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.
The firewall code in 2.4 and later kernels is called Netfilter. The user-space utility to manage a firewall is iptables(8). See the netfilter/iptables web site for details.
The basic constraint is that an IPsec gateway must have packet filters that allow IPsec packets, at least 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.
For nearly all FreeS/WAN applications, you must allow UDP port 500 and the ESP protocol.
There are two ways to set this up:
Both methods are described in more detail below.
It is 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
Since Pluto authenticates its partners during the negotiation, and KLIPS drops packets for which no tunnel has been negotiated, this may be all you need.
In simple cases, you need only a few rules, as in this example:
# allow IPsec # # IKE negotiations iptables -I INPUT -p udp --sport 500 --dport 500 -j ACCEPT iptables -I OUTPUT -p udp --sport 500 --dport 500 -j ACCEPT # ESP encryption and authentication iptables -I INPUT -p 50 -j ACCEPT iptables -I OUTPUT -p 50 -j ACCEPT
This should be all you need to allow IPsec through lokkit, which ships with Red Hat 9, on its medium security setting. Once you've tweaked to your satisfaction, save your active rule set with:
service iptables save
However, while it is certainly possible to create an elaborate set of 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.
The published rule sets we know of are described in the next section.
It is therefore reasonably straightforward to filter these packets in whatever way suits your situation.
In some cases rules that work fine before you add IPsec may require modification to work with IPsec.
This is especially likely for rules that deal with interfaces on the Internet side of your system. IPsec adds a new interface; often the rules must change to take account of that.
For example, consider the rules given in this section of the Netfilter documentation:
Most people just have a single PPP connection to the Internet, and don't
want anyone coming back into their network, or the firewall:
## Insert connection-tracking modules (not needed if built into kernel).
# insmod ip_conntrack
# insmod ip_conntrack_ftp
## Create chain which blocks new connections, except if coming from inside.
# iptables -N block
# iptables -A block -m state --state ESTABLISHED,RELATED -j ACCEPT
# iptables -A block -m state --state NEW -i ! ppp0 -j ACCEPT
# iptables -A block -j DROP
## Jump to that chain from INPUT and FORWARD chains.
# iptables -A INPUT -j block
# iptables -A FORWARD -j block
On an IPsec gateway, those rules may need to be modified. The above allows new connections from anywhere except ppp0. That means new connections from ipsec0 are allowed.
Do you want to allow anyone who can establish an IPsec connection to your gateway to initiate TCP connections to any service on your network? Almost certainly not if you are using opportunistic encryption. Quite possibly not even if you have only explicitly configured connections.
To disallow incoming connections from ipsec0, change the middle section above to:
## Create chain which blocks new connections, except if coming from inside.
# iptables -N block
# iptables -A block -m state --state ESTABLISHED,RELATED -j ACCEPT
# iptables -A block -m state --state NEW -i ppp+ -j DROP
# iptables -A block -m state --state NEW -i ipsec+ -j DROP
# iptables -A block -m state --state NEW -i -j ACCEPT
# iptables -A block -j DROP
The original rules accepted NEW connections from anywhere except ppp0. This version drops NEW connections from any PPP interface (ppp+) and from any ipsec interface (ipsec+), then accepts the survivors.
Of course, these are only examples. You will need to adapt them to your own situation.
Several sets of firewall rules that work with FreeS/WAN are available.
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.
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.
Another set of firewall scripts with IPsec support are the RCF or rc.firewall scripts. See their home page.
Asgard's Realm has set of firewall scripts with FreeS/WAN support, for 2.4 kernels and iptables.
One user gave considerable detail on his scripts, including supporting IPX through the tunnel. His message was too long to conveniently be quoted here, so I've put it in a separate file.
The ipsec.conf(5) 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 above 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.
One pair of parmeters are set in the config setup section of the ipsec.conf(5) file and affect all connections:
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.
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:
Note that only one of these should be used. You cannot sensibly use both. Since our default script is obsolete (designed for firewalls using ipfwadm(8) on 2.0 kernels), most users who need this service will need to write a custom script.
We supply a default script named _updown.
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.
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:
Your script should take the same arguments and use the same environment variables as _updown. See the "updown command" section of the ipsec_pluto(8) man page for details.
Note 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.
Our _updown is for firewalls using ipfwadm(8), the firewall code for the 2.0 series of Linux kernels. If you are using the more recent packages ipchains(8) (for 2.2 kernels) or iptables(8) (2.4 kernels), then you must do one of:
You can write a script to do whatever you need with firewalling. Specify its name in a [left|right]updown= parameter in ipsec.conf(5) and Pluto will automatically call it for you.
The arguments Pluto passes such a script are the same ones it passes to our default _updown script, so the best way to build yours is to copy ours and modify the copy.
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.
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.
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 Security 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:
In this case, no masquerading is done. Packets to or from the client subnet are encrypted or decrypted without any change to their client subnet addresses, although of course the encapsulating packets use gateway addresses in their headers. Clients behind the right security gateway see a route via that gateway to the left subnet.
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 documents which may be relevant include:
Of course simply allowing UDP 500 and ESP packets is not the whole story. Various other issues arise in making IPsec and packet filters co-exist and even co-operate. Some of them are summarised below.
Basic IPsec packet filtering rules deal only with packets addressed to or sent from your IPsec 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.
Where possible, allowing this is almost certainly a good idea. Using IPsec on an end-to-end basis is more secure than gateway-to-gateway.
Doing it is quite simple. You just need firewall rules that allow UDP port 500 and protocols 50 and 51 to pass through your gateway. If you wish, you can of course restrict this to certain hosts.
One application of this is for the telecommuter who might have:
Sunset==========West------------------East ================= firewall --- the Internet
home network untrusted net corporate network
The subnet on the right is 0.0.0.0/0, the whole Internet. The West gateway is set up so that it allows only IPsec packets to East in or out.
This configuration is used in AT&T Research's network. For details, see the papers links in our introduction.
Another application would be to set up firewall rules so that an internal machine, such as an employees-only web server, could not talk to the outside world except via specific IPsec tunnels.
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.
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 is the Internet Control Message Protocol. 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:
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.
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.
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.
IPsec uses three main types of packet:
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.
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.
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.
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
cables).
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.