84 lines
2.5 KiB
Python
84 lines
2.5 KiB
Python
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"""IPv4 mDNS resolver using only Python stdlib. Used as no-dep fallback."""
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import socket
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import struct
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import select
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import time
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MDNS_ADDR4 = "224.0.0.251"
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MDNS_PORT = 5353
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QTYPE_A = 1
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def build_query(hostname: str, qtype: int = QTYPE_A) -> bytes:
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header = struct.pack(">HHHHHH", 0, 0, 1, 0, 0, 0)
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q = b""
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for part in hostname.rstrip(".").split("."):
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q += bytes([len(part)]) + part.encode()
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q += b"\x00" + struct.pack(">HH", qtype, 1)
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return header + q
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def skip_name(data: bytes, off: int) -> int:
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while off < len(data):
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b = data[off]
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if b == 0:
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return off + 1
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if b & 0xC0 == 0xC0:
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return off + 2
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off += b + 1
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return off
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def parse_all_a(data: bytes) -> list[tuple[str, int]]:
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"""Return every A record (ip, ttl) in the response."""
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out: list[tuple[str, int]] = []
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if len(data) < 12:
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return out
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qd, an = struct.unpack(">HH", data[4:8])
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off = 12
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for _ in range(qd):
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off = skip_name(data, off) + 4
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for _ in range(an):
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off = skip_name(data, off)
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if off + 10 > len(data):
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break
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rtype, _, ttl, rdlen = struct.unpack(">HHIH", data[off:off + 10])
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off += 10
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if rtype == QTYPE_A and rdlen == 4 and off + 4 <= len(data):
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out.append((socket.inet_ntoa(data[off:off + 4]), ttl))
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off += rdlen
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return out
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def query_all(hostname: str, timeout: float) -> list[tuple[str, int]]:
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"""Collect all A records advertised for hostname during the time window."""
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s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
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s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
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try:
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mreq = socket.inet_aton(MDNS_ADDR4) + socket.inet_aton("0.0.0.0")
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s.setsockopt(socket.IPPROTO_IP, socket.IP_ADD_MEMBERSHIP, mreq)
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except OSError:
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pass
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s.setblocking(False)
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try:
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s.sendto(build_query(hostname, QTYPE_A), (MDNS_ADDR4, MDNS_PORT))
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except OSError:
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s.close()
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return []
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found: list[tuple[str, int]] = []
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end = time.time() + timeout
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while time.time() < end:
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r, _, _ = select.select([s], [], [], min(0.1, max(0.0, end - time.time())))
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if r:
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try:
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data, _ = s.recvfrom(4096)
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except OSError:
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continue
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recs = parse_all_a(data)
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if recs:
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found.extend(recs)
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# Give a brief grace window for additional A packets, then stop.
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end = min(end, time.time() + 0.15)
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s.close()
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return found
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