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Dynamic Host Configuration WG Olafur Gudmundsson
INTERNET DRAFT Trusted Information Systems
<draft-ietf-dhc-security-arch-01.txt> July 30, 1997
Security Architecture for DHCP
<draft-ietf-dhc-security-arch-01.txt>
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
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ftp.isi.edu (US West Coast).
Abstract
This document addresses the general security requirements of both
DHCPv4 and DHCPv6. This document lists security requirements and
proposes a security model, which meets scaling requirements,
security requirements and efficiency requirements.
The proposed security model uses public key cryptography and a
proposed trusted key distribution mechanism to authenticate clients
and servers. Once clients have authenticated themself less expensive
mechanishms can be used to protect subsequent communication. The
security model also addresses securing relay agents and server to
server protocols.
1. DHCP security requirements
One of the problems of designing a security model for DHCP[DHCP] is
the wide variety in use and preconditions that different sites/
clients have. The fact that sites deploy redundant servers and
the lack of a server to server protocol further complicates
things[Server,Intserver].
1.1. Authentication, confidentiality, data integrity
RFC-1825[RFC1825] contains a great description of these terms and
their uses. Authentication is the process of establishing the
identity of some entity. Once identity has been authenticated,
that identity can be used for access control, accounting etc.
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There are number of authentication technologies available.
Public key cryptography is a powerful tool that relies on complex
mathematical operations to provide information that only the holder
of the private key could have generated. This technology can be
used for all the functions named in the title of this section.
Shared secret authentication is the process of digesting the data
transmitted and obfuscating the digest by applying a transformation
by a key that is only used by the two entities. This technology
can be used to provide both authentication and data integrity.
Each pair can share multiple shared secrets, it is important
that each secret have an identifier attached to it.
Confidentiality can be accomplished by encrypting the data contents
of the outgoing packet. Shared secrets can be used as keys for
symmetric encryption.
1.2. Shared secrets
Shared secrets are between two entities; there is NEVER a need to
share these secrets with other entities. The hosts storing the
secrets MUST protect the secrets as well as possible.
1.3 Terminology and DHCP v6 considerations
This document uses DHCPv4 terminology as it is more familiar than
the new DHCPv6[DHCPv6] terminology. When this document talks
about DISCOVER messages the same will apply to DHCP v6 Solicit
Message. No changes are needed in the protocol section 6 to
support DHCPv6; some currently proposed DHCPv6[DHCPv6EXT]
security options need to be modified.
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
RFC 2119.
2. Proposed DHCP security requirements
The proposed requirements can be summarized in the following rules.
- Initial Client/Server Authentication
1. Server MUST authenticate client identity.
2. Client SHOULD authenticate the server identity as an
authorized server.
- Initial Relay Agent/Server Authentication
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3. Server MUST authenticate relay agent identity as an
authorized relay agent.
4. Relay Agent MUST authenticate server identity as an
authorized server.
HAND JUSTIFY
- Successive Client to Server and Relay Agent to Server Communication
5. Client and Server MUST have agred on security model for
protecting future communication.
- Server/Relay agent advertisements
6. Advertisements MUST be verifiable by all recipients.
- Server/Server communication
7. All communication MUST be protected for data integrity.
Servers MAY request that communication be encrypted.
DHCP security cannot be accomplished in a vacuum; as DHCP is not a
general purpose communication protocol. Fortunately there are
available (or soon will be) protocols that DHCP can take advantage
of. First and foremost DNSSEC[RFC2065] or some other key
distribution mechanism must be available. IPSEC[RFC1825,IPSEC]
will be able to handle requirement 5. It is not clear if IPSEC
for IPv6, in some cases using local link addresses, can address
requirement 1-4. In the case of IPv4 DHCP MUST perform the
initial authentication.
2.1. DHCP Identity
In order to secure DHCP all clients MUST have an identity, this
identity can possibly be one of the following: host name, user
identity, account code. The "prime" identity MUST have a public key
stored in the key distribution mechanism. The client MUST know its
identity before contacting the server. Each client MUST have access
to the correct private key before contacting the DHCP server.
If the identity selected for a host is its host name and the key
distribution mechanism is DNS, then the public key used to
authenticate the host is stored under the host name in its home
zone. The private key needs to be stored in the computer at all
times. If the identity selected is the user then the key is stored
under the user name in DNS (e.g.: ogud.tis.com for me), and the user
needs to load the computer with the private key before the host
can contact the server. If the identity of the host is just
there to uniquely identify the host, the host still needs a
private key.
