mdw@git.distorted.org.uk Git - firewall/blob - functions.m4

   1 ### -*-sh-*-
   2 ###
   3 ### Utility functions for firewall scripts
   4 ###
   5 ### (c) 2008 Mark Wooding
   6 ###
   7
   8 ###----- Licensing notice ---------------------------------------------------
   9 ###
  10 ### This program is free software; you can redistribute it and/or modify
  11 ### it under the terms of the GNU General Public License as published by
  12 ### the Free Software Foundation; either version 2 of the License, or
  13 ### (at your option) any later version.
  14 ###
  15 ### This program is distributed in the hope that it will be useful,
  16 ### but WITHOUT ANY WARRANTY; without even the implied warranty of
  17 ### MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  18 ### GNU General Public License for more details.
  19 ###
  20 ### You should have received a copy of the GNU General Public License
  21 ### along with this program; if not, write to the Free Software Foundation,
  22 ### Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
  23
  24 m4_divert(20)m4_dnl
  25 ###--------------------------------------------------------------------------
  26 ### Utility functions.
  27
  28 ## doit COMMAND ARGS...
  29 ##
  30 ## If debugging, print the COMMAND and ARGS.  If serious, execute them.
  31 run () {
  32   set -e
  33   if [ "$FW_DEBUG" ]; then echo "* $*"; fi
  34   if ! [ "$FW_NOACT" ]; then "$@"; fi
  35 }
  36
  37 ## trace MESSAGE...
  38 ##
  39 ## If debugging, print the MESSAGE.
  40 trace () {
  41   set -e
  42   if [ "$FW_DEBUG" ]; then echo "$*"; fi
  43 }
  44
  45 ## defport NAME NUMBER
  46 ##
  47 ## Define $port_NAME to be NUMBER.
  48 defport () {
  49   name=$1 number=$2
  50   eval port_$name=$number
  51 }
  52
  53 ## defproto NAME NUMBER
  54 ##
  55 ## Define $proto_NAME to be NUMBER.
  56 defproto () {
  57   name=$1 number=$2
  58   eval proto_$name=$number
  59 }
  60
  61 ## addword VAR WORD
  62 ##
  63 ## Adds WORD to the value of the shell variable VAR, if it's not there
  64 ## already.  Words are separated by a single space; no leading or trailing
  65 ## spaces are introduced.
  66 addword () {
  67   var=$1 word=$2
  68   eval val=\$$var
  69   case " $val " in
  70     *" $word "*) ;;
  71     *) eval "$var=\${$var:+\$val }\$word" ;;
  72   esac
  73 }
  74
  75 m4_divert(38)m4_dnl
  76 ###--------------------------------------------------------------------------
  77 ### Utility chains (used by function definitions).
  78
  79 m4_divert(20)m4_dnl
  80 ###--------------------------------------------------------------------------
  81 ### Basic chain constructions.
  82
  83 ## ip46tables ARGS ...
  84 ##
  85 ## Do the same thing for `iptables' and `ip6tables'.
  86 ip46tables () {
  87   set -e
  88   iptables "$@"
  89   ip6tables "$@"
  90 }
  91
  92 ## clearchain CHAIN CHAIN ...
  93 ##
  94 ## Ensure that the named chains exist and are empty.
  95 clearchain () {
  96   set -e
  97   for _chain; do
  98     case $_chain in
  99       *:*) table=${_chain%:*} _chain=${_chain#*:} ;;
 100       *) table=filter ;;
 101     esac
 102     run ip46tables -t $table -N $_chain 2>/dev/null || :
 103   done
 104 }
 105
 106 ## makeset SET TYPE [PARAMS]
 107 ##
 108 ## Ensure that the named ipset exists.  Don't clear it.
 109 makeset () {
 110   set -e
 111   name=$1; shift
 112   v=$(ipset --version)
 113   createp=t
 114   case "$v" in
 115     "ipset v4"*)
 116       if ipset -nL | grep -q "^Name: $name\$"; then createp=nil; fi
 117       ;;
 118     *)
 119       if ipset -n -L | grep -q "^$name\$"; then createp=nil; fi
 120       ;;
 121   esac
 122   case $createp in
 123     t) ipset -N "$name" "$@" ;;
 124   esac
 125 }
 126
 127 ## errorchain CHAIN ACTION ARGS ...
 128 ##
 129 ## Make a chain which logs a message and then invokes some other action,
 130 ## typically REJECT.  Log messages are prefixed by `fw: CHAIN'.
