Linux – Services binding to 127.0.0.2, not responding on 127.0.0.1

linuxnetworking

I have an OpenVZ host and running several services on it (mongodb, bitcoind). I only intend to connect to them locally. They all seem to bind to 127.0.0.2 – the venet interface. When I try to access a service at 127.0.0.1 (the default for the clients), I get get no response. However connecting on 127.0.0.2 works fine. I didn't realize it was possible for there to be a difference between the two. What's happening here?

Here's an output of ifconfig:

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:199957 errors:0 dropped:0 overruns:0 frame:0
      TX packets:199957 errors:0 dropped:0 overruns:0 carrier:0
      collisions:0 txqueuelen:0 
      RX bytes:15822725 (15.8 MB)  TX bytes:15822725 (15.8 MB)

venet0    Link encap:UNSPEC  HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00  
      inet addr:127.0.0.2  P-t-P:127.0.0.2  Bcast:0.0.0.0  Mask:255.255.255.255
      inet6 addr: 2a00:1dc0:2003::adb9:ca98/128 Scope:Global
      inet6 addr: 2a00:1dc0:2003::a59d:5802/128 Scope:Global
      inet6 addr: 2a00:1dc0:2003::8d9f:638b/128 Scope:Global
      inet6 addr: 2a00:1dc0:2003::8638:9f63/128 Scope:Global
      inet6 addr: 2a00:1dc0:2003::daef:a10a/128 Scope:Global
      inet6 addr: 2a00:1dc0:2003::92b9:5c2a/128 Scope:Global
      UP BROADCAST POINTOPOINT RUNNING NOARP  MTU:1500  Metric:1
      RX packets:389890 errors:0 dropped:0 overruns:0 frame:0
      TX packets:341645 errors:0 dropped:0 overruns:0 carrier:0
      collisions:0 txqueuelen:0 
      RX bytes:367108643 (367.1 MB)  TX bytes:98312087 (98.3 MB)

venet0:0  Link encap:UNSPEC  HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00  
      inet addr:178.17.169.91  P-t-P:178.17.169.91  Bcast:178.17.169.91  Mask:255.255.255.255
      UP BROADCAST POINTOPOINT RUNNING NOARP  MTU:1500  Metric:1

Here's my /etc/hosts file:

fe00::0     ip6-localnet
ff00::0     ip6-mcastprefix
ff02::1     ip6-allnodes
ff02::2     ip6-allrouters

127.0.0.2 localhost.localdomain localhost
# Auto-generated hostname. Please do not remove this comment.
178.17.169.91 btslots.com  btslots
::1     localhost ip6-localhost ip6-loopback

I changed localhost to have it point to 127.0.0.2. I'm surprised that it's possible for two addresses that are in the 127.0.0.1/8 subnet to give different results. All addresses besides 127.0.0.1 act the same (and as expected).

Best Answer

Having multiple, different loopback devices under 127.0.0.0/8 subnet is not uncommon. For example, you can implement them in FreeBSD jail setups to isolate loopback traffic to the specific jails.

127.0.0.0/8 is a subnet like any other subnet (with the exception that traffic destined to it should "loop" back inside the host), and 127.0.0.1 and 127.0.0.2 are distinct destinations when they are bound to two different interfaces.

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