Master CentOS7 Network Configuration: ifconfig, ifup/down, and Essential Commands

1. ifconfig network parameters

ifconfig can manually start, view, and modify network interface parameters.

[root@linux ~]# ifconfig {interface} {up|down}  <== observe and start interface
[root@linux ~]# ifconfig interface {options}    <== set and modify interface
Parameters:
Interface – network device name (e.g., eth0, eth1, ppp0)
Options – possible arguments such as:
  up or down – bring the interface up or down (no other parameters)
  mtu – set MTU size, e.g., mtu 1500 (bytes)
  netmask – subnet mask
  broadcast – broadcast address
Example – list all interfaces:
[root@linux ~]# ifconfig
eth0 Link encap:Ethernet  HWaddr 00:0F:EA:A3:06:A2
      inet addr:192.168.10.100  Bcast:192.168.10.255  Mask:255.255.255.0
      inet6 addr: fe80::20f:eaff:fe73:682/64 Scope:Link
      UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
      RX packets:3439 errors:0 dropped:0 overruns:0 frame:0
      TX packets:2735 errors:0 dropped:0 overruns:0 carrier:0
      collisions:0 txqueuelen:1000
      RX bytes:646935 (631.7 KiB)  TX bytes:562313 (549.1 KiB)
      Interrupt:209 Memory:fb000000-0

Running ifconfig without arguments lists all active interfaces, showing hardware address, IP, broadcast, netmask, and traffic statistics.

eth0 – interface name (also lo for loopback). HWaddr – MAC address. inet addr – IPv4 address; Bcast and Mask are broadcast and netmask. inet6 addr – IPv6 address (not covered here). RX – received packets, errors, dropped, etc. TX – transmitted packets and related stats. collisions – packet collisions count. txqueuelen – transmit queue length. RX Bytes / TX Bytes – total bytes received and transmitted. Interrupt, Memory – hardware IRQ and memory address.

2. ifup and ifdown

Use ifup and ifdown to apply settings from configuration files located in /etc/sysconfig/network-scripts, such as ifcfg-eth0.

[root@linux ~]# ifup {interface}
[root@linux ~]# ifdown {interface}

[root@linux ~]# ifup eth0

ifup and ifdown are simple scripts that read the corresponding ifcfg-ethX file and apply its settings. Ensure the config file exists; otherwise the operation fails. After modifying an interface with ifconfig , use ifconfig eth0 down to deactivate it, because ifdown compares current parameters with the config file and aborts if they differ.

3. CentOS7 network configuration files

/etc/resolv.conf – DNS configuration file /etc/hosts – hostname to IP mapping /etc/sysconfig/network – global network and routing information (only the last gateway is effective) /etc/sysconfig/network-scripts/ifcfg- interface – per‑interface configuration

Each NIC should have a single configuration file; if multiple files exist, the later one overrides earlier settings.

4. Default NIC configuration file (CentOS7)

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=eno16777736
UUID=f47bde51-fa78-4f79-b68f-d5dd90cfc698
DEVICE=ens33
ONBOOT=no
HWADDR="00:0C:29:EB:90:72"

5. Additional useful commands

ip [options] {link|addr|route...}
# ip addr show                     # show interface IP info
# ip addr add 192.168.0.1/24 dev eth0   # add IP address
# ip addr del 192.168.0.1/24 dev eth0   # delete IP address

# ip link show                     # show link info
# ip link set eth0 up               # bring interface up
# ip link set eth0 down             # bring interface down
# ip link set eth0 promisc on       # enable promiscuous mode
# ip link set eth0 promisc off      # disable promiscuous mode
# ip link set eth0 txqueuelen 1200  # set transmit queue length
# ip link set eth0 mtu 1400         # set MTU

# ip route show                     # display routing table
# ip route add 192.168.4.0/24 via 192.168.0.254 dev eth0   # add route
# ip route add default via 192.168.0.254 dev eth0          # set default gateway
# ip route del 192.168.4.0/24                               # delete route
# ip route del default                                      # delete default route

6. Summary

Cloned virtual machines often encounter network issues due to a new MAC address. Update the HWADDR in the configuration file under /etc/sysconfig/network-scripts/ and restart the network with systemctl start network . If problems persist, check or delete the MAC entry in /etc/udev/rules.d/70-persistent-ipoib.rules .