- Use the Source, Luke
Home | Register | News | Forums | Guide | MyLinks | Bookmark

Sponsored Links

Latest News
  General News
  Press Releases
  Off Topic

Back to files

Red Hat Linux 6.X as an Internet Gateway for a Home Network Paul Ramsey <>

June 22, 2000

A simple tutorial on configuring Red Hat 6 and related variants to operate as an internet gateway to a small home or office network. Top­ ics covered include masquerading, DNS, DHCP, and basic security.

Table of Contents

  1. Introduction

    1.1 Versions 1.2 Copyright

  2. Plugging Things In

    2.1 With a Hub 2.2 Without a Hub 2.3 With Only One Network Card

  3. Configuring Networking

    3.1 Configuring a Network Driver

    3.1.1 Two Identical Network Cards 3.2 Configuring the Inside Network

    3.2.1 The Network Device 3.2.2 The DHCP Server 3.2.3 The Client Computers 3.2.4 The DNS Server 3.2.5 Testing the Inside Network 3.3 Configuring the Outside Network

    3.3.1 With a Static IP 3.3.2 With DHCP 3.3.3 Quirks and Anomalies PPP Over Ethernet (PPPoE) Stupid DHCP Tricks Road Runner 3.3.4 Looking at the Network Entries 3.4 Security

  4. Configuring Masquerading
  5. Problems

    5.1 ICQ Does Not Work 5.2 I Have Caldera 2.X Not Red Hat 6.X 5.3 I Want One of My Internal Machines to be my Web Server

  1. Introduction

This page contains a simple cookbook for setting up Red Hat 6.X as an internet gateway for a home network or small office network. The instructions are very simplified: no special cases will be discussed, and some assumptions will be made about which network addresses are to be used. The most important assumptions are:

  • You have a fulltime Cable or ADSL connection to the Internet.
  • You can successfully install Red Hat 6.X on at least one of your computers. Note that these directions are also valid for Red Hat derivatives, such as Mandrake 6.X which is distributed by MacMillan Publishing under a variety of labels.
  • Your Linux computer has two network cards installed in it and both are compatible with Linux.
  • You have an ethernet hub if you are networking more than one computer or a cross-over cable if you are only networking one computer.
  • You know how to edit text files on your Linux machine.
  • You can log into your machine as root. You know how to install RPM packages from your Linux CDROM.

If you do not meet any of these assumptions, then this document probably isn't for you.

There is nothing special that you have to do during the installation process. Simply choose an installation which makes sense for you and go for it. This document gives directions on installing everything to do with networking from scratch, to avoid making any assumptions about what was installed or configured during installation. To ensure that things work and there is no confusion about what information goes where, all the configuration will be done by directly editing the system configuration files rather than using the GUI configuration tools provided with Red Hat. On the one hand, this might be a little harder than it has to be; on the other hand, your knowledge will be a good deal more transferable to different distributions and situations (like, what if X doesn't work, or you are setting up a headless server).

1.1. Versions

The latest version of this document should always be available at for the HTML version and for the SGML version.

  • December 21, 1999 : First version.
  • January 2, 2000 : Incorporated suggestions from John Mellor on outside networking quirks.
  • January 22, 2000 : Minor update about identical network cards and info on IP aliasing from Chris Lea.
  • March 16, 2000 : Some information on name server security and on supporting Caldera from Nelson Gibbs.
  • June 22, 1000 : Red Hat 6.2 configuration quirk documented. More PPPoE info from Kerr First.

1.2. Copyright

Copyright © 2000, Paul Ramsey.

This manual may be reproduced in whole or in part, without fee, subject to the following restrictions:

  • The copyright notice above and this permission notice must be preserved complete on all complete or partial copies.
  • Any translation or derived work must be approved by the author in writing before distribution.
  • If you distribute this work in part, instructions for obtaining the complete version of this manual must be included, and a means for obtaining a complete version provided.
  • Small portions may be reproduced as illustrations for reviews or quotes in other works without this permission notice if proper citation is given.

