Network Working Group B. Patel
Request for Comments: 3193 Intel
Category: Standards Track B. Aboba
W. Dixon
Microsoft
G. Zorn
S. Booth
Cisco Systems
November 2001
Securing L2TP using IPsec
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This document discusses how L2TP (Layer Two Tunneling Protocol) may
utilize IPsec to provide for tunnel authentication, privacy
protection, integrity checking and replay protection. Both the
voluntary and compulsory tunneling cases are discussed.
Table of Contents
1. IntrodUCtion .................................................. 2
1.1 Terminology .................................................. 3
1.2 Requirements language ........................................ 3
2. L2TP security requirements ................................... 4
2.1 L2TP security protocol ....................................... 5
2.2 Stateless compression and encryption ......................... 5
3. L2TP/IPsec inter-operability guidelines ....................... 6
3.1. L2TP tunnel and Phase 1 and 2 SA teardown ................... 6
3.2. Fragmentation Issues ........................................ 6
3.3. Per-packet security checks .................................. 7
4. IPsec Filtering details when protecting L2TP .................. 7
4.1. IKE Phase 1 Negotiations .................................... 8
4.2. IKE Phase 2 Negotiations .................................... 8
5. Security Considerations ....................................... 15
5.1 Authentication issues ........................................ 15
5.2 IPsec and PPP interactions ................................... 18
6. References .................................................... 21
Acknowledgments .................................................. 22
Authors' Addresses ............................................... 23
Appendix A: Example IPsec Filter sets ............................ 24
Intellectual Property Statement .................................. 27
Full Copyright Statement ......................................... 28
1. Introduction
L2TP [1] is a protocol that tunnels PPP traffic over variety of
networks (e.g., IP, SONET, ATM). Since the protocol encapsulates
PPP, L2TP inherits PPP authentication, as well as the PPP Encryption
Control Protocol (ECP) (described in [10]), and the Compression
Control Protocol (CCP) (described in [9]). L2TP also includes
support for tunnel authentication, which can be used to mutually
authenticate the tunnel endpoints. However, L2TP does not define
tunnel protection mechanisms.
IPsec is a protocol suite which is used to secure communication at
the network layer between two peers. This protocol is comprised of
IP Security Architecture document [6], IKE, described in [7], IPsec
AH, described in [3] and IPsec ESP, described in [4]. IKE is the key
management protocol while AH and ESP are used to protect IP traffic.
This document proposes use of the IPsec protocol suite for protecting
L2TP traffic over IP networks, and discusses how IPsec and L2TP
should be used together. This document does not attempt to
standardize end-to-end security. When end-to-end security is
required, it is recommended that additional security mechanisms (such
as IPsec or TLS [14]) be used inside the tunnel, in addition to L2TP
tunnel security.
Although L2TP does not mandate the use of IP/UDP for its transport
mechanism, the scope of this document is limited to L2TP over IP
networks. The exact mechanisms for enabling security for non-IP
networks must be addressed in appropriate standards for L2TP over
specific non-IP networks.
1.1. Terminology
Voluntary Tunneling
In voluntary tunneling, a tunnel is created by the user,
typically via use of a tunneling client. As a result, the
client will send L2TP packets to the NAS which will forward
them on to the LNS. In voluntary tunneling, the NAS does
not need to support L2TP, and the LAC resides on the same
machine as the client. Another example of voluntary
tunneling is the gateway to gateway scenario. In this case
the tunnel is created by a network device, typically a
router or network appliance. In this scenario either side
may start the tunnel on demand.
Compulsory Tunneling
In compulsory tunneling, a tunnel is created without any
action from the client and without allowing the client any
choice. As a result, the client will send PPP packets to
the NAS/LAC, which will encapsulate them in L2TP and tunnel
them to the LNS. In the compulsory tunneling case, the
NAS/LAC must be L2TP-capable.
Initiator The initiator can be the LAC or the LNS and is the device
which sends the SCCRQ and receives the SCCRP.
Responder The responder can be the LAC or the LNS and is the device
which receives the SCCRQ and replies with a SCCRP.
1.2. Requirements language
In this document, the key Words "MAY", "MUST, "MUST NOT", "OPTIONAL",
"RECOMMENDED", "SHOULD", and "SHOULD NOT", are to be interpreted as
described in [2].
2. L2TP security requirements
L2TP tunnels PPP traffic over the IP and non-IP public networks.
Therefore, both the control and data packets of L2TP protocol are
vulnerable to attack. Examples of attacks include:
[1] An adversary may try to discover user identities by snooping data
packets.
[2] An adversary may try to modify packets (both control and data).
[3] An adversary may try to hijack the L2TP tunnel or the PPP
connection inside the tunnel.
[4] An adversary can launch denial of service attacks by terminating
PPP connections, or L2TP tunnels.
[5] An adversary may attempt to disrupt the PPP ECP negotiation in
order to weaken or remove confidentiality protection.
Alternatively, an adversary may wish to disrupt the PPP LCP
authentication negotiation so as to weaken the PPP authentication
process or gain Access to user passwords.
To address these threats, the L2TP security protocol MUST be able to
provide authentication, integrity and replay protection for control
packets. In addition, it SHOULD be able to protect confidentiality
for control packets. It MUST be able to provide integrity and replay
protection of data packets, and MAY be able to protect
confidentiality of data packets. An L2TP security protocol MUST also
provide a scalable approach to key management.
The L2TP protocol, and PPP authentication and encryption do not meet
the security requirements for L2TP. L2TP tunnel authentication
provides mutual authentication between the LAC and the LNS at tunnel
origination. Therefore, it does not protect control and data traffic
on a per packet basis. Thus, L2TP tunnel authentication leaves the
L2TP tunnel vulnerable to attacks. PPP authenticates the client to
the LNS, but also does not provide per-packet authentication,
integrity, or replay protection. PPP encryption meets
confidentiality requirements for PPP traffic but does not address
authentication, integrity, replay protection and key management
requirements. In addition, PPP ECP negotiation, outlined in [10]
does not provide for a protected ciphersuite negotiation. Therefore,
PPP encryption provides a weak security solution, and in addition
does not assist in securing L2TP control channel.
