Unix Tip: The crossover LAN

First, you need a length of cat 5 or better cable. This generally gray cable contains four twisted copper wire pairs. The wire pairs are colored orange/orange and white, green/green and white, blue/blue and white and brown/brown and white. The frequency of the wire's twisting is critical to its resistance to electrical noise and crosstalk (signals leaking between the wires), so it's important that you straighten the wires as little as possible while working with it.

Next, you need a way to cut and strip the covering off the cable. There are special wire strippers that make this job easier, but you can manage with a pair of scissors if that's all you have on hand. Just be very careful to lightly bite the cable with the scissors and then turn them around by the handle until they have cut into the outer covering of cable just enough that you can pull it off. You want to end up with about a half inch of exposed wires, but start with more. You will trim the wires later in the process. Be very careful not to nick the covering of the wire pairs.

The third part is the more tedious. You need to rearrange the wires so that you end up with all eight wires lying flat beside in other in a particular order. On one end of the cable, fan the wires out in this order:

  1 green & white
  2 green
  3 orange & white
  4 blue
  5 blue & white
  6 orange
  7 brown & white
  8 brown

Then, grab the wires and try to line them up close to each other in the same order. I have found that moving my hands in a small "bicycle pedal" fashion seems to coax them out of their original orientation. After that, straighten and align the wires. Once they are reasonably cooperative, cut the ends off straight, leaving only half an inch of the previously twisted wires exposed. Be careful not to overdo the straightening of the wires in this process. Straighten only the exposed portion and don't tug too hard or you might straighten some length of the wires inside the cable covering as well.

Next, carefully glide the lined up wires into the RJ45 sleeve, holding the flat side of the cable toward you. You might have to tease the edge of the outer cover into the sleeve, but the wires should slide into their respective positions within the jack without too much objection.

No part of the individual wires should be exposed outside of the jack.

Then, gently slide the RJ45 jack into the appropriately shaped hole in your crimpers and squeeze. I have to squeeze very hard before my crimpers will open again and allow me to extract the completed connector but my crimpers were adopted from someone's trash pile so yours might work more easily.

Once that is done, do the same thing for the other end of the cable -- starting with stripping the out wire, fanning out the wires and so forth. When you are ready to align the wires, use the order listed in the "End 2" column below. This different wire arrangement is what makes the cable a crossover cable. Notice how we are switching the position of the 1-3 and 2-6 pairs:

  End 1 End 2
  ===== =====
  1 green & white 1 orange & white
  2 green 2 orange
  3 orange & white 3 green & white
  4 blue 4 blue
  5 blue & white 5 blue & white
  6 orange 6 green
  7 brown & white 7 brown & white
  8 brown 8 brown

When you're done with this, you have a crossover cable.

To connect two systems together, simply plug your new crossover cable into an available network adaptor on each of the systems and then issue some ifconfig commands to set up your network interface. In the commands below, I am connecting a Solaris system and a Linux box.

On the first (Solaris) system:

  # ifconfig hme1 plumb
  # ifconfig hme1 inet 192.168.11.10
  # ifconfig hme1 up

On the second (Linux) system:

  # ifconfig eth1 plumb
  # ifconfig eth1 inet 192.168.11.11
  # ifconfig eth1 up

At this point, you should see something like this in the ifconfig output on the first system and similar output (different IP address on the other).

  hme1: flags=1000843<UP,BROADCAST,RUNNING,MULTICAST,IPv4> mtu 1500 index 
  3
  inet 192.168.11.10 netmask ffffff00 broadcast 192.168.11.255
  ether 0:3:ba:4:be:11

You should also see a new route in your routing table on each end of the connection:

  # netstat -rn
  Routing Table: IPv4
  Destination Gateway Flags Ref Use Interface
  -------------------- -------------------- ----- ----- ------ ---------
  192.168.11.0 192.168.11.10 U 1 1 hme1

On the second system, your ifconfig output will look something like this:

  eth1 Link encap:Ethernet HWaddr 00:02:B3:CF:11:38
  inet addr:192.168.11.11 Bcast:192.168.11.255 Mask:255.255.255.0
  inet6 addr: fe80::202:beff:feff:1138/64 Scope:Link
  UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
  RX packets:0 errors:0 dropped:0 overruns:0 frame:0
  TX packets:43 errors:0 dropped:0 overruns:0 carrier:0
  collisions:0 txqueuelen:1000
  RX bytes:0 (0.0 b) TX bytes:6238 (6.0 KiB)

Your routing tables should show any entry more or less like this:

  # netstat -rn
  Kernel IP routing table
  Destination Gateway Genmask Flags MSS Window irtt Iface
  192.168.11.0 0.0.0.0 255.255.255.0 U 0 0 0 eth1

You can test your new network connection using ping or by logging in on each system from the other.