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Flush systemd DNS cache on startup and exit.

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It is possible for DNS requests to go through systemd's DNS resolution
system (which includes a cache) before sshuttle has an opportunity to
intercept the requests. The DNS entries in the cache may become
outdated when sshuttle starts or exits. This patch fixes the problem
by flushing the cache when sshuttle firewall starts and exits.
This commit is contained in:
Scott Kuhl 2021-05-05 12:24:58 -04:00
parent f6176d3581
commit cdfb4b7d71
1 changed files with 34 additions and 0 deletions
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34
sshuttle/firewall.py
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@ -5,6 +5,7 @@ import sys
import os
import platform
import traceback
import subprocess as ssubprocess
import sshuttle.ssyslog as ssyslog
import sshuttle.helpers as helpers
@ -89,6 +90,29 @@ def subnet_weight(s):
return (-s[-1] + (s[-2] or -65535), s[1], s[2])
def flush_systemd_dns_cache():
# If the user is using systemd-resolve for DNS resolution, it is
# possible for the request to go through systemd-resolve before we
# see it...and it may use a cached result instead of sending a
# request that we can intercept. When sshuttle starts and stops,
# this means that we should clear the cache!
#
# The command to do this was named systemd-resolve, but changed to
# resolvectl in systemd 239.
# https://github.com/systemd/systemd/blob/f8eb41003df1a4eab59ff9bec67b2787c9368dbd/NEWS#L3816
if helpers.which("resolvectl"):
debug2("Flushing systemd's DNS resolver cache: "
"resolvectl flush-caches")
ssubprocess.Popen(["resolvectl", "flush-caches"],
stdout=ssubprocess.PIPE, env=helpers.get_env())
elif helpers.which("systemd-resolve"):
debug2("Flushing systemd's DNS resolver cache: "
"systemd-resolve --flush-caches")
ssubprocess.Popen(["systemd-resolve", "--flush-caches"],
stdout=ssubprocess.PIPE, env=helpers.get_env())
# This is some voodoo for setting up the kernel's transparent
# proxying stuff. If subnets is empty, we just delete our sshuttle rules;
# otherwise we delete it, then make them from scratch.
@ -227,6 +251,7 @@ def main(method_name, syslog, ttl):
socket.AF_INET, subnets_v4, udp,
user, ttl)
flush_systemd_dns_cache()
stdout.write('STARTED\n')
try:
@ -288,3 +313,12 @@ def main(method_name, syslog, ttl):
debug1(traceback.format_exc())
except BaseException:
debug2('An error occurred, ignoring it.')
try:
flush_systemd_dns_cache()
except BaseException:
try:
debug1("Error trying to flush systemd dns cache.")
debug1(traceback.format_exc())
except BaseException:
debug2("An error occurred, ignoring it.")