Key management facilities are not provided by the L2TP protocol.
However, where L2TP tunnel authentication is desired, it is necessary
to distribute tunnel passwords.
Note that several of the attacks outlined above can be carried out on
PPP packets sent over the link between the client and the NAS/LAC,
prior to encapsulation of the packets within an L2TP tunnel. While
strictly speaking these attacks are outside the scope of L2TP
security, in order to protect against them, the client SHOULD provide
for confidentiality, authentication, replay and integrity protection
for PPP packets sent over the dial-up link. Authentication, replay
and integrity protection are not currently supported by PPP
encryption methods, described in [11]-[13].
2.1. L2TP Security Protocol
The L2TP security protocol MUST provide authentication, integrity and
replay protection for control packets. In addition, it SHOULD
protect confidentiality of control packets. It MUST provide
integrity and replay protection of data packets, and MAY protect
confidentiality of data packets. An L2TP security protocol MUST also
provide a scalable approach to key management.
To meet the above requirements, all L2TP security compliant
implementations MUST implement IPsec ESP for securing both L2TP
control and data packets. Transport mode MUST be supported; tunnel
mode MAY be supported. All the IPsec-mandated ciphersuites
(described in RFC2406 [4] and RFC2402 [3]), including NULL
encryption MUST be supported. Note that although an implementation
MUST support all IPsec ciphersuites, it is an operator choice which
ones will be used. If confidentiality is not required (e.g., L2TP
data traffic), ESP with NULL encryption may be used. The
implementations MUST implement replay protection mechanisms of IPsec.
L2TP security MUST meet the key management requirements of the IPsec
protocol suite. IKE SHOULD be supported for authentication, security
association negotiation, and key management using the IPsec DOI [5].
2.2. Stateless compression and encryption
Stateless encryption and/or compression is highly desirable when L2TP
is run over IP. Since L2TP is a connection-oriented protocol, use of
stateful compression/encryption is feasible, but when run over IP,
this is not desirable. While providing better compression, when used
without an underlying reliable delivery mechanism, stateful methods
magnify packet losses. As a result, they are problematic when used
over the Internet where packet loss can be significant. Although
L2TP [1] is connection oriented, packet ordering is not mandatory,
which can create difficulties in implementation of stateful
compression/encryption schemes. These considerations are not as
important when L2TP is run over non-IP media such as IEEE 802, ATM,
X.25, or Frame Relay, since these media guarantee ordering, and
packet losses are typically low.
3. L2TP/IPsec inter-operability guidelines
The following guidelines are established to meet L2TP security
requirements using IPsec in practical situations.
3.1. L2TP tunnel and Phase 1 and 2 SA teardown
Mechanisms within PPP and L2TP provide for both graceful and non-
graceful teardown. In the case of PPP, an LCP TermReq and TermAck
sequence corresponds to a graceful teardown. LCP keep-alive messages
and L2TP tunnel hellos provide the capability to detect when a non-
graceful teardown has occurred. Whenever teardown events occur,
causing the tunnel to close, the control connection teardown
mechanism defined in [1] must be used. Once the L2TP tunnel is
deleted by either peer, any phase 1 and phase 2 SA's which still
exist as a result of the L2TP tunnel between the peers SHOULD be
deleted. Phase 1 and phase 2 delete messages SHOULD be sent when
this occurs.
When IKE receives a phase 1 or phase 2 delete message, IKE should
notify L2TP this event has occurred. If the L2TP state is such that
a ZLB ack has been sent in response to a STOPCCN, this can be assumed
to be positive acknowledgment that the peer received the ZLB ack and
has performed a teardown of any L2TP tunnel state associated with the
peer. The L2TP tunnel state and any associated filters can now be
safely removed.
3.2. Fragmentation Issues
Since the default MRU for PPP connections is 1500 bytes,
fragmentation can become a concern when prepending L2TP and IPsec
headers to a PPP frame. One mechanism which can be used to reduce
this problem is to provide PPP with the MTU value of the
ingress/egress interface of the L2TP/IPsec tunnel minus the overhead
of the extra headers. This should occur after the L2TP tunnel has
been setup and but before LCP negotiations begin. If the MTU value
of the ingress/egress interface for the tunnel is less than PPP's
default MTU, it may replace the value being used. This value may
also be used as the initial value proposed for the MRU in the LCP
config req.
If an ICMP PMTU is received by IPsec, this value should be stored in
the SA as proposed in [6]. IPsec should also provide notification of
this event to L2TP so that the new MTU value can be reflected into
the PPP interface. Any new PTMU discoveries seen at the PPP
interface should be checked against this new value and processed
accordingly.
3.3. Per-packet security checks
When a packet arrives from a tunnel which requires security, L2TP
MUST:
[1] Check to ensure that the packet was decrypted and/or
authenticated by IPsec. Since IPsec already verifies that the
packet arrived in the correct SA, L2TP can be assured that the
packet was indeed sent by a trusted peer and that it did not
arrive in the clear.
[2] Verify that the IP addresses and UDP port values in the packet
match the socket information which was used to setup the L2TP
tunnel. This step prevents malicious peers from spoofing packets
into other tunnels.
4. IPsec Filtering details when protecting L2TP
Since IKE/IPsec is agnostic about the nuances of the application it
is protecting, typically no integration is necessary between the
application and the IPsec protocol. However, protocols which allow
the port number to float during the protocol negotiations (such as
L2TP), can cause problems within the current IKE framework. The L2TP
specification [1] states that implementations MAY use a dynamically
assigned UDP source port. This port change is reflected in the SCCRP
sent from the responder to the initiator.
