Is it possible to put a real IP on a loopback device?

Question

Is it possible to put a real IP (not in the 127.x.x.x) range on a loopback device?

Answer 1

On the current Linux kernel with the ip utility it is quite simple:

ip addr add 10.0.1.8 dev lo

This can be useful when you have a service that binds a port on an interface, and want to run a different program on the same port and network. I use it to enable both bind and dnsmasq to co-exist on the same server.

If you are using /etc/network/interfaces to configure your interfaces, then update the lo stanza to include:

up ip addr add 10.0.1.8 dev lo

Answer 2

Nothing forbids to do it.

# ifconfig lo:1 10.0.0.1/8
# ifconfig lo:1
lo:1      Link encap:Local Loopback  
          inet addr:10.0.0.1  Mask:255.0.0.0
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
# ping -c 1 10.0.0.1    
PING 10.0.0.1 (10.0.0.1) 56(84) bytes of data.
64 bytes from 10.0.0.1: icmp_seq=1 ttl=64 time=0.025 ms

--- 10.0.0.1 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.025/0.025/0.025/0.000 ms

---

Update:

For this address to persist after a reboot on Ubuntu 16.04, you can modify your /etc/network/interfaces file with these ethtool commands:

auto lo lo:1

iface lo inet loopback

iface lo:1 inet static
  address 10.0.0.1
  network 10.0.0.0
  netmask 255.0.0.0

Answer 3

As an alternative to using lo:0, you can also use dummy interfaces in Linux as in:

ifconfig dummy0 10.0.0.1/32
ifconfig dummy1 10.1.1.1/24

In addition to others answers:

• I do not recommend changing the usual/official loopback interface address as a lot of functionality depends on it;
• however, you can have/create several loopback/dummy interfaces - either lo:0 to lo:255 or dummyX interfaces;
• it has to be taken into account that lo:0 to lo:255 are aliases, whilst dummyX are full interfaces;
• moreover, one of the usual tactics in Linux for creating virtual IPs over BGP or OSPF is assigning them to loopback/dummy interfaces AND making pathways to them via routing;
• again, some daemons have problems with announcing addresses in aliases (for instance, quagga ) - so dummyX interfaces are advised in those cases;
• I would stress that without routing in the infrastructure, such addresses are only known/capable of being used in the server in question;
• assigning a private/public address to a loopback interface, without the proper routing can be a low computing cost measure to blacklist communications temporarily with an IP address/network.

For more details, see for instance a BIND anycast setup tutorial done with Quagga/BIRD.

routing clues here: OSPF: Migrating Quagga to BIRD

P.S. Linux by default only creates dummy0 and dummy1 and has to be instructed to create a bigger number of dummy interfaces.

Answer 4

Yes, but that doesn't mean it's a good idea. If you use an IP that is ever going to be accessed by your system, any data that it tries to send there will be redirected to the local system instead, which can cause all kinds of odd networking issues. This means in particular that you can't safely use anything outside of the following ranges:

127.0.0.0/8
192.0.2.0/24
198.51.100.0/24
203.0.113.0/24
240.0.0.0/4

With the possible exception of any of the following dependent on how your other network interfaces are configured:

10.0.0.0/8
192.168.0.0/16
172.16.0.0/12
169.254.0.0/16

This is a case where RFC 1925, section 2, item 3 applies.