Network Working Group M. Eisler

Network Working Group M. Eisler
Request for Comments: 2203 A. Chiu
Category: Standards Track L. Ling
September 1997

RPCSEC_GSS Protocol Specification

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Abstract

This memo describes an ONC/RPC security flavor that allows RPC
protocols to access the Generic Security Services Application
Programming Interface (referred to henceforth as GSS-API).

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. The ONC RPC Message Protocol . . . . . . . . . . . . . . . . . 2
3. Flavor Number Assignment . . . . . . . . . . . . . . . . . . . 3
4. New auth_stat Values . . . . . . . . . . . . . . . . . . . . . 3
5. Elements of the RPCSEC_GSS Security Protocol . . . . . . . . . 3
5.1. Version Selection . . . . . . . . . . . . . . . . . . . . . 5
5.2. Context Creation . . . . . . . . . . . . . . . . . . . . . . 5
5.2.1. Mechanism and QOP Selection . . . . . . . . . . . . . . . 5
5.2.2. Context Creation Requests . . . . . . . . . . . . . . . . 6
5.2.3. Context Creation Responses . . . . . . . . . . . . . . . . 8
5.2.3.1. Context Creation Response - Successful Acceptance . . . 8
5.2.3.1.1. Client Processing of Successful Context Creation
Responses . . . . . . . . . . . . . . . . . . . . . . 9
5.2.3.2. Context Creation Response - Unsuccessful Cases . . . . . 9
5.3. RPC Data Exchange . . . . . . . . . . . . . . . . . . . . 10
5.3.1. RPC Request Header . . . . . . . . . . . . . . . . . . . 10
5.3.2. RPC Request Data . . . . . . . . . . . . . . . . . . . . 11
5.3.2.1. RPC Request Data - No Data Integrity . . . . . . . . . 11
5.3.2.2. RPC Request Data - With Data Integrity . . . . . . . . 11
5.3.2.3. RPC Request Data - With Data Privacy . . . . . . . . . 12
5.3.3. Server Processing of RPC Data Requests . . . . . . . . . 12
5.3.3.1. Context Management . . . . . . . . . . . . . . . . . . 12
5.3.3.2. Server Reply - Request Accepted . . . . . . . . . . . 14
5.3.3.3. Server Reply - Request Denied . . . . . . . . . . . . 15

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5.3.3.4. Mapping of GSS-API Errors to Server Responses . . . . 16
5.3.3.4.1. GSS_GetMIC() Failure . . . . . . . . . . . . . . . . 16
5.3.3.4.2. GSS_VerifyMIC() Failure . . . . . . . . . . . . . . 16
5.3.3.4.3. GSS_Unwrap() Failure . . . . . . . . . . . . . . . . 16
5.3.3.4.4. GSS_Wrap() Failure . . . . . . . . . . . . . . . . . 16
5.4. Context Destruction . . . . . . . . . . . . . . . . . . . 17
6. Set of GSS-API Mechanisms . . . . . . . . . . . . . . . . . 17
7. Security Considerations . . . . . . . . . . . . . . . . . . 18
7.1. Privacy of Call Header . . . . . . . . . . . . . . . . . . 18
7.2. Sequence Number Attacks . . . . . . . . . . . . . . . . . 18
7.2.1. Sequence Numbers Above the Window . . . . . . . . . . . 18
7.2.2. Sequence Numbers Within or Below the Window . . . . . . 18
7.3. Message Stealing Attacks . . . . . . . . . . . . . . . . . 19
Appendix A. GSS-API Major Status Codes . . . . . . . . . . . . . 20
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23

1. Introduction

This document describes the protocol used by the RPCSEC_GSS security
flavor. Security flavors have been called authentication flavors for
historical reasons. This memo recognizes that there are two other
security services besides authentication, integrity, and privacy, and
so defines a new RPCSEC_GSS security flavor.

The protocol is described using the XDR language [Srinivasan-xdr].
The reader is assumed to be familiar with ONC RPC and the security
flavor mechanism [Srinivasan-rpc]. The reader is also assumed to be
familiar with the GSS-API framework [Linn]. The RPCSEC_GSS security
flavor uses GSS-API interfaces to provide security services that are
independent of the underlying security mechanism.

2. The ONC RPC Message Protocol

This memo refers to the following XDR types of the ONC RPC protocol,
which are described in the document entitled Remote Procedure Call
Protocol Specification Version 2 [Srinivasan-rpc]:

msg_type
reply_stat
auth_flavor
accept_stat
reject_stat
auth_stat
opaque_auth
rpc_msg
call_body
reply_body

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accepted_reply
rejected_reply

3. Flavor Number Assignment

The RPCSEC_GSS security flavor has been assigned the value of 6:

enum auth_flavor {
...
RPCSEC_GSS = 6 /* RPCSEC_GSS security flavor */
};

4. New auth_stat Values

RPCSEC_GSS requires the addition of two new values to the auth_stat
enumerated type definition:

enum auth_stat {
...
/*
* RPCSEC_GSS errors
*/
RPCSEC_GSS_CREDPROBLEM = 13,
RPCSEC_GSS_CTXPROBLEM = 14
};

The descriptions of these two new values are defined later in this
memo.

