Tag Archives: ssh

Fail2ban Debian 9

Scratch pad with conf files to configure Fail2ban on Debian 9

This setup will configure Fail2ban to monitor SSH and keep track of the bad guys. Every time an IP gets banned, it will be stored in /etc/fail2ban/ip.blacklist .
This files gets processed every time Fail2ban restarts.
A cron will sanitise the file daily.

HOW TO

1) Create a custom action file: /etc/fail2ban/action.d/iptables-allports-CUSTOM.conf

2) Create /etc/fail2ban/jail.local

3) Remove the default debian jail configuration (is integrated in the above custom jail.local file):

4) Set this cron:

5) Run the cron manually once, just to be sure all works AND to have an empty file

6) Restart Fail2ban … and good luck 😉

 

 

Email notification for successful SSH connection

If you manage a remote server, and you are a bit paranoiac about the bad guys outside, it could be nice to have some sort of notification every time a SSH connection is successful.

I found this post and it seems working pretty well for me as well.
I’ve installed this on my CentOS7 server and seems working good! Of course, this in addition to an aggressive Fail2Ban setup.

  1. Make sure you have your MTA (Postfix/Sendmail…) configured to deliver emails to the user root
  2. Make sure you get the emails for the user root (otherwise doesn’t make any sense 😛 )
  3. Create this script (this is a slightly modified version comparing with the one in the original post:
  4. Set the permission:
  5. Append this line to /etc/pam.d/sshd
  6.  …and that’s it! 😉

 

If you’d like to have a specific domain/IP whitelisted, for example if you don’t want to get notified when you connect from your office or your home (fixed IP or dynamic IP is required), you can use this version of the script:

The script will send an email ONLY if the source IP is not the one from myofficedomain.com; however, it will log the authentication in /var/log/messages using logger command.

Fail2ban notes

General notes about Fail2ban

How to “SSH” brute force

If you want to make safer your remote server, it is good practise to use a good combination of sshd setup and fail2ban.

Firstly, you should setup your server to allow only key auth, and no passwords. This will drastically reduce the risk. This means anyway that you need to keep your ssh key safe and you won’t be able to access your server unless you have this key. Most of the time is something possible 🙂

For this reason, I’m explaining here how I configured my server.

SSHD

/etc/ssh/sshd_config

Have these settings in the config file (NOTE: the verbosity is for Fail2ban)

(restart sshd)

/etc/fail2ban/jail.local

/etc/fail2ban/filter.d/sshd.conf

Add a custom section after the ddos one:

This line matches whoever tries to connect without a proper ssh key.

Add this line to include custom to the sshd-aggressive setup:

 

Linux SSH auth passwordless using key

Pretty basic, but handy for whoever start playing with Linux.

Here simple steps to follow in order to have box1 to be able to connect securely to box2 over SSH without being required to insert password.
This is very handy if you run scripts 😉

On BOX1

You can run this as any user.

This will generate  ~/.ssh/id_rsa (private key) and  ~/.ssh/id_rsa.pub (public key).
The .pub is the key that needs to be appended in ~/.ssh/authorized_keys on BOX2.

If the following command is available, that’s the best/safest way to setup BOX2.

Password for user on box2 will be requested.
Once completed, you can try to ssh user@box2 and theoretically you should be able to connect without need to insert the password again!

If ssh-copy-id does not exist (e.g. Mac or other Distros), you can scp the .pub file and append it as per below:

Then connect to box2 with user and run this:

After those 2 commands, the key should be added to the authorised ones, so  ssh user@box2 should work.

NOTE: if you are experiencing issues, please make sure that the permissions of id_rsa file is 600 on BOX1 and that sshd_conf on BOX2 is set to allow key auth connections

Remote port forwarding via SSH

Imagine that you want to access a specific port on a remote server from your local machine. Basically, a “remote port forwarding”.

This remote server is not accessible directly from internet. It is NAT’d behind firewall.
To access the remote server you need to connect firstly to a remote bastion server (accessible from internet) and from there, you will be able to access the server.
Your current machine is also within restricted network and unable to ssh out. You can ssh into a local bastion server only. From this local bastion you can ssh out.

As long as you have access to the 2 bastions servers, you will be able to run the following script.

The script points/links a local_port on your local machine to the ssh port of the remote bastion, via your local bastion.
After that, it will connect the remote port or the remote server to a new_local_port, ssh’ing via local_port.

Example below shows a way to have the VNC port 5900 available locally on port 5910.
I’m using port 8888 as local port.
Local Bastion ssh port is 8022.
Remote Bastion ssh port is 9022.

Example:

 

And here a full script:

 

SSH tunnel from A to B via jumpbox

Here a basic script that you can use if you want to connect from your local box, via a middle linux machine, to a third host.
It will also allow you to use FoxyProxy on your browser and browse the internal network of the destination box.

BOX_A <==== MIDDLE_BOX ====> BOX_B

The goal is having access from BOX_A to BOX_B via MIDDLE_BOX

MIDDLE_BOX is the only one that can talk withBOX_A and BOX_B