Internet DRAFT - draft-kaduk-kitten-gss-loop
draft-kaduk-kitten-gss-loop
Network Working Group B. Kaduk
Internet-Draft MIT
Intended status: Informational January 14, 2014
Expires: July 18, 2014
Structure of the GSS Negotiation Loop
draft-kaduk-kitten-gss-loop-02
Abstract
This document specifies the generic structure of the negotiation loop
to establish a GSS security context between initiator and acceptor.
The control flow of the loop is indicated for both parties, including
error conditions, and indications are given for where application-
specific behavior must be specified.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on July 18, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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1. Introduction
The Generic Security Service Application Program Intervace version 2
[RFC2743] provides a generic interface for security services, in the
form of an abstraction layer over the underlying security mechanisms
that an application may use. A GSS initiator and acceptor exchange
messages, called tokens, until a security context is established.
Such a security context allows for mutual authentication of the two
parties, the passing of confidential or integrity-protected messages
between the initiator and acceptor, the generation of identical
pseudo-random bit strings by both participants [RFC4401], and more.
The number of tokens which must be exchanged between initiator and
acceptor in order to establish the security context is dependent on
the underlying mechanism as well as the desired properties of the
security context, and is in general not known to the application.
Accordingly, the application's control flow must include a loop
within which GSS security context tokens are exchanged, which
terminates upon successful establishment of a security context (or an
error condition).
The GSS-API C bindings [RFC2744] provide some example code for such a
negotiation loop, but this code does not specify the application's
behavior on unexpected or error conditions. As such, individual
application protocol specifications have had to specify the structure
of their GSS negotiation loops, including error handling, on a per-
protocol basis. [RFC4462], [RFC3645], [RFC5801], [RFC4752],
[RFC2203] This represents a substantial duplication of effort, and
the various specifications go into different levels of detail and
describe different possible error conditions. It is therefore
preferable to have the structure of the GSS negotiation loop,
including error conditions and token passing, described in a single
specification, which can then be referred to from other documents in
lieu of repeating the structure of the loop each time. This document
will perform that role.
The necessary requirements for correctly performing a GSS negotiation
loop are essentially all included in [RFC2743], but they are
scattered in many different places. This document brings all the
requirements together into one place for the convenience of
implementors, even though the normative requirements remain in
[RFC2743]. In a few places, this document notes additional behavior
which is useful for applications but is not mandated by [RFC2743].
2. Loop Structure
The loop is begun by the appropriately named initiator, which calls
GSS_Init_sec_context() with an empty (zero-length) input_token and a
fixed set of input flags containing the desired attributes for the
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security context. The initiator should not change any of the input
parameters to GSS_Init_sec_context() between calls to it during the
loop, with the exception of the input_token parameter, which will
contain a message from the acceptor after the initial call, and the
input_context_handle, which must be the result returned in the
output_context_handle of the previous call to GSS_Init_sec_context()
(GSS_C_NO_CONTEXT for the first call). (In the C bindings, there is
only a single read/modify context_handle argument, so the same
variable should be passed for each call in the loop.) RFC 2743 only
requires that the claimant_cred_handle argument remain constant over
all calls in the loop, but the other non-excepted arguments should
also remain fixed for reliable operation.
The following subsections will describe the various steps of the
loop, without special consideration to whether a call to
GSS_Init_sec_context() or GSS_Accept_sec_context() is the first such
call in the loop. For the first call to each routine in the loop,
the major status code from the previous call to
GSS_Init_sec_context() or GSS_Accept_sec_context() should be taken as
GSS_S_CONTINUE_NEEDED.
2.1. Anonymous Initiators
If the initiator is requesting anonymity by setting the anon_req_flag
input to GSS_Init_sec_context(), then on non-error returns from
GSS_Init_sec_context() (that is, when the major status is
GSS_S_COMPLETE or GSS_S_CONTINUE_NEEDED), the initiator must verify
that the output value of anon_state from GSS_Init_sec_context() is
true before sending the security context token to the acceptor.
