Tuesday, November 13, 2012

The mysterious

Q. Lately, I have noticed several cases of machines having two loopback addresses configured: not only (standard), but also - why is this second local address being configured?

A. The first thing to realize is that all addresses are local by definition. Traffic sent to these addresses can never go out onto the network, whether LAN or WAN.

Over at linuxquestions.org, catkin had this to say:

127.x.x.x addresses are all local. Rarely is there any need to use more than one 
and is conventional. Netsearching showed others used/created:

1.  By Yast.
2.  For starting MySQL when no DNS system available
3.  a loopback device for a second NIC
4.  When using two DNS services on one system
5.  To give multiple names to the local system (not clear why they are not all 
given -- maybe it's a Novell/SUSE thing)
6.  To resolve DHCP initialisation problems (?)

So, it appears a second local address definition in /etc/hosts could be useful for several reasons.

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