Determine forwarding and reverse-path filtering from host definitions.

[firewall] / bookends.m4

### -*-sh-*-
###
### Initialization and finishing touches for firewall scripts
###
### (c) 2008 Mark Wooding
###

###----- Licensing notice ---------------------------------------------------
###
### This program is free software; you can redistribute it and/or modify
### it under the terms of the GNU General Public License as published by
### the Free Software Foundation; either version 2 of the License, or
### (at your option) any later version.
###
### This program is distributed in the hope that it will be useful,
### but WITHOUT ANY WARRANTY; without even the implied warranty of
### MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
### GNU General Public License for more details.
###
### You should have received a copy of the GNU General Public License
### along with this program; if not, write to the Free Software Foundation,
### Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.

m4_divert(30)m4_dnl
###--------------------------------------------------------------------------
### Clear existing firewall rules.

## The main chains: set policy to drop, and then clear the rules.  For a
## while, incoming packets will be silently dropped, but we should have got
## everything going before anyone actually hits a timeout.
##
## We don't control some of the chains, so we should preserve them.  This
## introduces a whole bunch of problems.

## Chains we're meant to preserve
preserve_chains="filter:fail2ban filter:fail2ban-* $preserve_chains"

## Take the various IP versions in turn.
unref=nil
for ip in ip ip6; do
  for table in $(cat /proc/net/${ip}_tables_names); do

    ## Step 1: clear out the builtin chains.
    ${ip}tables -nL -t $table |
    sed -n '/^Chain \([^ ]\+\) (policy .*$/ s//\1/p ' |
    while read chain; do
      case $table in
	nat) policy=ACCEPT ;;
	*) policy=DROP ;;
      esac
      run ${ip}tables -t $table -P $chain $policy
      run ${ip}tables -t $table -F $chain
    done

    ## Step 2: clear out user chains.  Unfortunately, we can only clear
    ## chains which have no references to them, so work through picking off
    ## unreferenced chains which aren't meant to be preserved until there are
    ## none left.
    while :; do
      progress=nil
      ${ip}tables -nL -t $table |
      sed -n '/^Chain \([^ ]\+\) (0 references)$/ s//\1/p ' \
	>/var/run/firewall-chains.tmp
      while read chain; do
	match=nil
	for pat in $preserve_chains; do
	  case "$table:$chain" in $pat) match=t ;; esac
	done
	case $match in
	  nil)
	    run ${ip}tables -t $table -F $chain
	    run ${ip}tables -t $table -X $chain
	    progress=t
	    ;;
	esac
      done </var/run/firewall-chains.tmp
      case $progress in nil) break ;; esac
    done

    ## Step 3: report on uncleared user chains.  This means that there's a
    ## serious problem.
    ${ip}tables -nL -t $table |
    sed -n '/^Chain \([^ ]\+\) (\([1-9][0-9]*\) references)$/ s//\1 \2/p ' \
      >/var/run/firewall-chains.tmp
    while read chain refs; do
      match=nil
      for pat in $preserve_chains; do
	case "$table:$chain" in $pat) match=t ;; esac
      done
      case $match in
	nil)
	  echo >&2 "$0: can't clear referenced $ip chain \`$table:$chain'"
	  unref=t
	  ;;
      esac
    done </var/run/firewall-chains.tmp
  done
done
rm -f /var/run/firewall-chains.tmp
case $unref in t) exit 1 ;; esac

m4_divert(32)m4_dnl
###--------------------------------------------------------------------------
### Set safe IP options.

