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Using KVM to securely host servers in a DMZ

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We host a number of web services and applications on the servers here in the PaperCut office. We’ve always planned on hosting these on an isolated server inside a demilitarized zone (DMZ) to ensure public applications are isolated from internal servers. This usually requires separate dedicated servers, however with the recent growth in virtualization technology, we decided to see if we could accomplish the same in a virtual environment.

There was not a lot of information out there so I embarked on a project to develop our own. The solution has worked very well over the past 6 months so I’ve decided to open source the configuration and control script so others in the Linux community can benefit (one of my Friday projects when I’m not working on print accounting software!).

The crux of the script is to host a Qemu or KVM virtual machine on an independent subnet via a tun/tap interface. iptables on the host (Dom0) is used to ensure that connections can not be instigated from the VM in the DMZ to any system in the internal network. They say a picture is worth a thousand words, so here’s a diagram:

Network Setup - KVM running in a DMZ

The key items are:

  • The host (dom0) hosted the VM on a tun/tap interface .
  • The VM is on a separate subnet.
  • A firewall on dom0 (important) prevents access to the internal network.
  • A static route has been added to the router so internal network can “find” the systems in the DMZ.
  • Public ports (e.g. port 80) on the router are forwarded into the server in the DMZ.

This strategy will provide an extra layer of protection as a compromise on the server in the DMZ (say hosting your website) will not automatically mean a compromise on your internal network. There are however come caveats to this: It may be possible to “jailbreak” from the VM into the host by exploiting vulnerabilities in the hypervisor/host. For example, some exploits were found in QEMU in 2007 .

The control script and its brief setup procedure should work on most modern Linux distributions.

file: dmz-vm-controller

#!/bin/sh

BEGIN INIT INFO

Provides: vm-dmz-controller

Required-Start: $local_fs $network

Required-Stop: $local_fs $network

Default-Start: 2 3 4 5

Default-Stop: 0 1 6

Short-Description: VM Management in a DMZ

Description: QEMU/KVM VM Management in a semi-secured DMZ.

END INIT INFO

##############################################################################

 

VM-DMZ-Controller is a wrapper script written to help with the management

and setup of a VM running inside a secured demilitarized zone (DMZ). The

objective is to ensure the host/vm inside the DMZ are firewalled in a way

that ensures connections from the DMZ to the internal network are not

possible.

 

Brief summary:

 

1. Install QEMU or KVM, and socat, iptables and tun/tap tunctl

(uml-utilities).

 

2. Create non-privileged user on your system called “vm”.

 

3. Create a sub-directory in the VM user’s home directory to host your VM

files.

 

4. Create your disk images (e.g. qemu-img) in this sub-directory.

 

5. Copy this script into the directory and modify configuration section

below.

 

6. Link in this script into /etc/init.d/ and configure runlevels as

appropriate.

 

7. Add a static route in your internal network default router so internal

systems can connect to the VM.

 

8. Start your VM and test. Confirm that the VM is unable to access your

internal network.

 

See here for details:

https://www.papercut.com/blog/print_tips/tech_and_dev/using-kvm-to-securely-host-servers-in-a-dmz/

 

 

Copyright (c) 2008, PaperCut Software International Pty. Ltd.

https://www.papercut.com/

All rights reserved.

 

Redistribution and use in source and binary forms, with or without

modification, are permitted provided that the following conditions are met:

* Redistributions of source code must retain the above copyright

notice, this list of conditions and the following disclaimer.

* Redistributions in binary form must reproduce the above copyright

notice, this list of conditions and the following disclaimer in the

documentation and/or other materials provided with the distribution.

* Neither the name of the nor the

names of its contributors may be used to endorse or promote products

derived from this software without specific prior written permission.

 

THIS SOFTWARE IS PROVIDED BY PAPERCUT SOFTWARE ‘‘AS IS’’ AND ANY

EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED

WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE

DISCLAIMED. IN NO EVENT SHALL BE LIABLE FOR ANY

DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES

(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;

LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND

ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT

(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS

SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

 

###############################################################################

###############################################################################

VM Configuration - modify below as appropriate

###############################################################################

The name of the VM instance (should be unique if hosting multiple VMs)

 

VM_NAME=external-web-server

The non-privileged user ID used to run the VM.

 

VM_USER=vm

The VM kernel module to load (e.g. kvm-intel, kvm-amd, qemu). Leave blank if

using QEmu as a kernel model is required.

 

VM_MODULE=kvm-intel

The name of the virtual network tap to bind/host to the DMZ network on.

 

IFNAME=tap0

The .1 gateway address that denotes the DMZ subnet.

 

DMZ_IP=192.168.100.1

The subnet range of the internal network (the range to firewall/protect)

 

INTERNAL_SUBNET=192.168.1.0/24

Your DMZ system may need DNS access provided by your internal network.

