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diff --git a/doc/src/background.html b/doc/src/background.html deleted file mode 100644 index e25b9da03..000000000 --- a/doc/src/background.html +++ /dev/null @@ -1,376 +0,0 @@ -<html> -<head> - <meta http-equiv="Content-Type" content="text/html"> - <title>FreeS/WAN background</title> - <meta name="keywords" content="Linux, IPSEC, VPN, security, FreeSWAN"> - <!-- - - Written by Sandy Harris for the Linux FreeS/WAN project - Freely distributable under the GNU General Public License - - More information at www.freeswan.org - Feedback to users@lists.freeswan.org - - CVS information: - RCS ID: $Id: background.html,v 1.1 2004/03/15 20:35:24 as Exp $ - Last changed: $Date: 2004/03/15 20:35:24 $ - Revision number: $Revision: 1.1 $ - - CVS revision numbers do not correspond to FreeS/WAN release numbers. - --> -</head> - -<body> -<h1><a name="background">Linux FreeS/WAN background</a></h1> - -<p>This section discusses a number of issues which have three things in -common:</p> -<ul> - <li>They are not specifically FreeS/WAN problems</li> - <li>You may have to understand them to get FreeS/WAN working right</li> - <li>They are not simple questions</li> -</ul> - -<p>Grouping them here lets us provide the explanations some users will need -without unduly complicating the main text.</p> - -<p>The explanations here are intended to be adequate for FreeS/WAN purposes -(please comment to the <a href="mail.html">users mailing list</a> if you -don't find them so), but they are not trying to be complete or definitive. If -you need more information, see the references provided in each section.</p> - -<h2><a name="dns.background">Some DNS background</a></h2> - -<p><a href="glossary.html#carpediem">Opportunistic encryption</a> requires -that the gateway systems be able to fetch public keys, and other -IPsec-related information, from each other's DNS (Domain Name Service) -records.</p> - -<p><a href="glossary.html#DNS">DNS</a> is a distributed database that maps -names to IP addresses and vice versa.</p> - -<p>Much good reference material is available for DNS, including:</p> -<ul> - <li>the <a href="http://www.linuxdoc.org/HOWTO/DNS-HOWTO.html">DNS - HowTo</a></li> - <li>the standard <a href="biblio.html#DNS.book">DNS reference</a> book</li> - <li><a href="http://www.linuxdoc.org/LDP/nag2/index.html">Linux Network - Administrator's Guide</a></li> - <li><a - href="http://www.nominum.com/resources/whitepapers/bind-white-paper.html">BIND - overview</a></li> - <li><a - href="http://www.nominum.com/resources/documentation/Bv9ARM.pdf">BIND 9 - Administrator's Reference</a></li> -</ul> - -<p>We give only a brief overview here, intended to help you use DNS for -FreeS/WAN purposes.</p> - -<h3><a name="forward.reverse">Forward and reverse maps</a></h3> - -<p>Although the implementation is distributed, it is often useful to speak of -DNS as if it were just two enormous tables:</p> -<ul> - <li>the forward map: look up a name, get an IP address</li> - <li>the reverse map: look up an IP address, get a name</li> -</ul> - -<p>Both maps can optionally contain additional data. For opportunistic -encryption, we insert the data need for IPsec authentication.</p> - -<p>A system named gateway.example.com with IP address 10.20.30.40 should have -at least two DNS records, one in each map:</p> -<dl> - <dt>gateway.example.com. IN A 10.20.30.40</dt> - <dd>used to look up the name and get an IP address</dd> - <dt>40.30.20.10.in-addr.arpa. IN PTR gateway.example.com.</dt> - <dd>used for reverse lookups, looking up an address to get the associated - name. Notice that the digits here are in reverse order; the actual - address is 10.20.30.40 but we use 40.30.20.10 here.</dd> -</dl> - -<h3>Hierarchy and delegation</h3> - -<p>For both maps there is a hierarchy of DNS servers and a system of -delegating authority so that, for example:</p> -<ul> - <li>the DNS administrator for example.com can create entries of the form - <var>name</var>.example.com</li> - <li>the example.com admin cannot create an entry for counterexample.com; - only someone with authority for .com can do that</li> - <li>an admin might have authority for 20.10.in-addr.arpa.