Failing to perform this check could cause the initiator to lose
anonymity.
2.2. GSS_Init_sec_context
The initiator calls GSS_Init_sec_context(), using the
input_context_handle for the current proto-security-context and its
fixed set of input parameters, and the input_token received from the
acceptor (if not the first iteration of the loop). The presence of a
nonempty output_token and the value of the major status code are the
indicators for how to proceed:
If the major status code is GSS_S_COMPLETE and the output_token is
empty, then the context negotiation is fully complete and ready
for use by the initiator with no further actions.
If the major status code is GSS_S_COMPLETE and the output_token is
nonempty, then the initiator's portion of the security context
negotiation is complete but the acceptor's is not. The initiator
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must send the output_token to the acceptor so that the acceptor
can establish its half of the security context.
If the major status code is GSS_S_CONTINUE_NEEDED and the
output_token is nonempty, the context negotiation is incomplete.
The initiator must send the output_token to the acceptor and await
another input_token from the acceptor.
If the major status code is GSS_S_CONTINUE_NEEDED and the
output_token is empty, the mechanism has produced an output which
is not compliant with [RFC2743]. However, there are some known
implementations of certain mechanisms which do produce empty
context negotiation tokens. For maximum interoperability,
applications should be prepared to accept such tokens, and should
transmit them to the acceptor if they are generated.
If the major status code is any other value, the context
negotiation has failed. If the output_token is nonempty, it is an
error token, and the initiator should send it to the acceptor. If
the output_token is empty, then the initiator should indicate the
failure to the acceptor if an appropriate channel to do so is
available.
2.3. Sending from Initiator to Acceptor
The establishment of a GSS security context between initiator and
acceptor requires some communication channel by which to exchange the
context negotiation tokens. The nature of this channel is not
specified by the GSS specification -- it could be a synchronous TCP
channel, a UDP-based RPC protocol, or any other sort of channel. In
many cases, the channel will be multiplexed with non-GSS application
data; the application protocol must provide some means by which the
GSS context tokens can be identified and passed through to the
mechanism accordingly. It is in such cases where the application
protocol has a means to indicate error conditions that the initiator
could indicate a failure to the acceptor, as mentioned in some of the
above cases conditional on "an appropriate channel to do so".
However, even the presence of a communication channel does not
necessarily indicate that it is appropriate for the initiator to
indicate such errors. For example, if the acceptor is a stateless or
near-stateless UDP server, there is probably no need for the
initiator to explicitly indicate its failure to the acceptor.
Conditions such as this can be treated in individual application
protocol specifications.
If a regular security context output_token is produced by the call to
GSS_Init_sec_context(), the initiator must transmit this token to the
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acceptor over the application's communication channel. If the call
to GSS_Init_sec_context() returns an error token as output_token, it
is recommended that the intiator transmit this token to the acceptor
over the application's communication channel.
2.4. Acceptor Sanity Checking
The acceptor's half of the negotiation loop is triggered by the
receipt of a context token from the initiator. Before calling
GSS_Accept_sec_context(), the acceptor may find it useful to perform
some sanity checks on the state of the negotiation loop.
If the acceptor receives a context token but was not expecting such a
token (for example, if the acceptor's previous call to
GSS_Accept_sec_context() returned GSS_S_COMPLETE), this is probably
an error condition indicating that the initiator's state is invalid.
See Section 3.2 for some exceptional cases. It is likely appropriate
for the acceptor to report this error condition to the acceptor via
the application's communication channel.
If the acceptor is expecting a context token (e.g., if the previous
call to GSS_Accept_sec_context() returned GSS_S_CONTINUE_NEEDED), but
does not receive such a token within a reasonable amount of time
after transmitting the previous output_token to the initiator, the
acceptor should assume that the initiator's state is invalid and fail
the GSS negotiation. Again, it is likely appropriate for the
acceptor to report this error condition to the initiator via the
application's communication channel.
[Are there other checks to perform here?]
