Client and common support for SASL authentication.

SERVER-7130, SERVER-7131, SERVER-7133

SConstruct

@@ -200,6 +200,8 @@ add_option( "gcov" , "compile with flags for gcov" , 0 , True )
 add_option("smokedbprefix", "prefix to dbpath et al. for smoke tests", 1 , False )
 add_option("smokeauth", "run smoke tests with --auth", 0 , False )
 
+add_option("use-sasl-client", "Support SASL authentication in the client library", 0, False)
+
 add_option( "use-system-tcmalloc", "use system version of tcmalloc library", 0, True )
 
 add_option( "use-system-pcre", "use system version of pcre library", 0, True )
@@ -819,6 +821,12 @@ def doConfigure(myenv):
         if not conf.CheckLib( v8_lib_choices ):
             Exit(1)
 
+    env['MONGO_BUILD_SASL_CLIENT'] = bool(has_option("use-sasl-client"))
+    if env['MONGO_BUILD_SASL_CLIENT'] and not conf.CheckLibWithHeader(
+        "gsasl", "gsasl.h", "C", "gsasl_check_version(GSASL_VERSION);", autoadd=False):
+
+        Exit(1)
+
     # requires ports devel/libexecinfo to be installed
     if freebsd or openbsd:
         if not conf.CheckLib("execinfo"):

src/mongo/SConscript

@@ -126,6 +126,13 @@ commonFiles = [ "pch.cpp",
                 "db/dbmessage.cpp"
                 ]
 
+commonSysLibdeps = []
+
+if env['MONGO_BUILD_SASL_CLIENT']:
+    commonFiles.extend(['client/sasl_client_authenticate.cpp',
+                        'util/gsasl_session.cpp'])
+    commonSysLibdeps.append('gsasl')
+
 # handle processinfo*
 processInfoFiles = [ "util/processinfo.cpp" ]
 
@@ -161,7 +168,8 @@ env.StaticLibrary('mongocommon', commonFiles,
                            'fail_point',
                            '$BUILD_DIR/third_party/pcrecpp',
                            '$BUILD_DIR/third_party/murmurhash3/murmurhash3',
-                           '$BUILD_DIR/third_party/shim_boost'],)
+                           '$BUILD_DIR/third_party/shim_boost'],
+                  SYSLIBDEPS=commonSysLibdeps)
 
 env.StaticLibrary("coredb", [
         "client/parallel.cpp",

src/mongo/base/error_codes.err

@@ -16,7 +16,11 @@ error_code("UnsupportedFormat", 12)
 error_code("Unauthorized", 13)
 error_code("TypeMismatch", 14)
 error_code("Overflow", 15)
-error_code("IllegalOperation", 16)
-error_code("EmptyArrayOperation", 17)
+error_code("InvalidLength", 16)
+error_code("ProtocolError", 17)
+error_code("AuthenticationFailed", 18)
+error_code("CannotReuseObject", 19)
+error_code("IllegalOperation", 20)
+error_code("EmptyArrayOperation", 21)
 
 error_class("NetworkError", ["HostUnreachable", "HostNotFound"])