 131 errorchain () {
 132   set -e
 133   chain=$1; shift
 134   case $chain in
 135     *:*) table=${chain%:*} chain=${chain#*:} ;;
 136     *) table=filter ;;
 137   esac
 138   clearchain $table:$chain
 139   run ip46tables -t $table -A $chain -j LOG \
 140           -m limit --limit 3/minute --limit-burst 10 \
 141           --log-prefix "fw: $chain " --log-level notice || :
 142   run ip46tables -t $table -A $chain -j "$@" \
 143           -m limit --limit 20/second --limit-burst 100
 144   run ip46tables -t $table -A $chain -j DROP
 145 }
 146
 147 m4_divert(20)m4_dnl
 148 ###--------------------------------------------------------------------------
 149 ### Basic option setting.
 150
 151 ## setopt OPTION VALUE
 152 ##
 153 ## Set an IP sysctl.
 154 setopt () {
 155   set -e
 156   opt=$1 val=$2
 157   any=nil
 158   for ver in ipv4 ipv6; do
 159     if [ -f /proc/sys/net/$ver/$opt ]; then
 160       run sysctl -q net/$ver/$opt="$val"
 161       any=t
 162     fi
 163   done
 164   case $any in
 165     nil) echo >&2 "$0: unknown IP option $opt"; exit 1 ;;
 166   esac
 167 }
 168
 169 ## setdevopt OPTION VALUE [INTERFACES ...]
 170 ##
 171 ## Set an IP interface-level sysctl.
 172 setdevopt () {
 173   set -e
 174   opt=$1 val=$2; shift 2
 175   case "$#,$1" in
 176     0, | 1,all)
 177       set -- $(
 178         seen=:
 179         for ver in ipv4 ipv6; do
 180           cd /proc/sys/net/$ver/conf
 181           for i in *; do
 182             [ -f $i/$opt ] || continue
 183             case "$seen" in (*:$i:*) continue ;; esac
 184             echo $i
 185           done
 186         done)
 187       ;;
 188   esac
 189   for i in "$@"; do
 190     any=nil
 191     for ver in ipv4 ipv6; do
 192       if [ -f /proc/sys/net/$ver/conf/$i/$opt ]; then
 193         any=t
 194         run sysctl -q net/$ver/conf/$i/$opt="$val"
 195       fi
 196     done
 197     case $any in
 198       nil) echo >&2 "$0: unknown device option $opt"; exit 1 ;;
 199     esac
 200   done
 201 }
 202
 203 m4_divert(20)m4_dnl
 204 ###--------------------------------------------------------------------------
 205 ### Packet filter construction.
 206
 207 ## conntrack CHAIN
 208 ##
 209 ## Add connection tracking to CHAIN, and allow obvious stuff.
 210 conntrack () {
 211   set -e
 212   chain=$1
 213   run ip46tables -A $chain -p tcp -m state \
 214           --state ESTABLISHED,RELATED -j ACCEPT
 215   run ip46tables -A $chain -p tcp ! --syn -g bad-tcp
 216 }
 217
 218 ## commonrules CHAIN
 219 ##
 220 ## Add standard IP filtering rules to the CHAIN.
 221 commonrules () {
 222   set -e
 223   chain=$1
 224
 225   ## Pass fragments through, assuming that the eventual destination will sort
 226   ## things out properly.  Except for TCP, that is, which should never be
 227   ## fragmented.  This is an extra pain for ip6tables, which doesn't provide
 228   ## a pleasant way to detect non-initial fragments.
 229   run iptables -A $chain -p tcp -f -g tcp-fragment
 230   run iptables -A $chain -f -j ACCEPT
 231   run ip6tables -A $chain -p tcp -g tcp-fragment \
 232           -m ipv6header --soft --header frag
 233   run ip6tables -A $chain -j accept-non-init-frag
 234 }
 235
 236 m4_divert(38)m4_dnl
 237 ## Accept a non-initial fragment.  This is only needed by IPv6, to work
 238 ## around a deficiency in the option parser.
 239 run ip6tables -N accept-non-init-frag
 240 run ip6tables -A accept-non-init-frag -j RETURN \
 241         -m frag --fragfirst
 242 run ip6tables -A accept-non-init-frag -j ACCEPT \
 243         -m ipv6header --header frag
 244
 245 m4_divert(20)m4_dnl
 246 ## allowservices CHAIN PROTO SERVICE ...
