Python: List all default gateways on host

get_default_gw.py

#!/usr/bin/env python3

"""
Determine IPv4/IPv6 default gateway(s) on a Linux machine via the socket interface.
"""

import socket
import struct

### Base netlink constants. See include/uapi/linux/netlink.h
NLM_F_REQUEST = 1
NLM_F_DUMP = 768
NLMSG_DONE = 3
### rtnetlink constants. See include/uapi/linux/rtnetlink.h
RTM_NEWROUTE = 24
RTM_GETROUTE = 26
RTN_UNICAST = 1
RTPROT_STATIC = 4
RTPROT_UNSPEC = 0
RT_SCOPE_UNIVERSE = 0
RT_TABLE_UNSPEC = 0
RTA_OIF = 4
RTA_GATEWAY = 5

flags = 0
flags |= NLM_F_REQUEST
flags |= NLM_F_DUMP
seq = 0
brk = 0

sock = socket.socket(socket.AF_NETLINK, socket.SOCK_RAW, socket.NETLINK_ROUTE)
pid = sock.getsockname()[1]
sock.bind((0,0))
rtnlpack_len = 28
nlpack_len = 16
nlmsg = struct.pack("=LHHLL", rtnlpack_len, RTM_GETROUTE, flags, seq, pid)
rtmsg = struct.pack("=BBBBBBBBI", 0, 0, 0, 0, RT_TABLE_UNSPEC, RTPROT_STATIC, RT_SCOPE_UNIVERSE, RTN_UNICAST, 0)
sock.send(nlmsg + rtmsg)
while data := sock.recv(65536):
    nlmsg_step = 0
    data_len = len(data)
    while nlmsg_step < data_len:
        nlmsg_len, nlmsg_type, *_ = struct.unpack("=LHHLL", data[nlmsg_step:nlmsg_step+nlpack_len])
        if nlmsg_type == NLMSG_DONE:
            brk = 1
            break
        if nlmsg_type == RTM_NEWROUTE:
            rtm_family, rtm_dst_len = struct.unpack("=BB", data[nlpack_len+nlmsg_step:nlpack_len+nlmsg_step+2])
            nla_step = 0
            while nla_step < nlmsg_len - (rtnlpack_len + 4):
                idx = rtnlpack_len + nlmsg_step + nla_step
                nla_len, nla_type = struct.unpack("=HH", data[idx:idx+4])
                if nla_type == RTA_GATEWAY and rtm_dst_len == 0:
                    print("default via {} dev {} metric {}".format(socket.inet_ntop(rtm_family, data[idx+4:idx+nla_len]),
                                                                   socket.if_indextoname(data[idx+nla_len+4]),
                                                                   struct.unpack("=H", data[idx-4:idx-2])[0]))
                nla_step += nla_len
        nlmsg_step += nlmsg_len
    if brk == 1:
        break