How is net connection selected?

[Jonathan Hutchins]

On Thursday, January 15, 2004 12:51 pm, Greg Kedrovsky wrote:

> I think this is where I don't understand the process (and maybe I don't
> need to). sendmail "attempts to make a connection" - how does it do
> that? How does it "know" to make that connection over eth0 (cable modem)
> first rather than dial-on-demand with /dev/modem and ppp? 

There's a routing table that indicates that eth0 (in this case) is the 
preferred route to all other networks.  In all the systems I've seen, 
something actually needs to detect the fact that the Cable connection has 
gone down, and switch the priority to the dial-up connection.

Most client programs don't actually initialise dial-up connections.  Some 
systems will detect the fact that a client is trying to send data out, and 
they will bring up the appropriate interface (dial the isp) to make a 
connection.

There are ways to automate this, but most people I know stop short of 
constantly checking to see if the connection's up.