Xpra failing on localhost

If you know Xpra then you know you can access a "background" X session remotely. It's reasonable to assume that you may also want to access it locally from time to time.

On Ubuntu Precise I was finding that attempting to access the local session failed silently. I have been using:

$ xpra attach -z0 ssh:user@localhost:99

This produces no output. The server log isn't very forthcoming either:

New connection received
Handshake complete; enabling connection
encoding set to rgb24, client supports ['rgb24', 'jpeg', 'png'], server supports ['rgb24', 'jpeg', 'png']
Unhandled error while processing packet from peer
Traceback (most recent call last):
  File "/usr/lib/xpra/xpra/protocol.py", line 338, in _process_packet
    self._process_packet_cb(self, decoded)
  File "/usr/lib/xpra/xpra/server.py", line 1957, in process_packet
    self._packet_handlers[packet_type](self, proto, packet)
  File "/usr/lib/xpra/xpra/server.py", line 1371, in _process_hello
    f = open(mmap_file, "r+b")
IOError: [Errno 13] Permission denied: '/tmp/xpra.QGv7UA.mmap'
connection lost: empty marker in read queue
Connection lost

The clue is in the .mmap. The server needs to be started with the --no-mmap option.

From the man page:

       --no-mmap
              Disables memory mapped pixel data transfer.  By default it is normally  enabled  automatically
              if  the  server  and  the client reside on the same filesystem namespace.

All good!

Start the Android media scanner from the command line

If you're like me, and let's hope for your sake that you're not, you find it much easier to run an SSH server on your Android device than using something like adb or USB Mass Storage or any of the other crap out there.

With SSH I can simply and easily check logcat, netstat, issue commands, copy files and generally bugger about.

Sometimes you have moved quite a lot of files around and you need your media scanner to launch to update its database. This can be done via the CLI.

Dropbear SSH server on Android

It's safe to say that if you're reading this then you know why one would want an SSH server running on your Android device. Personally I have SSH running on all my Linux boxes because it makes life so much easier, and I'm sure I do not need to explain why. Having an SSH server start on boot is simply a lot more convenient!

There are a number of apps on the Android market but most of them are utterly useless with one exception: SSHDroid.

If you are likely to use SSH once in a while then it may be easier for you to just install SSHDroid on your device and start it when needed, and I have been happy with it for a while.

However:

1. It's an ugly app that sits in your notification tray once it has been started and uses up RAM and CPU. I want a background app.
2. SSHDroid refuses to run if it cannot access adverts unless you pay for it. I block all adverts because they get on my nerves.
3. All it does is drop a copy of Dropbear anyway, so you may as well do that yourself.
4. Dropbear will happily run in the background and uses tiny amounts of CPU when active, so you can imagine how little it does when idle.
5. If you can compile Dropbear yourself you can choose the options that you want.

As I'm sure you are aware there is no package delivery system for this kind of application in Android, but Android runs on a modified Linux kernel - this makes things so much easier! The result being that the following process is ridiculously easy assuming that your OS has:

• su or sudo (root)
• init.d support (or another way to run scripts at startup)

The only problem, really, is that the filesystem and methods used in Android aren't exactly standard compared to other *nix systems, but its not massively different. The result being that there are a number of ways that you can get things to work, so I'll simply show you what I have done. If I was doing this to a number of devices or in a corporate environment then I would use a bit more planning.