</li> - <li>in either map, authority can be delegated - <ul> - <li>the example.com admin could give you authority for - westcoast.example.com</li> - <li>the 20.10.in-addr.arpa admin could give you authority for - 30.20.10.in-addr.arpa</li> - </ul> - </li> -</ul> - -<p>DNS zones are the units of delegation. There is a hierarchy of zones.</p> - -<h3>Syntax of DNS records</h3> - -<p>Returning to the example records:</p> -<pre> gateway.example.com. IN A 10.20.30.40 - 40.30.20.10.in-addr.arpa. IN PTR gateway.example.com.</pre> - -<p>some syntactic details are:</p> -<ul> - <li>the IN indicates that these records are for <strong>In</strong>ternet - addresses</li> - <li>The final periods in '.com.' and '.arpa.' are required. They indicate - the root of the domain name system.</li> -</ul> - -<p>The capitalised strings after IN indicate the type of record. Possible -types include:</p> -<ul> - <li><strong>A</strong>ddress, for forward lookups</li> - <li><strong>P</strong>oin<strong>T</strong>e<strong>R</strong>, for reverse - lookups</li> - <li><strong>C</strong>anonical <strong>NAME</strong>, records to support - aliasing, multiple names for one address</li> - <li><strong>M</strong>ail e<strong>X</strong>change, used in mail - routing</li> - <li><strong>SIG</strong>nature, used in <a href="glossary.html#SDNS">secure - DNS</a></li> - <li><strong>KEY</strong>, used in <a href="glossary.html#SDNS">secure - DNS</a></li> - <li><strong>T</strong>e<strong>XT</strong>, a multi-purpose record type</li> -</ul> - -<p>To set up for opportunistic encryption, you add some TXT records -to your DNS data. Details are in our <a href="quickstart.html">quickstart</a> -document.</p> - -<h3>Cacheing, TTL and propagation delay</h3> - -<p>DNS information is extensively cached. With no caching, a lookup by your -system of "www.freeswan.org" might involve:</p> -<ul> - <li>your system asks your nameserver for "www.freeswan.org"</li> - <li>local nameserver asks root server about ".org", gets reply</li> - <li>local nameserver asks .org nameserver about "freeswan.org", gets - reply</li> - <li>local nameserver asks freeswan.org nameserver about "www.freeswan.org", - gets reply</li> -</ul> - -<p>However, this can be a bit inefficient. For example, if you are in the -Phillipines, the closest a root server is in Japan. That might send you to a -.org server in the US, and then to freeswan.org in Holland. If everyone did -all those lookups every time they clicked on a web link, the net would grind -to a halt.</p> - -<p>Nameservers therefore cache information they look up. When you click on -another link at www.freeswan.org, your local nameserver has the IP address -for that server in its cache, and no further lookups are required. </p> - -<p>Intermediate results are also cached. If you next go to -lists.freeswan.org, your nameserver can just ask the freeswan.org nameserver -for that address; it does not need to query the root or .org nameservers -because it has a cached address for the freeswan.org zone server.</p> - -<p>Of course, like any cacheing mechanism, this can create problems of -consistency. What if the administrator for freeswan.org changes the IP -address, or the authentication key, for www.freeswan.org? If you use old -information from the cache, you may get it wrong. On the other hand, you -cannot afford to look up fresh information every time. Nor can you expect the -freeswan.org server to notify you; that isn't in the protocols.</p> - -<p>The solution that is in the protocols is fairly simple. Cacheable records -are marked with Time To Live (TTL) information. When the time expires, the -caching server discards the record. The next time someone asks for it, the -server fetches a fresh copy. Of course, a server may also discard records -before their TTL expires if it is running out of cache space.</p> - -<p>This implies that there will be some delay before the new version of a -changed record propagates around the net. Until the TTLs on all copies of the -old record expire, some users will see it because that is what is in their -cache. Other users may see the new record immediately because they don't have -an old one cached.