Although the current L2TP specification allows the responder to use a
new IP address when sending the SCCRP, implementations requiring
protection of L2TP via IPsec SHOULD NOT do this. To allow for this
behavior when using L2TP and IPsec, when the responder chooses a new
IP address it MUST send a StopCCN to the initiator, with the Result
and Error Code AVP present. The Result Code MUST be set to 2
(General Error) and the Error Code SHOULD be set to 7 (Try Another).
If the Error Code is set to 7, then the optional error message MUST
be present and the contents MUST contain the IP address (ASCII
encoded) that the Responder desires to use for subsequent
communications. Only the ASCII encoded IP address should be present
in the error message. The IP address is encoded in dotted decimal
format for IPv4 or in RFC2373 [17] format for IPv6. The initiator
MUST parse the result and error code information and send a new SCCRQ
to the new IP address contained in the error message. This approach
reduces complexity since now the initiator always knows precisely the
IP address of its peer. This also allows a controlled mechanism for
L2TP to tie IPsec filters and policy to the same peer.
The filtering details required to accommodate this behavior as well
as other mechanisms needed to protect L2TP with IPsec are discussed
in the following sections.
4.1. IKE Phase 1 Negotiations
Per IKE [7], when using pre-shared key authentication, a key must be
present for each peer to which secure communication is required.
When using Main Mode (which provides identity protection), this key
must correspond to the IP address for the peer. When using
Aggressive Mode (which does not provide identity protection), the
pre-shared key must map to one of the valid id types defined in the
IPsec DOI [5].
If the initiator receives a StopCCN with the result and error code
AVP set to "try another" and a valid IP address is present in the
message, it MAY bind the original pre-shared key used by IKE to the
new IP address contained in the error-message.
One may may wish to consider the implications for scalability of
using pre-shared keys as the authentication method for phase 1. As
the number of LAC and LNS endpoints grow, pre-shared keys become
increasingly difficult to manage. Whenever possible, authentication
with certificates is preferred.
4.2. IKE Phase 2 Negotiations
During the IKE phase 2 negotiations, the peers agree on what traffic
is to be protected by the IPsec protocols. The quick mode IDs
represent the traffic which the peers agree to protect and are
comprised of address space, protocol, and port information.
When securing L2TP with IPsec, the following cases must be
considered:
Cases:
+--------------------------------------------------+
Initiator Port Responder Addr Responder Port
+--------------------------------------------------+
1701 Fixed 1701
+--------------------------------------------------+
1701 Fixed Dynamic
+--------------------------------------------------+
1701 Dynamic 1701
+--------------------------------------------------+
1701 Dynamic Dynamic
+--------------------------------------------------+
Dynamic Fixed 1701
+--------------------------------------------------+
Dynamic Fixed Dynamic
+--------------------------------------------------+
Dynamic Dynamic 1701
+--------------------------------------------------+
Dynamic Dynamic Dynamic
+--------------------------------------------------+
By solving the most general case of the above permutations, all cases
are covered. The most general case is the last one in the list.
This scenario is when the initiator chooses a new port number and the
responder chooses a new address and port number. The L2TP message
flow which occurs to setup this sequence is as follows:
-> IKE Phase 1 and Phase 2 to protect Initial SCCRQ
SCCRQ -> (Fixed IP address, Dynamic Initiator Port)
<- STOPCCN (Responder chooses new IP address)
-> New IKE Phase 1 and Phase 2 to protect new SCCRQ
SCCRQ -> (SCCRQ to Responder's new IP address)
<- New IKE Phase 2 to for port number change by the responder
<- SCCRP (Responder chooses new port number)
SCCCN -> (L2TP Tunnel Establishment completes)
Although the Initiator and Responder typically do not dynamically
change ports, L2TP security must accommodate emerging applications
such as load balancing and QoS. This may require that the port and
IP address float during L2TP tunnel establishment.
To support the general case, mechanisms must be designed into L2TP
and IPsec which allow L2TP to inject filters into the IPsec filter
database. This technique may be used by any application which floats
ports and requires security via IPsec, and is described in the
following sections.
The responder is not required to support the ability to float its IP
address and port. However, the initiator MUST allow the responder to
float its port and SHOULD allow the responder to choose a new IP
address (see section 4.2.3, below).
Appendix A provides examples of these cases using the process
described below.
4.2.1. Terminology definitions used for filtering statements
I-Port The UDP port number the Initiator chooses to
originate/receive L2TP traffic on. This can be a static
port such as 1701 or an ephemeral one assigned by the
socket.
R-Port The UDP port number the Responder chooses to
originate/receive L2TP traffic on. This can be the port
number 1701 or an ephemeral one assigned by the socket.
This is the port number the Responder uses after
receiving the initial SCCRQ.
R-IPAddr1 The IP address the Responder listens on for initial
SCCRQ. If the responder does not choose a new IP
address, this address will be used for all subsequent
L2TP traffic.
R-IPAddr2 The IP address the Responder chooses upon receiving the
SCCRQ. This address is used to send the SCCRP and all
subsequent L2TP tunnel traffic is sent and received on
this address.
R-IPAddr The IP address which the responder uses for sending and
receiving L2TP packets. This is either the initial value
of R-IPAddr1 or a new value of R-IPAddr2.
I-IPAddr The IP address the Initiator uses to communicate with for
the L2TP tunnel.
Any-Addr The presence of Any-Address defines that IKE should
accept any single address proposed in the local address
of the quick mode IDs sent by the peer during IKE phase 2
negotiations. This single address may be formatted as an
IP Single address, an IP Netmask address with the Netmask
set to 255.255.255.255, and IP address Range with the
range being 1, or a hostname which can be resolved to one
address. Refer to [5] for more information on the format
for quick mode IDs.
Any-Port The presence of Any-Port defines that IKE should accept a
value of 0 or a specific port value for the port value in
the port value in the quick mode IDs negotiated during
IKE phase 2.