Exceptions to these rules may be granted for academic purposes: Write to the author and ask. These restrictions are here to protect us as authors, not to restrict you as learners and educators.

2. Plugging Things In

Depending on whether you are using a hub or not, your network topology will differ slightly. I am only covering networking with RJ45 cabling (the stuff that looks like phone cables on steroids) and not covering thin coax. With thin coax you can network multiple machines without requiring a hub, but have to be more careful about terminating connections and so on. If you know networking already, these instructions will be largely redundant.

2.1. With a Hub

If you have a hub, your network will look like this <>.

Connect the eth0 card on the Linux box to the cable modem or ADSL box using the cable supplied by the service provider during their install (or one you know works in that configuration. This is important because sometimes cable modems like to be connected with a crossover and sometimes they like a straight-through through cable: the one the company gives you is the one you want to use.

Connect the eth1 card on the Linux box to the hub with a straightthrough cable. Connect all your other computers to the hub with straight-through cables.

2.2. Without a Hub

If you do not have a hub, you can still connect one computer to your Linux box, using a crossover cable. Your topology will look like this <>.

Connect the eth0 card on the Linux box to the cable modem or ADSL box using the cable supplied by the service provider. Connect the eth1 card on the Linux box to the other computer with a crossover cable.

2.3. With Only One Network Card

This is not a recommended configuration (in this configuration your internal and external networks are on the same physical network, and are therefor theoretically more susceptible to cracking; in reality, the risk is probably very low), but it can be done. Your mileage may vary.

The Linux kernel includes support for "IP aliasing", which allows an ethernet card to service two different IP addresses simultaneously. The stock kernels shipped with Red Hat and Mandrake include support for IP aliasing by default. To set up your gateway with only one ethernet card, in all the subsequent code examples, simply replace eth1 with eth0:0.

In a single-card situation, running a DHCP server is not recommended.

Plug all your machines and your cable modem or ADSL box into the hub. Cross your fingers and continue.

3. Configuring Networking

OK, by now you have installed Linux on your gateway computer. You may have even configured one of your networking cards, and set up connectivity to the Internet. However, we are going to start from scratch and pretend that nothing is configured at all.

Log in as root. All the instructions given in this document assume you are logged in as root.

The Linux kernel refers to your two ethernet cards as eth0 and eth1, so that is how I'll be referring to them from now on too. The trouble is, which one is which? Here's a "simple" way of figuring out, guaranteed to work at least 50% of the time: lay your computer on the desk with the motherboard horizontal and the back panel facing you (as you would if you were going to open it and do some work on it). The leftmost card is eth0 -- you might want to label it with some masking tape. Now, write down on a piece of paper the make and model of both eth0 and eth1.

OK, let's see if eth0 and eth1 are recognized automatically by the kernel. Type ifconfig eth0 and ifconfig eth1. In both cases, if the kernel is recognizing your card, you should see something like this (bearing in mind that the numbers and whatnot will be different):

eth0 Link encap: Ethernet HWaddr 00:60:67:4A:02:0A

         inet addr:  Bcast:  Mask:
         RX packets:466 errors:0 dropped:0 overruns:0 frame:0
         TX packets:448 errors:0 dropped:0 overruns:0 carrier:0
         collisions:85 txqueuelen:100
         Interrupt:10 Base address:0xe400

If the kernel is not recognizing your network card you will see something like this:

eth0: error fetching interface information: Device not found.

3.1. Configuring a Network Driver

If both of your cards were found, skip to the next section. Otherwise, read this section.

OK, so one or both of your cards are not recognized by the kernel. This is not a problem, really. What we're going to have to do is tell the kernel more explicitly how to find your cards. There are lots of twists and turns here, and I'm not going to cover all of them. Remember, when the going gets tough, the tough turn to the Ethernet HOWTO. Here's some summary advice:

  • You have a PCI network card. You are probably sitting pretty, assuming it is not so new and cutting edge that no drivers exist. You can often find out a great deal about your network cards (and other things) by reading through /proc/pci and noting down makes and models.
  • You have an ISA network card. It is possible you will have to know the IO base address and the IRQ the card is operating on. You have manuals, right? Right? If not, this would be a good time to surf to the manufacturer's web site and see if they have any online references. Or if you have an old DOS configuration diskette, boot to DOS and see if there is a setup program which will read and set the address and IRQ.
  • You have an ISA Plug'n'Play card. You'll have to learn how to configure it first -- read the Plug'n'Play HOWTO. Fortunately, oncee you've configured your card you will know exactly what the IO base and IRQ are.

