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-<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>
projects / thirdparty / strongswan.git / blobdiff