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A Router With Just One Ethernet Port

By Silas Brown

If you want to connect multiple computers to a cable modem then you normally need a router with at least two network ports. One port talks to the cable modem, and the other port(s) talk to the rest of the network. The connections are multiplexed so that the cable modem sees only one machine, and the router can also act as a firewall.

Sometimes it's not feasible to obtain a machine with two Ethernet ports. This might be the case for example if you are working on a temporary and/or test setup and don't have the time or the resources to order extra hardware. My motivation for writing this article was Kapil Hari Paranjape's article "Debian on a Slug" in LG #138, in which he needed to use either two separate networks (Ethernet and wi-fi) or a network with an existing firewall/router in order to set up the NSLU2 (the "slug"), and I wanted to see if it's possible to do without that extra hardware. Especially if the slug is going to be a router/firewall, it seems a little excessive to require another one before you can set it up.

In this article I present some notes on how I constructed a router and firewall using just one Ethernet port on a Linux machine. It wasn't stable enough for long-term use, but it was adequate for temporary situations that call for sharing a cable modem between two or three machines.

Why One Ethernet Port Can Be Enough

A cable modem is basically a network bridge, repeating selected packets from your network to the provider's network and vice versa. It is usually a transparent learning bridge conforming to the IEEE 802.1d standard; it "learns" a number of your Ethernet addresses, and the ISP usually limits this number to one. If it sees additional Ethernet addresses then it will simply ignore them. That is why you usually need to switch the modem off and on again when moving it from one machine to another.

If the modem is going to ignore any machine except the first one it sees, then there is nothing in principle to stop that first machine from acting as a router for other machines even on the same Ethernet segment. For example, consider the following setup:

            4-port Unswitched Ethernet Hub
         port 1   port 2     port 3    port 4
           |        |          |         |
         Modem   Machine A  Machine B  Machine C

The hub simply repeats any traffic it receives, because it's unswitched. (In fact it's possible to make do with some simple wire connections; more on this later.) Therefore, the modem sees all traffic from all machines, and conversely all machines see the traffic from the modem. However, the modem will refuse to communicate with anything except the first machine it sees. Suppose that this is Machine A. If Machine B wants to send to the outside world, it first sends to Machine A (and the modem ignores this), then Machine A repeats it (and the modem takes it). Then the reply is addressed by the modem to Machine A (which Machine B will ignore unless its Ethernet interface is set to promiscuous mode) and Machine A can repeat it for Machine B. Note that no machine needs more than one ethernet port.

This approach is inefficient because everything has to be repeated twice on the same wire. Even though Ethernet is generally much faster than broadband, the repetitions can still reduce the speed because they always congest every transmission. In spite of this, the setup can still run at a reasonable speed, especially if your adapters are all 100Mbps or more.

Using A Simple Unpowered Hub

If you have a powered hub or switch then you can skip to the next section.

Some cheap devices called "ethernet splitters" are essentially passive hubs. Care should be taken though because sometimes other things are also sold as "ethernet splitters", such as devices to use the spare wires in an Ethernet cable for another connection, and that's not useful in this setting. If you do find (or even make) a simple unpowered passive hub or "ethernet splitter", you have to think about certain characteristics of Ethernet that can make this more complex than using a powered hub.

10-base-T wiring has different wires for transmit and receive. If several computers are connected using a hub, then what should happen is, if any computer sends data on its "transmit" line, then this data will be placed on the "receive" lines of all the other computers (or perhaps not all of them if it is a switched hub).

Some simple "ethernet splitters" merely connect all the "transmit" lines together and all the "receive" lines together, so none of the machines can actually exchange data unless one of them crosses its connection (receives on the shared "transmit" line and transmits on the shared "receive" line). Cable modems do normally cross their connections, so those "ethernet splitters" are intended to allow all the machines to communicate with the modem although not with each other. This is not very useful in the one-port router setup.