src/mongo/client/sasl_client_authenticate.cpp

@@ -0,0 +1,229 @@
+/*    Copyright 2012 10gen Inc.
+ *
+ *    Licensed under the Apache License, Version 2.0 (the "License");
+ *    you may not use this file except in compliance with the License.
+ *    You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *    Unless required by applicable law or agreed to in writing, software
+ *    distributed under the License is distributed on an "AS IS" BASIS,
+ *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *    See the License for the specific language governing permissions and
+ *    limitations under the License.
+ */
+
+#include "mongo/client/sasl_client_authenticate.h"
+
+#include <string>
+
+#include "mongo/base/string_data.h"
+#include "mongo/bson/util/bson_extract.h"
+#include "mongo/util/base64.h"
+#include "mongo/util/gsasl_session.h"
+#include "mongo/util/log.h"
+#include "mongo/util/mongoutils/str.h"
+#include "mongo/util/net/hostandport.h"
+
+namespace mongo {
+
+    using namespace mongoutils;
+
+    const char* const saslStartCommandName = "saslStart";
+    const char* const saslContinueCommandName = "saslContinue";
+    const char* const saslCommandAutoAuthorizeFieldName = "autoAuthorize";
+    const char* const saslCommandCodeFieldName = "code";
+    const char* const saslCommandConversationIdFieldName = "conversationId";
+    const char* const saslCommandDoneFieldName = "done";
+    const char* const saslCommandErrmsgFieldName = "errmsg";
+    const char* const saslCommandMechanismFieldName = "mechanism";
+    const char* const saslCommandMechanismListFieldName = "supportedMechanisms";
+    const char* const saslCommandPasswordFieldName = "password";
+    const char* const saslCommandPayloadFieldName = "payload";
+    const char* const saslCommandPrincipalFieldName = "principal";
+    const char* const saslCommandServiceHostnameFieldName = "serviceHostname";
+    const char* const saslCommandServiceNameFieldName = "serviceName";
+    const char* const saslDefaultDBName = "admin";
+    const char* const saslDefaultServiceName = "mongodb";
+
+    const char* const saslClientLogFieldName = "clientLogLevel";
+
+namespace {
+    // Default log level on the client for SASL log messages.
+    const int defaultSaslClientLogLevel = 4;
+}  // namespace
+
+    Status saslExtractPayload(const BSONObj& cmdObj, std::string* payload, BSONType* type) {
+        BSONElement payloadElement;
+        Status status = bsonExtractField(cmdObj, saslCommandPayloadFieldName, &payloadElement);
+        if (!status.isOK())
+            return status;
+
+        *type = payloadElement.type();
+        if (payloadElement.type() == BinData) {
+            const char* payloadData;
+            int payloadLen;
+            payloadData = payloadElement.binData(payloadLen);
+            if (payloadLen < 0)
+                return Status(ErrorCodes::InvalidLength, "Negative payload length");
+            *payload = std::string(payloadData, payloadData + payloadLen);
+        }
+        else if (payloadElement.type() == String) {
+            try {
+                *payload = base64::decode(payloadElement.str());
+            } catch (UserException& e) {
+                return Status(ErrorCodes::FailedToParse, e.what());
+            }
+        }
+        else {
+            return Status(ErrorCodes::TypeMismatch,
+                          (str::stream() << "Wrong type for field; expected BinData or String for "
+                           << payloadElement));
+        }
+
+        return Status::OK();
+    }
+
+namespace {
+
+    /**
+     * Configure "*session" as a client gsasl session for authenticating on the connection
+     * "*client", with the given "saslParameters".  "gsasl" and "sessionHook" are passed through
+     * to GsaslSession::initializeClientSession, where they are documented.
+     */
+    Status configureSession(Gsasl* gsasl,
+                            DBClientWithCommands* client,
+                            const BSONObj& saslParameters,
+                            void* sessionHook,
+                            GsaslSession* session) {
+
+        std::string mechanism;
+        Status status = bsonExtractStringField(saslParameters,
+                                               saslCommandMechanismFieldName,
+                                               &mechanism);
+        if (!status.isOK())
+            return status;
+
+        status = session->initializeClientSession(gsasl, mechanism, sessionHook);
+        if (!status.isOK())
+            return status;
+
+        std::string service;
+        status = bsonExtractStringFieldWithDefault(saslParameters,
+                                                   saslCommandServiceNameFieldName,
+                                                   saslDefaultServiceName,
