Clarifications and Extensions for the Bootstrap Protocol
draft-ietf-dhc-bootp-01
The information below is for an old version of the document that is already published as an RFC.
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| Author | Walter Wimer | ||
| Last updated | 2013-03-02 (Latest revision 1992-12-09) | ||
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draft-ietf-dhc-bootp-01
Internet Engineering Task Force W. Wimer
Internet Draft Carnegie Mellon University
September, 1992
DRAFT
Clarifications and Extensions for the Bootstrap Protocol
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 months.
Internet Drafts may be updated, replaced, or obsoleted by other
documents at any time. It is not appropriate to use Internet Drafts as
reference material or to cite them other than as a "working draft" or
"work in progress."
Please check the I-D abstract listing contained in each Internet Draft
directory to learn the current status of this or any other Internet
Draft. This Internet Draft expires on February 28, 1993.
This memo suggests several updates to the specification of the Bootstrap
Protocol (BOOTP) based on experience with the protocol.
This proposal is a product of the IETF Dynamic Host Configuration
Working Group. This draft document will be submitted to the RFC editor
as a protocol specification. Comments on this memo should be sent to
the IETF Dynamic Host Configuration Working Group mailing list,
host-conf@sol.bucknell.edu.
Distribution of this memo is unlimited.
Abstract
Some aspects of the BOOTP protocol were rather loosely defined in its
original specification. In particular, only a general description was
provided for the behavior of "BOOTP relay agents" (originally called
"BOOTP forwarding agents"). The client behavior description also
suffered in certain ways. This memo attempts to clarify and strengthen
the specification in these areas.
In addition, new issues have arisen since the original specification was
written. This memo also attempts to address some of these.
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Table of Contents
1. Introduction 3
1.1 Requirements 3
1.2 Terminology 4
1.3 Data Transmission Order 5
2. Definition of the 'flags' Field 6
3. BOOTP Relay Agents 7
3.1 General BOOTP Processing 8
3.1.1 BOOTREQUEST Messages 8
3.1.2 BOOTREPLY Messages 11
4. BOOTP Client Behavior 13
4.1 Client use of the 'flags' field 13
4.1.1 The BROADCAST flag 13
4.1.2 The remainder of the 'flags' field 14
4.2 Definition of the 'secs' field 14
4.3 Interpretation of the 'giaddr' field 14
4.4 Vendor information "magic cookie" 15
5. Bit Ordering of Hardware Addresses 16
6. BOOTP Over IEEE 802.5 Token Ring Networks 16
Security Considerations 18
Acknowledgements 18
References 19
Author's Address 19
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1. Introduction
The Bootstrap Protocol (BOOTP) is a UDP/IP-based protocol which allows a
booting host to configure itself dynamically and without user
supervision. BOOTP provides a means to notify a host of its assigned IP
address, the IP address of a boot server host, and the name of a file to
be loaded into memory and executed [1]. Other configuration information
such as the local subnet mask, the local time offset, the addresses of
default routers, and the addresses of various Internet servers can also
be communicated to a host using BOOTP [2,3].
Unfortunately, the original BOOTP specification [1] left some issues of
the protocol open to question. The exact behavior of BOOTP relay agents
(formerly called "BOOTP forwarding agents") was not clearly specified.
Some parts of the overall protocol specification actually conflict,
while other parts have been subject to misinterpretation, indicating
that clarification is needed. This memo addresses these problems.
Since the introduction of BOOTP, the IEEE 802.5 Token Ring Network has
been developed which presents a unique problem for BOOTP's particular
message-transfer paradigm. This memo also suggests a solution for this
problem.
1.1 Requirements
In this memo, the words that are used to define the significance of
particular requirements are capitalized. These words are:
o "MUST"
This word or the adjective "REQUIRED" means that the item
is an absolute requirement of the specification.
o "MUST NOT"
This phrase means that the item is an absolute prohibition
of the specification.
o "SHOULD"
This word or the adjective "RECOMMENDED" means that there
may exist valid reasons in particular circumstances to
ignore this item, but the full implications should be
understood and the case carefully weighed before choosing a
different course.