The filters defined in the following sections are listed from highest
priority to lowest priority.
4.2.2. Initial filters needed to protect the SCCRQ
The initial filter set on the initiator and responder is necessary to
protect the SCCRQ sent by the initiator to open the L2TP tunnel.
Both the initiator and the responder must either be pre-configured
for these filters or L2TP must have a method to inject this
information into the IPsec filtering database. In either case, this
filter MUST be present before the L2TP tunnel setup messages start to
flow.
Responder Filters:
Outbound-1: None. They should be be dynamically created by IKE
upon successful completion of phase 2.
Inbound-1: From Any-Addr, to R-IPAddr1, UDP, src Any-Port, dst
1701
Initiator Filters:
Outbound-1: From I-IPAddr, to R-IPAddr1, UDP, src I-Port,
dst 1701
Inbound-1: From R-IPAddr1, to I-IPAddr, UDP, src 1701,
dst I-Port
Inbound-2: From R-IPAddr1, to I-IPAddr, UDP, src Any-Port,
dst I-Port
When the initiator uses dynamic ports, L2TP must inject the filters
into the IPsec filter database, once its source port number is known.
If the initiator uses a fixed port of 1701, these filters MAY be
statically defined.
The Any-Port definition in the initiator's inbound-2 filter statement
is needed to handle the potential port change which may occur as the
result of the responder changing its port number.
If a phase 2 SA bundle is not already present to protect the SCCRQ,
the sending of a SCCRQ by the initiator SHOULD cause IKE to setup the
necessary SAs to protect this packet. Alternatively, L2TP may also
request IKE to setup the SA bundle. If the SA cannot be setup for
some reason, the packet MUST be dropped.
The port numbers in the Quick Mode IDs sent by the initiator MUST
contain the specific port numbers used to identify the UDP socket.
The port numbers would be either I-Port/1701 or 1701/1701 for the
initial SCCRQ. The quick mode IDs sent by the initiator will be a
subset of the Inbound-1 filter at the responder. As a result, the
quick mode exchange will finish and IKE should inject a specific
filter set into the IPsec filter database and associate this filter
set with the phase 2 SA established between the peers. These filters
should persist as long as the L2TP tunnel exists. The new filter set
at the responder will be:
Responder Filters:
Outbound-1: From R-IPAddr1, to I-IPAddr, UDP, src 1701,
dst I-Port
Inbound-1: From I-IPAddr, to R-IPAddr1, UDP, src I-Port,
dst 1701
Inbound-2: From Any-Addr, to R-IPAddr1, UDP, src Any-Port,
dst 1701
Mechanisms SHOULD exist between L2TP and IPsec such that L2TP is not
retransmitting the SCCRQ while the SA is being established. L2TP's
control channel retransmit mechanisms should start once the SA has
been established. This will help avoid timeouts which may occur as
the result of slow SA establishment.
Once the phase 2 SA has been established between the peers, the SCCRQ
should be sent from the initiator to the responder.
If the responder does not choose a new IP address or a new port
number, the L2TP tunnel can now proceed to establish.
4.2.3. Responder chooses new IP Address
This step describes the process which should be followed when the
responder chooses a new IP address. The only opportunity for the
responder to change its IP address is after receiving the SCCRQ but
before sending a SCCRP.
The new address the responder chooses to use MUST be reflected in the
result and error code AVP of a STOPCCN message. The Result Code MUST
be set to 2 (General Error) and the Error Code MUST be set to 7 (Try
Another). The optional error message MUST be present and the
contents MUST contain the IP address (ASCII encoded) the Responder
desires to use for subsequent communications. Only the ASCII encoded
IP address should be present in the error message. The IP address is
encoded in dotted decimal format for IPv4 or in RFC2373 [17] format
for IPv6.
The STOPCCN Message MUST be sent using the same address and UDP port
information which the initiator used to send the SCCRQ. This message
will be protecting using the initial SA bundle setup to protect the
SCCRQ.
Upon receiving the STOPCCN, the initiator MUST parse the IP address
from the Result and Error Code AVP and perform the necessary sanity
checks to verify this is a correctly formatted address. If no errors
are found L2TP should inject a new set of filters into the IPsec
filter database. If using pre-shared key authentication, L2TP MAY
request IKE to bind the new IP address to the pre-shared key which
was used for the original IP address.
Since the IP address of the responder changed, a new phase 1 and
phase 2 SA must be established between the peers before the new SCCRQ
is sent.
Assuming the initial tunnel has been torn down and the filters needed
to create the tunnel removed, the new filters for the initiator and
responder will be:
Initiator Filters:
Outbound-1: From I-IPAddr, to R-IPAddr2, UDP, src I-Port,
dst 1701
Inbound-1: From R-IPAddr2, to I-IPAddr, UDP, src 1701,
dst I-Port
Inbound-2: From R-IPAddr2, to I-IPAddr, UDP, src Any-Port,
dst I-Port
Once IKE phase 2 completes, the new filter set at the responder will
be:
Responder Filters:
Outbound-1: From R-IPAddr2, to I-IPAddr, UDP, src 1701,
dst I-Port
Inbound-1: From I-IPAddr, to R-IPAddr2, UDP, src I-Port,
dst 1701
Inbound-2: From Any-Addr, to R-IPAddr1, UDP, src Any-Port,
dst 1701
If the responder chooses not to move to a new port number, the L2TP
tunnel setup can now complete.
4.2.4. Responder chooses new Port Number
The responder MAY choose a new UDP source port to use for L2TP tunnel
traffic. This decision MUST be made before sending the SCCRP. If a
new port number is chosen, then L2TP must inject new filters into the
IPsec filter database. The responder must start new IKE phase 2
negotiations with the initiator.
The final filter set at the initiator and responder is as follows.