 247 ##
 248 ## Add rules to allow the SERVICES on the CHAIN.
 249 allowservices () {
 250   set -e
 251   chain=$1 proto=$2; shift 2
 252   count=0
 253   list=
 254   for svc; do
 255     case $svc in
 256       *:*)
 257         n=2
 258         left=${svc%:*} right=${svc#*:}
 259         case $left in *[!0-9]*) eval left=\$port_$left ;; esac
 260         case $right in *[!0-9]*) eval right=\$port_$right ;; esac
 261         svc=$left:$right
 262         ;;
 263       *)
 264         n=1
 265         case $svc in *[!0-9]*) eval svc=\$port_$svc ;; esac
 266         ;;
 267     esac
 268     case $svc in
 269       *: | :* | "" | *[!0-9:]*)
 270         echo >&2 "Bad service name"
 271         exit 1
 272         ;;
 273     esac
 274     count=$(( $count + $n ))
 275     if [ $count -gt 15 ]; then
 276       run ip46tables -A $chain -p $proto -m multiport -j ACCEPT \
 277              --destination-ports ${list#,}
 278       list= count=$n
 279     fi
 280     list=$list,$svc
 281   done
 282   case $list in
 283     "")
 284       ;;
 285     ,*,*)
 286       run ip46tables -A $chain -p $proto -m multiport -j ACCEPT \
 287               --destination-ports ${list#,}
 288       ;;
 289     *)
 290       run ip46tables -A $chain -p $proto -j ACCEPT \
 291               --destination-port ${list#,}
 292       ;;
 293   esac
 294 }
 295
 296 ## ntpclient CHAIN NTPSERVER ...
 297 ##
 298 ## Add rules to CHAIN to allow NTP with NTPSERVERs.
 299 ntpclient () {
 300   set -e
 301   ntpchain=$1; shift
 302
 303   clearchain ntp-servers
 304   for ntp; do run iptables -A ntp-servers -j ACCEPT -s $ntp; done
 305   run iptables -A $ntpchain -j ntp-servers \
 306           -p udp --source-port 123 --destination-port 123
 307 }
 308
 309 ## dnsresolver CHAIN
 310 ##
 311 ## Add rules to allow CHAIN to be a DNS resolver.
 312 dnsresolver () {
 313   set -e
 314   chain=$1
 315   for p in tcp udp; do
 316     run ip46tables -A $chain -j ACCEPT \
 317              -m state --state ESTABLISHED \
 318              -p $p --source-port 53
 319   done
 320 }
 321
 322 ## dnsserver CHAIN
 323 ##
 324 ## Add rules to allow CHAIN to be a DNS server.
 325 dnsserver () {
 326   set -e
 327   chain=$1
 328
 329   ## Allow TCP access.  Hitting us with SYNs will make us deploy SYN cookies,
 330   ## but that's tolerable.
 331   run ip46tables -A $chain -j ACCEPT -p tcp --destination-port 53
 332
 333   ## Avoid being a DDoS amplifier by rate-limiting incoming DNS queries.
 334   clearchain $chain-udp-dns
 335   run ip46tables -A $chain-udp-dns -j ACCEPT \
 336           -m limit --limit 20/second --limit-burst 300
 337   run ip46tables -A $chain-udp-dns -g dns-rate-limit
 338   run ip46tables -A $chain -j $chain-udp-dns \
 339           -p udp --destination-port 53
 340 }
 341
 342 ## openports CHAIN [MIN MAX]
 343 ##
 344 ## Add rules to CHAIN to allow the open ports.
 345 openports () {
 346   set -e
 347   chain=$1; shift
 348   [ $# -eq 0 ] && set -- $open_port_min $open_port_max
 349   run ip46tables -A $chain -p tcp -g interesting --destination-port $1:$2
 350   run ip46tables -A $chain -p udp -g interesting --destination-port $1:$2
 351 }
 352
 353 bcp38_setup=:
 354 bcp38 () {
 355   ipv=$1 ifname=$2; shift 2
 356   ## Add rules for BCP38 egress filtering for IP version IPV (either 4 or 6).
 357   ## IFNAME is the outgoing interface; the remaining arguments are network
 358   ## prefixes.