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o "SHOULD NOT"
This phrase means that there may exist valid reasons in
particular circumstances when the listed behavior is
acceptable or even useful, but the full implications should
be understood and the case carefully weighed before
implementing any behavior described with this label.
o "MAY"
This word or the adjective "OPTIONAL" means that this item
is truly optional. One vendor may choose to include the
item because a particular marketplace requires it or
because it enhances the product, for example; another
vendor may omit the same item.
1.2 Terminology
This memo uses the following terms:
BOOTREQUEST
A BOOTREQUEST message is a BOOTP message sent from a BOOTP
client to a BOOTP server, requesting configuration
information.
BOOTREPLY
A BOOTREPLY message is a BOOTP message sent from a BOOTP
server to a BOOTP client, providing configuration
information.
Silently discard
This memo specifies several cases where a BOOTP relay agent
is to "silently discard" a received BOOTP message. This
means that the relay agent should discard the message
without further processing, and that the relay agent will
not send any ICMP error message as a result. However, for
diagnosis of problems, the relay agent SHOULD provide the
capability of logging the error, including the contents of
the silently-discarded message, and SHOULD record the event
in a statistics counter.
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1.3 Data Transmission Order
The order of transmission of the header and data described in this
document is resolved to the octet level. Whenever a diagram shows a
group of octets, the order of transmission of those octets is the
normal order in which they are read in English. For example, in the
following diagram, the octets are transmitted in the order they are
numbered.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | 2 |
+-------------------------------+
| 3 | 4 |
+-------------------------------+
| 5 | 6 |
+-------------------------------+
Whenever an octet represents a numeric quantity, the leftmost bit in
the diagram is the high order or most significant bit. That is, the
bit labeled 0 is the most significant bit. For example, the
following diagram represents the value 170 (decimal).
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|1 0 1 0 1 0 1 0|
+---------------+
Similarly, whenever a multi-octet field represents a numeric
quantity the leftmost bit of the whole field is the most significant
bit. When a multi-octet quantity is transmitted the most
significant octet is transmitted first.
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2. Definition of the 'flags' Field
The standard BOOTP message format defined in [1] includes a two-octet
field located between the 'secs' field and the 'ciaddr' field. This
field is merely designated as "unused" and its contents left
unspecified, although Section 7.1 of [1] does offer the following
suggestion:
"Before setting up the packet for the first time, it is a good idea
to clear the entire packet buffer to all zeros; this will place all
fields in their default state."
This memo hereby designates this two-octet field as the 'flags' field.
The first 44 octets of a BOOTP message are shown below. The numbers in
parentheses indicate the size of each field in octets.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| op (1) | htype (1) | hlen (1) | hops (1) |
+---------------+---------------+---------------+---------------+
| xid (4) |
+-------------------------------+-------------------------------+
| secs (2) | flags (2) |
+-------------------------------+-------------------------------+
| ciaddr (4) |
+---------------------------------------------------------------+
| yiaddr (4) |
+---------------------------------------------------------------+
| siaddr (4) |
+---------------------------------------------------------------+
| giaddr (4) |
+---------------------------------------------------------------+
| |
| chaddr (16) |
| |
| |
+---------------------------------------------------------------+
This document hereby defines the most significant bit of the 'flags'
field as the BROADCAST (B) flag. The semantics of this flag are
discussed in Sections 3.1.2 and 4.1.1 of this memo.
The remaining bits of the 'flags' field are reserved for future use.
They MUST be set to zero by clients and ignored by servers and relay
agents.
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The 'flags' field, then, appears as follows:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|B| MBZ |
+-+-----------------------------+
where:
B BROADCAST flag (discussed in Sections 3.1.2 and 4.1.1)
MBZ MUST BE ZERO (reserved for future use)
3. BOOTP Relay Agents
In many cases, BOOTP clients and their associated BOOTP server(s) do not
reside on the same IP network or subnet. In such cases, some kind of
third-party agent is required to transfer BOOTP messages between clients
and servers. Such an agent was originally referred to as a "BOOTP
forwarding agent." However, in order to avoid confusion with the IP
forwarding function of an IP router, the name "BOOTP relay agent" is
hereby adopted instead.
DISCUSSION:
A BOOTP relay agent performs a task which is distinct from an IP
router's normal IP forwarding function. While a router normally
switches IP datagrams between networks more-or-less
transparently, a BOOTP relay agent may more properly be thought
to receive BOOTP messages as a final destination and then
generate new BOOTP messages as a result. One should resist the
notion of simply forwarding a BOOTP message "straight through
like a regular packet."