+                                                   &service);
+        if (!status.isOK())
+            return status;
+        session->setProperty(GSASL_SERVICE, service);
+
+        std::string hostname;
+        status = bsonExtractStringFieldWithDefault(saslParameters,
+                                                   saslCommandServiceHostnameFieldName,
+                                                   HostAndPort(client->getServerAddress()).host(),
+                                                   &hostname);
+        if (!status.isOK())
+            return status;
+        session->setProperty(GSASL_HOSTNAME, hostname);
+
+        BSONElement element = saslParameters[saslCommandPrincipalFieldName];
+        if (element.type() == String) {
+            session->setProperty(GSASL_AUTHID, element.str());
+        }
+        else if (!element.eoo()) {
+            return Status(ErrorCodes::TypeMismatch,
+                          str::stream() << "Expected string for " << element);
+        }
+
+        element = saslParameters[saslCommandPasswordFieldName];
+        if (element.type() == String) {
+            session->setProperty(GSASL_PASSWORD, element.str());
+        }
+        else if (!element.eoo()) {
+            return Status(ErrorCodes::TypeMismatch,
+                          str::stream() << "Expected string for " << element);
+        }
+
+        return Status::OK();
+    }
+
+    int getSaslClientLogLevel(const BSONObj& saslParameters) {
+        int saslLogLevel = defaultSaslClientLogLevel;
+        BSONElement saslLogElement = saslParameters[saslClientLogFieldName];
+        if (saslLogElement.trueValue())
+            saslLogLevel = 1;
+        if (saslLogElement.isNumber())
+            saslLogLevel = saslLogElement.numberInt();
+        return saslLogLevel;
+    }
+
+}  // namespace
+
+    Status saslClientAuthenticate(Gsasl *gsasl,
+                                  DBClientWithCommands* client,
+                                  const BSONObj& saslParameters,
+                                  void* sessionHook) {
+
+        GsaslSession session;
+
+        int saslLogLevel = getSaslClientLogLevel(saslParameters);
+
+        Status status = configureSession(gsasl, client, saslParameters, sessionHook, &session);
+        if (!status.isOK())
+            return status;
+
+        BSONObj saslFirstCommandPrefix = BSON(
+                saslStartCommandName << 1 <<
+                saslCommandMechanismFieldName << session.getMechanism());
+
+        BSONObj saslFollowupCommandPrefix = BSON(saslContinueCommandName << 1);
+        BSONObj saslCommandPrefix = saslFirstCommandPrefix;
+        BSONObj inputObj = BSON(saslCommandPayloadFieldName << "");
+        bool isServerDone = false;
+        while (!session.isDone()) {
+            std::string payload;
+            BSONType type;
+
+            status = saslExtractPayload(inputObj, &payload, &type);
+            if (!status.isOK())
+                return status;
+
+            LOG(saslLogLevel) << "sasl client input: " << base64::encode(payload) << endl;
+
+            std::string responsePayload;
+            status = session.step(payload, &responsePayload);
+            if (!status.isOK())
+                return status;
+
+            LOG(saslLogLevel) << "sasl client output: " << base64::encode(responsePayload) << endl;
+
+            BSONObjBuilder commandBuilder;
+            commandBuilder.appendElements(saslCommandPrefix);
+            commandBuilder.appendBinData(saslCommandPayloadFieldName,
+                                         int(responsePayload.size()),
+                                         BinDataGeneral,
+                                         responsePayload.c_str());
+            BSONElement conversationId = inputObj[saslCommandConversationIdFieldName];
+            if (!conversationId.eoo())
+                commandBuilder.append(conversationId);
+
+            if (!client->runCommand(saslDefaultDBName, commandBuilder.obj(), inputObj)) {
+                return Status(ErrorCodes::UnknownError,
+                              inputObj[saslCommandErrmsgFieldName].str());
+            }
+
+            int statusCodeInt = inputObj[saslCommandCodeFieldName].Int();
+            if (0 != statusCodeInt)
+                return Status(ErrorCodes::fromInt(statusCodeInt),
+                              inputObj[saslCommandErrmsgFieldName].str());
+
+            isServerDone = inputObj[saslCommandDoneFieldName].trueValue();
+            saslCommandPrefix = saslFollowupCommandPrefix;
+        }
+
+        if (!isServerDone)
+            return Status(ErrorCodes::ProtocolError, "Client finished before server.");
+        return Status::OK();
+    }
+
+}  // namespace mongo

