What you should check when SSH connection is refused in Linux

What you should check when SSH connection is refused in Linux ?

In initial settings of Linux server like CentOS or Ubuntu, sometimes you fail SSH connection.
This article shows what you should check when SSH connection is refused in Linux.

SSH connection is refused

When you try to connect to Linux by SSH application like Tera Term, you would see "connection refused".

What you should check

• Server side
  • SSH service software
  • Service is working or not
  • Server IP is known in the network or not
  • Accsess from outside is allowed or not
  • FW allows access or not
• Client side
  • Network path from client to server

SSH service software

There is a software openssh-server , you can install it by apt-get install .

$ sudo apt-get install openssh-server

ssh connection to ubuntu - Qiita

After installation, you can execute below.
$ ssh -V
Then you can see the result. So installation is done correctly.
OpenSSH_6.6.1p1 Ubuntu-2ubuntu2.6, OpenSSL 1.0.1f6 Jan 2014
Activate SSH on Ubuntu 14.04 - wide_snow’s blog

Service is working or not

In order to check openssh-server is working or not, You can try SSH connection from the server to the server itself.
If you could login, openssh-server is working.

$ ssh (IP address of server)

Server IP is known in the network or not

You can check IP address of server by ifconfig command.
You can use ip a command too.

But ifconfig command can't prove that IP is known in the network or not.
If other machine uses same IP address, IP address conflict happens on network.

Then you can use nmap command and check MAC address of IP that network(LAN) knows.
First you can activate nmap by apt install .

$ sudo apt install nmap

Then you check MAC address of IP on network.
In below case, network is 192.168.0.xxx.

$ sudo nmap 192.168.0.0/24 -sP

If MAC address that nmap shows is not same as the MAC address that you checked by ifconfig, it means that there is IP address conflict.

Port Connection refused in case of SSH connection | TOMMY NOTES
Install nmap

Accsess from outside is allowed or not

You can check access denied list in /etc/hosts.deny.
If client access is denied in the list, you can comment or delete it.

$ sudo nano /etc/hosts.deny
$ sudo reboot

settings (/etc/hosts.allow、/etc/hosts.deny)
How to apply hosts.allow hosts.deny | teratail
Access control by hosts file - renoji.com

FW allows access or not

You can check FW allows access or not by ufw status command.

$ sudo ufw status

If port 22 is not allowed, you can allow it.

$ sudo ufw allow ssh

How to open port in Ubuntu - Qiita

Network path from client to server

With using ping command, you can check that network path runs from client to server.
If response comes, network path is OK.

$ ping (IP address of server)

Finally

• If SSH connection is refused, check SSH software, network path, IP address conflict.