Check network connectivity on a Linux server

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This article describes how to check networking from outside the Linux® server by using tools such as ping,
traceroute, nmap, and so on.

Troubleshooting tools

Often, you need to troubleshoot network connectivity on a server, but you cannot log in to the server.
These tools help you to troubleshoot problems from outside the server.

`ping` and `traceroute`

These tools are the most basic tools for troubleshooting network connectivity from outside the server.

The ping command sends a packet request to the server. If the packet reaches the server, a reply
similar to the following example displays:

$ ping google.com
PING google.com (172.217.1.46) 56(84) bytes of data.
64 bytes from ord37s07-in-f46.1e100.net (172.217.1.46): icmp_seq=1 ttl=54 time=1.76 ms
64 bytes from ord37s07-in-f46.1e100.net (172.217.1.46): icmp_seq=2 ttl=54 time=1.84 ms
64 bytes from ord37s07-in-f46.1e100.net (172.217.1.46): icmp_seq=3 ttl=54 time=1.73 ms
64 bytes from ord37s07-in-f46.1e100.net (172.217.1.46): icmp_seq=4 ttl=54 time=1.79 ms
^C
--- google.com ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 9015ms
rtt min/avg/max/mdev = 1.683/1.859/2.444/0.203 ms

Note: Some servers have Internet Control Message Protocol (ICMP) disabled, which means that ping
probes are blocked. Thus, you don't get a response from ping, but that doesn't mean networking is down.

The traceroute command traces the network hops from your local machine to the host server. This
tool helps you to determine if there is an issue between your computer and the host server. The following
example shows the traceroute command and its output:

$ traceroute google.com
traceroute to google.com (172.217.1.142), 30 hops max, 60 byte packets
1  23.253.71.2 (23.253.71.2)  1.098 ms 23.253.71.3 (23.253.71.3)  1.004 ms 23.253.71.2 (23.253.71.2)  1.157 ms
2  aggr160a-4-core10.dfw3.rackspace.net (98.129.84.204)  1.074 ms  1.725 ms aggr160b-4-core10.dfw3.rackspace.net   (98.129.84.206)  1.712 ms
3  corec-core9.dfw3.rackspace.net (148.62.41.124)  1.009 ms corec-core10.dfw3.rackspace.net (148.62.41.134)  1.376 ms      corec-core9.dfw3.rackspace.net (148.62.41.136)  1.171 ms
4  dcpe3-corec.dfw3.rackspace.net (148.62.41.96)  2.009 ms dcpe4-cored.dfw3.rackspace.net (148.62.41.98)  1.994 ms dcpe3-cored.dfw3.rackspace.net (148.62.41.100)  2.063 ms
5  be2-mspe4.dfw30.rackspace.net (10.25.1.119)  2.724 ms  2.792 ms  2.881 ms
6  72.14.221.26 (72.14.221.26)  2.517 ms  1.317 ms  1.366 ms
7  108.170.252.129 (108.170.252.129)  1.406 ms  1.508 ms 108.170.252.161 (108.170.252.161)  2.480 ms
8  209.85.244.59 (209.85.244.59)  2.212 ms  2.399 ms 209.85.243.255 (209.85.243.255)  2.431 ms
9  atl14s07-in-f142.1e100.net (172.217.1.142)  1.768 ms  1.722 ms  1.817 ms

`curl`

The curl command, used to transfer data to or from a server, has many features and tools. For
troubleshooting a website, use the flags IL and ILk (for use with HTTPS sites). These flags return the
status codes of the sites. They also list any redirects that are in front of the server. The output
looks similar to the following example:

$ curl -IL https://google.com
HTTP/1.1 301 Moved Permanently
Location: https://www.google.com/
Content-Type: text/html; charset=UTF-8
Date: Sat, 06 Jun 2020 00:51:12 GMT
Expires: Mon, 06 Jul 2020 00:51:12 GMT
Cache-Control: public, max-age=2592000
Server: gws
Content-Length: 219
X-XSS-Protection: 0
X-Frame-Options: SAMEORIGIN

