For the weather buoy project we need a way to take data in over our Ministation’s serial port and push it upstream over the network. As the ministation is pretty limited in flash and ram, I assumed a compiled (as opposed to scripted) solution would be best.

I think my ideal system would be store-and-forward, with the serial port as a “push,” something along the lines of:

  1. Open the serial port.
  2. Forever:
    1. Read an buffer the data.
    2. When you have one “line” of data, try to poke it out over a network interface.
    3. If you succeed, forget that line of data.
    4. If you fail, save it to try next time. When you reach some maximum amount of backlog, start dropping the older messages.

Where the other end of the conversation would be a network-to-disk server:

  1. Open a network port
  2. As data comes in, write it to a file.

It sounds like a simple brief, but I couldn’t find a canned solution which does the job.

One obvious choice is ser2net, though it’s actually designed the other way around. As a “server,” it listens on a network connection (many, actually), opening serial ports as necessary when a connection comes in.

I’m a bit rusty on Linux network programming, so as an interim, I thought I’d try a serial-to-syslog gateway, operating as above but pushing data to syslog (who can then handle the network forwarding).

It’s not a terrible solution, with a couple of caveats:

  • It only really works because our data is read-only, and it’s NMEA-like strings, so it’s ASCII, nicely delimited into lines. Syslog wouldn’t work with binary data, I don’t think.
  • The Ministation SDK runs busybox’s syslogd, which only supports UDP forwarding, not TCP. OK, so it would only provide one additional ACK of security, not true store and forward, but it’s would be a start.

On the other end of the connection, I’ll have a full-fat computer running rsyslogd which can handle a bit of store and forward to servers off in the cloud.

In any case, I wrote a ser2syslog. I won’t pretend it’s polished, but works for me. As per the README, I was originally going to build it from the framework of ser2net, but given ser2net is a mature, featureful product, and arranged backwards (as above), I stripped 95% of the ser2net code out. I believe (hope!) the copyright and attribution are correct.

Next up is testing it on a Ministation.


Hm. I just realized I could do the same thing very simply on the command line. Something like:

$ cat /dev/ttyS0 | logger -p -t “/dev/ttyS0”

And I could probably achieve what I need network-wise with netcat. Sigh….

Building custom Ubiquiti firmwares

As noted previously, I’m mucking about with a Ubiquiti Ministation2 as a networked SBC for a project. The Ministation2 is Linux-based, and Ubiquiti provide an SDK for rolling your own firmware. I wanted a mechanism for introducing my own customizations while also keeping the tarball downloads from Ubiquiti at arms length.

After a bit of iteration, I ended up with the a project I’ve posted at Github. It’s a kind of reverse-patch-and-overlay environment, and no, I’m not a shell and Makefile genius, so I accept there’s lots of room for improvement.

Basically, you check out the the Remix,

$ git clone git://


$ cd Ubiquiti-SDK-Remix
$ ./

This will download the SDK from Ubiquiti (the version is currently hardcoded in, expand it on top of itself, and patch the SDK’s files as appropriate. You end up with a hybrid of the original SDK with the Remix’s improvements.

The Remix won’t check that you have the relevant tools, so make sure you’ve installed the MIPS toolchain and other dependencies. Then:

$ make clean xs2

as before.

The remix currently includes just one new package — ser2net-2.7 (it builds, but totally untested on the Ministation), but it provides a good template for how to include other packages in apps/local/.

Custom firmware on the Ubiquiti Ministation

We’re using a Ubiquiti Ministation as wifi-enabled SBC for a weather station project. There’s a lot to like, and a few things lacking. It has a single 802.11b/g interface, a single ethernet port, and a single console/serial port. Like? Most everything. Dislike? The single serial port, as we want to use it as a serial-to-network gateway, which means sacrificing the console. It also runs a custom Linux installation, which we can hack it to our devious needs.

Instructions for rebuilding the system are out there, but pretty brief. Here’s my experiences.

Ubiquiti provide the SDK as a tarball, as well as a MIPS toolchain as a i386 deb. That’s fine with me as I’m running Ubuntu, though sadly I’m running 64-bit, so I built up a small VirtualBox machine for development. Not optimal, but nothing wrong with it. It’s noted here that you could also build the MIPS compiler with buildroot. your mileage may vary.


  1. Download the SDK and toolchain from here, selecting the Ministation2 as the model. As of right now, the current firmware is v3.6.1.
  2. Install the toolchain on a i386 Debian/Ubuntu system.
  3. You’ll also need a few other packages. I got this list from here, so I can’t verify it’s correctness.

