[firewall] / classify.m4

### -*-sh-*-
###
### Classify packets according to source and destination networks.
###
### (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.

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###--------------------------------------------------------------------------
### Address classification.
###
### The objective of address classification is to work out what kind of
### networks a packet is travelling between, in order to make filtering
### decisions easier.
###
### Address classification is done in the mangle table, by attaching
### appropriate marks to the packet.  We split the Internet into a number of
### address classes, and make forwarding decisions based on the classes of
### the source and destination addresses.
###
### The mark word is split into three fields: the FROM and TO fields simply
### record the source and destination classes numerically; the MASK field is
### used to determine whether forwarding should occur.  There is a mask bit
### for each address class.  Source classification sets mask bits according
### to the forwarding policy for the source address class.  Destination
### classification clears all of the mask bits except for the one
### corresponding to the actual destination class.  Therefore, forwarding is
### permitted if and only if the mask bits are not all zero.
###
### The mangle chains are arranged as follows.
###
### The PREROUTING hook simply invokes in-classify and out-classify chains as
### subroutines.  These will tail-call appropriate classification chains.
###
### The in-classify chain is responsible for both source address
### classification and verifying that the packet arrived from the correct
### interface.  It does an initial dispatch on the source interface, to
### in-IFACE.  The in-IFACE chain dispatches to mark-from-CLASS when it
### recognizes an address belonging to the CLASS; if no matches succeed, it
### goes to bad-source-address, which logs a message and drops the packet.
### The default interface is special.  If no explicit matches are found, it
### dispatches to in-default which forbids a few obviously evil things and
### finally dispatches to mark-from-untrusted.
###
### The out-classify is simpler because it doesn't care about the interface.
### It simply checks each network range in turn, dispatching to mark-to-CLASS
### on a match or mark-to-DEFAULT (probably untrusted) if there is no match.

clearchain mangle:in-classify mangle:in-default mangle:out-classify
clearchain mangle:local-source

## An unpleasant hack.  We can't reject packets from the mangle table, so
## we mark packets with a bad destination and then detect this in the
## filter table.
clearchain mangle:bad-destination-address
BAD_DEST=0xf6f377d2
run ip46tables -t mangle -A bad-destination-address \
  -j MARK --set-mark $BAD_DEST
run ip46tables -t mangle -A bad-destination-address -j ACCEPT
for i in $inchains; do
  run ip46tables -A $i -m mark --mark $BAD_DEST -g bad-destination-address
done

## Packets over the loopback interface are automatically trusted.  All manner
## of weird stuff happens on lo, and it's best not to second-guess it.
run ip46tables -t mangle -A in-classify -i lo -j ACCEPT

## Local broadcast and link-local multicast packets sometimes have bizarre
## addresses.  Don't block them just because of this.
run iptables -t mangle -A in-classify -j RETURN \
        -s 0.0.0.0 -d 255.255.255.255 \
        -p udp
run iptables -t mangle -A in-classify -j RETURN \
        -s 0.0.0.0 -d 224.0.0.0/24 \
        -p udp

## Since packets with source and destination addresses both local will go
## over the loopback interface, I shouldn't see a packet from me over any
## other interface.  Except that I will if I sent a broadcast or multicast.
## Allow the broadcasts, and remember not to trust them.  There are no
## broadcast addresses in IPv6 (only link-local multicast)m so we don't have
## to worry about that.
run iptables -t mangle -A local-source -j RETURN \
        -m addrtype --dst-type BROADCAST
run iptables -t mangle -A local-source -j RETURN \
        -m addrtype --dst-type MULTICAST
run ip6tables -t mangle -A local-source -j RETURN \
        -d ff00::/8
run ip46tables -t mangle -A local-source -g bad-source-address
run iptables -t mangle -A in-classify -j local-source \
        -m addrtype --src-type LOCAL
for addr in $host_6addrs; do
  run ip6tables -t mangle -A in-classify -j local-source \
          -s $addr
done

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## Define the important networks.
for pass in 1 2; do
  netclassindex=0
m4_divert(42)m4_dnl
done

m4_divert(46)m4_dnl
## Special IPv4 source addresses.  Forbid broadcast and multicast sources.
## Mark the special zero address and link-local addresses as such.  (This
## also matches class-E addresses, which are probably permanently invalid.)
for i in 0.0.0.0 169.254.0.0/16; do
  run iptables -t mangle -A in-classify -g mark-from-link -s $i
done
run iptables -t mangle -A in-classify -g bad-source-address \
        -s 224.0.0.0/3
run iptables -t mangle -A in-classify -g bad-source-address \
        -m addrtype --src-type BROADCAST \