Now, since you know what the make and model of eth0 and eth1 are you can go to the compatibility page of the Ethernet HOWTO and look up your card. Take note of the recommended driver, and any information about special options your card may require. Write it down.

It's time to edit a configuration file! The file we will be editing is /etc/conf.modules. Open this file up in the text editor of your choice. Because there are so many possibilities and combinations of things which can go in this file, I'm going to give my own gateway as an example. I have a PCI 10/100Mb card based on the VIA Rhine chip, and a plain-jane 10Mb NE2000 ISA clone. I use the 100Mb card for the internal network and the 10Mb card for the external connection. My /etc/conf.modules file looks like this:

alias parport_lowlevel parport_pc
alias eth0 ne
options ne io=0x300 irq=10
alias eth1 via-rhine

My conf.modules file is laid out as follows:

  • The first line is there to configure my parallel port for printing. You probably have a similar line. Leave it alone.
  • The second line (alias eth0 ne) tells the kernel to use the ne driver for the eth0 device.
  • The third line (options ne io=0x300 irq=10) tells the ne driver at which io address and irq interrupt it will find the ISA card at. If you have ISA cards you will probably have to use this kind of directive, just replace the driver, io and irq directives with the correct information for your card.
  • The fourth line (alias eth1 via-rhine) tells the kernel to use the via-rhine driver for eth1. Because my eth1 card is a PCI card, I do not need to provide io or irq information: the PCI subsystem configures the device automatically.

You will want to ensure that you have alias entries in conf.modules for both your cards, and correct options lines for all your ISA cards. You may already have lines in conf.modules for any ethernet cards you configured during installation.
When you have finished editing conf.modules, try ifconfig eth0 and ifconfig eth1 again. You may have to apply some trial and error if you are messing with IO addresses and IRQs without a manufacturers manual.

3.1.1. Two Identical Network Cards

So, you were really really smart, bought two identical network cards for your Linux gateway, and now you cannot get them to work together? Do not worry, getting them to coexist is just a matter of using the correct syntax in /etc/conf.modules. For this example, the addresses and IRQ numbers are made up, and I will assume that you have bought a matched pair of NE2000 clones (a common choice). Your /etc/conf.modules file should look like this:

alias eth0 ne
alias eth1 ne
options ne io=0x330,0x360 irq=7,9

The addressing options are all given on the same line, and the first number for each addressing type is for eth0, the second number for eth1.

3.2. Configuring the Inside Network

The "inside network" is the network which all your home/office machines will talk on. The "outside network" is the big scary internet on the other side of the Linux box. By and large, the inside network will be completely insulated from the outside network by the Linux box, which will operate as a medium strength firewall.

3.2.1. The Network Device

Now that your drivers are working and you can see both eth0 and eth1 in ifconfig it is time to set up the internal home network. I am assuming that you are going to put your internal network on eth1 and your external device on eth0.

Your internal network is going to be a private network and will therefor be on a special network reserved for internal networking: This is a "private Class C network", in case you want to impress your friends.

First we need to make sure networking is turned on. Edit the file /etc/sysconfig/network and make sure the following lines exist:


The first line tells Linux that we want the network devices brought up at boot time. The second line tells Linux to enable IP forwarding. This is required when we start configuring masquerading in Section 4.