</p> - -<h2><a name="MTU.trouble">Problems with packet fragmentation</a></h2> - -<p>It seems, from mailing list reports, to be moderately common for problems -to crop up in which small packets pass through the IPsec tunnels just fine -but larger packets fail.</p> - -<p>These problems are caused by various devices along the way mis-handling -either packet fragments or <a href="glossary.html#pathMTU">path MTU -discovery</a>.</p> - -<p>IPsec makes packets larger by adding an ESP or AH header. This can tickle -assorted bugs in fragment handling in routers and firewalls, or in path MTU -discovery mechanisms, and cause a variety of symptoms which are both annoying -and, often, quite hard to diagnose.</p> - -<p>An explanation from project technical lead Henry Spencer:</p> -<pre>The problem is IP fragmentation; more precisely, the problem is that the -second, third, etc. fragments of an IP packet are often difficult for -filtering mechanisms to classify. - -Routers cannot rely on reassembling the packet, or remembering what was in -earlier fragments, because the fragments may be out of order or may even -follow different routes. So any general, worst-case filtering decision -pretty much has to be made on each fragment independently. (If the router -knows that it is the only route to the destination, so all fragments -*must* pass through it, reassembly would be possible... but most routers -don't want to bother with the complications of that.) - -All fragments carry roughly the original IP header, but any higher-level -header is (for IP purposes) just the first part of the packet data... so -only the first fragment carries that. So, for example, on examining the -second fragment of a TCP packet, you could tell that it's TCP, but not -what port number it is destined for -- that information is in the TCP -header, which appears in the first fragment only. - -The result of this classification difficulty is that stupid routers and -over-paranoid firewalls may just throw fragments away. To get through -them, you must reduce your MTU enough that fragmentation will not occur. -(In some cases, they might be willing to attempt reassembly, but have very -limited resources to devote to it, meaning that packets must be small and -fragments few in number, leading to the same conclusion: smaller MTU.)</pre> - -<p>In addition to the problem Henry describes, you may also have trouble with -<a href="glossary.html#pathMTU">path MTU discovery</a>.</p> - -<p>By default, FreeS/WAN uses a large <a href="glossary.html#MTU">MTU</a> for -the ipsec device. This avoids some problems, but may complicate others. -Here's an explanation from Claudia:</p> -<pre>Here are a couple of pieces of background information. Apologies if you -have seen these already. An excerpt from one of my old posts: - - An MTU of 16260 on ipsec0 is usual. The IPSec device defaults to this - high MTU so that it does not fragment incoming packets before encryption - and encapsulation. If after IPSec processing packets are larger than 1500, - [ie. the mtu of eth0] then eth0 will fragment them. - - Adding IPSec headers adds a certain number of bytes to each packet. - The MTU of the IPSec interface refers to the maximum size of the packet - before the IPSec headers are added. In some cases, people find it helpful - to set ipsec0's MTU to 1500-(IPSec header size), which IIRC is about 1430. - - That way, the resulting encapsulated packets don't exceed 1500. On most - networks, packets less than 1500 will not need to be fragmented. - -and... (from Henry Spencer) - - The way it *ought* to work is that the MTU advertised by the ipsecN - interface should be that of the underlying hardware interface, less a - pinch for the extra headers needed. - - Unfortunately, in certain situations this breaks many applications. - There is a widespread implicit assumption that the smallest MTUs are - at the ends of paths, not in the middle, and another that MTUs are - never less than 1500. A lot of code is unprepared to handle paths - where there is an "interior minimum" in the MTU, especially when it's - less than 1500. So we advertise a big MTU and just let the resulting - big packets fragment. - -This usually works, but we do get bitten in cases where some intermediate -point can't handle all that fragmentation. We can't win on this one.