diff options
| author | Rene Mayrhofer <rene@mayrhofer.eu.org> | 2006-06-03 23:37:13 +0000 |
|---|---|---|
| committer | Rene Mayrhofer <rene@mayrhofer.eu.org> | 2006-06-03 23:37:13 +0000 |
| commit | 42424656e873ad0da564131dbffb4b82ed3347c9 (patch) | |
| tree | 02b79fa4074fbdda73bb1953e4b54b6b918c3654 /doc/background.html | |
| parent | 996894ce26b9ea62e0881026c9cc5bc1b61abddc (diff) | |
| download | vyos-strongswan-42424656e873ad0da564131dbffb4b82ed3347c9.tar.gz vyos-strongswan-42424656e873ad0da564131dbffb4b82ed3347c9.zip | |
Remove these files from the repository, because they get created during the
build (and removed by make clean).
Diffstat (limited to 'doc/background.html')
| -rw-r--r-- | doc/background.html | 323 |
1 files changed, 0 insertions, 323 deletions
diff --git a/doc/background.html b/doc/background.html deleted file mode 100644 index 8f24cad4a..000000000 --- a/doc/background.html +++ /dev/null @@ -1,323 +0,0 @@ -<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" "http://www.w3.org/TR/REC-html40/loose.dtd"> -<HTML> -<HEAD> -<TITLE>Introduction to FreeS/WAN</TITLE> -<META HTTP-EQUIV="Content-Type" CONTENT="text/html; CHARSET=iso-8859-1"> -<STYLE TYPE="text/css"><!-- -BODY { font-family: serif } -H1 { font-family: sans-serif } -H2 { font-family: sans-serif } -H3 { font-family: sans-serif } -H4 { font-family: sans-serif } -H5 { font-family: sans-serif } -H6 { font-family: sans-serif } -SUB { font-size: smaller } -SUP { font-size: smaller } -PRE { font-family: monospace } ---></STYLE> -</HEAD> -<BODY> -<A HREF="toc.html">Contents</A> -<A HREF="config.html">Previous</A> -<A HREF="user_examples.html">Next</A> -<HR> -<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><A NAME="17_1_2">Hierarchy and delegation</A></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><A NAME="17_1_3">Syntax of DNS records</A></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><A NAME="17_1_4">Cacheing, TTL and propagation delay</A></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><A NAME="17_3_1">NAT to non-routable addresses</A></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="glossary.html#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><A NAME="17_3_2">NAT to routable addresses</A></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> -<HR> -<A HREF="toc.html">Contents</A> -<A HREF="config.html">Previous</A> -<A HREF="user_examples.html">Next</A> -</BODY> -</HTML> |