2.5. GSS_Accept_sec_context
The GSS acceptor responds to the actions of an initiator; as such,
there should always be a nonempty input_token to calls to
GSS_Accept_sec_context(). The input_context_handle parameter will
always be given as the output_context_handle from the previous call
to GSS_Accept_sec_context() in a given negotiation loop (or
GSS_C_NO_CONTEXT on the first call), but the acceptor_cred_handle and
chan_bindings arguments should remain fixed over the course of a
given GSS negotiation loop. [RFC2743] only requires that the
acceptor_cred_handle remain fixed throughout the loop, but the
chan_bindings argument should also remain fixed for reliable
operation.
The GSS acceptor calls GSS_Accept_sec_context(), using the
input_context_handle for the current proto-security-context and the
input_token received from the initiator. The presence of a nonempty
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output_token and the value of the major status code are the
indicators for how to proceed:
If the major status code is GSS_S_COMPLETE and the output_token is
empty, then the context negotiation is fully complete and ready
for use by the acceptor with no further actions.
If the major status code is GSS_S_COMPLETE and the output_token is
nonempty, then the acceptor's portion of the security context
negotiation is complete but the initiator's is not. The acceptor
must send the output_token to the initiator so that the initiator
can establish its half of the security context.
If the major status code is GSS_S_CONTINUE_NEEDED and the
output_token is nonempty, the context negotiation is incomplete.
The acceptor must send the output_token to the initiator and await
another input_token from the initiator.
If the major status code is GSS_S_CONTINUE_NEEDED and the
output_token is empty, the mechanism has produced an output which
is not compliant with [RFC2743]. However, there are some known
implementations of certain mechanisms which do produce empty
context negotiation tokens. For maximum interoperability,
applications should be prepared to accept such tokens, and should
transmit them to the initiator if they are generated.
If the major status code is any other value, the context
negotiation has failed. If the output_token is nonempty, it is an
error token, and the acceptor should send it to the initiator. If
the output_token is empty, then the acceptor should indicate the
failure to the initiator if an appropriate channel to do so is
available.
2.6. Sending from Acceptor to Initiator
The mechanism for sending the context token from acceptor to
initiator will depend on the nature of the communication channel
between the two parties. For a synchronous bidirectional channel, it
can be just another piece of data sent over the link, but for a
stateless UDP RPC acceptor, the token will probably end up being sent
as an RPC output parameter. Application protocol specifications will
need to specify the nature of this behavior.
If the application protocol has the initiator driving the
application's control flow (with the acceptor just responding to
actions from the initiator), it is particularly helpful for the
acceptor to indicate a failure to the initiator, as mentioned in some
of the above cases conditional on "an appropriate channel to do so".
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If a regular security context output_token is produced by the call to
GSS_Accept_sec_context(), the acceptor must transmit this token to
the initiator over the application's communication channel. If the
call to GSS_Accept_sec_context() returns an error token as
output_token, it is recommended that the acceptor transmit this token
to the initiator over the application's communication channel.
2.7. Initiator input validation
The initiator's half of the negotiation loop is triggered (after the
first call) by receipt of a context token from the acceptor. Before
calling GSS_Init_sec_context(), the initiator may find it useful to
perform some sanity checks on the state of the negotiation loop.
If the initiator receives a context token but was not expecting such
a token (for example, if the initiator's previous call to
GSS_Init_sec_context() returned GSS_S_COMPLETE), this is probably an
error condition indicating that the acceptor's state is invalid. See
Section 3.2 for some exceptional cases. It may be appropriate for
the initiator to report this error condition to the acceptor via the
application's communication channel.
If the initiator is expecting a context token (that is, the previous
call to GSS_Init_sec_context() returned GSS_S_CONTINUE_NEEDED), but
does not receive such a token within a reasonable amount of time
after transmitting the previous output_token to the acceptor, the
initiator should assume that the acceptor's state is invalid and fail
the GSS negotiation. Again, it may be appropriate for the initiator
to report this error condition to the acceptor via the application's
communication channel.
