ONC RPC Working Group M. Eisler
Internet Draft A. Chiu
Document: draft-oncrpc-rpcsec_gss-00.txt L. Ling
22 July 1996
RPCSEC_GSS Protocol Specification
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).
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
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Distribution of this memo is unlimited.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Review of the RPC Message Protocol . . . . . . . . . . . . . . 2
3. Flavor number assignment . . . . . . . . . . . . . . . . . . . 5
4. New auth_stat values . . . . . . . . . . . . . . . . . . . . . 6
5. Elements of the RPCSEC_GSS Security Protocol . . . . . . . . . 6
5.1. Version Negotiation . . . . . . . . . . . . . . . . . . . . 7
5.1.1. Version Inquiry Requests . . . . . . . . . . . . . . . . . 7
5.1.2. Version Inquiry Responses . . . . . . . . . . . . . . . . 8
5.1.2.1. Normal Version Inquiry Responses . . . . . . . . . . . . 8
5.1.2.2. Abnormal Version Inquiry Responses . . . . . . . . . . . 9
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5.2. Context Creation . . . . . . . . . . . . . . . . . . . . . 10
5.2.1. Context Creation Requests . . . . . . . . . . . . . . . 10
5.2.2. Context Creation Responses . . . . . . . . . . . . . . . 11
5.2.2.1. Context Creation Response - Successful Acceptance . . 11
5.2.2.1.1. Client Processing of Successful Context Creation
Responses . . . . . . . . . . . . . . . . . . . . . 12
5.2.2.2. Context Creation Response - Unsuccessful Cases . . . . 13
5.3. RPC Data Exchange . . . . . . . . . . . . . . . . . . . . 13
5.3.1. RPC Request Header . . . . . . . . . . . . . . . . . . . 13
5.3.2. RPC Request Data . . . . . . . . . . . . . . . . . . . . 14
5.3.2.1. RPC Request Data - No Data Integrity . . . . . . . . . 14
5.3.2.2. RPC Request Data - With Data Integrity . . . . . . . . 14
5.3.2.3. RPC Request Data - With Data Privacy . . . . . . . . . 15
5.3.3. Server Processing of RPC Data Requests . . . . . . . . . 16
5.3.3.1. Context Management . . . . . . . . . . . . . . . . . . 16
5.3.3.2. Server Reply - Request Accepted . . . . . . . . . . . 17
5.3.3.3. Server Reply - Request Denied . . . . . . . . . . . . 18
5.4. Context Destruction . . . . . . . . . . . . . . . . . . . 19
6. Security Considerations . . . . . . . . . . . . . . . . . . 19
6.1. Privacy of Call Header . . . . . . . . . . . . . . . . . . 19
6.2. Sequence number attacks . . . . . . . . . . . . . . . . . 20
6.3. Message stealing attacks . . . . . . . . . . . . . . . . . 20
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
9. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 21
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. Review of the RPC Message Protocol
This memo refers to several fields of the RPC protocol. For
convenience, the XDR language description of the RPC message protocol
is reproduced here.