src/mongo/client/sasl_client_authenticate.h

@@ -0,0 +1,129 @@
+/*    Copyright 2012 10gen Inc.
+ *
+ *    Licensed under the Apache License, Version 2.0 (the "License");
+ *    you may not use this file except in compliance with the License.
+ *    You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *    Unless required by applicable law or agreed to in writing, software
+ *    distributed under the License is distributed on an "AS IS" BASIS,
+ *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *    See the License for the specific language governing permissions and
+ *    limitations under the License.
+ */
+
+#pragma once
+
+#include "mongo/base/status.h"
+#include "mongo/bson/bsontypes.h"
+#include "mongo/client/dbclientinterface.h"
+
+struct Gsasl;
+
+namespace mongo {
+    class BSONObj;
+
+    /**
+     * Attempts to authenticate "client" using the SASL protocol.
+     *
+     * Requires an initialized instance of the "gsasl" library, as the first
+     * parameter.
+     *
+     * The "saslParameters" BSONObj should be initialized with zero or more of the
+     * fields below.  Which fields are required depends on the mechanism.  Consult the
+     * libgsasl documentation.
+     *
+     *     "mechanism": The string name of the sasl mechanism to use.  Mandatory.
+     *     "autoAuthorize": Truthy values tell the server to automatically acquire privileges on
+     *         all resources after successful authentication, which is the default.  Falsey values
+     *         instruct the server to await separate privilege-acquisition commands.
+     *     "database": The database target of the auth command.  Optional for GSSAPI/Kerberos.
+     *     "principal": The string name of the principal to authenticate, GSASL_AUTHID.
+     *     "password": The password data, GSASL_PASSWORD.
+     *     "serviceName": The GSSAPI service name to use.  Defaults to "mongodb".
+     *     "serviceHostname": The GSSAPI hostname to use.  Defaults to the name of the remote host.
+     *
+     * Other fields in saslParameters are silently ignored.
+     *
+     * "sessionHook" is a pointer to optional data, which may be used by the gsasl_callback
+     * previously set on "gsasl".  The session hook is set on an underlying Gsasl_session using
+     * gsasl_session_hook_set, and may be accessed by callbacks using gsasl_session_hook_get.
+     * See the gsasl documentation.
+     *
+     * Returns an OK status on success, and ErrorCodes::AuthenticationFailed if authentication is
+     * rejected.  Other failures, all of which are tantamount to authentication failure, may also be
+     * returned.
+     */
+    Status saslClientAuthenticate(Gsasl *gsasl,
+                                  DBClientWithCommands* client,
+                                  const BSONObj& saslParameters,
+                                  void* sessionHook);
+
+    /**
+     * Extracts the payload field from "cmdObj", and store it into "*payload".
+     *
+     * Sets "*type" to the BSONType of the payload field in cmdObj.
+     *
+     * If the type of the payload field is String, the contents base64 decodes and
+     * stores into "*payload".  If the type is BinData, the contents are stored directly
+     * into "*payload".  In all other cases, returns
+     */
+    Status saslExtractPayload(const BSONObj& cmdObj, std::string* payload, BSONType* type);
+
+    // Constants
+
+    /// String name of the saslStart command.
+    extern const char* const saslStartCommandName;
+
+    /// String name of the saslContinue command.
+    extern const char* const saslContinueCommandName;
+
+    /// Name of the saslStart parameter indicating that the server should automatically grant the
+    /// connection all privileges associated with the principal after successful authentication.
+    extern const char* const saslCommandAutoAuthorizeFieldName;
+
+    /// Name of the field contain the status code in responses from the server.
+    extern const char* const saslCommandCodeFieldName;
+
+    /// Name of the field containing the conversation identifier in server respones and saslContinue
+    /// commands.
+    extern const char* const saslCommandConversationIdFieldName;
+
+    /// Name of the field that indicates whether or not the server believes authentication has
+    /// completed successfully.
+    extern const char* const saslCommandDoneFieldName;
+
+    /// Field in which  to store error messages associated with non-success return codes.
+    extern const char* const saslCommandErrmsgFieldName;
+
+    /// Name of parameter to saslStart command indiciating the client's desired sasl mechanism.
+    extern const char* const saslCommandMechanismFieldName;
+
+    /// In the event that saslStart supplies an unsupported mechanism, the server responds with a
+    /// field by this name, with a list of supported mechanisms.
+    extern const char* const saslCommandMechanismListFieldName;
+
+    /// Field containing password information for saslClientAuthenticate().
+    extern const char* const saslCommandPasswordFieldName;
+
+    /// Field containing sasl payloads passed to and from the server.
+    extern const char* const saslCommandPayloadFieldName;
+
+    /// Field containing the string identifier of the principal to authenticate in
+    /// saslClientAuthenticate().
+    extern const char* const saslCommandPrincipalFieldName;
+
+    /// Field overriding the FQDN of the hostname hosting the mongodb srevice in
+    /// saslClientAuthenticate().
+    extern const char* const saslCommandServiceHostnameFieldName;
+
+    /// Field overriding the name of the mongodb service saslClientAuthenticate().
+    extern const char* const saslCommandServiceNameFieldName;
+
+    /// Default database against which sasl authentication commands should run.
+    extern const char* const saslDefaultDBName;
+
+    /// Default sasl service name, "mongodb".
+    extern const char* const saslDefaultServiceName;
+}