2.2. DHCP communication protection
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DHCP is a protocol that carries publicly known information, thus
there is limited need for confidentiality. DHCP requires data
integrity protection for communication. The option to allow DHCP
servers/clients to request confidentiality SHOULD be part of any
security architecture.
2.3. Policy issues.
This document does not address access control issues as that is a
policy issue for each site. Effective access control depends on
correct authentication, thus this work will make access control
simpler. This document does not address the issue of protecting the
private key on either server, agent or client.
2.4 Security threats to DHCP
2.4.1 Attacks against servers
There are many possible attacks possible against servers, including
denial of service by exhausting the servers allocated address space.
Another denial of service attack is to overload the server causing
it not to respond to clients.
Once servers start updating DNS and other directory services, DHCP
servers can be spoofed to register incorrect information in those
services.
Another possible attack is to gain unauthorized access to some
resources, such as network access.
2.4.2 Attacks against clients
This is a less known problem, but should not be ignored. Fake
servers can provide clients with partially correct information
that allows the attacker to route traffic through certain host
where critical information can be collected. This becomes
important to detect and prevent when encrypted traffic is
allowed to pass through firewalls.
Clients can be configured with bogus data, so that they will assume
that the network is down. In some cases it is hard to get a
client to reconfigure itself. Clients can also be configured
with addresses of other clients, causing address conflicts.
The bright side of this problem is that it is not that hard to
detect fake servers by monitoring the network for DHCP traffic.
2.5. Complications in implementing the security models
A Client that issues DISCOVER message does not have any IP address
that works outside the local network, and may not even work on
the local network. This prevents the clients from checking with
outside information sources. Servers on the other hand are fully
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configured and can use any information sources accessible.
Clients will not wait long for OFFER message, some security checks
may take longer than the DHCP retransmission timeout. If DHCP
servers had an option to inform clients that DISCOVER messages
are being worked on and client should expect an answer in short
order, then this problem would be solved.
Some DHCP servers do not have CPU cycles to spare to do security
checks. This is a bogus argument, since inexpensive powerful
computers are available and sites should upgrade if security is
of concern.
3. DHCP Security components
3.1 Authentication services
In order for DHCP servers to be able to determine if a client
request should be serviced it is essential for the server to be
able to establish the client's identity. There are two kinds of
identities that are possible, local mutually agreed upon
identities and global identities.
Local identity is sufficient if the client will only be configured
from a small set of servers, and if there are no expectations
that the client will migrate to another location. This is an
acceptable solution for a site where all computers are
stationary but are configured from DHCP for administrative
reasons. Solutions of this kind have certain scaling problems.
Global identity on the other hand is needed when a client can
connect to multiple servers and it provides some of its
identity.
An example of local identity is a name or number that is
configured in the client and server. This could be the name of the
client on the local network. A good example of global identity
is a DNS domain name.
3.2 Time service
To prevent replay attacks, DHCP messages must contain time
information that clients and servers check and act upon. Clock
synchronization service can be provided by an outside
entity[RFC1305] once a client is configured, but bounds must be
placed on acceptable skew while a client is off line or migrates
between locations. Clients SHOULD not trust time information
from servers until after servers have been validated as such.
Clients should always assume that the network is insecure.
3.3 Data confidentiality
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This is not desired service at this point, but it can be added at a
later point for all communication except DISCOVER and possibly OFFER
and REQUEST. All subsequent communication can be encrypted.
3.4 Possible security models for DHCP
This section will present a few possible security models and the
reasons why each one may be useful. This section IS NOT an
advocacy for any of the descibed models
3.4.1 The "No security" model
This is the current situation. The motivation for not changing
anything is that security is hard. Considering that DHCP for IPv4 is
a hack built on an older hack (BOOTP), there is not enough
flexibility in the protocol to add security.
A smart client attached to a broadcast network can learn everything
it needs to know to configure itself by listening to network
traffic. The client can either monitor DHCP traffic and/or all
network traffic to find gateways, servers and unused addresses.
There is no protection against this.
DHCPv6 can on the other hand be extended and modified to fit any
security model selected. Sites will migrate to IPv6 soon, and
the ones that do not deserve what they get.
In this model DHCP clients will be able to do harm and be harmed
by bogus servers. This model is not acceptable when DHCP servers
perform update operations on a client's behalf. Sites MAY
select this model but this is strongly discouraged.
