HIPRG H. Tschofenig
Internet-Draft A. Nagarajan
Expires: August 25, 2005 Siemens
V. Torvinen
J. Ylitalo
Ericsson
M. Shanmugam
Siemens
February 21, 2005
NAT and Firewall Traversal for HIP
draft-tschofenig-hiprg-hip-natfw-traversal-01.txt
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
The Host Identity Protocol is a signaling protocol which adds another
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layer to the Internet model and establishes IPsec ESP SAs to protect
subsequent data traffic. HIP also aims to interwork with middleboxes
(such as NATs and Firewalls). This document investigates this aspect
in more detail.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 5
4. Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.1 Same Firewall at Initiator for both outgoing and
incoming packets . . . . . . . . . . . . . . . . . . . . . 8
4.2 Different Firewalls at Initiator for outgoing and
incoming packets . . . . . . . . . . . . . . . . . . . . . 9
4.3 Different Firewalls at Initiator and Receiver . . . . . . 11
5. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6. Solution Approach . . . . . . . . . . . . . . . . . . . . . . 15
6.1 Flow identifier interception . . . . . . . . . . . . . . . 15
6.2 Sender Invariance . . . . . . . . . . . . . . . . . . . . 16
6.3 Authentication and Authorization . . . . . . . . . . . . . 17
6.3.1 What is SPKI? . . . . . . . . . . . . . . . . . . . . 17
6.3.2 SAML Usage in HIP . . . . . . . . . . . . . . . . . . 18
6.3.3 SPKI usage for HIP . . . . . . . . . . . . . . . . . . 20
6.3.4 Authentication and authorization for Base Exchange . . 20
6.3.5 Authentication and authorization for Readdressing . . 24
7. Security Considerations . . . . . . . . . . . . . . . . . . . 27
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 28
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
9.1 Normative References . . . . . . . . . . . . . . . . . . . 29
9.2 Informative References . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 30
Intellectual Property and Copyright Statements . . . . . . . . 32
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1. Introduction
An IP address serves the dual role of a locator and an identifier for
every host on the Internet. End systems that use IP addresses as
identifiers cannot support dynamic changes in the mapping between the
identifier and the locator in case of multi-homing and mobility.
The Host Identity Protocol (HIP) [1] proposes to separate the
identifier from the locator by adding an additional layer between the
transport layer and the network layer. The transport layer uses a
new, mobility-unrelated identifier, Host Identity Tags (HITs), in
place of IP addresses, while the network layer uses conventional IP
addresses. IPsec security associations are bound to the HITs and are
not modified with IP address changes. In other words, a host despite
being mobile or multi-homed can use a single transport layer
connection associated to one HIT and multiple IP addresses.
One of the integral features of HIP protocol is, it establishes IPsec
ESP which are subsequently used to encrypt data traffic between the
two end hosts. HIP being a mobility protocol also supports changes
in IP addresses. Because of this, HIP is liable to all known
incompatibilities of IPsec with middleboxes as NATs [3] and
firewalls. This draft investigates problems with the HIP protocol
when supporting the secure traversal of NATs and Firewalls.
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [2].
This draft used the terminology defined in [4], [1] and [5] and [6].
The term SPI refers to the Security Parameter Index value used in
IPsec packets. The initiator selects one SPI(I) which is then used
by the responder to create an IPsec packet (ESP packet in this case)
for traffic sent to the initiator. The responder selects one SPI(R)
which is used by the initiator to encrypt all data sent to the
responder.
Other relevant abbreviations can be found in [1].
The concept of a flow identifier is described in [7].
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3. Problem Statement
This version of the document assumes that the data traffic following
the HIP base exchange is IPsec protected. Besides the communicating
hosts, the entities such as NATs and Firewalls play a major role to
allow the packets to traverse through them. The NAT traversal
approach described in [8] and [9] allows the NAT to be detected and
IPsec protected packets to experience UDP encapsulation (see also
[10] with regard to UDP encapsulation). HIP also provides a way to
deal with legacy NATs, as described in [11]. To support this
functionality, it is necessary to provide UDP encapsulation for both
HIP signaling and IPsec packets. Legacy NAT traversal does not
require NATs to be HIP aware or to understand the HIP message
exchange. HIP, however, aims to interact with middleboxes actively
whereby these devices need to understand the HIP protocol and they
need to be involved in the protocol exchange.
In the context of middlebox signaling a few goals can be
accomplished:
o Add some authentication and authorization capabilities to NAT
traversal. Many NAT/Firewall traversal solutions do not allow the
end host to interact with the middlebox. As a consequence, some
security vulnerabilities are introduced.
o Add secure firewall traversal functionality as another type of
middlebox signaling by using <destination IP address, SPI and
protocol> triplet. as a substitute for the typical < source IP,
destination IP, source port, destination port, transport protocol>
information.
The HIP protocol is a signaling protocol that carries (what NSIS
calls a flow identifier) inside the signaling protocol payloads.
Since HIP uses IPsec ESP to encrypt all its payload messages, the
flow identifier takes the shape of a <destination IP address, SPI and
ESP>. Although HIP is described as a two-party protocol, middle
boxes are supposed to intercept these messages in order to learn the
flow identifier and to process them correctly. In other words, a
multi party protocol is created such that the flow identifier is
available to middle boxes between the HIP hosts. To provide proper
security, middleboxes should not be subject to denial of service
attacks and might want to authenticate or authorize entities which
create state. Note that the IPsec SA is unidirectional and therefore
two IPsec SAs (with two different SPIs) have to be established.