It would be more useful if all machines could communicate with the router rather than with the modem. This can be arranged by connecting the router to the hub with a cross-over cable (or cross-over adaptor), and using normal straight-through cable everywhere else. That way the router's "transmit" is connected to the "receive" of all the other machines and vice versa. It might be necessary to use a second cross-over cable to connect the modem to the hub, in order to cancel out the modem's own crossing-over (or, equivalently, everything except the router and the modem), but most modern cable modems will automatically adapt anyway; just make sure the router is the first machine to boot up.

Further problems can be caused by the polarity auto-detection that's done by Ethernet devices with auto-MDIX ports, and unfortunately there's no way to turn that off other than to use old hardware that doesn't have such auto-detection. (Many Ethernet cards have commands to turn off auto-negotiation of speed and duplex, but not polarity.) There should be no problems when connecting the first two devices to the hub (i.e. the router and the modem), but when the third device is added, if that third device has an auto-MDIX port then it may or may not guess the correct polarity depending on which device it sees first (remember they are opposites). You might have to repeatedly disconnect and reconnect the new device until it sees the router, and if that new device is an NSLU2 which you are trying to connect to a desktop router during the initial setup stage then you'll have to reboot it on every attempt.

To save the hassle of repeated connections (and possibly reboots) of the new device, you could try the following: After connecting only the router and the modem to the hub and verifying that the router can reach the outside world (i.e. the modem has learned its MAC address), disconnect the modem from the hub (but without powering it off) and connect only the new device to the hub. Wait until you can ping the new device, and then reconnect the modem (you may have to reconnect the modem more than once before it will work). This allows the device to negotiate polarity in a less confusing environment, while still allowing the modem to see the router first.

You may have to follow this proceduce if the router itself has an auto-MDIX port and the new device does not, because it's then possible that the router and the modem will have negotiated a polarity that won't work with the new device no matter how many times it's re-connected, so the only option is to disconnect the modem while the router detects the polarity of the new device.

If the new device does have auto-MDIX then in some cases it may help to make sure that the network is as quiet as possible when connecting it. This is because return traffic from the modem is likely to increase the probability of the new device guessing the wrong polarity.

Finally, you may experience problems with the cables themselves. If the hub has no power then it can't amplify the signal, so if the cables are somewhat too long, or if there are cables that are connected at the hub but not connected to anything at the other end (or the device they are connected to is switched off), then this can harm the signal too much and the network will stop working, so try to use shorter cables and don't put any cable on the hub until it's active. Also, beware of fiddly connectors: it took me many hours to track down a fault that was caused by one of the Ethernet plugs working intermittently because I had damaged it while connecting things.

Linux Commands For One-port Routing

Linux lets you run multiple IP addresses on the same interface, using "aliases". This is useful if you want the router to appear with a private 192.168 address for the local network, but with whatever address it is assigned for the ISP. After the router has DHCP-negotiated with the ISP using dhclient or equivalent, you can do this:

ifconfig eth0:1 netmask

replacing eth0 with whatever other interface you are using if necessary. The :1 can also be :2, :3 etc to add more IP addresses; you can have up to 256 different IP addresses on the same interface if you want.

Note: This article assumes that your upstream DHCP server does not give you an IP address that is within the 192.168 subnet. If it does (which may be the case if your outgoing connection is shared privately) then you could replace 192.168 with 172.16 throughout this article, because 172.16 is also reserved for private use. You may have difficulty performing initial set-up of an NSLU2 if you cannot control 192.168 however.

Once you have a local-network IP, you can then you can switch on NAT connection forwarding:

modprobe iptable_nat
iptables -t nat -A POSTROUTING -j MASQUERADE
echo 1 > /proc/sys/net/ipv4/ip_forward

It may also help to allow the local 192.168 network to access any services running on the router, if its firewall does not already allow this:

iptables -A INPUT -d -j ACCEPT

You may also wish to run a DHCP server for the local network, to save having to manually configure your other machines' IP addresses. This usually means installing a package like dhcp and putting something like the following into /etc/dhcpd.conf:

subnet netmask {
  option routers;
  option domain-name-servers;

and then run or restart dhcpd. When putting all this in the system startup scripts, check that the above commands run before dhcpd (in a default Debian installation it suffices to put them at the end of the start) section of /etc/init.d/networking).