3.4.2 The "Simple" model
A DHCP client is configured with a token that allows it to
authenticate itself to the servers in the DHCP DISCOVER message.
If servers can authenticate the token and the client associated
with the token is allowed to communicate with the server the
server will reply with OFFER message.
In this model servers will know with which client they are dealing,
and that should be sufficient protection against most of the
attacks against the servers. If a client is able to authenticate
the server response, the client might be protected against bad
servers.
With minor extensions to DHCP, all subsequent communication can be
protected.
3.4.3 The "Comprehensive" Model
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In this model DHCP servers and clients have the ability to
authenticate each other. The requirement here is that clients must
be able to authenticate the server without any communication as
they can not trust the information from the server. This model
also must prevent replay attacks.
This model protects all traffic between clients and servers, making
it impossible to stage any attacks other than denial of service
attacks due to CPU overload of servers.
4. Client Authentication:
Initial authentication is the most important step. Once server and
client have established each other's identity the remaining
problems can solved.
The problem of initial client authentication cannot be solved by
IPSEC, as the client does not have an IP identity when it requests
service for the first time from the server. Once the client has
been configured it can enter IPSEC security associations with
other DHCP servers during the lifetime of the IP address lease.
4.1 Types of DHCP clients and their identification needs.
In DHCP there are two types of clients: clients that request some of
their net identity from DHCP, and clients that request all of their
net identity from DHCP. From a security point of view, the
second type of client is no different, because these clients
must have some identity (for example MAC address) that can be
used to uniquely identify them. Previous DHCP security
proposals[DHCAUTH] have suggested the use of shared secrets and
passwords to identify clients. It is also possible to use some
form of challenge/response system to identify clients. These
approaches have limited scaling ability and require a server to
server protocol. But in many environments these weaker
authentication mechanisms are adequate.
The most general case is the identification of a computer that
connects to a world wide ISP network and expects the same identity
regardless of location. In this case it is unlikely that the same
DHCP server serves both India and Iceland. A network of this
kind can have a collaborative agreement between a number of
different ISPs, with multiple administrative domains. It is not
reasonable/scalable that all DHCP servers in this network know
shared secrets, or passwords for all computers that are allowed
to connect. From a security standpoint it is a bad practice to
distribute shared secrets or passwords to many places.
4.2. Motivation for single strong authentication schema.
It is better to mandate ONE strong authentication protocol for all
DHCP interaction, rather than allow for multiple ones and allow
sites to choose the wrong one. The protocol below uses strong
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authentication with public key signatures and encryption. The
security of the protocol depends on the difficulty in breaking
the private keys used. The site security depends on the sites
protecting the private keys. By mandating one protocol at this
point, we also eliminate the need for negotiating what
authentication protocol to use. At this point, the public key
algorithm is MUST be RSA.
4.3 Motivation for global DNS identities for DHCP clients
Once the global identity is registered with an information service,
this identity is available within the limits of the information
service. DNS is the most common information service used by
computers.
DNSSEC[RFC2065] strengthens DNS[RFC1035] against information
corruption and provides distribution of public keys. If every host
that is configured by DHCP has a public key stored in DNS then
servers can verify digital signatures generated by that key.
Once clients are configured it is possible for client to verify
that the server it was configured by is a good DHCP server. In
order to do this, either SRV[SRV] records or ALLOC[DHCPVERSERV]
DNS record must list the DHCP servers for each domain.
IPSEC can be preconfigured with SPI's but there is no definition for
the format of the 'destination address'. If it is DNS format, DHCP
entities MAY enter IPSEC relationship without a key exchange once
client has received DHCP ACK message.
5. DHCP security options and their processing
5.1 DNS Identity option
This option allows the DHCP entities to advertise their own public
keys which are stored in DNS and DNSSEC provides secure key rerival
mechanism.
Field value size in bytes
------------- ------ -----------
option TBD 1
length 0-255 1
selector 0-64K 2
name variable < 250
The name specified in this option does not have to be the same name
that the client is requesting/using.
5.2 Signature option
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This is an option that carries any type of signature that can be
specified.
Field value size in bytes
------------- ------ -----------
option TBD 1
length 0-255 1
algorithm 1-253 1
ID 0-2^16 2
current time 0-2^32 4
signature data binary variable < 246
The following algorithms are defined and must be supported by all
implementations.