## Set forwarding options.  Apparently setting ip_forward clobbers other
## settings, so put this first.
case $host_type_<::>FWHOST in
  router) forward=1 ;;
  *) forward=0 ;;
esac
setopt ip_forward $forward
setdevopt forwarding $forward

## Set dynamic port allocation.
setopt ip_local_port_range $open_port_min $open_port_max

## Deploy SYN-cookies if necessary.
setopt tcp_syncookies 1

## Allow broadcast and multicast ping, because it's a useful diagnostic tool.
setopt icmp_echo_ignore_broadcasts 0

## Turn off iptables filtering for bridges.  We'll use ebtables if we need
## to; but right now the model is that we do filtering at the borders, and
## are tolerant of things which are local.
if [ -x /sbin/brctl ]; then
  modprobe bridge || :
  if [ -d /proc/sys/net/bridge ]; then
    for filter in arptables iptables ip6tables; do
      run sysctl -q net.bridge.bridge-nf-call-$filter=0
    done
  fi
fi

## Turn off the reverse-path filter.  It's basically useless: the filter does
## nothing at all for single-homed hosts; and multi-homed hosts tend to have
## routing aysmmetries if there's any kind of cycle.
setdevopt rp_filter 0
setdevopt log_martians 0

## Turn off things which can mess with our routing decisions.
setdevopt accept_source_route 0
setdevopt accept_redirects 0

## If we're maent to stop the firewall, then now is the time to do it.
$exit_after_clearing

m4_divert(34)m4_dnl
###--------------------------------------------------------------------------
### Establish error chains.

errorchain forbidden REJECT
## Generic `not allowed' chain.

errorchain tcp-fragment REJECT
## Chain for logging fragmented TCP segements.

errorchain bad-tcp REJECT -p tcp --reject-with tcp-reset
## Bad TCP segments (e.g., for unknown connections).  Sends a TCP reset.

errorchain mangle:bad-source-address DROP
errorchain bad-source-address DROP
## Packet arrived on wrong interface for its source address.  Drops the
## packet, since there's nowhere sensible to send an error.

errorchain bad-destination-address REJECT
## Packet arrived on non-loopback interface with loopback destination.

errorchain interesting ACCEPT
## Not an error, just log interesting packets.

m4_divert(50)m4_dnl
###--------------------------------------------------------------------------
### Standard filtering.

## Don't clobber local traffic
run ip46tables -A INPUT -i lo -j ACCEPT

## We really shouldn't see packets destined for localhost on any interface
## other than the loopback.
run iptables -A INPUT -g bad-destination-address \
	-d 127.0.0.0/8
run ip6tables -A INPUT -g bad-destination-address \
	-d ::1

## We shouldn't be asked to forward things with link-local addresses.
case $forward in
  1)
    run iptables -A FORWARD -g bad-source-address \
	    -s 169.254.0.0/16
    run iptables -A FORWARD -g bad-destination-address \
	    -d 169.254.0.0/16
    run ip6tables -A FORWARD -g bad-source-address \
	    -s fe80::/10
    run ip6tables -A FORWARD -g bad-destination-address \
	    -d fe80::/10
    ;;
esac

## Also, don't forward link-local broadcast or multicast.
case $forward in
  1)
    run iptables -A FORWARD -g bad-destination-address \
	    -d 255.255.255.255
    run iptables -A FORWARD -g bad-destination-address \
	    -m addrtype --dst-type BROADCAST
    run iptables -A FORWARD -g bad-destination-address \
	    -d 224.0.0.0/24
    for x in 0 1 2 3 4 5 6 7 8 9 a b c d e f; do
      run ip6tables -A FORWARD -g bad-destination-address \
	    -d fe${x}2::/16
    done
    ;;
esac

## Add a hook for fail2ban.
clearchain fail2ban
run ip46tables -A INPUT -j fail2ban

m4_divert(90)m4_dnl
###--------------------------------------------------------------------------
### Finishing touches.

m4_divert(94)m4_dnl
## Locally generated packets are all OK.
run ip46tables -P OUTPUT ACCEPT

## Other incoming things are forbidden.
for chain in INPUT FORWARD; do
  run ip46tables -A $chain -g forbidden
done

## Allow stuff through unknown tables.
for ip in ip ip6; do
  for table in $(cat /proc/net/${ip}_tables_names); do
    case $table in mangle | filter) continue ;; esac
    ${ip}tables -nL -t $table |
    sed -n '/^Chain \([^ ]\+\) (policy .*$/ s//\1/p ' |
    while read chain; do
      run ${ip}tables -t $table -P $chain ACCEPT
    done
  done
done

m4_divert(-1)
###----- That's all, folks --------------------------------------------------