[Are there other checks to perform here?]
2.8. Continue the Loop
If the loop is in neither a success or failure condition, then the
loop must continue. Control flow returns to Section 2.2.
3. After Security Context Negotiation
Once a party has completed its half of the security context and
fulfilled its obligations to the other party, the context is
complete, but it is not necessarily ready and appropriate for use.
(In some cases the context may be ready for use earlier than this,
see Section 3.1.) In particular, the security context flags may not
be appropriate for the given application's use.
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The initiator specifies as part of its fixed set of inputs to
GSS_Init_sec_context() values for the following booleans:
deleg_req_flag, mutual_req_flag, replay_det_req_flag,
sequence_req_flag, conf_req_flag, and integ_req_flag. Upon
completion of security context negotiation, the initiator must verify
that the values of the deleg_state, mutual_state, replay_det_state,
sequence_state, conf_avail, and integ_avail flags from the last call
to GSS_Init_sec_context() corresponding to the requested flags. If a
flag was requested but is not available, and that feature is
necessary for the appplication protocol, the initiator must destroy
the security context and not use the security context for application
traffic.
Application protocol specifications citing this document should
indicate which context flags are required for their application
protocol.
The acceptor receives as output the following booleans: deleg_state,
mutual_state, replay_det_state, sequence_state, anon_state,
trans_state, conf_avail, and integ_avail. The acceptor must verify
that any flags necessary for the application protocol are set. If a
necessary flag is not set, the acceptor must destroy the security
context and not use the security context for application traffic.
3.1. Using Partially Complete Security Contexts
For mechanism/flag combinations that require multiple token
exchanges, an application protocol may find it desirable to begin
sending application data protected with GSS per-message operations
while continuing to exchange security context tokens to complete the
security context negotiation. For example, an application may wish
to reduce the number of round trips before application data is
transmitted. The prot_ready_state output value from
GSS_Init_sec_context() and GSS_Accept_sec_context() indicates when
per-message operations are avaialble. Applications using per-message
operations on a partially complete security context must ensure that
there is some mechanism in place to prevent replays of those
messages.
Applications requiring confidentiality and/or integrity protection
from such messages must check the value of the conf_avail and/or
integ_avail output flags from GSS_Init_sec_context()/
GSS_Accept_sec_context() as well as the conf_state output of
GSS_Wrap() (if GSS_Wrap() is used).
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3.2. Additional Context Tokens
Under some (rare) conditions, a context token will be received by a
party to a security context negotiation after that party has
completed the negotiation (i.e., after GSS_Init_sec_context() or
GSS_Accept_sec_context() has returned GSS_S_COMPLETE). Such tokens
must be passed to GSS_Process_context_token() for processing.
The most common cause of such tokens is security context deletion
tokens, emitted when the remote party called GSS_Delete_sec_context()
with a non-null output_context_token parameter. With the GSS-API
version 2, it is not recommended to use security context deletion
tokens.
Extra security context tokens can also be emitted if the selected
mechanism specifies some functionality (such as per-message
confidentiality protection) as optional-to-implement, and the
acceptor's implementation does not implement the optional
functionality, but the functionality was requested by the initiator.
In this case, the acceptor's GSS implementation is required to emit
at least one context token (even when one would not otherwise be
needed to complete the context negotiation), and this can result in
an "extra" token.
In the rare case when an application receives an extra context token,
GSS_Inquire_context() should be used after processing the extra token
to re-verify that the context does support the features necessary for
the application protocol. This will also indicate whether the token
was a deletion token, in which case the major status will be
GSS_S_NO_CONTEXT.
4. Sample Code
This section gives sample code for the GSS negotiation loop, both for
a regular application and for an application where the initiator
wishes to remain anonymous. Since the code for the two cases is very
similar, the anonymous-specific additions are wrapped in a
conditional check; that check and the conditional code may be ignored
if anonymous processing is not needed.
