Resolving ping6 Issues on macOS with Multiple Network Interfaces

When using macOS with both Ethernet and Wi-Fi enabled, you might encounter issues with the ping6 command failing to receive packets unless you explicitly specify a network interface. For instance, running ping6 dns.google may yield no response, but ping6 dns.google -I en1 works correctly. This problem is primarily caused by the system’s inability to determine the correct default network interface for IPv6 communication when multiple interfaces are active.

Understanding the Issue

1. Multiple Active Interfaces:
   When both Ethernet (e.g., en0) and Wi-Fi (e.g., en1) are enabled, macOS needs to decide which interface to use for outgoing IPv6 packets. If the routing table lacks a clear default route, the system may choose an incorrect or non-functional interface.
2. IPv6 Routing Table:
   IPv6 relies on the routing table to direct packets. If no specific interface is chosen, ping6 may not function as expected, leading to packet loss or no response from the target.

Solutions

1. Manually Specify an Interface:
Use the -I option to explicitly define the interface for the ping6 command. For example:

ping6 -I en1 dns.google

To identify your interface names, run:

ifconfig

2. Adjust Default Route Priority:

Modify the IPv6 routing table to ensure the desired interface is prioritized:

• View the current routing table:

netstat -rn -f inet6

• Change or delete routes to prioritize the preferred interface:

sudo route change -inet6 default <gateway_address>
sudo route delete -inet6 default
sudo route add -inet6 default -interface en1

3. Configure Network Preferences (Best Recommendation):

Adjust the network service order to prioritize a specific interface:

• Open System Settings > Network.
• Click the gear icon in the bottom-left corner and select Set Service Order.
• Drag Wi-Fi or Ethernet to the desired priority position.

Verification

After applying these changes, test with ping6 dns.google to confirm functionality. Ensure that all active interfaces have valid IPv6 addresses and that the target address supports IPv6.

By understanding and managing macOS’s routing behavior, you can effectively resolve issues with ping6 in multi-interface setups.