What is 127.0.0.2 IP address for?

Question

I'm running the example in hiredis, which is using "127.0.0.2" as the redis server IP, and it is running properly. Actually, the redis server is running on the same machine. I know that "127.0.0.1" is the IP address of "lo", but how about "127.0.0.2"? Is it the same as "127.0.0.1"?

Answer 1

Yes:

• http://en.wikipedia.org/wiki/Loopback
• https://en.wikipedia.org/wiki/Localhost

IPv4 network standards reserve the entire 127.0.0.0/8 address block for loopback purposes. That means any packet sent to one of those 16,777,214 addresses (127.0.0.1 through 127.255.255.254) is looped back. IPv6 has just a single address, ::1.

Various Internet Engineering Task Force (IETF) standards reserve the IPv4 address block 127.0.0.0/8, in CIDR notation and the IPv6 address ::1 for this purpose. The most common IPv4 address used is 127.0.0.1. Commonly these loopback addresses are mapped to the hostnames, localhost or loopback.

or from the RFC itself:

• RFC 3330 - Special-Use IPv4 Addresses

127.0.0.0/8 - This block is assigned for use as the Internet host loopback address. A datagram sent by a higher level protocol to an address anywhere within this block should loop back inside the host. This is ordinarily implemented using only 127.0.0.1/32 for loopback, but no addresses within this block should ever appear on any network anywhere [RFC1700, page 5].

For fun, try by pinging:

$ ping 127.127.127.127
PING 127.127.127.127 (127.127.127.127) 56(84) bytes of data.
64 bytes from 127.127.127.127: icmp_req=1 ttl=64 time=0.110 ms
64 bytes from 127.127.127.127: icmp_req=2 ttl=64 time=0.065 ms
^C
--- 127.127.127.127 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 999ms
rtt min/avg/max/mdev = 0.065/0.087/0.110/0.024 ms

Answer 2

• “Are all 127.x.x.x addresses restricted to the local machine?” Yes
• “Are all 127.x.x.x addresses bound to the lo interface” Yes
• “Are 127.x.x.x addresses routed over the network?” No

127.0.0.0/8 - This block is assigned for use as the Internet host loopback address. A datagram sent by a higher-level protocol to an
address anywhere within this block loops back inside the host. This
is ordinarily implemented using only 127.0.0.1/32 for loopback. As
described in [RFC1122], Section 3.2.1.3, addresses within the entire 127.0.0.0/8 block do not legitimately appear on any network anywhere. — RFC5735

• “Is 127.0.0.2 the same as 127.0.0.1?” NO

While 127.0.0.1 to 127.255.255.254 are all local addresses bound to interface lo. They are not the same. You can use each address to bind a different service onto the same port. E.g 16 Million web-servers on port 80, only accessible from the local machine (If you don't run out of memory, or other resource first)

I have just set up a docker service to bind to 127.0.0.2:80. I have then added an alias to /etc/hosts. Now I can connect to it via http://myserver, but not via http://127.0.0.1 or http://localhost. However it is only available to this machine. As it is, only, on the lo interface.

I then set up another docker service to bind to 127.0.0.3:80, and a python service on localhost:80 and another on 127.0.0.4:80.

---

This may not work on all operating systems. I am using Debian(9) Gnu/Linux, Linux kernel 4.9.0-3-amd64. Some OSs may treat all addresses 127.0.0.1 → 127.255.255.254 the same. Some may only work with 127.0.0.1.

see also

• https://serverfault.com/a/157508/111338

Answer 3

Not a comprehensive general answer (there's one already). This answer of mine shows an example where 127.0.0.2 was used to solve the problem.

Extract:

The OP there attempted to test some software in a case when its connection to a server was rejected. This was done on the server by a temporary iptables rule that rejects all traffic from the client IP. The client was immediately able to "see" the connection was rejected.

The problem appeared when this person moved the server software to the same machine as the client and tried to use loopback interface. The rule was set to block communication from 127.0.0.1 but the information a connection was rejected underwent the same rule and never got to the client software which hanged (presumably till timeout).

The solution was to use 127.0.0.2 as the server address and set a rule that rejects connections to it. The information about a rejection went to 127.0.0.1 and was able to pass to the client software.