Figure 1 shows the HIP base exchange traversing a NAT.
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I1 +-----------+ I1
+-------------------->| |----------------------+
| | | |
| | | |
R1 | Intercept | R1 v
+---------+ <-------------| the flow |<---------------- +---------+
|Initiator| I2 | identifer | I2 |Responder|
+---------+ ------------->| <Dest IP, |----------------> +---------+
^ | SPI,ESP> |
| | | |
| R2 | | R2 |
+---------------------| |<----------------------+
+-----------+
NAT
Figure 1: NAT and HIP Base Exchange
Subsequently, the HIP base exchange is described in more detail.
I -> R: I1: Trigger exchange
I <- R: R1: {Puzzle, D-H(R), HI(R), ESP Transform,
HIP Transform }SIG
I -> R: I2: {Solution, LSI(I), SPI(I), D-H(I),
ESP Transform, HIP Transform, {H(I)}SK }SIG
I <- R: R2: {LSI(R), SPI(R), HMAC}SIG
A potential responsibility of the NAT, as shown in Figure 1, can be
the following
o Intercept the signaling messages
o Authenticate and authorize the HIP nodes by verifying the
signatures.
o Process the flow identifier information
o Perform actions according to the state machine
o Create state based on the content of message I2 (SPI(I)) and R2
(SPI(R)). Additionally, it might be necessary to include support
for storing the respective HITs and host identities.
If HIP should also consider firewall traversal then the routing
asymmetry needs to be looked into and the fact that the messages I1
and I2 do not necessarily traverse the same devices as R1 and R2.
The same is true with more complex network topologies with a mixture
of NATs and Firewalls. This is an assumption made in the NSIS
working group (and therefore also with NAT/Firewall traversal). Pure
NAT traversal is therefore simpler to handle in comparison to
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middlebox traversal which also includes devices such as Firewalls.
Figure 3 shows this circumstance graphically:
I1 +----------+ I1
+--------------------> | Firewall | -----------------------+
| I2 | 1 | I2 |
| +-----------------> | | ------------------+ |
| | +----------+ v v
+---------+ +---------+
|Initiator| |Responder|
+---------+ +---------+
^ ^ R1 +----------+ R1 | |
| +------------------ | Firewall | <-------------------+ |
| R2 | 2 | R2 |
+--------------------- | | <-----------------------+
+----------+
............... IPsec ESP protected traffic (SPI(R)).........>
<.............. IPsec ESP protected traffic (SPI(I))..........
Legend:
--- = HIP signaling
... = IPsec protected data traffic
Figure 3: NAT and HIP Base Exchange
With one single NAT between the HIP nodes, all messages of the base
exchange are forced through it. With firewalls, it becomes obvious
that the nice property of a NAT with respect to the symmetric
forwarding path is lost and the individual firewalls (Firewall 1 and
Firewall 2) are unable to create the necessary firewall pinholes.
SPI(I) is exchanged in I2 message through firewall 1, however
firewall 2 only needs it. Similarly firewall 2 needs SPI (R) which
is sent in message R2 through firewall 1.
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4. Scenarios
The following section describes some sample scenarios and the
possible solutions to learn the flow identifier:
4.1 Same Firewall at Initiator for both outgoing and incoming packets
This scenario assumes that the initiator I alone is behind a firewall
named FW(I). This firewall is both for the outgoing and incoming
packets and hence can look into all the base exchange messages. The
FW(I) is expected to authenticate and authorize the initiator to send
out going packets, receiver if necessary to let incoming packets and
intercept the flow identifier from the base exchange. With the E2M
messages, it can be achieved as follows. This is illustrated in
Figure 4
FW(I)
I1 +-----+ I1
+----------> | |--------------------------------------+
| I2 | | I2 |
| +-----> | |---------------------------------+ |
| | | | | |
| | | | v v
---------+ | | +--------+
Initiator| | | |Receiver|
---------+ | | +--------+
^ ^ | |
| | R2 | | R2 | |
| +------ | |< --------------------------------+ |
| R1 | | R1 |
+---------- | |< -------------------------------------+
+-----+
Figure 4: One FW only at initiator end
1. I1 packet is sent from the initiator I to receiver R.
2. FW(I) drops the packet and sends a R1' message back to I. This
is the End host-to-Middlebox or E2M message exchange initiation.
3. I sends I2' message with CERT(I) parameters to FW(I). It
requests the FW(I) to open up a pinhole.
4. FW verifies SPKI certificate and the signature of I.
Accordingly, it either sends a R2' to acknowledge I that it can
continue with the base exchange with message I1 or drops packet
if verification fails.
5. On receiving R2',I sends message I1 to R again. Now the FW(I)
will let the packet through.
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6. R sends the message R1 to I.
7. On receiving R1, if FW(I) wishes to authenticate/authorize the
receiver R, it should initiate E2M exchange here. It sends
message R1'to R forcing R to send an I2' in exchange.
8. R sends the CERT(R) parameter in I2'.
9. FW verifies SPKI certificate and the signature of R.
Accordingly, it either sends a R2' to acknowledge R that it can
continue with the base exchange with message R1 or drops packet
if verification fails.