Redhat 6.2 Note: In order to properly support IP forwarding and masquerading, Red Hat 6.2 requires changes to the /etc/sysctl.conf file. Make sure the following lines exist and are set to the correct


net.ipv4.ip_forward = 1
net.ipv4.ip_always_defrag = 1

All the network interface settings for Red Hat and Red Hat derivatives are contained in files in the /etc/sysconfig/network-scripts directory. Enter that directory, and create a new file ifcfg-eth1. Put the following into the ifcfg-eth1 file:


This code tells the networking scripts to configure eth1 at boot time and to give it a particular IP address. Activate your network with the new settings with the following command: /etc/rc.d/init.d/network restart

3.2.2. The DHCP Server

A DHCP server will automatically configure devices on your internal home network with IP addresses. This is very useful for people with laptops: they can simply plug their machines in and be immediately properly configured. If you do not want a DHCP server on your internal network, just skip to the next section.

First you need to be sure you have the DHCP server installed. Mount your Linux CD and install the dhcp RPM. Now edit the /etc/dhcpd.conf file and put the following (and only the following) in it:

subnet netmask { range;
default-lease-time 86400;
max-lease-time 86400;
option routers;
option ip-forwarding off;
option broadcast-address; option subnet-mask;

If you are going to set up your Linux box as a caching domain name server, insert the following option:

option domain-name-servers;

If you know your outside DNS addresses and you are not going to use the Linux box for DNS, insert the following option, where x.x.x.x and y.y.y.y are IP numbers of the DNS servers:

option domain-name-servers x.x.x.x, y.y.y.y;

If you are going to run Samba file sharing on the Linux box for your Windows computers, add the following options to use the Linux box as the default WINS and browsing server:

option netbios-name-servers; option netbios-dd-server; option netbios-node-type 8;
option netbios-scope "";

Configuring Samba and WINS is well beyond the scope of this document. If you need some pointers, start with the SMB HOWTO and go on from there.

There are still a few more steps. Next, edit the /etc/rc.d/init.d/dhcpd file and look for the following line:

/sbin/route add -host dev eth1

Windows DHCP clients require a particular broadcast address in DHCP responses, and this command forces the Linux TCP/IP stack to produce it. If you cannot find that line in the file, add it. If you do find a line like that one, make sure that the device it references is eth1.

The next step is to alter the /etc/rc.d/init.d/dhcpd file to use eth1 as the default device. Replace the line:

daemon /usr/sbin/dhcpd


daemon /usr/sbin/dhcpd eth1

OK, now we are ready to start up DHCP. First start the DHCP server with the command: /etc/rc.d/init.d/dhcpd start.

Finally, we have to make sure that the DHCP server will start at reboot time. Some RPM packages of the DHCP server do not include directives to ensure the server starts every time, so we'll make sure it gets started by invoking the command chkconfig dhcpd on.

This command causes RedHat to add the dhcp startup script to the various runlevel directories under /etc/rc.d. In runlevels 3 and 5 (multiuser console and multiuser X) the DHCP server is started. In runlevels 0, 1 and 6 (shutdown, single user and reboot) the DHCP server is stopped.
3.2.3. The Client Computers

If you have set DHCP up, configuring your client computers is very easy: just enable DHCP configuration. For Windows computers, this involves opening the "Control Panel" and then the "Networking" option. Find the "TCP/IP" protocol and opt to "Configure" it. Check the box that says to "Configure TCP/IP address automatically", apply your changes, and reboot.

Before you reboot, you might want to type the following command: tail -f /var/log/messages. This will watch the Linux system log continuously. If all goes well, when you reboot your Windows computer, you will see it request an IP address and see the DHCP server respond. Control-C exits the tail -f command.

If you have not set up DHCP, configuration is still fairly easy. Again, open the "Networking" option from the "Control Panel", and choose to configure the TCP/IP protocol. You can assign your client computers any address in the network except (the network address), (the broadcast address) or (your Linux server). Never give two computers the same IP address. Set the "Gateway" address to, so that outgoing traffic is routed through your Linux gateway.

The IP Masquerading HOWTO has very detailed information on client configuration in the Configuration Section.

In general, to configure a client computer, either enable DHCP configuration, or manually assign it an address in the 192.168.1.X network with a gateway of Let the DNS server be either if you are running a caching DNS server (see below) or point the DNS at the addresses assigned by your network provider.

