UDPのconnect()は何も送らず、getsockname()で自分のIPが分かる

import socket

def source_ip_for(dest: str) -> str:
    s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    try:
        s.connect((dest, 9))      # 経路の検索と相手の記憶だけ。何も送らない
        return s.getsockname()[0]
    finally:
        s.close()

import socket, sys

def source_ip_for(dest: str, family=socket.AF_INET) -> str:
    """The local address the kernel would use to reach `dest`. Sends nothing."""
    s = socket.socket(family, socket.SOCK_DGRAM)
    try:
        s.connect((dest, 9))                 # UDP connect = route lookup + remember the peer; no packet leaves
        return s.getsockname()[0]
    finally:
        s.close()

if __name__ == "__main__":
    for dest in sys.argv[1:]:
        try:    print(f"{dest:>14} -> {source_ip_for(dest)}")
        except OSError as e: print(f"{dest:>14} -> OSError: {e.strerror} (errno {e.errno})")

#!/bin/bash
# Needs root (ip netns). Builds a host with a Wi-Fi-like interface, a VPN-like interface and a docker-like bridge.
set -e
HERE=$(cd "$(dirname "$0")" && pwd)
ip netns del b1 2>/dev/null || true
ip netns add b1
mk() { ip link add name $1 type veth peer name $1p; ip link set $1 netns b1; ip netns exec b1 ip addr add $2 dev $1; ip netns exec b1 ip link set $1 up; ip link set $1p up; }
mk wlan0 192.168.1.5/24
mk br0   172.17.0.1/16
mk tun0  10.8.0.2/24
ip netns exec b1 ip link set lo up
ip netns exec b1 ip route add default via 192.168.1.1 dev wlan0
tx() { ip netns exec b1 cat /sys/class/net/{wlan0,br0,tun0}/statistics/tx_packets | tr '\n' ' '; }
echo "interfaces:"; ip netns exec b1 ip -br addr | grep -v '^lo'
echo "routes:";     ip netns exec b1 ip route | sed 's/^/  /'
echo
echo "tx_packets before (wlan0 br0 tun0): $(tx)"
echo "-- default route is the Wi-Fi gateway"
ip netns exec b1 python3 "$HERE/lan_ip.py" 8.8.8.8 192.0.2.1 192.168.1.77 172.17.0.9 10.8.0.50
echo "tx_packets after:                   $(tx)"
echo "-- control: a sniffer on the other end of each interface for 2 s, with NO lookups at all"
sleep 3   # let the interfaces finish their own IPv6 housekeeping first
python3 "$HERE/sniff.py" 2 wlan0p br0p tun0p
echo "-- the same sniffer while the five lookups run"
python3 "$HERE/sniff.py" 2 wlan0p br0p tun0p &
SN=$!; sleep 0.5
ip netns exec b1 python3 "$HERE/lan_ip.py" 8.8.8.8 192.0.2.1 192.168.1.77 172.17.0.9 10.8.0.50 >/dev/null
wait $SN
echo
echo "-- the VPN takes over the default route"
ip netns exec b1 ip route replace default dev tun0
ip netns exec b1 python3 "$HERE/lan_ip.py" 8.8.8.8 192.168.1.77
echo
echo "-- no default route at all"
ip netns exec b1 ip route del default
ip netns exec b1 python3 "$HERE/lan_ip.py" 8.8.8.8 192.168.1.77
echo
echo "-- tx_packets at the end:              $(tx)"
ip netns del b1
for i in wlan0p br0p tun0p; do ip link del $i 2>/dev/null || true; done

import socket, sys, time, collections, select
# usage: sniff.py <seconds> <iface>...   counts every frame arriving on the given (host-side) interfaces, by Ethernet type
secs, ifaces = float(sys.argv[1]), sys.argv[2:]
socks = {}
for i in ifaces:
    s = socket.socket(socket.AF_PACKET, socket.SOCK_RAW, socket.htons(3)); s.bind((i, 0)); socks[s] = i
seen = collections.Counter(); end = time.monotonic() + secs
while time.monotonic() < end:
    r, _, _ = select.select(list(socks), [], [], 0.1)
    for s in r:
        f = s.recv(2048); et = int.from_bytes(f[12:14], 'big')
        what = f"0x{et:04x}"
        if et == 0x86dd:                                   # IPv6: show the next-header byte, and the ICMPv6 type if it is ICMPv6
            what += f" ipv6 next-header={f[20]}" + (f" icmpv6-type={f[54]}" if f[20] == 58 else "")
        seen[(socks[s], what)] += 1
print("frames seen:", dict(seen) if seen else "none")

interfaces:
wlan0@if68       UP             192.168.1.5/24 fe80::1d:81ff:feab:5f0e/64 
br0@if70         UP             172.17.0.1/16 fe80::488:d7ff:fefa:fb7a/64 
tun0@if72        UP             10.8.0.2/24 fe80::5480:f9ff:fe59:a0e7/64 
routes:
  default via 192.168.1.1 dev wlan0 
  10.8.0.0/24 dev tun0 proto kernel scope link src 10.8.0.2 
  172.17.0.0/16 dev br0 proto kernel scope link src 172.17.0.1 
  192.168.1.0/24 dev wlan0 proto kernel scope link src 192.168.1.5 

tx_packets before (wlan0 br0 tun0): 1 1 0 
-- default route is the Wi-Fi gateway
       8.8.8.8 -> 192.168.1.5
     192.0.2.1 -> 192.168.1.5
  192.168.1.77 -> 192.168.1.5
    172.17.0.9 -> 172.17.0.1
     10.8.0.50 -> 10.8.0.2
tx_packets after:                   1 1 1 
-- control: a sniffer on the other end of each interface for 2 s, with NO lookups at all
frames seen: {('wlan0p', '0x86dd ipv6 next-header=58 icmpv6-type=133'): 1}
-- the same sniffer while the five lookups run
frames seen: {('br0p', '0x86dd ipv6 next-header=58 icmpv6-type=133'): 2, ('tun0p', '0x86dd ipv6 next-header=58 icmpv6-type=133'): 2, ('wlan0p', '0x86dd ipv6 next-header=58 icmpv6-type=133'): 1}

-- the VPN takes over the default route
       8.8.8.8 -> 10.8.0.2
  192.168.1.77 -> 192.168.1.5

-- no default route at all
       8.8.8.8 -> OSError: Network is unreachable (errno 101)
  192.168.1.77 -> 192.168.1.5

-- tx_packets at the end:              7 7 7