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FreeS/WAN and firewalls

FreeS/WAN, or other IPSEC implementations, frequently run on gateway machines, the same machines running firewall or packet filtering code. This document discusses the relation between the two.

IPSEC packets

IPSEC uses three main types of packet:

IKE uses the UDP protocol and port 500.
Unless you are using only (less secure, not recommended) manual keying, you need IKE to negotiate connection parameters, acceptable algorithms, key sizes and key setup. IKE handles everything required to set up, rekey, repair or tear down IPSEC connections.
ESP is protocol number 50
This is required for encrypted connections.
AH is protocol number 51
This can be used where only authentication, not encryption, is required. That can also be done with ESP and null encryption.

All of those packets should have appropriate IPSEC gateway addresses in both the to and from IP header fields. Firewall rules can check this if you wish, though it is not strictly necessary. This is discussed in more detail later.

IPSEC processing of incoming packets authenticates them then removes the ESP or AH header and decrypts if necessary. Successful processing exposes an inner packet which is then delivered back to the firewall machinery, marked as having arrived on an ipsec[0-3] interface. Firewall rules can use that interface label to distinguish these packets from unencrypted packets which are labelled with the physical interface they arrived on (or perhaps with a non-IPSEC virtual interface such as ppp0).

One of our users sent a mailing list message with a diagram of the packet flow.

ESP and AH do not have ports

Some protocols, such as TCP and UDP, have the notion of ports. Others protocols, including ESP and AH, do not. Quite a few IPSEC newcomers have become confused on this point. There are no ports in the ESP or AH protocols, and no ports used for them. For these protocols, the idea of ports is completely irrelevant.

Header layout

The protocol numbers for ESP or AH are used in the 'next header' field of the IP header. On most non-IPSEC packets, that field would have one of:

Each header in the sequence tells what the next header will be. IPSEC adds headers for ESP or AH near the beginning of the sequence. The original headers are kept and the 'next header' fields adjusted so that all headers can be correctly interpreted.

For example, using [ ] to indicate data protected by ESP and unintelligible to an eavesdropper between the gateways:

Part of the ESP header itself is encrypted, which is why the [ indicating protected data appears in the middle of some lines above. The next header field of the ESP header is protected. This makes traffic analysis more difficult. The next header field would tell an eavesdropper whether your packet was UDP to the gateway, TCP to the gateway, or encapsulated IP. It is better not to give this information away. A clever attacker may deduce some of it from the pattern of packet sizes and timings, but we need not make it easy.

IPSEC allows various combinations of these to match local policies, including combinations that use both AH and ESP headers or that nest multiple copies of these headers.

For example, suppose my employer has an IPSEC VPN running between two offices so all packets travelling between the gateways for those offices are encrypted. If gateway policies allow it (The admins could block UDP 500 and protocols 50 and 51 to disallow it), I can build an IPSEC tunnel from my desktop to a machine in some remote office. Those packets will have one ESP header throughout their life, for my end-to-end tunnel. For part of the route, however, they will also have another ESP layer for the corporate VPN's encapsulation. The whole header scheme for a packet on the Internet might be:

The first ESP (outermost) header is for the corporate VPN. The inner ESP header is for the secure machine-to-machine link.

Filtering rules for IPSEC packets

As a consequence of the above, an IPSEC gateway should have packet filters that allow the following protocols when talking to other IPSEC gateways:

Your gateway and the other IPSEC gateways it communicates with must be able to exchange these packets for IPSEC to work. Firewall rules must allow UDP 500 and at least one of AH or ESP on the interface that communicates with the other gateway.

IPSEC through the gateway

The preceeding paragraph deals with packets addressed to or sent from your gateway. It is a separate policy decision whether to permit such packets to pass through the gateway so that client machines can build end-to-end IPSEC tunnels of their own. This may not be practical if you are using NAT (IP masquerade) on your gateway, and may conflict with some corporate security policies. Other than that, it is likely a good idea.