/* RPC message type */
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enum msg_type {
CALL = 0,
REPLY = 1
};
/* Reply types */
enum reply_stat {
MSG_ACCEPTED = 0,
MSG_DENIED = 1
};
/* Security flavors */
enum auth_flavor {
AUTH_NONE = 0,
AUTH_SYS = 1,
AUTH_SHORT = 2
};
/* Status of accepted messages */
enum accept_stat {
SUCCESS = 0,
PROG_UNAVAIL = 1,
PROG_MISMATCH = 2,
PROC_UNAVAIL = 3,
GARBAGE_ARGS = 4,
SYSTEM_ERR = 5
};
/* Status of rejected messages */
enum reject_stat {
RPC_MISMATCH = 0,
AUTH_ERROR = 1
};
/* Why authentication failed */
enum auth_stat {
AUTH_OK = 0,
/* failed at remote end */
AUTH_BADCRED = 1,
AUTH_REJECTEDCRED = 2,
AUTH_BADVERF = 3,
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AUTH_REJECTEDVERF = 4,
AUTH_TOOWEAK = 5,
/* failed locally */
AUTH_INVALIDRESP = 6,
AUTH_FAILED = 7
};
/* Opaque structure of credential and verifier */
struct opaque_auth {
auth_flavor flavor;
opaque body<400>;
};
/* The RPC message */
struct rpc_msg {
unsigned int xid;
union switch (msg_type mtype) {
case CALL:
call_body cbody;
case REPLY:
reply_body rbody;
} body;
};
/* Body of RPC call */
struct call_body {
unsigned int rpcvers;
unsigned int prog;
unsigned int vers;
unsigned int proc;
opaque_auth cred;
opaque_auth verf;
/* procedure specific parameters start here */
};
/* Body of RPC reply */
union reply_body switch (reply_stat stat) {
case MSG_ACCEPTED:
accepted_reply areply;
case MSG_DENIED:
rejected_reply rreply;
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} reply;
/* Accepted reply */
struct accepted_reply {
opaque_auth verf;
union switch (accept_stat stat) {
case SUCCESS:
opaque results[0];
/* procedure-specific results start here */
case PROG_MISMATCH:
struct {
unsigned int low;
unsigned int high;
} mismatch_info;
default:
/*
* Void. Cases include PROG_UNAVAIL,
* PROC_UNAVAIL, GARBAGE_ARGS, and
* SYSTEM_ERR.
*/
void;
} reply_data;
};
/* Rejected reply */
union rejected_reply switch (reject_stat stat) {
case RPC_MISMATCH:
struct {
unsigned int low;
unsigned int high;
} mismatch_info;
case AUTH_ERROR:
auth_stat stat;
};
3. Flavor number assignment
The RPCSEC_GSS security flavor has been assigned the value of 6:
enum auth_flavor {
...
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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 {
...
/*
* GSS related errors
*/
RPCSEC_GSS_NOCRED = 13,
RPCSEC_GSS_FAILED = 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
four phases: version negotiation, context creation, RPC data
exchange, and context destruction. In the following discussion,
protocol elements for these four phases are described.
The following description of the RPCSEC_GSS protocol uses some of the
definitions within XDR language description of the RPC protocol.
Version negotiation, 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 service program and version numbers.
The procedure number used is NULLPROC (zero). A field in the
credential information (the gss_proc field) 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 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.
The following definitions are used for describing the protocol.
/* RPCSEC_GSS control procedures */
#define RPCSEC_GSS_DATA 0
#define RPCSEC_GSS_INIT 1
#define RPCSEC_GSS_CONTINUE_INIT 2
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#define 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 */
#define RPCSEC_GSS_INQUIRE_VERS 0 /* version negotiation */
#define RPCSEC_GSS_VERS_1 1
struct rpc_gss_cred_t {
union switch (unsigned int version) { /* version of RPCSEC_GSS */
case RPCSEC_GSS_INQUIRE_VERS
struct {
unsigned int inquire_vers;
} rpc_gss_cred_vers_0_t;
case RPCSEC_GSS_VERS_1:
struct {
unsigned int 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;
}
};
/* Maximum sequence number value */
#define MAXSEQ 0x80000000
5.1. Version Negotiation
Before establishing a context, the RPC client needs to know which
version of the RPCSEC_GSS protocol the client and server both use.
Currently, aside from version zero (a control version) there is only
one version (RPCSEC_GSS_VERS_1), so this part of RPCSEC_GSS is
allowing for the possibility of future versions.
5.1.1. Version Inquiry Requests
The first RPC request from the client to the server inquires the
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versions of RPCSEC_GSS supported on the server. 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
structured defined previously. The version field of rpc_gss_cred_t
struct is set to RPCSEC_GSS_INQUIRE_VERS (zero). The inquire_vers
field is set to version of RPCSEC_GSS that the client wishes to use.
inquire_ver must be equal to 1. As is normal for NULLPROC requests,
the call arguments are void.
