Linux Network Administrator's Guide, 2nd Edition By Olaf Kirch & Terry Dawson 2nd Edition June 2000 1-56592-400-2, Order Number: 4002 506 pages, $34.95

Chapter 11
IP Masquerade and Network Address Translation

Contents:
Side Effects and Fringe Benefits
Configuring the Kernel for IP Masquerade
Configuring IP Masquerade
Handling Name Server Lookups
More About Network Address Translation

You don't have to have a good memory to remember a time when only large organizations could afford to have a number of computers networked together by a LAN. Today network technology has dropped so much in price that two things have happened. First, LANs are now commonplace, even in many household environments. Certainly many Linux users will have two or more computers connected by some Ethernet. Second, network resources, particularly IP addresses, are now a scarce resource and while they used to be free, they are now being bought and sold.

Most people with a LAN will probably also want an Internet connection that every computer on the LAN can use. The IP routing rules are quite strict in how they deal with this situation. Traditional solutions to this problem would have involved requesting an IP network address, perhaps a class C address for small sites, assigning each host on the LAN an address from this network and using a router to connect the LAN to the Internet.

In a commercialized Internet environment, this is quite an expensive proposition. First, you'd be required to pay for the network address that is assigned to you. Second, you'd probably have to pay your Internet Service Provider for the privilege of having a suitable route to your network put in place so that the rest of the Internet knows how to reach you. This might still be practical for companies, but domestic installations don't usually justify the cost.

Fortunately, Linux provides an answer to this dilemma. This answer involves a component of a group of advanced networking features called Network Address Translation (NAT). NAT describes the process of modifying the network addresses contained with datagram headers while they are in transit. This might sound odd at first, but we'll show that it is ideal for solving the problem we've just described and many have encountered. IP masquerade is the name given to one type of network address translation that allows all of the hosts on a private network to use the Internet at the price of a single IP address.

IP masquerading allows you to use a private (reserved) IP network address on your LAN and have your Linux-based router perform some clever, real-time translation of IP addresses and ports. When it receives a datagram from a computer on the LAN, it takes note of the type of datagram it is, "TCP," "UDP," "ICMP," etc., and modifies the datagram so that it looks like it was generated by the router machine itself (and remembers that it has done so). It then transmits the datagram onto the Internet with its single connected IP address. When the destination host receives this datagram, it believes the datagram has come from the routing host and sends any reply datagrams back to that address. When the Linux masquerade router receives a datagram from its Internet connection, it looks in its table of established masqueraded connections to see if this datagram actually belongs to a computer on the LAN, and if it does, it reverses the modification it did on the forward path and transmits the datagram to the LAN computer.

A simple example is illustrated in Figure 11.1.

Figure 11.1: A typical IP masquerade configuration

We have a small Ethernet network using one of the reserved network addresses. The network has a Linux-based masquerade router providing access to the Internet. One of the workstations on the network (192.168.1.3) wishes to establish a connection to the remote host 209.1.106.178 on port 8888. The workstation routes its datagram to the masquerade router, which identifies this connection request as requiring masquerade services. It accepts the datagram and allocates a port number to use (1035), substitutes its own IP address and port number for those of the originating host, and transmits the datagram to the destination host. The destination host believes it has received a connection request from the Linux masquerade host and generates a reply datagram. The masquerade host, upon receiving this datagram, finds the association in its masquerade table and reverses the substution it performed on the outgoing datagram. It then transmits the reply datagram to the originating host.

The local host believes it is speaking directly to the remote host. The remote host knows nothing about the local host at all and believes it has received a connection from the Linux masquerade host. The Linux masquerade host knows these two hosts are speaking to each other, and on what ports, and performs the address and port translations necessary to allow communication.

This might all seem a little confusing, and it can be, but it works and is really quite simple to configure. So don't worry if you don't understand all the details yet.

Side Effects and Fringe Benefits

The IP masquerade facility comes with its own set of side effects, some of which are useful and some of which might become bothersome.

None of the hosts on the supported network behind the masquerade router are ever directly seen; consequently, you need only one valid and routable IP address to allow all hosts to make network connections out onto the Internet. This has a downside; none of those hosts are visible from the Internet and you can't directly connect to them from the Internet; the only host visible on a masqueraded network is the masquerade machine itself. This is important when you consider services such as mail or FTP. It helps determine what services should be provided by the masquerade host and what services it should proxy or otherwise treat specially.

Second, because none of the masqueraded hosts are visible, they are relatively protected from attacks from outside; this could simplify or even remove the need for firewall configuration on the masquerade host. You shouldn't rely too heavily on this, though. Your whole network will be only as safe as your masquerade host, so you should use firewall to protect it if security is a concern.

