Friday, May 12, 2017

Persistent/resilient ssh sessions for unstable internet connections.

Long back SSH had introduced an experimental 'roaming' feature where changes to the ssh client IP resulted in resuming the session on the server regardless of the changed IP. This feature was never implemented on the server, rending it useless on the client but causing a vulnerability.

Instead of using roaming, a much better approach is using screen with shell scripting. This has serious advantages like resuming the session over a different client machine, the program running in foreground won't slow down even if the terminal (or Internet connection) is slow etc...

Just install screen on the server and run the following commands for a presistant session --

while [[ j != k ]]; do ssh -tt screen -r / -p 0; done

This'll reconnect on disconnecting. You can use tabs in screen and take multiple sessions over the same screen instance. Open the other tabs using --

while [[ j != k ]]; do ssh -tt screen -r / -p 1; done

while [[ j != k ]]; do ssh -tt screen -r / -p 2; done

while [[ j != k ]]; do ssh -tt screen -r / -p 3; done

For tabs numbered 1, 2, 3 etc...

I use Gentoo's default config for the screen on the server, it works great!

Friday, May 5, 2017

Incremental backup system of your Android app settings and your data.

Phone is an unreliable device which can always be stolen and get bricked regardless of how expensive the phone is or how must reliable the manufacture claims to be. This's primarily because of the fact that the storage cannot be detached from the phone and the storage's content is highly integrated to the phone's hardware.

So I've created a system to regularly backup your app data  in an incremental way -- so the old data gets retained and snapshot of the latest backups is also taken all using less space. You can restore all this data to a new phone or revert an older version of the data to your existing phone (maybe to get it unbricked without loosing all your settings).

Of course I know about Google's cloud backup, but in my experience it's unreliable, requires a lot of bandwidth and works only on select (Google only) apps. This works on all apps. I also know about adb backup and restore feature, but that also does not work on all apps.

This system requires sshelper app and it must run in the background all the time. You must configure key based login as specified in this (Public-key (passwordless) logins) tutorial. After configuring that, you can disable password based login and disable the 'keep device awake' checkbox to improve on the battery and security.

Other things that is requires is root access.

sshelper installs a busybox. You need to use the tar command cron command of that. The scripts I've deployed use exactly that --

Place this script in /system/bin/ -- 

# backups data only if the latest one is less than 12 hours old
#! /system/bin/sh
SD_CARD="Your sdcard mount point"
mkdir $SD_CARD/custom_backup
latest=`ls -tr $SD_CARD/custom_backup/ | tail -1`
if test \( -z "$latest" \) -o \( `date +%s` -gt $(($latest + $SECONDS)) \)
 cd /data/data && /data/data/com.arachnoid.sshelper/bin/tar -cpf $SD_CARD/custom_backup/`date +%s` *

This can be done by the command (as root) --

 vim /system/bin/

And then pressing 'i' to got to edit mode. Then paste, make changes, then press ESC a few times and type ':x' (without the single quotes).

Modify SD_CARD variable to point to the mount point where your sdcard is mounted. Use the mount command to see the various mount points. One of these must be your SDcard. cd to that place and verify by looking at it's contents if it is indeed the place.

It happens that Android has a bug or a problem etc... the system call which these basic utilities use to seep for a certain period of time is inaccurate. This systemcall never returns when sleep is done for a long period of time. This system works around this problem.

Next you need to setup cron.

Run these commands root -- 

mkdir /data/data/com.arachnoid.sshelper/spool
vim /data/data/com.arachnoid.sshelper/spool/root

Now press i, then copy paste the following text -- 

*/30 * * * *

Then press ESC a few times and type ':x' (without the single quotes).

Then run -- 

vim /etc/init.d/

Now press i, then copy paste the following text -- 

mount -o remount,rw /
ln -s /system/bin /bin
mount -o remount,ro /
/data/data/com.arachnoid.sshelper/bin/crond -c /data/data/com.arachnoid.sshelper/spool

Then press ESC a few times and type ':x' (without the single quotes).

Then run -- 

chmod 755 /etc/init.d/

Install universal init.d and enable init.d scripts support. If you've a rom which has inbuilt support of init.d, you will not require this.

After this you must see backups created in directory /custom_backup. The file name is the timestamp of the date at which the backup was taken. These files will be created ~ every 12 hours.

Next configure your desktop system to take incremental backups. This's the script which runs a daemon and keeps running until the backups are complete. --

Modify the variables ($*) as per your needs and as per your phone.

$sdcard is the directory where backups are placed (that custom_backup directory) on your phone. It must be a full path.

$sshkey is the ssh key which you generated for key-based login to your phone.

$ip is the IP of your phone on the wi-fi network. You must configure your phone for static IP.

$backupDest is the path of your desktop system where the backups will be placed.

$sdcardInternal is the full path of your internal SDcard in your phone. $sdcardExternal is similar but for your external SDcard.

Your SDcards will be incrementally backed up too.

This script is resilient to failure -- so even if your phone is gone, it'll wait until your backup is complete after you've come back and your phone's presence is detected. Setup a cron job once a day which triggers this script. If you wanna know -- the script is released under GPL v3 license.