We often need to check connectivity to troubleshoot network issues, or want to check whether our server is online. We created this article to show you how to do this.
There are many handy Linux tools for this:
1. ping
Ping is one of the most widely used commands to check whether a server or remote host is up and responding. Ping sends ICMP ECHO_REQUEST to the server. If ICMP is blocked from the server-side or cloud provider, then the ping command may not work as expected (like AWS ec2 instance). Here, we assume that cloud providers firewalls do not block ICMP.
Syntax: ping <destination>
root@ubuntu:~# ping example.com
PING example.com(2606:2800:220:1:248:1893:25c8:1946 (2606:2800:220:1:248:1893:25c8:1946)) 56 data bytes
64 bytes from 2606:2800:220:1:248:1893:25c8:1946 (2606:2800:220:1:248:1893:25c8:1946): icmp_seq=1 ttl=59 time=2.04 ms
64 bytes from 2606:2800:220:1:248:1893:25c8:1946 (2606:2800:220:1:248:1893:25c8:1946): icmp_seq=2 ttl=59 time=1.60 ms
^C
--- example.com ping statistics —
4 packets transmitted, 4 received, 0% packet loss, time 3005ms
rtt min/avg/max/mdev = 1.443/1.698/2.044/0.220 ms2. telnet
Telnet is a network protocol that allows you to connect to a remote host. Telnet is usually installed on most Linux OS by default. Otherwise, you can run the following command in the terminal:
For Redhat/CentOS/Rocky Linux
yum -y install telnetor for Ubuntu/Debian
apt-get install telnetUsing telnet, you can identify the availability of certain ports. You can use telnet to make sure no firewalls in between are blocking incoming connections to the server.
System or network administrators use Telnet to configure network devices like routers and switches.
root@ubuntu:~# telnet localhost 22
Trying 127.0.0.1…
Connected to localhost.
Escape character is ‘^]’.
SSH-2.0-OpenSSH_8.2p1 Ubuntu-4ubuntu0.3The above command shows that port 22 is open on the localhost.
Let’s now check whether the webserver port is open.
root@ubuntu:~# telnet example.com 443
Trying 2606:2800:220:1:248:1893:25c8:1946...
Connected to example.com.
Escape character is ‘^]’.If the port is firewalled on the remote host, then you will see something like this
root@ubuntu:~# telnet example.com 22
Trying 2606:2800:220:1:248:1893:25c8:1946...3. nslookup
Simply speaking, nslookup is used for DNS record checks. You can use nslookup to check if your website is connected to the internet. It is also used to troubleshoot DNS-related problems.
Syntax:
nslookup <domain>It will show the IP address and other details
root@ubuntu:~# nslookup example.com
Server: 127.0.0.53
Address: 127.0.0.53#53Non-authoritative answer:
Name: example.com
Address: 93.184.216.34
Name: example.com
Address: 2606:2800:220:1:248:1893:25c8:1946
Otherwise you will see the following:
root@ubuntu:~# nslookup example.com1
Server: 127.0.0.53
Address: 127.0.0.53#53** server can’t find example.com1: NXDOMAIN
By default, nslookup will show "A" records.
4. nmap
Nmap or Network Mapper is a powerful tool for finding open ports and network audits.
In case the nmap command is not available, you can install it executing the following command in terminal:
sudo apt install nmapNmap is also used to identify whether the host is up or not.
nmap example.comThis command will show open port and host status as below:
root@ubuntu:~# nmap example.com
Starting Nmap 7.80 ( https://nmap.org ) at 2021-11-05 14:48 UTC
Nmap scan report for example.com (93.184.216.34)
Host is up (0.0016s latency).
Other addresses for example.com (not scanned): 2606:2800:220:1:248:1893:25c8:1946
Not shown: 996 filtered ports
PORT STATE SERVICE
80/tcp open http
443/tcp open https
1119/tcp closed bnetgame
1935/tcp closed rtmpNmap done: 1 IP address (1 host up) scanned in 4.91 seconds
You can easily check whether your server is running using the following command:
Syntax:
nmap -sn <hostname>root@ubuntu:~# nmap -sn example.com
Starting Nmap 7.80 ( https://nmap.org ) at 2021-11-05 14:54 UTC
Nmap scan report for example.com (93.184.216.34)
Host is up (0.0023s latency).5. nc
Netcat is known as an nc command used for network scanning like Port Scanning, open Remote connections, Network debugging, read and write operation on the network, and many more.
You can easily check the reachability of remote ports using nc Command.
Syntax
nc -vz $HOSTNAME $PORTnc -vz example.com 443
If port 443 is running on the server, we will get the following:
Connection to example.com 443 port [tcp/https] succeeded!
root@ubuntu:~# nc -vz example.com
nc: missing port number
root@ubuntu:~# nc -vz example.com 443
Connection to example.com 443 port [tcp/https] succeeded!
root@ubuntu:~# nc -vz example.com 80
Connection to example.com 80 port [tcp/https] succeeded!6. wget
Wget is a command-line download tool that supports HTTP, HTTPS and FTP. Wget is very useful for those who want to check any network-related issue or test connection from a Linux terminal. It will help you in web-related issues debugging.
Syntax:
wget <URL>root@ubuntu:~# wget example.com
--2021-11-05 17:48:52– http://example.com
Resolving example.com (example.com)... 2606:2800:220:1:248:1893:25c8:1946, 93.184.216.34
Connecting to example.com (example.com)| 2606:2800:220:1:248:1893:25c8:1946|:80… connected.
HTTP request sent, awaiting response… 200 OK
Length: 1256 (1.2K) [text/html]
Saving to: índex.html.2index.html.2 100%[=========================>] 1.23K --.-KB\s in 0s2021-11-05 17:48:52 (74.6 MB/s) - ’index.html.2’saved [1256/1256]
Here we get the “200” response code from the web server and index.html file downloaded.
7. curl
You can use the curl command to transfer data either from or to a server. It supports proxy support, user authentication, FTP upload, HTTP post, SSL connections, cookies, and many more.
You can use curl command to test TCP ports connectivity.
Let’s check our SSH port connection with CURL
Syntax:
curl -v telnet://127.0.0.1:22root@ubuntu:~# curl -v telnet://127.0.0.1.22
* Trying 127.0.0.1:22...
* TCP_NODELAY set
* Connected to 127.0.0.1 (127.0.0.1) port 22 (#0)Now let’s check whether our web server is running on port 80 using curl
root@ubuntu:~# curl -v telnet://example.com:80
* Trying 2606:2800:220:1:248:1893:25c8:1946:80
* TCP_NODELAY set
* Connected to example.com (2606:2800:220:1:248:1893:25c8:1946) port 80 (#0)Let’s check the SMTP port
root@ubuntu:~# curl -v telnet://127.0.0.1:25
* Trying 127.0.0.1:25...
* TCP_NODELAY set
* Connected to 127.0.0.1 port 25 failed: Connection refused
* Failed to connect to 127.0.0.1 port 25: Connection refused
* Closing connection 0
curl: (7) Failed to connect to 127.0.0.1 port 25: Connection refusedAs you see, CURL is a useful command-line tool available in Linux to check the connection with the server and port.
We have examined the Linux command-line tools for checking server health and network connectivity in detail. These are essential tools for system and network administrators to troubleshoot any network-related issues.