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Manage IP Alias on CentOS 6

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1. About IP Alias

IP alias / IP aliasing is assigning multiple IP addresses to a single physical network interface. Alias network interface is configured on top of existing physical network interface.

By IP aliasing we can make applications and services running on linux server available on multiple IP addresses by using only one physical network interfase.

Requirements for IP alias:

  • Physical Network Interface Connectivity
  • Available Multiple IP Addresses

Limitations of IP alias:

  • Subnet Aliasing (alias network interface IP address is preffered to be in the same network subnet as physical network interface below – if not proper network infrastructure configuration is needed)
  • DHCP (alias interfaces do not support DHCP)

Example: If physical network interface is named eth0, alias interfaces on top of this one are named eth0:0, eth0:1, … and if physical network interface is named eth1, alias interfaces on top of this one are names eth1:0, eth1:1 … and so on.

IP Alias Network Interface
IP Alias Network Interface

2. Non-Persistent IP Alias Configuration

Non-persistent IP aliases do not survive linux server reboot. This means IP alias is only configured for the time the server is up and running and will dissapear if our linux server is rebooted. The following is the linux command line syntax to configure non-persistent alias network interface where X numbers the configured physical network interface, Y numbers the desired alias interface starting with 0 and IPADDRESS is the IP address we want to assign to our alias network interface:

[root@foo1 ~]# ifconfig ethX:Y IPADDRESS up

 

We can check if alias network interface is up and running with the following linux command (look for the ethX:Y interface):

[root@foo1 ~]# ifconfig

 

Example: In the following example a physical interface eth0 is properly configured with tested network connectivity and has an IP address 192.168.1.100. We can configure an alias network interface called eth0:0 and IP address 192.168.1.101 with the following command:

[root@foo1 ~]# ifconfig eth0:0 192.168.1.101 up

 

We can check if alias network interface is up with ifconfig, looking for the eth0:0 and IP address 192.168.1.101:

[root@foo1 ~]# ifconfig
eth0      Link encap:Ethernet  HWaddr 08:00:27:A9:01:61  
          inet addr:192.168.1.100  Bcast:192.168.1.255  Mask:255.255.255.0
          inet6 addr: fe80::a00:27ff:fea9:161/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:4726 errors:0 dropped:0 overruns:0 frame:0
          TX packets:732 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:542695 (529.9 KiB)  TX bytes:115702 (112.9 KiB)

eth0:0    Link encap:Ethernet  HWaddr 08:00:27:A9:01:61  
          inet addr:192.168.1.101  Bcast:192.168.1.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:140 errors:0 dropped:0 overruns:0 frame:0
          TX packets:140 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:11760 (11.4 KiB)  TX bytes:11760 (11.4 KiB)

3. Persistent IP Alias Configuration

Persistent IP aliases survive linux server reboot and are configured in alias network configuration file. Network configuration files are stored in /etc/sysconfig/network-scripts/ directory and are named ifcfg-ethX, where X is a number of the physical network interface. Alias network configuration files are named ifcfg-ethX:Y, where X is the number of the physical network interface and Y is the number of the alias network interface.

The quickest and easiest way to create alias network interface configuration file is to copy an existing physical network interface configuration file with working network connectivity by running the following command:

[root@foo1 ~]# cp /etc/sysconfig/network-scripts/ifcfg-ethX /etc/sysconfig/network-scripts/ifcfg-ethX:Y

 

…where X is the number of the physical network interface and Y is the number of the alias network interface.

Next we need to edit the newly created file /etc/sysconfig/network-scripts/ifcfg-ethX:Y with the preferred file editor and replace the following physical network interface (ethX) entries with the desired new alias network interface (ethX:Y) entries.

Replace:

DEVICE=ethX

With:

DEVICE=ethX:Y

Find the IPADDR line of your ethX physical network interface:

IPADDR=XXX.XXX.XXX.XXX

And replace it with the desired alias network interface IP address:

IPADDR=YYY.YYY.YYY.YYY

When alias network interface configuration file is configured, we can bring the alias network interface up by running the following command:

[root@foo1 ~]# ifup ethX:Y

 

We can check if alias network interface is up with ifconfig command (look for the ethX:Y interface):

[root@foo1 ~]# ifconfig

 

Example: This is a working example of the physical network interface configuration file (eth0:0):

/etc/sysconfig/network-scripts/ifcfg-eth0

DEVICE=eth0
TYPE=Ethernet
ONBOOT=yes
BOOTPROTO=none
IPADDR=192.168.1.100
NETMASK=255.255.255.0
GATEWAY=192.168.1.1

 

And a working example of the alias network interface (eth0:0) on top of physical network interface (eth0):

/etc/sysconfig/network-scripts/ifcfg-eth0:0

DEVICE=eth0:0
TYPE=Ethernet
ONBOOT=yes
BOOTPROTO=none
IPADDR=192.168.1.149
NETMASK=255.255.255.0
GATEWAY=192.168.1.1

 

By running ifup command, we bring alias network interface up and running:

[root@foo1 ~]# ifup eth0:0

 

Checking for alias network interface eth0:0 with the IP address 192.168.1.101 with ifconfig command:

[root@foo1 ~]# ifconfig
eth0      Link encap:Ethernet  HWaddr 08:00:27:A9:01:61  
          inet addr:192.168.1.100  Bcast:192.168.1.255  Mask:255.255.255.0
          inet6 addr: fe80::a00:27ff:fea9:161/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:4726 errors:0 dropped:0 overruns:0 frame:0
          TX packets:732 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:542695 (529.9 KiB)  TX bytes:115702 (112.9 KiB)

eth0:0    Link encap:Ethernet  HWaddr 08:00:27:A9:01:61  
          inet addr:192.168.1.101  Bcast:192.168.1.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:140 errors:0 dropped:0 overruns:0 frame:0
          TX packets:140 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:11760 (11.4 KiB)  TX bytes:11760 (11.4 KiB)

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