This relay-agent functionality is most conveniently located in the
routers which interconnect the clients and servers, but may
alternatively be located in a host which is directly connected to the
client subnet.
Any Internet host or router which provides BOOTP relay-agent capability
MUST conform to the specifications in this memo.
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3.1 General BOOTP Processing
All locally delivered UDP messages whose UDP destination port number
is BOOTPS (67) are considered for special processing by the host or
router's logical BOOTP relay agent.
In the case of a host, locally delivered datagrams are simply all
datagrams normally received by that host, i.e. broadcast and
multicast datagrams as well as unicast datagrams addressed to IP
addresses of that host.
In the case of a router, local delivery has a similar but somewhat
more careful definition for which [4] should be consulted.
Hosts and routers are usually required to silently discard incoming
datagrams containing illegal IP source addresses. This is generally
known as "Martian address filtering." One of these illegal
addresses is 0.0.0.0 (or actually anything on network 0). However,
hosts or routers which support a BOOTP relay agent MUST accept for
local delivery to the relay agent BOOTREQUEST messages whose IP
source address is 0.0.0.0. BOOTREQUEST messages from legal IP
source addresses MUST also be accepted, of course.
The following consistency checks SHOULD be performed on BOOTP
messages:
o The IP Total Length and UDP Length must be large enough to
contain the minimal BOOTP header of 300 octets (in the UDP
data field) specified in [1].
NOTE: Future extensions to the BOOTP protocol may increase
the size of BOOTP messages. Therefore, BOOTP messages
which, according to the IP Total Length and UDP Length
fields, are larger than the minimum size specified by [1]
MUST also be accepted.
o The 'op' (opcode) field of the message must contain either
the code for a BOOTREQUEST (1) or the code for a BOOTREPLY
(2).
BOOTP messages not meeting these consistency checks MUST be silently
discarded.
3.1.1 BOOTREQUEST Messages
Some configuration mechanism MUST exist to enable or disable the
relaying of BOOTREQUEST messages. Relaying MUST be disabled by
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default.
When the BOOTP relay agent receives a BOOTREQUEST message, it
MAY use the value of the 'secs' (seconds since client began
booting) field of the request as a factor in deciding whether to
relay the request. If such a policy mechanism is implemented,
its threshold SHOULD be configurable.
DISCUSSION:
To date, this feature of the BOOTP protocol has not
necessarily been shown to be useful. See Section 4.2
for a discussion.
The relay agent MUST silently discard BOOTREQUEST messages whose
'hops' field exceeds the value 16. A configuration option
SHOULD be provided to set this threshold to a smaller value if
desired by the network manager. The default setting for a
configurable threshold SHOULD be 4.
If the relay agent does decide to relay the request, it MUST
examine the 'giaddr' ("gateway" IP address) field. If this
field is zero, the relay agent MUST fill this field with the IP
address of the logical interface on which the request was
received. In addition, the relay agent MAY insert the subnet
mask of that logical interface into the vendor area (see the
next paragraph for details). If the 'giaddr' field contains
some non-zero value, the 'giaddr' field MUST NOT be modified and
the subnet mask MUST NOT be inserted into the vendor area nor
modified. The relay agent MUST NOT, under any circumstances,
fill the 'giaddr' field with a broadcast address as is suggested
in [1] (Section 8, sixth paragraph).
To insert the subnet mask into the vendor area as suggested
above, the relay agent MUST examine the first four octets of the
'vend' field (these first four octets are usually referred to as
the "vendor magic number" or "vendor magic cookie"). If these
four octets do not contain the dotted decimal value 99.130.83.99
as specified in [2,3], the subnet mask MUST NOT be inserted. If
these four octets do contain the value 99.130.83.99, it is safe
to insert the subnet mask. The relay agent MUST use the data
format as specified in [2,3] and MUST use the "Subnet Mask
Field" (Tag 1) specified in [2,3] to express the subnet mask.
