IPSEC using Linux Kernel 2.6
Tom
Eastep
2004
2005
2006
Thomas M. Eastep
Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License, Version
1.2 or any later version published by the Free Software Foundation; with
no Invariant Sections, with no Front-Cover, and with no Back-Cover
Texts. A copy of the license is included in the section entitled
GNU Free Documentation
License
.
This article applies to Shorewall 3.0 and
later. If you are running a version of Shorewall earlier than Shorewall
3.0.0 then please see the documentation for that
release.
The information in this article is only applicable if you plan to
have IPSEC end-points on the same system where Shorewall is used.
To use the features described in this article, your kernel must be
2.6.16 or later or your kernel and
iptables must include the Netfilter+ipsec patches and policy match
support. The Netfilter patches are available from Netfilter
Patch-O-Matic-NG and are also included in some commercial distributions
(most notably SUSE 9.1 through 10.0).
As of this writing, the Netfilter+ipsec and policy match support are
broken when used with a bridge device. The problem has been reported to
the responsible Netfilter developer who has confirmed the problem. The
problem was presumably corrected in Kernel 2.6.20 as a result of the
removal of defered FORWARD/OUTPUT processing of traffic destined for a
bridge. See the "Bridging without
using physdev match support" article.
Shorewall 3.0 and Kernel 2.6 IPSEC
This is not a HOWTO for Kernel 2.6
IPSEC -- for that, please see http://www.ipsec-howto.org/.
The 2.6 Linux Kernel introduces new facilities for defining
encrypted communication between hosts in a network. The network
administrator defines a set of Security Policies
which are stored in the kernel as a Security Policy
Database (SPD). Security policies determine which traffic is
subject to encryption. Security Associations are
created between pairs of hosts in the network (one SA for traffic in each
direction); these SAs define how traffic is to be encrypted. Outgoing
traffic that is to be encrypted according to the contents of the SPD
requires an appropriate SA to exist. SAs may be created manually using
setkey(8) but most often, they are created by a
cooperative process involving the ISAKMP protocol and daemons such
as racoon or isakmpd. Incoming
traffic is verified against the SPD to ensure that no unencrypted traffic
is accepted in violation of the administrator's policies.
There are three ways in which IPSEC traffic can interact with
Shorewall policies and rules:
Traffic that is encrypted on the firewall system. The traffic
passes through Netfilter twice -- first as unencrypted then
encrypted.
Traffic that is decrypted on the firewall system. The traffic
passes through Netfilter twice -- first as encrypted then as
unencrypted.
Encrypted traffic that is passed through the firewall system.
The traffic passes through Netfilter once.
In cases 1 and 2, the encrypted traffic is handled by entries in
/etc/shorewall/tunnels (don't be mislead by the name
of the file -- transport mode encrypted traffic is
also handled by entries in that file). The unencrypted traffic is handled
by normal rules and policies.
Under the 2.4 Linux Kernel, the association of unencrypted traffic
and zones was made easy by the presense of IPSEC pseudo-interfaces with
names of the form ipsecn (e.g.
ipsec0). Outgoing unencrypted
traffic (case 1.) was send through an ipsecn device while incoming unencrypted
traffic (case 2) arrived from an ipsecn device. The 2.6 kernel-based
implementation does away with these pseudo-interfaces. Outgoing traffic
that is going to be encrypted and incoming traffic that has been decrypted
must be matched against policies in the SPD and/or the appropriate
SA.
Shorewall provides support for policy matching in three ways:
In /etc/shorewall/masq, traffic that will
later be encrypted is exempted from MASQUERADE/SNAT using existing
entries. If you want to MASQUERADE/SNAT outgoing traffic that will
later be encrypted, you must include the appropriate indication in the
new IPSEC column in that file.
The /etc/shorewall/zones
file allows you to associate zones with traffic that will be encrypted
or that has been decrypted.
A new option (ipsec) has been
provided for entries in /etc/shorewall/hosts.
