Table of Contents
In this article I will share multiple commands and examples to check if connected to internet for your Linux machine. At the end of the article I will also share some example to test internet connection from the Linux to external network using shell script.
But before we do that there are certain things you must take care of, such as
- You have IP Address assigned to your network interface (static or dhcp)
- Your gateway is reachable from the Linux system
- If you are on virtual machine then make sure your VM is configured to be able to connect external network
- Your DNS is properly configured
Now there are multiple commands which are available in Linux which can be used to check if connected to internet, we will try to discuss some of them here in brief.
1. Ping Test
The very first tool I use to check if connected to internet is via
ping utility. But do you know how to check ping or perform ping check to check if connected to internet? Ping is part of
iputils rpm so make sure
iputils is installed on your setup
To perform ping check, try to ping any page on internet such as
# ping -c 1 google.com PING google.com (184.108.40.206) 56(84) bytes of data. 64 bytes from maa05s09-in-f14.1e100.net (220.127.116.11): icmp_seq=1 ttl=49 time=46.9 ms --- google.com ping statistics --- 1 packets transmitted, 1 received, 0% packet loss, time 0ms rtt min/avg/max/mdev = 46.923/46.923/46.923/0.000 ms
Here we use
-c 1 with
ping tool to send only one packet. Now check the statistics section, we see 1 packet was transmitted and received and there was
0% packet loss so it means we are connected to internet.
If you plan to check if connected to internet via shell script then you can use below, anything in the output other than “0” means not connected to internet.
# ping -c 1 -q google.com >&/dev/null; echo $? 0
2. Check port availability using cat, echo..
There are various tools which can be used to check port availability which I will share in this article. But many of them are not installed by default so you may have to install them manually. Alternatively you can use below command to check if connected to internet without installing any additional rpm
# cat < /dev/null > /dev/tcp/18.104.22.168/53; echo $? 0
If the output is 0 then it means you are connected to internet, but if the output is something like below
# cat < /dev/null > /dev/tcp/22.214.171.124/53; echo $? -bash: connect: Network is unreachable -bash: /dev/tcp/126.96.36.199/53: Network is unreachable 1
Then your linux node is not connected to internet.
Alternatively you can also use
# echo -n >/dev/tcp/188.8.131.52/53; echo $? 0
3. DNS lookup using nslookup, host etc..
You can perform a DNS lookup any web page address to check if connected to internet. With a successful DNS lookup you should get a response something like below.
# nslookup google.com Server: 192.168.0.10 Address: 192.168.0.10#53 Non-authoritative answer: Name: google.com Address: 184.108.40.206 Name: google.com Address: 2404:6800:4007:811::200e
For a failed DNS lookup you should get something like
# nslookup google.com1 Server: 220.127.116.11 Address: 18.104.22.168#53 ** server can't find google.com1: NXDOMAIN
server can't find XXXX: NXDOMAIN” doesn’t specifically mean that the node is not connected to internet, it may also mean that the provided server XXX is invalid. So try to lookup a domain which you know is UP to avoid false positive.
There are many more commands to perform DNS lookup such as host, dig etc
# host www.google.com www.google.com has address 22.214.171.124 www.google.com has IPv6 address 2404:6800:4007:812::2004
curl is a tool to transfer data from or to a server, using one of the many supported protocols such as HTTP, FTP etc. We can also use this tool to query a webpage and test internet connection in Linux.
# curl -Is http://www.google.com | head -n 1 HTTP/1.1 200 OK
Here if you receive anything other than
200 OK then it means the server failed to connect to the provided page. So unless you provide an invalid webpage, your node is connected to internet
Telnet is another tool to check port connectivity so you can check port availability of any webpage from the Linux node to check if connected to internet. We can try connecting port 53 of Google DNS to check internet connection.
# telnet 126.96.36.199 53 Trying 188.8.131.52... Connected to 184.108.40.206. Escape character is '^]'. Connection closed by foreign host.