 359
 360   ## Sort out which command we're using
 361   case $ipv in
 362     4) ipt=iptables ;;
 363     6) ipt=ip6tables ;;
 364     *) echo >&2 "Unknown IP version $ipv"; exit 1 ;;
 365   esac
 366
 367   ## If we've not set up the error chain then do that.
 368   case $bcp38_setup in
 369     :)
 370       errorchain bcp38 DROP
 371       clearchain bcp38-check
 372       ip46tables -A bcp38-check -g bcp38
 373       ;;
 374   esac
 375
 376   ## Stitch our egress filter into the outbound chains if we haven't done
 377   ## that yet.  Do this for both IP versions: if we're only ever given
 378   ## IPv6 addresses for a particular interface then we assume that IPv4
 379   ## packets aren't allowed on it at all.
 380   case $bcp38_setup in
 381     *:$ifname:*) ;;
 382     *)
 383       run ip46tables -A OUTPUT -j bcp38-check -o $ifname
 384       case $forward in
 385         1) run ip46tables -A FORWARD -j bcp38-check -o $ifname ;;
 386       esac
 387       bcp38_setup=$bcp38_setup$ifname:
 388       ;;
 389   esac
 390
 391   ## Finally, add in our allowed networks.
 392   for i in "$@"; do
 393     run $ipt -I bcp38-check -j RETURN -s $i
 394   done
 395 }
 396
 397 m4_divert(20)m4_dnl
 398 ###--------------------------------------------------------------------------
 399 ### Packet classification.
 400 ###
 401 ### See `classify.m4' for an explanation of how the firewall machinery for
 402 ### packet classification works.
 403 ###
 404 ### A list of all network names is kept in `allnets'.  For each network NET,
 405 ### shell variables are defined describing their properties.
 406 ###
 407 ### net_class_NET       The class of the network, as defined by
 408 ###                     `defnetclass'.
 409 ### net_inet_NET        List of IPv4 address ranges in the network.
 410 ### net_inet6_NET       List of IPv6 address ranges in the network.
 411 ### net_via_NET         List of other networks that this one forwards via.
 412 ### net_hosts_NET       List of hosts known to be in the network.
 413 ### host_inet_HOST      IPv4 address of the named HOST.
 414 ### host_inet6_HOST     IPv6 address of the named HOST.
 415 ###
 416 ### Similarly, a list of hosts is kept in `allhosts', and for each host HOST,
 417 ### a shell variables are defined:
 418 ###
 419 ### host_ifaces_HOST    List of interfaces for this host and the networks
 420 ###                     they attach to, in the form IFACE=NET.
 421
 422 ## defbitfield NAME WIDTH
 423 ##
 424 ## Defines MASK_NAME and BIT_NAME symbolic constants for dealing with
 425 ## bitfields: x << BIT_NAME yields the value x in the correct position, and
 426 ## ff & MASK_NAME extracts the corresponding value.
 427 defbitfield () {
 428   set -e
 429   name=$1 width=$2
 430   eval MASK_$name=$(( (1 << $width) - 1 << $bitindex ))
 431   eval BIT_$name=$bitindex
 432   bitindex=$(( $bitindex + $width ))
 433 }
 434
 435 ## Define the layout of the bitfield.
 436 bitindex=0
 437 defbitfield MASK 16
 438 defbitfield FROM 4
 439 defbitfield TO 4
 440
 441 ## defnetclass NAME FORWARD-TO...
 442 ##
 443 ## Defines a netclass called NAME, which is allowed to forward to the
 444 ## FORWARD-TO netclasses.
 445 ##
 446 ## For each netclass, constants from_NAME and to_NAME are defined as the
 447 ## appropriate values in the FROM and TO fields (i.e., not including any mask
 448 ## bits).
 449 ##
 450 ## This function also establishes mangle chains mark-from-NAME and
 451 ## mark-to-NAME for applying the appropriate mark bits to the packet.
 452 ##
 453 ## Because it needs to resolve forward references, netclasses must be defined
 454 ## in a two-pass manner, using a loop of the form
 455 ##
 456 ##   for pass in 1 2; do netclassindex=0; ...; done
 457 netclassess=
 458 defnetclass () {
 459   set -e
 460   name=$1; shift
 461   case $pass in
 462     1)
 463
 464       ## Pass 1.  Establish the from_NAME and to_NAME constants, and the
 465       ## netclass's mask bit.