Initiator Filters:
Outbound-1: From I-IPAddr, to R-IPAddr, UDP, src I-Port, dst
R-Port
Outbound-2: From I-IPAddr, to R-IPAddr, UDP, src I-Port, dst
1701
Inbound-1: From R-IPAddr, to I-IPAddr, UDP, src R-Port, dst
I-Port
Inbound-2: From R-IPAddr, to I-IPAddr, UDP, src 1701, dst
I-Port
Inbound-3: From R-IPAddr, to I-IPAddr, UDP, src Any-Port, dst
I-Port
The Inbound-1 filter for the initiator will be injected by IKE
upon successful completion of the phase 2 negotiations
initiated by the peer.
Responder Filters:
Outbound-1: From R-IPAddr, to I-IPAddr, UDP, src R-Port, dst
I-Port
Outbound-2: From R-IPAddr, to I-IPAddr, UDP, src 1701, dst
I-Port
Inbound-1: From I-IPAddr, to R-IPAddr, UDP, src I-Port, dst
R-Port
Inbound-2: From I-IPAddr, to R-IPAddr, UDP, src I-Port, dst
1701
Inbound-3: From Any-Addr, to R-IPAddr1, UDP, src Any-Port, dst
1701
Once the negotiations have completed, the SCCRP is sent and the L2TP
tunnel can complete establishment. After the L2TP tunnel has been
established, any residual SAs and their associated filters may be
deleted.
4.2.5. Gateway-gateway and L2TP Dial-out considerations
In the gateway-gateway or the L2TP dial-out scenario, either side may
initiate L2TP. The process outlined in the previous steps should be
followed with one addition. The initial filter set at both sides
MUST include the following filter:
Inbound Filter:
1: From Any-Addr, to R-IPAddr1, UDP, src Any-Port, dst 1701
When either peer decides to start a tunnel, L2TP should inject the
necessary inbound and outbound filters to protect the SCCRQ. Tunnel
establishment then proceeds exactly as stated in the previous
sections.
5. Security Considerations
5.1. Authentication issues
IPsec IKE negotiation MUST negotiate an authentication method
specified in the IKE RFC2409 [7]. In addition to IKE
authentication, L2TP implementations utilize PPP authentication
methods, such as those described in [15]-[16]. In this section, we
discuss authentication issues.
5.1.1. Differences between IKE and PPP authentication
While PPP provides initial authentication, it does not provide per-
packet authentication, integrity or replay protection. This implies
that the identity verified in the initial PPP authentication is not
subsequently verified on reception of each packet.
With IPsec, when the identity asserted in IKE is authenticated, the
resulting derived keys are used to provide per-packet authentication,
integrity and replay protection. As a result, the identity verified
in the IKE conversation is subsequently verified on reception of each
packet.
Let us assume that the identity claimed in PPP is a user identity,
while the identity claimed within IKE is a machine identity. Since
only the machine identity is verified on a per-packet basis, there is
no way to verify that only the user authenticated within PPP is using
the tunnel. In fact, IPsec implementations that only support machine
authentication typically have no way to enforce traffic segregation.
As a result, where machine authentication is used, once an L2TP/IPsec
tunnel is opened, any user on a multi-user machine will typically be
able to send traffic down the tunnel.
If the IPsec implementation supports user authentication, this
problem can be averted. In this case, the user identity asserted
within IKE will be verified on a per-packet basis. In order to
provide segregation of traffic between users when user authentication
is used, the client MUST ensure that only traffic from that
particular user is sent down the L2TP tunnel.
5.1.2. Certificate authentication in IKE
When X.509 certificate authentication is chosen within IKE, the LNS
is eXPected to use an IKE Certificate Request Payload (CRP) to
request from the client a certificate issued by a particular
certificate authority or may use several CRPs if several certificate
authorities are trusted and configured in its IPsec IKE
authentication policy.
The LNS SHOULD be able to trust several certificate authorities in
order to allow tunnel client end-points to connect to it using their
own certificate credential from their chosen PKI. Client and server
side certificate revocation list checking MAY be enabled on a per-CA
basis, since differences in revocation list checking exist between
different PKI providers.
L2TP implementations MAY use dynamically assigned ports for both
source and destination ports only if security for each source and
destination port combination can be successfully negotiated by IKE.
5.1.3. Machine versus user certificate authentication in IKE
The certificate credentials provided by the L2TP client during the
IKE negotiation MAY be those of the machine or of the L2TP user.
When machine authentication is used, the machine certificate is
typically stored on the LAC and LNS during an enrollment process.
When user certificates are used, the user certificate can be stored
either on the machine or on a smartcard.
Since the value of a machine certificate is inversely proportional to
the ease with which an attacker can oBTain one under false pretenses,
it is advisable that the machine certificate enrollment process be
strictly controlled. For example, only administrators may have the
ability to enroll a machine with a machine certificate.
While smartcard certificate storage lessens the probability of
compromise of the private key, smartcards are not necessarily
desirable in all situations. For example, some organizations
deploying machine certificates use them so as to restrict use of
non-approved hardware. Since user authentication can be provided
within PPP (keeping in mind the weaknesses described earlier),
support for machine authentication in IPsec makes it is possible to
authenticate both the machine as well as the user.
In circumstances in which this dual assurance is considered valuable,
enabling movement of the machine certificate from one machine to
another, as would be possible if the machine certificate were stored
on a smart card, may be undesirable.
Similarly, when user certificate are deployed, it is advisable for
the user enrollment process to be strictly controlled. If for
example, a user password can be readily used to obtain a certificate
(either a temporary or a longer term one), then that certificate has
no more security value than the password. To limit the ability of an
attacker to obtain a user certificate from a stolen password, the
enrollment period can be limited, after which password access will be
turned off. Such a policy will prevent an attacker obtaining the
password of an unused account from obtaining a user certificate once
the enrollment period has expired.