Preventing non-IPSEC traffic

You can of course also filter everything but UDP port 500 and ESP or AH to restrict traffic to IPSEC only, either for anyone communicating with your host or just for specific partners.

Filtering packets from unknown gateways

It is possible to use firewall rules to restrict UDP 500, ESP and AH packets so that these packets are accepted only from known gateways. This is not strictly necessary since FreeS/WAN will discard packets from unknown gateways. You might, however, want to do it for any of a number of reasons. For example:

It is not possible to use only static firewall rules for this filtering if you do not know the other gateways' IP addresses in advance, for example if you have "road warriors" who may connect from a different address each time or if want to do opportunistic encryption to arbitrary gateways. In these cases, you can accept UDP 500 IKE packets from anywhere, then use the updown script feature of pluto(8) to dynamically adjust firewalling for each negotiated tunnel.

Firewall packet filtering does not much reduce the risk of a denial of service attack on FreeS/WAN. The firewall can drop packets from unknown gateways, but KLIPS does that quite efficiently anyway, so you gain little. The firewall cannot drop otherwise legitmate packets that fail KLIPS authentication, so it cannot protect against an attack designed to exhaust resources by making FreeS/WAN perform many expensive authentication operations.

In summary, firewall filtering of IPSEC packets from unknown gateways is possible but not strictly necessary.

Other packet filters

When the IPSEC gateway is also acting as your firewall, other packet filtering rules will be in play. In general, those are outside the scope of this document. See our Linux firewall links for information. There are a few types of packet, however, which can affect the operation of FreeS/WAN or of diagnostic tools commonly used with it. These are discussed below.

ICMP filtering

ICMP is the I nternet Control Message P rotocol. It is used for messages between IP implementations themselves, whereas IP used is used between the clients of those implementations. ICMP is, unsurprisingly, used for control messages. For example, it is used to notify a sender that a desination is not reachable, or to tell a router to reroute certain packets elsewhere.

ICMP handling is tricky for firewalls.

ICMP does not use ports. Messages are distinguished by a "message type" field and, for some types, by an additional "code" field. The definitive list of types and codes is on the IANA site.

One expert uses this definition for ICMP message types to be dropped at the firewall.

# ICMP types which lack socially redeeming value.
#  5     Redirect
#  9     Router Advertisement
# 10     Router Selection
# 15     Information Request
# 16     Information Reply
# 17     Address Mask Request
# 18     Address Mask Reply

badicmp='5 9 10 15 16 17 18'

A more conservative approach would be to make a list of allowed types and drop everything else.

Whichever way you do it, your ICMP filtering rules on a FreeS/WAN gateway should allow at least the following ICMP packet types:

echo (type 8)
echo reply (type 0)
These are used by ping(1). We recommend allowing both types through the tunnel and to or from your gateway's external interface, since ping(1) is an essential testing tool.

It is fairly common for firewalls to drop ICMP echo packets addressed to machines behind the firewall. If that is your policy, please create an exception for such packets arriving via an IPSEC tunnel, at least during intial testing of those tunnels.

destination unreachable (type 3)
This is used, with code 4 (Fragmentation Needed and Don't Fragment was Set) in the code field, to control path MTU discovery. Since IPSEC processing adds headers, enlarges packets and may cause fragmentation, an IPSEC gateway should be able to send and receive these ICMP messages on both inside and outside interfaces.

UDP packets for traceroute

The traceroute(1) utility uses UDP port numbers from 33434 to approximately 33633. Generally, these should be allowed through for troubleshooting.

Some firewalls drop these packets to prevent outsiders exploring the protected network with traceroute(1). If that is your policy, consider creating an exception for such packets arriving via an IPSEC tunnel, at least during intial testing of those tunnels.

UDP for L2TP

Windows 2000 does, and products designed for compatibility with it may, build L2TP tunnels over IPSEC connections.

