Some people change their SSH port on their servers so that it is slightly harder to find for bots or other nasties and while that is generally viewed as an action of security through obscurity it does work very well at killing a lot of the automated logins you always see in /var/log/auth.log.
However what if we could go take this to a ridiculous level? What if we could use TOTP codes that are normally used as 2nd factor codes to login to websites to actually know what port the sshd server is listening on?
For this I present totp-ssh-flux: a way to make sure your sshd port changes every 30 seconds, possibly causing your adversaries a small period of frustration.
Demo:
What you can see here is my phone (using a generic TOTP client) generating codes that I can then use as the port to SSH into on a server.
The software behind it is fairly simple - It runs in a loop that does the following:
- Generates a TOTP token
- Takes the last digit, if the result is above 65536, do that again
- Adds a iptables PREROUTING rule to redirect that number generated above
- Waits 30 seconds, removes that rule, repeat.
The neat thing is that because this is done in PREROUTING even if the code expires established connections stay connected.
You will most likely find more up to date instructions on the totp-ssh-flux project readme
At the time of writing the project is just a single file. You will need to install golang and then go get and go build.
Run the program as root ( it needs to, sorry, it's editing iptables )
On the first run the program will generate a token for the host in /etc/ssh-flux-key ( you can use the -keypath option to change that ) which you can then input into your phone or other clients.
You can confirm that it works by running watch iptables -vL -t nat and waiting for the iptables rules to be inserted and removed.
Want to see more insanity like this? Follow me on twitter @benjojo12