5.1.4. Pre-shared keys in IKE
Use of pre-shared keys in IKE main mode is vulnerable to man-in-the-
middle attacks when used in remote access situations. In main mode
it is necessary for SKEYID_e to be used prior to the receipt of the
identification payload. Therefore the selection of the pre-shared
key may only be based on information contained in the IP header.
However, in remote access situations, dynamic IP address assignment
is typical, so that it is often not possible to identify the required
pre-shared key based on the IP address.
Thus when pre-shared keys are used in remote access scenarios, the
same pre-shared key is shared by a group of users and is no longer
able to function as an effective shared secret. In this situation,
neither the client nor the server identifies itself during IKE phase
1; it is only known that both parties are a member of the group with
knowledge of the pre-shared key. This permits anyone with access to
the group pre-shared key to act as a man-in-the-middle.
This vulnerability does not occur in aggressive mode since the
identity payload is sent earlier in the exchange, and therefore the
pre-shared key can be selected based on the identity. However, when
aggressive mode is used the user identity is exposed and this is
often considered undesirable.
As a result, where main mode is used with pre-shared keys, unless PPP
performs mutual authentication, the server is not authenticated.
This enables a rogue server in possession of the group pre-shared key
to successfully masquerade as the LNS and mount a dictionary attack
on legacy authentication methods such as CHAP [15]. Such an attack
could potentially compromise many passwords at a time. This
vulnerability is present in some existing IPsec tunnel mode
implementations.
To avoid this problem, L2TP/IPsec implementations SHOULD NOT use a
group pre-shared key for IKE authentication to the LNS. IKE pre-
shared authentication key values SHOULD be protected in a manner
similar to the user's account password used by L2TP.
5.2. IPsec and PPP security interactions
When L2TP is protected with IPsec, both PPP and IPsec security
services are available. Which services are negotiated depends on
whether the tunnel is compulsory or voluntary. A detailed analysis
of voluntary and compulsory tunneling scenarios is included below.
These scenarios are non-normative and do not create requirements for
an implementation to be L2TP security compliant.
In the scenarios below, it is assumed that both L2TP clients and
servers are able to set and get the properties of IPsec security
associations, as well as to influence the IPsec security services
negotiated. Furthermore, it is assumed that L2TP clients and servers
are able to influence the negotiation process for PPP encryption and
compression.
5.2.1. Compulsory tunnel
In the case of a compulsory tunnel, the client sends PPP frames to
the LAC, and will typically not be aware that the frames are being
tunneled, nor that any security services are in place between the LAC
and LNS. At the LNS, a data packet will arrive, which includes a PPP
frame encapsulated in L2TP, which is itself encapsulated in an IP
packet. By obtaining the properties of the Security Association set
up between the LNS and the LAC, the LNS can obtain information about
security services in place between itself and the LAC. Thus in the
compulsory tunneling case, the client and the LNS have unequal
knowledge of the security services in place between them.
Since the LNS is capable of knowing whether confidentiality,
authentication, integrity and replay protection are in place between
itself and the LAC, it can use this knowledge in order to modify its
behavior during PPP ECP [10] and CCP [9] negotiation. Let us assume
that LNS confidentiality policy can be described by one of the
following terms: "Require Encryption," "Allow Encryption" or
"Prohibit Encryption." If IPsec confidentiality services are in
place, then an LNS implementing a "Prohibit Encryption" policy will
act as though the policy had been violated. Similarly, an LNS
implementing a "Require Encryption" or "Allow Encryption" policy will
act as though these policies were satisfied, and would not mandate
use of PPP encryption or compression. This is not the same as
insisting that PPP encryption and compression be turned off, since
this decision will depend on client policy.
Since the client has no knowledge of the security services in place
between the LAC and the LNS, and since it may not trust the LAC or
the wire between itself and the LAC, the client will typically want
to ensure sufficient security through use of end-to-end IPsec or PPP
encryption/compression between itself and the LNS.
A client wishing to ensure security services over the entire travel
path would not modify this behavior even if it had knowledge of the
security services in place between the LAC and the LNS. The client
negotiates confidentiality services between itself and the LNS in
order to provide privacy on the wire between itself and the LAC. The
client negotiates end-to-end security between itself and the end-
station in order to ensure confidentiality on the portion of the path
between the LNS and the end-station.
The client will typically not trust the LAC and will negotiate
confidentiality and compression services on its own. As a result,
the LAC may only wish to negotiate IPsec ESP with null encryption
with the LNS, and the LNS will request replay protection. This will
ensure that confidentiality and compression services will not be
duplicated over the path between the LAC and the LNS. This results
in better scalability for the LAC, since encryption will be handled
by the client and the LNS.
The client can satisfy its desire for confidentiality services in one
of two ways. If it knows that all end-stations that it will
communicate with are IPsec-capable (or if it refuses to talk to non-
IPsec capable end-stations), then it can refuse to negotiate PPP
encryption/compression and negotiate IPsec ESP with the end-stations
instead. If the client does not know that all end-stations it will
contact are IPsec capable (the most likely case), then it will
negotiate PPP encryption/compression. This may result in duplicate
compression/encryption which can only be eliminated if PPP
compression/encryption can be turned off on a per-packet basis. Note
that since the LNS knows that the client's packets are being tunneled
but the client does not, the LNS can ensure that stateless
compression/encryption is used by offering stateless
compression/encryption methods if available in the ECP and CCP
negotiations.
5.2.2. Voluntary tunnel
In the case of a voluntary tunnel, the client will be send L2TP
packets to the NAS, which will route them to the LNS. Over a dialup
link, these L2TP packets will be encapsulated in IP and PPP.
Assuming that it is possible for the client to retrieve the
properties of the Security Association between itself and the LNS,
the client will have knowledge of any security services negotiated
between itself and the LNS. It will also have knowledge of PPP
encryption/compression services negotiated between itself and the
NAS.