0 No signature data
1 RSA/MD5 as specified in PKCS1[NETSEC], this is
identical to algorithm 1 in DNSSEC.
100 HMAC-MD5 as specified in RFC2104[RFC2014]
The ID is a 16 bit number that is identical to the key indentifier
in the DNSSEC SIG record. This identifier is used to select a
key from the key set of the name. If there are multiple keys in
the key set that match this ID and can be used for DHCP and are
the same size, the verifier must be ready to try all of the keys
until verification succeeds.
The current time value states at what time the signature is
generated, in Universal Time. DHCP entity SHOULD accept signatures
that are within 60 seconds of local time. If the signature is not
within these bounds the whole packet should be rejected.
The size of the signature data field depends on the algorithm used
and for some algorithms, the key size. MD5 digests are 16 bytes.
RSA signatures are always the same size as the modulus of the
key. Signature data can never exceed 246 bytes, this restricts
the key sizes used to about 1968 bits.
The data covered by signatures is defined in section 5.3.1. The
Signature option MUST be the LAST option in the DHCP packet, adding
a Signature option MUST NOT result in too large of a packet,
other options MUST be removed to make space for the signature
option.
There are special considerations for Relay agents. A Relay Agent
that adds a relay agent option(s) to a signed DHCP packet MUST
add a Signature option after its option(s) and its signature
MUST cover the whole outgoing packet. If the incoming
signature is addressed to this Relay Agent, the Relay Agent MUST
remove that signature from the outgoing packet before adding its
option(s) to the packet. If the incoming signature is a digital
signature (alg=1) it MUST be retained.
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5.3.1 Data covered by signature option
The whole outgoing DHCP packet is covered, including the signature
option.
For DISCOVER/SOLICIT the signature is calculated over
Modified outgoing packet
For all other messages the signature is calculated over
digest of last valid incoming packet | Modified outgoing packet
The modified outgoing packet is the whole DHCP packet with following
differences:
'giaddr' and 'hops' fields must be set to all zero.
All bytes in the signature data must be set 0xa6.
The digest of the last incoming packet from the other entity is to
associate the outgoing packet to the request or last answer.
5.3 Wait option
This option allows the server to inform the client that it is
currently validating the clients identity and request that the
client wait the specified time before retransmitting the query.
Field value size in bytes
------------- ------ -----------
option TBD 1
length 3 1
seconds 1-6 1
This option is to throttle back security aware clients while server
is authenticating the clients identity. The client MUST ignore
this option if it has received 2 previous ones from same server
for the same message.
6. "Simple" DHCP authentication protocol
This protocol is along the lines of the simple model described in
section 3.4.2. The foundation that this protocol offers can be used
to build a comprehensive protocol.
This protocol depends on a reliable certified public key
distribution mechanism like DNSSEC[DNSSEC]. Each client supplies
its identity in the initial DISCOVER message. This identity
indicates where the associated public key is stored. For DNS the
identity is the FQDN (Fully Qualified Domain Name), accompanied
by the key algorithm number and public key footprint. For other
key distribution mechanisms there must be enough information to
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retrieve the key from that source.
To successfully validate a server public key, clients must be
configured with the root key(s) for the key distribution
certification tree.
The protocols below make use of currently undefined options, these
options must be specified before this proposal can be adopted.
6.1 DHCP authentication protocol overview
In the following discussion, DHCP options not important to the
overall schema are not included.
6.1.1 Client: DISCOVER message MUST include
IDENTITY option (contains identity type, name, unique selector)
Signature option (alg=1) signed by public key in IDENTITY option.
6.1.2 Server: DHCP-OFFER message
If server is able to validate DISCOVER message from a client it
shares a secret with it MUST include following options.
IDENTITY option
Signature option (alg=100)
If the client is able to verify this message, it has authenticated
the server and the authentication protocol is complete. Future
communication can be protected by this secret.
If the server is able to validate DISCOVER message from a client
that it does not share a secret with, the following options MUST
be included.
IDENTITY option
Signature option (alg=1)
If the server needs more time to complete authentication it can send
back
WAIT option
Signature option 0
If the server refuses to offer service, as the time in request is
out of bounds, the server sends back OFFER which MUST contain
only the following options: (EMPTY OFFER).
IDENTITY option
Signature option
If server is unable to authenticate the identity of the client, the
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server MUST ignore the client messages. The server SHOULD log the
event and CAN ignore requests from the same hardware address for a
fixed time.
