /etc/init.d/nginx start; \ sleep inf" &

The command configures the bridge device and the IP address. It starts a syslog server and nginx inside the sandbox. Use sleep inf to keep the session open indefinitely.

Sandbox isolation:

Filesystem: The sandbox has isolated the filesystem by making all directories read-only. Option –private removes /root, /home and /tmp directories. Among other things, this also isolates X11 socket and prevents any kind of snooping on X11 sessions.

The sandbox has isolated the filesystem by making all directories read-only. Option removes /root, /home and /tmp directories. Among other things, this also isolates X11 socket and prevents any kind of snooping on X11 sessions. Process space: The only processes visible in the sandbox are the processes started in the sandbox (syslog and web server). This prevents attacks such as strace attack. You can get a list of the processes running in sandbox using firejail –list command:

# firejail --list 3867:root:firejail --private --seccomp --net=br0 --ip=10.10.20.10 /etc/init... 3868:root:bash -c /etc/init.d/rsyslog start; /etc/init.d/nginx st... 3885:root:/usr/sbin/rsyslogd -c5 3912:root:nginx: master process /usr/sbin/nginx 3913:www-data:nginx: worker process 3914:www-data:nginx: worker process 3916:www-data:nginx: worker process 3917:www-data:nginx: worker process 3915:root:sleep inf #

The only processes visible in the sandbox are the processes started in the sandbox (syslog and web server). This prevents attacks such as strace attack. You can get a list of the processes running in sandbox using command: Network stack: The sandbox uses a separate network stack, with different interfaces, its own routing table and firewall, and its own set of socket connections. The host firewall was set to forward TCP port 80 traffic to our sandbox, and to drop any connections originated on 10.10.20.0/24 network segment.

The sandbox uses a separate network stack, with different interfaces, its own routing table and firewall, and its own set of socket connections. The host firewall was set to forward TCP port 80 traffic to our sandbox, and to drop any connections originated on 10.10.20.0/24 network segment. Seccomp: Seccomp (alias for “secure computing”) is a filtering mechanism that allows processes to specify an arbitrary filter of system calls (expressed as a Berkeley Packet Filter program) that should be forbidden. Berkeley Packet Filter support for seccomp was introduced in Linux kernel 3.5. It greatly reduces kernel attack surface. The feature is enabled using –seccomp option.

Step 5: Monitoring the web server

Check the server using the log files in /var/log directory inside the sandbox. To reach them you would have to join the sandbox using firejail –join command:

# firejail --join=3868 [root@debian ~]$

Specify the PID number for one of the processes running in sandbox as –join argument. The option will only work on Linux kernels 3.8 or newer, and it is equivalent to a terminal login. If you are using an earlier kernel, add an ssh server to your sandbox:

# firejail --private --seccomp --net=br0 --ip=10.10.20.10 \ "/etc/init.d/rsyslog start; \ /etc/init.d/ssh start; \ /etc/init.d/nginx start; \ sleep inf" &

Conclusion

I start with a mainstream web server (nginx) running on one of the most popular web server platforms (Debian). This provides a secure baseline for my setup. I sandbox the server using Linux namespaces feature in Linux kernel, thus increasing the security of the setup.

This is a comparison of a regular server setup and a restricted server setup in a security sandbox:

Regular setup Security sandbox setup Filesystem Read-write Configurable, mostly read-only Process table Access to all running processes Access only to processes running in the sandbox Network Full local network access Controlled local network access

The same solid server security practices are required for both the regular and the sandbox case. An attacker gaining unauthorized root access is bad in both cases.

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