Third, IP masquerade will have some impact on the performance of your networking. In typical configurations this will probably be barely measurable. If you have large numbers of active masquerade sessions, though, you may find that the processing required at the masquerade machine begins to impact your network throughput. IP masquerade must do a good deal of work for each datagram compared to the process of conventional routing. That 386SX16 machine you have been planning on using as a masquerade machine supporting a dial-up link to the Internet might be fine, but don't expect too much if you decide you want to use it as a router in your corporate network at Ethernet speeds.

Last, some network services just won't work through masquerade, or at least not without a lot of help. Typically, these are services that rely on incoming sessions to work, such as some types of Direct Communications Channels (DCC), features in IRC, or certain types of video and audio multicasting services. Some of these services have specially developed kernel modules to provide solutions for these, and we'll talk about those in a moment. For others, it is possible that you will find no support, so be aware,it won't be suitable in all situations.

Configuring the Kernel for IP Masquerade

To use the IP masquerade facility, your kernel must be compiled with masquerade support. You must select the following options when configuring a 2.2 series kernel:

Networking options  --->
        [*] Network firewalls
        [*] TCP/IP networking
        [*] IP: firewalling
        [*] IP: masquerading
        --- Protocol-specific masquerading support will be built as modules.
        [*] IP: ipautofw masq support
        [*] IP: ICMP masquerading

Note that some of the masquerade support is available only as a kernel module. This means that you must ensure that you "make modules" in addition to the usual "make zImage" when building your kernel.

The 2.4 series kernels no longer offer IP masquerade support as a kernel compile time option. Instead, you should select the network packet filtering option:

Networking options  --->
    [M] Network packet filtering (replaces ipchains)

In the 2.2 series kernels, a number of protocol-specific helper modules are created during kernel compilation. Some protocols begin with an outgoing request on one port, and then expect an incoming connection on another. Normally these cannot be masqueraded, as there is no way of associating the second connection with the first without peering inside the protocols themselves. The helper modules do just that; they actually look inside the datagrams and allow masquerading to work for supported protocols that otherwise would be impossible to masquerade. The supported protocols are:

Module	Protocol
ip_masq_ftp	FTP
ip_masq_irc	IRC
ip_masq_raudio	RealAudio
ip_masq_cuseeme	CU-See-Me
ip_masq_vdolive	For VDO Live
ip_masq_quake	IdSoftware's Quake

You must load these modules manually using the insmod command to implement them. Note that these modules cannot be loaded using the kerneld daemon. Each of the modules takes an argument specifying what ports it will listen on. For the RealAudio(TM) module you might use:[1]

# insmod ip_masq_raudio.o ports=7070,7071,7072           

The ports you need to specify depend on the protocol. An IP masquerade mini-HOWTO written by Ambrose Au explains more about the IP masquerade modules and how to configure them.[2]

[1] RealAudio is a trademark of the Progressive Networks Corporation.

[2] You can contact Ambrose at ambrose@writeme.com.

The netfilter package includes modules that perform similar functions. For example, to provide connection tracking of FTP sessions, you'd load and use the ip_conntrack_ftp and ip_nat_ftp.o modules.

Configuring IP Masquerade

If you've already read the firewall and accounting chapters, it probably comes as no surprise that the ipfwadm, ipchains, and iptables commands are used to configure the IP masquerade rules as well.

Masquerade rules are a special class of filtering rule. You can masquerade only datagrams that are received on one interface that will be routed to another interface. To configure a masquerade rule you construct a rule very similar to a firewall forwarding rule, but with special options that tell the kernel to masquerade the datagram. The ipfwadm command uses the -m option, ipchains uses -j MASQ, and iptables uses -j MASQUERADE to indicate that datagrams matching the rule specification should be masqueraded.

Let's look at an example. A computing science student at Groucho Marx University has a number of computers at home internetworked onto a small Ethernet-based local area network. She has chosen to use one of the reserved private Internet network addresses for her network. She shares her accomodation with other students, all of whom have an interest in using the Internet. Because student living conditions are very frugal, they cannot afford to use a permanent Internet connection, so instead they use a simple dial-up PPP Internet connection. They would all like to be able to share the connection to chat on IRC, surf the Web, and retrieve files by FTP directly to each of their computers -- IP masquerade is the answer.

The student first configures a Linux machine to support the dial-up link and to act as a router for the LAN. The IP address she is assigned when she dials up isn't important. She configures the Linux router with IP masquerade and uses one of the private network addresses for her LAN: 192.168.1.0. She ensures that each of the hosts on the LAN has a default route pointing at the Linux router.

