├── nat ├── TODO ├── README ├── 21-nat └── LICENSE /nat: -------------------------------------------------------------------------------- 1 | # put this file into /etc/default/ - it will be sourced by 2 | # the script 3 | 4 | # set to '0' to disable the script 5 | ENABLED=1 6 | 7 | # set to 'ULA' for a cascaded router that has ULA networks on 8 | # both the WAN and LAN side, and which you have configured 9 | # to isolate the ULA network on the LAN side 10 | MAPTO=GUA 11 | 12 | # set to 1 for debugging output on the console; the script 13 | # does a dry run in this case, so no ip6tables rules and no 14 | # routes will be changed 15 | DEBUG=0 16 | -------------------------------------------------------------------------------- /TODO: -------------------------------------------------------------------------------- 1 | - DONE 2 | all variables used in awk functions are *global* unless declared 3 | as function parameters; check/modify code accordingly 4 | 5 | "localized" all function variables now, by declaring them in 6 | function parameter list 7 | 8 | - DONE 9 | seen packets w/ destination address "fd00::1" going out my WAN 10 | interface; that doesn't really make sense inside my network but 11 | should be prohibited anyway, possibly by firewall or routing 12 | rules; application/system misconfiguration? 13 | 14 | ok, those are LAN applications using bogus addresses to open 15 | connections, has nothing to do with our setup; route 16 | ip -6 route add unreachable fc00::/7 metric 2147483647 17 | prevents those packets from leaving our network 18 | -------------------------------------------------------------------------------- /README: -------------------------------------------------------------------------------- 1 | owrt-ipv6-nat 2 | ============= 3 | 4 | What? 5 | ----- 6 | 7 | A hotplug script that implements an IPv6 full cone NAT on OpenWrt. 8 | 9 | Why? 10 | ---- 11 | 12 | If your ISP provides you with changing IPv6 prefixes, like in my case, that 13 | might potentially cause you some trouble. Examples. 14 | 15 | You aren't able to configure global static addresses. Because those change when 16 | the prefix changes. 17 | 18 | If you run a stateful DHCPv6 network w/o SLAAC, as soon as the prefix changes 19 | your clients will have invalid IPv6 addresses and keep them until their lease 20 | time expires. 21 | 22 | If you want to allow incoming connections on your firewall the corresponding 23 | rules have to take the currently active prefix into account, and reconfigure 24 | themselves upon prefix change. 25 | 26 | Etcetera etcetera. 27 | 28 | So if you don't like that, and don't mind IPv6 purists chastising you for 29 | violating dogmas, NAT might actually provide some relief. 30 | 31 | How? 32 | ---- 33 | 34 | This script implements a full cone NAT by means of the netfilter/ip6tables 35 | NETMAP target. It does this as a hotplug script that gets triggered whenever the 36 | wan6 interface comes up. 37 | 38 | When this happens it looks up the currently active prefixes on the WAN and LAN 39 | side, installs two simple rules in the ip6tables nat table which map interface 40 | identifiers 1-1 between those prefixes, and installs a corresponding route. And 41 | that's it. 42 | 43 | More details, please. 44 | --------------------- 45 | 46 | Ok. What I've done, and you should do when going down this road, is to configure 47 | your LAN as pure ULA network. That means that 48 | option ula_prefix 49 | is set and you've configured your router like this 50 | list ip6class 'local' 51 | option ra_default '1' 52 | 53 | That means that your router will advertise only the ULA prefix on your LAN and 54 | announce itself as default gateway in any case. So no global addresses on LAN at 55 | all, and you're free to do what you want, like in a private IPv4 network. 56 | Configure static and DHCPv6 addresses. Whatever SLAAC identerface identifier 57 | generation approach you want (I'm using the net.ipv6.conf.default.stable_secret 58 | approach, to prevent MAC address leakage to the Internet). Whatever. 59 | 60 | Now, to reach the Internet you need a route and a global return address for 61 | each connection. And this is what this script provides you with. 62 | 63 | The ip6tables rules just rewrite the addresses on outgoing/incoming packets by 64 | changing the prefix, while leaving the host identifier part intact. Resulting in 65 | a 1-1 mapping between ULA and global addresses. And a route gets installed that 66 | tells your router to forward those packets to the next hop configured for your 67 | global prefix. 