When an entry has this option specified, traffic to/from the hosts
described in the entry is assumed to be encrypted.
In summary, Shorewall provides the facilities to replace the use of
ipsec pseudo-interfaces in zone and MASQUERADE/SNAT definition.
There are two cases to consider:
Encrypted communication is used to/from all hosts in a
zone.
The value ipsec is placed in
the TYPE column of the /etc/shorewall/zones entry
for the zone.
By default, encrypted communication is not used to communicate
with the hosts in a zone.
The value ipv4 is placed in the
TYPE column of the /etc/shorewall/zones entry for
the zone and the new ipsec option is
specified in /etc/shorewall/hosts for any hosts
requiring secure communication.
For simple zones such as are shown in the following examples, the
two techniques are equivalent and are used interchangably.
It is redundent to have ipsec in
the TYPE column of the /etc/shorewall/zones entry
for a zone and to also have the ipsec
option in /etc/shorewall/hosts entries for that
zone.
Finally, the OPTIONS, IN OPTIONS and OUT OPTIONS columns in
/etc/shorewall/zones can be used to match the zone to a particular (set
of) SA(s) used to encrypt and decrypt traffic to/from the zone and the
security policies that select which traffic to encrypt/decrypt.
This article assumes the use of ipsec-tools (http://ipsec-tools.sourceforge.net).
As of this writing, I recommend that you run at least version 0.5.2.
Debian users, please note that there are separate Debian packages for
ipsec-tools and racoon although the ipsec-tools project releases them as a
single package.
For more information on IPSEC, Kernel 2.6 and Shorewall see my presentation on the subject given at LinuxFest NW
2005. Be warned though that the presentation is based on Shorewall
2.2 and there are some differences in the details of how IPSEC is
configured.
IPSec Gateway on the Firewall System
Suppose that we have the following sutuation:
![]()
We want systems in the 192.168.1.0/24 sub-network to be able to
communicate with systems in the 10.0.0.0/8 network. We assume that on both
systems A and B, eth0 is the internet interface.
To make this work, we need to do two things:
Open the firewall so that the IPSEC tunnel can be established
(allow the ESP and AH protocols and UDP Port 500).
Allow traffic through the tunnel.
Opening the firewall for the IPSEC tunnel is accomplished by adding
an entry to the /etc/shorewall/tunnels file.
In /etc/shorewall/tunnels on system A, we need
the following
/etc/shorewall/tunnels —
System A:
#TYPE ZONE GATEWAY GATEWAY ZONE
ipsec net 134.28.54.2
#LAST LINE -- ADD YOUR ENTRIES ABOVE THIS LINE -- DO NOT REMOVE
/etc/shorewall/tunnels —
System B:
#TYPE ZONE GATEWAY GATEWAY ZONE
ipsec net 206.162.148.9
#LAST LINE -- ADD YOUR ENTRIES ABOVE THIS LINE -- DO NOT REMOVE
If either of the endpoints is behind a NAT gateway then the
tunnels file entry on the other
endpoint should specify a tunnel type of ipsecnat rather than ipsec and
the GATEWAY address should specify the external address of the NAT
gateway.
You need to define a zone for the remote subnet or include it in
your local zone. In this example, we'll assume that you have created a
zone called vpn
to represent the remote subnet.
/etc/shorewall/zones —
Systems A and B:
#ZONE TYPE OPTIONS IN OUT
# OPTIONS OPTIONS
vpn ipv4
net ipv4
#LAST LINE -- ADD YOUR ENTRIES ABOVE THIS LINE -- DO NOT REMOVE
Remember the assumption that both systems A and B have eth0 as their
internet interface.
You must define the vpn zone using the
/etc/shorewall/hosts file. The hosts file entries
below assume that you want the remote gateway to be part of the vpn zone —
If you don't wish the remote gateway included, simply omit its IP address
from the HOSTS column.