 466       trace "netclass $name = $netclassindex"
 467       eval from_$name=$(( $netclassindex << $BIT_FROM ))
 468       eval to_$name=$(( $netclassindex << $BIT_TO ))
 469       eval fwd_$name=$(( 1 << ($netclassindex + $BIT_MASK) ))
 470       nets="$nets $name"
 471       ;;
 472     2)
 473
 474       ## Pass 2.  Compute the actual from and to values.  This is fiddly:
 475       ## we want to preserve the other flags.
 476       from=$(( ($netclassindex << $BIT_FROM) ))
 477       frommask=$(( $MASK_FROM | $MASK_MASK ))
 478       for net; do
 479         eval bit=\$fwd_$net
 480         from=$(( $from + $bit ))
 481       done
 482       to=$(( ($netclassindex << $BIT_TO) ))
 483       tomask=$(( $MASK_TO | $MASK_MASK ^ (1 << ($netclassindex + $BIT_MASK)) ))
 484       trace "from $name --> set $(printf %08x/%08x $from $frommask)"
 485       trace "  to $name --> set $(printf %08x/%08x $to $tomask)"
 486
 487       ## Now establish the mark-from-NAME and mark-to-NAME chains.
 488       clearchain mangle:mark-from-$name mangle:mark-to-$name
 489       run ip46tables -t mangle -A mark-from-$name -j MARK \
 490               --set-xmark $from/$frommask
 491       run ip46tables -t mangle -A mark-to-$name -j MARK \
 492               --set-xmark $to/$tomask
 493       ;;
 494   esac
 495   netclassindex=$(( $netclassindex + 1 ))
 496 }
 497
 498 ## defnet NET CLASS
 499 ##
 500 ## Define a network.  Follow by calls to `addr', `via', etc. to define
 501 ## properties of the network.  Networks are processed in order, so if their
 502 ## addresses overlap then the more specific addresses should be defined
 503 ## earlier.
 504 defnet () {
 505   net=$1 class=$2
 506   addword allnets $net
 507   eval net_class_$1=\$class
 508 }
 509
 510 ## addr ADDRESS/LEN ...
 511 ##
 512 ## Define addresses for the network being defined.  ADDRESSes are in
 513 ## colon-separated IPv6 or dotted-quad IPv4 form.
 514 addr () {
 515   for i in "$@"; do
 516     case "$i" in
 517       *:*) addword net_inet6_$net $i ;;
 518       *) addword net_inet_$net $i ;;
 519     esac
 520   done
 521 }
 522
 523 ## via NET ...
 524 ##
 525 ## Declare that packets from this network are forwarded to the other NETs.
 526 via () {
 527   eval "net_via_$net=\"$*\""
 528 }
 529
 530 ## noxit NET ...
 531 ##
 532 ## Declare that packets from this network must not be forwarded to the other
 533 ## NETs.
 534 noxit () {
 535   eval "net_noxit_$net=\"$*\""
 536 }
 537
 538 ## host HOST ADDR ...
 539 ##
 540 ## Define the address of an individual host on the current network.  The
 541 ## ADDRs may be full IPv4 or IPv6 addresses, or offsets from the containing
 542 ## network address, which is a simple number for IPv4, or a suffix beginning
 543 ## with `::' for IPv6.  If an IPv6 base address is provided for the network
 544 ## but not for the host then the host's IPv4 address is used as a suffix.
 545 host () {
 546   name=$1; shift
 547
 548   ## Work out which addresses we've actually been given.
 549   unset a6
 550   for i in "$@"; do
 551     case "$i" in ::*) a6=$i ;; *) a=$i ;; esac
 552   done
 553   case "${a+t}" in
 554     t) ;;
 555     *) echo >&2 "$0: no address for $name"; exit 1 ;;
 556   esac
 557   case "${a6+t}" in t) ;; *) a6=::$a ;; esac
 558
 559   ## Work out the IPv4 address.
 560   eval nn=\$net_inet_$net
 561   for n in $nn; do
 562     addr=${n%/*}
 563     base=${addr%.*}
 564     offset=${addr##*.}
 565     case $a in *.*) aa=$a ;; *) aa=$base.$(( $offset + $a )) ;; esac
 566     eval host_inet_$name=$aa
 567   done
 568
 569   ## Work out the IPv6 address.
 570   eval nn=\$net_inet6_$net
 571   for n in $nn; do
 572     addr=${n%/*}
 573     base=${addr%::*}
 574     case $a6 in ::*) aa=$base$a6 ;; *) aa=$a6 ;; esac
 575     eval host_inet6_$name=$aa
 576   done
 577
 578   ## Remember the host in the list.