For this to work, you must allow UDP protocol 1701 packets coming out of your tunnels to continue to their destination. You can, and probably should, block such packets to or from your external interfaces, but allow them from ipsec0.

See also our Windows 2000 interoperation discussion.


Network Address Translation, also known as IP masquerading, is a method of allocating IP addresses dynamically, typically in circumstances where the total number of machines which need to access the Internet exceeds the supply of IP addresses.

Any attempt to perform NAT operations on IPSEC packets between the IPSEC gateways creates a basic conflict:

For AH, which authenticates parts of the packet header including source and destination IP addresses, this is fatal. If NAT changes those fields, AH authentication fails.

For IKE and ESP it is not necessarily fatal, but is certainly an unwelcome complication.

NAT on or behind the IPSEC gateway works

This problem can be avoided by having the masquerading take place on or behind the IPSEC gateway.

This can be done physically with two machines, one physically behind the other. A picture, using SG to indicate IPSEC S ecurity Gateways, is:

      clients --- NAT ----- SG ---------- SG
                  two machines

In this configuration, the actual client addresses need not be given in the leftsubnet= parameter of the FreeS/WAN connection description. The security gateway just delivers packets to the NAT box; it needs only that machine's address. What that machine does with them does not affect FreeS/WAN.

A more common setup has one machine performing both functions:

      clients ----- NAT/SG ---------------SG
                  one machine
Here you have a choice of techniques depending on whether you want to make your client subnet visible to clients on the other end:

NAT between gateways is problematic

We recommend not trying to build IPSEC connections which pass through a NAT machine. This setup poses problems:

      clients --- SG --- NAT ---------- SG
If you must try it, some references are:

Other references on NAT and IPSEC

Other documents which may be relevant include:

Calling firewall scripts, named in ipsec.conf(5)

The ipsec.conf configuration file has three pairs of parameters used to specify an interface between FreeS/WAN and firewalling code.

Note that using these is not required if you have a static firewall setup. In that case, you just set your firewall up at boot time (in a way that permits the IPSEC connections you want) and do not change it thereafter. Omit all the FreeS/WAN firewall parameters and FreeS/WAN will not attempt to adjust firewall rules at all. See below for some information on appropriate scripts.

However, if you want your firewall rules to change when IPSEC connections change, then you need to use these parameters.

Scripts called at IPSEC start and stop

One pair of parmeters are set in the config setup section of the ipsec.conf(5) file and affect all connections:

specify scripts to be called before our pluto(8) IKE daemon is started and after it is stopped.
These parameters allow you to change firewall parameters whenever IPSEC is started or stopped.

They can also be used in other ways. For example, you might have prepluto add a module to your kernel for the secure network interface or make a dialup connection, and then have postpluto remove the module or take the connection down.

Scripts called at connection up and down

The other parameters are set in connection descriptions. They can be set in individual connection descriptions, and could even call different scripts for each connection for maximum flexibility. In most applications, however, it makes sense to use only one script and to call it from conn %default section so that it applies to all connections.

You can either set [left|right]firewall=yes to use our supplied default script or assign a name in a [left|right]updown= line to use your own script.

For details of when Pluto calls these scripts, what arguments it passes to them, and what the default script does with those arguments, see the ipsec_pluto(8) man page.

Note that only one of these should be used. You cannot sensibly use both.

The default script

We supply a default script named _updown.
indicates that the gateway is doing firewalling and that pluto(8) should poke holes in the firewall as required.
Set these to yes and Pluto will call our default script _updown with appropriate arguments whenever it: The supplied default _updown script is appropriate for simple cases using the ipfwadm(8) firewalling package.

User-written scripts

You can also write your own script and have Pluto call it. Just put the script's name in one of these ipsec.conf(5) lines:
specifies a script to call instead of our default script _updown.
Your script should take the same arguments and use the same environment variables as _updown. These are described in the pluto(8) man page.