5.1.2. Version Inquiry Responses
5.1.2.1. Normal Version Inquiry Responses
A normal response to a version inquiry request has the RPC message's
stat field in the reply_body set to an MSG_ACCEPTED response with a
status of SUCCESS. The verifier (the verf element in the response)
has the flavor field set to RPCSEC_GSS, and body field encodes the
following structure:
struct rpc_gss_vers_verf_t {
union switch (bool_t inquire_vers_supported)
case TRUE:
void;
case FALSE:
boot_t inquire_vers_in_hole;
unsigned int low;
unsigned int high;
};
The server must use the following algorithm to compute the contents
of the verifier:
if (version in credential is one of server's supported versions) {
inquire_vers_supported = TRUE;
} else if (version < server's lowest supported version) {
inquire_vers_supported = FALSE;
inquire_vers_in_hole = FALSE;
low = server's lowest supported version;
high = server's highest support version;
} else if (version > server's highest supported version) {
inquire_vers_supported = FALSE;
inquire_vers_in_hole = FALSE;
low = server's lowest supported version;
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high = server's highest supported version;
} else {
/*
* client has hit a "hole" in range of server's
* support versions
*/
inquire_vers_supported = FALSE;
inquire_vers_in_hole = TRUE;
low = server's highest supported version
that is < inquire_vers;
high = server's lowest supported version
that is > inquire_vers;
}
The server must never return values for low and high that are equal
to 0. As is normal for NULLPROC responses, the server must encode
void results after the verifier.
In the event inquire_ver_supported is FALSE, the client selects a
version it supports within the range specified by low and high, and
retries the version inquiry request. If there are holes in range of
versions the server supports, it is possible another mismatch
indication will be returned. The client and server can then iterate
to find a common version, or until the client determines that it and
the server have no common version of the RPCSEC_GSS protocol. The
client should initially be optimistic and use the highest version of
RPCSEC_GSS it supports when setting the inquire_vers field in the
version inquiry request.
5.1.2.2. Abnormal Version Inquiry Responses
An MSG_ACCEPTED reply (to a version inquiry request) with an
acceptance status of other than SUCCESS has a NULL verifier (flavor
of AUTH_NONE and zero octets of data in the body field), and is
formulated as usual for different status values.
An MSG_DENIED reply (to a version inquiry 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. Solaris 2 implementations return a
rejection status of AUTH_ERROR with an auth_stat of
AUTH_REJECTEDCRED. Even though two new values (RPCSEC_GSS_NOCRED and
RPCSEC_GSS_FAILED) have been defined for the auth_stat type, neither
of these two can be returned in responses to version inquiry
requests. The new values are relevant to for responses to normal
(data) requests. This is described later.
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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. Context Creation Requests
The first RPC request from the client to the server initiates context
creation for those mechanisms that require context creation messages.
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
structured 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 that was determined in the version negotiation
phase. 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,
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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.
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<>;
unsigned int qop;
rpc_gss_service_t service;
};
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.
The qop and service fields must be set to the GSS-API quality-of-
protection (QOP) and service the client wishes to use for the
session. Some services will need this information before they will
allow the context to be setup. Although the client can change the
QOP and service used on a per request basis, this may not be
acceptable to all RPC based services - such services may choose to
enforce the QOP and service specified during context creation for the
rest of the session. If there is more than one creation request, all
of them must use the same values for qop and service.
Note that QOPs are specific to the security mechanism that the call
to gss_init_sec_context() specified when the gss_token was created.
Because the specification in this memo is meant to be generic, this
memo does not specifically define valid QOPs. It is up to the
definers of GSS-API mechanisms to define valid QOPs.
