Re: Primary network interface?
On 10/19/2010 11:51 AM, Spud wrote:
java.net.NetworkInterface can return a big list of all the network
connections that a system has, both physical and virtual.
Is there any way to determine which one is the primary interface?
There really isn't any such thing as a primary interface. For example,
on my laptop, I have three interfaces that I use regularly: my ethernet
card, my wireless card, and my loopback interface (eth1, wlan0, and lo,
respectively, if it really matters). I could have any combination of
these up that I wanted to. On servers, it's actually not unlikely that
they would have multiple ethernet ports to talk to multiple distinct
LANs, and this is actually necessary for routers to function correctly.
What is rather easy to tell is whether or not a network interface can
contact the "outside" world (i.e., is it not the loopback interface?).
If I were to enumerate my interfaces, one has an IP address of
128.xxx.xxx.xxx, and another has a 127.x.x.x (and ::1/128, but I
digress). The one which has the 127.x.x.x IP address is the loopback
interface, and is not likely to be useful most of the time. The sets of
loopback addresses are well-known (it's 127.x.y.z for any values x,y,z
in IPv4 or ::1 in IPv6 [1]), so you can easily check if an interface has
that address.
Googling around I found some code that suggests that the primary
interface is always named "eth0", but that's not true on my laptop.
It sounds like you're on Windows, where this is pretty much always not
the case. Even in Linux or other Unix-esque systems (is ifconfig
mandated by POSIX?), eth0 is not necessarily the primary interface. I
have eth1 and wlan0 for my ethernet and wireless, respectively, and I
have seen other interface names.
[1] And, presumably, the 127.0.0.0/8 block of the IPv4-in-IPv6 portion
of IPv6 as well.
--
Beware of bugs in the above code; I have only proved it correct, not
tried it. -- Donald E. Knuth