Raspberry Pi + ser2net = Cheap NM16A (Serial Console Server)

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Yesterday I was talking with a customer who had installed a network device in their DC racks that wasn’t talking to the outside world anymore. It’s management is either by SSH or serial port. The SSH was part of the problem, so serial needed to be the solution.

A small, easy to install box to allow network connectivity to a serial port? This did seem like a job for the RPi. I grabbed the latest Debian Wheezy SD image from the RPi website, and the USB-serial adapter from my bag, and got to work.

Booting the RPi with the serial adapter installed Just Worked, like USB stuff is supposed to. It’s an FTDI-chipset adapter and it just comes up as /dev/ttyUSB0 in Linux.

To get access to the serial port remotely, I could have just installed minicom on the RPi and then SSH to a shell before running it, but I was interested in how this might scale to more serial ports. You can get the USB serial gizmos for £2 each on ebay if you hunt around, and a couple of 8 port powered hubs would run to perhaps £15 each. That makes an ugly but usable 16-port console server for under £100. If you are building a lab environment for Cisco CCNP SWITCH or CCIE study, then this is a pretty decent deal.

The alternatives are Cisco’s NM-16A and NM-32A modules, plus the special cables to connect them, plus the router to put them in, or the ancient Cisco 2509 (so old it doesn’t have 10BaseT ethernet), or other random ebay scrap. I currently have a Lantronix 8 port device, but it was made before Cisco completely dominated the network world, and everyone else took up their console pinout – that means making up special cables to use it in my rack, which is kind of a pain. NM-16A modules go for around £150-200 on ebay, and you still need a pair of £50 cables to connect all the ports, and a router to put the module in.

The RPi has the added bonus that it’s still a linux box – so if you want to have an NTP server, or TFTP server or DNS, or RADIUS, then it’ll do that for your lab network too!

ser2net is a small application that listens for incoming telnet connections and connects them to serial ports. You configure a TCP port per serial port, so that ‘telnet rpi-ip-address 2001’ goes to /dev/ttyUSB01. You can preset the speed and other settings of the serial port, and you can also change them on the fly using the control interface (on a different TCP port). It also understands RFC2217, an extension to the telnet protocol that allows a client to control serial port settings with special codes. You can also get software like Serial Port Redirector which makes the remote serial port available as a local one under Windows, complete with port control. ser2net compiles simply on RPi Debian, and a single line added to the ser2net.conf has you up and running with your new serial console server.

First, grab the Debian Wheezy SD image from the Raspberry Pi Foundation’s Download Page and write it to a fresh 2GB SD card as described on their website.

Boot the RPi from the card, while connected to a monitor and network – Linux is preconfigured to use DHCP to get an IP address, so you’ll need to know that somehow to get access to the system. It tells you at the end of the boot process. Once booted for the first time, you’ll get the configurator utility, which allows you to enable SSH access. That should be the last time you need the monitor.

Check what device name your serial adapter has:

And test by running the server:

With that running, you should be able to open another window, telnet to port 3001 on the RPi and get a welcome banner. If you have something connected to the serial port, you should be able to talk to it.

The final step is to make sure that the ser2net service starts when the RPi boots. Simply add the following line to the bottom of /etc/rc.local, just before the ‘exit 0’ line:

and it will be started automatically on boot.

You can add additional lines to /etc/ser2net.conf for multiple serial devices.

For an added bonus in a shared environment, you can log all output from the serial devices (while someone is connected). You get a file per session, with a timestamp for the start and finish, and the source IP. This is another couple of config lines in ser2net.conf

i.e. add tr=tr1 and timestamp to the end of end of each telnet line. Then create the /var/log/ser2net directory and you are off and running.

Now it’s time to figure out how to ‘package’ this into less of a mess. A 1U box with 16 serial ports in Cisco pinout and a simple IEC power connector would be very handy! I think it’s do-able for about £200.


Filed under Network, Projects & Hacking, Tech

9 Responses to Raspberry Pi + ser2net = Cheap NM16A (Serial Console Server)

  1. D. Gowda


    I just tried it on my Raspberry Pi 2 to connect to one of my Arm based amplifier (with USB CDC interface) and everything worked perfectly.

  2. Looks like a great idea, I was trying to peice everything together and your guide has helped a lot thank you.

  3. Pingback: Expanding the home lab.

  4. André L.

    Great tutorial the best I have found I was already using ser2net, but I discover new useful things.

  5. Vijay

    Hi ,

    I am also trying to connect to my respberry pi2 board. But It’s not working. can you able to help me?

    Following are my ser2net.conf.

    BANNER:banner1:this is ser2net TCP port \p device \d serial parms \s\r\n

    # Don’t do this by default
    2000:telnet:20:/dev/ttyUSB0:115200 8DATABITS NONE 1STOPBIT banner
    2001:telnet:20:/dev/ttyUSB1:115200 8DATABITS NONE 1STOPBIT banner

    Thanks advance.

  6. Rajeev

    I have a headless server, I need to reboot and watch console and also do some operations while I am away. So I started looking for console servers. They are all very expensive.
    With your solution, will I be able to pass mouse and keyboard strokes and monitor the console output on remote ssh/telnet client?
    Or do you know any cheap console server for just one or two servers?

    kindly advice.

    • hjadmin

      This is for serial consoles, so no mouse, just keyboard and text mode characters. If your servers are Linux, you can set them up to use a serial console in addition to the normal monitor (using getty).

      Some “real” servers with management processors (e.g. HP ILO and Dell DRAC) will let you access the BIOS during boot this way, too. This solution isn’t much use for Windows though.

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