 579   addword net_hosts_$net $name
 580 }
 581
 582 ## defhost NAME
 583 ##
 584 ## Define a new host.  Follow by calls to `iface' to define the host's
 585 ## interfaces.
 586 defhost () {
 587   host=$1
 588   addword allhosts $host
 589   eval host_type_$host=server
 590 }
 591
 592 ## hosttype TYPE
 593 ##
 594 ## Declare the host to have the given type.
 595 hosttype () {
 596   type=$1
 597   case $type in
 598     router | server | client) ;;
 599      *) echo >&2 "$0: bad host type \`$type'"; exit 1 ;;
 600   esac
 601   eval host_type_$host=$type
 602 }
 603
 604 ## iface IFACE NET ...
 605 ##
 606 ## Define a host's interfaces.  Specifically, declares that the host has an
 607 ## interface IFACE attached to the listed NETs.
 608 iface () {
 609   name=$1; shift
 610   for net in "$@"; do
 611     addword host_ifaces_$host $name=$net
 612   done
 613 }
 614
 615 ## matchnets OPT WIN FLAGS PREPARE BASE SUFFIX NEXT NET [NET ...]
 616 ##
 617 ## Build rules which match a particular collection of networks.
 618 ##
 619 ## Specifically, use the address-comparison operator OPT (typically `-s' or
 620 ## `-d') to match the addresses of each NET, writing the rules to the chain
 621 ## BASESUFFIX.  If we find a match, dispatch to WIN-CLASS, where CLASS is the
 622 ## class of the matching network.  In order to deal with networks containing
 623 ## negative address ranges, more chains may need to be constructed; they will
 624 ## be named BASE#Q for sequence numbers Q starting with NEXT.  All of this
 625 ## happens on the `mangle' table, and there isn't (currently) a way to tweak
 626 ## this.
 627 ##
 628 ## The FLAGS gather additional interesting information about the job,
 629 ## separated by colons.  The only flag currently is :default: which means
 630 ## that the default network was listed.
 631 ##
 632 ## Finally, there is a hook PREPARE which is called just in advance of
 633 ## processing the final network, passing it the argument FLAGS.  (The PREPARE
 634 ## string will be subjected to shell word-splitting, so it can provide some
 635 ## arguments of its own if it wants.)  It should set `mode' to indicate how
 636 ## the chain should be finished.
 637 ##
 638 ## goto         If no networks matched, then issue a final `goto' to the
 639 ##              chain named by the variable `fail'.
 640 ##
 641 ## call         Run `$finish CHAIN' to write final rules to the named CHAIN
 642 ##              (which may be suffixed from the original BASE argument if
 643 ##              this was necessary).  This function will arrange to call
 644 ##              these rules if no networks match.
 645 ##
 646 ## ret          If no network matches then return (maybe by falling off the
 647 ##              end of the chain).
 648 matchnets () {
 649   local opt win flags prepare base suffix next net lose splitp
 650   opt=$1 win=$2 flags=$3 prepare=$4 base=$5 suffix=$6 next=$7 net=$8
 651   shift 8
 652
 653   ## If this is the default network, then set the flag.
 654   case "$net" in default) flags=${flags}default: ;; esac
 655
 656   ## Do an initial pass over the addresses to see whether there are any
 657   ## negative ranges.  If so, we'll need to split.  See also the standard
 658   ## joke about soup.
 659   splitp=nil
 660   eval "addrs=\"\$net_inet_$net \$net_inet6_$net\""
 661   for a in $addrs; do case $a in !*) splitp=t; break ;; esac; done
 662
 663   trace "MATCHNETS [splitp $splitp] $opt $win $flags [$prepare] $base $suffix $next : $net $*"
 664
 665   ## Work out how to handle matches against negative address ranges.  If this
 666   ## is the last network, invoke the PREPARE hook to find out.  Otherwise, if
 667   ## we have to split the chain, recursively build the target here.
