From aaa0331ecf95ced1e913ac9be50168cf0e7cbb82 Mon Sep 17 00:00:00 2001 From: Rene Mayrhofer Date: Tue, 30 Jan 2007 12:21:07 +0000 Subject: [svn-upgrade] Integrating new upstream version, strongswan (2.8.2) --- doc/background.html | 323 ---------------------------------------------------- 1 file changed, 323 deletions(-) delete mode 100644 doc/background.html (limited to 'doc/background.html') 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 @@ - - - -Introduction to FreeS/WAN - - - - -Contents -Previous -Next -
-

Linux FreeS/WAN background

-

This section discusses a number of issues which have three things in - common:

- -

Grouping them here lets us provide the explanations some users will - need without unduly complicating the main text.

-

The explanations here are intended to be adequate for FreeS/WAN - purposes (please comment to the users mailing list - 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.

-

Some DNS background

-

Opportunistic encryption - 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.

-

DNS is a distributed database that - maps names to IP addresses and vice versa.

-

Much good reference material is available for DNS, including:

- -

We give only a brief overview here, intended to help you use DNS for - FreeS/WAN purposes.

-

Forward and reverse maps

-

Although the implementation is distributed, it is often useful to - speak of DNS as if it were just two enormous tables:

- -

Both maps can optionally contain additional data. For opportunistic - encryption, we insert the data need for IPsec authentication.

-

A system named gateway.example.com with IP address 10.20.30.40 should - have at least two DNS records, one in each map:

-
-
gateway.example.com. IN A 10.20.30.40
-
used to look up the name and get an IP address
-
40.30.20.10.in-addr.arpa. IN PTR gateway.example.com.
-
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.
-
-

Hierarchy and delegation

-

For both maps there is a hierarchy of DNS servers and a system of - delegating authority so that, for example:

- -

DNS zones are the units of delegation. There is a hierarchy of zones.

-

Syntax of DNS records

-

Returning to the example records:

-
        gateway.example.com. IN A 10.20.30.40
-        40.30.20.10.in-addr.arpa. IN PTR gateway.example.com.
-

some syntactic details are:

- -

The capitalised strings after IN indicate the type of record. - Possible types include:

- -

To set up for opportunistic encryption, you add some TXT records to - your DNS data. Details are in our quickstart - document.

-

Cacheing, TTL and propagation delay

-

DNS information is extensively cached. With no caching, a lookup by - your system of "www.freeswan.org" might involve:

- -

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.

-

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.

-

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.

-

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.

-

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.

-

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.

-

Problems with packet fragmentation

-

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.

-

These problems are caused by various devices along the way - mis-handling either packet fragments or - path MTU discovery.

-

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.

-

An explanation from project technical lead Henry Spencer:

-
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.)
-

In addition to the problem Henry describes, you may also have trouble - with path MTU discovery.

-

By default, FreeS/WAN uses a large MTU - for the ipsec device. This avoids some problems, but may complicate - others. Here's an explanation from Claudia:

-
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.
-

The MTU can be changed with an overridemtu= statement in - the config setup section of - ipsec.conf.5.

-

For a discussion of MTU issues and some possible solutions using - Linux advanced routing facilities, see the - Linux 2.4 Advanced Routing HOWTO. For a discussion of MTU and NAT - (Network Address Translation), see - James Carter's MTU notes.

-

Network address translation (NAT)

-

Network Address T -ranslation is a service provided by some gateway machines. Calling it - NAPT (adding the word Port) would be more precise, but - we will follow the widespread usage.

-

A gateway doing NAT rewrites the headers of packets it is forwarding, - changing one or more of:

- -

On Linux 2.4, NAT services are provided by the - netfilter(8) firewall code. There are several - Netfilter HowTos including one on NAT.

-

For older versions of Linux, this was referred to as "IP masquerade" - and different tools were used. See this - resource page.

-

Putting an IPsec gateway behind a NAT gateway is not recommended. See - our firewalls document.

-

NAT to non-routable addresses

-

The most common application of NAT uses private - non-routable addresses.

-

Often a home or small office network will have:

- -

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.

-

A common solution is to have:

- -

The client machines are set up with reserved - non-routable 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.

-

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.

-

For IPsec on such a gateway, you can entirely ignore the NAT in:

- -

Those can be set up exactly as they would be if your gateway had no - other systems behind it.

-

You do, however, have to take account of the NAT in firewall rules - which affect packet forwarding.

-

NAT to routable addresses

-

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 - Netfilter HowTos.

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