</pre> - -<p>The MTU can be changed with an <var>overridemtu=</var> statement in the -<var>config setup</var> section of <a -href="manpage.d/ipsec.conf.5.html">ipsec.conf.5</a>.</p> - -<p>For a discussion of MTU issues and some possible solutions using Linux -advanced routing facilities, see the <a -href="http://www.linuxguruz.org/iptables/howto/2.4routing-15.html#ss15.6">Linux -2.4 Advanced Routing HOWTO</a>. - -For a discussion of MTU and NAT (Network Address Translation), see -<A HREF="http://harlech.math.ucla.edu/services/ipsec.html">James Carter's MTU -notes</A>.</p> - -<h2><a name="nat.background">Network address translation (NAT)</a></h2> - -<p><strong>N</strong>etwork <strong>A</strong>ddress -<strong>T</strong>ranslation is a service provided by some gateway machines. -Calling it NAPT (adding the word <strong>P</strong>ort) would be more -precise, but we will follow the widespread usage.</p> - -<p>A gateway doing NAT rewrites the headers of packets it is forwarding, -changing one or more of:</p> -<ul> - <li>source address</li> - <li>source port</li> - <li>destination address</li> - <li>destination port</li> -</ul> - -<p>On Linux 2.4, NAT services are provided by the <a -href="http://netfilter.samba.org">netfilter(8)</a> firewall code. There are -several <a -href="http://netfilter.samba.org/documentation/index.html#HOWTO">Netfilter -HowTos</a> including one on NAT.</p> - -<p>For older versions of Linux, this was referred to as "IP masquerade" and -different tools were used. See this <a -href="http://www.e-infomax.com/ipmasq/">resource page</a>.</p> - -<p>Putting an IPsec gateway behind a NAT gateway is not recommended. See our -<a href="firewall.html#NAT">firewalls document</a>.</p> - -<h3>NAT to non-routable addresses</h3> - -<p>The most common application of NAT uses private <a -href="glossary.html#non-routable">non-routable</a> addresses.</p> - -<p>Often a home or small office network will have:</p> -<ul> - <li>one connection to the Internet</li> - <li>one assigned publicly visible IP address</li> - <li>several machines that all need access to the net</li> -</ul> - -<p>Of course this poses a problem since several machines cannot use one -address. The best solution might be to obtain more addresses, but often this -is impractical or uneconomical.</p> - -<p>A common solution is to have:</p> -<ul> - <li><a href="glossary.html#non-routable">non-routable</a> addresses on the - local network</li> - <li>the gateway machine doing NAT</li> - <li>all packets going outside the LAN rewritten to have the gateway as - their source address</li> -</ul> - -<p>The client machines are set up with reserved <a -href="#non-routable">non-routable</a> IP addresses defined in RFC 1918. The -masquerading gateway, the machine with the actual link to the Internet, -rewrites packet headers so that all packets going onto the Internet appear to -come from one IP address, that of its Internet interface. It then gets all -the replies, does some table lookups and more header rewriting, and delivers -the replies to the appropriate client machines.</p> - -<p>As far as anyone else on the Internet is concerned, the systems behind the -gateway are completely hidden. Only one machine with one IP address is -visible.</p> - -<p>For IPsec on such a gateway, you can entirely ignore the NAT in:</p> -<ul> - <li><a href="manpage.d/ipsec.conf.5.html">ipsec.conf(5)</a></li> - <li>firewall rules affecting your Internet-side interface</li> -</ul> - -<p>Those can be set up exactly as they would be if your gateway had no other -systems behind it.</p> - -<p>You do, however, have to take account of the NAT in firewall rules which -affect packet forwarding.</p> - -<h3>NAT to routable addresses</h3> - -<p>NAT to routable addresses is also possible, but is less common and may -make for rather tricky routing problems. We will not discuss it here. See the -<a href="http://netfilter.samba.org/documentation/index.html#HOWTO">Netfilter -HowTos</a>.</p> -</body> -</html> |