dns-proxy/init/dns-proxy.conf

# Copyright 2021 The Chromium OS Authors. All rights reserved.
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.

description     "DNS proxy daemon"
author          "chromium-os-dev@chromium.org"

start on starting system-services
stop on stopping system-services

respawn
respawn limit 3 5

# Do not respawn if the service is terminated on purpose.
normal exit 0

# Sacrifice before OOM panic.
# TODO(garrick): Add vm size limit.
oom score 0

# Minijail actually forks off the desired process.
expect fork

script
  # Start constructing minijail0 args...
  args=""

  # Make sure minijail0 exits right away and won't block upstart.
  args="${args} -i"

  # Create a UTS namespace to isolate changes to the host / domain name.
  args="${args} --uts"

  # Create an IPC namespace (isolate System V IPC objects/POSIX message queues).
  args="${args} -l"

  # Remount /proc read-only (prevents any messing with it).
  args="${args} -r"

  # Creates new, empty tmp directory (technically, mounts tmpfs).
  args="${args} -t"

  # Prevent that execve gains privileges, required for seccomp filters.
  args="${args} -n"

  # Apply seccomp policy.
  args="${args} -S /usr/share/policy/dns-proxy-seccomp.policy"

  # Use a minimalistic mount namespace.
  args="${args} --profile minimalistic-mountns"

  # Mount /run as tmpfs read-only.
  args="${args} -k tmpfs,/run,tmpfs,MS_NOSUID|MS_NODEV|MS_NOEXEC"

  # Bind-mount /run/dbus read-only for D-Bus to work.
  args="${args} -b /run/dbus"

  # Bind-mount /sbin read-only to start sandboxed processes using minijail0.
  args="${args} -b /sbin"

  # Bind-mount /run/shill for DNS resolution.
  args="${args} -b /run/shill"

  # Run as dns-proxy user and group.
  args="${args} -u dns-proxy -g dns-proxy"

  # Run with root permissions so that the daemon can start sandboxed processes.
  # Run with CAP_NET_BIND_SERVICE so that it can bind to default DNS port (53).
  args="${args} -c cap_sys_admin,cap_net_bind_service=e"

  # Execute dns-proxy.
  args="${args} /usr/sbin/dnsproxyd"

  # -e is not specified because the service needs to connect to servers.

  exec minijail0 ${args}
end script