What can you do with a second Ethernet port?

By: Nathan Willis

Purchase a new PC or motherboard soon, and the chances are good that it will come with two built-in network interfaces -- either two Ethernet jacks or one Ethernet and one Wi-Fi. Tossing in a second adapter is an inexpensive way for the manufacturer to add another bullet point to the product description -- but what exactly are you supposed to do with it? If you are running Linux, you have several alternatives.

Plugging another Ethernet cable into the second jack and hoping for the best will accomplish nothing; you have to configure Linux's networking subsystem to recognize both adapters, and you must tell the OS how to use them to send and receive traffic. You can do the latter step in several different ways, which is where all the fun comes in.

The big distinction between your options lies in the effect each has on the other devices on your network (computers, routers, and other appliances) -- intelligently routing network traffic between them, linking them together transparently, and so on. In some cases, the simplest end result is not the easiest to set up, so it pays to read through all of the alternatives before you decide which to tackle.
Bonding

From your network's perspective, the simplest option is channel bonding or "port trunking" -- combining both of the computer's interfaces into a single interface that looks like nothing out of the ordinary to your applications.

A combined logical interface can provide load balancing and fault tolerance. The OS can alternate which interface it uses to send traffic, or it can gracefully fail over between them in the event of a problem. You can even use it to balance your traffic between multiple wide area network (WAN) connections, such as DSL and cable, or dialup and your next door neighbor's unsecured Wi-Fi.

To bond two Ethernet interfaces, you must have the bonding module compiled for your kernel (which on a modern distro is almost a certainty), and the ifenslave package (which is a standard utility, although you might need to install it from from your distro's RPM or APT repository).

On a typical two-port motherboard, the Ethernet adapters are named eth0 and eth1, so we will use that for our example commands. With ifenslave installed, take both Ethernet adapters offline by running sudo ifdown eth0 and sudo ifdown eth1. Load the bonding module into the Linux kernel with modprobe. There are two important options to pass to the module: mode and miimon. Mode establishes the type of bond (round-robin, failover, and so on), and miimon establishes how often (in milliseconds) the links will be checked for failure. sudo modprobe bonding mode=0 miimon=100 will set up a round-robin configuration in which network packets alternate between the Ethernet adapters as they are sent out. The miimon value of 100 is a standard place to begin; you can adjust if it you really want to tweak your network.

To create an actual bond (which for convenience we'll call bond0), run sudo ifconfig bond0 192.168.1.100 up to assign an IP address to the bond, then run ifenslave bond0 eth0 followed by ifenslave bond0 eth1 to tie the physical Ethernet interfaces into it.

Round-robin mode is good for general purpose load balancing between the adapters, and if one of them fails, the link will stay active via the other. The other six mode options provide features for different setups. Mode 1, active backup, uses just one adapter until it fails, then switches to the other. Mode 2, balance XOR, tries to balance traffic by splitting up outgoing packets between the adapters, using the same one for each specific destination when possible. Mode 3, broadcast, sends out all traffic on every interface. Mode 4, dynamic link aggregation, uses a complex algorithm to aggregate adapters by speed and other settings. Mode 5, adaptive transmit load balancing, redistributes outgoing traffic on the fly based on current conditions. Mode 6, adaptive load balancing, does the same thing, but attempts to redistribute incoming traffic as well by sending out ARP updates.

The latter, complex modes are probably unnecessary for home use. If you have a lot of network traffic you are looking to manage, consult the bonding driver documentation. For most folks, bonding's fault tolerance and failover is a bigger gain than any increased link speed. For example, bonding two WAN links gives you load balancing and fault tolerance between them, but it does not double your upstream throughput, since each connection (such as a Web page HTTP request) has to take one or the other route.
Bridging

The bonding solution is unique in that both network adapters act like a single adapter for the use of the same machine. The other solutions use the two adapters in a manner that provides a new or different service to the rest of your network.

Bridging, for example, links the two network adapters so that Ethernet frames flow freely between them, just as if they were connected on a simple hub. All of the traffic heard on one interface is passed through to the other.

You can set up a bridge so that the computer itself does not participate in the network at all, essentially transforming the computer into an overpriced Ethernet repeater. But more likely you will want to access the Internet as well as bridge traffic between the ports. That isn't complicated, either.

Bridging requires the bridge-utils package, a standard component of every modern Linux distribution that provides the command-line utility brctl.

To create a bridge between your network adapters, begin by taking both adapters offline with the ifdown command. In our example eth0/eth1 setup, run sudo ifdown eth0 and sudo ifdown eth1 from the command line.

Next, create the bridge with sudo brctl addbr bridge0. The addbr command creates a new "virtual" network adapter named bridge0. You then connect your real network adapters to the bridge with addif: sudo brctl addif bridge0 eth0 adds the first adapter, and sudo brctl addif bridge0 eth1 adds the second.

Once configured, you activate the bridge0 virtual adapter just as you would a normal, physical Ethernet card. You can assign it a static IP address with a command like sudo ifconfig bridge0 192.168.1.100 netmask 255.255.255.0, or tell it to retrieve its configuration via DHCP with sudo dhclient bridge0.

