Gratuitous IPv6 Neighbor Advertisement of GUAs using Scapy

gratuitousna.py

#!/usr/bin/env -S uv run --script
# /// script
# requires-python = ">=3.14"
# dependencies = [
#     "scapy>=2.7.0",
# ]
# ///

import socket
import time

from scapy.all import *


def fix_lladdr(lladdr):
    """
    Remove interface-id from the second quad of a link-local address.
    """
    mask = inet_pton(socket.AF_INET6, "ffff::ffff:ffff:ffff:ffff")
    addr = inet_pton(socket.AF_INET6, lladdr)
    return inet_ntop(socket.AF_INET6, in6_and(addr, mask))


def resolve_ndp(dst, src, iface):
    """
    Do an NDP MAC address resolution.
    The implementation in Scapy is broken on macOS due to interface ids
    in the second quad.
    """
    res = neighsol(dst, src, iface)
    if res is not None:
        if ICMPv6NDOptDstLLAddr in res:
            return res[ICMPv6NDOptDstLLAddr].lladdr
        else:
            return res.src


def do_ping(addrs, mac, iface, dst="2001:7fd::1", timeout=2):
    """
    Send ICMP echo request from each of addrs using Ether header mac
    to destination dst with timeout timeout.
    Return tuple of lists of working and broken source IP addresses
    """

    ping = ICMPv6EchoRequest()
    pkts = (mac/IPv6(src=a, dst=dst)/ping for a in addrs)

    ans, unans = srp(pkts, iface=iface, timeout=timeout)
    working = [a.answer[IPv6].dst for a in ans]
    broken = [a[IPv6].src for a in unans]
    return working, broken


def gratuitous_na(addrs, src, hwaddr, iface):
    """
    Send Neighbor advertisements for addresses in the list addrs
    from a Link-local address src, pointing at hwaddr MAC address.
    """
    dst = "ff02::2"
    gwmac = getmacbyip6(dst)
    mac = Ether(src=hwaddr, dst=gwmac)
    ip = IPv6(src=src, dst=dst)
    na = ICMPv6ND_NA(O=1, R=0, S=0)
    opt = ICMPv6NDOptDstLLAddr(lladdr=hwaddr)
    for a in addrs:
        na.tgt = a
        pkt = mac/ip/na/opt
        print(a)
        sendp(pkt, iface=iface)


def main():
    """
    Enumerate all non-link-local IPv6 addresses on the default interface.
    Send a gratious Neighbor advertisement with each address to the
    default gateway.
    """
    conf.verb = 0
    iface, src, dst = conf.route6.route("::/0")
    src, dst = (fix_lladdr(a) for a in [src, dst])
    hwaddr = get_if_hwaddr(iface)
    gwmac = resolve_ndp(dst, src, iface)

    print(f"Iface {iface}")
    print(f"Src {src}\t{hwaddr}")
    print(f"Dst {dst}\t{gwmac}")

    addrs = (x[0] for x in in6_getifaddr()
             if x[1] == IPV6_ADDR_GLOBAL and x[2] == iface)

    print("Priming router's neighbor cache by pinging k.root-servers.net")
    mac = Ether(src=hwaddr, dst=gwmac)
    working, broken = do_ping(addrs, mac, iface)
    if len(working) > 0:
        print("Following addresses seem to be working well:")
        print("\n".join(working))

    if len(broken) > 0:
        print("Sending gratuitous NAs for broken addresses:")
        gratuitous_na(broken, src, hwaddr, iface)

        print("Checking ping again")
        time.sleep(1)
        working, broken = do_ping(broken, mac, iface)
        if len(broken) > 0:
            print("These addresses are still broken:")
            print("\n".join(broken))
        else:
            print("All addresses are working now.")
    else:
        print("All addresses are working. Nothing to do.")


if __name__ == '__main__':
    main()