In developing your own script, you can of course use our scripts (either the default _updown or the ipchains-based example given below) as a starting point. Note, however, that you should not modify our _updown script in place. If you did that, then upgraded FreeS/WAN, the upgrade would install a new default script, overwriting your changes.

Scripts for ipchains

Our _updown is for firewalls using ipfwadm(8) . If you are using the more recent package ipchains(8), you must do one of:

We provide an example script for use with ipchains(8) below.

DHR on the updown script

Here are some mailing list comments from pluto(8) developer Hugh Redelmeier on an earlier draft of this document:

There are many important things left out

- firewalling is important but must reflect (implement) policy.  Since
  policy isn't the same for all our customers, and we're not experts,
  we should concentrate on FW and MASQ interactions with FreeS/WAN.

- we need a diagram to show packet flow WITHIN ONE MACHINE, assuming
  IKE, IPsec, FW, and MASQ are all done on that machine.  The flow is
  obvious if the components are run on different machines (trace the

  IKE input:
        + packet appears on public IF, as UDP port 500
        + input firewalling rules are applied (may discard)
        + Pluto sees the packet.

  IKE output:
        + Pluto generates the packet & writes to public IF, UDP port 500
        + output firewalling rules are applied (may discard)
        + packet sent out public IF

  IPsec input, with encapsulated packet, outer destination of this host:
        + packet appears on public IF, protocol 50 or 51.  If this
          packet is the result of decapsulation, it will appear
          instead on the paired ipsec IF.
        + input firewalling rules are applied (but packet is opaque)
        + KLIPS decapsulates it, writes result to paired ipsec IF
        + input firewalling rules are applied to resulting packet
          as input on ipsec IF
        + if the destination of the packet is this machine, the
          packet is passed on to the appropriate protocol handler.
          If the original packet was encapsulated more than once
          and the new outer destination is this machine, that
          handler will be KLIPS.
        + otherwise:
          * routing is done for the resulting packet.  This may well
            direct it into KLIPS for encoding or encrypting.  What
            happens then is described elsewhere.
          * forwarding firewalling rules are applied
          * output firewalling rules are applied
          * the packet is sent where routing specified

 IPsec input, with encapsulated packet, outer destination of another host:
        + packet appears on some IF, protocol 50 or 51
        + input firewalling rules are applied (but packet is opaque)
        + routing selects where to send the packet
        + forwarding firewalling rules are applied (but packet is opaque)
        + packet forwarded, still encapsulated

  IPsec output, from this host or from a client:
        + if from a client, input firewalling rules are applied as the
          packet arrives on the private IF
        + routing directs the packet to an ipsec IF (this is how the
          system decides KLIPS processing is required)
        + if from a client, forwarding firewalling rules are applied
        + KLIPS eroute mechanism matches the source and destination
          to registered eroutes, yielding a SPI group.  This dictates
          processing, and where the resulting packet is to be sent
          (the destinations SG and the nexthop).
        + output firewalling is not applied to the resulting
          encapsulated packet

- Until quite recently, KLIPS would double encapsulate packets that
  didn't strictly need to be.  Firewalling should be prepared for
  those packets showing up as ESP and AH protocol input packets on
  an ipsec IF.

- MASQ processing seems to be done as if it were part of the
  forwarding firewall processing (this should be verified).

- If a firewall is being used, it is likely the case that it needs to
  be adjusted whenever IPsec SAs are added or removed.  Pluto invokes
  a script to do this (and to adjust routing) at suitable times.  The
  default script is only suitable for ipfwadm-managed firewalls.  Under
  LINUX 2.2.x kernels, ipchains can be managed by ipfwadm (emulation),
  but ipchains more powerful if manipulated using the ipchains command.
  In this case, a custom updown script must be used.

  We think that the flexibility of ipchains precludes us supplying an
  updown script that would be widely appropriate.
We do provide a sample script in the next section. It is essentially a transliteration of the version we supply for ipfwadm. Because it doesn't process the command line argument, it cannot be directly subsituted -- it won't support the semantics of *firewall=no. It can be used in [left|right]updown=.