/etc/shorewall/hosts — System A
#ZONE HOSTS OPTIONS
vpn eth0:10.0.0.0/8,134.28.54.2 ipsec
#LAST LINE -- ADD YOUR ENTRIES ABOVE THIS LINE -- DO NOT REMOVE
/etc/shorewall/hosts — System B
#ZONE HOSTS OPTIONS
vpn eth0:192.168.1.0/24,206.162.148.9 ipsec
#LAST LINE -- ADD YOUR ENTRIES ABOVE THIS LINE -- DO NOT REMOVE
Assuming that you want to give each local network free access to the
remote network and vice versa, you would need the following
/etc/shorewall/policy entries on each system:
#SOURCE DESTINATION POLICY LEVEL BURST:LIMIT
loc vpn ACCEPT
vpn loc ACCEPT
If you need access from each firewall to hosts in the other network,
then you could add:
#SOURCE DESTINATION POLICY LEVEL BURST:LIMIT
$FW vpn ACCEPT
If you need access between the firewall's, you should describe the
access in your /etc/shorewall/rules file. For example, to allow SSH access
from System B, add this rule on system A:
#ACTION SOURCE DESTINATION PROTO POLICY
ACCEPT vpn:134.28.54.2 $FW
Note that your Security Policies must also be set up to send traffic
between 134.28.54.2 and 206.162.148.9 through the tunnel (see
below).
Once you have these entries in place, restart Shorewall (type
shorewall restart); you are now ready to configure IPSEC.
For full encrypted connectivity in this configuration (between the
subnets, between each subnet and the opposite gateway, and between the
gateways), you will need eight policies in
/etc/racoon/setkey.conf. For example, on gateway
A:
# First of all flush the SPD and SAD databases
spdflush;
flush;
# Add some SPD rules
spdadd 192.168.1.0/24 10.0.0.0/8 any -P out ipsec esp/tunnel/206.162.148.9-134.28.54.2/require;
spdadd 192.168.1.0/24 134.28.54.2/32 any -P out ipsec esp/tunnel/206.162.148.9-134.28.54.2/require;
spdadd 206.162.148.9/32 134.28.54.2/32 any -P out ipsec esp/tunnel/206.162.148.9-134.28.54.2/require;
spdadd 206.162.148.9/32 10.0.0.0/8 any -P out ipsec esp/tunnel/206.162.148.9-134.28.54.2/require;
spdadd 10.0.0.0/8 192.168.1.0/24 any -P in ipsec esp/tunnel/134.28.54.2-206.162.148.9/require;
spdadd 10.0.0.0/8 206.162.148.9/32 any -P in ipsec esp/tunnel/134.28.54.2-206.162.148.9/require;
spdadd 134.28.54.2/32 192.168.1.0/24 any -P in ipsec esp/tunnel/134.28.54.2-206.162.148.9/require;
spdadd 134.28.54.2/32 206.162.148.9/32 any -P in ipsec esp/tunnel/134.28.54.2-206.162.148.9/require;
The setkey.conf file on gateway B would be
similar.