68 | 69 | For the mapping to work it needs a key prerequisite though: the prefix length on 70 | your WAN side needs to be less or equal than the one on your LAN side. Because 71 | if for example your ISP provides you with a /64 prefix but your ULA is 72 | configured as /60 prefix there's obviously a 4 byte long network part that can't 73 | be mapped. In such a case you may want to adjust 74 | option ip6assign 75 | accordingly. 76 | 77 | Pitfalls 78 | -------- 79 | 80 | The usual ones with NAT, which can break end-to-end connectivity. Which is 81 | particularly true for application protocols that encode IP addresses themselves, 82 | like SIP. Which is why there's been lots of jumping through hoops to make those 83 | work w/ IPv4 NAT. And since IPv6 NAT isn't even supposed to exist the situation 84 | here might actually be worse. 85 | 86 | Application developers seem to catch up though, and acceptance of a desire for 87 | IPv6 NAT solutions has become more widespread. That's why we have according 88 | netfilter code on Linux at all, anyway. 89 | 90 | Technical overview 91 | ------------------ 92 | 93 | The script uses 'ifstatus' to retrieve interface configuration data for the 94 | wan6 and lan interfaces. It then checks if there's a GUA prefix on the wan6 and 95 | and an ULA prefix on the LAN side, and compares the prefix lengths. If the 96 | lengths match up, i.e. if the GUA prefix length is less or equal than the ULA 97 | one, the mapping is established. 98 | 99 | For this, the ip6tables nat table PRE/POSTROUTING chains are flushed. So any 100 | rules you might have had there will be gone. And then two new rules are 101 | installed, one on each chain. Which implement the NETMAP DNAT/SNAT. 102 | 103 | Finally, the next hop for the given GUA prefix gets looked up in the routing 104 | table. And a route is installed that designs this hop as default route for 105 | packets directed at global addresses and coming from the ULA network. 106 | 107 | Understood. What now? 108 | --------------------- 109 | 110 | Put the 'nat' file in /etc/default/. That's where you comfortably en/disable the 111 | script. And put the 21-nat script itself in /etc/hoplug.d/iface/. Finally make 112 | sure you have the iptables-mod-nat-extra package installed, for NETMAP. 113 | 114 | Next time wan6 comes up the script will run. And logread should show you entries 115 | like this 116 | # logread | grep nat 117 | user.notice nat: Mapping fdee:dead:beef::/60 -> 2003:60:daed:beef::/60 on device pppoe-wan 118 | user.notice nat: Installing route - default from fdee:dead:beef::/60 via fe80::xxxx:xxxx:xxxx:xxxx dev pppoe-wan 119 | 120 | If it doesn't something went wrong. And you'll have to figure out the reason(s) 121 | yourself, I'm afraid. -------------------------------------------------------------------------------- /21-nat: -------------------------------------------------------------------------------- 1 | #!/bin/sh 2 | # 3 | # Script to implement a IPv6 full cone NAT on OpenWrt, w/ the aid of the 4 | # netfilter NETMAP target. 5 | # 6 | # This script is meant to be used in /etc/hotplug.d/iface, for users who suffer 7 | # from changing IPv6 prefixes their ISP provides them with. It assumes that the 8 | # LAN side is a pure ULA network 9 | # (network.lan.ip6class`" = "local"). 10 | # 11 | # What it does is implement a 1:1 mapping from ULA to GUA addresses in the 12 | # ip6tables PRE/POSTROUTING chains, and install a default route for ULA 13 | # addresses, so that those can connect to the Internet with the source addresses 14 | # of their packets accordingly modified. 15 | # 16 | # Note that the ip6tables nat chains are flushed before installment of our 17 | # rules, so any other rules will be deleted. 18 | # 19 | # For the ip6tables NETMAP target the package iptables-mod-nat-extra needs to be 20 | # installed. 