Red Hat Linux 6.X as an Internet Gateway for a Home Network
Paul Ramsey <pramsey@refractions.net>
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
3.3.3.1 PPP Over Ethernet (PPPoE)
3.3.3.2 Stupid DHCP Tricks
3.3.3.3 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
http://www.coastnet.com/~pramsey/linux/homenet.html for the HTML
version and http://www.coastnet.com/~pramsey/linux/homenet.sgml 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
<http://www.coastnet.com/~pramsey/linux/w_hub.gif>.
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 straight-
through 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
<http://www.coastnet.com/~pramsey/linux/wo_hub.gif>.
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:0.0.0.0 Bcast:0.0.0.0 Mask:255.255.255.255
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
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:
192.168.1.0. 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:
NETWORKING=yes
FORWARD_IPV4=yes
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
values:
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:
DEVICE=eth1
IPADDR=192.168.1.1
ONBOOT=yes
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 192.168.1.0 netmask 255.255.255.0 {
range 192.168.1.2 192.168.1.60;
default-lease-time 86400;
max-lease-time 86400;
option routers 192.168.1.1;
option ip-forwarding off;
option broadcast-address 192.168.1.255;
option subnet-mask 255.255.255.0;
}
If you are going to set up your Linux box as a caching domain name
server, insert the following option:
option domain-name-servers 192.168.1.1;
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 192.168.1.1;
option netbios-dd-server 192.168.1.1;
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 255.255.255.255 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
With:
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 re-
boot 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 192.168.1.0 network except 192.168.1.0
(the network address), 192.168.1.255 (the broadcast address) or
192.168.1.1 (your Linux server). Never give two computers the same IP
address. Set the "Gateway" address to 192.168.1.1, 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 192.168.1.1. Let the DNS server be either
192.168.1.1 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; 127.0.0.1/32; }; 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 192.168.1.1. 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:
DEVICE=eth0
IPADDR=x.x.x.x
NETMASK=y.y.y.y
ONBOOT=yes
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 127.0.0.1. 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/network-
scripts/ifcfg-eth0and put the following in it:
DEVICE=eth0
BOOTPROTO=dhcp
ONBOOT=yes
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 www.yahoo.com 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.
3.3.3.1. 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)
3.3.3.2. 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:
HOSTNAME=
To read this:
HOSTNAME=your_isp_assigned_name
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.
3.3.3.3. 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:24.65.182.43 Bcast:24.65.182.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
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:192.168.1.1 Bcast:192.168.1.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
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:127.0.0.1 Mask:255.0.0.0
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
255.255.255.255 * 255.255.255.255 UH 0 0 0 eth1
192.168.1.0 * 255.255.255.0 U 0 0 0 eth1
24.65.182.0 * 255.255.255.0 U 0 0 0 eth0
127.0.0.0 * 255.0.0.0 U 0 0 0 lo
default 24.65.182.1 0.0.0.0 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 255.255.255.255
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!
ALL: ALL
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: 127.0.0.1
ALL: 192.168.1.
This tells the "TCP wrappers" that they can allow connections to all
services from the local device (127.0.0.1) 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 192.168.1.0/24 -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 pramsey@refractions.net.
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 (ngibbs@pacbell.net) sends good news, because most of these
instructions will work for you. There are some important changes to
note however:
1. A GATEWAY=xxx.xxx.xxx.xxx statement in /etc/sysconfig/network-
scripts/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
216.102.154.201 netmask 255.255.255.255 { } 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 255.255.255.255, 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.