5. Elements of the RPCSEC_GSS Security Protocol

An RPC session based on the RPCSEC_GSS security flavor consists of
three phases: context creation, RPC data exchange, and context
destruction. In the following discussion, protocol elements for
these three phases are described.

The following description of the RPCSEC_GSS protocol uses some of the
definitions within XDR language description of the RPC protocol.

Context creation and destruction use control messages that are not
dispatched to service procedures registered by an RPC server. The
program and version numbers used in these control messages are the
same as the RPC service's program and version numbers. The procedure
number used is NULLPROC (zero). A field in the credential
information (the gss_proc field which is defined in the
rpc_gss_cred_t structure below) specifies whether a message is to be
interpreted as a control message or a regular RPC message. If this
field is set to RPCSEC_GSS_DATA, no control action is implied; in

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this case, it is a regular data message. If this field is set to any
other value, a control action is implied. This is described in the
following sections.

Just as with normal RPC data exchange messages, the transaction
identifier (the xid field in struct rpc_msg), should be set to unique
values on each call for context creation and context destruction.

The following definitions are used for describing the protocol.

/* RPCSEC_GSS control procedures */

enum rpc_gss_proc_t {
RPCSEC_GSS_DATA = 0,
RPCSEC_GSS_INIT = 1,
RPCSEC_GSS_CONTINUE_INIT = 2,
RPCSEC_GSS_DESTROY = 3
};

/* RPCSEC_GSS services */

enum rpc_gss_service_t {
/* Note: the enumerated value for 0 is reserved. */
rpc_gss_svc_none = 1,
rpc_gss_svc_integrity = 2,
rpc_gss_svc_privacy = 3
};

/* Credential */

/*
* Note: version 0 is reserved for possible future
* definition of a version negotiation protocol
*
*/
#define RPCSEC_GSS_VERS_1 1

struct rpc_gss_cred_t {
union switch (unsigned int version) { /* version of
RPCSEC_GSS */
case RPCSEC_GSS_VERS_1:
struct {
rpc_gss_proc_t gss_proc; /* control procedure */
unsigned int seq_num; /* sequence number */
rpc_gss_service_t service; /* service used */
opaque handle<>; /* context handle */
} rpc_gss_cred_vers_1_t;

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}
};

/* Maximum sequence number value */

#define MAXSEQ 0x80000000

5.1. Version Selection

This document defines just one protocol version (RPCSEC_GSS_VERS_1).
The client should assume that the server supports RPCSEC_GSS_VERS_1
and issue a Context Creation message (as described in the section
RPCSEC_GSS_VERS_1, the RPC response will have a reply_stat of
MSG_DENIED, a rejection status of AUTH_ERROR, and an auth_stat of
AUTH_REJECTED_CRED.

5.2. Context Creation

Before RPC data is exchanged on a session using the RPCSEC_GSS
flavor, a context must be set up between the client and the server.
Context creation may involve zero or more RPC exchanges. The number
of exchanges depends on the security mechanism.

5.2.1. Mechanism and QOP Selection

There is no facility in the RPCSEC_GSS protocol to negotiate GSS-API
mechanism identifiers or QOP values. At minimum, it is expected that
implementations of the RPCSEC_GSS protocol provide a means to:

* specify mechanism identifiers, QOP values, and RPCSEC_GSS
service values on the client side, and to

* enforce mechanism identifiers, QOP values, and RPCSEC_GSS
service values on a per-request basis on the server side.

It is necessary that above capabilities exist so that applications
have the means to conform the required set of required set of
<mechanism, QOP, service> tuples (See the section entitled Set of
GSS-API Mechanisms). An application may negotiate <mechanism, QOP,
service> selection within its protocol or via an out of band
protocol. Hence it may be necessary for RPCSEC_GSS implementations to
provide programming interfaces for the specification and enforcement
of <mechanism, QOP, service>.

Additionally, implementations may depend on negotiation schemes
constructed as pseudo-mechanisms under the GSS-API. Because such
schemes are below the GSS-API layer, the RPCSEC_GSS protocol, as
specified in this document, can make use of them.

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5.2.2. Context Creation Requests

The first RPC request from the client to the server initiates context
creation. Within the RPC message protocol's call_body structure,
rpcvers is set to 2. prog and vers are always those for the service
being accessed. The proc is always set to NULLPROC (zero).

Within the RPC message protocol's cred structure, flavor is set to
RPCSEC_GSS (6). The opaque data of the cred structure (the body
field) constituting the credential encodes the rpc_gss_cred_t
structure defined previously.

The values of the fields contained in the rpc_gss_cred_t structure
are set as follows. The version field is set to the version of the
RPCSEC_GSS protocol the client wants to use. The remainder of this
memo documents version RPCSEC_GSS_VERS_1 of RPCSEC_GSS, and so the
version field would be set to RPCSEC_GSS_VERS_1. The gss_proc field
must be set to RPCSEC_GSS_INIT for the first creation request. In
subsequent creation requests, the gss_proc field must be set to
RPCSEC_GSS_CONTINUE_INIT. In a creation request, the seq_num and
service fields are undefined and both must be ignored by the server.
In the first creation request, the handle field is NULL (opaque data
of zero length). In subsequent creation requests, handle must be
equal to the value returned by the server. The handle field serves
as the identifier for the context, and will not change for the
duration of the context, including responses to
RPCSEC_GSS_CONTINUE_INIT.

