Initial commit of fancy firewall infrastructure.

[firewall] / bookends.m4

### -*-m4-*-
###
### 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.
for t in mangle filter; do
  for i in PREROUTING INPUT FORWARD OUTPUT POSTROUTING; do
    run iptables -t $t -P $i DROP 2>/dev/null || :
    run iptables -t $t -F $i 2>/dev/null || :
  done
  run iptables -t $t -F
  run iptables -t $t -X
done

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

## Set forwarding options.  Apparently setting ip_forward clobbers other
## settings, so put this first.
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

## Turn on the reverse-path filter, and log weird things.
setdevopt rp_filter 1
setdevopt log_martians 1

## 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 --reject-with icmp-host-prohibited
## Generic `not allowed' chain.  Rejects with ICMP host-prohibited.

errorchain tcp-fragment REJECT --reject-with icmp-host-prohibited
## Chain for logging fragmented TCP segements.  Rejects with ICMP
## host-prohibited.

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

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

m4_divert(36)m4_dnl
###--------------------------------------------------------------------------
### Don't clobber local traffic.

run iptables -A INPUT -i lo -j ACCEPT

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

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

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

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