21 | # 22 | 23 | #set -x 24 | 25 | # set to 1 to actually enable this script; preferably done in 26 | # /etc/default/nat 27 | ENABLED=0 28 | # set DEBUG to 1 for a dry run w/ verbose output; in this case the 29 | # router configuration is not changed 30 | DEBUG=0 31 | # map LAN ULA addresses to global addresses on WAN by default; 32 | # for a 2nd tier, cascaded router w/ different ULA prefixes on both 33 | # sides, LAN and WAN, use 'ULA' here 34 | MAPTO=GUA 35 | 36 | # users may set ENABLED/MAPTO/DEBUG in this file 37 | . /etc/default/nat 38 | 39 | [ "$ENABLED" = "1" ] || [ "$ENABLED" = "y" ] || exit 0 40 | 41 | # this script gets triggered when the wan6 iface comes up 42 | [ "$DEBUG" = 1 ] || ( [ "$ACTION" = ifup ] && [ "$INTERFACE" = wan6 ] ) || exit 0 43 | # make sure the LAN side is configured as pure ULA network 44 | [ "`uci get network.lan.ip6class`" = "local" ] || exit 0 45 | 46 | debug () { 47 | [ "$DEBUG" = 1 ] && echo "$@" >&2 48 | : 49 | } 50 | log() { 51 | [ -t ] && echo "$@" 52 | logger -t nat "$@" 53 | } 54 | 55 | # helper function, prints addresses that match a given bitmask; like: 56 | # cat addresses | get_addresses_for_bitmask 57 | get_addresses_for_bitmask() { 58 | 59 | awk -v mask=$1 -v debug=$DEBUG -e ' 60 | 61 | function debug(msg) { 62 | if ( debug == 1 ) 63 | print msg | "cat >&2" 64 | } 65 | 66 | # convert a string of hex numbers to a string of bits; 67 | # inspired by bits2str function from gawk manual 68 | function hexstr2bitstr(hexstring, chars, ret, num, bits) 69 | { 70 | split(hexstring, chars, "") 71 | ret = "" 72 | 73 | # each individual hex char converts to 4 bits 74 | for (i=1; i <= length(hexstring); i++) { 75 | 76 | # convert character to decimal number 77 | num = sprintf("%d", "0x" chars[i]) 78 | bits = "" 79 | 80 | # bitshift through the number and add 1/0s accordingly 81 | for (; num != 0; num = rshift(num, 1)) 82 | bits = (and(num, 1) ? "1" : "0") bits 83 | 84 | # fill up to 4 bits if need be 85 | while (length(bits) < 4) 86 | bits = "0" bits 87 | 88 | ret = ret bits 89 | } 90 | return ret 91 | } 92 | 93 | # expand potentially compressed ipv6 address; idea from here: 94 | # https://github.com/chmduquesne/wg-ip/blob/master/wg-ip 95 | function expand_ipv6_addr(addr, tmp, count, colons, len, a, ret) 96 | { 97 | # prepend 0 if leading char is : 98 | if(addr ~ /^:/) 99 | addr = "0" addr 100 | 101 | # replace double colon with :0 for each colon that is missing; 102 | # a potential extra leading 0 will be eliminated by sprintf later 103 | if(addr ~ /::/) { 104 | tmp = addr 105 | count = gsub(/:/, ":", tmp) 106 | for (i = 1; i <= 9 - count; i++) 107 | colons = ":0" colons 108 | sub(/::/, colons, addr) 109 | } 110 | 111 | # now format each field as 4 digit hex number 112 | len = split(addr, a , ":") 113 | for (i=1; i <= len; i++) 114 | a[i] = sprintf("%04x", "0x" a[i]) 115 | 116 | # join the array of formatted numbers back together 117 | ret = a[1] 118 | for (i=2; i <= len; i++) 119 | ret = ret ":" a[i] 120 | 121 | return ret 122 | } 123 | 124 | { 125 | addr = $0 126 | if (addr == "") exit 127 | 128 | mlength=length(mask) 129 | l = 26 130 | 131 | addr = expand_ipv6_addr(addr) 132 | debug(sprintf(" %-" l "s %s %s", "expanded address", ":", addr)) 133 | 134 | # get a pure hex string from expanded address 135 | gsub(":","",addr) 136 | debug(sprintf(" %-" l "s %s %s", "colons stripped", ":", addr)) 137 | 138 | # shorten hex string to the max length we need for mask comparison 139 | strlen = int(mlength/4) + 1 140 | hexstr = substr(addr, 1, strlen) 141 | debug(sprintf(" %-" l "s %s %s", "hexstr for mask comparison", ":", hexstr)) 142 | 143 | # the bit string 144 | bitstr = hexstr2bitstr(hexstr) 145 | debug(sprintf(" %-" l "s %s %s", "hexstr2bitstr result", ":", bitstr)) 146 | 147 | # now a precise shortening to the given mask length 148 | bitstr = substr(bitstr, 1, mlength) 149 | debug(sprintf(" %-" l "s %s %s", "bits to be compared", ":", bitstr)) 150 | debug(sprintf(" %-" l "s %s %s", "mask", ":", mask)) 151 | 152 | if(bitstr == mask) { 153 | # address is a match; 154 | # reduce line num by one, for use in jsonfilter indexing 155 | printf("%d| %s\n", NR-1, $0) 156 | } 157 | } 158 | ' 159 | } 160 | 161 | # bitmasks we use to recognize IPv6 address classes 162 | guabits=001 # GUA 163 | ulabits=11111101 # ULA 164 | 165 | # the classes we want to use for nat mapping 166 | lanbits=$ulabits # map ula addresses on lan ... 