The relay agent MUST be careful to preserve any and all existing
data in the 'vend' field. The subnet mask MUST either be placed
at the beginning of the data portion of the 'vend' field
(immediately after the four-octet magic cookie), or the relay
agent MUST be careful to replace any existing subnet mask
entries (Tag 1) with the correct subnet mask value. This is to
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avoid any ambiguity in the event that the client supplied one or
more subnet mask entries somewhere in the 'vend' field. If the
subnet mask cannot be inserted without loss of data in the
'vend' field, the subnet mask MUST NOT be inserted.
DISCUSSION:
Having the relay agent insert the subnet mask into the
vendor area is an optimization for the proposed Dynamic
Host Configuration Protocol (DHCP) [5]. This
optimization should not be strictly necessary for
correct operation of the protocol, but it should make
configuration of the DHCP server much easier. It is
strongly encouraged that relay agents provide this
subnet mask feature, but it is not absolutely required.
The value of the 'hops' field MUST be incremented.
All other fields MUST be preserved intact.
At this point, the request is relayed to its new destination (or
destinations). This destination MUST be configurable. Further,
this destination configuration SHOULD be independent of the
destination configuration for any other so-called "broadcast
forwarders" (e.g. for the UDP-based TFTP, DNS, Time, etc.
protocols).
DISCUSSION:
The network manager may wish the relaying destination to
be an IP unicast, multicast, broadcast, or some
combination. A configurable list of destination IP
addresses provides good flexibility. More flexible
configuration schemes are encouraged. For example, it
may be desirable to send to the limited broadcast
address (255.255.255.255) on specific logical
interfaces. However, if the BOOTREQUEST message was
received as a broadcast, the relay agent MUST NOT
rebroadcast the BOOTREQUEST on the logical interface
from whence it came.
A relay agent MUST use the same destination (or set of
destinations) for all BOOTREQUEST messages it relays from a
given client.
DISCUSSION:
At least one known relay agent implementation uses a
round-robin scheme to provide load balancing across
multiple BOOTP servers. Each time it receives a new
BOOTREQUEST message, it relays the message to the next
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BOOTP server in a list of servers. Thus, with this
relay agent, multiple consecutive BOOTREQUEST messages
from a given client will be delivered to different
servers.
Unfortunately, this well-intentioned scheme reacts badly
with certain variations of the BOOTP protocol which
depend on multiple exchanges of BOOTREQUEST and
BOOTREPLY messages between clients and servers.
Therefore, all BOOTREQUEST messages from a given client
MUST be relayed to the same destination (or set of
destinations).
One way to meet this requirement while providing some
load-balancing benefit is to hash the client's
link-layer address (or some other reliable
client-identifying information) and use the resulting
hash value to select the appropriate relay destination
(or set of destinations). The simplest solution, of
course, is to not use a load-balancing scheme and just
relay ALL received BOOTREQUEST messages to the same
destination (or set of destinations).
When transmitting the request to its next destination, the relay
agent may set the IP Time-To-Live field to either the default
value for new datagrams originated by the relay agent, or to the
TTL of the original BOOTREQUEST decremented by (at least) one.
DISCUSSION:
As an extra precaution against BOOTREQUEST loops, it is
preferable to use the decremented TTL from the original
BOOTREQUEST. Unfortunately, this may be difficult to do
in some implementations.
The UDP checksum must be recalculated before transmitting the
request.
3.1.2 BOOTREPLY Messages
BOOTP relay agents relay BOOTREPLY messages only to BOOTP
clients. It is the responsibility of BOOTP servers to send
BOOTREPLY messages directly to the relay agent identified in the
'giaddr' field. Therefore, a relay agent may assume that all
BOOTREPLY messages it receives are intended for BOOTP clients on
its directly-connected networks.
When a relay agent receives a BOOTREPLY message, it should
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examine the BOOTP 'giaddr', 'yiaddr', 'chaddr', 'htype', and
'hlen' fields. These fields should provide adequate information
for the relay agent to deliver the BOOTREPLY message to the
client.
The 'giaddr' field can be used to identify the logical interface
to which the reply must be sent (i.e. the host or router
interface connected to the same network as the BOOTP client).
If the content of the 'giaddr' field does not match one of the
relay agent's directly-connected logical interfaces, the
BOOTREPLY messsage MUST be silently discarded.
The 'htype', 'hlen', and 'chaddr' fields supply the link-layer
hardware type, hardware address length, and hardware address of
the client as defined in the ARP protocol [6] and the Assigned
Numbers document [7]. The 'yiaddr' field is the IP address of
the client, as assigned by the BOOTP server.
