Network Configuration in CentOS

Introduction

To better understand this article, especially if you have concerns about IP address, netmask, subnet, etc. I recommend to go through the

«Networks» article.

Network Configuration

To study connections execute one of the following commands:

• sudo ls /sys/class/net/
• ip a s
• ifconfig
• nmcli conn show

E.g.:

nmcli conn show

NAME UUID TYPE DEVICE enp0s3 64486a76-a4b0-4693-9110-205da942c9c5 ethernet enp0s3 enp0s8 fc26857f-4cb0-3756-a7fa-95ec246b3981 ethernet enp0s8

If some interface, for example enp0s3, is disabled, it will have a -- in the DEVICE column

Usually the inactive interface is highlighted in gray and the active one in green.

NAME UUID TYPE DEVICE enp0s3 64486a76-a4b0-4693-9110-205da942c9c5 ethernet -- enp0s8 fc26857f-4cb0-3756-a7fa-95ec246b3981 ethernet enp0s8

You can enable the interface with the command

nmcli conn up enp0s3

Enable the interface when loading

To not turn on the network interface manually each time, you can specify ONBOOT=yes in the settings

Alternatively, you can go to

/etc/sysconfig/network-scripts/

And edit ifcfg-INTERFACE_NAME file - e.g. ifcfg-enp0s3 e.g. with vi

sudo vi /etc/sysconfig/network-scripts/ifcfg-enp0s3

Or use sed as root

sudo sed -i s/ONBOOT=no/ONBOOT=yes/ /etc/sysconfig/network-scripts/ifcfg-enp0s3

If the command above is not fully clear to you, study the article «sed basics»

To check whether it was possible to change ONBOOT to yes, you can use the command

grep ONBOOT !$

Detailed information about each network interface can be obtained by running nmcli connection show interface name

For example:

nmcli connection show ens192

connection.id: ens192 connection.uuid: 5c2584c5-7d87-4826-ba2a-79713eb62a9a connection.stable-id: -- connection.type: 802-3-ethernet connection.interface-name: ens192 connection.autoconnect: yes connection.autoconnect-priority: 0 connection.autoconnect-retries: -1 (default) connection.multi-connect: 0 (default) connection.auth-retries: -1 connection.timestamp: 1618914913 connection.read-only: no connection.permissions: -- connection.zone: -- connection.master: -- connection.slave-type: -- connection.autoconnect-slaves: -1 (default) connection.secondaries: -- connection.gateway-ping-timeout: 0 connection.metered: unknown connection.lldp: default connection.mdns: -1 (default) connection.llmnr: -1 (default) 802-3-ethernet.port: -- 802-3-ethernet.speed: 0 802-3-ethernet.duplex: -- 802-3-ethernet.auto-negotiate: no 802-3-ethernet.mac-address: -- 802-3-ethernet.cloned-mac-address: -- 802-3-ethernet.generate-mac-address-mask:-- 802-3-ethernet.mac-address-blacklist: -- 802-3-ethernet.mtu: auto 802-3-ethernet.s390-subchannels: -- 802-3-ethernet.s390-nettype: -- 802-3-ethernet.s390-options: -- 802-3-ethernet.wake-on-lan: default 802-3-ethernet.wake-on-lan-password: -- ipv4.method: auto ipv4.dns: -- ipv4.dns-search: -- ipv4.dns-options: "" ipv4.dns-priority: 0 ipv4.addresses: -- ipv4.gateway: --

Set Static IP

nmcli conn show

Setting up a static IP address for enp0s8

The network settings are stored in the directory

/etc/sysconfig/network-scripts/

You need to edit the ifcfg-enp0s8 flle

Basic parameters:
TYPE - connection type, wired (Ethernet), Wireless(Wired), etc;
BOOTPROTO - a way to get an IP address, static, dhcp or none;
NAME - the name of the connection;
DEVICE - name of the network interface;
ONBOOT - is it necessary to start at system startup;
IPADDR - The IP address that will be used for this computer;
GATEWAY - gateway for Internet access;
NETMASK - network mask;
DNS1 is a DNS domain name resolution server.

sudo vi /etc/sysconfig/network-scripts/ifcfg-enp0s8

TYPE=Ethernet PROXY_METHOD=none BROWSER_ONLY=no BOOTPROTO=dhcp DEFROUTE=yes IPV4_FAILURE_FATAL=no IPV6INIT=yes IPV6_AUTOCONF=yes IPV6_DEFROUTE=yes IPV6_FAILURE_FATAL=no IPV6_ADDR_GEN_MODE=stable-privacy NAME=enp0s8 UUID=939a0551-f5b5-46d8-87fe-24e5e7b1a0c1 DEVICE=enp0s8 ONBOOT=yes

Change IP

You can quickly change the IP using ifconfig

First you need to install the package containing ifconfig - according to the instructions

You need to change the IP with the command

sudo ifconfig enp0s8 192.168.56.111

Instead of enp0s8, enter the desired interface

Instead of 192.168.56.111, enter the IP you want to set

Если нужно поменять ещё и маску подсети - execute

sudo ifconfig enp0s8 192.168.56.111 netmask 255.255.255.0

Change Mask

If you need to change only the subnet mask execute

sudo ifconfig enp0s8 netmask 255.255.255.0

Add IP

To add an IP to the interface execute

ip addr add 172.17.67.3/16 dev enp0s8

IP address is added in a dynamic way - it will disappear after reboot.