10. On receiving R2', R sends message R1 to I again. Now the FW(I)
will let it through.
11. The base exchange continues until complete. Since all messages
I1,R1,I2 and R2 traverse through FW(I), it can look into these
messages to learn the flow identifier information.
4.2 Different Firewalls at Initiator for outgoing and incoming packets
This scenario assumes that the initiator I alone is behind firewalls
named FW1(I) and FW2(I).FW1(I) is for the incoming packets to I and
FW2(I) is for the outgoing packets to R. The FW(I) is expected to
authenticate and authorize the initiator to send out going packets,
while FW2(I) would authenticate and authorize the receiver, if
necessary to let incoming packets. It is sufficient that FW2(I)
alone learns the flow identifier information of I. It should store
the state <SPI(I),IP(I),HIT(I)> to forward IPsec protected payload
packets. This scenario is illustrated in Figure 5
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FW1(I)
I1 +-----+ I1
+----------> | |--------------------------------------+
| I2 | | I2 |
| +-----> | |---------------------------------+ |
| | +-----+ | |
| | v v
+---------+ +--------+
|Initiator| |Receiver|
+---------+ FW2(I) +--------+
^ ^ +-----+
| | R2 | | R2 | |
| +------ | |< --------------------------------+ |
| R1 | | R1 |
+---------- | |< -------------------------------------+
+-----+
Figure 5: Two FWs at initiator's end
1. I1 packet is sent from the initiator I to receiver R.
2. FW1(I) drops the packet and sends a R1' message back to I. This
is the E2M message exchange initiation.
3. I sends I2' message with CERT(I) parameters to FW1(I). It
requests the FW1(I) to open up a pinhole.
4. FW1(I) verifies SPKI certificate and the signature of I.
Accordingly, it either sends a R2' to acknowledge I that it can
continue with the base exchange with message I1 or drops packet
if verification fails.
5. On receiving R2',I sends message I1 to R again. Now the FW1(I)
will let the packet through.
6. R sends the message R1 to I.
7. On receiving R1, if FW2(I) wishes to authenticate/authorize the
receiver R, it should initiate E2M exchange here. It sends
message R1'to R forcing R to send an I2' in exchange.
8. R sends the CERT(R) parameter in I2'.
9. FW2(I) verifies SPKI certificate and the signature of R.
Accordingly, it either sends a R2' to acknowledge R that it can
continue with the base exchange with message R1 or drops packet
if verification fails.
10. On receiving R2', R sends message R1 to I again. Now the FW2(I)
will let it through.
11. Since FW2(I) needs the store the state, once the base exchange
is complete, the initiator should inform the FW2(I) about the
SPI it has chosen for the exchange. This way, FW2(I) can
forward further IPsec payload packets from R to I
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4.3 Different Firewalls at Initiator and Receiver
This scenario looks into a more complicated situation. Initiator I
is behind multiple firewalls FW1(I) for outgoing packets and FW2(I)
and FW3(I) are for incoming packets. Similarly, receiver R is behind
FW1(R) and FW2(R) for incoming packets and FW3(R) for outgoing
packets. The incoming firewalls are chosen based on the type of the
application and the hosts are unaware from which firewall they
receive packets. Here, however for our scenario we assume that
FW2(R) and FW2(I) are chosen about which also the hosts are unaware
of. The FW1(I) is expected to authenticate and authorize the
initiator to send outgoing packets to R, while FW2(R) would
authenticate and authorize the receiver to let outgoing packets to I.
FW2(R) should store the state <SPI(R),IP(R),HIT(R)> for the receiver
while FW2(I) should store the state <SPI(I),IP(I),HIT(I)> for the
initiator. This scenario is illustrated in Figure 6
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+-----+
| |
|FW1-R|
| |
+-----+ +-----+
I1 | | I1 +-----+
+------------| | -------------------> | |---------+
| I2 |FW1-I| I2 |FW2-R| |
| +-------| | -------------------> | |----+ |
| | | | +-----+ | |
| | +-----+ v v
+---------+ +--------+
|Initiator| |Receiver|
+---------+ +--------+
^ ^ +-----+
| | R2 | | R2 +-----+ | |
| +------ |FW2-I| <--------------------| |-----+ |
| R1 | | R1 |FW3-R| |
+---------- | | <--------------------| |----------+
+-----+ | |
+-----+ | |
| | +-----+
|FW3-I|
| |
+-----+
Figure 6: Multiple FWs at initiator's and receiver's end
1. I1 packet is sent from the initiator I to receiver R.
2. FW1(I) drops the packet and sends a R1' message back to I. This
is the E2M message exchange initiation.
3. I sends I2' message with CERT(I) parameters to FW1(I). It
requests the FW1(I) to open up a pinhole.
4. FW1(I) verifies SPKI certificate and the signature of I.
Accordingly, it either sends a R2' to acknowledge I that it can
continue with the base exchange with message I1 or drops packet
if verification fails.
5. On receiving R2',I sends message I1 to R again. Now the FW1(I)
will let the packet through.
6. This packet would reach FW2(R). If this firewall wishes to
authenticate and authorize the initiator I, then it can start a
E2M exchange with I. After this is successfully completed,
FW2(R) would open up a pinhole to send packets to R.