167 | if [ "$MAPTO" = "GUA" ]; then 168 | wanbits=$guabits # ... to gua addresses on wan 169 | elif [ "$MAPTO" = "ULA" ]; then 170 | wanbits=$ulabits # ... to ula addresses on wan 171 | else 172 | log "No suitable WAN mapping - MAPTO: $MAPTO" 173 | exit 0 174 | fi 175 | 176 | # get lists of prefix addresses that match our bitmasks 177 | waddr_candidates=`ifstatus wan6 | jsonfilter -e '@["ipv6-prefix"][*].address' | get_addresses_for_bitmask $wanbits` 178 | laddr_candidates=`ifstatus lan | jsonfilter -e '@["ipv6-prefix-assignment"][*].address' | get_addresses_for_bitmask $lanbits` 179 | debug "wan prefix candidates": 180 | debug "$waddr_candidates" 181 | debug "lan prefix candidates": 182 | debug "$laddr_candidates" 183 | 184 | # take the first matching prefix on each side, WAN and LAN 185 | windex=`echo $waddr_candidates | head -1 | cut -d'|' -f1` 186 | lindex=`echo $laddr_candidates | head -1 | cut -d'|' -f1` 187 | debug "json indices - windex: $windex lindex: $lindex" 188 | 189 | [ -n "$windex" ] && [ -n "$lindex" ] || { 190 | log "No suitable prefixes - windex: $windex lindex: $lindex" 191 | exit 1 192 | } 193 | 194 | # prefix addresses 195 | waddr=`ifstatus wan6 | jsonfilter -e "@['ipv6-prefix'][$windex].address"` 196 | laddr=`ifstatus lan | jsonfilter -e "@['ipv6-prefix-assignment'][$lindex].address"` 197 | debug "prefixes - waddr: $waddr laddr: $laddr" 198 | [ -n "$waddr" ] && [ -n "laddr" ] && ! [ "$waddr" = "$laddr" ] || { 199 | log "No suitable prefixes - waddr: $waddr laddr: $laddr" 200 | exit 1 201 | } 202 | 203 | # prefix masks 204 | wmask=`ifstatus wan6 | jsonfilter -e "@['ipv6-prefix'][$windex].mask"` 205 | lmask=`ifstatus lan | jsonfilter -e "@['ipv6-prefix-assignment'][$lindex].mask"` 206 | debug "prefix masks - wmask: $wmask lmask: $lmask" 207 | 208 | [ -n "$wmask" ] && [ -n "$lmask" ] && [ $wmask -le $lmask ] || { 209 | log "No suitable prefix masks - wmask: $wmask lmask: $lmask" 210 | exit 1 211 | } 212 | 213 | # ok, we found a valid pair of prefixes, now determine the default 214 | # route for the prefix we've chosen on the WAN side; this will then 215 | # also serve as default route for the ULA network on the LAN side 216 | nexthop=`ifstatus wan6 | jsonfilter -e "$.route[@.target='::' && @.source='$waddr/$wmask'].nexthop"` 217 | debug "nexthop - $nexthop for $waddr/$wmask" 218 | [ -n "$nexthop" ] || { 219 | log "No suitable route - nexthop: $nexthop" 220 | exit 1 221 | } 222 | 223 | # wan interface device 224 | wan_iface=`ifstatus wan6 | jsonfilter -e '$.l3_device'` 225 | debug "wan device - $wan_iface" 226 | 227 | # let's get to work - this implements the full cone NAT 228 | log "Mapping $laddr/$lmask -> $waddr/$lmask on device $wan_iface" 229 | [ "$DEBUG" = 1 ] || ip6tables-restore -n <<-EOF; 230 | *nat 231 | -F PREROUTING 232 | -F POSTROUTING 233 | -A PREROUTING -i $wan_iface -d $waddr/$lmask -j NETMAP --to $laddr/$lmask 234 | -A POSTROUTING -o $wan_iface -s $laddr/$lmask -j NETMAP --to $waddr/$lmask 235 | COMMIT 236 | EOF 237 | 238 | # now the route, so that packets with ULA source address can actually leave 239 | # the network 240 | route="default from $laddr/$lmask via $nexthop dev $wan_iface metric 512" 241 | log "Installing route - $route" 242 | [ "$DEBUG" = 1 ] && exit 0 243 | ip -6 route del default from $laddr/$lmask 244 | ip -6 route add $route 245 | -------------------------------------------------------------------------------- /LICENSE: -------------------------------------------------------------------------------- 1 | GNU GENERAL PUBLIC LICENSE 2 | Version 3, 29 June 2007 3 | 4 | Copyright (C) 2007 Free Software Foundation, Inc. 5 | Everyone is permitted to copy and distribute verbatim copies 6 | of this license document, but changing it is not allowed. 7 | 8 | Preamble 9 | 10 | The GNU General Public License is a free, copyleft license for 11 | software and other kinds of works. 12 | 13 | The licenses for most software and other practical works are designed 14 | to take away your freedom to share and change the works. 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Patents. 472 | 473 | A "contributor" is a copyright holder who authorizes use under this 474 | License of the Program or a work on which the Program is based. The 475 | work thus licensed is called the contributor's "contributor version". 476 | 477 | A contributor's "essential patent claims" are all patent claims 478 | owned or controlled by the contributor, whether already acquired or 479 | hereafter acquired, that would be infringed by some manner, permitted 480 | by this License, of making, using, or selling its contributor version, 481 | but do not include claims that would be infringed only as a 482 | consequence of further modification of the contributor version. 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"Knowingly relying" means you have 508 | actual knowledge that, but for the patent license, your conveying the 509 | covered work in a country, or your recipient's use of the covered work 510 | in a country, would infringe one or more identifiable patents in that 511 | country that you have reason to believe are valid. 