To delete this IP address execute

ip addr del 172.17.67.3/16 dev enp0s8

Release IP

If IP was obtained via DHCP you can release it by running

sudo dhclient -r enp0s8

Type your own adapter name instead of enp0s8

Get Gateway

To get the gateway IP address execute

ip r

default via 10.0.2.1 dev enp0s3 proto dhcp metric 100

IP address of the gea 10.0.2.1

Install ifconfig

yum provides ifconfig

or

yum whatprovides ifconfig

Loaded plugins: fastestmirror Loading mirror speeds from cached hostfile * base: ftp.funet.fi * epel: www.nic.funet.fi * extras: ftp.funet.fi * updates: ftp.funet.fi base/7/x86_64/filelists_db | 7.2 MB 00:00:01 epel/x86_64/filelists_db | 12 MB 00:00:03 extras/7/x86_64/filelists_db | 224 kB 00:00:00 ssh-products/x86_64/filelists_db | 89 kB 00:00:00 updates/7/x86_64/filelists_db | 3.4 MB 00:00:01 net-tools-2.0-0.25.20131004git.el7.x86_64 : Basic networking tools Repo : base Matched from: Filename : /sbin/ifconfig

ifconfig is a part of net-tools

Install net-tools

sudo yum install net-tools

Open Ports

To get list of open ports execute

ss -tulpn

Netid State Recv-Q Send-Q Local Address:Port Peer Address:Port

Use grep to check a specific port

ss -tulpn | grep 1234

tcp LISTEN 0 128 [::]:1234 [::]:* users:(("andrei",pid=5226,fd=14))

Open port

To open port XXXX in the firewall execute

sudo firewall-cmd --add-port=XXXX/tcp --permanent
sudo firewall-cmd --reload

You can find more details here: Centos firewall

hostname

Step 1. Check current hostname

To get host info execute

hostnamectl

Static hostname: localhost.localdomain Icon name: computer-vm Chassis: vm Machine ID: a714bee8007d114a84177dbb15072f2e Boot ID: b02d9498c1764a338c781672aac1be29 Virtualization: kvm Operating System: CentOS Linux 7 (Core) CPE OS Name: cpe:/o:centos:centos:7 Kernel: Linux 3.10.0-1160.el7.x86_64 Architecture: x86-64

Required info is in the first line.

Static hostname: localhost.localdomain

Step 2. Change hostname

Since CentOS 7 only supports Fully Qualified Domain Name (Fqdn), I advise you to carefully check the hostname that you plan to use.

Acceptable values:

• Lowercase letters from a to z
• Numbers from 0 to 9
• Dots and hyphens
• The hostname can be from 2 to 63 characters
• Hostnames must start and end with a number or letter

Execute

hostnamectl set-hostname my.new-hostname.server

You can find more details here: hostname

NetworkManager

You can check the status with the command

systemctl status NetworkManager

● NetworkManager.service - Network Manager Loaded: loaded (/usr/lib/systemd/system/NetworkManager.service; enabled; vendor preset: enabled) Active: active (running) since Thu 2021-02-18 17:30:13 EET; 1 months 30 days ago Docs: man:NetworkManager(8) Main PID: 841 (NetworkManager) Tasks: 5 CGroup: /system.slice/NetworkManager.service ├─ 841 /usr/sbin/NetworkManager --no-daemon └─9599 /sbin/dhclient -d -q -sf /usr/libexec/nm-dhcp-helper -pf /var/run/dhclient-ens192.pid -lf /var/lib/NetworkManager/dhclient-5c2584c5-7d87-4826-ba2a-79713eb62a9a-ens192.lease -cf /var/lib/Net... Aug 30 17:17:26 localhost.localdomain NetworkManager[841]: <info> [1618907115.2914] dhcp4 (ens192): gateway 10.1.103.254 Aug 30 17:17:26 localhost.localdomain NetworkManager[841]: <info> [1618907115.2914] dhcp4 (ens192): lease time 7200 Aug 30 17:17:26 localhost.localdomain NetworkManager[841]: <info> [1618907115.2915] dhcp4 (ens192): nameserver '10.10.10.1' Aug 30 17:17:26 localhost.localdomain NetworkManager[841]: <info> [1618907115.2915] dhcp4 (ens192): nameserver '10.1.0.2' Aug 30 17:17:26 localhost.localdomain NetworkManager[841]: <info> [1618907115.2915] dhcp4 (ens192): domain name 'eth1.ru' Aug 30 17:17:26 localhost.localdomain NetworkManager[841]: <info> [1618907115.2915] dhcp (ens192): domain search 'eth1.ru.' Aug 30 17:17:26 localhost.localdomain NetworkManager[841]: <info> [1618907115.2915] dhcp (ens192): domain search 'hel.fi.eth1.ru.' Aug 30 17:17:26 localhost.localdomain NetworkManager[841]: <info> [1618907115.2915] dhcp (ens192): domain search 'ad.eth1.ru.' Aug 30 17:17:26 localhost.localdomain NetworkManager[841]: <info> [1618907115.2915] dhcp4 (ens192): state changed bound -> bound Aug 30 17:17:26 localhost.localdomain dhclient[9599]: bound to 10.1.102.211 -- renewal in 2903 seconds.