Set this if required. This will leave a hole in the firewall allowing

DNZ access (UDP source port 53).

 

INTERNAL_DNS_IP=

The directory with disk images (and pid files, etc.) are hosted

 

VM_DIR=/home/${VM_USER}/${VM_NAME}

MONITORFILE=${VM_DIR}/.${VM_NAME}.monitor PIDFILE=${VM_DIR}/.${VM_NAME}.pid LOGFILE=${VM_DIR}/${VM_NAME}.log

VM Start command-line. No need to define:

-pidfile, -net, or -monitor

as these are all appended as part of this script.

Add -cdrom and -boot d to boot and install your VM off a CD.

 

VM_START_CMD=“kvm \ -hda disk1.qcow2 \ -m 384 \ -vnc :0”

The maximum time to provide the VM to conduct a graceful shutdown.

 

SHUTDOWN_TIMEOUT=20

###############################################################################

End Configuration - DO NOT MODIFY BELOW THIS LINE

###############################################################################

start_vm() {

echo\_n "Starting VM ${VM\_NAME}..."
if isrunning; then
    echo "ALREADY RUNNING"
    exit 0
fi

setup\_networking
start\_firewall

if \[ ! -z "${VM\_MODULE}" \]; then
    modprobe "${VM\_MODULE}"
fi
cd "${VM\_DIR}"
su "${VM\_USER}" -c "${VM\_START\_CMD} \\
            -net nic -net tap,ifname=${IFNAME},script=no \\

            -pidfile ${PIDFILE} \\
            -monitor unix:${MONITORFILE},server,nowait \\
            >> ${LOGFILE} 2>&1 &"

for i in 0 1 2 3; do
    sleep 2
    if isrunning; then
        echo "Started ${VM\_NAME} at: \`date\`" >> ${LOGFILE}
        echo "started."
        exit 0
    else
        echo\_n "."
    fi
done

echo "ERROR"
exit 1

}

stop_vm() {

echo\_n "Stopping VM ${VM\_NAME}..."
if isrunning; then
    # Send nice powerdown command
    echo "system\_powerdown" | socat - UNIX-CONNECT:${MONITORFILE} \\
            >/dev/null
    clean\_shutdown=
    for (( i = 0 ; i <= ${SHUTDOWN\_TIMEOUT} ; i++ )); do sleep 1 if isrunning; then echo\_n "." else clean\_shutdown=y break; fi done if \[ -z "${clean\_shutdown}" \]; then echo\_n "forcing..." kill -TERM "${pid}" sleep 2 fi if isrunning; then echo "problem stopping!" exit 1 fi rm ${MONITORFILE} rm ${PIDFILE} stop\_firewall stop\_networking fi echo "Stopped ${VM\_NAME} at: \`date\`" >> ${LOGFILE}
echo "stopped."

}

status() {

if isrunning; then
    echo "Running (pid: ${pid})."
else
    echo "Not Running."
fi

}

forcekill() {

if isrunning; then
    kill -9 "${pid}"
else
    echo "Not running!"
fi

}

isrunning() {

if \[ -r ${PIDFILE} \]; then
    pid=\`cat ${PIDFILE} 2>/dev/null\`
    if \[ ! -z "${pid}" -a -d /proc/${pid} \]; then
        return 0 #Success - running
    else
        return 1 #Failure - not running
    fi
else
    return 1 #Failure - not running
fi

}

setup_networking() {

tunctl -u ${VM\_USER} -t ${IFNAME} >/dev/null

ifconfig ${IFNAME} ${DMZ\_IP} netmask 255.255.255.0 up >/dev/null

}

start_firewall() {

modprobe ip\_tables
modprobe iptable\_nat

echo "1" > /proc/sys/net/ipv4/ip\_forward

#
# Deny new connections to internal network (forwarded) and Dom0 (input)
#
iptables -A FORWARD -d $INTERNAL\_SUBNET -i $IFNAME -p tcp --syn \\
        -m limit --limit 6/h --limit-burst 5 -j LOG

iptables -A FORWARD -d $INTERNAL\_SUBNET -i $IFNAME -p tcp --syn \\
        -j DROP

iptables -A INPUT -d $INTERNAL\_SUBNET -i $IFNAME -p tcp --syn \\
        -m limit --limit 6/h --limit-burst 5 -j LOG

iptables -A INPUT -d $INTERNAL\_SUBNET -i $IFNAME -p tcp --syn \\
        -j DROP

# Also need to protect the DMZ side of host box.
iptables -A INPUT -d $DMZ\_IP -i $IFNAME -p tcp --syn \\
        -m limit --limit 6/h --limit-burst 5 -j LOG

iptables -A INPUT -d $DMZ\_IP -i $IFNAME -p tcp --syn \\
        -j DROP

#
# Allow DNS UDP packets to DNS server (required if on internal network)
#
if \[ ! -z "${INTERNAL\_DNS\_IP}" \]; then
    iptables -A FORWARD -p udp -d $INTERNAL\_DNS\_IP \\
        --dport 53 -i $IFNAME -j ACCEPT
fi