HTTP/1.1 200 OK
Content-Type: text/html; charset=ISO-8859-1
P3P: CP="This is not a P3P policy! See g.co/p3phelp for more info."
Date: Sat, 06 Jun 2020 00:51:12 GMT
Server: gws
X-XSS-Protection: 0
X-Frame-Options: SAMEORIGIN
Transfer-Encoding: chunked
Expires: Sat, 06 Jun 2020 00:51:12 GMT
Cache-Control: private
Set-Cookie: 1P_JAR=2020-06-06-00; expires=Mon, 06-Jul-2020 00:51:12 GMT; path=/; domain=.google.com; Secure
Set-Cookie:    NID=204=TAwW41Xd8265avwh4_xx_BkbXwhIgpKFV4pADCq00xw4drIPhdWNJaKlJplrxby8xJksIntdx3uBERj9S1gHTrf56S4XPpCAEjI0uZmpaQIeS79ZfYeMGh33hq4cnGZLU6g0ATclpSK7ZNOEIroqLuDMYvfUuJ6FgeFbYJaQ1fQ; expires=Sun, 06-Dec-2020 00:51:12 GMT; path=/; domain=.google.com; HttpOnly


$ curl -ILk https://google.com
HTTP/1.1 301 Moved Permanently
Location: https://www.google.com/
Content-Type: text/html; charset=UTF-8
Date: Sat, 06 Jun 2020 00:51:28 GMT
Expires: Mon, 06 Jul 2020 00:51:28 GMT
Cache-Control: public, max-age=2592000
Server: gws
Content-Length: 220
X-XSS-Protection: 0
X-Frame-Options: SAMEORIGIN
Alt-Svc: h3-27=":443"; ma=2592000,h3-25=":443"; ma=2592000,h3-T050=":443"; ma=2592000,h3-Q050=":443"; ma=2592000,h3-Q049=":443"; ma=2592000,h3-Q048=":443"; ma=2592000,h3-Q046=":443"; ma=2592000,h3-Q043=":443"; ma=2592000,quic=":443"; ma=2592000; v="46,43"

HTTP/1.1 200 OK
Content-Type: text/html; charset=ISO-8859-1
P3P: CP="This is not a P3P policy! See g.co/p3phelp for more info."
Date: Sat, 06 Jun 2020 00:51:28 GMT
Server: gws
X-XSS-Protection: 0
X-Frame-Options: SAMEORIGIN
Transfer-Encoding: chunked
Expires: Sat, 06 Jun 2020 00:51:28 GMT
Cache-Control: private
Set-Cookie: 1P_JAR=2020-06-06-00; expires=Mon, 06-Jul-2020 00:51:28 GMT; path=/; domain=.google.com; Secure
Set-Cookie: NID=204=A4RYGnwtQmMAh8q4rF6bajtuadHU9OU0AbEfw529GZ74aeLM2voJmdhhG67rBGPmzWKPEea38LBu0FcSOEV1Fjiqnet26amxXQaCsRigOMuhOO4RCkSozOFZCn6uX00DWvDZ4ps3n0aoupaDQh1GrwGtEmkpfJ7_7sO4VHIaUmg; expires=Sun, 06-Dec-2020 00:51:28 GMT; path=/; domain=.google.com; HttpOnly
Alt-Svc: h3-27=":443"; ma=2592000,h3-25=":443"; ma=2592000,h3-T050=":443"; ma=2592000,h3-Q050=":443"; ma=2592000,h3-Q049=":443"; ma=2592000,h3-Q048=":443"; ma=2592000,h3-Q046=":443"; ma=2592000,h3-Q043=":443"; ma=2592000,quic=":443"; ma=2592000; v="46,43"

`nmap`

Use the nmap command to scan the ports for a specific host. Some servers do not have this utility
installed by default, so you might need to install it on your local machine. If you cannot connect
to a server by using a specific protocol (even though the server is running), you can run the nmap
command to see if the port is closed or filtered. Here is an example of the output:

$ nmap x.x.x.x

Starting Nmap 6.40 ( https://nmap.org ) at 2020-06-06 00:56 UTC
Nmap scan report for x.x.x.x
Host is up (0.037s latency).
Not shown: 992 closed ports
PORT     STATE SERVICE
22/tcp   open  ssh
25/tcp   open  smtp
80/tcp   open  http
443/tcp  open  https
8080/tcp open  http-proxy
8443/tcp open  https-alt

Nmap done: 1 IP address (1 host up) scanned in 0.61 seconds

If you try all of these methods and still cannot reach the server, see
Troubleshoot a downed Linux cloud server
for more steps.

Updated about 1 year ago

Troubleshooting tools

ping and traceroute

curl

nmap

`ping` and `traceroute`

`curl`

`nmap`