5 simple methods to test ssh connection in Linux & Unix

In this article I will share different commands and methods to Test SSH connection in Linux and Unix with real time practical examples.

Method 1: Use timeout with bash utility to test SSH connection

/usr/bin/timeout utility is installed by default in most distros which is part of coreutils rpm in Linux

Check if coreutils is installed on your server

# rpm -q coreutils
coreutils-8.22-24.el7.x86_64

We can use bash utility with timeout to test SSH connection by checking port 22 status.
If you are using a different port for 22 then you can replace it in the below syntax

Syntax:

# timeout <value> bash -c "</dev/tcp/<server>/<port>"

Here server2 is my target host, I will execute the command with a timeout value of 5s on port 22

[root@server1 ~]# timeout 5 bash -c "</dev/tcp/server2/22"

If the exit status is <strong>0</strong>, it means test ssh connection was successful

[root@server1 ~]# echo $?
0

Or if you get “connection refused” with non-zero exit status then test SSH connection has failed

[root@server1 ~]# timeout 5 bash -c "</dev/tcp/server2/22"
bash: connect: Connection refused
bash: /dev/tcp/10.10.10.10/22: Connection refused
[root@server1 ~]# echo $?
1

Shell Script Example

We can use this tool in a shell script to test SSH connection over port 22

# cat /tmp/check_connectivity.sh
#!/bin/bash

server=10.10.10.10      # server IP
port=22                 # port
connect_timeout=5       # Connection timeout

timeout $connect_timeout bash -c "</dev/tcp/$server/$port"
if [ $? == 0 ];then
   echo "SSH Connection to $server over port $port is possible"
else
   echo "SSH connection to $server over port $port is not possible"
fi

Method 2: Use nmap to test SSH connection

• /usr/bin/nmap is provided by nmap rpm.
• nmap is widely used to check port status so we can use nmap to check port 22 status on target host
• nmap is not installed by default in most distros and you must install it before using it
• On RHEL/CentOS environment use yum or dnf to install nmap

# yum -y install nmap

Syntax:

# nmap <server> -PN -p ssh | egrep 'open|closed|filtered'

Here,

-Pn        Treat all hosts as online -- skip host discovery
-p ssh      Only scan the default SSH port
open        means that an application on the target machine is listening for connections/packets on that port
closed      ports have no application listening on them, though they could open up at any time
filtered    means that a firewall, filter, or other network obstacle is blocking the port so that Nmap cannot tell whether it is open or closed.

Here server2 is my target host and we are looking for nmap port status

[root@server1 ~]# nmap server2 -Pn -p ssh | egrep -io 'open|closed|filtered'
closed

[root@server1 ~]# nmap server2 -Pn -p ssh | egrep -io 'open|closed|filtered'
open

Shell script Example

We can use this command in our shell script to test SSH connection over port 22

[root@server1 ~]# cat /tmp/check_connectivity.sh
#!/bin/bash

server=10.10.10.10      # server IP
port=22                 # port
connect_timeout=5       # Connection timeout

status=`nmap $server -Pn -p $port | egrep -io 'open|closed|filtered'`
if [ $status == "open" ];then
   echo "SSH Connection to $server over port $port is possible"
elif [ $status == "filtered" ]; then
   echo "SSH Connection to $server over port $port is possible but blocked by firewall"
elif [ $status == "closed" ]; then
   echo "SSH connection to $server over port $port is not possible"
else
   echo "Unable to get port $port status from $server"
fi

Method 3: Use netcat or nc to test SSH connection

• In my earlier article I had shared the steps to use nc and ncat to transfer files between Linux server.
• We can also use nc and ncat utility to check port status from target hosts and test SSH connection
• nc and ncat is provided by nmap-ncat rpm

To check if nmap-ncat is installed on your server

# rpm -q nmap-ncat
nmap-ncat-6.40-19.el7.x86_64

Syntax:

# nc --wait <value> <server> <port> < /dev/null &> /dev/null

Here we have defined a connection timeout period of 5 second which you can change based on your environment

Check the exit status of nc command in this command. For 0 exit status we know that port 22 is open and SSH connection will be successful.

