apache/support/logresolve.py.in

#!@pythonbin@
#
# Licensed to the Apache Software Foundation (ASF) under one or more
# contributor license agreements.  See the NOTICE file distributed with
# this work for additional information regarding copyright ownership.
# The ASF licenses this file to You 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.
#
# logresolve.py -- Python port of the historical Perl logresolve.pl.
#
# usage: logresolve.py <infile >outfile
#
# input = Apache/NCSA/.. logfile with IP numbers at start of lines
# output = same logfile with IP addresses resolved to hostnames where
#  name lookups succeeded.
#
# this differs from the C based 'logresolve' in that this script
# resolves a number (CHILDREN) of addresses concurrently and sets a
# short timeout (TIMEOUT) for each lookup in order to keep things moving
# quickly.
#
# The original Perl version forked CHILDREN subprocesses and exchanged
# IPs/hostnames over Unix sockets, with a per-lookup alarm(TIMEOUT).
# This Python port reproduces the *semantics* without the fork/socket
# IPC machinery: a concurrent.futures.ThreadPoolExecutor with
# max_workers=CHILDREN performs up to CHILDREN reverse lookups in
# parallel, and the per-lookup timeout is enforced via
# socket.setdefaulttimeout(TIMEOUT) (socket.gethostbyaddr() honours the
# process default socket timeout; it has no timeout argument of its own).
#
# Results are cached in a dict so each unique IP is resolved only once,
# and output lines are emitted in the exact same order as the input
# (logfiles must stay in order).
#
# Concurrency / timeout / order-preservation:
#   * Concurrency: ThreadPoolExecutor(max_workers=CHILDREN). Each unique
#     IP is submitted exactly once; the cache dict guards against
#     resolving the same IP twice.
#   * Timeout: socket.setdefaulttimeout(TIMEOUT) bounds each DNS lookup.
#     As a belt-and-braces measure the worker also bounds itself via
#     future.result(timeout=...) when collecting results.
#   * Order: we buffer all input lines (remembering each line's leading
#     IP), resolve the unique IPs concurrently, then walk the buffered
#     lines in their original order substituting the cached hostname.

import socket
import sys
from concurrent.futures import ThreadPoolExecutor

CHILDREN = 40
TIMEOUT = 5


def nslookup(ip):
    """Reverse-resolve an IP to a hostname.

    Equivalent to the Perl gethostbyaddr(gethostbyname($ip), AF_INET).
    Returns the resolved hostname, or the original IP on any failure or
    timeout (matching the Perl behaviour of leaving the IP as-is).
    """
    try:
        hostname = socket.gethostbyaddr(ip)[0]
    except Exception:
        return ip
    return hostname if hostname else ip


def main():
    # Bound every DNS lookup to TIMEOUT seconds. socket.gethostbyaddr()
    # has no timeout argument, so we rely on the process-wide default
    # socket timeout, which it honours.
    socket.setdefaulttimeout(TIMEOUT)

    # Read the whole logfile, buffering each line and the IP that starts
    # it.  Order is preserved by replaying this buffer at the end.
    lines = []          # list of (ip, rest, had_space); rest keeps its newline
    unique_ips = []     # unique IPs in first-seen order (for stable submit)
    seen = set()

    for line in sys.stdin:
        # split on the FIRST space only; a line with no space is all IP.
        parts = line.split(' ', 1)
        if len(parts) > 1:
            ip = parts[0]
            rest = parts[1]
            had_space = True
        else:
            # No space: the whole line is the IP. Strip the trailing
            # newline so it resolves cleanly, but remember the line
            # ending so we can reproduce it verbatim on output.
            stripped = line.rstrip('\n')
            ip = stripped
            rest = line[len(stripped):]
            had_space = False
        lines.append((ip, rest, had_space))
        if ip not in seen:
            seen.add(ip)
            unique_ips.append(ip)

    # Resolve all unique IPs concurrently, up to CHILDREN at a time.
    cache = {}
    if unique_ips:
        with ThreadPoolExecutor(max_workers=CHILDREN) as pool:
            futures = {ip: pool.submit(nslookup, ip) for ip in unique_ips}
            for ip, fut in futures.items():
                try:
                    cache[ip] = fut.result(timeout=TIMEOUT + 1)
                except Exception:
                    # On timeout/failure leave the IP unchanged.
                    cache[ip] = ip

    # Emit lines in the original input order, substituting hostnames.
    out = sys.stdout
    for ip, rest, had_space in lines:
        host = cache.get(ip, ip)
        if had_space:
            out.write("%s %s" % (host, rest))
        else:
            # No space in the original line: emit host then the original
            # line ending (rest is "\n", "" with EOF no-newline, etc.).
            out.write(host)
            if rest:
                out.write(rest)


if __name__ == "__main__":
    main()