The verifier field in the RPC message header is also described by the
opaque_auth structure. All creation requests have the NULL verifier
(AUTH_NONE flavor with zero length opaque data).

Following the verifier are the call data (procedure specific
parameters). Note that the proc field of the call_body structure is
set to NULLPROC, and thus normally there would be zero octets
following the verifier. However, since there is no RPC data exchange
during a context creation, it is safe to transfer information
following the verifier. It is necessary to "overload" the call data
in this way, rather than pack the GSS-API token into the RPC header,
because RPC Version 2 restricts the amount of data that can be sent
in the header. The opaque body of the credential and verifier fields
can be each at most 400 octets long, and GSS tokens can be longer
than 800 octets.

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The call data for a context creation request is described by the
following structure for all creation requests:

struct rpc_gss_init_arg {
opaque gss_token<>;
};

Here, gss_token is the token returned by the call to GSS-API's
GSS_Init_sec_context() routine, opaquely encoded. The value of this
field will likely be different in each creation request, if there is
more than one creation request. If no token is returned by the call
to GSS_Init_sec_context(), the context must have been created
(assuming no errors), and there will not be any more creation
requests.

When GSS_Init_sec_context() is called, the parameters
replay_det_req_flag and sequence_req_flag must be turned off. The
reasons for this are:

* ONC RPC can be used over unreliable transports and provides no
layer to reliably re-assemble messages. Thus it is possible for
gaps in message sequencing to occur, as well as out of order
messages.

* RPC servers can be multi-threaded, and thus the order in which
GSS-API messages are signed or wrapped can be different from the
order in which the messages are verified or unwrapped, even if
the requests are sent on reliable transports.

* To maximize convenience of implementation, the order in which an
ONC RPC entity will verify the header and verify/unwrap the body
of an RPC call or reply is left unspecified.

The RPCSEC_GSS protocol provides for protection from replay attack,
yet tolerates out-of-order delivery or processing of messages and
tolerates dropped requests.

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5.2.3. Context Creation Responses

5.2.3.1. Context Creation Response - Successful Acceptance

The response to a successful creation request has an MSG_ACCEPTED
response with a status of SUCCESS. The results field encodes a
response with the following structure:

struct rpc_gss_init_res {
opaque handle<>;
unsigned int gss_major;
unsigned int gss_minor;
unsigned int seq_window;
opaque gss_token<>;
};

Here, handle is non-NULL opaque data that serves as the context
identifier. The client must use this value in all subsequent requests
whether control messages or otherwise). The gss_major and gss_minor
fields contain the results of the call to GSS_Accept_sec_context()
executed by the server. The values for the gss_major field are
defined in Appendix A of this document. The values for the gss_minor
field are GSS-API mechanism specific and are defined in the
mechanism's specification. If gss_major is not one of GSS_S_COMPLETE
or GSS_S_CONTINUE_NEEDED, the context setup has failed; in this case
handle and gss_token must be set to NULL by the server. The value of
gss_minor is dependent on the value of gss_major and the security
mechanism used. The gss_token field contains any token returned by
the GSS_Accept_sec_context() call executed by the server. A token
may be returned for both successful values of gss_major. If the
value is GSS_S_COMPLETE, it indicates that the server is not
expecting any more tokens, and the RPC Data Exchange phase must begin
on the subsequent request from the client. If the value is
GSS_S_CONTINUE_NEEDED, the server is expecting another token. Hence
the client must send at least one more creation request (with
gss_proc set to RPCSEC_GSS_CONTINUE_INIT in the request's credential)
carrying the required token.

In a successful response, the seq_window field is set to the sequence
window length supported by the server for this context. This window
specifies the maximum number of client requests that may be
outstanding for this context. The server will accept "seq_window"
requests at a time, and these may be out of order. The client may
use this number to determine the number of threads that can
simultaneously send requests on this context.

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If gss_major is GSS_S_COMPLETE, the verifier's (the verf element in
the response) flavor field is set to RPCSEC_GSS, and the body field
set to the checksum of the seq_window (in network order). The QOP
used for this checksum is 0 (zero), which is the default QOP. For
all other values of gss_major, a NULL verifier (AUTH_NONE flavor with
zero-length opaque data) is used.

5.2.3.1.1. Client Processing of Successful Context Creation Responses

If the value of gss_major in the response is GSS_S_CONTINUE_NEEDED,
then the client, per the GSS-API specification, must invoke
GSS_Init_sec_context() using the token returned in gss_token in the
context creation response. The client must then generate a context
creation request, with gss_proc set to RPCSEC_GSS_CONTINUE_INIT.