 668   case $splitp,$# in
 669     t,0 | nil,0)
 670       $prepare $flags
 671       case $splitp,$mode in
 672         *,goto)
 673           lose="-g $fail"
 674           ;;
 675         *,ret)
 676           lose="-j RETURN"
 677           ;;
 678         t,call)
 679           clearchain mangle:$base#$next
 680           lose="-g $base#$next"
 681           ;;
 682         nil,call)
 683           ;;
 684       esac
 685       ;;
 686     t,*)
 687       clearchain mangle:$base#$next
 688       matchnets $opt $win $flags "$prepare" \
 689         $base \#$next $(( $next + 1 )) "$@"
 690       lose="-g $base#$next" mode=goto
 691       ;;
 692     *)
 693       mode=continue
 694       ;;
 695   esac
 696
 697   ## Populate the chain with rules to match the necessary networks.
 698   eval addr=\$net_inet_$net addr6=\$net_inet6_$net class=\$net_class_$net
 699   for a in $addr; do
 700     case $a in
 701       !*) run iptables -t mangle -A $base$suffix $lose $opt ${a#!} ;;
 702       *) run iptables -t mangle -A $base$suffix -g $win-$class $opt $a ;;
 703     esac
 704   done
 705   for a in $addr6; do
 706     case $a in
 707       !*) run ip6tables -t mangle -A $base$suffix $lose $opt ${a#!} ;;
 708       *) run ip6tables -t mangle -A $base$suffix -g $win-$class $opt $a ;;
 709     esac
 710   done
 711
 712   ## Wrap up the chain appropriately.  If we didn't split and there are more
 713   ## networks to handle then append the necessary rules now.  (If we did
 714   ## split, then we already wrote the rules for them above.)  If there are no
 715   ## more networks then consult the `mode' setting to find out what to do.
 716   case $splitp,$#,$mode in
 717     *,0,ret) ;;
 718     *,*,goto) run ip46tables -t mangle -A $base$suffix $lose ;;
 719     t,0,call) $finish $base#$next ;;
 720     nil,0,call) $finish $base$suffix ;;
 721     nil,*,*)
 722       matchnets $opt $win $flags "$prepare" $base "$suffix" $next "$@"
 723       ;;
 724   esac
 725 }
 726
 727 ## net_interfaces HOST NET
 728 ##
 729 ## Determine the interfaces on which packets may plausibly arrive from the
 730 ## named NET.  Returns `-' if no such interface exists.
 731 ##
 732 ## This algorithm is not very clever.  It's just about barely good enough to
 733 ## deduce transitivity through a simple routed network; with complicated
 734 ## networks, it will undoubtedly give wrong answers.  Check the results
 735 ## carefully, and, if necessary, list the connectivity explicitly; use the
 736 ## special interface `-' for networks you know shouldn't send packets to a
 737 ## host.
 738 net_interfaces () {
 739   host=$1 startnet=$2
 740
 741   ## Determine the locally attached networks.
 742   targets=:
 743   eval ii=\$host_ifaces_$host
 744   for i in $ii; do targets=$targets$i:; done
 745
 746   ## Determine the transitivity.
 747   seen=:
 748   nets=$startnet
 749   while :; do
 750
 751     ## First pass.  Determine whether any of the networks we're considering
 752     ## are in the target set.  If they are, then return the corresponding
 753     ## interfaces.
 754     found=""
 755     for net in $nets; do
 756       tg=$targets
 757       while :; do
 758         any=nil
 759         case $tg in
 760           *"=$net:"*)
 761             n=${tg%=$net:*}; tg=${n%:*}:; n=${n##*:}
 762             addword found $n
 763             any=t
 764             ;;
 765         esac
 766         case $any in nil) break ;; esac
 767       done
 768     done
 769     case "$found" in ?*) echo $found; return ;; esac
 770
 771     ## No joy.  Determine the set of networks which (a) these ones can
 772     ## forward to, and (b) that we've not considered already.  These are the
 773     ## nets we'll consider next time around.
 774     nextnets=""
 775     any=nil
 776     for net in $nets; do
 777       eval via=\$net_via_$net
 778       for n in $via; do
 779         case $seen in *":$n:"*) continue ;; esac
 780         seen=$seen$n:
 781         eval noxit=\$net_noxit_$n
 782         case " $noxit " in *" $startnet "*) continue ;; esac
 783         case " $nextnets " in
 784           *" $n "*) ;;
 785           *) addword nextnets $n; any=t ;;
 786         esac
 787       done
 788     done
 789
 790     ## If we've run out of networks then there's no reachability.  Return a
 791     ## failure.
 792     case $any in nil) echo -; return ;; esac
 793     nets=$nextnets
 794   done
 795 }
 796
 797 m4_divert(-1)
 798 ###----- That's all, folks --------------------------------------------------