Securing Linux ssh server with ssh key-based login and using a custom port for the SSH server daemon. The instructions here will work for any Linux. Any special cases are highlighted. 

Securing your SSH server.

# Warning
This can potentially cause damage if you make mistakes. You can lose access to remove servers so Behave and Check before executing or saving.
# Changing Default Port
Plain point: You dont want those bad guys who run scripts on IP ranges trying out SSH logins on Port 22. Let it find common guys.
The first thing you should do when you fire up a remote server that is to accessed via SSH is to change the default port `22` to any custom port like `2229`. 

## Firewall consideration (optional)

In case you server is configure with UFW or FirewallD on Centos, you first need to allow the port to be accessible to the outside world.

### For UFW (Ubuntu)
```shell
sudo ufw allow 2229
```

### For Centos with SELinux and FirewallD

Allow port on FirewallD

```shell
sudo firewall-cmd --add-port 2229/tcp --permanent
```
Adding port to SELinux

```shell
sudo semanage port -a -t ssh_port_t -p tcp 2229
```

Now you are ready to change the port.

Open ssh config file

```shell
 sudo nano /etc/ssh/sshd_config
 ```
Add a new Port. Dont replace port as if something goes wrong 
```shell
..........
# What ports, IPs and protocols we listen for
Port 22
Port 2229
.........
```

Reload ssh server 

```shell
sudo syetemctl reload sshd
```

Test the new port using

```shell
ssh user@YOUR_IP -p 2229
```

If you can login, then you can now safely remove port 22 from the bind list.

Open ssh config file

```shell
 sudo nano /etc/ssh/sshd_config
 ```
Remove port 22

```shell
..........
# What ports, IPs and protocols we listen for
Port 2229
.........
```

And reload the server 
```shell
sudo syetemctl reload sshd
```

# Using SSH Key for login(Only if you still use password logins).

Next thing you wanna do is to disable password login on your server and only use public key based login. This also goes hand in hand with disabling remote logins for `root` user.

## Copy public key to server.

```shell
ssh-copy-id user@YOUR_IP -p 2229
```
You get prompts, just follow through and put your password.

Now try to login to the server. You should not get a password prompt. If so, then you are ready to disable password logins.

Open ssh config file

```shell
 sudo nano /etc/ssh/sshd_config
 ```

Find ChallengeResponseAuthentication and set to no:

```shell
........
ChallengeResponseAuthentication=no
........
```
Find PasswordAuthentication set to no:

```shell
.......
PasswordAuthentication=no
.......
```
Find UsePAM and set to no:

```shell
......
UsePAM=no
......
```
Find PermitRootLogin and set to no:

```shell
.......
PermitRootLogin no
.......
```

And if you are no  Centos, find PubkeyAuthentication and set to yes:

```shell
.........
PubkeyAuthentication yes
........
```

Now you can reboot the server and try login with password. You can try any user that has no key added.

Cheers!

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Securing your SSH server.

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Securing your SSH server.