Example updown script for ipchains

Here is an example updown script for use with ipchains. It is intended to be called via an updown= statement in ipsec.conf.

#! /bin/sh
# sample updown script for ipchains
# Copyright (C) 2000  D. Hugh Redelmeier, Henry Spencer
# This program is free software; you can redistribute it and/or modify it
# under the terms of the GNU General Public License as published by the
# Free Software Foundation; either version 2 of the License, or (at your
# option) any later version.  See .
# This program is distributed in the hope that it will be useful, but
# WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
# or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
# for more details.
# RCSID $Id: firewall.html,v 1.20 2001/06/12 05:14:54 sandy Exp $

# check interface version
case "$PLUTO_VERSION" in
1.0)    ;;
*)      echo "$0: unknown interface version \`$PLUTO_VERSION'" >2        exit 2

# check parameter(s)
case "$*" in
'')     ;;
*)      echo "$0: parameters unexpected" >2        exit 2

# utility functions for route manipulation
# Meddling with this stuff should never be necessary and is most unwise.
uproute() {
        route add -net $PLUTO_PEER_CLIENT_NET netmask $PLUTO_PEER_CLIENT_MASK \
                dev $PLUTO_INTERFACE gw $PLUTO_NEXT_HOP
downroute() {
        route del -net $PLUTO_PEER_CLIENT_NET netmask $PLUTO_PEER_CLIENT_MASK \
                dev $PLUTO_INTERFACE gw $PLUTO_NEXT_HOP

# the big choice
case "$PLUTO_VERB" in
        # delete possibly-existing route (preliminary to adding a route)
        oops="`route del -net $PLUTO_PEER_CLIENT_NET \
                                        netmask $PLUTO_PEER_CLIENT_MASK 2>1"
        if test " $oops" = " " -a " $status" != " 0"
                oops="silent error in route command, exit status $status"
        case "$oops" in
        'SIOCDELRT: No such process')
                # This is what route (currently -- not documented!) gives
                # for "could not find such a route".
        exit $status
        # connection to this host or client being routed
        # connection to this host or client being unrouted
        # connection to this host coming up
        # connection to this host going down
        # connection to client subnet, through forwarding firewall, coming up
        ipchains -I forward -j ACCEPT -b \
        # connection to client subnet, through forwarding firewall, going down
        ipchains -D forward -j ACCEPT -b \
*)      echo "$0: unknown verb \`$PLUTO_VERB' or parameter \`$1'" >2        exit 1

Ipchains firewall configuration at boot

It is also possible to set up both firewalling and IPSEC with appropriate scripts at boot and then not use leftupdown= and rightupdown=, or use them only for simple up and down operations.

Basically, the technique is

Firewall rules can recognise packets emerging from IPSEC. They are marked as arriving on an interface such as ipsec0, rather than eth0, ppp0 or whatever.

While it is possible to create such rules yourself (please let us know via the mailing list if you do), it may be both easier and more secure to use a set which has already been published and tested. Those we know of are described below.

Scripts based on Ranch's work

One user, Rob Hutton, posted his boot time scripts to the mailing list, and we included them in previous versions of this documentation. They are still available from our web site. However, they were for an earlier FreeS/WAN version so we no longer recommend them. Also, they had some bugs. See this message.

Those scripts were based on David Ranch's scripts for his "Trinity OS" for setting up a secure Linux. Check his home page for the latest version and for information on his book on securing Linux. If you are going to base your firewalling on Ranch's scripts, we recommend using his latest version, and sending him any IPSEC modifications you make for incorporation into later versions.

The Seattle firewall

We have had several mailing lists reports of good results using FreeS/WAN with Seawall (the Seattle Firewall). See that project's home page on Sourceforge.

The RCF scripts

Another set of firewall scripts with IPSEC support are the RCF or rc.firewall scripts. See their home page.
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