Network Configuration in Linux

Introduction

Network Configuration is one of the most common activities of a Linux administrator.

About setting up a network in Red Hat, CentOS and other .rpm distributions read here

Network Configuration

You can study a file with a list of interfaces involved using commands

vi /etc/network/interfaces

It depends on the version of Linux you are using.

New versions. Debian . For example, on Debian 9 it is used. Iproute2

If you need to set up a network on the old version of Debian using utilities ifconfig scroll down the page or go down reference

Network Configuration with Iproute2

Relevant for Debian 9 and above.

To get a list of interfaces you need to use command

ip a

There will be something like this coming out.

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: enp3s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000 link/ether 30:f9:ed:d9:ea:d3 brd ff:ff:ff:ff:ff:ff inet 192.0.2.131/24 brd 192.0.2.255 scope global enp3s0 valid_lft forever preferred_lft forever inet6 fe80::32f9:edff:fed9:ead3/64 scope link valid_lft forever preferred_lft forever 3: wlp1s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000 link/ether b8:76:3f:f5:c4:1f brd ff:ff:ff:ff:ff:ff inet 192.168.43.4/24 brd 192.168.43.255 scope global dynamic noprefixroute wlp1s0 valid_lft 2800sec preferred_lft 2800sec inet6 fe80::ba76:3fff:fef5:c41f/64 scope link noprefixroute valid_lft forever preferred_lft forever

Previously, interfaces were called eth0, eth1 and so on, but now they have more diverse names.



1: The first interface. lo It is directed to a local machine and has an IP. 127.0.0.1/8



2: Second interface enp3s0 This is my Ethernet connection to another PC.

In this example, a computer with Debian (1) is connected to a computer. on Windows (2) Directly via Ethernet cable

Through this connection, I go to SSH computer number 1.

IP computer with Debian you can see above 192.0.2.131/24 And a Windows IP computer. 192.0.2.130/24 Of course you can't see him here.

How to set up this IP read further



3: Third inteface. wlp1s0 It's a local Wi-Fi connection from a mobile phone. IP received dynamically 192.168.43.4/24

How to Install a Static IP

To tap into another computer via Ethernet you need to set in Windows Adapter parameters through the control panel. The same thing had to be done and Linux.

To do this, you need to execute command.

sudo vi /etc/network/interfaces

And add it.

auto enp3s0 iface enp3s0 inet static address 192.0.2.131 netmask 255.255.255.0 dns-nameservers 8.8.8.8,8.8.4.4 auto wlp1s0 iface wlp1s0 inet dhcp

Please note that the gateway is not listed.

And restart the network.

/etc/init.d/networking restart

or

sudo service networking restart

The second option is easier to remember, but to make it work need to have a service

Now if you have SSH installed, you can go to a computer with Linux on ssh using Putty or MobaXterm

If you want to install SSH execute

sudo apt install openssh-server

sudo apt-get install openssh-client

Insert / Turn off the network interface

Include the enp0s25 interface:

ip link set dev enp0s25 up

Turn off the enp0s25 interface:

ip link set dev enp0s25 down

Change the Gateway

Removing the Old Gateway

route delete default

Adding a new gateway

route add default gw IP_Address

If there are many old gateways, you need to specify which IP from which interface. remove

route delete default gw 10.0.0.1 eth0

Specific data can also be provided to create

route add default gw 192.168.0.1 eth0

Change netmas k

What is netmask or subnet mask you can find in the article «Networks»

Know your mask

ip r

default via 10.6.0.1 dev eth0
10.6.0.0/8 dev eth0 src 10.6.1.68

Remove the old one.

ip address del 10.6.1.100/8 dev eth0

Add new

ip address add 10.6.1.68/22 dev eth0

Listening ports

lsof

The lsof utility allows you to see everything open connections in the system.

To learn exactly the network connections use the option i

To display the ports, not the names of network services P

lsof -i -P

ss

The question of how to view the list of open ports can also be answered by the utility sss

ss -lntup

If you want to check if a specific port is being used, for example 5,000

ss -na | grep :5000

If the output is empty, the port is not used.

To find out which program listens to a particular port, you need to pre-installe one of them. instruments:

netstat, fuser or lsof

Let's start with netstat.

Netstat

sudo apt install net-tools

With commands. netstat You can see the list of active connections on the network.

netstat

Usually netstat is used with a set of tulpn options. This allows you to know which program is listening to which port.

netstat -tulpn

(Not all processes could be identified, non-owned process info will not be shown, you would have to be root to see it all.) Active Internet connections (only servers) Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN - tcp 0 0 127.0.0.1:631 0.0.0.0:* LISTEN - tcp 0 0 127.0.0.1:6010 0.0.0.0:* LISTEN - tcp 0 0 127.0.0.1:6011 0.0.0.0:* LISTEN - tcp6 0 0 :::22 :::* LISTEN - tcp6 0 0 ::1:631 :::* LISTEN - tcp6 0 0 ::1:6010 :::* LISTEN - tcp6 0 0 ::1:6011 :::* LISTEN - tcp6 0 0 :::80 :::* LISTEN - udp 0 0 0.0.0.0:52756 0.0.0.0:* - udp 0 0 0.0.0.0:68 0.0.0.0:* - udp 0 0 0.0.0.0:631 0.0.0.0:* - udp 0 0 0.0.0.0:5353 0.0.0.0:* - udp6 0 0 :::45815 :::* - udp6 0 0 :::5353 :::* -

Network Configuration with ifconfig

You can get information about existing interfaces on older versions of Debian using commands.

ifconfig

Study the output of this commands and see what interface is available to you. Usually. eth0. If you need to receive ip automatically

iface eth0 inet dhcp

If you need to set a static IP

iface eth0 inet static
address 192.168.1.1
netmask 255.255.255.0

The changes have come into force and must be implemented.

ifup eth0

Turn off wi-fi.

ifconfig wlan0 down

Enable/disable the network

ifconfig eth0 up/down

Restart/stop/start network services

/etc/init.d/networking restart/stop/start

You can configure DNS through a file /etc/resolv.conf

nameserver 8.8.8.8

Prix fitting You need to remember which network interfaces I suggest you configure Debian.

In older versions of Dbian it was simple, they were called eth0, eth1 and so on.

Therefore, to enable the network interface, in the above command, you need to change 0 to 1, etc.

In newer versions of Debian and other Unix-like distributions, for example OpenBSD Network interfaces are called more diverse (msk0, sk0, etc.).

In order to avoid difficulties with this, it is useful to write down what is proposed to be configured when installing a distribution.

Enable automatic receipt of ip

dhclient eth0

You can turn off the automatic receiving ip by pulling the network cord and inserting it back. They say that He should drop.

I suggest you do.

man interfaces

and study the conclusion.

iftop

You can monitor the volume of incoming and outgoing traffic on a certain interface. command command command command

iftop -i eth0