6.1.3 Client DHCP-REQUEST processing
If the server has accepted the client's identity, the client can now
send the REQUEST message, and this message MUST be signed by its
public key in order for other servers to verify that their
offers were declined. The following options MUST be present:
IDENTITY
Security option (alg=1)
6.2 Future communication
Once a client that does not share a secret with the server selected
has been configured, it can optionally authenticate the server as
specified in[DHCPVERSERV]. All future communiction between the
client and the server MUST contain data protection. It can also
attempt to exchange secrets with the server via an optional
protocol extension "To Be Determined". If IPSEC is available it
CAN be used to protect future communication until the client is
renumbered. The client and server can also elect to use RSA
signatures on all communication.
6.3 Computational complexity of Simple Authentication Protocol
This protocol places most of the cost of the expensive public key
operations on the client. Servers need to generate signatures on all
messages to clients that do not share secrets with them.
The cost of verifying the public keys of the client can be to a
large extent offloaded to the DNSSEC server if a DNS transaction
signature mechanism[RFC2065,TSIG] is used to protect the
communication.
6.4 Client security requirements
Security enabled DHCP clients MUST be able to store their identity
and private key between reboots. These same clients SHOULD have
a clock that keeps reasonable good time. The client SHOULD be
able to store multiple Server Identities and Shared secrets
between reboots. Clients MUST be able to perform the following
security operations: RSA/MD5 digital signatures and HMAC-MD5.
6.5 Server security requirements
Security enabled DHCP servers MUST be able to store identities and
shared secrets with a large number of clients. Servers MUST be able
to perform RSA/MD5 and HMAC-MD5 operations. Servers MUST be
configured with either secure DNS resolver, or other form of
trusted communication with DNSSEC server.
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7. Security considerations
This document addresses how to add security features to the
unsecured DHCP protocol.
7. References
[DHCP] R. Droms, "Dynamic Host Configuration Protocol", RFC
2131, Bucknell University, April 1997.
[DHCAUTH] R. Droms, "Authentication for DHCP Messages",
Internet Draft <draft-ietf-dhc-authentication-03.txt> November
1996
[DHCPv6] J. Bound, C. Perkins,
"Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
Internet Draft <draft-ietf-dhc-dhcpv6-10.txt> May 1997
[DHCPv6EXT] C. Perkins, "Extensions for DHCPv6", Internet Draft
<draft-ietf-dhc-v6exts-06.txt> May 1997
[DHCPVERSERV] R. Watson, O. Gudmundsson,
"DHCP Server verification by client via DNSSEC",
<draft-watson-dhc-serv-ver-00.txt> July 1997.
[HMAC] H. Krawczyk, M. Bellare, R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February 1997
[Intserver] R. Droms, K. Kinnear, "An Inter-server Protocol for
DHCP", Internet Draft <draft-ietf-dhc-interserver-alt-00.txt>,
April 1997
[Server] R. Droms, R. Cole, "An Inter-server Protocol for
DHCP", Internet Draft <draft-ietf-dhc-interserver-01.txt>
March 1997.
[IPSEC] R. Atkinson, "Security Architecture for the Internet
Protocol", Internet Draft <draft-ietf-ipsec-arch-sec-01.txt>,
November 1996.
[RFC1035] P. Mockapetris, "Domain Names - Implementation and
Specification," RFC 1034, ISI, November 1987.
[RFC1305] Mills, D., "Network Time Protocol (v3)", RFC 1305,
March 1992.
[RFC1825] R. Atkinson, "Security Architecture for the Internet
Protocol", RFC 1825, September 1995.
[RFC2065] D. Eastlake, C. Kaufman, "Domain Name System Security
Extensions", RFC 2065, January 1997.
[SRV] A. Gulbrandsen, P. Vixie, "A DNS RR for specifying the
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location of services (DNS SRV)", RFC 2052, October 1996.
[RFC2132] S. Alexander, R. Droms, "DHCP Options and BOOTP
Vendor Extensions", RFC 2132, March 1997.
[NETSEC] C. Kaufman, R. Perlman, M. Speniner, "Network
Security: PRIVATE Communications in a PUBLIC World", Prentice
Hall 1995.
9. Author address
Olafur Gudmundsson
Trusted Information System
3060 Washington Road
Glenwood, MD 21738
+1 301 854 5794
<ogud@tis.com>
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