If the value of gss_major in the response is GSS_S_COMPLETE, and if
the client's previous invocation of GSS_Init_sec_context() returned a
gss_major value of GSS_S_CONTINUE_NEEDED, then the client, per the
GSS-API specification, must invoke GSS_Init_sec_context() using the
token returned in gss_token in the context creation response. If
GSS_Init_sec_context() returns GSS_S_COMPLETE, the context is
successfully set up, and the RPC data exchange phase must begin on
the subsequent request from the client.

5.2.3.2. Context Creation Response - Unsuccessful Cases

An MSG_ACCEPTED reply (to a creation request) with an acceptance
status of other than SUCCESS has a NULL verifier (flavor set to
AUTH_NONE, and zero length opaque data in the body field), and is
formulated as usual for different status values.

An MSG_DENIED reply (to a creation request) is also formulated as
usual. Note that MSG_DENIED could be returned because the server's
RPC implementation does not recognize the RPCSEC_GSS security flavor.
RFC 1831 does not specify the appropriate reply status in this
instance, but common implementation practice appears to be to return
a rejection status of AUTH_ERROR with an auth_stat of
AUTH_REJECTEDCRED. Even though two new values (RPCSEC_GSS_CREDPROBLEM
and RPCSEC_GSS_CTXPROBLEM) have been defined for the auth_stat type,
neither of these two can be returned in responses to context creation
requests. The auth_stat new values can be used for responses to
normal (data) requests. This is described later.

MSG_DENIED might also be returned if the RPCSEC_GSS version number in
the credential is not supported on the server. In that case, the
server returns a rejection status of AUTH_ERROR, with an auth_stat of

AUTH_REJECTED_CRED.

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5.3. RPC Data Exchange

The data exchange phase is entered after a context has been
successfully set up. The format of the data exchanged depends on the
security service used for the request. Although clients can change
the security service and QOP used on a per-request basis, this may
not be acceptable to all RPC services; some RPC services may "lock"
the data exchange phase into using the QOP and service used on the
first data exchange message. For all three modes of service (no data
integrity, data integrity, data privacy), the RPC request header has
the same format.

5.3.1. RPC Request Header

The credential has the opaque_auth structure described earlier. The
flavor field is set to RPCSEC_GSS. The credential body is created by
XDR encoding the rpc_gss_cred_t structure listed earlier into an
octet stream, and then opaquely encoding this octet stream as the
body field.

Values of the fields contained in the rpc_gss_cred_t structure are
set as follows. The version field is set to same version value that
was used to create the context, which within the scope of this memo
will always be RPCSEC_GSS_VERS_1. The gss_proc field is set to
RPCSEC_GSS_DATA. The service field is set to indicate the desired
service (one of rpc_gss_svc_none, rpc_gss_svc_integrity, or
rpc_gss_svc_privacy). The handle field is set to the context handle
value received from the RPC server during context creation. The
seq_num field can start at any value below MAXSEQ, and must be
incremented (by one or more) for successive requests. Use of
sequence numbers is described in detail when server processing of the
request is discussed.

The verifier has the opaque_auth structure described earlier. The
flavor field is set to RPCSEC_GSS. The body field is set as follows.
The checksum of the RPC header (up to and including the credential)
is computed using the GSS_GetMIC() call with the desired QOP. This
returns the checksum as an opaque octet stream and its length. This
is encoded into the body field. Note that the QOP is not explicitly
specified anywhere in the request. It is implicit in the checksum or
encrypted data. The same QOP value as is used for the header
checksum must also be used for the data (for checksumming or
encrypting), unless the service used for the request is
rpc_gss_svc_none.

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5.3.2. RPC Request Data

5.3.2.1. RPC Request Data - No Data Integrity

If the service specified is rpc_gss_svc_none, the data (procedure
arguments) are not integrity or privacy protected. They are sent in
exactly the same way as they would be if the AUTH_NONE flavor were
used (following the verifier). Note, however, that since the RPC
header is integrity protected, the sender will still be authenticated
in this case.

5.3.2.2. RPC Request Data - With Data Integrity

When data integrity is used, the request data is represented as
follows:

struct rpc_gss_integ_data {
opaque databody_integ<>;
opaque checksum<>;
};

The databody_integ field is created as follows. A structure
consisting of a sequence number followed by the procedure arguments
is constructed. This is shown below as the type rpc_gss_data_t:

struct rpc_gss_data_t {
unsigned int seq_num;
proc_req_arg_t arg;
};

Here, seq_num must have the same value as in the credential. The
type proc_req_arg_t is the procedure specific XDR type describing the
procedure arguments (and so is not specified here). The octet stream
corresponding to the XDR encoded rpc_gss_data_t structure and its
length are placed in the databody_integ field. Note that because the
XDR type of databody_integ is opaque, the XDR encoding of
databody_integ will include an initial four octet length field,
followed by the XDR encoded octet stream of rpc_gss_data_t.