## Special IPv6 addresses.  Format multicast sources, and mark zero and
## link local addresses.
for i in :: fe80::/10; do
  run ip6tables -t mangle -A in-classify -g mark-from-link -s $i
done
run ip6tables -t mangle -A in-classify -g bad-source-address \
        -s ff00::/8

## Special IPv4 destination addresses.  The zero address is invalid; mark
## link-local and recognized broadcast addresses as link-local.  We leave
## multicast for later.
for i in 0.0.0.0 240.0.0.0/4; do
  run iptables -t mangle -A out-classify -g bad-destination-address -d $i
done
run iptables -t mangle -A out-classify -g mark-to-link -d 169.254.0.0/16
run iptables -t mangle -A out-classify -g mark-to-link \
        -m addrtype --dst-type BROADCAST

## Special IPv6 destination addressses.  The zero address is again invalid;
## mark link local addresses.  We do multicast later.
run ip6tables -t mangle -A out-classify -g bad-destination-address \
        -d ::
run ip6tables -t mangle -A out-classify -g mark-to-link -d fe80::/10

## Now deal with multicast.  Link-local multicast is detected as being
## link-local, so that we can prevent it being forwarded correctly.
clearchain mangle:out-classify-mcast
run iptables -t mangle -A out-classify-mcast -g mark-to-link \
        -d 224.0.0.0/24
for i in 0 1 2 3 4 5 6 7 8 9 a b c d e f; do
  run ip6tables -t mangle -A out-classify-mcast -g mark-to-link \
          -d ff${i}2::/16
done
run ip46tables -t mangle -A out-classify-mcast -g mark-to-mcast
run iptables -t mangle -A out-classify -g out-classify-mcast \
        -d 224.0.0.0/4
run ip6tables -t mangle -A out-classify -g out-classify-mcast \
        -d ff00::/8

## Build the input classification chains.  There's one chain `in-IFACE' for
## each local interface.  This chain does a further dispatch on the source
## address to the appropriate `mark-from-CLASS' chain for the source network
## class.  We also build a table mapping interface names to numbers (since
## the names are so unhelpful).
seen=:
ifq=0
ifmap=""
for iface in $host_ifaces_<::>FWHOST; do
  ifname=${iface%=*}
  case $seen in *:$ifname:*) continue ;; esac
  seen=$seen$ifname:
  addword ifmap $ifname=$ifq
  ifq=$(( $ifq + 1 ))
  clearchain mangle:in-$ifname
  run ip46tables -t mangle -A in-classify -i $ifname -g in-$ifname
done

## We do a first pass over nets first, and then the interfaces which those
## networks reach.  During this pass, we populate the `out-classify' chains,
## and we also build some lists so that we can do later passes over
## interfaces first and then reaching networks.  This is complicated by
## interface names being unhelpful.
##
## Here are the variables we maintain.
##
## ifmap                A list of entries IFACE=N mapping interface names to
##                      numbers.
##
## ifnets_N             A space-separated list of networks reaching interface
##                      number N.  This is used for building the matching
##                      chains.
##
## ifaddrs_N            A bang-separated list of address ranges reaching
##                      interface number N.  This is used for filtering out
##                      known networks if the default network reaches the
##                      interface.
for net in $allnets; do

  ## Work through the interfaces that this network reaches.
  for iface in $(net_interfaces FWHOST $net); do
    case $iface in -) break ;; esac

    ## Find a sequence number for this interface.
    q=nil
    for i in $ifmap; do
      case "$i" in "$iface"=*) q=${i##*=}; break ;; esac
    done
    case $q in
      nil)
        echo >&2 "$0 INTERNAL ERROR: missing interface \`$iface'!"
        exit 1
        ;;
    esac

    ## Remember the reachability information.
    addword ifnets_$q $net
  done
done

## Build the `ifaddr_N' map and an `all-addresses' list.
alladdrs=!
trace "ifmap = $ifmap"
for entry in $ifmap; do
  iface=${entry%=*} q=${entry##*=}
  eval nets=\$ifnets_$q
  aa=!
  for n in $nets; do
    eval "addrs=\"\$net_inet_$n \$net_inet6_$n\""
    trace "$iface $n addrs = $addrs"
    for a in $addrs; do
      case $aa in *!$a!*) ;; *) aa=$aa$a! ;; esac
      case $alladdrs in *!$a!*) ;; *) alladdrs=$alladdrs$a! ;; esac
    done
  done
  eval ifaddrs_$q=\$aa
  trace "iface $q = $iface; nets = $nets; addrs = $aa"
done
trace "alladdrs = $alladdrs"