7. R sends the message R1 to I.
8. When R sends R1 to I, FW3(R) would initiate a E2M message to
authenticate and authorize the receiver R. After this is
complete, it will forward the packet to the initiator. On
receiving R1, if FW2(I) wishes to authenticate/authorize the
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receiver R, it should initiate E2M exchange here.
9. FW2(I) verifies SPKI certificate and the signature of R.
Accordingly, it either sends a R2' to acknowledge R that it can
continue with the base exchange with message R1 or drops packet
if verification fails.
10. On receiving R2', R sends message R1 to I again. Now the FW2(I)
will let it through.
11. This has completed only one round of authentication and
authorization. However, the states are still not established at
the firewalls. For this, the hosts have to signal their
incoming firewalls about the SPI that they have chosen for IPsec
ESP packets to follow.
When hosts are behind multiple incoming firewalls, there are uble to
decide to which firewall they have to inform their SPI values to.
The first option would be to somehow make the chosen FW to signal the
host about its requirement for a state to forward IPsec protected
packets (similar to a pull model). This could be possibly done along
with the first incoming packet which is R1. R1 packet could include
extra signaling as record route to the initiator. The second option
would be to inform firewall about the SPI values (like the push
model). Here, however it would be necessary to send an extra message
I3 from the initiator to the receiver which would include the SPI(I)
for FW(I) and to resend the SPI(R) in I2 message for FW(R).
The second problem is to secure the SPI signalling message from the
end host to the FW. Since the endhosts authenticate and authorize to
the FW that lets outgoing packets, they share keys only with them.
However, they need to signal the SPI value to the FW on the other end
which forwards incoming packets . For the sake of securing the SPI
value, it might be necessary that the end hosts have to run a E2M
exchange with the firewalls on the receiving end also.
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5. Goals
The main goal of the draft is to find a suitable NAT/FW traversal
solution for the Host Identity Protocol. Such a solution for
HIP-based middlebox signaling has to provide the following
properties:
o A HIP-aware NAT/FW MUST be able to authenticate the entity
requesting a NAT binding or a firewall pinhole.
o A HIP-aware NAT/FW MUST authorize the entity requesting a NAT
binding or a firewall pinhole before storing state information.
This requirement might be accomplished by identity based
authorization or an identity independent authorization mechanism.
o A HIP-aware NAT/FW MUST be able to intercept HIP messages in order
to extract the flow identifier information and other related
information. HIP messages are base exchange messages during
context establishment and readdressing messages during IP address
changes. A NAT/FW MUST be able to distinguish these messages.
o A NAT/FW node MUST NOT introduce new denial of service attacks
based on authentication or key management mechanisms.
o A potential solution MUST respect the property of some middleboxes
which do not allow traffic (data and signaling traffic) to
traverse this middlebox without proper authorization.
Some requirements are taken from [12].
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6. Solution Approach
6.1 Flow identifier interception
The most important issue with the HIP NAT/FW traversal is to make the
flow identifier <destination IP address, SPI and ESP> available to
the middleboxes. In the presence of NATs, we are always sure that
the forward path and the backward path are same, since the NAT forces
the IP packets to flow through these devices. Hence all the 4
messages I1, R1, I2 and R2 traverse through a single NAT. This makes
it possible for the NATs to intercept the messages for the relevant
flow identifier information. But, in the presence of firewalls,
routing asymmetry has to be taken into consideration.
To enable the firewalls intercept the correct mapping triplet < dest
IP, SPI, ESP > certain values have to be resent with the base
exchange messages. This is illustrated in the Figure 7. While the
IP value of the flow identifier can be intercepted from the IP header
of any base exchange message, the SPI value can be intercepted only
in messages I2 and R2. I generates its SPI(I) and sends it to R
through FW-R. However, FW-I needs this information to forward all
packets from R to I. Therefore there has to be someway FW-I can
learn this information. One possible method would be that message R2
could include the SPI(I) value. However, changes to the base
exchange are not desired and we try to keep the base exchange
unaffected. The only other possibility would be that once the base
exchange is complete, the HIP host I could inform the FW-I in its
domain about its SPI(I) value. Similarly, the receiver R could
inform the FW-R local to it about its SPI(R).This way, the firewalls
will be able to learn the SPI values needed to create the state.
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FW-R
+-+
1.I1 | | I1
-------------------------------------> | |--------->
| |
FW-I +-+
+-+
2.R1 | | R1
<---------| | <------------------------------------
| |
+-+ FW-R
+-+
3.I2 | | I2
-------------------------------------->| |--------->
| |
FW-I +-+
+-+
4.R2 | | R2
<---------| | <------------------------------------
| |
+-+
FW-I FW-R
+-+ +-+
5.SPI(I) | | | | 5.SPI(R)
-------->| | | |<--------
| | | |
+-+ +-+
Figure 7: Firewalls and mapping information during Base exchange
6.2 Sender Invariance
The NAT/Firewall HIP node establishes state at possibly several
entities between the HIP Initiator and the HIP Responder. Providing
authentication of the signaling initiator to each individual HIP node
along the path might be possible but not particularly useful, since
the initiator is most likely unknown to some middlebox along the
path. Hence, authentication per se does not solve the security
problem.
With mobility, it might be possible that the intermediate HIP-aware
middlebox need some assurance that a particular node is the allowed
to modify existing state. No other entity should be allowed to
modify state since this would allow certain attacks (such as denial
of service or third party flooding). In some respect this issue is
similar to the authorization property in Mobile IP where the mobility
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binding state established at the CN needs to be protected against
unauthorized modifications.