512 | 513 | If, pursuant to or in connection with a single transaction or 514 | arrangement, you convey, or propagate by procuring conveyance of, a 515 | covered work, and grant a patent license to some of the parties 516 | receiving the covered work authorizing them to use, propagate, modify 517 | or convey a specific copy of the covered work, then the patent license 518 | you grant is automatically extended to all recipients of the covered 519 | work and works based on it. 520 | 521 | A patent license is "discriminatory" if it does not include within 522 | the scope of its coverage, prohibits the exercise of, or is 523 | conditioned on the non-exercise of one or more of the rights that are 524 | specifically granted under this License. You may not convey a covered 525 | work if you are a party to an arrangement with a third party that is 526 | in the business of distributing software, under which you make payment 527 | to the third party based on the extent of your activity of conveying 528 | the work, and under which the third party grants, to any of the 529 | parties who would receive the covered work from you, a discriminatory 530 | patent license (a) in connection with copies of the covered work 531 | conveyed by you (or copies made from those copies), or (b) primarily 532 | for and in connection with specific products or compilations that 533 | contain the covered work, unless you entered into that arrangement, 534 | or that patent license was granted, prior to 28 March 2007. 535 | 536 | Nothing in this License shall be construed as excluding or limiting 537 | any implied license or other defenses to infringement that may 538 | otherwise be available to you under applicable patent law. 539 | 540 | 12. No Surrender of Others' Freedom. 541 | 542 | If conditions are imposed on you (whether by court order, agreement or 543 | otherwise) that contradict the conditions of this License, they do not 544 | excuse you from the conditions of this License. If you cannot convey a 545 | covered work so as to satisfy simultaneously your obligations under this 546 | License and any other pertinent obligations, then as a consequence you may 547 | not convey it at all. For example, if you agree to terms that obligate you 548 | to collect a royalty for further conveying from those to whom you convey 549 | the Program, the only way you could satisfy both those terms and this 550 | License would be to refrain entirely from conveying the Program. 551 | 552 | 13. Use with the GNU Affero General Public License. 553 | 554 | Notwithstanding any other provision of this License, you have 555 | permission to link or combine any covered work with a work licensed 556 | under version 3 of the GNU Affero General Public License into a single 557 | combined work, and to convey the resulting work. The terms of this 558 | License will continue to apply to the part which is the covered work, 559 | but the special requirements of the GNU Affero General Public License, 560 | section 13, concerning interaction through a network will apply to the 561 | combination as such. 562 | 563 | 14. Revised Versions of this License. 564 | 565 | The Free Software Foundation may publish revised and/or new versions of 566 | the GNU General Public License from time to time. Such new versions will 567 | be similar in spirit to the present version, but may differ in detail to 568 | address new problems or concerns. 569 | 570 | Each version is given a distinguishing version number. If the 571 | Program specifies that a certain numbered version of the GNU General 572 | Public License "or any later version" applies to it, you have the 573 | option of following the terms and conditions either of that numbered 574 | version or of any later version published by the Free Software 575 | Foundation. If the Program does not specify a version number of the 576 | GNU General Public License, you may choose any version ever published 577 | by the Free Software Foundation. 578 | 579 | If the Program specifies that a proxy can decide which future 580 | versions of the GNU General Public License can be used, that proxy's 581 | public statement of acceptance of a version permanently authorizes you 582 | to choose that version for the Program. 