Since the communication channel between the initiator and acceptor is
a matter for individual application protocols, it is inherently
unspecified at the GSS-API level, which can lead to examples that are
less satisfying than may be desired. For example, the sample code in
[RFC2744] uses an unspecified send_token_to_peer() routine. Fully
correct and general code to frame and transmit tokens requires a
substantial amount of error checking and would detract from the core
purpose of this document, so we only present the function signature
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for one example of what such functions might be, and leave some
comments in the otherwise-empty function bodies.
This sample code is written in C, using the GSS-API C bindings
[RFC2744]. It uses the macro GSS_ERROR() to help unpack the various
sorts of information which can be stored in the major status field;
supplementary information does not necessarily indicate an error.
Applications written in other languages will need to exercise care
that checks against the major status value are written correctly.
This sample code should be compilable as a standalone program, linked
against a GSS-API library. In addition to supplying implementations
for the token transmission/receipt routines, in order for the program
to successfully run when linked against most GSS-API libraries, the
initiator will need to specify an explicit target name for the
acceptor (which must match the credentials available to the
acceptor). A skeleton for how this may be done is provided, in a
disabled block of code.
This sample code assumes v2 of the GSS-API. Applications wishing to
remain compatible with v1 of the GSS-API may need to perform
additional checks in some locations.
4.1. GSS Application Sample Code
#include <unistd.h>
#include <assert.h>
#include <err.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <gssapi/gssapi.h>
/*
* This helper is used only on buffers that we allocate ourselves (e.g.,
* from receive_token()). Buffers allocated by GSS routines must use
* gss_release_buffer().
*/
static void
release_buffer(gss_buffer_t buf)
{
free(buf->value);
buf->value = NULL;
buf->length = 0;
}
/*
* Helper to send a token on the specified fd.
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*
* We must warnx() instead of errx() because compliant GSS applications must
* release resources allocated by the GSS library before exiting. (These
* resources may be non-local to the current process.)
*/
static int
send_token(int fd, gss_buffer_t token)
{
/*
* Supply token framing and transmission code here.
*
* It is advisable for the application protocol to specify the length
* of the token being transmitted, unless the underlying transit
* does so implicitly.
*
* In addition to checking for error returns from whichever syscall(s)
* are used to send data, applications should have a loop to handle
* EINTR returns.
*/
return 0;
}
/*
* Helper to receive a token on the specified fd.
*
* We must warnx() instead of errx() because compliant GSS applications must
* release resources allocated by the GSS library before exiting. (These
* resources may be non-local to the current process.)
*/
static int
receive_token(int fd, gss_buffer_t token)
{
/*
* Supply token framing and transmission code here.
*
* In addition to checking for error returns from whichever syscall(s)
* are used to receive data, applications should have a loop to handle
* EINTR returns.
*
* This routine is assumed to allocate memory for the local copy of the
* received token, which must be freed with release_buffer().