It seems that the property of "sender Invariance" is required in this
case: "A party is assured that the source of the communication has
remained the same as the one that started the communication, although
the actual identity of the source is not important to the recipient."
This property is particularly important in the context of mobility
which requires a change in the NAT binding or the packet filter. An
outline for solutions have been presented in [13]. SPINAT (see [14]
and [15]) provided innovative aspects by using a hash chain approach
in combination with delayed authorization to secure state
modifications at NAT devices.
A future version of this document will address the aspect of sender
invariance in more details.
6.3 Authentication and Authorization
Before a middlebox can allocate a NAT binding or a pin hole, the HIP
nodes requesting them may need to be authenticated. Middleboxes
could potentially use information stored in the DNS to authenticate
the HIP end points. Since Host Identities are used to identify HIP
nodes, middleboxes can also use signature verification at relevant
HIP messages for authentication. This might raise some issues on
denial of service attacks at the middleboxes and these need to be
determined. Authorization is certainly more important than
authentication particularly since HIP supports ephemeral host
identities as a mechanism to preserve privacy. As such it would be
useful to use identity independent authorization assertions. SPKI
certificates, attribute certificates or similar mechanisms could be
of particular use, especially in cases where the HIP nodes prefer to
remain anonymous.
6.3.1 What is SPKI?
SPKI authorization certificates are used in access control and are
identity independent. Issuing and receiving an SPKI certificate is
completely local to the network domain and there is no need for a
higher certification authority to issue them. For a HIP protocol
this would mean whenever a HIP host wishes to create a NAT binding or
a FW pinhole, it can locally obtain the SPKI certificate for
authorization at middleboxes. The structure of the SPKI certificate
is shown in Figure 8.
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+--------+---------+-----------------+
| Key 1 | Key 2 | |
| Issuer | Receiver| Can delegate ? |
| | | |
+--------+---------+-----------------+
| |
| Rights |
| |
+------------------------------------+
| |
| Dates |Certificate signed
| |by issuer
+------------------------------------+
Figure 8: SPKI certificate structure
o Key 1 is the public key of the certificate issuer.
o Key 2 is the public key of the certificate receiver.
o If a subject gets the right to re-delegate its rights, it can
re-delegate its certificates to other subjects. In addition, he
can freely sign new certificates to other subjects.
o Rights define access control of the receiver.
o Dates define the validity period of the certificate.
o The complete certificate is signed by the private key of the
issuer.
When a subject wishes to use his certificates, it sends a request
that is signed by the subject's private key. Attached are a chain of
certificates that belong not only to it but also to those of its
delegates. When a verifier receives requests along with a chain of
certificates from a subject, the verifier verifies the requests and
the certificates. If the verifier is satisfied with the
certificates, then the requested operation is allowed. Otherwise,
the requests will be refused.
6.3.2 SAML Usage in HIP
Security Assertion Markup Language (SAML) [16] is an XML extension
for security information exchange. It is being developed by OASIS.
SAML enables entities to access resources by providing assertions
which allow authorization.
6.3.2.1 Assertions
An Assertion is a package of information including authentication
statements, attribute statements and authorization decision
statements. All kinds of statements do not have to be present, but
at least one. An Assertion contains several elements:
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Issuing information:
Who issued the assertion, when was it issued and the assertion
identifier.
Subject information:
The name of the subject, the security domain and optional subject
information, like public key.
Conditions under which the assertion is valid:
special kind of conditions like assertion validity period,
audience restriction and target restriction.
Additional advice:
explaining how the assertion was made, for example.
In an authentication statement, an issuing authority asserts that a
certain subject was authenticated by certain means at a certain time.
In an attribute statement, an issuing authority asserts that a
certain subject is associated with certain attributes which has
certain values. For example, user jon@cs.example.com is associated
with the attribute 'Department', which has the value 'Computer
Science'.
In an authorization decision statement, a certain subject with a
certain access type to a certain resource has given certain evidence
that the identity is correct. Based on this, the relying party then
makes the decision on giving access or not. The subject could be a
human or a program, the resource could be a webpage or a web service,
for example.
6.3.2.2 Artifact
The Artifact is a base-64 encoded string which is 40 bytes long. 20
bytes consists of the typecode, which is the source id. The
remaining 20 bytes consists of a 20-byte random number that servers
use to look up an assertion. The entity creating an Assertion stores
it temporarily. The entity performing the authorization decision
uses the received Artifact to retrieve the assertion. The purpose of
the Artifact is to act as a token which references an Assertion.
SAML also defines a request/response protocol for obtaining
Assertions. The request asks for an Assertion or makes queries for
authentication, attribute and authorization decisions. The response
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is carrying back the requested Assertion. The XML format for
protocol messages are defined within an XML schema.
A HIP-aware NAT/Firewall can use this request/response protocol to
fetch assertions from the indicated place.
HIP can use SAML Assertions in CER payloads to provide a mechanism
for HIP end points to authorize them towards middlebox using an
emerging technology. Furthermore, SAML Assertions can be used to
bind the authorization decision of different protocols sessions from
different layers in the ISO-OSI model together. As an example, the
authorization decision by an application layer entity can be used to
bind it to a subsequent HIP exchange. SAML provides a complete
solution for authorization using Artifacts and Assertions and the
corresponding protocols to obtain them. The assertions are based on
XML which allows extensibility beyond the initially envisioned
deployment area.