The checksum field represents the checksum of the XDR encoded octet
stream corresponding to the XDR encoded rpc_gss_data_t structure
(note, this is not the checksum of the databody_integ field). This
is obtained using the GSS_GetMIC() call, with the same QOP as was
used to compute the header checksum (in the verifier). The

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GSS_GetMIC() call returns the checksum as an opaque octet stream and
its length. The checksum field of struct rpc_gss_integ_data has an
XDR type of opaque. Thus the checksum length from GSS_GetMIC() is
encoded as a four octet length field, followed by the checksum,
padded to a multiple of four octets.

5.3.2.3. RPC Request Data - With Data Privacy

When data privacy is used, the request data is represented as
follows:

struct rpc_gss_priv_data {
opaque databody_priv<>
};

The databody_priv field is created as follows. The rpc_gss_data_t
structure described earlier is constructed again in the same way as
for the case of data integrity. Next, the GSS_Wrap() call is invoked
to encrypt the octet stream corresponding to the rpc_gss_data_t
structure, using the same value for QOP (argument qop_req to
GSS_Wrap()) as was used for the header checksum (in the verifier) and
conf_req_flag (an argument to GSS_Wrap()) of TRUE. The GSS_Wrap()
call returns an opaque octet stream (representing the encrypted
rpc_gss_data_t structure) and its length, and this is encoded as the
databody_priv field. Since databody_priv has an XDR type of opaque,
the length returned by GSS_Wrap() is encoded as the four octet
length, followed by the encrypted octet stream (padded to a multiple
of four octets).

5.3.3. Server Processing of RPC Data Requests

5.3.3.1. Context Management

When a request is received by the server, the following are verified
to be acceptable:

* the version number in the credential

* the service specified in the credential

* the context handle specified in the credential

* the header checksum in the verifier (via GSS_VerifyMIC())

* the sequence number (seq_num) specified in the credential (more
on this follows)

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The gss_proc field in the credential must be set to RPCSEC_GSS_DATA
for data requests (otherwise, the message will be interpreted as a
control message).

The server maintains a window of "seq_window" sequence numbers,
starting with the last sequence number seen and extending backwards.
If a sequence number higher than the last number seen is received
(AND if GSS_VerifyMIC() on the header checksum from the verifier
returns GSS_S_COMPLETE), the window is moved forward to the new
sequence number. If the last sequence number seen is N, the server
is prepared to receive requests with sequence numbers in the range N
through (N - seq_window + 1), both inclusive. If the sequence number
received falls below this range, it is silently discarded. If the
sequence number is within this range, and the server has not seen it,
the request is accepted, and the server turns on a bit to "remember"
that this sequence number has been seen. If the server determines
that it has already seen a sequence number within the window, the
request is silently discarded. The server should select a seq_window
value based on the number requests it expects to process
simultaneously. For example, in a threaded implementation seq_window
might be equal to the number of server threads. There are no known
security issues with selecting a large window. The primary issue is
how much space the server is willing to allocate to keep track of
requests received within the window.

The reason for discarding requests silently is that the server is
unable to determine if the duplicate or out of range request was due
to a sequencing problem in the client, network, or the operating
system, or due to some quirk in routing, or a replay attack by an
intruder. Discarding the request allows the client to recover after
timing out, if indeed the duplication was unintentional or well
intended. Note that a consequence of the silent discard is that
clients may increment the seq_num by more than one. The effect of
this is that the window will move forward more quickly. It is not
believed that there is any benefit to doing this.

Note that the sequence number algorithm requires that the client
increment the sequence number even if it is retrying a request with
the same RPC transaction identifier. It is not infrequent for
clients to get into a situation where they send two or more attempts
and a slow server sends the reply for the first attempt. With
RPCSEC_GSS, each request and reply will have a unique sequence
number. If the client wishes to improve turn around time on the RPC
call, it can cache the RPCSEC_GSS sequence number of each request it
sends. Then when it receives a response with a matching RPC
transaction identifier, it can compute the checksum of each sequence
number in the cache to try to match the checksum in the reply's
verifier.

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The data is decoded according to the service specified in the
credential. In the case of integrity or privacy, the server ensures
that the QOP value is acceptable, and that it is the same as that
used for the header checksum in the verifier. Also, in the case of
integrity or privacy, the server will reject the message (with a
reply status of MSG_ACCEPTED, and an acceptance status of
GARBAGE_ARGS) if the sequence number embedded in the request body is
different from the sequence number in the credential.

5.3.3.2. Server Reply - Request Accepted

An MSG_ACCEPTED reply to a request in the data exchange phase will
have the verifier's (the verf element in the response) flavor field
set to RPCSEC_GSS, and the body field set to the checksum (the output
of GSS_GetMIC()) of the sequence number (in network order) of the
corresponding request. The QOP used is the same as the QOP used for
the corresponding request.

If the status of the reply is not SUCCESS, the rest of the message is
formatted as usual.

If the status of the message is SUCCESS, the format of the rest of
the message depends on the service specified in the corresponding
request message. Basically, what follows the verifier in this case
are the procedure results, formatted in different ways depending on
the requested service.

If no data integrity was requested, the procedure results are
formatted as for the AUTH_NONE security flavor.