network

You can check status with the following command

systemctl status network

● network.service - LSB: Bring up/down networking Loaded: loaded (/etc/rc.d/init.d/network; bad; vendor preset: disabled) Active: active (exited) since Tue 2021-04-20 12:57:04 EEST; 7min ago Docs: man:systemd-sysv-generator(8) Process: 13493 ExecStop=/etc/rc.d/init.d/network stop (code=exited, status=0/SUCCESS) Process: 13652 ExecStart=/etc/rc.d/init.d/network start (code=exited, status=0/SUCCESS) Aug 30 17:17:26 localhost.localdomain systemd[1]: Starting LSB: Bring up/down networking... Aug 30 17:17:26 localhost.localdomain network[13652]: Bringing up loopback interface: [ OK ] Aug 30 17:17:26 localhost.localdomain network[13652]: Bringing up interface ens192: Connection successfully activated (D-Bus active path: /org/freedesktop/NetworkManager/ActiveConnection/5) Aug 30 17:17:26 localhost.localdomain network[13652]: [ OK ] Aug 30 17:17:26 localhost.localdomain systemd[1]: Started LSB: Bring up/down networking.

Virtual Interface

A virtual interface is an alias to an existing interface. Let's say you have ens192

Do the following

cp /etc/sysconfig/network-scripts/ens192 /etc/sysconfig/network-scripts/ens192:0
vi /etc/sysconfig/network-scripts/ens192:0

Change something there, for example, set a different IP from the same subnet

IPADDR=10.1.10.2

Restart network

systemctl restart network

Since the virtual interface can only be inside the same subnet, it will not give you the opportunity to listen to another session.

Create a new network adapter

Adapter list

nmcli d

DEVICE TYPE STATE CONNECTION wlp0s20f3 wifi connected SSH-office docker0 bridge connected docker0 p2p-dev-wlp0s20f3 wifi-p2p disconnected -- enp0s31f6 ethernet unavailable -- vboxnet0 ethernet unmanaged -- lo loopback unmanaged --

Errors and Warnings

sudo systemctl restart networking

Job for network.service failed because the control process exited with error code. See "systemctl status network.service" and "journalctl -xe" for details.

systemctl status network.service

● network.service - LSB: Bring up/down networking Loaded: loaded (/etc/rc.d/init.d/network; bad; vendor preset: disabled) Active: failed (Result: exit-code) since Tue 2021-01-19 19:09:42 EET; 7s ago Docs: man:systemd-sysv-generator(8) Process: 2448 ExecStop=/etc/rc.d/init.d/network stop (code=exited, status=0/SUCCESS) Process: 8268 ExecStart=/etc/rc.d/init.d/network start (code=exited, status=1/FAILURE) Aug 30 17:17:26 server1.example.com network[8268]: RTNETLINK answers: File exists Aug 30 17:17:26 server1.example.com network[8268]: RTNETLINK answers: File exists Aug 30 17:17:26 server1.example.com network[8268]: RTNETLINK answers: File exists Aug 30 17:17:26 server1.example.com network[8268]: RTNETLINK answers: File exists Aug 30 17:17:26 server1.example.com network[8268]: RTNETLINK answers: File exists Aug 30 17:17:26 server1.example.com network[8268]: RTNETLINK answers: File exists Aug 30 17:17:26 server1.example.com systemd[1]: network.service: control process exited, code=exited status=1 Aug 30 17:17:26 server1.example.com systemd[1]: Failed to start LSB: Bring up/down networking. Aug 30 17:17:26 server1.example.com systemd[1]: Unit network.service entered failed state. Aug 30 17:17:26 server1.example.com systemd[1]: network.service failed.