Using glibc `res_ninit()` to query DNS name servers and searches

query_dns_nameserver.py

#!/usr/bin/python3

# Copyright 2020 Red Hat
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
#
# Author: Gris Ge <fge@redhat.com>


from ctypes import byref
from ctypes import c_char
from ctypes import c_char_p
from ctypes import c_int
from ctypes import c_uint
from ctypes import c_uint16
from ctypes import c_uint32
from ctypes import c_uint8
from ctypes import c_ulong
from ctypes import c_ulonglong
from ctypes import c_ushort
from ctypes import c_void_p
from ctypes import cdll
from ctypes import POINTER
from ctypes import Structure
from ctypes import Union
from ctypes.util import find_library
import ipaddress

MAXNS = 3
SOCKADDR_IN_PAD_LENGTH = 8
MAXDNSRCH = 6
MAXRESOLVSORT = 10
AF_INET = 2
AF_INET6 = 10


class _SortList(Structure):
    _fields_ = [("addr", c_uint32), ("mask", c_uint32)]


class _SockaddrIn(Structure):
    _fields_ = [
        ("sin_family", c_ushort),
        ("sin_port", c_uint16),
        ("sin_addr", c_uint8 * 4),
        ("sin_zero", c_uint8 * SOCKADDR_IN_PAD_LENGTH),
    ]


class _SockaddrIn6(Structure):
    _fields_ = [
        ("sin6_family", c_ushort),
        ("sin6_port", c_uint16),
        ("sin6_flowinfo", c_uint32),
        ("sin6_addr", c_uint8 * 16),
        ("sin6_scope_id", c_uint32),
    ]


class _ResStateExt(Structure):
    _fields_ = [
        ("nscount", c_uint16),
        ("nsmap", c_uint16 * MAXNS),
        ("nssocks", c_int * MAXNS),
        ("nscount6", c_uint16),
        ("nsinit", c_uint16),
        ("nsaddrs", POINTER(_SockaddrIn6) * MAXNS),
        ("__glibc_extension_index", c_ulonglong),
    ]


class _ResStateUnion(Union):
    _fields_ = [
        ("pad", c_char * 52),
        ("_ext", _ResStateExt),
    ]


class _ResState(Structure):
    _fields_ = [
        ("retrans", c_int),
        ("retry", c_int),
        ("options", c_ulong),
        ("nscount", c_int),
        ("nsaddr_list", _SockaddrIn * MAXNS),
        ("id", c_ushort),
        ("dnsrch", c_char_p * (MAXDNSRCH + 1)),
        ("defdname", c_char * 256),
        ("pfcode", c_ulong),
        ("ndots", c_uint, 4),
        ("nsort", c_uint, 4),
        ("ipv6_unavail", c_uint, 1),
        ("unused", c_uint, 23),
        ("sort_list", _SortList * MAXRESOLVSORT),
        ("__glibc_unused_qhook", c_void_p),
        ("__glibc_unused_rhook", c_void_p),
        ("res_h_errno", c_int),
        ("_vcsock", c_int),
        ("_flags", c_uint),
        ("_u", _ResStateUnion),
    ]


def main():
    libc = cdll.LoadLibrary(find_library("c"))

    libc.__res_ninit.restype = c_int
    libc.__res_ninit.argtypes = (POINTER(_ResState),)

    libc.__res_nclose.restype = None
    libc.__res_nclose.argtypes = (POINTER(_ResState),)

    c_res_state_obj = _ResState()

    libc.__res_ninit(byref(c_res_state_obj))
    for i in range(0, c_res_state_obj.nscount):
        if c_res_state_obj.nsaddr_list[i].sin_family == AF_INET:
            sock_addr4 = c_res_state_obj.nsaddr_list[i]
            print(ipaddress.IPv4Address(bytes(sock_addr4.sin_addr)))
        elif c_res_state_obj._u._ext.nsaddrs:
            sock_addr6 = c_res_state_obj._u._ext.nsaddrs[i].contents
            if sock_addr6.sin6_family == AF_INET6:
                print(ipaddress.IPv6Address(bytes(sock_addr6.sin6_addr)))
    for i in range(0, MAXDNSRCH):
        if c_res_state_obj.dnsrch[i]:
            print(c_res_state_obj.dnsrch[i].decode("utf-8"))
        else:
            break

    libc.__res_nclose(byref(c_res_state_obj))


main()