The relay agent SHOULD examine the newly-defined BROADCAST flag
(see Sections 2 and 4.1.1 for more information). If this flag
is set to 1, the reply SHOULD be sent as an IP broadcast using
an IP broadcast address (preferably 255.255.255.255) as the IP
destination address and the link-layer broadcast address as the
link-layer destination address. If the BROADCAST flag is
cleared (0), the reply SHOULD be sent as an IP unicast to the IP
address specified by the 'yiaddr' field and the link-layer
address specified in the 'chaddr' field. If unicasting is not
possible, the reply MAY be sent to the link-layer broadcast
address using an IP broadcast address (preferably
255.255.255.255) as the IP destination address.
DISCUSSION:
The addition of the BROADCAST flag to the protocol is a
workaround to help promote interoperability with certain
client implementations.
Note that since the 'flags' field was previously defined
in [1] simply as an "unused" field, it is possible that
old client or server implementations may accidentally
and unknowingly set the new BROADCAST flag. It is
actually expected that such implementations will be rare
(most implementations seem to zero-out this field), but
interactions with such implementations must nevertheless
be considered. If an old client or server does set the
BROADCAST flag to 1 incorrectly, conforming relay agents
will generate broadcast BOOTREPLY messages to the
corresponding client. The BOOTREPLY messages should
still properly reach the client, at the cost of one
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(otherwise unnecessary) additional broadcast. This,
however, is no worse than a server or relay agent which
always broadcasts its BOOTREPLY messages.
The reply MUST have its UDP destination port set to BOOTPC (68).
The UDP checksum must be recalculated before transmitting the
reply.
4. BOOTP Client Behavior
This section clarifies various issues regarding BOOTP client behavior.
4.1 Client use of the 'flags' field
4.1.1 The BROADCAST flag
Normally, BOOTP servers and relay agents attempt to deliver
BOOTREPLY messages directly to a client using unicast delivery.
The IP destination address (in the IP header) is set to the
BOOTP 'yiaddr' address and the link-layer destination address is
set to the BOOTP 'chaddr' address. Unfortunately, some client
implementations are unable to receive such unicast IP datagrams
until they know their own IP address (thus we have a "chicken
and egg" issue). Often, however, they can receive broadcast IP
datagrams (those with a valid IP broadcast address as the IP
destination and the link-layer broadcast address as the
link-layer destination).
If a client falls into this category, it SHOULD set (to 1) the
newly-defined BROADCAST flag in the 'flags' field of BOOTREPLY
messages it generates. This will provide a hint to BOOTP
servers and relay agents that they should attempt to broadcast
their BOOTREPLY messages to the client.
If a client does not have this limitation (i.e. it is perfectly
able to receive unicast BOOTREPLY messages), it SHOULD NOT set
the BROADCAST flag (i.e. it SHOULD clear the BROADCAST flag to
0).
DISCUSSION:
This addition to the protocol is a workaround for old
host implementations. It is strongly recommended that
such implementations be modified so that they may
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receive unicast BOOTREPLY messages, thus making use of
this workaround unnecessary. In general, the use of
this mechanism is discouraged.
4.1.2 The remainder of the 'flags' field
The remaining bits of the 'flags' field are reserved for future
use. A client MUST set these bits to zero in all BOOTREQUEST
messages it generates. A client MUST ignore these bits in all
BOOTREPLY messages it receives.
4.2 Definition of the 'secs' field
The 'secs' field of a BOOTREQUEST message SHOULD represent the
elapsed time, in seconds, since the client sent its first
BOOTREQUEST message. Note that this implies that the 'secs' field
of the first BOOTREQUEST message SHOULD be set to zero.
Clients SHOULD NOT set the 'secs' field to a value which is constant
for all BOOTREQUEST messages.
DISCUSSION:
The original definition of the 'secs' field was vague. It
was not clear whether it represented the time since the
first BOOTREQUEST message was sent or some other time period
such as the time since the client machine was powered-up.
This has limited its usefulness as a policy control for
BOOTP servers and relay agents. Furthermore, certain client
implementations have been known to simply set this field to
a constant value or use incorrect byte-ordering (usually
resulting in very inflated figures).
