30 May 2023 Macbooks Networking

Mac OS X: Find Which Network Interface Routes Traffic

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Running `route get ` in Terminal reveals which network interface handles traffic to a specific address on Mac OS X. To find the default interface, use `route get default`, which displays the gateway, interface name, and flags. If multiple connections exist, macOS routes traffic through whichever interface ranks first in System Preferences Network Service Order.

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1.
What it is
Shows how to use command-line tools to determine which network interface your Mac will use to route traffic to specific IP addresses or DNS names.
2.
Why it matters
Essential for troubleshooting network connectivity issues when you have multiple network connections active, such as VPN clients or multiple adapters.
3.
Key takeaway
Use route get commands to identify which interface handles traffic routing on macOS systems with multiple network connections.

~1 min read

If you have multiple connections on your device (and maybe you have a zero trust client installed); how do you find out which network interface on your device will be used to route the traffic?

Below is a route get request for googles DNS service:

$ route get 8.8.8.8

   route to: dns.google
destination: dns.google
    gateway: 100.64.0.1
  interface: utun3
      flags: 
 recvpipe  sendpipe  ssthresh  rtt,msec    rttvar  hopcount      mtu     expire
       0         0         0         0         0         0      1400         0

If you have multiple interfaces enabled, then the first item in the Service Order will be used. If you want to see the default interface for your device:

$ route -n get 0.0.0.0 | grep interface
  interface: en0

Lets go an see whats going on in my default interface:

$ netstat utun3 | grep ESTABLISHED
tcp4       0      0  100.64.0.1.65271       jnb02s11-in-f4.1.https ESTABLISHED
tcp4       0      0  100.64.0.1.65269       jnb02s02-in-f14..https ESTABLISHED
tcp4       0      0  100.64.0.1.65262       192.0.73.2.https       ESTABLISHED
tcp4       0      0  100.64.0.1.65261       192.0.73.2.https       ESTABLISHED
tcp4       0      0  100.64.0.1.65260       192.0.73.2.https       ESTABLISHED
tcp4       0      0  100.64.0.1.65259       192.0.73.2.https       ESTABLISHED
tcp4       0      0  100.64.0.1.65258       192.0.73.2.https       ESTABLISHED
tcp4       0      0  100.64.0.1.65257       192.0.73.2.https       ESTABLISHED
tcp4       0      0  100.64.0.1.65256       192.0.73.2.https       ESTABLISHED
tcp4       0      0  100.64.0.1.65255       192.0.73.2.https       ESTABLISHED
tcp4       0      0  100.64.0.1.65254       192.0.78.23.https      ESTABLISHED
tcp4       0      0  100.64.0.1.65253       192.0.76.3.https       ESTABLISHED
tcp4       0      0  100.64.0.1.65252       192.0.78.23.https      ESTABLISHED
tcp4       0      0  100.64.0.1.65251       192.0.76.3.https       ESTABLISHED
tcp4       0      0  100.64.0.1.65250       192.0.78.23.https      ESTABLISHED
tcp4       0      0  100.64.0.1.65249       192.0.76.3.https       ESTABLISHED
tcp4       0      0  100.64.0.1.65248       ec2-13-244-140-3.https ESTABLISHED
tcp4       0      0  100.64.0.1.65247       192.0.73.2.https       ESTABLISHED