If data integrity was requested, the results are encoded in exactly
the same way as the procedure arguments were in the corresponding
request. See the section 'RPC Request Data - With Data Integrity.'
The only difference is that the structure representing the
procedure's result - proc_res_arg_t - must be substituted in place of
the request argument structure proc_req_arg_t. The QOP used for the
checksum must be the same as that used for constructing the reply
verifier.

If data privacy was requested, the results are encoded in exactly the
same way as the procedure arguments were in the corresponding
request. See the section 'RPC Request Data - With Data Privacy.' The
QOP used for encryption must be the same as that used for
constructing the reply verifier.

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5.3.3.3. Server Reply - Request Denied

An MSG_DENIED reply (to a data request) is formulated as usual. Two
new values (RPCSEC_GSS_CREDPROBLEM and RPCSEC_GSS_CTXPROBLEM) have
been defined for the auth_stat type. When the reason for denial of
the request is a reject_stat of AUTH_ERROR, one of the two new
auth_stat values could be returned in addition to the existing
values. These two new values have special significance from the
existing reasons for denial of a request.

The server maintains a list of contexts for the clients that are
currently in session with it. Normally, a context is destroyed when
the client ends the session corresponding to it. However, due to
resource constraints, the server may destroy a context prematurely
(on an LRU basis, or if the server machine is rebooted, for example).
In this case, when a client request comes in, there may not be a
context corresponding to its handle. The server rejects the request,
with the reason RPCSEC_GSS_CREDPROBLEM in this case. Upon receiving
this error, the client must refresh the context - that is,
reestablish it after destroying the old one - and try the request
again. This error is also returned if the context handle matches
that of a different context that was allocated after the client's
context was destroyed (this will be detected by a failure in
verifying the header checksum).

If the GSS_VerifyMIC() call on the header checksum (contained in the
verifier) fails to return GSS_S_COMPLETE, the server rejects the
request and returns an auth_stat of RPCSEC_GSS_CREDPROBLEM.

When the client's sequence number exceeds the maximum the server will
allow, the server will reject the request with the reason
RPCSEC_GSS_CTXPROBLEM. Also, if security credentials become stale
while in use (due to ticket expiry in the case of the Kerberos V5
mechanism, for example), the failures which result cause the
RPCSEC_GSS_CTXPROBLEM reason to be returned. In these cases also,
the client must refresh the context, and retry the request.

For other errors, retrying will not rectify the problem and the
client must not refresh the context until the problem causing the
client request to be denied is rectified.

If the version field in the credential does not match the version of
RPCSEC_GSS that was used when the context was created, the
AUTH_BADCRED value is returned.

If there is a problem with the credential, such a bad length, illegal
control procedure, or an illegal service, the appropriate auth_stat
status is AUTH_BADCRED.

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Other errors can be returned as appropriate.

5.3.3.4. Mapping of GSS-API Errors to Server Responses

During the data exchange phase, the server may invoke GSS_GetMIC(),
GSS_VerifyMIC(), GSS_Unwrap(), and GSS_Wrap(). If any of these
routines fail to return GSS_S_COMPLETE, then various unsuccessful
responses can be returned. The are described as follows for each of
the aforementioned four interfaces.

5.3.3.4.1. GSS_GetMIC() Failure

When GSS_GetMIC() is called to generate the verifier in the response,
a failure results in an RPC response with a reply status of
MSG_DENIED, reject status of AUTH_ERROR and an auth status of
RPCSEC_GSS_CTXPROBLEM.

When GSS_GetMIC() is called to sign the call results (service is
rpc_gss_svc_integrity), a failure results in no RPC response being
sent. Since ONC RPC server applications will typically control when a
response is sent, the failure indication will be returned to the
server application and it can take appropriate action (such as
logging the error).

5.3.3.4.2. GSS_VerifyMIC() Failure

When GSS_VerifyMIC() is called to verify the verifier in request, a
failure results in an RPC response with a reply status of MSG_DENIED,
reject status of AUTH_ERROR and an auth status of
RPCSEC_GSS_CREDPROBLEM.

When GSS_VerifyMIC() is called to verify the call arguments (service
is rpc_gss_svc_integrity), a failure results in an RPC response with
a reply status of MSG_ACCEPTED, and an acceptance status of
GARBAGE_ARGS.

5.3.3.4.3. GSS_Unwrap() Failure

When GSS_Unwrap() is called to decrypt the call arguments (service is
rpc_gss_svc_privacy), a failure results in an RPC response with a
reply status of MSG_ACCEPTED, and an acceptance status of
GARBAGE_ARGS.

5.3.3.4.4. GSS_Wrap() Failure

When GSS_Wrap() is called to encrypt the call results (service is
rpc_gss_svc_privacy), a failure results in no RPC response being
sent. Since ONC RPC server applications will typically control when a

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response is sent, the failure indication will be returned to the
application and it can take appropriate action (such as logging the
error).

5.4. Context Destruction

When the client is done using the session, it must send a control
message informing the server that it no longer requires the context.
This message is formulated just like a data request packet, with the
following differences: the credential has gss_proc set to
RPCSEC_GSS_DESTROY, the procedure specified in the header is
NULLPROC, and there are no procedure arguments. The sequence number
in the request must be valid, and the header checksum in the verifier
must be valid, for the server to accept the message. The server
sends a response as it would to a data request. The client and
server must then destroy the context for the session.

