When working with multihomed Linux systems (machines with multiple network interfaces), determining which interface will be used to reach a specific host is crucial for network programming and troubleshooting. The system's routing table decides this path selection, but directly parsing /proc/net/route or ip route output can be error-prone.

The most reliable approach is to use the SO_BINDTODEVICE socket option combined with routing lookups. However, Linux provides more straightforward methods:

#include <stdio.h>
#include <netdb.h>
#include <ifaddrs.h>
#include <linux/rtnetlink.h>

// Alternative using getifaddrs (simpler)
void get_interface_for_host(const char *host) {
    struct ifaddrs *ifaddr, *ifa;
    if (getifaddrs(&ifaddr) == -1) {
        perror("getifaddrs");
        return;
    }
    // Iterate through interfaces
    for (ifa = ifaddr; ifa != NULL; ifa = ifa->ifa_next) {
        if (ifa->ifa_addr && ifa->ifa_addr->sa_family == AF_INET) {
            printf("%s\n", ifa->ifa_name);
        }
    }
    freeifaddrs(ifaddr);
}

For more accurate results, we can use the NETLINK socket:

#include <linux/netlink.h>
#include <linux/rtnetlink.h>
#include <net/if.h>

int query_route_table(const char *dest_ip) {
    // Create NETLINK socket
    int sock = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);
    if (sock < 0) return -1;

    // Prepare request
    struct {
        struct nlmsghdr nh;
        struct rtmsg rt;
        char buf[1024];
    } req;

    memset(&req, 0, sizeof(req));
    req.nh.nlmsg_len = NLMSG_LENGTH(sizeof(struct rtmsg));
    req.nh.nlmsg_flags = NLM_F_REQUEST;
    req.nh.nlmsg_type = RTM_GETROUTE;
    req.rt.rtm_family = AF_INET;
    req.rt.rtm_table = RT_TABLE_MAIN;

    // Send request and process response
    // ... (full implementation would include address parsing and response handling)
    
    close(sock);
    return 0;
}

For quick scripting needs, you can combine ip commands:

#!/bin/bash
# Get interface for specific host
TARGET="8.8.8.8"
INTERFACE=$(ip route get $TARGET | grep -oP 'dev \K\S+')
echo "Interface for $TARGET: $INTERFACE"

Using Python's socket and fcntl modules:

import socket
import fcntl
import struct

def get_interface_for_host(ip):
    s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    try:
        # This IOCTL call returns the interface index
        iface = fcntl.ioctl(
            s.fileno(),
            0x8915,  # SIOCGIFADDR
            struct.pack('256s', ip[:15].encode('utf-8'))
        )
        return iface
    except IOError:
        return None
    finally:
        s.close()

When working with multihomed Linux systems, determining the egress interface for traffic destined to a specific host is a common networking challenge. The routing subsystem makes this decision based on the routing table, but accessing this information programmatically requires careful consideration.

The most robust approach is to leverage the ip route get command, which precisely answers our question by performing a route lookup:

ip route get 192.168.1.100 | grep -oP 'dev \K\S+'

This command returns just the interface name (e.g., eth0, wlan1) that would be used to reach the specified host.

For native C programs, we can use netlink sockets to query route information:

#include <linux/netlink.h>
#include <linux/rtnetlink.h>
#include <net/if.h>

int get_egress_interface(const char *dest_ip) {
    struct nlmsghdr *nlmsg;
    struct rtmsg *rtm;
    char buf[4096];
    // Netlink socket setup code here...
    // Full implementation would include:
    // 1. Creating netlink socket
    // 2. Building RTM_GETROUTE message
    // 3. Parsing response for oif (output interface)
    return oif; // Returns interface index
}

Here's a complete Python solution using subprocess:

import subprocess
import re

def get_outbound_interface(host):
    cmd = f"ip route get {host}"
    result = subprocess.run(cmd, shell=True, 
                          capture_output=True, text=True)
    match = re.search(r'dev (\S+)', result.stdout)
    return match.group(1) if match else None

# Example usage:
print(get_outbound_interface("8.8.8.8"))

Other approaches worth considering include:

• getsockopt() with SO_BINDTODEVICE (requires CAP_NET_RAW)
• Parsing /proc/net/route (less reliable for complex routing)
• Using libmnl for lower-level netlink communication

Remember that:

• Results may vary based on network namespaces
• Routing decisions can change dynamically
• Some methods require root privileges
• ICMP-based solutions (like traceroute) aren't reliable for this purpose

Here's how you might implement interface detection in a network monitoring application:

import psutil
from collections import defaultdict

class RouteMonitor:
    def __init__(self):
        self.interface_map = defaultdict(list)
    
    def update_routes(self):
        for conn in psutil.net_connections():
            if conn.status == 'ESTABLISHED':
                dest = conn.raddr.ip
                iface = get_outbound_interface(dest)
                self.interface_map[iface].append(dest)