As you see the session was “
connected“. If you get an output like below
# telnet 220.127.116.11 53 Trying 18.104.22.168... telnet: connect to address 22.214.171.124: Network is unreachable
then it means there is a problem with internet connectivity.
nmap is normally a port scanner to check the list of open ports on a system. We will use this to connect to external network to scan the port. If it is able to connect to the external network for port scanning then we can check if connected to internet.
# nmap -p 443 google.com Starting Nmap 7.70 ( https://nmap.org ) at 2019-11-19 07:40 IST Nmap scan report for google.com (126.96.36.199) Host is up (0.054s latency). Other addresses for google.com (not scanned): 2404:6800:4007:811::200e DNS record for 188.8.131.52: maa05s09-in-f14.1e100.net PORT STATE SERVICE 443/tcp open https Nmap done: 1 IP address (1 host up) scanned in 0.77 seconds
Here I am scanning google.com on port 443. As you see highlighted section, nmap was able to establish connection with google.com
7. netcat or nc
In some variant of Linux you will find
netcat while in others
nc, you can use either of these tools for port scanning.
ncat is part of
nmap-ncat rpm. Here we use
nc command to check connection to google.com on port 443
# nc -vz google.com 443 Ncat: Version 7.70 ( https://nmap.org/ncat ) Ncat: Connected to 184.108.40.206:443. Ncat: 0 bytes sent, 0 bytes received in 0.07 seconds.
GNU Wget is a free utility for non-interactive download of files from the Web. But we can also use this to check if connected to internet.
# wget -q --spider http://google.com ; echo $? 0
--spider Wget will behave as a Web spider, which means that it will not download the pages, just check that they are there
echo $? we check the exit status, anything other than
0 in the output means your system is not connected to internet
traceroute command shows every step taken on the route from your machine to a specified host. Assuming you are having problems connecting to
www.google.com so you can use
traceroute to check all the hubs taken to reach the destination server. This will also test internet connection.
# traceroute www.google.com traceroute to www.google.com (220.127.116.11), 30 hops max, 60 byte packets 1 _gateway (192.168.0.1) 3.046 ms 2.873 ms 3.291 ms 2 10.249.0.1 (10.249.0.1) 4.148 ms 4.676 ms 4.678 ms 3 broadband.actcorp.in (18.104.22.168) 5.153 ms 5.445 ms 6.305 ms 4 broadband.actcorp.in (22.214.171.124) 4.952 ms 6.212 ms 6.195 ms 5 * * * 6 126.96.36.199 (188.8.131.52) 48.202 ms 49.669 ms 49.504 ms 7 184.108.40.206 (220.127.116.11) 50.061 ms 18.104.22.168 (22.214.171.124) 48.480 ms 48.512 ms 8 126.96.36.199 (188.8.131.52) 49.911 ms 56.915 ms 184.108.40.206 (220.127.116.11) 57.610 ms 9 * * * 10 * * * 11 * * * 12 * * *
Here we were able to trace the route to google.com upto a certain point.
What do those * * * mean?
Each one indicates a five-second timeout at that hop. Sometimes that could indicate that the machine simply doesn’t understand how to cope with that traceroute packet due to a bug, but a consistent set of * indicates that there’s a problem somewhere with the router to which 18.104.22.168 hands off packets.
On a Linux system with no internet connection, you would face problem with DNS resolution itself
# traceroute www.google.com www.google.com: Name or service not known Cannot handle "host" cmdline arg `www.google.com' on position 1 (argc 1)
How To check if connected to internet using shell script?
You can easily use any or all the above commands/methods to check if connected to internet using bash/shell script.
For example by using
wget you can test internet connection in the below shell script:
#!/bin/bash wget -q --spider https://google.com if [ $? -eq 0 ]; then echo "Online" else echo "Offline" fi
Here this script will print the status of the internet, similarly for other scripts you can check the exit status and perform tasks accordingly.
Lastly I hope the steps from the article to check if connected to internet (test internet connection) on Linux was helpful. So, let me know your suggestions and feedback using the comment section.