3.2.4. The DNS Server

Setting up your Linux box as a caching DNS server will (slightly) improve your netsurfing speed, because commonly used DNS addresses will get cached inside your network and not have to be retrieved from the outside.

If you are interesting in doing full blown DNS, there is a great deal of complexity to be learned. There is a DNS HOWTO available, and the book DNS and BIND is a good (and very comprehensive) paper reference.

In order for your client machines to take advantage of the caching server, they must be configured to use the Linux gateway as their primary DNS server. The DHCP directives given in section 3.2.2 are one way to accomplish this. If you are configuring your client computers by hand, you can change the DNS configurations in the same control tabs you used to set the IP address of the machine.

To install the DNS server, first install the bind RPM, then install the caching-nameserver RPM. At this point, you are almost ready.

As installed, the caching server will work fine, but if you know the IP addresses of the internet providers DNS servers you can improve performance slightly by editing the /etc/named.conf file and adding the following line after the directory line (where x.x.x.x and y.y.y.y are the primary and secondary DNS servers):

forwarders { x.x.x.x; y.y.y.y; };

This change makes your DNS server first query the ISPs DNS servers before traversing the internet in search of a given address. The ISPs servers often have a rich cache of DNS information and can provide a much faster answer than your server could.

The named daemon has had some security problems over the past 12 months, so it is very important that you have the latest version running, and make some changes to the default settings to enhance security.

  1. Check your version of bind and make sure it is at least 8.2.2. Go to the Red Hat Updates or Mandrake Updates sites to check for the latest version.
  2. Restrict access to your name server to just the local network by adding the line allow-query { 192.168.1/24;; }; to the /etc/named.conf file after the forwarders line.
  3. Avoid running your name server as root. If your server is running as root, an exploit of the server will grant the exploiter root privledges. If you run the server as a powerless user, like nobody, you can lower the risk of a name server exploit. To run your name server as nobody, edit the /etc/rc.d/init.d/named file and change the line daemon named to daemon named -u nobody -g nobody.

Make sure your DNS server will start at boot time: chkconfig named on. Again, this ensures that the server will start in the usual runlevels (3 and 5) at boot time.

OK, now you can start your DNS server: /etc/rc.d/init.d/named start

3.2.5. Testing the Inside Network

Until we configure the outside network, the DNS service will not work (since it has to communicate with other DNS servers on the internet), but we can test out the basic internal connectivity with the ping program.

On one of your client computers, open up a terminal (MSDOS) window, and type ping This will send out packets to your Linux computer at regular intervals, and your Linux computer will reflect the packets back. If things are working right, you should see a set of packet return times.

3.3. Configuring the Outside Network

Now we're ready to configure the outside network. Sometimes this will be difficult, depending on how well your internet provider supports Linux. If you have difficulty, there is an ADSL mini-HOWTO which covers ADSL issues in some detail. If I can find a Cable Modem HOWTO, I will link to it also.

The main problem with most outside connections is getting an IP address. Some internet providers hand out static IP addresses to cable or ADSL subscribers, and in that case configuration is easy. However, most providers have now moved to dynamic configuration via (you guessed it) DHCP. This means that your Linux computer will likely be a DHCP server on your eth1 interface, and a DHCP client on your eth0 interface.

Additionally, many providers have taken to providing their services in specialized non-standard ways which assume their customers will be using Windows. Some of those cases will be discussed at the end of section 3.3.2.

3.3.1. With a Static IP

If your internet provider has assigned you a static IP address, you are sitting pretty. First, create a new interface configuration file, /etc/sysconfig/network-scripts/ifcfg-eth0 and put the following in it:


Just fill in x.x.x.x and y.y.y.y with the values given by your internet provider. Now edit the /etc/resolv.conf file and enter the following information:

search provider_domain_here
nameserver n.n.n.n
nameserver m.m.m.m

The provider_domain should be supplied by your internet provider. Also enter the primary and secondary DNS servers in the n.n.n.n and m.m.m.m lines. If you have set up the Linux box as a DNS server, you can add a line before the other nameserver entries: nameserver This will make your Linux server use the caching server before asking the outside servers for DNS information.

