Android Tips


WARNING: This information comes with no warranty or guarantee expressed or implied. Unix does not protect you from doing something stupid and turning your fancy equipment into a smoldering brick. This page is more guidelines and advice than a canned recipe. If you don't understand what your doing, please read up first.

Android phones have become cheap and rather ubiquitous. Here's some info on making them bearable for old *nix hands.

Table of Contents

1 Android for Linux Users

Android is based on the Linux kernel, but its userland deviates significantly from typical conventions, the POSIX standard, and perhaps even good sense. Here's an outline of some various quirks:

  • Basic utilities are provided by BusyBox, which omits many features and GNU extensions to minimize size.
  • Android uses a nonstandard init. It runs the /init.rc file which has a nonstandard syntax. This process seems to handle things that are often done using an initrd. At some point, it executes scripts under /etc/init.d. You can hook into it with a script at /sd-ext/ (this is an ext2/3/4 partition on the sdcard), which will get called by Android's init scripts. This gets run by the /etc/init.d/20userinit script.
  • The filesystem hierarchy is rather bizarre. Most of a normal filesystem is under /system. The vfat partition on the sdcard is mounted at /mnt/sdcard with a symlink from /sdcard. An ext2/3/4 partition on the sdcard will be mounted at /sd-ext
  • The sdcard must have a vfat partition for Marketplace apps to use it (security be damned). You can add an ext2/3/4 partition as well. The extX needs to be the second partition. The /sd-ext gets setup by the /etc/init.d/05mountsd script.
  • There is no /etc/{passwd,shadow,group}. If have no idea where the OS is mapping uids to usernames.
  • Android uses unix users and groups to manage permissions. Individual applications downloaded through Android Marketplace run as separate users. Group membership is required for things such as sdcard or network access.

2 Debian Chroot


This is perhaps the laziest way to get a standard *nix setup running on an Android phone. We use a stock or (preferably) third-party Android OS for the kernel and to mange touchscreen, cell-phone capabilities, etc.; this means the phone still works as a ''phone'' without any (too much) fiddling around on our part. Then, we install a Debian filesystem on a subdirectory of the sdcard. This lets us chroot into the debian install and then have a pretty much typical Linux setup.