It may also help to ensure that dhclient waits to be assigned an address by an outside DHCP server, not by the DHCP server running on the same machine (which may respond first, especially if the outside server goes down for a while). To do this, put


into dhclient.conf (usually in the /etc/dhcp3 directory). This is not necessary if you are setting up a one-off router manually and do not need it to work at system startup, because in that case the DHCP client will likely have obtained an outside address before you can type the commands.

Note that the above option domain-name-servers in dhcpd.conf will work only if the router is running a DNS cache such as pdnsd (available as a Debian package); if you don't want to run that extra server then you'll have to arrange for the upstream DNS server to be copied into dhcpd.conf.

Security Considerations

The above commands are reasonably secure by default. While it is relatively easy for someone on the outside Internet to send a packet to your router with a fake 192.168 source IP, they will not normally be able to set the destination to anything other than your router's public IP address, which means they will not be able to access any private servers that are open only on your router's 192.168 address (that's why the above INPUT rule uses -d to specify the destination IP address as being, rather than simply constraining the source IP). Also because of the natural constraint on the destination IP, they will not be able to access any of your computers other than the router (even if you have a hub that allows everything to see the modem traffic), nor will they be able to get your router to help forward their packets either to your network or to elsewhere.

However, there are some circumstances in which it is possible for an attacker to deliver packets into your network with a destination IP address other than that of your public IP. This can happen if the attacker takes over your ISP's equipment, or if your ISP's equipment allows source routing, or if the attacker breaks into the connection on your side of the ISP. If you wish, you can take extra steps to protect your private network from this kind of attack.

Such steps involve both protecting the router itself, and ensuring that it is not possible to place packets on the network that bypass the router.

The router itself can be protected by adding MAC-address rules so that it accepts packets only from the known MAC addresses of your network adapters and does not accept inappropriate packets from the modem. See the iptables(8) man page for details. Note that cable modems' MAC addresses have been known to change at power-cycling, so it's better to make a list of all your other MAC addresses. In order to get past this test, an attacker would either have to get packets onto your network by some means other than via the cable modem, or else break into the modem itself or (in some cases) the ISP's head node.

You could get the other machines on your network to also recognise unwanted packets by MAC address, but you may have some machines that cannot run firewalls, and if they can see the traffic from the cable modem before it gets to the router then you could have a problem if an attacker can fake destination IP addresses. The best workaround to this is probably to use a simple "ethernet splitter" hub (see the above section) which physically prevents traffic from bypassing the router even though the router has only one port.

Routing Packets Between The Private Machines

If the private network is such that all the machines can see the router but they cannot see each other, then, if you need them to be able to communicate with each other, you need to arrange for this to go through the router even though they're on the same subnet. This is not too difficult if you can manipulate their route tables by hand, but it's slightly more difficult with DHCP (and note that most DHCP client implementations don't support all the extra options). You could simply set up port-forwarding rules on the router and have the other machines explicitly connect to the router rather than to each other (more on port-forwarding below). A more transparent solution (but more complex to set up) is to arrange for the router to listen on many different IP addresses, each on a very small subnet (with a very narrow network mask) and to allow only one DHCP-allocable address on each of these subnets. At any rate, expect the network speed to be less than what it would be if you had a proper switch, because everything is being repeated.

Port Forwarding

One final thing you may wish to do is to forward ports, both to make allowances for running public servers such as a Web server, and to facilitate communication between the machines on the private network if they can't see each other (see previous section).

If the server is run on the router itself then this is simply a matter of ensuring the router's firewall allows incoming connections on that port, if it does not already do so. (When specifying that the server is for the private network only, remember that restricting the destination to is more secure than restricting the source for the reasons discussed in the security section above.)

