IPSEC using Linux Kernel 2.6
Tom
Eastep
2004-08-19
2004
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
.
To use this support, your kernel and iptables must include the
Netfilter+ipsec patches and policy match support and you must be running
Shorewall 2.1.5 or later.
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.
Shorewall 2.1 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). Outgoing traffic is encrypted
according to the contents of the SPD and 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.
Shorewall provides support for policy matching in two 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 an entry in the new IPSEC column
in that file.
A new /etc/shorewall/ipsec
file allows you to associate zones with traffic that will be encrypted
or that has been decrypted.
In summary, Shorewall 2.1.5 and later versions provide 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 Yes is placed in the
IPSEC column of the /etc/shorewall/ipsec entry
for the zone.
Encrypted communication is used to/from only part of the hosts
in a zone.
The value No is placed in the
IPSEC column of the /etc/shorewall/ipsec entry
for the zone and the new ipsec option
is specified in /etc/shorewall/hosts for those
hosts requiring secure communication.
For simple zones such as are shown in the following examples, the
two techniques are equivalent and are used interchangably.
Finally, the OPTIONS, IN OPTIONS and OUT OPTIONS columns in
/etc/shorewall/ipsec 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.
IPSec Gateway on the Firewall System
Suppose that we have the following sutuation:
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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 206.161.148.9
#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 DISPLAY COMMENTS
net Internet The big bad internet
vpn VPN Virtual Private Network
#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.
/etc/shorewall/hosts — System A
#ZONE HOSTS OPTIONS
vpn eth0:10.0.0.0/8 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 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
Once you have these entries in place, restart Shorewall (type
shorewall restart); you are now ready to configure IPSEC.
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.
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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 DISPLAY COMMENTS
net Internet The big bad internet
vpn VPN Road Warriors
loc local Local Network (192.168.1.0/24)
#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 ipsec
#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.
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.
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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 aggressive ;
my_identifier user_fqdn "teastep@shorewall.net" ;
lifetime time 24 hour ;
proposal {
encryption_algorithm 3des;
hash_algorithm sha1;
authentication_method pre_shared_key ;
dh_group 2 ;
}
}
sainfo anonymous
{
pfs_group 2;
lifetime time 12 hour ;
encryption_algorithm 3des, blowfish, des, rijndael ;
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:
teastep@shorewall.net <key>
Shorewall configuration goes as follows:
/etc/shorewall/zones:
#ZONE DISPLAY COMMENTS
net Net Internet
loc Local Local Network
#LAST LINE -- ADD YOUR ENTRIES ABOVE THIS LINE -- DO NOT REMOVE
/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/ipsec:
#ZONE IPSEC OPTIONS IN OUT
# ONLY OPTIONS OPTIONS
loc Yes mode=transport
/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.