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Setting up IP Masquerading

If you are using Linux as your host operating system and running VMware GSX Server virtual machines with host-only networking, IP masquerade gives you a convenient way to connect those virtual machines to the Internet. (IP masquerade also works with virtual machines that use bridged networking.)

To set up IP masquerading for virtual machines:

  1. Verify that your Linux kernel is version is 2.2.0 or higher using this command in a shell window:
    uname -a

  2. Verify that you have IP Chains installed using this command in a shell window:
    ls /sbin/ip*

    If you see a listing similar to
    /sbin/ipchains
    you have IP Chains installed. If you do not, please install it first.

  3. Verify that your host's kernel has IP forwarding enabled using this command in a shell window:
    cat /proc/sys/net/ipv4/ip_forward

    If it shows a value of 1, then it is enabled. If the value is 0, then it is not. To enable it without having to reboot your host, issue this command in a shell window:
    echo 1 > /proc/sys/net/ipv4/ip_forward

  4. Compile your Linux kernel to support certain options, based on the version of the kernel.
  5. Compile and install the modules.
  6. Configure two network interfaces — either two network interface cards (NICs) or one NIC and one modem.
  7. Configure TCP/IP settings in the guest operating system(s).

The network you set up looks like the following diagram.

Compiling the Linux kernel to support options

The following table lists the options you should enable when compiling your Linux kernel.

Compile option

Description

Linux 2.2.x

Linux 2.0.x

CONFIG_EXPERIMENTAL

Allows you to select experimental IP Masquerade code compiled into the kernel.

Required

Required

CONFIG_MODULES

Allows you to load ipmasq modules such as ip_masq_ftp.o.

Required

Required

CONFIG_NET

Provides networking support.

Required

Required

CONFIG_FIREWALL

Provides network firewall support.

Required

Required

CONFIG_INET

Provides TCP/IP networking.

Required

Required

CONFIG_IP_FORWARD

Provides IP forwarding and gateway capabilities.

Required

Required

CONFIG_IP_FIREWALL

Provides IP firewall support.

Required

Required

CONFIG_IP_MASQUERADE

Provides support for IP masquerading.

Required

Required

CONFIG_IP_MASQUERADE_IPPORTFW

Provides support for IP port forwarding.

Recommended

N/A

CONFIG_IP_MASQUERADE_IPAUTOFW

Provides support for automated IP forwarding during masquerading.

Optional

Required

CONFIG_IP_MASQUERADE_ICMP

Provides support for masquerading ICMP packets.

Recommended

Optional

CONFIG_IP_ALWAYS_DEFRAG

Provides support for defragmenting.

Highly recommended

Highly recommended

CONFIG_DUMMY

Provides support for the dummy net driver.

Recommended

Recommended

CONFIG_IP_MASQUERADE_MFW

Provides support for ip fwmark masq-forwarding.

Optional

N/A

Note: These are just the components you need for IP masquerade. You also need to select various options in other categories to support your specific setup.

Compiling and installing the modules

After compiling the kernel, you should compile and install the modules:
make modules; make modules_install

Then you should add a few lines to your /etc/rc.d/rc.local file (or another appropriate configuration file) to load the required modules from /lib/modules/2.x.x/ipv4/ automatically during each reboot:

.
.
/sbin/depmod -a
/sbin/modprobe ip_masq_ftp
/sbin/modprobe ip_masq_raudio
/sbin/modprobe ip_masq_irc

Reboot the Linux machine.

Configuring Your Host System

To configure your host system, you must perform these steps:

  1. Set up a generic Class C network for your virtual machines.
  2. On SuSE Linux 6.2, edit configuration files.
  3. Make appropriate firewall settings.
  4. Run ipchains.

Setting up a generic network for the virtual machines

For this example, if your virtual machines are configured for host-only networking, you can identify the network address range by looking in the file /etc/vmware/config and finding the line that starts with vmnet1.HostOnlyAddress=. In the following example, the number there is 172.16.16.1. You then need to add entries in the following format in your /etc/hosts file:

    127.0.0.1	  localhost	localhost.localdomain
    172.16.16.2	  win95		win95.linuxbogus.com 
    172.16.16.3	  winnt		winnt.linuxbogus.com
    172.16.16.4	  linux		linux.linuxbogus.com
    172.16.16.5	  bsd	    bsd.linuxbogus.com

If your virtual machines are configured for bridged networking: 

IP Address: 10.0.1.1/5
Subnet 255.255.255.0

You should follow this format when editing the /etc/hosts file: 

    127.0.0.1	localhost   localhost.localdomain
    10.0.1.1	gateway	    gateway.linuxbogus.com
    10.0.1.2	win95	    win95.linuxbogus.com
    10.0.1.3	winnt	    winnt.linuxbogus.com
    10.0.1.4	linux	    linux.linuxbogus.com
    10.0.1.5	bsd	        bsd.linuxbogus.com

Note: If you run a name service, you should not add these machines into your DNS files.

Editing configuration files on SuSE Linux 6.2

On SuSE Linux 6.2, you must edit the /etc/rc.config file to use firewall and IP masquerade. The relevant lines for the configuration are shown next:

Note: The backslash character (\) indicates that the line in the script should continue without a line break. Do not type the backslash character.

