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Determining interface to access IP

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# 1  
Old 02-17-2010
Determining interface to access IP
Hello

I've got a server with multiple NICS. In a script I want to log the outbound interface. Is there an easy way I can do this so that the output looks something like this:

host(xxx.xxx.xxx.xxx): Opening connection to ...

Obviously, getting the host is simple with hostname. But how can I determine the interface. Some netstat invocation?
Last edited by brsett; 02-17-2010 at 04:26 PM..
# 2  
Old 02-17-2010
If this script is being called by the daemon or what have you, maybe the information is reported to the script in environment vars? Try 'set' for a list of them. Otherwise you have the problem of finding out which socket is your socket on top of everything else...
# 3  
Old 02-17-2010
I don't know why I need the socket, I just need the route. On modern Linux I can do the following as it turns out (just learned this):

/sbin/ip route get <remote_ip>

And it is in that output. But that is a little platform dependent. I was hoping there was a System V mostly-portable way to do this.
Last edited by brsett; 02-17-2010 at 04:26 PM..
# 4  
Old 02-18-2010
Quote:
Originally Posted by brsett
I don't know why I need the socket, I just need the route.
You failed to mention that you had the remote IP. My thoughts were along the lines of how in the heck do I get that without knowing anything about what program made what connection where?

Another complication is that there may be several valid routes, not just one.
# 5  
Old 02-18-2010
Quote:
Originally Posted by Corona688
You failed to mention that you had the remote IP. My thoughts were along the lines of how in the heck do I get that without knowing anything about what program made what connection where?

Another complication is that there may be several valid routes, not just one.
Yeah, there is no route without a destination endpoint.

I don't think multiple routes should matter, UNIX will use the first valid entry in the routing table. I want the interface used to get to that route.

I guess there isn't any clearly (or even mostly) portable way to do this from the shell. Darn.
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