*/
return 0;
}
static void
do_initiator(int readfd, int writefd, int anon)
{
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int context_established = 0;
gss_ctx_id_t ctx = GSS_C_NO_CONTEXT;
OM_uint32 major, minor, req_flags, ret_flags;
gss_buffer_desc input_token, output_token;
gss_name_t target_name = GSS_C_NO_NAME;
OM_uint32 ret;
memset(&input_token, 0, sizeof(input_token));
memset(&output_token, 0, sizeof(output_token));
/* Applications should set target_name to a real value. */
gss_buffer_desc name_buf;
name_buf.value = "<service>@<hostname.domain>";
name_buf.length = strlen(name_buf.value);
major = gss_import_name(&minor, &name_buf, GSS_C_NT_HOSTBASED_SERVICE,
&target_name);
/* target_name must be released with gss_release_name() at cleanup. */
/* Mutual authentication will require a token from acceptor to initiator,
* and thus a second call to gss_init_sec_context(). */
req_flags = GSS_C_MUTUAL_FLAG | GSS_C_CONF_FLAG | GSS_C_INTEG_FLAG;
if (anon)
req_flags |= GSS_C_ANON_FLAG;
while (!context_established) {
/* The initiator_cred_handle, mech_type, time_req, input_chan_bindings,
* actual_mech_type, and time_rec parameters are not needed in many
* cases. We pass GSS_C_NO_CREDENTIAL, GSS_C_NO_OID, 0, NULL, NULL,
* and NULL for them, respectively. */
major = gss_init_sec_context(&minor, GSS_C_NO_CREDENTIAL, &ctx,
target_name, GSS_C_NO_OID, req_flags, 0,
NULL, &input_token, NULL, &output_token,
&ret_flags, NULL);
/* This memory is no longer needed. */
release_buffer(&input_token);
if (anon) {
/* Initiators which wish to remain anonymous must check whether
* their request has been honored before sending each token. */
if ((ret_flags & GSS_C_ANON_FLAG) != GSS_C_ANON_FLAG) {
warnx("Anonymous processing not available\n");
goto cleanup;
}
}
/* Always send a token if we are expecting another input token
* (GSS_S_CONTINUE_NEEDED) or if it is nonempty. */
if ((major & GSS_S_CONTINUE_NEEDED) != 0 ||
output_token.length > 0) {
ret = send_token(writefd, &output_token);
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if (ret != 0)
goto cleanup;
}
/* Check for errors after sending the token so that we will send
* error tokens. */
if (GSS_ERROR(major)) {
warnx("gss_init_sec_context() error major 0x%x\n", major);
goto cleanup;
}
/* Having sent any output_token, release the storage for it. */
(void)gss_release_buffer(&minor, &output_token);
if ((major & GSS_S_CONTINUE_NEEDED) != 0) {
ret = receive_token(readfd, &input_token);
if (ret != 0)
goto cleanup;
} else if (major == GSS_S_COMPLETE) {
context_established = 1;
} else {
/* This situation is forbidden by RFC 2743. Bail out. */
warnx("major not complete or continue-needed but not error\n");
goto cleanup;
}
} /* while(!context_established) */
if ((ret_flags & req_flags) != req_flags) {
warnx("Negotiated context does not support requested flags\n");
goto cleanup;
}
printf("Initiator's context negotiation successful\n");
cleanup:
/* It is safe to call gss_release_buffer twice on the same buffer. */
(void)gss_release_buffer(&minor, &output_token);
/* Do not request a context deletion token; pass NULL. */
(void)gss_delete_sec_context(&minor, &ctx, NULL);
}
static void
do_acceptor(int readfd, int writefd)
{
int context_established = 0, ret;
gss_ctx_id_t ctx = GSS_C_NO_CONTEXT;
OM_uint32 major, minor, ret_flags;
gss_buffer_desc input_token, output_token;
gss_name_t client_name;
memset(&input_token, 0, sizeof(input_token));
memset(&output_token, 0, sizeof(output_token));
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context_established = 0;
major = GSS_S_CONTINUE_NEEDED;
while(!context_established) {
if ((major & GSS_S_CONTINUE_NEEDED) != 0) {
ret = receive_token(readfd, &input_token);
if (ret != 0)
goto cleanup;
} else if (major == GSS_S_COMPLETE) {
context_established = 1;
break;
} else {
/* This situation is forbidden by RFC 2743. Bail out. */
warnx("major not complete or continue-needed but not error\n");
goto cleanup;
}
/* We can use the default behavior or do not need the returned
* information for the parameters acceptor_cred_handle,
* input_chan_bindings, mech_type, time_rec, and delegated_cred_handle
* and pass the values GSS_C_NO_CREDENTIAL, NULL, NULL, NULL, and NULL,
* respectively. In some cases the src_name will not be needed, but
