How can I tell which network interface my computer is using?

The definitive reference for questions such as 'Which interface will be used to reach host w.x.y.z?' for any given time or situation is to refer to the routing table.

[mini-nevie:~] nevinwilliams% route get 10.10.10.10
   route to: 10.10.10.10
destination: default
       mask: default
    gateway: 192.168.2.1
  interface: en0
      flags: <UP,GATEWAY,DONE,STATIC,PRCLONING>
 recvpipe  sendpipe  ssthresh  rtt,msec    rttvar  hopcount      mtu     expire
       0         0         0         0         0         0      1500         0

I've both en0 and en1 enabled, and en0 is first in Service Order. Unplugging my ethernet, making no other changes, the same command gives:

mini-nevie:~] nevinwilliams% route get 10.10.10.10
   route to: 10.10.10.10
destination: default
       mask: default
    gateway: 192.168.2.1
  interface: en1
      flags: <UP,GATEWAY,DONE,STATIC,PRCLONING>
 recvpipe  sendpipe  ssthresh  rtt,msec    rttvar  hopcount      mtu     expire
       0         0         0         0         0         0      1500         0

On Mac, I use this:

if=$(route -n get 0.0.0.0 2>/dev/null | awk '/interface: / {print $2}')

if [ -n "$if" ]; then
    echo "Default route is through interface $if"
else
    echo "No default route found"
fi

On Linux, the first line would be slightly different:

if=$(ip route show 0.0.0.0/0 | awk '/ dev / {print $NF}')
# or
if=$(awk '$2 == 00000000 {print $1}' /proc/net/route)

Here's a dump of ifconfig -a in each situation (I'll highlight the differences afterward):

Ethernet cable is plugged in:

lo0: flags=8049 mtu 16384
        inet6 ::1 prefixlen 128
        inet6 xxxx::1%lo0 prefixlen 64 scopeid 0x1
        inet 127.0.0.1 netmask 0xff000000
gif0: flags=8010 mtu 1280
stf0: flags=0 mtu 1280
en0: flags=8863 mtu 1500
        inet6 xxxx%en0 prefixlen 64 scopeid 0x4
        inet 192.168.0.110 netmask 0xffffff00 broadcast 192.168.0.255
        ether xx:xx:xx:xx:xx:xx
        media: autoselect (100baseTX ) status: active
        supported media: none autoselect 10baseT/UTP  10baseT/UTP  10baseT/UTP  10baseT/UTP  100baseTX  100baseTX  100baseTX  100baseTX  1000baseT  1000baseT  1000baseT 
fw0: flags=8863 mtu 4078
        lladdr xx:xx:xx:xx:xx:xx
        media: autoselect  status: inactive
        supported media: autoselect 
en1: flags=8863 mtu 1500
        inet6 xxxx%en1 prefixlen 64 scopeid 0x6
        inet 192.168.0.110 netmask 0xffffff00 broadcast 192.168.0.255
        ether xx:xx:xx:xx:xx:xx
        media: autoselect status: active
        supported media: autoselect

Ethernet cable is unplugged:

lo0: flags=8049 mtu 16384
        inet6 ::1 prefixlen 128
        inet6 xxxx::1%lo0 prefixlen 64 scopeid 0x1
        inet 127.0.0.1 netmask 0xff000000
gif0: flags=8010 mtu 1280
stf0: flags=0 mtu 1280
en0: flags=8863 mtu 1500
        ether xx:xx:xx:xx:xx:xx
        media: autoselect status: inactive
        supported media: none autoselect 10baseT/UTP  10baseT/UTP  10baseT/UTP  10baseT/UTP  100baseTX  100baseTX  100baseTX  100baseTX  1000baseT  1000baseT  1000baseT 
fw0: flags=8863 mtu 4078
        lladdr xx:xx:xx:xx:xx:xx
        media: autoselect  status: inactive
        supported media: autoselect 
en1: flags=8863 mtu 1500
        inet6 xxxx%en1 prefixlen 64 scopeid 0x6
        inet 192.168.0.110 netmask 0xffffff00 broadcast 192.168.0.255
        ether xx:xx:xx:xx:xx:xx
        media: autoselect status: active
        supported media: autoselect

diff plugged unplugged shows that the wired connection does become active when it is available:

8,9d7
<   inet6 xxxx%en0 prefixlen 64 scopeid 0x4 
<   inet 192.168.0.110 netmask 0xffffff00 broadcast 192.168.0.255
11c9
<   media: autoselect (100baseTX <full-duplex,flow-control>) status: active
---
>   media: autoselect status: inactive

A couple of things. Mac will always use the network connections in order that's listed in your "Network Connections" profile. For example, if Ethernet is on top of Wireless, Ethernet will take over.

I'm not sure on how SSH works that much, but from my experience. If one connection fails or disconnects, SSH will then use the other connection.

If you really want to see if the connections are active, you should use a bandwidth monitor utility. A great one is iStat (http://www.islayer.com/apps/istatmenus/).

How can I tell which network interface my computer is using?

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