The general way to do port forwarding with iptables (which is more lightweight than setting up some process to listen on the port and forward connections) is this:

iptables -t nat -A PREROUTING -p tcp -d $PUBLIC_ADDRESS --dport $PUBLIC_PORT -j DNAT --to $REAL_ADDRESS:$REAL_PORT

where PUBLIC_ADDRESS and PUBLIC_PORT are set to the public IP address of the router and the port you want the server to appear on, and REAL_ADDRESS and REAL_PORT are where the server is actually running on the local network. Note that this method can only make the server visible on one interface: either the router's public IP address, or the private address, or localhost if you use OUTPUT instead of PREROUTING, but not more than one of these at the same time. If you require more than one interface to forward to the same server then you'll have to set up a process to listen and forward connections, such as by connecting inetd to nc, or even by using ssh, which is rather overkill for a private network but it's probably the least difficult way to set up forwarding).

If you do use iptables to forward ports for the public interface then you need to set PUBLIC_ADDRESS to the IP that your ISP gives you. You will likely find that your distribution's /etc/dhcp3/ has a dhclient-exit-hooks script or dhclient-exit-hooks.d directory (see man dhclient-script) in which you can place commands such as:

if [ $reason = BOUND -o $reason = RENEW -o $reason = REBIND -o $reason = REBOOT ]; then

  # new IP will be placed in $new_ip_address
  # may need to flush the tables (e.g. changed IP)
  iptables -t nat -F PREROUTING
  iptables -t nat -F POSTROUTING

  # ... then add the rules here, using $new_ip_address

  # and finally re-add the masquerade rule
  # (because it would have been deleted by the above flush)
  iptables -t nat -A POSTROUTING -j MASQUERADE


You may also need to take some care when setting REAL_ADDRESS to make sure that it is always the same, either by configuring that machine manually (without DHCP) or by noting its MAC address and giving that a fixed-address in dhcpd.conf (see man page for details).

If you use the voice-over-IP application "Skype", it can sound better if you open a port for it in this way and thus avoid the need for your connections to be relayed. For best results open UDP as well as TCP (i.e. repeat the above forwarding setup twice, once as-is and once substituting -p udp for -p tcp in both commands), and tell Skype about the port in Tools/Options/Advanced, but don't tell Skype until after the port is open, because Skype may not save the setting if it does not appear to work at the time. If you use Skype from more than one machine then you can give each one a different public port.

Concluding Remarks

The above discussion of routing commands is probably more in-depth than you need for a temporary setup, but it is included because you might find that your setup is actually stable and you don't need a multi-port router after all. If you do need a multi-port router then you can use your one-port router settings as a reference when configuring the multi-port version.

In my trials with the NSLU2, a desktop, a laptop and a cable modem on a simple "Ethernet splitter" unpowered hub, when the NSLU2 was fully set up as a slug and was itself acting as the one-port router, it often lost connectivity, sometimes to the point of having to be rebooted (not just disconnected and reconnected from the network), if it processed large amounts of traffic for too long ("too long" being anything from seconds to minutes). Extreme throttling of the traffic and packet sizes did help to avoid this, but it made the connection worse than dial-up. However, when the desktop PC was acting as the one-port router on the same network, that problem did not occur. As is so often the case, "Your Mileage May Vary". But it may at least be useful to know that, if you're in a desperate situation, you can rig up a router/firewall on a machine with only one Ethernet port.

Talkback: Discuss this article with The Answer Gang


Silas Brown is a legally blind computer scientist based in Cambridge UK. He has been using heavily-customised versions of Debian Linux since 1999.

Copyright © 2007, Silas Brown. Released under the Open Publication License unless otherwise noted in the body of the article. Linux Gazette is not produced, sponsored, or endorsed by its prior host, SSC, Inc.

Published in Issue 140 of Linux Gazette, July 2007