3.3.2. With DHCP

If your internet provider uses DHCP configuration, you need to create a new interface configuration file, /etc/sysconfig/networkscripts /ifcfg-eth0and put the following in it:


Now make sure that the dhcpcd client daemon is installed on your system. Go to your Linux CD and install the dhcpcd RPM package.

It's time to test your new network configuration. Just use the command /etc/rc.d/init.d/network restart. Now test your outside connection with ping. Ping a computer on the internet, like and see if anything comes back.

3.3.3. Quirks and Anomalies

Your situation may differ from the very simple situations described above. Here are some short remarks on the various difficulties and links to more authoritative resources and addressing them. Thanks to John Mellor for supplying the links and impetus for adding this section. PPP Over Ethernet (PPPoE)

Several ADSL providers (Bell Atlantic, for example) are now insisting that their new customers connect to the service using the "PPP over Ethernet" protocol (PPPoE). To this end, they provide a Windows client program: not very useful for Linux users. Fortunately, PPPoE is a simple protocol and several efforts are underway to support it under Linux.

  • The Roaring Penguin PPPoE Client comes highly recommended by reader Kerr First.
  • PPPoE on Linux for Bell Sympatico
  • PPPoE on Linux for Sympatico (General Info) (Linux Info) Stupid DHCP Tricks

One of the favorite tricks network providers play is to tie your service to a unique hostname, or even a unique network interface card. This is presumably to keep you from plugging multiple computers into your ethernet port using a hub (of course, by using Linux and Masquerading you're getting the same effect with better security and the cable company has no way of knowing!).

If the provider has given you a hostname and insisted that you set your Windows box with that name in order you use their service, then you'll have to make sure that your Linux box sends in that hostname when requesting an address from the DHCP server.

The Red Hat DHCP client is called when you set the BOOTPROTO to dhcp in the interface configuration file, but it is called without reference to a hostname. To call the program with a hostname, in Red Hat 6.1, edit the /etc/sysconfig/network file, and change the line:


To read this:


This may not work in some of the Red Hat variants. If it does not work, check the /sbin/ifup script and see if the call to dhcpcd and pump include a -h $HOSTNAME switch. If they do not, add them, so the calls look like /sbin/dhcpcd -i $DEVICE -h $HOSTNAME and /sbin/pump -i $DEVICE -h $HOSTNAME. Road Runner

The Road Runner cable service has a special login process which must be run before the server can be used. Fortunately, a detailed Linux Road Runner HOWTO is available.

3.3.4. Looking at the Network Entries

Now you can admire your work. Type ifconfig to see all your configured devices. On my gateway computer, it looks like this:

eth0 Link encap:Ethernet HWaddr 00:60:67:4A:02:0A

        inet addr:  Bcast:  Mask:
        RX packets:487167 errors:0 dropped:0 overruns:0 frame:0
        TX packets:467064 errors:0 dropped:0 overruns:0 carrier:0
        collisions:89 txqueuelen:100
        Interrupt:10 Base address:0xe400
  eth1  Link encap:Ethernet  HWaddr 00:80:C8:D3:30:2C
        inet addr:  Bcast:  Mask:
        RX packets:284112 errors:0 dropped:0 overruns:0 frame:1
        TX packets:311533 errors:0 dropped:0 overruns:0 carrier:0
        collisions:37938 txqueuelen:100
        Interrupt:5 Base address:0xe800
  lo    Link encap:Local Loopback
        inet addr:  Mask:
        UP LOOPBACK RUNNING  MTU:3924  Metric:1
        RX packets:12598 errors:0 dropped:0 overruns:0 frame:0
        TX packets:12598 errors:0 dropped:0 overruns:0 carrier:0
        collisions:0 txqueuelen:0

Note that the eth0 interface has a fancy outside IP address, and the eth1 address has a private internal address.

