Wireless and Copper Routing on Windows
Ever used a laptop with a wireless and a copper network interface? Practically every laptop have both, so chances are you have. Have you ever wondered what happens if you plug in with copper, turn wireless on and have both interfaces configured with an IP address. I decided to do this and have have a quick look at the routing table to see what goes on.
TL;DNR windows is sane and uses copper is you configure both.
Here is a sample ipconfig with some slight anonymization:
>ipconfig
Windows IP Configuration
Wireless LAN adapter Wireless Network Connection:
Connection-specific DNS Suffix . : example.com
IPv4 Address. . . . . . . . . . . : 10.0.2.24
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : 10.0.2.1
Ethernet adapter Local Area Connection:
Connection-specific DNS Suffix . : example.com
IPv4 Address. . . . . . . . . . . : 10.0.1.24
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : 10.0.1.1
Tunnel adapter isatap.example.com:
Media State . . . . . . . . . . . : Media disconnected
Connection-specific DNS Suffix . : example.com
Tunnel adapter Local Area Connection* 11:
Media State . . . . . . . . . . . : Media disconnected
Connection-specific DNS Suffix . :
>
and here is the routing table:
>netstat -rn
===========================================================================
Interface List
14...10 0b a9 96 fc 2c ......Intel(R) Centrino(R) Advanced-N 6205
11...00 21 cc ba 7f 38 ......Intel(R) 82579LM Gigabit Network Connection
1...........................Software Loopback Interface 1
13...00 00 00 00 00 00 00 e0 Microsoft ISATAP Adapter
10...00 00 00 00 00 00 00 e0 Teredo Tunneling Pseudo-Interface
===========================================================================
IPv4 Route Table
===========================================================================
Active Routes:
Network Destination Netmask Gateway Interface Metric
0.0.0.0 0.0.0.0 10.0.1.1 10.0.1.24 10
0.0.0.0 0.0.0.0 10.0.2.1 10.0.2.24 30
127.0.0.0 255.0.0.0 On-link 127.0.0.1 306
127.0.0.1 255.255.255.255 On-link 127.0.0.1 306
127.255.255.255 255.255.255.255 On-link 127.0.0.1 306
10.0.1.0 255.255.255.0 On-link 10.0.1.24 266
10.0.1.24 255.255.255.255 On-link 10.0.1.24 266
10.0.1.255 255.255.255.255 On-link 10.0.1.24 266
10.0.2.0 255.255.255.0 On-link 10.0.2.24 286
10.0.2.24 255.255.255.255 On-link 10.0.2.24 286
10.0.2.255 255.255.255.255 On-link 10.0.2.24 286
224.0.0.0 240.0.0.0 On-link 127.0.0.1 306
224.0.0.0 240.0.0.0 On-link 10.0.1.24 266
224.0.0.0 240.0.0.0 On-link 10.0.2.24 286
255.255.255.255 255.255.255.255 On-link 127.0.0.1 306
255.255.255.255 255.255.255.255 On-link 10.0.1.24 266
255.255.255.255 255.255.255.255 On-link 10.0.2.24 286
===========================================================================
Persistent Routes:
None
===========================================================================
Persistent Routes:
None
>
As you can see there are two default gateways (ie destination 0.0.0.0) and the one with the lowest metric uses the 10.0.1.24 interface, which is the copper one. So it turns out that Windows seems sane and favours the faster copper interface when both are configured and working.
As an aside on Linux using a default network-manager setup, is also seems sane. You get one default gateway but the copper interface is used if it is available. The default gateway changes when the network interface go up and down.
