feat(client): re-resolve the server hostnames at the DNS TTL
hbc, hbc_mini.py and hbc_mini.c resolved the server once at startup, so an address change needed a client restart. They now re-resolve every TTL seconds (smallest A/AAAA TTL, floor 30s, 300s when unknown) and re-point, add or drop connections to follow the new addresses; a failed lookup keeps the current ones. Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
This commit is contained in:
Claude Sonnet 5.5
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"""Re-resolution of the server hostnames at the TTL of their A/AAAA records.
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getaddrinfo() hides the TTL, so a minimal stub-resolver query (resolv.conf nameservers, search
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list) is made only to learn it. The addresses themselves still come from getaddrinfo(). When
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the TTL cannot be learned (IP literal, /etc/hosts name, no resolv.conf, resolver timeout)
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DEFAULT_TTL is used.
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"""
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import ipaddress
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import logging
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import os
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import socket
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import struct
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from typing import List, Optional, Tuple
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logger = logging.getLogger("hbc.dns")
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DEFAULT_TTL = 300 # used when the real TTL is unknown
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MIN_TTL = 30 # a TTL of 0 must not turn into a resolution loop
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FAILED_RETRY = 60 # re-try delay after a failed resolution
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QUERY_TIMEOUT = 2.0
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TYPE_A, TYPE_CNAME, TYPE_AAAA = 1, 5, 28
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def parse_resolv_conf(path: str = "/etc/resolv.conf") -> Tuple[List[str], List[str]]:
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"""Return (nameservers, search domains)."""
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servers: List[str] = []
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search: List[str] = []
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try:
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with open(path) as f:
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for line in f:
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parts = line.split()
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if len(parts) < 2:
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continue
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if parts[0] == "nameserver":
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servers.append(parts[1].split("%")[0])
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elif parts[0] in ("search", "domain"):
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search = parts[1:]
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except OSError:
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pass
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return servers, search
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def build_query(name: str, qtype: int, qid: int) -> bytes:
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out = struct.pack("!HHHHHH", qid, 0x0100, 1, 0, 0, 0)
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for label in name.rstrip(".").split("."):
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out += bytes([len(label)]) + label.encode("idna")
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return out + b"\x00" + struct.pack("!HH", qtype, 1)
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def _skip_name(buf: bytes, off: int) -> int:
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while True:
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n = buf[off]
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if n == 0:
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return off + 1
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if n & 0xC0:
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return off + 2
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off += n + 1
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def min_answer_ttl(resp: bytes, qid: int, qtype: int) -> Optional[int]:
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"""Smallest TTL among the answer RRs (CNAMEs included), or None if there is no qtype answer."""
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try:
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rid, flags, qd, an = struct.unpack("!HHHH", resp[:8])
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if rid != qid or flags & 0x000F:
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return None
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off = 12
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for _ in range(qd):
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off = _skip_name(resp, off) + 4
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ttls: List[int] = []
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found = False
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for _ in range(an):
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off = _skip_name(resp, off)
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rtype, _cls, ttl, rdlen = struct.unpack("!HHIH", resp[off:off + 10])
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off += 10 + rdlen
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if rtype in (qtype, TYPE_CNAME):
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ttls.append(ttl)
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found = found or rtype == qtype
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return min(ttls) if found else None
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except (struct.error, IndexError):
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return None
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def _query_ttl(server: str, name: str, qtype: int) -> Optional[int]:
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qid = struct.unpack("!H", os.urandom(2))[0]
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try:
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af = socket.AF_INET6 if ":" in server else socket.AF_INET
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with socket.socket(af, socket.SOCK_DGRAM) as s:
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s.settimeout(QUERY_TIMEOUT)
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s.sendto(build_query(name, qtype, qid), (server, 53))
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return min_answer_ttl(s.recv(4096), qid, qtype)
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except (OSError, UnicodeError):
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return None
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def lookup_ttl(host: str, af_filter: int = 0) -> Optional[int]:
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"""Blocking. Smallest A/AAAA TTL for host, or None when it cannot be determined."""
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try:
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ipaddress.ip_address(host)
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return None
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except ValueError:
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pass
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servers, search = parse_resolv_conf()
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if host.endswith("."):
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names = [host]
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elif "." in host:
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names = [host] + [f"{host}.{d}" for d in search]
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else:
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names = [f"{host}.{d}" for d in search] + [host]
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qtypes = [t for t, af in ((TYPE_A, socket.AF_INET), (TYPE_AAAA, socket.AF_INET6))
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if af_filter in (0, af)]
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for name in names:
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ttls = []
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for qtype in qtypes:
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for server in servers:
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ttl = _query_ttl(server, name, qtype)
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if ttl is not None:
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ttls.append(ttl)
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break
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if ttls:
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return min(ttls)
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return None
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def refresh_delay(ttls: List[Optional[int]]) -> int:
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"""Seconds until the next resolution: the smallest known TTL, floored at MIN_TTL."""
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known = [t for t in ttls if t is not None]
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return max(min(known), MIN_TTL) if known else DEFAULT_TTL
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def resolve(hosts: List[str], port: int, af_filter: int) -> Optional[List[Tuple[int, str]]]:
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"""Blocking. Every (family, address) the hosts resolve to, deduplicated, or None if any lookup fails."""
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found: List[Tuple[int, str]] = []
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for host in hosts:
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try:
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infos = socket.getaddrinfo(host, port, af_filter, 0, socket.SOL_UDP)
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except socket.gaierror as e:
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logger.warning(f"Cannot resolve {host}: {e}")
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return None
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for info in infos:
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entry = (info[0], str(info[4][0]))
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if entry not in found:
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found.append(entry)
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return found or None
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def retarget(connections: list, resolved: List[Tuple[int, str]]) -> Tuple[List[Tuple[int, str]], list]:
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"""Point the connections at the resolved addresses, in place.
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A connection whose address is gone is re-pointed at an address that is new, in list order, so
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the first connection (the one plugin data uses) survives as long as anything resolves.
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Returns (new addresses still needing a connection, connections to drop).
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"""
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current = {c.addr for c in connections}
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wanted = {addr for _, addr in resolved}
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new = [(af, addr) for af, addr in resolved if addr not in current]
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gone = [c for c in connections if c.addr not in wanted]
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paired = min(len(gone), len(new))
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for conn, (af, addr) in zip(gone, new):
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logger.info(f"Server address {conn.addr} -> {addr}")
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conn.set_target(addr, af)
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drop = gone[paired:]
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if connections and connections[0] in drop:
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# plugin data goes out on the first connection: hand it a surviving address and drop that one instead
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keep = next(c for c in connections if c not in gone)
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connections[0].set_target(keep.addr, keep.af)
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drop.remove(connections[0])
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drop.append(keep)
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return new[paired:], drop
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