4.3 Interpretation of the 'giaddr' field
The 'giaddr' field is rather poorly named. It exists to facilitate
the transfer of BOOTREQUEST messages from a client, through BOOTP
relay agents, to servers on different networks than the client.
Similarly, it facilitates the delivery of BOOTREPLY messages from
the servers, through BOOTP relay agents, back to the client. In no
case does it represent a general IP router to be used by the client.
A BOOTP client MUST set the 'giaddr' field to zero (0.0.0.0) in all
BOOTREQUEST messages it generates.
A BOOTP client MUST NOT consider the 'giaddr' field of a BOOTREPLY
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message to represent an IP router. A BOOTP client SHOULD completely
ignore the contents of the 'giaddr' field in BOOTREPLY messages.
DISCUSSION:
The semantics of the 'giaddr' field were poorly defined.
Section 7.5 of [1] states:
"If 'giaddr' (gateway address) is nonzero, then the
packets should be forwarded there first, in order to
get to the server."
In that sentence, "get to" refers to communication from the
client to the server subsequent to the BOOTP exchange, such
as a TFTP session. Unfortunately, the 'giaddr' field may
contain the address of a BOOTP relay agent that is not
itself an IP router (according to [1], Section 8, fifth
paragraph), in which case, it will be useless as a first-hop
for TFTP packets sent to the server (since, by definition,
non-routers don't forward datagrams at the IP layer).
Although now prohibited by Section 3.1.1 of this memo, the
'giaddr' field might contain a broadcast address according
to Section 8, sixth paragraph of [1]. Not only would such
an address be useless as a router address, it might also
cause the client to ARP for the broadcast address (since, if
the client didn't receive a subnet mask in the BOOTREPLY
message, it would be unable to recognize a subnet broadcast
address). This is clearly undesirable.
To reach a non-local server, clients can obtain a first-hop
router address from the "Gateway" subfield of the "Vendor
Information Extensions" [2,3] (if present), or from some
other router discovery protocol.
4.4 Vendor information "magic cookie"
It is RECOMMENDED that a BOOTP client always fill the first four
octets of the 'vend' (vendor information) field of a BOOTREQUEST
with a four-octet identifier called a "magic cookie." A BOOTP
client SHOULD do this even if it has no special information to
communicate to the BOOTP server using the 'vend' field. This aids
the BOOTP server in determining what vendor information format it
should use in its BOOTREPLY messages.
If a special vendor-specific magic cookie is not being used, a BOOTP
client SHOULD use the dotted decimal value 99.130.83.99 as specified
in [2,3]. In this case, if the client has no information to
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communicate to the server, the octet immediately following the magic
cookie SHOULD be set to the "End" tag (255) and the remaining octets
of the 'vend' field SHOULD be set to zero.
DISCUSSION:
Sometimes different operating systems or networking packages
are run on the same machine at different times (or even at
the same time!). Since the hardware address placed in the
'chaddr' field will likely be the same, BOOTREQUESTs from
completely different BOOTP clients on the same machine will
likely be difficult for a BOOTP server to differentiate. If
the client includes a magic cookie in its BOOTREQUEST
messages, the server will at least know what format the
client expects and can understand in corresponding BOOTREPLY
messages.
5. Bit Ordering of Hardware Addresses
The bit ordering used for link-level hardware addresses in the 'chaddr'
field SHOULD be the same as the ordering used for the ARP protocol [6]
on the client's network (assuming ARP is defined for that network).
DISCUSSION:
One of the primary reasons the 'chaddr' field exists is to
enable BOOTP servers and relay agents to communicate directly
with clients without the use of broadcasts. In practice, the
contents of the 'chaddr' field is often used to create an
ARP-cache entry in exactly the same way the normal ARP protocol
would have. Clearly, interoperability can only be achieved if a
consistent interpretation of the 'chaddr' field is used.
6. BOOTP Over IEEE 802.5 Token Ring Networks
Special consideration of the client/server and client/relay agent
interactions must be given to 802.5 networks because of non-transparent
bridging. In the simplest case, an unbridged, single ring network, the
broadcast behavior of the BOOTP protocol is identical to that of
Ethernet networks. However, a BOOTP client cannot know, a priori, that
an 802.5 network is not bridged. In fact, the likelihood is that the
server, or relay agent, will not know either.