You can look at the network routes by typing the route command. On my gateway computer it looks like this:

Kernel IP routing table

    Destination     Gateway      Genmask         Flags Metric Ref Use Iface *   UH    0      0     0 eth1     *     U     0      0     0 eth1     *     U     0      0     0 eth0       *         U     0      0     0 lo
    default         UG    0      0     0 eth0

Here we can see the outside network is set up, the inside network is set up, the local device is set up, the special broadcast address is set up, and the default route is set up to point to the internet providers gateway. Perfect!

Now you have the outside, and the inside. All the remains is to open the door between the two. First though, we have to make sure no monsters can get in from the outside.

3.4. Security

One of the drawbacks of being permanently connected to the internet via ADSL or cable is that your computer is exposed to potential security threats 24 hours a day, 7 days a week. Using Linux as a gateway reduces the risks, because it hides all your other computers: as far as the rest of the internet is concerned, only your Linux box is available for connections. This means that your network is only as secure as your Linux box, so at this point I'll give a few basic tips to make your box more secure.

First, you need to shut out all the bad guys. To do this, edit the file /etc/hosts.deny and make sure it looks just like this:

# hosts.deny This file describes the names of the hosts which are

  #             not allowed to use the local INET services, as decided
  #             by the '/usr/sbin/tcpd' server.
  #            The portmap line is redundant, but it is left to remind you that
  #        the new secure portmap uses hosts.deny and hosts.allow. In particular
  #             you should know that NFS uses portmap!


This tells the "TCP wrappers" -- which control 95% of incoming connections -- to deny all connections from all hosts. That's a pretty good rule! But, it will also keep you from connecting to your Linux box from inside your home network, which is annoying, so we will make one exception. Edit the file /etc/hosts.allow and make sure it looks just like this:

# hosts.allow This file describes the names of the hosts which are

  #              allowed to use the local INET services, as decided
  #              by the '/usr/sbin/tcpd' server.

ALL: 192.168.1.

This tells the "TCP wrappers" that they can allow connections to all services from the local device ( and from your home network (192.168.1.).

You have now locked the monsters outside, with a strong padlock. If you want to put up bars and alarm systems, you will have to be alot more sophisticated. The Security HOWTO is a good place to start if you want to learn more about securing your Linux box.

4. Configuring Masquerading

All right! The preliminaries are over, this is where the magic begins. IP masquerading is one of the truly magical services Linux provides. There are commercial products for Windows which do the same thing, but not nearly as efficiently: an ancient 386 can merrily provide IP masquerading services to a whole medium sized office, but cannot even run Windows 95, let alone the add on masquerading package. (As an addendum, I read in some recent reviews that Windows 2000 will support "connection sharing" without addon software. It looks like the companies which sold connection sharing software have been "embraced and extended" by MicroSoft. However, I wouldn't recommend you try the Windows 2000 solution on a 386.)

Linux has an extremely versatile firewalling capability, and we are going to be using it in the simplest and crudest possible manner. If you want to learn how to do firewalling like an expert, you should read both the Firewalling HOWTO for an understanding of the theory and the IPChains HOWTO for instructions on the new ipchains firewalling tool which ships with the Linux 2.2.X kernel (and by extension Red Hat 6.X). There is also now a very good IP Masquerading HOWTO available which has more details on masquerading tweaks.

Configuring simple masquerading is very very easy once your internal and external networking is operational. Edit the /etc/rc.d/rc.local file and add the following lines to the bottom:

# 1) Flush the rule tables.
/sbin/ipchains -F input
/sbin/ipchains -F forward
/sbin/ipchains -F output
# 2) Set the MASQ timings and allow packets in for DHCP configuration. /sbin/ipchains -M -S 7200 10 60
/sbin/ipchains -A input -j ACCEPT -i eth0 -s 0/0 68 -d 0/0 67 -p udp # 3) Deny all forwarding packets except those from local network. # Masquerage those.
/sbin/ipchains -P forward DENY
/sbin/ipchains -A forward -s -j MASQ # 4) Load forwarding modules for special services. /sbin/modprobe ip_masq_ftp
/sbin/modprobe ip_masq_raudio

