Wondering how to check connectivity between two network endpoints?

One of the common tasks for sysadmin is to check the connectivity to troubleshoot networking issues. It could be anything like application can’t connect to backend service, unable to fetch data from external URL, verify if the flow is opened, etc.

Whatever it might be, the following utility/commands would help you. They are tested on CentOS, and I don’t see any reason not to work on another Linux distro.

Let’s explore…

telnet

One of the widely used commands to test essential connectivity between servers, server to another network device’s IP. The syntax for the command is easy.

telnet $destinationIP $PORT

Let’s say you want to test if you can connect to port 8080 on 10.0.0.1 IP address; then the command would be.

telnet 10.0.0.1 8080

If there is no issue in connecting, then you should see the connected message.

Trying 10.0.0.1...
Connected to 10.0.0.1.
Escape character is '^]'.

Note: if you get a command not found while executing telnet then you need to install telnet as I explained here.

In most of the scenarios, telnet should help. However, if you need some other option then here are some telnet alternatives.

ncat or nc

Ncat (a.k.a. nc) is a powerful network utility with many features like bind and accept a connection, execute commands remotely, write and read data, etc. It works on IPv4 and IPv6, both.

To do a simple test to check if the port is opened or not, you will execute the following.

nc -vz $HOSTNAME $PORT

Let’s take an example of testing 443 port on geekflare.com.

[root@geekflare-lab ~]# nc -vz geekflare.com 443
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connected to 104.25.133.107:443.
Ncat: 0 bytes sent, 0 bytes received in 0.02 seconds.
[root@geekflare-lab ~]#

As mentioned, you can also use nc to bind the connection to listen on a particular port. This can be handy when you don’t have actual services running but want to ensure connectivity exists.

To start listening on a port:

nc -l $PORTNUMBER

It will bind the port to a given number.

If ncat is not installed, then you can get it done with yum install nc on CentOS/RHEL servers.

wget

wget is a useful command to download/test HTTP, HTTPS, and FTP. If you are working as a web engineer or often dealing with web-related issues then wget is your friend. Testing using wget is straightforward.

wget $URL

Here is an example of testing tools.geekflare.com

[root@geekflare-lab ~]# wget tools.geekflare.com
--2019-05-09 20:40:01--  http://tools.geekflare.com/
Resolving tools.geekflare.com (tools.geekflare.com)... 104.25.134.107, 104.25.133.107, 2606:4700:20::6819:866b, ...
Connecting to tools.geekflare.com (tools.geekflare.com)|104.25.134.107|:80... connected.
HTTP request sent, awaiting response... 301 Moved Permanently
Location: https://tools.geekflare.com/ [following]
--2019-05-09 20:40:01--  https://tools.geekflare.com/
Connecting to tools.geekflare.com (tools.geekflare.com)|104.25.134.107|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: unspecified [text/html]
Saving to: 'index.html.2'

    [ <=>                                                                                                                                  ] 15,139      --.-K/s   in 0.001s  

2019-05-09 20:40:02 (12.8 MB/s) - 'index.html.2' saved [15139]

[root@geekflare-lab ~]#

If it shows connected means there is no connectivity issue.

Check out this to see some of the frequently used wget command examples.

curl

A curl is a multipurpose tool.

Do you know you can telnet to a port using curl?

Well, now you know.

curl -v telnet://$IP:$PORT

The following is a working example.

[root@geekflare-lab ~]# curl -v telnet://chandan.io:443
* About to connect() to chandan.io port 443 (#0)
*   Trying 104.31.68.106...
* Connected to chandan.io (104.31.68.106) port 443 (#0)

And, when there is no listening port or firewall issue, then you will see trying…

[root@geekflare-lab ~]# curl -v telnet://chandan.io:4434
* About to connect() to chandan.io port 4434 (#0)
*   Trying 104.31.68.106...

You can also use curl to download the data. It supports multiple protocols – HTTP, HTTPS, FTP, IMAP, LDAP, POP3, SCP, SFTP, GOPHER, etc.

nmap

A popular tool with hundreds of features. Often this is considered as a security tool, nmap lets you test a single IP/port or in the range.

To test a single port

nmap -p $PORT $IP

An example of testing port 443 on siterelic.com

[root@geekflare-lab ~]# nmap -p 443 siterelic.com
Starting Nmap 7.70 ( https://nmap.org ) at 2019-05-10 06:55 UTC
Nmap scan report for siterelic.com (104.27.174.50)
Host is up (0.0079s latency).
Other addresses for siterelic.com (not scanned): 104.27.175.50 2606:4700:30::681b:ae32 2606:4700:30::681b:af32

PORT    STATE SERVICE
443/tcp open  https

Nmap done: 1 IP address (1 host up) scanned in 0.13 seconds
[root@geekflare-lab ~]#

Look at the state column. If you see open means connection is ok. And, if the state is filtered that means connectivity doesn’t exist.

Ping

One of the widely used commands is to check if a remote host is responding to ICMP ECHO_REQUEST or not. Keep in mind, this may not give you accurate results when ICMP is blocked at the remote network’s firewall. Assuming that’s not the case, you can ping to IPv4 or IPv4 network endpoint as below.

ping $ipaddress
ping $url

For example, success result of geekflare.com

chandan@192 ~ % ping geekflare.com
PING geekflare.com (104.27.119.115): 56 data bytes
64 bytes from 104.27.119.115: icmp_seq=0 ttl=53 time=7.944 ms
64 bytes from 104.27.119.115: icmp_seq=1 ttl=53 time=8.870 ms

For example, failure result of internal IP.

chandan@192 ~ % ping 192.168.0.1
PING 192.168.0.1 (192.168.0.1): 56 data bytes
Request timeout for icmp_seq 0
Request timeout for icmp_seq 1

If the endpoint supports IPv6, you can use the ping6 command as below.

chandan@192 ~ % ping6 geekflare.com
PING6(56=40+8+8 bytes) 2a01:4b00:88e4:8700:d8ca:bf50:159c:2a1d --> 2606:4700:20::681b:7673
16 bytes from 2606:4700:20::681b:7673, icmp_seq=0 hlim=250 time=8.650 ms
16 bytes from 2606:4700:20::681b:7673, icmp_seq=1 hlim=250 time=8.738 ms

If you need to do remote ping over the Internet, you can use the online ping tool.

Conclusion

telnet is phasing out in the latest Linux version. Thanks to the above telnet alternative.

If you are new to Linux and looking to learn then check out this Udemy course.