Linux System Network Management with ifconfig, ip, and nmcli

Network Basics

IP addresses identify machines on a network (IPv4/IPv6). DNS resolves domain names to IP addresses, and a gateway (router) connects local networks to external networks.

Viewing Interface Information

Use ifconfig to list all interfaces, ifconfig ens33 for a specific NIC, or ip addr for detailed address information.

Network Interface Configuration Files

Configuration files are located in /etc/sysconfig/network-scripts. For example, ens33 corresponds to ifcfg-ens33 and ens35 to ifcfg-ens35.

TYPE="Ethernet"<br/>PROXY_METHOD="none"<br/>BROWSER_ONLY="no"<br/>BOOTPROTO="none"  ## none static static, dhcp dynamic<br/>DEFROUTE="yes"   ## default route<br/>IPV4_FAILURE_FATAL="no"<br/>IPV6INIT="yes"<br/>IPV6_AUTOCONF="yes"<br/>IPV6_DEFROUTE="yes"<br/>IPV6_FAILURE_FATAL="no"<br/>IPV6_ADDR_GEN_MODE="stable-privacy"<br/>NAME="ens33"   ## interface name<br/>UUID="e20ba92f-11c2-49b4-8542-36b5a3879173"  ## unique UUID<br/>DEVICE="ens33"<br/>ONBOOT="yes"  ## enable on boot<br/>IPADDR=192.168.1.199  ## IP address<br/>NETMASK=255.255.255.0  ## subnet mask

On CentOS 7, restart the network service with systemctl restart network.

Using nmcli nmcli c reload<br/>nmcli c up xxx Experiment: Adding a Test NIC

Copy an existing configuration file, modify the IP address, netmask, and gateway, then bring the interface up.

cp ifcfg-ens33 ifcfg-ens37

[root@myserver network-scripts]# cat ifcfg-ens37<br/>TYPE="Ethernet"<br/>PROXY_METHOD="none"<br/>BROWSER_ONLY="no"<br/>BOOTPROTO="none"<br/>DEFROUTE="yes"<br/>IPV4_FAILURE_FATAL="no"<br/>IPV6INIT="yes"<br/>IPV6_AUTOCONF="yes"<br/>IPV6_DEFROUTE="yes"<br/>IPV6_FAILURE_FATAL="no"<br/>IPV6_ADDR_GEN_MODE="stable-privacy"<br/>NAME="ens37"<br/>DEVICE="ens37"<br/>ONBOOT="yes"<br/>IPADDR=192.168.1.188<br/>NETMASK=255.255.255.0<br/>GATEWAY=192.168.1.1

nmcli c reload<br/>ifdown ens37<br/>ifup ens37

After bringing the interface up, ifconfig ens37 shows the new IP configuration.

[root@myserver ~]# cd /etc/sysconfig/network-scripts/<br/>[root@myserver network-scripts]# ls<br/>ifcfg-ens33  ifcfg-ens37  ifcfg-ens37-1<br/>[root@myserver network-scripts]# mv ifcfg-ens37 ifcfg-ens37~~~~<br/>[root@myserver network-scripts]# ls<br/>ifcfg-ens33  ifcfg-ens37~~~~  ifcfg-ens37-1<br/>[root@myserver ~]# nmcli connection modify ens37 ipv4.addresses 192.168.1.189<br/>[root@myserver ~]# ifconfig ens37<br/>ens37: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500<br/>        inet 192.168.1.188  netmask 255.255.255.255  broadcast 0.0.0.0<br/>        inet6 fe80::20c:29ff:fe94:173a  prefixlen 64  scopeid 0x20<link><br/>        ether 00:0c:29:94:17:3a  txqueuelen 1000  (Ethernet)<br/>        RX packets 449  bytes 47699 (46.5 KiB)<br/>        TX packets 530  bytes 59159 (57.7 KiB)<br/>[root@myserver ~]# nmcli connection reload<br/>[root@myserver ~]# nmcli connection up ens37<br/>Connection successfully activated (D-Bus active path: /org/freedesktop/NetworkManager/ActiveConnection/13)<br/>[root@myserver ~]# ifconfig ens37<br/>ens37: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500<br/>        inet 192.168.1.189  netmask 255.255.255.255  broadcast 0.0.0.0<br/>        inet6 fe80::20c:29ff:fe94:173a  prefixlen 64  scopeid 0x20<link><br/>        ether 00:0c:29:94:17:3a  txqueuelen 1000  (Ethernet)<br/>        RX packets 500  bytes 52058 (50.8 KiB)<br/>        TX packets 582  bytes 64767 (63.2 KiB)

Common nmcli Commands

# nmcli connection               # list connections<br/># nmcli con xxx                 # show details of a connection<br/># nmcli connection show xxx     # show interface details<br/># nmcli connection show --active# list active connections<br/># nmcli connection delete ens37 # delete a connection<br/># nmcli connection modify ens37 ipv4.addresses 192.168.1.189<br/># nmcli connection modify ens37 ipv4.addresses 192.168.1.189/24   # set netmask<br/># nmcli connection modify ens37 ipv4.method manual          # set static method<br/># nmcli connection modify ens37 +ipv4.addresses 192.168.1.189/24<br/># nmcli connection modify ens37 -ipv4.addresses 192.168.1.189/24<br/># nmcli connection modify ens37 ipv4.gateway 192.168.0.2<br/># nmcli connection modify ens37 ipv4.dns 114.114.114.114<br/># nmcli connection modify ens37 -ipv4.dns 114.114.114.114<br/># nmcli c reload<br/># nmcli c up ens37

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