#
# Firewall settings - See /usr/doc/packages/firewall
# for a detailed description
#
FW_START="yes"
FW_LOCALNETS="192.168.2.0/24"
FW_FTPSERVER=""
FW_WWWSERVER=""
FW_SSLSERVER=""
FW_SSLPORT="443"
FW_MAILSERVER=""
FW_DNSSERVER=""
FW_NNTPSERVER=""
FW_NEWSFEED=""
FW_WORLD_DEV="tr0"
FW_INT_DEV="vmnet1"
FW_LOG_ACCEPT="no"
FW_LOG_DENY="yes"
FW_ROUTER="192.168.2.0/24"
FW_FRIENDS="no"
FW_INOUT="yes"
FW_SSH="no"
FW_TRANSPROXY_OUT=""
FW_TRANSPROXY_IN=""
FW_REDIRECT=""
FW_TCP_LOCKED_PORTS="1:1023"
FW_UDP_LOCKED_PORTS="1:1023"
#
# Masquerading settings - See /usr/doc/packages/firewall
# for a detailed description
#
MSQ_START="yes"
MSQ_NETWORKS="192.168.2.0/24"
MSQ_DEV="tr0"
MSQ_MODULES="ip_masq_cuseeme ip_masq_ftp ip_masq_irc ip_masq_quake \
ip_masq_raudio ip_masq_vdolive"

Making appropriate firewall settings

SuSE Linux 6.2 installs a firewall init script in the directory /sbin/init.d. To be sure that the init script is run automatically whenever your reboot your SuSE host, do the following:

  1. At the console or in a terminal window, list the firewall services using this command:
    ls -al /sbin/init.d/rc*.d/*firewall

    This command should return a list. As you may have noticed, the services listed in the command are for both run level 2 and run level 3 and are symbolic links to the /sbin/init.d/firewall init script.

  2. Reboot your host and watch for any errors.
  3. If the host boots without errors, run the following command as root to verify that the firewall started successfully:
    /sbin/init.d/firewall status

    Red Hat Linux 6.1 should install a firewall init script in the directory /etc/rc.d/init.d. If not, you may create one named firewall.rh.

    You must also create a new file on Red Hat Linux hosts to enable the host to use firewall and IP masquerade.

    To be sure that the init script is run automatically whenever you reboot your Red Hat Linux host:

  4. Add the following line to the /etc/rc.d/rc.local file:
    /etc/rc.d/init.d/firewall.rh start

  5. Reboot your host and watch for any errors.
  6. If the host boots without errors, run the following command as root to verify that the firewall started successfully:
    /etc/rc.d/init.d/firewall.rh status

Running ipchains

Next, you need to run ipchains. For 2.2.x kernels and host-only networking, the commands would look like this:

ipchains -P forward DENY
ipchains -A forward -s 172.16.16.2/24 -j MASQ

or

ipchains -P forward DENY
ipchains -A forward -s 172.16.16.2/255.255.255.0 -j MASQ

If you are using bridged networking, in a terminal window type:

ipchains -P forward DENY
ipchains -A forward -s 10.0.1.0/24 -j MASQ

or

ipchains -P forward DENY
ipchains -A forward -s 10.0.1.0/255.255.255.0 -j MASQ

This applies to the whole subnet. You can do it on a per machine basis if you prefer. For example, if you are using bridged networking, type:

ipchains -P forward DENY
ipchains -A forward -s 10.0.1.2/32 -j MASQ
ipchains -A forward -s 10.0.1.5/32 -j MASQ

For 2.0.x kernels and bridged networking, in a terminal window type:

ipfwadm -F -p deny
ipfwadm -F -a m -S 10.0.1.0/24 -D 0.0.0.0/0

Note: ipfwadm is broken in the 2.2.x kernels. You must update to ipchains.

Warning: Do not make masquerading your default policy. If you do, someone who can manipulate his or her routing can tunnel straight back through your gateway, using it to masquerade their identity!

Configuring Your Virtual Machines

Perform the network installation the same way you would if you were trying to connect to a real network.

Note: Be sure that vmnet0, vmnet-bridge and so on are loaded properly.

Windows 95 or Windows 98

  1. Select Control Panel > Network.
  2. If the TCP/IP protocol is not installed, install it.
  3. Select the TCP/IP protocol and click Properties.
  4. Click IP Address and set the IP addresses as follows:
  5. Click DNS and make the same entries as the original host uses. So if the DNS server on the host is 205.178.44.3, use the same IP address for the virtual machine.
  6. Leave all the other settings at their defaults.
  7. Restart the virtual machine.

The procedure was successful if you can ping the virtual machine.

Windows NT 4.0

  1. Select Control Panel > Network.
  2. If the TCP/IP protocol is not installed, install it.
  3. Select the TCP/IP protocol, then click Properties.
  4. Do not enable Automatic DHCP Configuration and do not add entries in the WINS Server section.
  5. Click IP Address and set the IP addresses as follows:
  6. Click DNS and make the same entries as the original host uses. So if the DNS server on the host is 205.178.44.3, use the same IP address for the virtual machine.
  7. Leave all the other settings at their defaults.
  8. Restart the virtual machine.

The procedure is successful if you can ping the virtual machine.

For more information, refer to the Linux IP Masquerade HOWTO ( http://www.linuxdoc.org/HOWTO/IP-Masquerade-HOWTO.html).

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