* most likely it will be needed for some authorization or logging
* functionality. */
major = gss_accept_sec_context(&minor, &ctx, GSS_C_NO_CREDENTIAL,
&input_token, NULL, &client_name, NULL,
&output_token, &ret_flags, NULL, NULL);
/* Release memory no longer needed. */
release_buffer(&input_token);
/* Always send a token if we are expecting another input token
* (GSS_S_CONTINUE_NEEDED) or if it is nonempty. */
if ((major & GSS_S_CONTINUE_NEEDED) != 0 ||
output_token.length > 0) {
ret = send_token(writefd, &output_token);
if (ret != 0)
goto cleanup;
}
/* Check for errors after sending the token so that we will send
* error tokens. */
if (GSS_ERROR(major)) {
warnx("gss_accept_sec_context() error major 0x%x\n", major);
goto cleanup;
}
/* Release the output token's storage; we don't need it anymore. */
(void)gss_release_buffer(&minor, &output_token);
} /* while(!context_established) */
if ((ret_flags & GSS_C_INTEG_FLAG) != GSS_C_INTEG_FLAG) {
warnx("Negotiated context does not support integrity\n");
goto cleanup;
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}
printf("Acceptor's context negotiation successful\n");
cleanup:
/* It is safe to call gss_release_buffer twice on the same buffer. */
release_buffer(&input_token);
/* Do not request a context deletion token, pass NULL. */
(void)gss_delete_sec_context(&minor, &ctx, NULL);
(void)gss_release_name(&minor, &client_name);
}
int main(int argc, char **argv)
{
pid_t pid;
int fd1 = -1, fd2 = -1;
/* Create fds for reading/writing here. */
pid = fork();
if (pid == 0)
do_initiator(fd1, fd2, 0);
else if (pid > 0)
do_acceptor(fd2, fd1);
else
err(1, "fork() failed\n");
exit(0);
}
5. Security Considerations
This document provides a (reasonably) concise description and example
for correct construction of the GSS-API security context negotiation
loop. Since everything relating to the construction and use of a GSS
security context is security-related, there are security-relevant
considerations throughout the document. It is useful to call out a
few things in this section, though.
The GSS-API uses a request-and-check model for features. An
application using the GSS-API requests that certain features
(confidentiality protection for messages, or anonymity), but such a
request does not require the GSS implementation to provide that
feature. The application must check the returned flags to verify
whether a requested feature is present; if the feature was non-
optional for the application, the application must generate an error.
Phrased differently, the GSS-API will not generate an error if it is
unable to satisfy the features requested by the application.
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6. References
6.1. Normative References
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[RFC2744] Wray, J., "Generic Security Service API Version 2 :
C-bindings", RFC 2744, January 2000.
6.2. Informational References
[RFC4462] Hutzelman, J., Salowey, J., Galbraith, J., and V. Welch,
"Generic Security Service Application Program Interface
(GSS-API) Authentication and Key Exchange for the Secure
Shell (SSH) Protocol", RFC 4462, May 2006.
[RFC4401] Williams, N., "A Pseudo-Random Function (PRF) API
Extension for the Generic Security Service Application
Program Interface (GSS-API)", RFC 4401, February 2006.
[RFC3645] Kwan, S., Garg, P., Gilroy, J., Esibov, L., Westhead, J.,
and R. Hall, "Generic Security Service Algorithm for
Secret Key Transaction Authentication for DNS (GSS-TSIG)",
RFC 3645, October 2003.
[RFC5801] Josefsson, S. and N. Williams, "Using Generic Security
Service Application Program Interface (GSS-API) Mechanisms
in Simple Authentication and Security Layer (SASL): The
GS2 Mechanism Family", RFC 5801, July 2010.
[RFC4752] Melnikov, A., "The Kerberos V5 ("GSSAPI") Simple
Authentication and Security Layer (SASL) Mechanism", RFC
4752, November 2006.
[RFC2203] Eisler, M., Chiu, A., and L. Ling, "RPCSEC_GSS Protocol
Specification", RFC 2203, September 1997.
Appendix A. Acknowledgements
Thanks to Nico Williams and Jeff Hutzleman for prompting me to write
this document.
Author's Address
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Benjamin Kaduk
MIT Kerberos Consortium
Email: kaduk@mit.edu
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