2.1 Basic Installation

  1. Get the Android SDK. You'll need this to access the device over USB in order to copy files to the internal flash and get a shell.
  2. Connect the Android device to your PC via usb.
  3. Root your phone, and (preferably) install cyanogenmod.
  4. Partition the sdcard with one vfat and one ext2/3/4 partition. This can be done through the Clockwork Recovery, or manually by mounting the card on your PC.
  5. Check that the partitions mount on android
    PC$ adb shell
    android# df
    tmpfs                   189796        32    189764   0% /dev
    tmpfs                   189796         0    189796   0% /mnt/asec
    tmpfs                   189796         0    189796   0% /mnt/obb
                            419028    149740    269288  36% /system
                            1340488     67316   1205076   5% /data
                            203070     40284    152301  21% /cache
                             20336     17232      3104  85% /devlog
    /dev/block/mmcblk1p2   3937268    693896   3043364  19% /sd-ext
    none                    189796         4    189792   0% /tmp
                           3754360    523660   3230700  14% /mnt/sdcard
                           3754360    523660   3230700  14% /mnt/secure/asec
    android# exit
  6. Get the debootstrap package. (If you're not on Debian or Ubuntu, this will be harder)
    PC$ sudo apt-get install debootstrap
  7. Mount the sdcard on the pc again and prepare the initial filesystem.
    PC# mkdir /media/sd-ext/debian
    PC# debootstrap --arch=armel --foreign squeeze /mnt/sd-ext
  8. Unmount the sdcard on the PC. You may need to also reboot the phone at this time.
  9. On the phone, setup /sd-ext/ to bind mount proc and tmp in the debian chroot
    # configure debian chroot
    if [ -d /sd-ext/debian/proc ] && [ ! -d /sd-ext/debian/proc/1 ] ; then
       mount -o bind /proc /sd-ext/debian/proc
    if [ -d /sd-ext/debian/dev/.udev ] ; then
       mount -o bind /dev /sd-ext/debian/dev
       mount -o bind /dev/pts /sd-ext/debian/dev/pts
    if [ -d /sd-ext/debian/mnt/sdcard/ ] && [ ! -d /sd-ext/debian/mnt/sdcard/Android ] ;
       mount -o bind /mnt/sdcard /sd-ext/debian/mnt/sdcard
  10. Run the to get the filesystems mounted
    android# /sd-ext/
  11. Finish the debootstrap process
    android# chroot /sd-ext/debian /debootstrap/debootstrap --second-stage
  12. Copy DNS servers
    android# cp /etc/resolv.conf /sd-ext/debian/etc/
  13. Chroot into the debian.
    android# chroot /sd-ext/debian /bin/bash
  14. Configure apt sources. Add the following to /etc/apt/sources.list
    deb squeeze main non-free
    deb squeeze/updates main contrib non-free
    deb squeeze-updates main
  15. apt-get update
    debian# apt-get update
  16. Fix Locales
    debian# apt-get install locales
    debian# dpkg-reconfigure locales
  17. Create a user account. Android-land doesn't seem to use UID 500, so let's give that to the debian user.
    debian# apt-get install sudo
    debian# adduser --uid 500 ntd
    debian# adduser ntd sudo
  18. Inform debian about Android-specific groups. We need this for our user account to use the sdcard and network. Put the following in /etc/groups
  19. Install ssh server
    debian# apt-get install openssh-server
  20. Add the following to /sd-ext/ (back outside the chroot) to start the debian sshd.
    # start debian ssh
    chroot /sd-ext/debian service ssh start
  21. That should do it. Reboot the phone and make sure you can ssh in and everything works. Now you can install all your favorite packages and never be without emacs, nethack, and a C compiler ever again!

2.2 Using Inetd

To conserve a bit of memory, you can run daemons from inetd. This isn't a huge improvement, since unix the daemons here are generally well written and don't consume gobbs and gobbs of memory like a typical Android app. But it should be a small improvement nonetheless.

  1. Install openbsd-inetd on the phone
    debian # apt-get install openbsd-inetd
  2. Configure /etc/inetd.conf in the debian chroot. Here's a setup for ssh and samba:
    # SSH
    22   stream  tcp nowait root /usr/sbin/sshd sshd -i
    # SAMBA
    139  stream tcp nowait root /usr/sbin/smbd smbd
    137  dgram  udp wait   root /usr/sbin/nmbd nmbd
  3. Configure Android's /sd-ext/ to start inetd. Note that you will NOT want to start ssh and samba as separate services.
    # start debian ssh
    chroot /sd-ext/debian /usr/sbin/service openbsd-inetd start

3 Scripts

4 Unresolved Issues

4.1 Name Resolution

  • How to resolve the hostname of our phone as we move between networks?
  • Mostly, I want to ssh/sftp to the phone from both my laptop and my lab workstation.

4.1.1 Options

  • Sounds like a job for: mDNS
    • Android doesn't support multicast, maybe?
  • Dyndns: doesn't update fast enough
  • ipv6 link local addresses: still flaky support in applications
    • kernel/application can't figure which interface to use for link-local connections (sounds ironic, right?)
    • can sometimes specify interface manually, but it's going to change, ie laptop on wired vs. wifi

4.2 WIFI

  • WIFI Sleep Policy: seems to change nothing on CM7
  • Screen On:
    • Ping Time: ~10ms
    • Bandwidth: 25 Mbps
  • Screen Off:
    • Ping Time: ~300MS
    • Bandwidth: 5 Mbps

Author: Neil T. Dantam <>

Date: 2014-04-14 11:12:39 EDT

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