/* Portions of this file are subject to the following copyright(s).  See

 * the Net-SNMP's COPYING file for more details and other copyrights

 * that may apply:

 */

/* 

 * See the following web pages for useful documentation on this transport:

 * http://www.net-snmp.org/wiki/index.php/TUT:Using_TLS

 * http://www.net-snmp.org/wiki/index.php/Using_DTLS

 */

#include <net-snmp/net-snmp-config.h>

#include <net-snmp/net-snmp-features.h>

netsnmp_feature_require(cert_util)

#include <stdio.h>

#include <sys/types.h>

#include <ctype.h>

#include <errno.h>

#if HAVE_STRING_H

#include <string.h>

#else

#include <strings.h>

#endif

#if HAVE_STDLIB_H

#include <stdlib.h>

#endif

#if HAVE_UNISTD_H

#include <unistd.h>

#endif

#if HAVE_SYS_SOCKET_H

#include <sys/socket.h>

#endif

#if HAVE_NETINET_IN_H

#include <netinet/in.h>

#endif

#if HAVE_ARPA_INET_H

#include <arpa/inet.h>

#endif

#if HAVE_NETDB_H

#include <netdb.h>

#endif

#if HAVE_SYS_UIO_H

#include <sys/uio.h>

#endif

#if HAVE_ARPA_INET_H

#include <arpa/inet.h>

#endif

#if HAVE_DMALLOC_H

#include <dmalloc.h>

#endif

#include <net-snmp/types.h>

#include <net-snmp/output_api.h>

#include <net-snmp/config_api.h>

#include <net-snmp/library/snmp_assert.h>

#include <net-snmp/library/snmpIPv4BaseDomain.h>

#include <net-snmp/library/snmpSocketBaseDomain.h>

#include <net-snmp/library/snmpTLSBaseDomain.h>

#include <net-snmp/library/snmpTLSTCPDomain.h>

#include <net-snmp/library/system.h>

#include <net-snmp/library/tools.h>

#include <net-snmp/library/cert_util.h>

#include <net-snmp/library/snmp_openssl.h>

#include <net-snmp/library/callback.h>

#include "openssl/bio.h"

#include "openssl/ssl.h"

#include "openssl/err.h"

#ifndef INADDR_NONE

#define INADDR_NONE	-1

#endif

#define WE_ARE_SERVER 0

#define WE_ARE_CLIENT 1

oid             netsnmpTLSTCPDomain[] = { TRANSPORT_DOMAIN_TLS_TCP_IP };

size_t          netsnmpTLSTCPDomain_len = OID_LENGTH(netsnmpTLSTCPDomain);

static netsnmp_tdomain tlstcpDomain;

/*

 * Return a string representing the address in data, or else the "far end"

 * address if data is NULL.  

 */

static char *

netsnmp_tlstcp_fmtaddr(netsnmp_transport *t, const void *data, int len)

{

    if (t && !data) {

        data = t->data;

        len = t->data_length;

    }

    switch (data ? len : 0) {

    case sizeof(netsnmp_indexed_addr_pair):

        return netsnmp_ipv4_fmtaddr("TLSTCP", t, data, len);

    case sizeof(netsnmp_tmStateReference): {

        const netsnmp_tmStateReference *r = data;

        const netsnmp_indexed_addr_pair *p = &r->addresses;

        return netsnmp_ipv4_fmtaddr("TLSTCP", t, p, sizeof(*p));

    }

    case sizeof(_netsnmpTLSBaseData): {

        const _netsnmpTLSBaseData *b = data;

        char *buf;

        if (asprintf(&buf, "TLSTCP: %s", b->addr_string) < 0)

            buf = NULL;

        return buf;

    }

    case 0:

        return strdup("TLSTCP: unknown");

    default: {

        char *buf;

        if (asprintf(&buf, "TLSTCP: len %d", len) < 0)

            buf = NULL;

        return buf;

    }

    }

}

static void netsnmp_tlstcp_get_taddr(struct netsnmp_transport_s *t,

                                     void **addr, size_t *addr_len)

{

    *addr_len = t->remote_length;

    *addr = netsnmp_memdup(t->remote, *addr_len);

}

/*

 * You can write something into opaque that will subsequently get passed back 

 * to your send function if you like.  For instance, you might want to

 * remember where a PDU came from, so that you can send a reply there...  

 */

static int

netsnmp_tlstcp_copy(const netsnmp_transport *oldt, netsnmp_transport *newt)

{

    _netsnmpTLSBaseData *oldtlsdata = (_netsnmpTLSBaseData *) oldt->data;

    _netsnmpTLSBaseData *newtlsdata = (_netsnmpTLSBaseData *) newt->data;

    oldtlsdata->accepted_bio = NULL;

    oldtlsdata->ssl = NULL;

    newtlsdata->ssl_context = NULL;

    if (oldtlsdata->addr_string)

        newtlsdata->addr_string = strdup(oldtlsdata->addr_string);

    if (oldtlsdata->securityName)

        newtlsdata->securityName = strdup(oldtlsdata->securityName);

    if (oldtlsdata->our_identity)

        newtlsdata->our_identity = strdup(oldtlsdata->our_identity);

    if (oldtlsdata->their_identity)

        newtlsdata->their_identity = strdup(oldtlsdata->their_identity);

    if (oldtlsdata->their_fingerprint)

        newtlsdata->their_fingerprint = strdup(oldtlsdata->their_fingerprint);

    if (oldtlsdata->their_hostname)

        newtlsdata->their_hostname = strdup(oldtlsdata->their_hostname);

    if (oldtlsdata->trust_cert)

        newtlsdata->trust_cert = strdup(oldtlsdata->trust_cert);

    if (oldtlsdata->addr)

        newtlsdata->addr = netsnmp_memdup(oldtlsdata->addr,

                                          sizeof(*oldtlsdata->addr));

    return 0;

