Master CentOS 7 Network Configuration: ifconfig, ifup/ifdown, and Essential Commands

1. ifconfig Network Parameters

ifconfig allows you to manually start, observe, 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 – up/down, mtu, netmask, broadcast, etc.

Example – display 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

Key fields: eth0 – interface name (lo is loopback) HWaddr – MAC address inet addr – IPv4 address (Bcast and Mask follow) inet6 addr – IPv6 address (often omitted) RX/TX – packets received/sent, errors, drops, collisions, txqueuelen, byte counters Interrupt/Memory – hardware IRQ and memory address

2. ifup and ifdown

These scripts read the configuration files under /etc/sysconfig/network‑scripts (e.g., ifcfg‑eth0) and bring the interface up or down accordingly.

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

[root@linux ~]# ifup eth0

Notes: ifup/ifdown are simple wrappers that locate the matching ifcfg‑ethX file. The file must exist in the correct directory; otherwise the operation fails. If you modify an interface with ifconfig , you must bring it down with ifconfig eth0 down because ifdown compares the current state with the ifcfg‑eth0 file and will abort if they differ.

3. CentOS 7 Network Configuration Files

Important system files:

/etc/resolv.conf – DNS configuration

/etc/hosts – hostname‑to‑IP mappings

/etc/sysconfig/network – global interface and routing information (only the last gateway is effective)

/etc/sysconfig/network-scripts/ifcfg‑* – per‑interface configuration

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

4. Default Network Interface Configuration File (example)

TYPE=Ethernet               # Interface type
PROXY_METHOD=none           # Proxy disabled
BROWSER_ONLY=no             # Not browser‑only
BOOTPROTO=dhcp              # DHCP boot protocol
DEFROUTE=yes                # Default route enabled
IPV4_FAILURE_FATAL=no       # IPv4 fatal error detection disabled
IPV6INIT=yes                # IPv6 auto‑initialization
IPV6_AUTOCONF=yes           # IPv6 auto‑configuration
IPV6_DEFROUTE=yes           # IPv6 default route
IPV6_FAILURE_FATAL=no       # IPv6 fatal error detection disabled
IPV6_ADDR_GEN_MODE=stable-privacy # IPv6 address generation mode
NAME=eno16777736            # Interface name (derived from file name)
UUID=f47bde51-fa78-4f79-b68f-d5dd90cfc698   # Unique identifier
DEVICE=ens33                # Device name
ONBOOT=no                   # Not started automatically; set to yes to enable at boot or use systemctl restart network
HWADDR="00:0C:29:EB:90:72"  # Physical MAC address

5. Additional Useful Commands

ip [options] {link|addr|route...}

# Show IP information
ip addr show
# Assign IP address
ip addr add 192.168.0.1/24 dev eth0
# Delete IP address
ip addr del 192.168.0.1/24 dev eth0

# Show link (interface) information
ip link show
# Bring interface up/down
ip link set eth0 up
ip link set eth0 down
# Enable/disable promiscuous mode
ip link set eth0 promisc on
ip link set eth0 promisc off
# Set queue length and MTU
ip link set eth0 txqueuelen 1200
ip link set eth0 mtu 1400

# Show routing table
ip route show   # or ip route list or route -n
# Add a specific route
ip route add 192.168.4.0/24 via 192.168.0.254 dev eth0
# Set default gateway
ip route add default via 192.168.0.254 dev eth0
# Delete a route
ip route del 192.168.4.0/24
# Delete default route
ip route del default

6. Summary and Troubleshooting

When cloning a virtual machine, network problems often stem from the new MAC address. After cloning, check the MAC in the VM settings → Network Adapter → Advanced. Update the HWADDR field in the appropriate /etc/sysconfig/network-scripts/ifcfg‑* file, then restart the network with systemctl start network . If issues persist, verify or delete the MAC entry in /etc/udev/rules.d/70-persistent-ipoib.rules .

Link for reference: https://www.cnblogs.com/hongdada/p/9883107.html