583 | 584 | Later license versions may give you additional or different 585 | permissions. However, no additional obligations are imposed on any 586 | author or copyright holder as a result of your choosing to follow a 587 | later version. 588 | 589 | 15. Disclaimer of Warranty. 590 | 591 | THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY 592 | APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT 593 | HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY 594 | OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, 595 | THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 596 | PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM 597 | IS WITH YOU. SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF 598 | ALL NECESSARY SERVICING, REPAIR OR CORRECTION. 599 | 600 | 16. Limitation of Liability. 601 | 602 | IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING 603 | WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS 604 | THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY 605 | GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE 606 | USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF 607 | DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD 608 | PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS), 609 | EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF 610 | SUCH DAMAGES. 611 | 612 | 17. Interpretation of Sections 15 and 16. 613 | 614 | If the disclaimer of warranty and limitation of liability provided 615 | above cannot be given local legal effect according to their terms, 616 | reviewing courts shall apply local law that most closely approximates 617 | an absolute waiver of all civil liability in connection with the 618 | Program, unless a warranty or assumption of liability accompanies a 619 | copy of the Program in return for a fee. 620 | 621 | END OF TERMS AND CONDITIONS 622 | 623 | How to Apply These Terms to Your New Programs 624 | 625 | If you develop a new program, and you want it to be of the greatest 626 | possible use to the public, the best way to achieve this is to make it 627 | free software which everyone can redistribute and change under these terms. 628 | 629 | To do so, attach the following notices to the program. It is safest 630 | to attach them to the start of each source file to most effectively 631 | state the exclusion of warranty; and each file should have at least 632 | the "copyright" line and a pointer to where the full notice is found. 633 | 634 | 635 | Copyright (C) 636 | 637 | This program is free software: you can redistribute it and/or modify 638 | it under the terms of the GNU General Public License as published by 639 | the Free Software Foundation, either version 3 of the License, or 640 | (at your option) any later version. 641 | 642 | This program is distributed in the hope that it will be useful, 643 | but WITHOUT ANY WARRANTY; without even the implied warranty of 644 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 645 | GNU General Public License for more details. 646 | 647 | You should have received a copy of the GNU General Public License 648 | along with this program. If not, see . 649 | 650 | Also add information on how to contact you by electronic and paper mail. 651 | 652 | If the program does terminal interaction, make it output a short 653 | notice like this when it starts in an interactive mode: 654 | 655 | Copyright (C) 656 | This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'. 657 | This is free software, and you are welcome to redistribute it 658 | under certain conditions; type `show c' for details. 659 | 660 | The hypothetical commands `show w' and `show c' should show the appropriate 661 | parts of the General Public License. Of course, your program's commands 662 | might be different; for a GUI interface, you would use an "about box". 663 | 664 | You should also get your employer (if you work as a programmer) or school, 665 | if any, to sign a "copyright disclaimer" for the program, if necessary. 666 | For more information on this, and how to apply and follow the GNU GPL, see 667 | . 668 | 669 | The GNU General Public License does not permit incorporating your program 670 | into proprietary programs. If your program is a subroutine library, you 671 | may consider it more useful to permit linking proprietary applications with 672 | the library. If this is what you want to do, use the GNU Lesser General 673 | Public License instead of this License. But first, please read 674 | . 675 | --------------------------------------------------------------------------------