Approaches of interprocess communication

[Steve Holden]

Donn Cave wrote:
> Quoth Steve Holden <steve at holdenweb.com>:
> | Ben Finney wrote:
> ...
> | > If a programmer decides on behalf of the user that "localhost" should
> | > be treated specially, that programmer is making an error.
> |
> | Inter-process TCP/IP communication between two processes on the same 
> | host invariably uses the loopback interface (network 127.0.0.0). 
> | According to standards, all addresses in that network space refer to the 
> | local host, though 127.0.0.1 is conventionally used.
> |
> | The transmit driver for the loopback interface receives a datagram from 
> | the local network layer and immediately announces its reception back to 
> | the local network layer.
> 
> Are you saying, in that first paragraph, that if for example I telnet to
> my local host's external IP address, to a service that bound explicitly to
> the external interface -- I'll actually be going through the loopback
> interface anyway, invariably regardless of host network implementation?
> 
I wasn't specifically saying that, no. However on Solaris I have 
observed local connections to an external interface actually increasing 
the packet count on the loopback, but I can't confirm whether those 
connections were to services specifically bound only to the external 
interface.

Certainly on Windows XP there is a host-specific route via 127.0.0.1 to 
the external interfaces as well as the network route via the external 
interface. I wouldn't necessarily expect this to extend to other 
platforms. This is demonstrated by the following output from "route 
print". I have chopped the metric column to avoid line wrapping.

=====================================================================
Active Routes:
Network Destination        Netmask          Gateway       Interface
           0.0.0.0          0.0.0.0  192.168.123.254  192.168.123.123
         127.0.0.0        255.0.0.0        127.0.0.1       127.0.0.1
     192.168.123.0    255.255.255.0  192.168.123.123  192.168.123.123
   192.168.123.123  255.255.255.255        127.0.0.1       127.0.0.1
   192.168.123.255  255.255.255.255  192.168.123.123  192.168.123.123
     192.168.174.0    255.255.255.0    192.168.174.1   192.168.174.1
     192.168.174.1  255.255.255.255        127.0.0.1       127.0.0.1
   192.168.174.255  255.255.255.255    192.168.174.1   192.168.174.1
         224.0.0.0        240.0.0.0  192.168.123.123  192.168.123.123
         224.0.0.0        240.0.0.0    192.168.174.1   192.168.174.1
   255.255.255.255  255.255.255.255  192.168.123.123               3
   255.255.255.255  255.255.255.255  192.168.123.123  192.168.123.123
   255.255.255.255  255.255.255.255    192.168.174.1   192.168.174.1
Default Gateway:   192.168.123.254
=====================================================================

regards
  Steve
-- 
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