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Internet-Draft O. Gudmundsson, R. Droms
DHC Working Group TIS, Bucknell University
<draft-ietf-dhc-security-requirements-00.txt> March 1998
Security Requirements for the DHCP protocol
<draft-ietf-dhc-security-requirements-00.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,
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Distribution of this memo is unlimited.
Abstract
This document addresses the general security requirements of both
DHCPv4 and DHCPv6. This document lists security requirements and
the the reasons for each requirement. This document does not
address how to implement the security requirements.
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1. Background
This section presents some concepts and definitions used throughout
this document.
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. 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.
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
DHCP v4 DISCOVER messages the same will apply to DHCP v6 Solicit
Message. Subsequent v6 messages are similar to v4 messages. Both
v4 and v6 take advantage of RELAY agents, in some cases these
agents can add to the messages from servers, it is important that
the added information is treated the same way as data from servers.
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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 be able to authenticate client identity.
2. Client MUST be able authenticate the server identity as an
authorized server.
- Initial Relay Agent/Server Authentication
3. Server MUST be able to authenticate relay agent identity as an
authorized relay agent.
4. Relay Agent MUST be able to authenticate server identity as an
authorized server.
- Successive Client to Server and Relay Agent to Server Communication
5. Client and Server MUST agree 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 requirements 5 and 7.
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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.
Traditional identifiers such as MAC addresses, are not suitable in
all cases in identify clients, first there is no database of what
MAC goes with what host, secondly some MACs are portable and can
easily migrate between hosts such as Ethernet PC CARDS.
2.2. DHCP communication protection
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.
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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
Fake servers[DHCPVERSERV] 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 fake servers are easy to
detect 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
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,
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then this problem would be solved.
Some DHCP servers do not have CPU cycles to spare to do security
checks. Computational load on server in verifying the identity of
client can be significant. Different authentication mechanisms have
different computational overhead, similarly network delays have to
be taken into account if DHC server needs to query remote data
source for more data.
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. An example of global identity is a DNS name.
3.2 Replay prevention
In order to protect replay attacks, all communication to servers
should contain some variable data that never repeats and both
server and client can agree on. A simple approach is to use time of
day 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
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should always assume that the network is insecure.
Another approach is to use counters but this requires clients to
keep state for each server they talk and has synchronization
issues.
3.3 Data confidentiality
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 described models there may be other models
possible. It is expected that any security proposal put forward
state which model is used as its bases.
3.4.1 The ``No security'' model
This is the current situation. Below are few arguments that can be
made for the status quo.
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 DNS update operations on a client's behalf. Sites MAY
select this model but this is strongly discouraged.
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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
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 a client has been
configured it can enter IPSEC security associations with other DHCP
servers during the lifetime of the IP address lease.
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4.1 Identification of DHCP clients and scaling issues
From a DHCP servers perspective it needs a ''handle'' that can be
used to uniquely identify each client, to the server it should not
matter what kind of handle is used. From a security point of view,
it is important that the ''handle'' be always the same and no
possibility of confusion. In DHCP there are at least 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, but
from the security requirements standpoint these are identical.
MAC addresses are frequently proposed as ''handles'', but in many
cases they are not suitable. For example most laptop computers have
network connectivity via a PCCARD, these cards are easy to swap and
thus are not static. Similarly laptops at different times connect
via Ethernet, modem, infrared or wireless all with different MAC
addresses but the laptop may ask for the same Identity regardless
of connection.
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.
DHCP SHOULD require all servers and clients to support at least one
mandatory authentication protocol, and allow other ones. This will
ensure interoperabilty of all servers and clients.
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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, DHCP servers for each domain must be listed.
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. Sever verification by clients
When a client receives an DHCPOFFER message it should try to
authenticate the server. For stationary clients this can be as
simple as verifying that this is one of the servers it knows about,
and trusts. For mobile clients and in adverse networks this is
more difficult, there must be a mechanism for identifying the
servers that are authorized to allocate addresses in a range. This
could be accomplished by adding an RP record at delectation points
in the inverse DNS tree or at every node that points the
authorities for that address(es), the <mbox-dname> is the mail
address of the responsible party and the <txt-dname> is the
authorized server. There can be as many RP records as there are
servers. If the inverse address map is protected by DNSSEC then
this is a convenient mechanism to authenticate this is a good
server. For clients that have host name configured they should
perform similar lookup to make sure the server is authorized to
allocate names in that space.
6. Security considerations
This document addresses how to add security features to the
unsecured DHCP protocol.
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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-04.txt>
August 1997
[DHCPv6] J. Bound, C. Perkins, "Dynamic Host Configuration
Protocol for IPv6 (DHCPv6)", Internet Draft
<draft-ietf-dhc-dhcpv6-10.txt> May 1997
[DHCPVERSERV]
R. Watson, O. Gudmundsson, "DHCP Server verification
by client via DNSSEC", <draft-watson-dhc-serv-ver-00.txt>
July 1997.
[IPSEC] R. Atkinson, "Security Architecture for the
Internet Protocol", Internet Draft
<draft-ietf-ipsec-arch-sec-03.txt>, February 1998.
[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.
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9. Author address
Olafur Gudmundsson Ralph Droms
Trusted Information System Computer Science Department
3060 Washington Road 323 Dana Engineering
Bucknell University
Glenwood, MD 21738 Lewisburg, PA 17837
+1 301 854 6889 +1 717 524 1145
ogud@tis.com droms@bucknell.edu
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