If the request to destroy the context fails for some reason, the
client need not take any special action. The server must be prepared
to deal with situations where clients never inform the server that
they no longer are in session and so don't need the server to
maintain a context. An LRU mechanism or an aging mechanism should be
employed by the server to clean up in such cases.

6. Set of GSS-API Mechanisms

RPCSEC_GSS is effectively a "pass-through" to the GSS-API layer, and
as such it is inappropriate for the RPCSEC_GSS specification to
enumerate a minimum set of required security mechanisms and/or
quality of protections.

If an application protocol specification references RPCSEC_GSS, the
protocol specification must list a mandatory set of { mechanism, QOP,
service } triples, such that an implementation cannot claim
conformance to the protocol specification unless it implements the
set of triples. Within each triple, mechanism is a GSS-API security
mechanism, QOP is a valid quality-of-protection within the mechanism,
and service is either rpc_gss_svc_integrity or rpc_gss_svc_privacy.

For example, a network filing protocol built on RPC that depends on
RPCSEC_GSS for security, might require that Kerberos V5 with the
default QOP using the rpc_gss_svc_integrity service be supported by
implementations conforming to the network filing protocol
specification.

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RFC 2203 RPCSEC_GSS Protocol Specification September 1997

7. Security Considerations

7.1. Privacy of Call Header

The reader will note that for the privacy option, only the call
arguments and results are encrypted. Information about the
application in the form of RPC program number, program version
number, and program procedure number is transmitted in the clear.
Encrypting these fields in the RPC call header would have changed the
size and format of the call header. This would have required revising
the RPC protocol which was beyond the scope of this proposal. Storing
the encrypted numbers in the credential would have obviated a
protocol change, but would have introduced more overloading of fields
and would have made implementations of RPC more complex. Even if the
fields were encrypted somehow, in most cases an attacker can
determine the program number and version number by examining the
destination address of the request and querying the rpcbind service
on the destination host [Srinivasan-bind]. In any case, even by not
encrypting the three numbers, RPCSEC_GSS still improves the state of
security over what existing RPC services have had available
previously. Implementors of new RPC services that are concerned about
this risk may opt to design in a "sub-procedure" field that is
included in the service specific call arguments.

7.2. Sequence Number Attacks

7.2.1. Sequence Numbers Above the Window

An attacker cannot coax the server into raising the sequence number
beyond the range the legitimate client is aware of (and thus engineer
a denial of server attack) without constructing an RPC request that
will pass the header checksum. If the cost of verifying the header
checksum is sufficiently large (depending on the speed of the
processor doing the checksum and the cost of checksum algorithm), it
is possible to envision a denial of service attack (vandalism, in the
form of wasting processing resources) whereby the attacker sends
requests that are above the window. The simplest method might be for
the attacker to monitor the network traffic and then choose a
sequence number that is far above the current sequence number. Then
the attacker can send bogus requests using the above window sequence
number.

7.2.2. Sequence Numbers Within or Below the Window

If the attacker sends requests that are within or below the window,
then even if the header checksum is successfully verified, the server
will silently discard the requests because the server assumes it has
already processed the request. In this case, a server can optimize by

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skipping the header checksum verification if the sequence number is
below the window, or if it is within the window, not attempt the
checksum verification if the sequence number has already been seen.

7.3. Message Stealing Attacks

This proposal does not address attacks where an attacker can block or
steal messages without being detected by the server. To implement
such protection would be tantamount to assuming a state in the RPC
service. RPCSEC_GSS does not worsen this situation.

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RFC 2203 RPCSEC_GSS Protocol Specification September 1997

Appendix A. GSS-API Major Status Codes

The GSS-API definition [Linn] does not include numerical values for
the various GSS-API major status codes. It is expected that this will
be addressed in future RFC. Until then, this appendix defines the
values for each GSS-API major status code listed in the GSS-API
definition. If in the future, the GSS-API definition defines values
for the codes that are different than what follows, then implementors
of RPCSEC_GSS will be obliged to map them into the values defined
below. If in the future, the GSS-API definition defines additional
status codes not defined below, then the RPCSEC_GSS definition will
subsume those additional values.

Here are the definitions of each GSS_S_* major status that the
implementor of RPCSEC_GSS can expect in the gss_major major field of
rpc_gss_init_res. These definitions are not in RPC description
language form. The numbers are in base 16 (hexadecimal):

