The -c 1 (or count) option tells ping to send only one ping. Remove it to
keep pinging once per second.
Unix Networking
Learn the basics of Unix networking and how to make TCP connections.
You will need
- A Unix CLI
- An Ubuntu server with a public IP address to connect to
Recommended reading
On this page
Table of contents
Useful commands
Useful commands for unix networking
The ip command
The ip command is used to manipulate and display IP network
information:
$> ip address
1: `lo`: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue ...
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet `127.0.0.1`/8 scope host lo
valid_lft forever preferred_lft forever
inet6 `::1`/128 scope host
valid_lft forever preferred_lft forever
2: `eth0`: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9001 ...
link/ether 06:5f:44:85:36:92 brd ff:ff:ff:ff:ff:ff
inet `172.31.39.219`/20 brd 172.31.47.255 scope global dynamic eth0
valid_lft 2665sec preferred_lft 2665sec
inet6 `fe80::45f:44ff:fe85:3692`/64 scope link
valid_lft forever preferred_lft forever
In this sample output, there are two network interfaces:
- The virtual loopback interface (
lo) through which applications can communicate on the computer itself without actually hitting the network - A physical Ethernet interface (
eth0) which has the private IP address172.31.39.219(i.e. the computer’s address in its local network)
The ping command
The ping command tests the reachability of a host on an IP network. It
measures the round-trip time (rtt) for messages sent to a computer
and echoed back. The name comes from active sonar terminology that
sends a pulse of sound and listens for the echo to detect objects under water.
It uses the Internet Control Message Protocol (ICMP), a network layer protocol (OSI layer 3).
$> ping -c 1 google.com
PING `google.com` (`172.217.21.238`) 56(84) bytes of data.
64 bytes from 172.217.21.238: icmp_seq=1 ttl=53 time=`1.12 ms`
--- google.com ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 1.125/1.125/1.125/0.000 ms
In this example, you can see that the domain name google.com was translated to the public IP address 172.217.21.238 by the Domain Name System,
and that the round-trip to that computer took about 1.12 milliseconds.
Tip
The ss command
The socket statistics (ss) command (or the older netstat command) displays information about the open network sockets on the computer.
A socket is the software representation of a network communication’s endpoint. For a TCP connection in an IP network, it corresponds to a connection made on an IP address and port number.
$> ss -tlpn
State Recv-Q Send-Q Local Address:Port Peer Address:Port Process
LISTEN 0 80 127.0.0.1:3306 0.0.0.0:* mysqld...
LISTEN 0 128 127.0.0.53%lo:53 0.0.0.0:* systemd-resolve...
LISTEN 0 128 0.0.0.0:22 0.0.0.0:* sshd...
LISTEN 0 128 [::]:22 [::]:* sshd...
The above command lists the processes listening for TCP connections. In this example, we can see that there is a MySQL database listening on port 3306, a DNS resolver on port 53, and an SSH server on port 22.
More information
On some systems, you may need to add the -e (extended) option to display
process information. You can remove the -n (or --numeric) option to see
service names (e.g. ssh instead of 22). The other options are -t for
TCP, -l to only display listening sockets, and -p to show the
processes.
The nc command
The netcat (nc) command can read from and write to network
connections using TCP or UDP.
$> nc -zv -w 2 google.com 80
Connection to google.com 80 port [tcp/http] succeeded!
$> nc -zv -w 2 google.com 81
nc: connect to google.com port 81 (tcp) timed out: Operation now in progress
nc: connect to google.com port 81 (tcp) failed: Network is unreachable
$> nc -zv -w 2 google.com 443
Connection to google.com 443 port [tcp/http] succeeded!
For example, the above two commands check whether ports 80, 81 and 443 are open
on the computer reached by resolving the domain name google.com.
More information
The -z (zero bytes) option tells netcat to close the connection as soon as
it opens, the -v option enables more verbose output, and the -w 2 tels
netcat to wait at most 2 seconds before giving up.