[root@server1 ~]# nc --wait 5 server2 22 < /dev/null &> /dev/null
[root@server1 ~]# echo $?
0

For <strong>non-zero</strong> exit status we know that SSH connection will fail for respective target host

[root@server1 ~]# nc --wait 5 server2 22 < /dev/null &> /dev/null
[root@server1 ~]# echo $?
1

Shell Script Example

We can use this command in our shell script example to automate the verification

# cat /tmp/check_connectivity.sh
#!/bin/bash

server=10.10.10.10      # server IP
port=22                 # port
connect_timeout=5       # Connection timeout

nc --wait $connect_timeout $server $port < /dev/null &> /dev/null
if [ $? == 0 ];then
   echo "SSH Connection to $server over port $port is possible"
else
   echo "SSH connection to $server over port $port is not possible"
fi

Method 4: Use SSH to check SSH connection

• I know we are looking for SSH alternatives to check SSH connection but if you have a setup configured with password less connection then you can also use SSH for this verification
• We will use ConnectTimeout to make sure our SSH don’t get stuck waiting for connection to become active
• StrictHostKeyChecking is used to avoid any security and fingerprint prompt
• If BatchMode is yes then passphrase/password querying will be disabled

Here we are using ‘exit 0’ as the remote command to be called on successful SSH

[root@server1 ~]# ssh -q -o BatchMode=yes  -o StrictHostKeyChecking=no -o ConnectTimeout=5 server2 'exit 0'
[root@server1 ~]# echo $?
0

While if the output exit status is non-zero so we know the test SSH connection has failed

[root@server1 ~]# ssh -q -o BatchMode=yes  -o StrictHostKeyChecking=no -o ConnectTimeout=5 server2 'exit 0'
[root@server1 ~]# echo $?
255

Shell Script Example

We can use this command in our existing shell script for automation purpose:

# cat /tmp/check_connectivity.sh
#!/bin/bash

server=10.10.10.10      # server IP
port=22                 # port
connect_timeout=5       # Connection timeout

ssh -q -o BatchMode=yes  -o StrictHostKeyChecking=no -o ConnectTimeout=$connect_timeout $server 'exit 0'
if [ $? == 0 ];then
   echo "SSH Connection to $server over port $port is possible"
else
   echo "SSH connection to $server over port $port is not possible"
fi

Method 5: Use telnet to test SSH connection

• telnet is another very handy tool to check port status
• /usr/bin/telnet is provided by telnet rpm which is part of default repositories and you do not need any third party repository

Check if telnet is installed

# rpm -q telnet
telnet-0.17-65.el7_8.x86_64

Syntax:

# telnet <server> <port>

But since our end goal is to automate so we will tweak the syntax

# echo quit | telnet <server> <port> 2>/dev/null | egrep -qi Connected

Let us use this to test SSH connection in Linux. if we are able to grep for “Connected” then port 22 is reachable and SSH connection is possible

[root@server1 ~]# echo quit | telnet server2 22 2>/dev/null | egrep -qi Connected
[root@server1 ~]# echo $?
0

If we get a non-zero exit status, this means that we were unable to grep “Connected” in the output hence SSH connection is not possible

[root@server1 ~]# echo quit | telnet server2 22 2>/dev/null | egrep -qi Connected
[root@server1 ~]# echo $?
1

Shell Script Example

We will use telnet with our existing sample shell script to test SSH connection

# cat /tmp/check_connectivity.sh
#!/bin/bash

server=10.10.10.10      # server IP
port=22                 # port
connect_timeout=5       # Connection timeout

echo quit | telnet $server $port 2>/dev/null | egrep -qi "Connected"
if [ $? == 0 ];then
   echo "SSH Connection to $server over port $port is possible"
else
   echo "SSH connection to $server over port $port is not possible"
fi

Conclusion

In this tutorial guide we learned about different internal tools within Linux and Unix which can be used to test and verify SSH connection before actually attempting the SSH. We do not need to rely on additional third party tools for such verification. It is always a good idea to first check the network connectivity and port 22 availability before performing the SSH to avoid un-necessary wait time and timeout scenarios

Lastly I hope the commands from this article to test SSH connection on Linux and Unix was helpful. So, let me know your suggestions and feedback using the comment section.

References

I have used below external references for this tutorial guide
How to create a bash script to check the SSH connection?