A sample /etc/racoon/racoon.conf file using
X.509 certificates might look like:
path certificates "/etc/certs" ;
listen
{
isakmp 206.162.148.9;
}
remote 134.28.54.2
{
exchange_mode main ;
certificate_type x509 "GatewayA.pem" "GatewayA_key.pem" ;
verify_cert on;
my_identifier asn1dn ;
peers_identifier asn1dn ;
verify_identifier on ;
lifetime time 24 hour ;
proposal {
encryption_algorithm blowfish;
hash_algorithm sha1;
authentication_method rsasig ;
dh_group 2 ;
}
}
sainfo address 192.168.1.0/24 any address 10.0.0.0/8 any
{
pfs_group 2;
lifetime time 12 hour ;
encryption_algorithm blowfish ;
authentication_algorithm hmac_sha1, hmac_md5 ;
compression_algorithm deflate ;
}
sainfo address 206.162.148.9/32 any address 10.0.0.0/8 any
{
pfs_group 2;
lifetime time 12 hour ;
encryption_algorithm blowfish ;
authentication_algorithm hmac_sha1, hmac_md5 ;
compression_algorithm deflate ;
}
sainfo address 206.162.148.9/32 any address 134.28.54.2/32 any
{
pfs_group 2;
lifetime time 12 hour ;
encryption_algorithm blowfish ;
authentication_algorithm hmac_sha1, hmac_md5 ;
compression_algorithm deflate ;
}
sainfo address 192.168.1.0/24 any address 134.28.54.2/32 any
{
pfs_group 2;
lifetime time 12 hour ;
encryption_algorithm blowfish ;
authentication_algorithm hmac_sha1, hmac_md5 ;
compression_algorithm deflate ;
}
If you have hosts that access the internet through an IPSEC
tunnel, then it is a good idea to set the MSS value for traffic from
those hosts explicitly in the
/etc/shorewall/zones file. For example, if hosts
in the sec zone access the internet
through an ESP tunnel then the following entry would be
appropriate:
#ZONE TYPE OPTIONS IN OUT
# OPTIONS OPTIONS
sec ipsec mode=tunnel mss=1400
Note that CLAMPMSS=Yes in shorewall.conf
isn't effective with the 2.6 native IPSEC implementation because there
is no separate ipsec device with a lower mtu as there was under the
2.4 and earlier kernels.
Mobile System (Road Warrior)
Suppose that you have a laptop system (B) that you take with you
when you travel and you want to be able to establish a secure connection
back to your local network.
![]()
Road Warrior VPN
You need to define a zone for the laptop or include it in your
local zone. In this example, we'll assume that you have created a zone
called vpn
to represent the remote host.
/etc/shorewall/zones — System A
#ZONE TYPE OPTIONS IN OUT
# OPTIONS OPTIONS
vpn ipsec
net ipv4
loc ipv4
#LAST LINE -- ADD YOUR ENTRIES ABOVE THIS LINE -- DO NOT REMOVE
In this instance, the mobile system (B) has IP address 134.28.54.2
but that cannot be determined in advance. In the
/etc/shorewall/tunnels file on system A, the
following entry should be made:
#TYPE ZONE GATEWAY GATEWAY ZONE
ipsec net 0.0.0.0/0 vpn
#LAST LINE -- ADD YOUR ENTRIES ABOVE THIS LINE -- DO NOT REMOVE
the GATEWAY ZONE column contains the name of the zone
corresponding to peer subnetworks. This indicates that the gateway
system itself comprises the peer subnetwork; in other words, the
remote gateway is a standalone system.
The VPN zone is defined using the /etc/shorewall/hosts
file:
/etc/shorewall/hosts — System A:
#ZONE HOSTS OPTIONS
vpn eth0:0.0.0.0/0
#LAST LINE -- ADD YOUR ENTRIES ABOVE THIS LINE -- DO NOT REMOVE
You will need to configure your through the tunnel
policy as shown under the first example above.
On the laptop:
/etc/shorewall/zones - System B:
#ZONE TYPE OPTIONS IN OUT
# OPTIONS OPTIONS
vpn ipsec
net ipv4
loc ipv4
#LAST LINE -- ADD YOUR ENTRIES ABOVE THIS LINE -- DO NOT REMOVE
/etc/shorewall/tunnels - System B:
#TYPE ZONE GATEWAY GATEWAY ZONE
ipsec net 206.162.148.9 vpn
#LAST LINE -- ADD YOUR ENTRIES ABOVE THIS LINE -- DO NOT REMOVE
/etc/shorewall/hosts - System B:
#ZONE HOSTS OPTIONS
vpn eth0:0.0.0.0/0
#LAST LINE -- ADD YOUR ENTRIES ABOVE THIS LINE -- DO NOT REMOVE
On system A, here are the IPSEC files:
/etc/racoon/racoon.conf - System A:
path certificate "/etc/certs" ;
listen
{
isakmp 206.162.148.9;
}
remote anonymous
{
exchange_mode main ;
generate_policy on ;
passive on ;
certificate_type x509 "GatewayA.pem" "GatewayA_key.pem" ;
verify_cert on;
my_identifier asn1dn ;
peers_identifier asn1dn ;
verify_identifier on ;
lifetime time 24 hour ;
proposal {
encryption_algorithm blowfish ;
hash_algorithm sha1;
authentication_method rsasig ;
dh_group 2 ;
}
}
sainfo anonymous
{
pfs_group 2;
lifetime time 12 hour ;
encryption_algorithm blowfish ;
authentication_algorithm hmac_sha1, hmac_md5 ;
compression_algorithm deflate ;
}
/etc/racoon/setkey.conf - System A:
flush;
spdflush;
If system A is running kernel 2.6.10 or later then it must also be
running ipsec-tools (racoon) 0.5rc1 or later.