GSS_S_COMPLETE 0x00000000
GSS_S_CONTINUE_NEEDED 0x00000001
GSS_S_DUPLICATE_TOKEN 0x00000002
GSS_S_OLD_TOKEN 0x00000004
GSS_S_UNSEQ_TOKEN 0x00000008
GSS_S_GAP_TOKEN 0x00000010
GSS_S_BAD_MECH 0x00010000
GSS_S_BAD_NAME 0x00020000
GSS_S_BAD_NAMETYPE 0x00030000
GSS_S_BAD_BINDINGS 0x00040000
GSS_S_BAD_STATUS 0x00050000
GSS_S_BAD_MIC 0x00060000
GSS_S_BAD_SIG 0x00060000
GSS_S_NO_CRED 0x00070000
GSS_S_NO_CONTEXT 0x00080000
GSS_S_DEFECTIVE_TOKEN 0x00090000
GSS_S_DEFECTIVE_CREDENTIAL 0x000a0000
GSS_S_CREDENTIALS_EXPIRED 0x000b0000
GSS_S_CONTEXT_EXPIRED 0x000c0000
GSS_S_FAILURE 0x000d0000
GSS_S_BAD_QOP 0x000e0000
GSS_S_UNAUTHORIZED 0x000f0000
GSS_S_UNAVAILABLE 0x00100000
GSS_S_DUPLICATE_ELEMENT 0x00110000
GSS_S_NAME_NOT_MN 0x00120000
GSS_S_CALL_INACCESSIBLE_READ 0x01000000
GSS_S_CALL_INACCESSIBLE_WRITE 0x02000000
GSS_S_CALL_BAD_STRUCTURE 0x03000000

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RFC 2203 RPCSEC_GSS Protocol Specification September 1997

Note that the GSS-API major status is split into three fields as
follows:

Most Significant Bit Least Significant Bit
|------------------------------------------------------------|
| Calling Error | Routine Error | Supplementary Info |
|------------------------------------------------------------|
Bit 31 24 23 16 15 0

Up to one status in the Calling Error field can be logically ORed
with up to one status in the Routine Error field which in turn can be
logically ORed with zero or more statuses in the Supplementary Info
field. If the resulting major status has a non-zero Calling Error
and/or a non-zero Routine Error, then the applicable GSS-API
operation has failed. For purposes of RPCSEC_GSS, this means that
the GSS_Accept_sec_context() call executed by the server has failed.

If the major status is equal GSS_S_COMPLETE, then this indicates the
absence of any Errors or Supplementary Info.

The meanings of most of the GSS_S_* status are defined in the GSS-API
definition, which the exceptions of:

GSS_S_BAD_MIC This code has the same meaning as GSS_S_BAD_SIG.

GSS_S_CALL_INACCESSIBLE_READ
A required input parameter could not be read.

GSS_S_CALL_INACCESSIBLE_WRITE
A required input parameter could not be written.

GSS_S_CALL_BAD_STRUCTURE
A parameter was malformed.

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RFC 2203 RPCSEC_GSS Protocol Specification September 1997

Acknowledgements

Much of the protocol was based on the AUTH_GSSAPI security flavor
developed by Open Vision Technologies [Jaspan]. In particular, we
acknowledge Barry Jaspan, Marc Horowitz, John Linn, and Ellen
McDermott.

Raj Srinivasan designed RPCSEC_GSS [Eisler] with input from Mike
Eisler. Raj, Roland Schemers, Lin Ling, and Alex Chiu contributed to
Sun Microsystems' implementation of RPCSEC_GSS.

Brent Callaghan, Marc Horowitz, Barry Jaspan, John Linn, Hilarie
Orman, Martin Rex, Ted Ts'o, and John Wroclawski analyzed the
specification and gave valuable feedback.

Steve Nahm and Kathy Slattery reviewed various drafts of this
specification.

Much of content of Appendix A was excerpted from John Wray's Work in
Progress on GSS-API Version 2 C-bindings.

References

[Eisler] Eisler, M., Schemers, R., and Srinivasan, R.
(1996). "Security Mechanism Independence in ONC
RPC," Proceedings of the Sixth Annual USENIX
Security Symposium, pp. 51-65.

[Jaspan] Jaspan, B. (1995). "GSS-API Security for ONC
RPC," `95 Proceedings of The Internet Society
Symposium on Network and Distributed System
Security, pp. 144- 151.

[Linn] Linn, J., "Generic Security Service Application
Program Interface, Version 2", RFC 2078, January
1997.

[Srinivasan-bind] Srinivasan, R., "Binding Protocols for
ONC RPC Version 2", RFC 1833, August 1995.

[Srinivasan-rpc] Srinivasan, R., "RPC: Remote Procedure Call
Protocol Specification Version 2", RFC 1831,
August 1995.

[Srinivasan-xdr] Srinivasan, R., "XDR: External Data
Representation Standard", RFC 1832, August 1995.

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RFC 2203 RPCSEC_GSS Protocol Specification September 1997

Authors' Addresses

Michael Eisler
Sun Microsystems, Inc.
M/S UCOS03
2550 Garcia Avenue
Mountain View, CA 94043

Phone: +1 (719) 599-9026
EMail: [email protected]

Alex Chiu
Sun Microsystems, Inc.
M/S UMPK17-203
2550 Garcia Avenue
Mountain View, CA 94043

Phone: +1 (415) 786-6465
EMail: [email protected]

Lin Ling
Sun Microsystems, Inc.
M/S UMPK17-201
2550 Garcia Avenue
Mountain View, CA 94043

Phone: +1 (415) 786-5084
EMail: [email protected]

Eisler, et. al. Standards Track [Page 23]