6.3.3 SPKI usage for HIP
HIP has already defined the CERT parameter that can carry
certificates. The HIP nodes requesting a NAT/FW traversal can send
their base exchange message with the CERT parameter. The CERT will
carry the SPKI certificate and the packet will be signed by the
requesting HIP node. This would mean, messages I2 and R2 should
include the CERT parameter to get them authorized at the middleboxes.
The structure of the SPKI certificate for HIP is shown in Figure 9.
+------+-------+---------------+
| Key1 |HI(I/R)|Can Delegate? |
+------+-------+---------------+
| |
| Rights for NAT/FW traversal |
+------------------------------+
| |
| Date and further info |
+------------------------------+
| |
| Digital Signature |
+------------------------------+
Figure 9: SPKI certificate structure for HIP
6.3.4 Authentication and authorization for Base Exchange
When a HIP host requests a NAT binding or a FW pinhole, it has to be
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first authenticated and authorized by the middleboxes.
Authentication is necessary in many cases,because, in case of
mobility, the middlebox should be authorized to change the flow id
and no other party forge the middlebox to change. Since all HIP
packets are signed using the private keys of the HIP hosts,
middleboxes can verify these packets using the signature
verifications. This, of course, will introduce certain kinds of
denial of service attacks. Denial of service attacks for signature
verification at middleboxes can be prevented by using the client
puzzle concept used by the HIP protocol. For more details the HIP
protocol [1] can be consulted. This will force the middleboxes to
delay state creation and to also delay expensive computational
operations. As explained in the previous sections, we seek to use
non-identity based authorization mechanisms that can be verified by
the middleboxes before creating a NAT binding or FW pinhole. Since
NATs force the outbound and inbound packets to flow through them,
they are much easier to handle. For instance, the mechanism used by
SPINAT [14] can be used for authorization of state modifications by
utilizing hash chains and delayed authentication with NATS. However,
this is not presently suitable for firewalls with asymmetric paths.
More work needs to be done towards extending this idea for a
combination of NATs and firewalls with routing asymmetry.
A HIP host behind a firewall might need to register itself with local
middleboxes before the base exchange can be initiated or completed.
Firewalls might not allow the traffic to bypass the firewall. For
this, we consider using messages I1',R1',I2' and R2' which are an
extended version of the normal base exchange messages used in HIP.
However, these messages are exclusively used only for configuring the
HIP host with the firewalls such that authentication and
authorization is complete before the firewall opens up a pinhole.
With this approach, we make fewer changes to the base exchange by
avoiding the inclusion of certain authorization parameters into them.
We refer to this exchange as 'Registration Procedure', defined in
[17], as shown in Figure 10 which provides more details of this
lightweight protocol exchange.
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End host-to-Middlebox or E2M messages
I -> R: I1: Trigger exchange
OR
I -> FW1: I1': Trigger exchange
I <- FW1: R1': {Puzzle, D-H(R), HI(R),HIP Transform}SIG
I -> FW1: I2': {Solution,D-H(I),HIP Transform,{H(I)},CERT(I)}SIG
I <- FW1: R2': {HMAC}SIG
Figure 10: HIP NAT/Firewall Registration Procedure
As an overview, we modify the HIP exchange protocol to authenticate
the middlebox towards the initiator, to authorize (and possibly
authenticate) the initiator towards the firewall and to establish a
security association between the initiator and the responder. We
reuse the HIP protocol for this purpose to use the same
infrastructure and to benefit from a lightweight protocol. Note that
the message flow in Figure 10 does not establish IPsec security
associations. These security associations are not necessary in most
scenarios.
When a host I wishes to create a pinhole with a FW on its side (named
as FW-I), it has two choices:
o It sends a regular I1 message to the firewall. This assumes that
the end host knows that a firewall is located in the network and
additionally the address of this firewall is also known to the end
host. This might be the case in a corporate network environment.
This is shown as the I1' message.
o The initiator I can also send a regular HIP I1 message towards a
destination host (denoted as R). This message will then be
intercepted by the firewall and a R1' is returned.
With R1' the firewall sends a puzzle to the initiator similar to the
one sent from a HIP receiver to a HIP initiator. The initiator
solves the puzzle and sends the solution back to the FW along with
its SPKI certificate using the I2' message. Note that the Initiator
can send its certificate in the I1/I1' message. This will, however,
create a state even before the client puzzle solution is obtained
from the initiator. This raises some denial of service concerns.
The FW can validate this SPKI certificate and authorize the HIP host
I1. This packet is not liable to any denial of service or replay
attacks as the solution is dependent on the cookie that R1' included.
Hence, the FW can look into the cookie index to avoid unwanted
signature verifications. The ESP transforms are also dropped here as
the there will be no IPsec ESP packets exchanged between the HIP host
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and the FW. There is also no need for the SPI(I) values in I2' and
R2' messages.
Once the FW receives the I2' packet, it verifies the solution to make
sure that it is the entity to which it sent the R1' packet. It sends
a R2' packet back to the initiator as an acknowledgement for
authorization. The R2' packet however should include a HMAC to
prevent denial of service attacks on I.
After I receives the R2' packet, it can now initiate the normal base
exchange that the FW will forward to R.