src/mongo/util/gsasl_session.cpp

@@ -0,0 +1,94 @@
+/*    Copyright 2012 10gen Inc.
+ *
+ *    Licensed under the Apache License, Version 2.0 (the "License");
+ *    you may not use this file except in compliance with the License.
+ *    You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *    Unless required by applicable law or agreed to in writing, software
+ *    distributed under the License is distributed on an "AS IS" BASIS,
+ *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *    See the License for the specific language governing permissions and
+ *    limitations under the License.
+ */
+
+#include "mongo/util/gsasl_session.h"
+
+#include <cstdlib>
+#include <gsasl.h>
+
+#include "mongo/util/assert_util.h"
+
+namespace mongo {
+
+    GsaslSession::GsaslSession() : _gsaslSession(NULL), _done(false) {}
+
+    GsaslSession::~GsaslSession() {
+        if (_gsaslSession)
+            gsasl_finish(_gsaslSession);
+    }
+
+    std::string GsaslSession::getMechanism() const {
+        return gsasl_mechanism_name(_gsaslSession);
+    }
+
+    void GsaslSession::setProperty(Gsasl_property property, const StringData& value) {
+        gsasl_property_set_raw(_gsaslSession, property, value.data(), value.size());
+    }
+
+    Status GsaslSession::initializeClientSession(Gsasl* gsasl,
+                                                 const StringData& mechanism,
+                                                 void* sessionHook) {
+        return _initializeSession(&gsasl_client_start, gsasl, mechanism, sessionHook);
+    }
+
+    Status GsaslSession::initializeServerSession(Gsasl* gsasl,
+                                                 const StringData& mechanism,
+                                                 void* sessionHook) {
+        return _initializeSession(&gsasl_server_start, gsasl, mechanism, sessionHook);
+    }
+
+    Status GsaslSession::_initializeSession(
+            GsaslSessionStartFn sessionStartFn,
+            Gsasl* gsasl, const StringData& mechanism, void* sessionHook) {
+
+        if (_done || _gsaslSession)
+            return Status(ErrorCodes::CannotReuseObject, "Cannot reuse GsaslSession.");
+
+        int rc = sessionStartFn(gsasl, mechanism.data(), &_gsaslSession);
+        switch (rc) {
+        case GSASL_OK:
+            gsasl_session_hook_set(_gsaslSession, sessionHook);
+            return Status::OK();
+        case GSASL_UNKNOWN_MECHANISM:
+            return Status(ErrorCodes::BadValue, gsasl_strerror(rc));
+        default:
+            return Status(ErrorCodes::ProtocolError, gsasl_strerror(rc));
+        }
+    }
+
+    Status GsaslSession::step(const StringData& inputData, std::string* outputData) {
+        char* output;
+        size_t outputSize;
+        int rc = gsasl_step(_gsaslSession,
+                            inputData.data(), inputData.size(),
+                            &output, &outputSize);
+
+        if (GSASL_OK == rc)
+            _done = true;
+
+        switch (rc) {
+        case GSASL_OK:
+        case GSASL_NEEDS_MORE:
+            *outputData = std::string(output, output + outputSize);
+            free(output);
+            return Status::OK();
+        case GSASL_AUTHENTICATION_ERROR:
+            return Status(ErrorCodes::AuthenticationFailed, gsasl_strerror(rc));
+        default:
+            return Status(ErrorCodes::ProtocolError, gsasl_strerror(rc));
+        }
+    }
+
+}  // namespace mongo