## Populate the `out-classify' chain, matching networks.
prepare_to () { mode=goto fail=mark-to-$net_class_default; }
matchnets -d mark-to : prepare_to out-classify "" 0 $allnets

## A `finish' hook for rejecting known address ranges arriving on a
## default-reachable interface.
finish_from_default () {
  q=$1 chain=$2
  eval addrs=\$ifaddrs_$q

  for n in $allnets; do
    eval addr=\$net_inet_$n addr6=\$net_inet6_$n
    for a in $addr; do
      case $a in !*) continue ;; esac
      case $addrs in *"!$a!"*) continue ;; esac
      run iptables -t mangle -A $chain -s $a -g bad-source-address
    done
    for a in $addr6; do
      case $a in !*) continue ;; esac
      case $addrs in *"!$a!"*) continue ;; esac
      run ip6tables -t mangle -A $chain -s $a -g bad-source-address
    done
  done
  run ip46tables -t mangle -A $chain -g in-default
}

## A `prepare' hook for input classification.  If the interface is
## default-reachable, then we need to reject known address ranges before
## dispatching to the default chain; otherwise just reject the packet.
prepare_from () {
  q=$1 flags=$2
  case $flags in
    *:default:*) mode=call finish="finish_from_default $q" ;;
    *) mode=goto fail=bad-source-address ;;
  esac
}

## Populate the `in-IFACE' chains.
for entry in $ifmap; do
  iface=${entry%=*} q=${entry##*=}
  eval nets=\$ifnets_$q

  case $iface in
    *-+)
      ## A special marker indicating a collection of point-to-point
      ## interfaces.  We should match an address to a particular interface.
      chains=""
      for net in $nets; do
        eval hosts=\$net_hosts_$net class=\$net_class_$net
        for host in $hosts; do
          eval ha=\$host_inet_$host ha6=\$host_inet6_$host
          trace "$host : $class -> $iface"
          for a in $ha; do
            run iptables -t mangle -A in-$iface \
                    -i ${iface%+}$host -s $a -g mark-from-$class
          done
          for a in $ha6; do
            run ip6tables -t mangle -A in-$iface \
                    -i ${iface%+}$host -s $a -g mark-from-$class
          done
        done
      done
      run ip46tables -t mangle -A in-$iface -g bad-source-address
      ;;
    *)
      matchnets -s mark-from : "prepare_from $q" in-$iface "" 0 $nets
      ;;
  esac
done

## Fill in the black holes in the network.  Some of these might actually be
## known networks, so don't fill those in again.
for addr in \
        10.0.0.0/8 172.16.0.0/12 192.168.0.0/16 \
        127.0.0.0/8 \
        192.0.2.0/24 198.51.100.0/24 203.0.113.0/24
do
  case $alladdrs in *!$addr!*) continue ;; esac
  run iptables -t mangle -A in-default -s $addr -g bad-source-address
done
for addr in \
        fc00::/7 \
        2001:db8::/32
do
  case $alladdrs in *!$addr!*) continue ;; esac
  run ip6tables -t mangle -A in-default -s $addr -g bad-source-address
done
run ip46tables -t mangle -A in-default -g mark-from-$net_class_default

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## Put the final default decision on the in-default chain, and attach the
## classification chains to the PREROUTING hook.
for iface in $defaultifaces; do
  run ip46tables -t mangle -A in-$iface -g in-default
done
run ip46tables -t mangle -A PREROUTING -j in-classify
run ip46tables -t mangle -A PREROUTING -j out-classify

## Incoming stuff to or from a link-local address is OK.
run ip46tables -t mangle -A INPUT \
        -m mark --mark $to_link/$MASK_TO \
        -j MARK --or-mark $fwd_link
run ip46tables -t mangle -A INPUT \
        -m mark --mark $from_link/$MASK_FROM \
        -j MARK --or-mark $fwd_link

## Now it's safe to let stuff through.
for i in PREROUTING INPUT FORWARD OUTPUT POSTROUTING; do
  run ip46tables -t mangle -P $i ACCEPT
done

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