5.2.2. Context Creation Responses
5.2.2.1. 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 result
argument that has the following structure:
struct rpc_gss_init_res {
opaque handle<>;
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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 server must generate
handles such that they will be generated again for the same pair of
client and server principals. Otherwise, the server could be
vulnerable to a replay attacker by an attacker that sends requests
from a previous context. The gss_major and gss_minor fields contain
the results of the call to gss_accept_sec_context() executed by the
server. 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.
The verifier used for a successful context creation response is the
NULL verifier (AUTH_NONE flavor with zero-length opaque data).
5.2.2.1.1. 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.
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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_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.
5.2.2.2. 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. Solaris 2 implementations return a rejection status of
AUTH_ERROR with an auth_stat of AUTH_REJECTEDCRED. Even though two
new values (RPCSEC_GSS_NOCRED and RPCSEC_GSS_FAILED) have been
defined for the auth_stat type, neither of these two can be returned
in responses to context creation requests. The new values are
relevant to 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 the case, the
server returns a rejection status of AUTH_ERROR, with an auth_stat of
AUTH_REJECTED_CRED.
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. 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
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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_sign() 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.
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<>;
};
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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_sign() call, with the same QOP as was used
to compute the header checksum (in the verifier). The gss_sign() 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_sign() 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_seal() call is invoked
to encrypt the octet stream corresponding to the rpc_gss_data_t
structure, using the same value for QOP as was used for the header
checksum (in the verifier). The gss_seal() 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
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gss_seal() 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
* the sequence number (seq_num) specified in the credential (more
on this follows)
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,
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
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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.
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 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
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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.
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_NOCRED and RPCSEC_GSS_FAILED) 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_NOCRED 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).
When the client's sequence number exceeds the maximum the server will
allow, the server will reject the request with the reason
RPCSEC_GSS_FAILED. 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_FAILED reason to be returned. In these cases also, the
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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.
Other errors can be returned as appropriate.
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. Security Considerations
6.1. 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
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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.
6.2. Sequence number attacks
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 whereby the
attacker sends requests that are outside the window. The most trivial
one, would be one where the attacker sends a request with a sequence
number that is below the window. In this case, even if the header
checksum were successfully verified, the server would silently
discard it. To guard against this kind of denial of service (stealing
processing resources), one might implement a server to examine
request's the sequence number to see if it is below the window before
verifying the checksum.
6.3. 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.
7. Acknowledgements
Much of 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
SunSoft's implementation of RPCSEC_GSS.
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Barry Jaspan, Marc Horowitz, Hilarie Orman, Martin Rex, and John
Wroclawski analyzed the specification and gave valuable feedback.
Kathy Slattery proof read the specification.
8. References
[Eisler] Eisler, M., Schemers, R., and Srinivasan, R. (1996).
"Security Mechanism Independence in ONC RPC," To be
published in the Proceedings of 1996 Usenix Security
Symposium.
[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. (1993). RFC 1508, "Generic Security Service
Application Program Interface."
[Srinivasan-bind] Srinivasan, R. (1995). RFC 1833, "Binding Protocols
for ONC RPC Version 2."
[Srinivasan-rpc] Srinivasan, R. (1995). RFC 1831, "RPC: Remote
Procedure Call Protocol Specification Version 2."
[Srinivasan-xdr] Srinivasan, R. (1995). RFC 1832, "XDR: External Data
Representation Standard."
9. Authors' Addresses
Michael Eisler
Sun Microsystems, Inc.
M/S UCOS03
2550 Garcia Avenue
Mountain View, CA 94043
Phone: +1 (719) 599-9026
E-mail: mre@eng.sun.com
Alex Chiu
Sun Microsystems, Inc.
M/S UMPK17-203
2550 Garcia Avenue
Mountain View, CA 94043
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Phone: +1 (415) 786-6465
E-mail: hacker@eng.sun.com
Lin Ling
Sun Microsystems, Inc.
M/S UMPK17-201
2550 Garcia Avenue
Mountain View, CA 94043
Phone: +1 (415) 786-5084
E-mail: lling@eng.sun.com
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