When working with multiple network interfaces on macOS (or any Unix-like system), you might need to determine which local interface and IP address are being used to communicate with a specific remote host. This is particularly relevant in scenarios involving VPN connections, multi-homed systems, or complex network configurations.

The most straightforward approach combines route and ifconfig commands. Here's the basic version that answers the question:

route get example.com | grep interface
ifconfig en0 | grep inet

Here's a more robust solution that combines these operations into a single command pipeline:

ifconfig $(route get example.com | awk '/interface:/{print $2}') | awk '/inet /{print $2}'

Let's analyze the components:

1. route get example.com - Queries the routing table for the specified host
2. awk '/interface:/{print $2}' - Extracts the interface name
3. ifconfig $(...) - Gets configuration for the identified interface
4. awk '/inet /{print $2}' - Filters for the IPv4 address

For those preferring different tools or needing additional information:

Using netstat

netstat -rn | grep -m 1 "default"

Python Script Alternative

import socket
import fcntl
import struct

def get_interface_ip(ifname):
    s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    return socket.inet_ntoa(fcntl.ioctl(
        s.fileno(),
        0x8915,  # SIOCGIFADDR
        struct.pack('256s', ifname[:15].encode())
    )[20:24])

print(get_interface_ip('en0'))

To handle IPv6 addresses, modify the grep/awk patterns:

ifconfig $(route get example.com | awk '/interface:/{print $2}') | awk '/inet6 /{print $2}'

• Ensure you have proper permissions (may need sudo)
• Verify network interfaces are properly configured
• Check for VPN connections that might override routing
• Consider DNS resolution if using hostnames

When working with multiple network interfaces on a macOS system (e.g., Ethernet, Wi-Fi, VPN), you might need to identify which local interface and IP address are used to communicate with a specific remote host. This is particularly useful in scenarios where you have multiple routes configured and need to debug network connectivity or configure firewall rules.

The most straightforward method involves combining route and ifconfig commands. Here's how it works:

# Get the interface used to reach the target host
INTERFACE=$(route get example.com | grep "interface: " | awk '{print $2}')

# Get the IP address assigned to that interface
IP_ADDR=$(ifconfig $INTERFACE | grep "inet " | awk '{print $2}')

echo "Interface: $INTERFACE"
echo "IP Address: $INTERFACE"

For those who prefer a one-liner, here's a more compact version of the solution:

ifconfig $(route get example.com | awk '/interface: /{print $2}') | awk '/inet /{print $2}'

Another approach is to use netstat to find established connections:

netstat -anv | grep ESTABLISHED | grep example.com

This will show you all active connections to the host, including the local IP and interface being used.

For those who prefer a programmatic solution, here's a Python script that accomplishes the same thing:

import subprocess
import re

def get_local_interface_and_ip(host):
    # Get the interface
    route_cmd = f"route get {host}"
    route_output = subprocess.check_output(route_cmd, shell=True).decode()
    interface = re.search(r"interface: (.+)", route_output).group(1)
    
    # Get the IP address
    ifconfig_cmd = f"ifconfig {interface}"
    ifconfig_output = subprocess.check_output(ifconfig_cmd, shell=True).decode()
    ip_addr = re.search(r"inet (\d+\.\d+\.\d+\.\d+)", ifconfig_output).group(1)
    
    return interface, ip_addr

if __name__ == "__main__":
    host = "example.com"
    interface, ip = get_local_interface_and_ip(host)
    print(f"Interface: {interface}")
    print(f"IP Address: {ip}")

Keep in mind that:

• This method won't work if there's no route to the host
• The interface might change if you have multiple possible routes
• VPN connections might show up as utun interfaces
• IPv6 addresses require modifying the grep/awk patterns

While there are multiple ways to solve this problem, the route/ifconfig combination provides the most reliable method on macOS. The Python implementation offers more flexibility if you need to integrate this functionality into a larger application.