On receiving I1, receiver R will send a R1 message back to the
initiator. However, since the FW-R at the receiver end also needs to
authenticate and authorize the receiver, we run the registration
procedure with the E2M messages similar to the previous step between
FW-R and receiver R. Once receiver R receives the acknowledgement
R2', it now sends packet R1 to the initiator that the FW-R will
forward. The rest of the base exchange continues as usual. However
for the sake of the SPI interception at the firewalls, as mentioned
before signaling messages have to be sent from the HIP hosts to their
local middleboxes about the SPI values they have agreed on.
I1' FW-I
----------------> +-+
R1' | |
<---------------- | |
I2' | |
----------------> | |
R2' | |
<---------------- | |
I1 | | 1.I1
---------------> | | ----------------------------------->
+-+
FW-R R1
+-+<---------------
| | R1'
| |---------------->
| | I2'
| |<----------------
| | R2'
| |---------------->
2.R1 | | R1
<--------------------------------------| |<---------------
+-+
FW-I
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+-+
3.I2 | | I2
---------------> | | ------------------------------------>
| |
+-+
FW-R
+-+
4.R2 | | R2
<-------------------------------------| |<------------------
| |
+-+
FW-I FW-R
+-+ +-+
5.SPI(I) | | | | 5.SPI(R)
--------------->| | | |<------------------
| | | |
+-+ +-+
Figure 11: Authentication and authorization for base exchange
messages
6.3.5 Authentication and authorization for Readdressing
After the base exchange is complete, IPsec payload packets are
exchanged among the HIP hosts. The middleboxes use the state that is
established with them to forward such packets to the HIP hosts. The
state at FW-R is < SPI(R), IP(R), HIT(R) > and state at FW-I will be
< SPI(I), IP(I), HIT(I) >.When one of the HIP hosts moves, it sends
an UPDATE message to its peer informing about the new IP addresses.
The peer will send a new SPI value back to the initiator to make a
return routability check. If the peer receives data from the
initiator on the new security association with this new SPI, it
confirms the mobile node has moved and is indeed reachable at the new
IP address. For middleboxes that use <destination IP address, SPI
and ESP> as the flow identifier to forward HIP packets, this
information needs to be updated with every UPDATE message. FW-I
(assuming that I is mobile) has to intercept the new IP address of I
while FW-R (behind which is the peer R) has to update the new SPI(R)
to forward packets correctly. This is illustrated in Figure 12.
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FW-R <SPI(R), IP(R), HIT(R)>
+-+
1.UPDATE with REA[new IP(I)] | |
---------------------------------+ |---------------->
| |
FW-I +-+
+-+
| | 2.UPDATE[new SPI(R)] to IP(I)
<---------| |----------------------------------------
| |
| |
+-+
FW-R
+-+
| | 3.new SPI(R)
| |<------------------
| |
+-+
FW-I
+-+
| | 4.Data on the new SA
---------| |---------------------------------------->
| |
| |
+-+
Figure 12: Authentication and authorization for UPDATE messages
As seen, FW-R has the flow identifier information for receiver R and
FW-I has the flow identifier information for initiator I. When I
sends a UPDATE message with a REA parameter, R sends a new SPI(R) to
check the reachability of the new IP address. FW-I can intercept the
destination IP address from this message and can update its
information. After both the UPDATE and UPDATE reply messages have
been sent out, the receiver needs to signal the FW-R about it new
SPI(R). Denial of service attacks and replay attacks are
considerably reduced at firewalls if the firewalls keep track of the
UPDATE ID that is sent in the UPDATE messages. Every UPDATE REPLY
message carries the same number as the UPDATE message and hence the
middleboxes are able to keep up the sequence. Issues as to how the
receiver can inform the FW-R about its new SPI(R) even before it has
received a confirmation on the return routability test have to be
considered.
However, even this is true only if the new access point has the same
set of middleboxes. If the mobile node is behind a new firewall
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while sending an UPDATE message, the firewall does not have any state
information to create a pin hole. Hence, it should send a trigger
message that will reinitiate the extended E2M messages between the
mobile node and the firewall as in Figure 11.
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7. Security Considerations
In this approach, we have tried to extend the HIP base exchange to
create a state at the NAT/FW securely. Though it is possible to make
this configuration along with the base exchange messages itself, the
middlebox traversal will significantly alter the original base
exchange messages as including the sequence numbers and SPI values
for the middleboxes. By extending the base exchange messages as
I1',R1',I2' and R2', we also effectively make use of the security
features that comes with the HIP protocol to protect the
configuration information between the HIP hosts and the middleboxes.
We will now quickly look into some possible security threats at the
middleboxes and how extended HIP base exchange mechanism can protect
the configuration information.
Extended Base Exchange messages for configuration
o Message I1' is only a trigger message sent from the initiator to
the FW. FW has support to precompute many R1' messages and to
send them in response to the I1' messages. Since the FW does not
create any state at this point in time, it is quite difficult to
launch a DoS attack here.
o Message R1' can be spoofed by a MITM and can tie up an initiator
with solving puzzles for a long time. However, this is avoided by
solving puzzles in R1' messages that correspond to a previously
sent I1' message only.
o On receiving an I2' message, a FW is expected to verify the
signature and validate the certificates. There can be possible
DoS and replay attacks here either to create multiple false states
at the firewall or to reuse the certificates. However, the FW
maintains a cookie index and the corresponding cookie that was
sent in the R1' packet. The firewall can choose to validate the
certificate only if the cookie index and the cookie value both
match the expected values. These verifications can considerably
prevent such attacks.
o Hosts are protected against replays to R2' by use of a less
expensive HMAC verification preceding HIP signature verification.
o Hosts can prevent denial of service attacks and replay attacks
with the UPDATE message with the use of the UPDATE ID in the
UPDATE packets. These UPDATE messages are sequence numbers which
the middleboxes can keep a track of. They can simply precede any
signature verification by checking the UPDATE ID first.
