On Mar 12, 2006, at 14:23 , John Turnipseed wrote:
Maybe I'm missing something obvious... or I'm trying to do the
impossible... (neither would be a first for me unfortunately ;^)
Not entirely obvious; you have to understand how routing works, for
IP, both inside the kernel and across the network.
Is there an easy way... or I guess even a hard way if necessary...
to determine which network interface a connection is using?
You can tell, given a destination address, what the kernel will
choose as the next-hop router, and by inference, which interface it
will use to send the packet.
Getting this info is a bit, um, involved. Your best source is
Stevens's website (<http://www.kohala.com/>). Look for the source
tarball for his "Unix Network Programming", V1, 2nd Ed. It's a book
worth the investment if you're doing a bunch of this kind of
programming. There is a 3rd ed., written by others, that should also
be worth the bucks.
The scenario is a hierarchy of interfaces are defined in system
preferences Ethernet, then AIrport, then one or more modem dial-up
configurations. I need to know which one is being used to connect
to a specified host.
I'd hoped that the SCNetworkReachability APIs might help, but it
only appears to tell if a destination is reachable and if it would
require some type of connection.
This API is a hold-over from Mac OS 9, and is useful only if you are
in a fairly simple environment, where you have a dialup connection
that you want to use only sparingly. It's not a general-purpose "can
I get from here to there" indicator.
My application already makes periodic network calls using
CFNetwork. Is it possible to determine which interface a
CFReadStream is using? And if so, can that result then be resolved
back to the user visible name from System Preferences... especially
if the interface used is a dial-up connection.
You will need to open a "raw" AF_ROUTE socket, with which you can
query the state of the kernel's routing table. Stevens has a chunk
of code (route/getrt.c) that does essentially what you want.
Regards,
Justin
--
Justin C. Walker, Curmudgeon-At-Large
Institute for the Absorption of Federal Funds
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Men are from Earth.
Women are from Earth.
Deal with it.
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