ifpeer

#!/usr/bin/env python3
# ifpeer -- show the peer IP address of a point-to-point link
import argparse
import ipaddress
import json
import subprocess
import sys

def warn(text):
    print(f"ifpeer: {text}", file=sys.stderr)

def get_ip_addrs(iface):
    res = subprocess.run(["ip", "--json", "addr", "list", "dev", args.interface],
                         stdout=subprocess.PIPE,
                         check=True)
    res = json.loads(res.stdout)
    for ifdata in res:
        for adata in ifdata["addr_info"]:
            if adata["family"] == "inet":
                addr = ipaddress.IPv4Interface("%(local)s/%(prefixlen)s" % adata)
                peer = None
                if adata.get("address"):
                    peer = ipaddress.IPv4Interface("%(address)s/%(prefixlen)s" % adata)
                yield (addr, peer)
            elif adata["family"] == "inet6":
                if adata["scope"] == "link":
                    continue
                addr = ipaddress.IPv6Interface("%(local)s/%(prefixlen)s" % adata)
                peer = None
                yield (addr, peer)
            else:
                continue

parser = argparse.ArgumentParser()
parser.add_argument("-4", dest="ip4", action="store_true", help="show the IPv4 address")
parser.add_argument("-6", dest="ip6", action="store_true", help="show the IPv6 address")
parser.add_argument("-1", dest="one", action="store_true", help="show at most one result")
parser.add_argument("interface", help="network interface to use")
args = parser.parse_args()

if args.ip4 + args.ip6 > 1:
    exit(f"ifpeer: Contradictory options -4 and -6 given.")

if "/" in args.interface:
    # Convenience shortcut to output the peer of a /30 or /31
    ifname = None
    addrs = [(ipaddress.ip_interface(args.interface), None)]
else:
    ifname = args.interface
    addrs = [*get_ip_addrs(args.interface)]

ip4 = [(a, p) for (a, p) in addrs if a.version == 4]
ip6 = [(a, p) for (a, p) in addrs if a.version == 6]

addrs = (ip4 if args.ip4 else
         ip6 if args.ip6 else
         ip4 + ip6)

if not addrs:
    if ifname:
        exit(f"ifpeer: Interface has no IP addresses of requested version")
    else:
        exit(f"ifpeer: Requested IP version contradicts specified address")

peers = []

for addr, peer in addrs:
    maxlen = addr.network.max_prefixlen
    if peer:
        # Bird treats this as a non-PtP interface despite presence of a peer address.
        if peer.network.prefixlen < maxlen:
            warn(f"Non-/{maxlen} peer address will confuse Bird: {peer}")
        peers.append(peer.ip)
    elif addr.network.prefixlen == maxlen:
        warn(f"Missing peer address for /{maxlen} address: {addr}")
    elif addr.network.prefixlen == maxlen - 1:
        a, b = addr.network
        peers.append(b if (addr.ip == a) else a)
    elif addr.network.prefixlen == maxlen - 2:
        _, a, b, _ = addr.network
        # Warn about /126 three-address shenanigans and assume that 'a' is our peer
        if addr.ip not in {a, b}:
            warn(f"Ambiguous configuration: {addr} <--> [{addr.network}]")
        peers.append(b if (addr.ip == a) else a)
    else:
        if addr.version == 4:
            a = addr.network.network_address + 1
            b = addr.network.broadcast_address - 1
            warn(f"Not a point-to-point link: {addr} [{a} ~ {b}]")
        else:
            warn(f"Not a point-to-point link: {addr}")

for peer in peers:
    print(peer)
    if args.one:
        break

if not peers:
    # Don't show a redundant message if we were explicitly given one address
    if ifname:
        exit(f"ifpeer: No point-to-point links found on interface '{ifname}'")
    else:
        exit(1)