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.
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.
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:
For both maps there is a hierachy of DNS servers and a system of delegating authority so that, for example:
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:
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.
Here is an example of the difficulty of diagnosing an MTU-related problem, from the mailing list:
Date: Mon, 3 Apr 2000
From: "Michael H. Warfield" <mhw@wittsend.com>
Paul Koning wrote:
> Chris> It appears that the Osicom router discards IP
> Chris> fragments...
> Amazing. A device that discards fragments isn't a router, it's at
> best a boat anchor.
It may not be exactly what it appears. I ran into a similar problem
with an ISDN link a while ago giving similar symptoms. Turned out that
the device was negotiating an MTU that it really couldn't handle and the
device in front of it (a Linux box with always defragment enabled) was
defragmenting the huge IPSec datagrams and then refragmenting them into
hunks that the ISDN PPP thought it could handle but couldn't. Problem was
solved by manually capping the MTU on the ISDN link to a smaller value.
I gave the FreeSwan guys a hard time until tracking it down since
FreeSwan was the only thing that appeared to be able to tickle the bug.
Nothing else seemed to be broken. What it really was that MTU discovery
was avoiding the problem on normal links and it was only the IPsec tunnels
that were creating huge datagrams that went through the defragment/refragment
process.
Point here is that it "appeared" as though the ISDN link was
failing to handle fragments when it was really a configuration error and
a software bug resulting in a bad MTU that was really the culprit. So
it may not be that the router is not handling fragments. It may be that
it's missconfigured or has some other bug that only FreeSwan is tripping
over.
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.
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:
You do, however, have to take account of the NAT in firewall rules which affect packet forwarding.