    $ sudo apt-get install sharutils fakeroot zlib1g-dev patch lzma flex bison

    If a package is missing, you’ll notice pretty quickly when the compile blows up.

  4. Unpack the SDK someplace convenient:

    $ mkdir ministation
    $ cd ministation
    $ tar -xjvf ~/SDK.UBNT.v3.6.1.4873.tar.bz2
    $ cd SDK.UBNT.v3.6.1.4873

First, let’s see how the SDK works out of the box. Note, I already have my ministation up, running, and configured properly. So what happens when I “upgrade” my ministation with a filesystem I’ve built myself.

First, build it:

$ make xs2

And go get a refreshing drink.

The resulting package will end up in a directory with a name like rootfs/XS2.ar2316.v3.6.1.unknown.110707.1210 which gives the Ubiquiti model (XS2), the chip (ar2316), version number (v3.6.1) and a date/timestamp (110707.1210). The unkown is where Ubiquiti puts a patch number (like 4873 in this case), not sure why that doesn’t make it through.

Conveniently, they also symlink in rootfs/XS2.ar2316.v3.6.1.latest which points to the latest build.

In that directory are a number of intermediate results, as well as a few .bin packages:

$ ls -al
total 38976
drwxr-xr-x 2 aaron aaron 4096 2011-07-14 16:17 .
drwxr-xr-x 14 aaron aaron 4096 2011-07-14 16:22 ..
-rw-r–r– 1 aaron aaron 678439 2011-07-14 16:17 bzImage
-rwxr-xr-x 1 aaron aaron 2605056 2011-07-14 16:17 squashfs.bin
-rwxr-xr-x 1 aaron aaron 493554 2011-07-14 16:17 vmlinux.lzma
-rwxr-xr-x 1 aaron aaron 25055859 2011-07-14 16:17 vmlinux.notstripped
-rw-r–r– 1 aaron aaron 3099018 2011-07-14 16:17 XS2.ar2316.v3.6.1.unknown.110714.1614-8M.bin
-rw-r–r– 1 aaron aaron 3099018 2011-07-14 16:17 XS2.ar2316.v3.6.1.unknown.110714.1614.bin
-rw-r–r– 1 aaron aaron 4858567 2011-07-14 16:17 XS2.ar2316.v3.6.1.unknown.110714.1614.debug_binaries.notstripped.tgz

Comparing XS2.ar2316.v3.6.1.unknown.110714.1614.bin to the “stock” Ubiquiti filesystem:

$ ls -l
total 9856
-rw-r–r– 1 amarburg amarburg 3244841 2011-07-19 10:50 MiniStation2-v3.6.1.build4866.bin

my version is a bit smaller. Hmm. Well, never mind.

Install the new version of the s/w through the the web interace. Luckily, the Ministation warns me I’m about to break things:

Well, let’s go. I monitored the process on the serial console (which is a separate topic).

Writing ‘FIS directory ‘ to /dev/mtd4(FIS directory ) … [[1300398.770000] Restarting system.
+Ethernet eth0: MAC address 00:15:6d:a4:14:ed
IP:, Gateway:
Default server:

RedBoot(tm) bootstrap and debug environment [ROMRAM]
Ubiquiti Networks certified release, version 0.9.00483.1103151313 – built 13:14:44, Mar 15 2011

Copyright (C) 2000, 2001, 2002, 2003, 2004 Red Hat, Inc.

Board: Ubiquiti Ubiquiti AR2315/6/7/8 based board (Ubiquiti PicoStation2 detected)

Arch: ar2316
RAM: 0x80000000-0x82000000, [0x80040b80-0x80fe1000] available
Flash: 0xbfc00000-0xbfff0000, in 64 blocks of 0x00010000 bytes each
== Executing boot script in 1.000 seconds – enter ^C to abort
RedBoot> cache off
RedBoot> fis load -d -e kernel
Trying LZMA decompression…
Image loaded from 0x80041000-0x801df23c
Entry point: 0x80196040, address range: 0x80042000-0x801bf000
RedBoot> go
[ 0.000000] CPU revision is: 00019064
[ 0.000000] Primary instruction cache 16kB, physically tagged, 4-way, linesize 16 bytes.
[ 0.000000] Primary data cache 16kB 4-way, linesize 16 bytes.
[ 0.000000] Linux version 2.4.27-ubnt0 (aaron@moa) (gcc version 3.3.3) #2 Tue Jul 19 10:55:22 NZST 2011

Much to my surprise, it worked. And, much to my surprise, my configuration has been maintained. Sweet.