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

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

Using nmcli

nmcli c reload
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
TYPE="Ethernet"
PROXY_METHOD="none"
BROWSER_ONLY="no"
BOOTPROTO="none"
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="ens37"
DEVICE="ens37"
ONBOOT="yes"
IPADDR=192.168.1.188
NETMASK=255.255.255.0
GATEWAY=192.168.1.1

nmcli c reload
ifdown ens37
ifup ens37

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

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

Common nmcli Commands

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

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