The last two lines insert kernel modules which allow FTP and RealAudio to work for computers on the inside network. There are other modules for special services which you can tack on if you need them:

  • CUSeeMe (/sbin/modprobe ip_masq_cuseeme)
  • Internet Relay Chat (/sbin/modprobe ip_masq_irc)
  • Quake (/sbin/modprobe ip_masq_quake)
  • VDOLive (/sbin/modprobe ip_masq_vdolive)

Now you're ready to try masquerading! Run the rc.local script with the command /etc/rc.d/rc.local and you are ready to go! Sit down at one of your other computers and try some web surfing. With any luck, everything is now hunky dory.

5. Problems

There are lots and lots of things which can go wrong using a simple document like this, because there are plenty of special cases. The majority of possible problems adhere to the configuration of the internal and external network devices. I will try and respond to people with problems, figure out what went wrong and add links down here so that people with special case problems can track down help. Feel free to contact me at

5.1. ICQ Does Not Work

Some portions of ICQ work fine over masquerading. Other portions do not work well at all. There is a beta quality ICQ module under development, however, which addresses some (but not all) of the deficiencies of running ICQ over masquerading. The README file in the source code distribution describes how to compile the module. Once you have it compiled and installed, invoke /sbin/modprobe ip_masq_icq.

5.2. I Have Caldera 2.X Not Red Hat 6.X

Well, firstly congratulations for bucking the trend! Secondly, Nelson Gibbs ( sends good news, because most of these instructions will work for you. There are some important changes to note however:

  1. A statement in /etc/sysconfig/networkscripts /ifcfg-eth0 & eth1 for the interface (local interface uses remote interface IP address and remote interface uses service provider's gateway IP).
  2. Make sure /etc/sysconfig/daemons/dhcpd script lists ROUTE_DEVICE as eth1 not eth0.
  3. /etc/dhcpd.conf requires a subnet statement for both interfaces (I'm not entirely sure why as I made my second statement : subnet netmask { } with no other options and the DHCP server listens and sends on eth0 and eth1 as well as the fallback). The DHCP server errors out if only one subnet is listed.
  4. Do not add host route, the /etc/rc.d/init.d/dhcpd script Caldera uses already fixes the problem. DO make sure to change all references to eth0 in the script to eth1.

5.3. I Want One of My Internal Machines to be my Web Server

Piece of cake! However, you need to have a static IP address for this easy set of directions to work. If you have a dynamic IP address, you will need some additional scripting to ensure that your IP address gets updated in the port forwarding commands when the address changes.

Bear in mind, forwarding an external port to an inside machine makes your "internal" machine less "internal" than before, but it can be done very transparently and with little or no performance degredation. One of the side effects of the IP masquerading code in the Linux kernel is the ability to do some pretty funky stuff with packets as they hit the network layer, and the ipmasqadm utility is built to take advantage of that.

For some reason ipmasqadm is not shipped with all the Red Hat and Mandrake variants, so will probably have to retrieve it from the maintainer's web site -- there is an RPM available there as well as source code.

Once you have the RPM, install it, and then add the following lines to your /etc/rc.d/rc.local file:

/usr/sbin/ipmasqadm portfw -f
/usr/sbin/ipmasqadm portfw -a -P tcp -L x.x.x.x 80 -R 192.168.1.x 80

The first command flushes the port forwarding rules and the second command adds a forward from port 80 on the external interface to port 80 on the internal machine. Note that the external static IP address goes in the x.x.x.x space and the internal machine IP address goes in the 192.168.1.x space.

Now external requests for port 80 will be transparently sent to port 80 of the internal machine. Note that you cannot test this by telnetting or connecting to your gateway's port 80 from one of your inside machine: the port forwarder only honors requests coming in on the external interface.

Sponsored Links

Discussion Groups
  Networking / Security

About | FAQ | Privacy | Awards | Contact
Comments to the webmaster are welcome.
Copyright 2006 All rights reserved.