Of the four possible scenerios, only two are interesting: where the
assumption is that the 802.5 network is not bridged and it is, and the
assumption that the network is bridged and it is not. In the former
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case, the Routing Information Field (RIF) will not be used; therefore,
if the server/relay agent are on another segment of the ring, the client
cannot reach it. In the latter case, the RIF field will be used,
resulting in a few extraneous bytes on the ring. It is obvious that an
almost immeasurable inefficiency is to be preferred over a complete
failure to communicate.
Given that the assumption is that RIF fields will be needed, it is
necesary to determine the optimum method for the client to reach the
server/relay agent, and the optimum method for the response to be
returned.
The client SHOULD send its broadcast BOOTREQUEST with an All Routes
Explorer RIF. This will enable servers/relay agents to cache the return
route if they choose to do so. For those server/relay agents which
cannot cache the return route (because they are stateless, for example),
the BOOTREPLY message is sent to the client's hardware address, as taken
from the BOOTP message, with a Spanning Tree Rooted RIF. The actual
bridge route will be recorded by the client and server/relay agent by
normal ARP processing code.
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Security Considerations
BOOTP is built directly upon UDP and IP which are as yet inherently
insecure. Furthermore, BOOTP is generally intended to make maintenance
of remote and/or diskless hosts easier. While perhaps not impossible,
configuring such hosts with passwords or keys may be difficult and
inconvenient. Therefore, BOOTP in its current form is quite insecure.
Unauthorized BOOTP servers may easily be set up. Such servers can then
send false and potentially disruptive information to clients such as
incorrect or duplicate IP addresses, incorrect routing information
(including spoof routers, etc.), incorrect domain nameserver addresses
(such as spoof nameservers), and so on. Clearly, once this "seed"
mis-information is planted, an attacker can further compromise the
affected systems.
BOOTP relay agents suffer some of the same problems as BOOTP servers.
Malicious BOOTP clients could masquerade as legitimate clients and
retrieve information intended for those legitimate clients. Where
dynamic allocation of resources is used, a malicious client could claim
all resources for itself, thereby denying resources to legitimate
clients.
Acknowledgements
The author would like to thank Gary Malkin of FTP Software, Inc. for his
contribution of the "BOOTP over IEEE 802.5 Token Ring Networks" section,
and Steve Deering of Xerox PARC for his observations on the problems
associated with the 'giaddr' field.
Ralph Droms of Bucknell University and the many members of the IETF
Dynamic Host Configuration and Router Requirements working groups
provided ideas for this memo as well as encouragement to write it.
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References
[1] Croft, B., and J. Gilmore. Bootstrap Protocol (BOOTP). Request
For Comments (RFC) 951, DDN Network Information Center, SRI
International, Menlo Park, California, USA, September, 1985.
[2] Prindeville, P. BOOTP Vendor Information Extensions. Request For
Comments (RFC) 1048, DDN Network Information Center, SRI
International, Menlo Park, California, USA, February, 1988.
[3] Reynolds, J. BOOTP Vendor Information Extensions. Request For
Comments (RFC) 1084, DDN Network Information Center, SRI
International, Menlo Park, California, USA, December, 1988.
[4] Almquist, P. Requirements for IP Routers. Internet Draft,
Corporation for National Research Initiatives, Reston, Virginia,
USA, May, 1991.
[5] Droms, R. Dynamic Host Configuration Protocol. Internet Draft,
Corporation for National Research Initiatives, Reston, Virginia,
USA, June, 1992.
[6] Plummer, D. An Ethernet Address Resolution Protocol. Request For
Comments (RFC) 826, DDN Network Information Center, SRI
International, Menlo Park, California, USA, November, 1982.
[7] Reynolds, J., and J. Postel. Assigned Numbers. Request For
Comments (RFC) 1340, DDN Network Information Center, SRI
International, Menlo Park, California, USA, July, 1992. This RFC
is periodically reissued with a new number. Please be sure to
consult the latest version.
Author's Address
Walt Wimer
Network Development
Carnegie Mellon University
4910 Forbes Avenue
Pittsburgh, PA 15213-3890
Phone: (412) 268-6252
EMail: Walter.Wimer@ANDREW.CMU.EDU
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