From the LNS point of view, it will note a PPP frame encapsulated in
L2TP, which is itself encapsulated in an IP packet. This situation
is identical to the compulsory tunneling case. If LNS retrieves the
properties of the Security Association set up between itself and the
client, it can be informed of the security services in place between
them. Thus in the voluntary tunneling case, the client and the LNS
have symmetric knowledge of the security services in place between
them.
Since the LNS is capable of knowing whether confidentiality,
authentication, integrity check or replay protection is in place
between the client and itself, it is able to use this knowledge to
modify its PPP ECP and CCP negotiation stance. If IPsec
confidentiality is in place, the LNS can behave as though a "Require
Encryption" directive had been fulfilled, not mandating use of PPP
encryption or compression. Typically the LNS will not insist that
PPP encryption/compression be turned off, instead leaving this
decision to the client.
Since the client has knowledge of the security services in place
between itself and the LNS, it can act as though a "Require
Encryption" directive had been fulfilled if IPsec ESP was already in
place between itself and the LNS. Thus, it can request that PPP
encryption and compression not be negotiated. If IP compression
services cannot be negotiated, it will typically be desirable to turn
off PPP compression if no stateless method is available, due to the
undesirable effects of stateful PPP compression.
Thus in the voluntary tunneling case the client and LNS will
typically be able to avoid use of PPP encryption and compression,
negotiating IPsec Confidentiality, Authentication, and Integrity
protection services instead, as well as IP Compression, if available.
This may result in duplicate encryption if the client is
communicating with an IPsec-capable end-station. In order to avoid
duplicate encryption/compression, the client may negotiate two
Security Associations with the LNS, one with ESP with null
encryption, and one with confidentiality/compression. Packets going
to an IPsec- capable end-station would run over the ESP with null
encryption security association, and packets to a non-IPsec capable
end-station would run over the other security association. Note that
many IPsec implementations cannot support this without allowing L2TP
packets on the same tunnel to be originated from multiple UDP ports.
This requires modifications to the L2TP specification.
Also note that the client may wish to put confidentiality services in
place for non-tunneled packets traveling between itself and the NAS.
This will protect the client against eavesdropping on the wire
between itself and the NAS. As a result, it may wish to negotiate
PPP encryption and compression with the NAS. As in compulsory
tunneling, this will result in duplicate encryption and possibly
compression unless PPP compression/encryption can be turned off on a
per-packet basis.
6. References
[1] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G., and
B. Palter, "Layer Two Tunneling Protocol L2TP", RFC2661,
August 1999.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.
[3] Kent, S. and R. Atkinson, "IP Authentication Header", RFC2402,
November 1998.
[4] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
(ESP)", RFC2406, November 1998.
[5] Piper, D., "The Internet IP Security Domain of Interpretation
of ISAKMP", RFC2407, November 1998.
[6] Atkinson, R. and S. Kent, "Security Architecture for the
Internet Protocol", RFC2401, November 1998.
[7] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
RFC2409, November 1998.
[8] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC
1661, July 1994.
[9] Rand, D., "The PPP Compression Control Protocol (CCP)", RFC
1962, June 1996.
[10] Meyer, G., "The PPP Encryption Control Protocol (ECP)", RFC
1968, June 1996.
[11] Sklower, K. and G. Meyer, "The PPP DES Encryption Protocol
(DESE)", RFC1969, June 1996.
[12] Sklower, K. and G. Meyer, "The PPP DES Encryption Protocol,
Version 2 (DESE-bis)", RFC2419, September 1998.
[13] Hummert, K., "The PPP Triple-DES Encryption Protocol (3DESE)",
RFC2420, September 1998.
[14] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
2246, November 1998.
[15] Simpson, W., "PPP Challenge Handshake Authentication Protocol
(CHAP)," RFC1994, August 1996.
[16] Blunk, L. and J. Vollbrecht, "PPP Extensible Authentication
Protocol (EAP)," RFC2284, March 1998.
[17] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC2373, July 1998.
Acknowledgments
Thanks to Gurdeep Singh Pall, David Eitelbach, Peter Ford, and Sanjay
Anand of Microsoft, John Richardson of Intel and Rob Adams of Cisco
for useful discussions of this problem space.
Authors' Addresses
Baiju V. Patel
Intel Corp
2511 NE 25th Ave
Hillsboro, OR 97124
Phone: +1 503 702 2303
EMail: baiju.v.patel@intel.com
Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
Phone: +1 425 706-6605
EMail: bernarda@microsoft.com
William Dixon
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
Phone: +1 425 703 8729
EMail: wdixon@microsoft.com
Glen Zorn
Cisco Systems, Inc.
500 108th Avenue N.E., Suite 500
Bellevue, Washington 98004
Phone: +1 425 438 8218
Fax: +1 425 438 1848
EMail: gwz@cisco.com
Skip Booth
Cisco Systems
7025 Kit Creek Road
RTP, NC 27709
Phone: +1 919 392 6951
EMail: ebooth@cisco.com
Appendix A: Example IPsec Filter sets for L2TP Tunnel Establishment
This section provides examples of IPsec filter sets for L2TP tunnel
establishment. While example filter sets are for IPv4, similar
examples could just as easily be constructed for IPv6.
A.1 Initiator and Responder use fixed addresses and ports
This is the most simple of the cases since nothing changes during
L2TP tunnel establishment. Since the initiator does not know whether
the responder will change its port number, it still must be prepared
for this case. In this example, the initiator will use an IPv4
address of 1.1.1.1 and the responder will use an IPv4 address of
2.2.2.1.