On the mobile system (system B), it is not possible to create a
static IPSEC configuration because the IP address of the laptop's
internet connection isn't static. I have created an 'ipsecvpn' script
and included in the tarball and in the RPM's documentation directory;
this script can be used to start and stop the connection.
The ipsecvpn script has some variable assignments at the top -- in
the above case, these would be as follows:
#
# External Interface
#
INTERFACE=eth0
#
# Remote IPSEC Gateway
#
GATEWAY=206.162.148.9
#
# Networks behind the remote gateway
#
NETWORKS="192.168.1.0/24"
#
# Directory where X.509 certificates are stored.
#
CERTS=/etc/certs
#
# Certificate to be used for this connection. The cert
# directory must contain:
#
# ${CERT}.pem - the certificate
# ${CERT}_key.pem - the certificates's key
#
CERT=roadwarrior
#
# The setkey binary
#
SETKEY=/usr/sbin/setkey
#
# The racoon binary
#
RACOON=/usr/sbin/racoon
The ipsecvpn script can be installed in /etc/init.d/ but it is
probably best installed in /usr/local/sbin and run manually:
ipsecvpn start # Starts the tunnel
ipsecvpn stop # Stops the tunnel
Although the ipsecvpn script allows you to specify multiple remote
NETWORKS as a space-separated list, SAs are created on the gateway only
during ISAKMP negotiation. So in practice, only the first remote network
accessed will be accessible from the roadwarrior.
Transport Mode
In today's wireless world, it is often the case that individual
hosts in a network need to establish secure connections with the other
hosts in that network. In that case, IPSEC transport mode is an
appropriate solution.
![]()
Here's an example
using the ipsec-tools package. The files shown are from host
192.168.20.10; the configuration of the other nodes is similar.
/etc/racoon/racoon.conf:
path pre_shared_key "/etc/racoon/psk.txt" ;
remote anonymous
{
exchange_mode main ;
my_identifier address ;
lifetime time 24 hour ;
proposal {
encryption_algorithm blowfish ;
hash_algorithm sha1;
authentication_method pre_shared_key ;
dh_group 2 ;
}
}
sainfo anonymous
{
pfs_group 2;
lifetime time 12 hour ;
encryption_algorithm blowfish ;
authentication_algorithm hmac_sha1, hmac_md5 ;
compression_algorithm deflate ;
}
/etc/racoon/setkey.conf:
# First of all flush the SPD database
spdflush;
# Add some SPD rules
spdadd 192.168.20.10/32 192.168.20.20/32 any -P out ipsec esp/transport/192.168.20.10-192.168.20.20/require;
spdadd 192.168.20.20/32 192.168.20.10/32 any -P in ipsec esp/transport/192.168.20.20-192.168.20.10/require;
spdadd 192.168.20.10/32 192.168.20.30/32 any -P out ipsec esp/transport/192.168.20.10-192.168.20.30/require;
spdadd 192.168.20.30/32 192.168.20.10/32 any -P in ipsec esp/transport/192.168.20.30-192.168.20.10/require;
spdadd 192.168.20.10/32 192.168.20.40/32 any -P out ipsec esp/transport/192.168.20.10-192.168.20.40/require;
spdadd 192.168.20.40/32 192.168.20.10/32 any -P in ipsec esp/transport/192.168.20.40-192.168.20.10/require;
/etc/racoon/psk.txt:
192.168.20.20 <key for 192.168.20.10<->192.168.20.20>
192.168.20.30 <key for 192.168.20.10<->192.168.20.30>
192.168.20.40 <key for 192.168.20.10<->192.168.20.40>
Note that the same keymust be
used in both directions.