src/mongo/util/gsasl_session.h

@@ -0,0 +1,138 @@
+/*    Copyright 2012 10gen Inc.
+ *
+ *    Licensed under the Apache License, Version 2.0 (the "License");
+ *    you may not use this file except in compliance with the License.
+ *    You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *    Unless required by applicable law or agreed to in writing, software
+ *    distributed under the License is distributed on an "AS IS" BASIS,
+ *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *    See the License for the specific language governing permissions and
+ *    limitations under the License.
+ */
+
+#pragma once
+
+#include <gsasl.h>
+#include <string>
+
+#include "mongo/base/disallow_copying.h"
+#include "mongo/base/status.h"
+#include "mongo/base/string_data.h"
+
+namespace mongo {
+
+    /**
+     * C++ wrapper around Gsasl_session.
+     */
+    class GsaslSession {
+        MONGO_DISALLOW_COPYING(GsaslSession);
+    public:
+        GsaslSession();
+        ~GsaslSession();
+
+        /**
+         * Initializes "this" as a client sasl session.
+         *
+         * May only be called once on an instance of GsaslSession, and may not be called on an
+         * instance on which initializeServerSession has been called.
+         *
+         * "gsasl" is a pointer to a Gsasl library context that will exist for the rest of
+         * the lifetime of "this".
+         *
+         * "mechanism" is a SASL mechanism name.
+         *
+         * "sessionHook" is user-supplied data associated with this session.  If is accessible in
+         * the gsasl callback set on "gsasl" using gsasl_session_hook_get().  May be NULL.  Owned
+         * by caller, and must stay in scope as long as this object.
+         *
+         * Returns Status::OK() on success, some other status on errors.
+         */
+        Status initializeClientSession(Gsasl* gsasl,
+                                       const StringData& mechanism,
+                                       void* sessionHook);
+
+        /**
+         * Initializes "this" as a server sasl session.
+         *
+         * May only be called once on an instance of GsaslSession, and may not be called on an
+         * instance on which initializeClientSession has been called.
+         *
+         * "gsasl" is a pointer to a Gsasl library context that will exist for the rest of
+         * the lifetime of "this".
+         *
+         * "mechanism" is a SASL mechanism name.
+         *
+         * "sessionHook" is user-supplied data associated with this session.  If is accessible in
+         * the gsasl callback set on "gsasl" using gsasl_session_hook_get().  May be NULL.  Owned
+         * by caller, and must stay in scope as long as this object.
+         *
+         * Returns Status::OK() on success, some other status on errors.
+         */
+        Status initializeServerSession(Gsasl* gsasl,
+                                       const StringData& mechanism,
+                                       void* sessionHook);
+
+        /**
+         * Returns the string name of the SASL mechanism in use in this session.
+         *
+         * Not valid before initializeServerSession() or initializeClientSession().
+         */
+        std::string getMechanism() const;
+
+        /**
+         * Sets a property on this session.
+         *
+         * Not valid before initializeServerSession() or initializeClientSession().
+         */
+        void setProperty(Gsasl_property property, const StringData& value);
+
+        /**
+         * Performs one more step on this session.
+         *
+         * Receives "inputData" from the other side and produces "*outputData" to send.
+         *
+         * Both "inputData" and "*outputData" are logically strings of bytes, not characters.
+         *
+         * For the first step by the authentication initiator, "inputData" should have 0 length.
+         *
+         * Returns Status::OK() on success.  In that case, isDone() can be queried to see if the
+         * session expects another call to step().  If isDone() is true, the authentication has
+         * completed successfully.
+         *
+         * Any return other than Status::OK() means that authentication has failed, but the specific
+         * code or reason message may provide insight as to why.
+         */
+        Status step(const StringData& inputData, std::string* outputData);
+
+        /**
+         * Returns true if this session has completed successfully.
+         *
+         * That is, returns true if the session expects no more calls to step(), and all previous
+         * calls to step() and initializeClientSession()/initializeServerSession() have returned
+         * Status::OK().
+         */
+        bool isDone() const { return _done; }
+
+    private:
+        // Signature of gsas session start functions.
+        typedef int (*GsaslSessionStartFn)(Gsasl*, const char*, Gsasl_session**);
+
+        /**
+         * Common helper code for initializing a session.
+         *
+         * Uses "sessionStartFn" to initialize the underlying Gsasl_session.
+         */
+        Status _initializeSession(GsaslSessionStartFn sessionStartFn,
+                                  Gsasl* gsasl, const StringData& mechanism, void* sessionHook);
+
+        /// Underlying C-library gsasl session object.
+        Gsasl_session* _gsaslSession;
+
+        /// See isDone(), above.
+        bool _done;
+    };
+
+}  // namespace mongo