The filters for this scenario are:
A.1.1 Protect the SCCRQ
Initiator Filters:
Outbound-1: From 1.1.1.1, to 2.2.2.1, UDP, src 1701, dst 1701
Inbound-1: From 2.2.2.1, to 1.1.1.1, UDP, src 1701, dst 1701
Inbound-2: From 2.2.2.1, to 1.1.1.1, UDP, src Any-Port, dst 1701
Responder Filters:
Outbound-1: None, dynamically injected when IKE Phase 2 completes
Inbound-1: From Any-Addr, to 2.2.2.1, UDP, src Any-Port, dst 1701
After IKE Phase 2 completes the filters at the initiator and
responder will be:
Initiator Filters:
Outbound-1: From 1.1.1.1, to 2.2.2.1, UDP, src 1701, dst 1701
Inbound-1: From 2.2.2.1, to 1.1.1.1, UDP, src 1701, dst 1701
Inbound-2: From 2.2.2.1, to 1.1.1.1, UDP, src Any-Port, dst 1701
Responder Filters:
Outbound-1: From 2.2.2.1, to 1.1.1.1, UDP, src 1701, dst 1701
Inbound-1: From 1.1.1.1, to 2.2.2.1, UDP, src 1701, dst 1701
Inbound-2: From Any-Addr, to 2.2.2.1, UDP, src Any-Port, dst 1701
A.2 Gateway to Gateway Scenario where Initiator and Responder use
dynamic ports
In this scenario either side is allowed to initiate the tunnel.
Since dynamic ports will be used, an extra phase 2 negotiation must
occur to protect the SCCRP sent from the responder to the initiator.
Other than the additional phase 2 setup, the only other difference is
that L2TP on the responder must inject an additional filter into the
IPsec database once the new port number is chosen.
This example also shows the additional filter needed by the initiator
which allows either side to start the tunnel. In either the dial-out
or the gateway to gateway scenario this additional filter is
required.
For this example, assume the dynamic port given to the initiator is
5000 and his IP address is 1.1.1.1. The responder will use an IP
address of 2.2.2.1 and a port number of 6000.
The filters for this scenario are:
A.2.1 Initial Filters to allow either side to respond to negotiations
In this case both peers must be able to accept phase 2 negotiations
to from L2TP peers. My-IPAddr is defined as whatever IP address the
device is willing to accept L2TP negotiations on.
Responder Filters present at both peers:
Inbound-1: From Any-Addr, to My-IPAddr, UDP, src Any-Port, dst 1701
Note: The source IP in the inbound-1 filter above for gateway to
gateway tunnels can be IP specific, such as 1.1.1.1, not necessarily
Any-Addr.
A.2.2 Protect the SCCRQ, one peer is now the initiator
Initiator Filters:
Outbound-1: From 1.1.1.1, to 2.2.2.1, UDP, src 5000, dst 1701
Inbound-1: From 2.2.2.1, to 1.1.1.1, UDP, src 1701, dst 5000
Inbound-2: From 2.2.2.1, to 1.1.1.1, UDP, src Any-Port, dst 5000
Inbound-3: From Any-Addr, to 1.1.1.1, UDP, src Any-Port, dst 1701
Responder Filters:
Outbound-1: None, dynamically injected when IKE Phase 2 completes
Inbound-1: From Any-Addr, to 2.2.2.1, UDP, src Any-Port, dst 1701
After IKE Phase 2 completes the filters at the initiator and
responder will be:
Initiator Filters:
Outbound-1: From 1.1.1.1, to 2.2.2.1, UDP, src 5000, dst 1701
Inbound-1: From 2.2.2.1, to 1.1.1.1, UDP, src 1701, dst 5000
Inbound-2: From 2.2.2.1, to 1.1.1.1, UDP, src Any-Port, dst 5000
Inbound-3: From Any-Addr, to 1.1.1.1, UDP, src Any-Port, dst 1701
Responder Filters:
Outbound-1: From 2.2.2.1, to 1.1.1.1, UDP, src 1701, dst 5000
Inbound-1: From 1.1.1.1, to 2.2.2.1, UDP, src 5000, dst 1701
Inbound-2: From Any-Addr, to 2.2.2.1, UDP, src Any-Port, dst 1701
A.2.3 Protect the SCCRP after port change
At this point the responder knows which port number it is going to
use. New filters should be injected by L2TP to reflect this new port
assignment.
The new filter set at the responder is:
Responder Filters:
Outbound-1: From 2.2.2.1, to 1.1.1.1, UDP, src 6000, dst 5000
Outbound-2: From 2.2.2.1, to 1.1.1.1, UDP, src 1701, dst 5000
Inbound-1: From 1.1.1.1, to 2.2.2.1, UDP, src 5000, dst 6000
Inbound-2: From 1.1.1.1, to 2.2.2.1, UDP, src 5000, dst 1701
Inbound-3: From Any-Addr, to 2.2.2.1, UDP, src Any-Port, dst 1701
The second phase 2 will start once L2TP sends the SCCRP. Once the
phase 2 negotiations complete, the new filter set at the initiator
and the responder will be:
Initiator Filters:
Outbound-1: From 1.1.1.1, to 2.2.2.1, UDP, src 5000, dst 6000
Outbound-2: From 1.1.1.1, to 2.2.2.1, UDP, src 5000, dst 1701
Inbound-1: From 2.2.2.1, to 1.1.1.1, UDP, src 6000, dst 5000
Inbound-2: From 2.2.2.1, to 1.1.1.1, UDP, src 1701, dst 5000
Inbound-3: From 2.2.2.1, to 1.1.1.1, UDP, src Any-Port, dst 1701
Responder Filters:
Outbound-1: From 2.2.2.1, to 1.1.1.1, UDP, src 6000, dst 5000
Outbound-2: From 2.2.2.1, to 1.1.1.1, UDP, src 1701, dst 5000
Inbound-1: From 1.1.1.1, to 2.2.2.1, UDP, src 5000, dst 6000
Inbound-2: From 1.1.1.1, to 2.2.2.1, UDP, src 5000, dst 1701
Inbound-3: From Any-Addr, to 2.2.2.1, UDP, src Any-Port, dst 1701
Once the L2TP tunnel has been successfully established, the original
phase 2 may be deleted. This allows the Inbound-2 and Outbound-2
filter statements to be removed as well.
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