Shorewall configuration goes as follows:
/etc/shorewall/interfaces:
#ZONE INTERFACE BROADCAST OPTIONS
net eth0 detect routefilter,dhcp,tcpflags
#LAST LINE -- ADD YOUR ENTRIES BEFORE THIS ONE -- DO NOT REMOVE
/etc/shorewall/tunnels:
#TYPE ZONE GATEWAY GATEWAY
# ZONE
ipsec:noah net 192.168.20.0/24 loc
/etc/shorewall/zones:
#ZONE TYPE OPTIONS IN OUT
# OPTIONS OPTIONS
loc ipsec mode=transport
net ipv4
/etc/shorewall/hosts:
#ZONE HOST(S) OPTIONS
loc eth0:192.168.20.0/24
#LAST LINE -- ADD YOUR ENTRIES BEFORE THIS LINE -- DO NOT REMOVE
It is worth noting that although loc is a
sub-zone of net, because loc
is an IPSEC-only zone it does not need to be defined before
net in
/etc/shorewall/zones.
/etc/shorewall/policy:
#SOURCE DEST POLICY LOG LEVEL LIMIT:BURST
$FW all ACCEPT
loc $FW ACCEPT
net loc NONE
loc net NONE
net all DROP info
# The FOLLOWING POLICY MUST BE LAST
all all REJECT info
#LAST LINE -- ADD YOUR ENTRIES ABOVE THIS LINE -- DO NOT REMOVE
Since there are no cases where net<->loc traffic should
occur, NONE policies are used.
IPSEC and <trademark>Windows</trademark> XP
I have successfully configured my work laptop to use IPSEC with
X.509 certificates for wireless IP communication when it is undocked at
home. I looked at dozens of sites and the one I found most helpful was
http://ipsec.math.ucla.edu/services/ipsec-windows.html.
The instructions on that site are directed to students at UCLA but they
worked fine for me (once I followed them very carefully).
The instructions found on the UCLA site are complex and do not
include any information on the generation of X.509 certificates. There
are lots of sites however that can tell you how to generate
certificates, including http://www.ipsec-howto.org/.
One piece of information that may not be so easy to find is "How
do I generate a PKCS#12 certificate to import into Windows?". Here's the
openssl command that I used:
openssl pkcs12 -export -in eastepnc6000.pem -inkey eastepnc6000_key.pem -out eastepnc6000.pfx -name "IPSEC Cert for Home Wireless"
I was prompted for a password to associate with the certificate.
This password is entered on the Windows system during import.
In the above command:
eastepnc6000.pem was the laptop's
certificate in PEM format.
eastepnc6000_key.pem was the laptop's
private key (actually, it's the original signing request which
includes the private key).
eastepnc6000.pfx is the PKCS#12 output
file.
"IPSEC Cert for Home Wireless" is the friendly name for the
certificate.
I started to write an article about how to do this, complete with
graphics captured from my laptop. I gave up. I had captured 12 images
and hadn't really started yet. The Windows interface for configuring
IPSEC is the worst GUI that I have ever used. What can be displayed on
one split Emacs screen (racoon.conf plus setkey.conf) takes 20+
different dialog boxes on Windows XP!!!
Source of Additional Samples
Be sure to check out the src/racoon/samples subdirectory in the
ipsec-tools source tree. It has a wide variety of sample racoon
configuration files.