#
# Deny UDP packets to internal network
#
iptables -A FORWARD -d $INTERNAL\_SUBNET -i $IFNAME -p udp \\
        -m limit --limit 6/h --limit-burst 5 -j LOG

iptables -A FORWARD -d $INTERNAL\_SUBNET -i $IFNAME -p udp -j DROP

iptables -A INPUT -d $INTERNAL\_SUBNET -i $IFNAME -p udp \\
        -m limit --limit 6/h --limit-burst 5 -j LOG

iptables -A INPUT -d $INTERNAL\_SUBNET -i $IFNAME -p udp -j DROP

# Don't log Windows/Samba name broadcasts as they will occure often
iptables -A INPUT -d $DMZ\_IP -i $IFNAME -p udp --dport 137 -j DROP

iptables -A INPUT -d $DMZ\_IP -i $IFNAME -p udp \\
        -m limit --limit 6/h --limit-burst 5 -j LOG

iptables -A INPUT -d $DMZ\_IP -i $IFNAME -p udp -j DROP

#
# Deny selected ICMP to internal network
#
iptables -A FORWARD -d $INTERNAL\_SUBNET -i $IFNAME -p icmp \\
        --icmp-type echo-request -j DROP
iptables -A FORWARD -d $INTERNAL\_SUBNET -i $IFNAME -p icmp \\
        --icmp-type redirect -j DROP
iptables -A FORWARD -d $INTERNAL\_SUBNET -i $IFNAME -p icmp \\
        --icmp-type router-advertisement -j DROP

iptables -A INPUT  -d $INTERNAL\_SUBNET -i $IFNAME -p icmp \\
        --icmp-type echo-request -j DROP
iptables -A INPUT -d $INTERNAL\_SUBNET -i $IFNAME -p icmp \\
        --icmp-type redirect -j DROP
iptables -A INPUT -d $INTERNAL\_SUBNET -i $IFNAME -p icmp \\
        --icmp-type router-advertisement -j DROP

iptables -A INPUT  -d $DMZ\_IP -i $IFNAME -p icmp \\
        --icmp-type echo-request -j DROP
iptables -A INPUT -d $DMZ\_IP -i $IFNAME -p icmp \\
        --icmp-type redirect -j DROP
iptables -A INPUT -d $DMZ\_IP -i $IFNAME -p icmp \\
        --icmp-type router-advertisement -j DROP

#
# Deny spoofed packets from DMZ
#
iptables -A INPUT -s ! ${DMZ\_IP}/24 -i $IFNAME -j DROP
iptables -A FORWARD -s ! ${DMZ\_IP}/24 -i $IFNAME -j DROP

}

stop_firewall() {

#
# Remove all rules added on the IFNAME interface
#
iptables -S | \\
        egrep "${IFNAME}" | \\
        egrep "^-A " | \\
        sed "s/-A //" | \\
        while read rulespec; do
            iptables -D ${rulespec}
        done

}

stop_networking() {

tunctl -d ${IFNAME} >/dev/null

}

Hack for POSIX echo -n support on all platforms

if [ “X`echo -n`” = “X-n” ]; then echo_n() { echo ${1+"$@"}"\c"; } else echo_n() { echo -n ${1+"$@"}; } fi

 

Begin Main

 

userid=`id | sed “s/^uid=\([0-9][0-9]*\).*$/\1/”` if test “${userid}” -ne 0; then echo “Error: You must be root to run this program” 1>&2 exit 1 fi

if [ -z `which iptables` ]; then echo “Error: Please install iptables.” 1>&2 exit 1 fi

if [ -z `which socat` ]; then echo “Error: Please install socat.” 1>&2 exit 1 fi

if [ -z `which tunctl` ]; then echo “Error: Please install tunctl.” 1>&2 exit 1 fi

case “${1}” in start) start_vm ;;

stop)
    stop\_vm
    ;;

forcekill)
    forcekill
    ;;

restart)
    stop\_vm
    sleep 1
    start\_vm
    ;;

stopfirewall)
    stop\_firewall
    ;;

startfirewall)
    start\_firewall
    ;;

status)
    status
    ;;

\*)
    echo "Usage: vm-dmz-controller start|stop|restart|status" >&2
    echo "Advanced Options: stopfirewall|startfirewall|forcekill" >&2
    exit 1
    ;;

esac

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Chris Dance
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Chris comes with many titles: world champion sailor, coffee snob, environmentalist and electric car fan. He’s also our co-founder and CEO soooo guess we better make this the best bio EVER.