The following ipfwadm commands are all that are required to make masquerading work in her configuration:

# ipfwadm -F -p deny
# ipfwadm -F -a accept -m -S 192.168.1.0/24 -D 0/0

or with ipchains:

# ipchains -P forward -j deny
# ipchains -A forward -s 192.168.1.0/24 -d 0/0 -j MASQ

or with iptables:

# iptables -t nat -P POSTROUTING DROP
# iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE 

Now whenever any of the LAN hosts try to connect to a service on a remote host, their datagrams will be automatically masqueraded by the Linux masquerade router. The first rule in each example prevents the Linux machine from routing any other datagrams and also adds some security.

To list the masquerade rules you have created, use the -l argument to the ipfwadm command, as we described in earlier while discussing firewalls.

To list the rule we created earlier we use:

# ipfwadm -F -l -e

which should display something like:

# ipfwadm -F -l -e
IP firewall forward rules, default policy: accept
 pkts bytes type  prot opt  tosa tosx ifname  ifaddress  ...
    0     0 acc/m all  ---- 0xFF 0x00 any     any        ...

The "/m" in the output indicates this is a masquerade rule.

To list the masquerade rules with the ipchains command, use the -L argument. If we list the rule we created earlier with ipchains, the output will look like:

# ipchains -L
Chain input (policy ACCEPT):
Chain forward (policy ACCEPT):
target     prot opt     source                destination           ports
MASQ       all  ------  192.168.1.0/24        anywhere              n/a

Chain output (policy ACCEPT):

Any rules with a target of MASQ are masquerade rules.

Finally, to list the rules using iptables you need to use:

# iptables -t nat -L
Chain PREROUTING (policy ACCEPT)
target     prot opt source               destination         

Chain POSTROUTING (policy DROP)
target     prot opt source               destination         
MASQUERADE  all  --  anywhere             anywhere           MASQUERADE 

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination         

Again, masquerade rules appear with a target of MASQUERADE.

Setting Timing Parameters for IP Masquerade

When each new connection is established, the IP masquerade software creates an association in memory between each of the hosts involved in the connection. You can view these associations at any time by looking at the /proc/net/ip_masquerade file. These associations will timeout after a period of inactivity, though.

You can set the timeout values using the ipfwadm command. The general syntax for this is:

ipfwadm -M -s <tcp> <tcpfin> <udp>

and for the ipchains command it is:

ipchains -M -S <tcp> <tcpfin> <udp>

The iptables implementation uses much longer default timers and does not allow you to set them.

Each of these values represents a timer used by the IP masquerade software and are in units of seconds. The following table summarizes the timers and their meanings:

Name	Description
tcp	TCP session timeout. How long a TCP connection may remain idle before the association for it is removed.
tcpfin	TCP timeout after FIN. How long an association will remain after a TCP connection has been disconnected.
udp	UDP session timeout. How long a UDP connection may remain idle before the association for it is removed.

Handling Name Server Lookups

Handling domain name server lookups from the hosts on the LAN with IP masquerading has always presented a problem. There are two ways of accomodating DNS in a masquerade environment. You can tell each of the hosts that they use the same DNS that the Linux router machine does, and let IP masquerade do its magic on their DNS requests. Alternatively, you can run a caching name server on the Linux machine and have each of the hosts on the LAN use the Linux machine as their DNS. Although a more aggressive action, this is probably the better option because it reduces the volume of DNS traffic travelling on the Internet link and will be marginally faster for most requests, since they'll be served from the cache. The downside to this configuration is that it is more complex. "Caching-only named Configuration" in Chapter 6 describes how to configure a caching name server.

More About Network Address Translation

The netfilter software is capable of many different types of Network Address Translation. IP Masquerade is one simple application of it.

It is possible, for example, to build NAT rules that translate only certain addresses or ranges of addresses and leave all others untouched, or to translate addresses into pools of addresses rather than just a single address, as masquerade does. You can in fact use the iptables command to generate NAT rules that map just about anything, with combinations of matches using any of the standard attributes, such as source address, destination address, protocol type, port number, etc.

Translating the Source Address of a datagram is referred to as "Source NAT," or SNAT, in the netfilter documentation. Translating the Destination Address of a datagram is known as "Destination NAT," or DNAT. Translating the TCP or UDP port is known by the term REDIRECT. SNAT, DNAT, and REDIRECT are targets that you may use with the iptables command to build more complex and sophisticated rules.

The topic of Network Address Translation and its uses warrants at least a whole chapter of its own.[3] Unfortunately we don't have the space in this book to cover it in any greater depth. You should read the IPTABLES-HOWTO for more information, if you're interested in discovering more about how you might use Network Address Translation.

[3] ... and perhaps even a whole book!

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