Checking network settings (Linux)
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This article lists the most important commands you need to check the network configuration.
Performing a Port Scan
To perform a port scan, you can use the netcat program. Netcat (nc), is a universal command line tool. It can be used in the terminal or in scripts for communication via TCP and UDP network connections (Internet Protocol Version 4 and Version 6).
The program netcat is part of every Ubuntu installation, but can be installed using the following commands, if necessary:
Ubuntu 18.04, Ubuntu 20.04, Ubuntu 22.04
[root@localhost ~]# apt-get update
[root@localhost ~]# apt-get install netcat
To install netcat on a server with the distribution CentOS, enter the following command:
CentOS 7, CentOS Stream 8 and 9
[root@localhost ~]# yum update
[root@localhost ~]# yum install -y nc
To perform a port scan with netcat, type the following command:
nc -v IP_ADDRESS_OF_THE_SERVER PORT_NUMBER
Example:
[root@localhost ~]# nc -v 192.168.1.1 22
Checking the Network Services
To get a list of listening network services, daemons, and programs, type the following command:
netstat –tulpen
If netstat is not installed, you can install it using the following command:
CentOS 7, CentOS Stream 8 and 9
[root@localhost ~]# sudo yum install net-tools
Ubuntu
[root@localhost ~]# sudo apt-get install net-tools
Then you can check if the relevant network services, daemons and programs are active and listening on the correct port. After entering the command, you can also determine whether the required port needs to be unblocked.
Example:
[root@localhost ~]# netstat -tulpen
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address Foreign Address State User Inode PID/Program name
tcp 0 0 0.0.0.0:5355 0.0.0.0:* LISTEN 193 27635 1368/systemd-resolv
tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN 0 29477 1582/sshd
tcp6 0 0 :::5355 :::* LISTEN 193 27638 1368/systemd-resolv
tcp6 0 0 :::22 :::* LISTEN 0 29479 1582/sshd
udp 0 0 0.0.0.0:5355 0.0.0.0:* 193 27634 1368/systemd-resolv
udp 0 0 127.0.0.53:53 0.0.0.0:* 193 27640 1368/systemd-resolv
udp 0 0 0.0.0.0:68 0.0.0.0:* 0 27510 1314/dhclient
udp 0 0 127.0.0.1:323 0.0.0.0:* 0 25505 1174/chronyd
udp6 0 0 :::5355 :::* 193 27637 1368/systemd-resolv
udp6 0 0 ::1:323 :::* 0 25506 1174/chronydChecking the Network Configuration
To display the configured interfaces, enter the command ip addr. After entering the command, the status of each interface is displayed:
Example:
root@localhost:~# ip addr
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
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: ens192: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
link/ether 00:50:56:00:f1:5b brd ff:ff:ff:ff:ff:ff
inet 217.160.173.123/32 brd 217.160.173.123 scope global ens192
valid_lft forever preferred_lft forever
inet6 fe80::250:56ff:fe00:f15b/64 scope link
valid_lft forever preferred_lft forever
You can reactivate a deactivated interface with the following command:
Ubuntu 22.04, CentOS Stream 8, CentOS Stream 9:
root@localhost:~# sudo ip link set NAME_OF_THE_INTERFACE up
Example:
root@localhost:~# sudo ip link set ens192 up
Ubuntu 20.04, CentOS 7
sudo ifup NAME_OF_THE_INTERFACE
Example:
[root@localhost ~]# sudo ifup ens192
If this command fails, it is possible that the interface is in a state unknown to the command script. In this case, enter the same command with the --force parameter:
sudo ifup --force NAME_OF_THE_INTERFACE
Example:
[root@localhost ~]# sudo ifup --force ens192
Then check, if you can establish an encrypted network connection to your server. If this is not possible, check the network configuration of the server.
For this purpose, open the configuration files listed below with the vi editor. Check and edit the settings and then restart the network if necessary to apply the changes to the network configuration:
Ubuntu 18.04, Ubuntu 20.04
/etc/network/interfaces
Ubuntu 22.04
/etc/netplan
CentOS 7
/etc/sysconfig/network-scripts/
CentOS Stream 8
/etc/sysconfig/network-scripts/ifcfg-ens192
CentOS Stream 9
/etc/NetworkManager/system-connections/ens192.nmconnection
To reboot the network, enter the following command(s):
Ubuntu 18.04, Ubuntu 20.04
[root@localhost ~]# /etc/init.d/networking restart
Ubuntu 22.04
If you are using Ubuntu 22.04, reboot the server. To do this, enter the following command:
[root@localhost ~]# systemctl reboot
CentOS 7
[root@localhost ~]# /etc/init.d/network restart
CentOS Stream 8 and CentOS Stream 9
If you are using CentOS Stream 8 or CentOS Stream 9, reboot the server. To do this, enter the following command:
[root@localhost ~]# systemctl reboot
Display and Configure IP Routing Table With Route
With the program route you can display and configure the IP routing table. To do so, enter the following command:
[root@localhost ~]# route
After entering the command, you will see results similar to the following example:
[root@localhost home]# route
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
default gateway 0.0.0.0 UG 100 0 0 ens192
gateway 0.0.0.0 255.255.255.255 UH 100 0 0 ens192
169.254.169.254 gateway 255.255.255.255 UGH 100 0 0 ens192
172.17.0.0 0.0.0.0 255.255.0.0 U 0 0 0 docker0
localhost 0.0.0.0 255.255.255.255 UH 100 0 0 ens192