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8. Acknowledgements
The authors would like to thank Pekka Nikander, Dieter Gollmann and
Thomas Aura for their feedback to this document.
This document is a byproduct of the Ambient Networks Project,
partially funded by the European Commission under its Sixth Framework
Programme. It is provided "as is" and without any express or implied
warranties, including, without limitation, the implied warranties of
fitness for a particular purpose. The views and conclusions
contained herein are those of the authors and should not be
interpreted as necessarily representing the official policies or
endorsements, either expressed or implied, of the Ambient Networks
Project or the European Commission.
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9. References
9.1 Normative References
[1] Moskowitz, R., "Host Identity Protocol",
Internet-Draft draft-ietf-hip-base-00, June 2004.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", March 1997.
9.2 Informative References
[3] Srisuresh, P. and K. Egevang, "Traditional IP Network Address
Translator (Traditional NAT)", RFC 3022, January 2001.
[4] Srisuresh, P. and M. Holdrege, "IP Network Address Translator
(NAT) Terminology and Considerations", Request For Comments RFC
2663, August 1999.
[5] Moskowitz, R. and P. Nikander, "Host Identity Protocol
Architecture", Internet-Draft draft-moskowitz-hip-arch-05 (work
in progress), September 2003.
[6] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[7] Hancock, R., "Next Steps in Signaling: Framework",
Internet-Draft draft-ietf-nsis-fw-07, November 2004.
[8] Kivinen, T., A. Huttunen, A., Swander, B. and V. Volpe,
"Negotiation of NAT-Traversal in the IKE", RFC 3947, January
2005.
[9] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
Internet-Draft draft-ietf-ipsec-ikev2-17, September 2004.
[10] A. Huttunen, A., Swander, B., Volpe, V., DiBurro, L. and M.
Stenberg, "UDP Encapsulation of IPsec Packets", RFC 3948,
January 2005.
[11] Nikander, P., Tschofenig, H., Henderson, T., Eggert, L. and J.
Laganier, "Preferred Alternatives for Tunnelling HIP (PATH)",
Internet-Draft draft-nikander-hip-path-00.txt (work in
progress), February 2005.
[12] Stiemerling, M., "A NAT/Firewall NSIS Signaling Layer Protocol
(NSLP)", Internet-Draft draft-ietf-nsis-nslp-natfw-04, October
2004.
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[13] Tschofenig, H., "Security Implications of the Session
Identifier", Internet-Draft draft-tschofenig-nsis-sid-00, June
2003.
[14] Ylitalo, J., Melen, J., Nikander, P. and V. Torvinen,
"Re-thinking Security in IP based Micro-Mobility", 7th
Information Security Conference (ISC-04), Palo Alto,",
September 2004.
[15] Ylitalo, J., Melen, J. and P. Nikander, "SPINAT: A Security
Framework for Local IP Mobility Management, unpublished
manuscript", November 2003.
[16] Maler, E. and J. Hughes, "Technical Overview of the OASIS
Security Assertion Markup Language (SAML) V1.1", March 2004.
[17] Laganier, J., Koponen, T. and L. Eggert, "Host Identity
Protocol (HIP) Registration Extension",
Internet-Draft draft-koponen-hip-registration-00.txt, February
2005.
[18] Kent, S., "IP Encapsulating Security Payload (ESP)",
Internet-Draft draft-ietf-ipsec-esp-v3-08 (work in progress),
March 2004.
[19] Srisuresh, P. and K. Egevang, "Traditional IP Network Address
Translator (Traditional NAT)", Request For Comments RFC 3022,
January 2001.
[20] Nikander, P. and J. Arkko, "End-Host Mobility and Multi-Homing
with Host Identity Protocol",
Internet-Draft draft-nikander-hip-mm-01.txt (work in progress),
October 2004.
[21] Crocker, D., "Choices for Multiaddressing",
Internet-Draft draft-crocker-multiaddr-analysis-01.txt, October
2003.
Authors' Addresses
Hannes Tschofenig
Siemens
Otto-Hahn-Ring 6
Munich, Bayern 81739
Germany
Email: Hannes.Tschofenig@siemens.com
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Aarthi Nagarajan
Siemens
Otto-Hahn-Ring 6
Munich, Bayern 81739
Germany
Email: aarthi.nagarajan@tuhh.de
Vesa Torvinen
Ericsson
Joukahaisenkatu 1
Turku FIN 20520
Finland
Email: vesa.torvinen@ericsson.com
Jukka Ylitalo
Ericsson Research Nomadiclab
Jorvas FIN 02420
Finland
Phone: +358 9 299 1
Email: jukka.ylitalo@ericsson.com
Murugaraj Shanmugam
Siemens
Otto-Hahn-Ring 6
Munich, Bayern 81739
Germany
Email: murugaraj.shanmugam@tuhh.de
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