You can then attach as many computers, hub, switches, and other devices as you want through the machine's Ethernet port, and they will all be able to see and communicate with each other. On the downside, if you have a lot of traffic, your computer will spend some extra energy passing all of those Ethernet frames back and forth across the two adapters.
Firewalling and gateway-ing

As long as you have excess traffic zipping through your computer, the OS might as well look at it and do something useful, such as filter it based on destination address, or cache repeatedly requested Web pages. And indeed, you can place your dual-port computer between your upstream cable or DSL connection and the rest of your local network, to serve as a simple Internet-connection-sharing gateway, or as a firewall that exerts control over the packets passing between the network interfaces.

First, you will need to bring both network adapters up and assign each a different IP address -- and, importantly, IP addresses that are on different subnets. For example, sudo ifconfig eth0 192.168.1.100 netmask 255.255.255.0 and sudo ifconfig eth1 192.168.2.100 netmask 255.255.255.0. Note that eth0's address is within the 192.168.1.x range, while eth1's is within 192.168.2.x. Maintain this separation when you add other devices to your network and you will keep things running smoothly.

Forwarding the packets between the Internet on one adapter and your LAN on the other is the purview of iptables, a tool for configuring the Linux kernel's IP filtering subsystem. The command sudo iptables -A FORWARD --in-interface eth1 --out-interface eth0 --source 192.168.2.0/255.255.255.0 -m state --state NEW -j ACCEPT allows computers on the LAN interface eth1 to start new connections, and forwards them to the outside world via the eth0 interface. Following that with sudo iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT keeps subsequent packets from those connections flowing smoothly as well.

Next, sudo iptables -A POSTROUTING -t nat -j MASQUERADE activates Network Address Translation (NAT), secretly rewriting the IP addresses of traffic from the LAN so that when it goes out to the Internet, it appears to originate from the Linux box performing the routing. This is a necessary evil for most home Internet connections, both because it allows you to use the private 192.168.x.x IP address block, and because many ISPs frown upon traffic coming from multiple computers.

Finally, run sudo sh -c "echo 1 > /proc/sys/net/ipv4/ip_forward to activate the kernel's packet forwarding.

This setup will pass traffic from your LAN to your Internet connection, but it does not configure the network settings on the LAN computers themselves. Each of them needs an IP address, gateway and network information, and some working DNS server addresses. If your dual-adapter Linux box is serving as a NAT gateway, you could easily have it provide that information to the clients as well, using DHCP. Your distro probably comes with the dhcpd package. Configuring dhcpd is beyond the scope of the subject here, but check your distro's documentation for Internet connection sharing and you will likely find the instructions you need.

Once you are comfortable using iptables to set up basic NAT and packet forwarding, you can dig a little deeper and learn how to use your box as a first-rate firewall by writing rules that filter traffic based on source and destination address, port, and protocol.
Isolating

Finally, you can always configure your secondary network adapter to work in complete isolation from the rest of your LAN.

Sure, there is little gain to such a setup for general-purpose computers, but it is a popular choice for certain Ethernet-connected devices that only need to send data to one destination. Homebrew digital video recorder builders use the technique to connect the HDHomerun HDTV receiver directly to a MythTV back end, thereby isolating the bandwidth-hogging MPEG streams from the LAN. The same traffic separation idea might also come in handy for other single-purpose devices, such as a dedicated network-attached storage (NAS) box, a networked security camera, or your Ethernet-connected houseplant.

For most devices, isolating your second adapter entails setting up the computer to act as a DHCP server as in the gateway example above, but without worrying about NAT rules routing between the secondary client and the rest of the network.
Caveat emptoring

So which technique is right for you? My advice is to think about what network trouble you most need to prepare for. If your dual-adapter box is a server with heavy traffic to handle, or you need to balance your traffic across two WAN connections, bonding is for you. On the other hand, if you just bought an HDHomeRun to add to your MythTV back end, think about attaching it directly to the spare interface.

Bridging and gatewaying are most similar, in that they use the dual-adapter box to connect multiple other devices into a single network. If that is what you need to do, consider that bridging works at the Ethernet link level, well below IP and TCP in the protocol stack. At the Ethernet level, the only sort of traffic shaping you can do is that based on the hardware MAC address of the computer. You have significantly more control when you run a full-fledged NAT gateway.

But whichever option you choose, remember that messing around with your network configuration can get you disconnected in a hurry if you make a mistake. For that reason, all of the above examples use commands that change the "live" system, but don't alter the configuration files Linux reads in at startup. If you make a mistake, a reboot should bring you back to a known working state.

If you decide you want to make your changes permanent, your best bet is to consult your distro's documentation. Distros vary slightly in where and how they store network configuration scripts (Red Hat uses /etc/sysconfig/network-scripts/, for example, while Ubuntu uses /etc/network/).

One you start digging into the details, you'll find even more possibilities for utilizing that second network adapter under Linux. But you should now be armed with a general idea of how to make both adapters talk to your network at the same time -- and you can do your part to eliminate network adapter wastefulness.