1.What is IP Alias:
IP alias / IP aliasing is associating more than one IP address to a physical network interface. With this, one node on a network can have multiple connections to a network, each serving a different purpose.Alias network interface is configured on top of existing physical network interface.
2.What needs to IP alias:
Physical Network Interface Connectivity in server.
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
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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