}

static int

netsnmp_tlstcp_recv(netsnmp_transport *t, void *buf, int size,

                    void **opaque, int *olength)

{

    int             rc = -1;

    netsnmp_tmStateReference *tmStateRef = NULL;

    _netsnmpTLSBaseData *tlsdata;

    if (NULL == t || t->sock < 0 || NULL == t->data) {

        snmp_log(LOG_ERR,

                 "tlstcp received an invalid invocation with missing data\n");

        DEBUGMSGTL(("tlstcp", "recvfrom fd %d err %d (\"%s\")\n",

                    (t ? t->sock : -1), errno, strerror(errno)));

        if (t)

            DEBUGMSGTL(("tlstcp", "  tdata = %p", t->data));

        DEBUGMSGTL(("tlstcp", "\n"));

        return -1;

    }

    /* RFC5953 Section 5.1.2 step 1:

    1) Determine the tlstmSessionID for the incoming message. The

       tlstmSessionID MUST be a unique session identifier for this

       (D)TLS connection.  The contents and format of this identifier

       are implementation-dependent as long as it is unique to the

       session.  A session identifier MUST NOT be reused until all

       references to it are no longer in use.  The tmSessionID is

       equal to the tlstmSessionID discussed in Section 5.1.1.

       tmSessionID refers to the session identifier when stored in the

       tmStateReference and tlstmSessionID refers to the session

       identifier when stored in the LCD.  They MUST always be equal

       when processing a given session's traffic.

     */

    /* For this implementation we use the t->data memory pointer as

       the sessionID.  As it's a pointer to session specific data tied

       with the transport object we know it'll never be realloated

       (ie, duplicated) until release by this transport object and is

       safe to use as a unique session identifier. */

    tlsdata = t->data;

    if (NULL == tlsdata->ssl) {

        snmp_log(LOG_ERR,

                 "tlstcp received an invalid invocation without ssl data\n");

        return -1;

    }

    /* RFC5953 Section 5.1.2 step 1, part2:

     * This part (incrementing the counter) is done in the

       netsnmp_tlstcp_accept function.

     */

    /* RFC5953 Section 5.1.2 step 2:

     * Create a tmStateReference cache for the subsequent reference and

       assign the following values within it:

       tmTransportDomain  = snmpTLSTCPDomain or snmpDTLSUDPDomain as

                            appropriate.

       tmTransportAddress = The address the message originated from.

       tmSecurityLevel    = The derived tmSecurityLevel for the session,

                            as discussed in Section 3.1.2 and Section 5.3.

       tmSecurityName     = The fderived tmSecurityName for the session as

                            discussed in Section 5.3.  This value MUST

                            be constant during the lifetime of the

                            session.

       tmSessionID        = The tlstmSessionID described in step 1 above.

     */

    /* Implementation notes:

     * - The tmTransportDomain is represented by the transport object

     * - The tmpSessionID is represented by the tlsdata pointer (as

         discussed above)

     * - The following items are handled later in netsnmp_tlsbase_wrapup_recv:

         - tmSecurityLevel

         - tmSecurityName

         - tmSessionID

    */

    /* create a tmStateRef cache for slow fill-in */

    tmStateRef = SNMP_MALLOC_TYPEDEF(netsnmp_tmStateReference);

    if (tmStateRef == NULL) {

        *opaque = NULL;

        *olength = 0;

        return -1;

    }

    /* Set the transportDomain */

    memcpy(tmStateRef->transportDomain,

           netsnmpTLSTCPDomain, sizeof(netsnmpTLSTCPDomain[0]) *

           netsnmpTLSTCPDomain_len);

    tmStateRef->transportDomainLen = netsnmpTLSTCPDomain_len;

    /* Set the tmTransportAddress */

    tmStateRef->have_addresses = 1;

    /* RFC5953 Section 5.1.2 step 1:

     * 3)  The incomingMessage and incomingMessageLength are assigned values

       from the (D)TLS processing.

     */

    /* Implementation notes:

       - incomingMessage       = buf pointer

       - incomingMessageLength = rc

    */

    /* read the packet from openssl */

    do {

        rc = SSL_read(tlsdata->ssl, buf, size);

        if (rc == 0) {

            /* XXX closed connection */

            DEBUGMSGTL(("tlstcp", "remote side closed connection\n"));

            /* XXX: openssl cleanup */

            SNMP_FREE(tmStateRef);

            return -1;

        }

        if (rc == -1) {

            int err = SSL_get_error(tlsdata->ssl, rc);

            if (err != SSL_ERROR_WANT_READ && err != SSL_ERROR_WANT_WRITE) {

                /* error detected */

                _openssl_log_error(rc, tlsdata->ssl, "SSL_read");

                SNMP_FREE(tmStateRef);

                return rc;

            }

        }

        /* retry read for SSL_ERROR_WANT_READ || SSL_ERROR_WANT_WRITE */

    } while (rc <= 0); 

    DEBUGMSGTL(("tlstcp", "received %d decoded bytes from tls\n", rc));

    /* log the packet */

    DEBUGIF("tlstcp") {

        char *str = netsnmp_tlstcp_fmtaddr(t, NULL, 0);

        DEBUGMSGTL(("tlstcp",

                    "recvfrom fd %d got %d bytes (from %s)\n",

                    t->sock, rc, str));

        free(str);

    }

    /* Other wrap-up things common to TLS and DTLS */

    if (netsnmp_tlsbase_wrapup_recv(tmStateRef, tlsdata, opaque, olength) !=

        SNMPERR_SUCCESS)

        return SNMPERR_GENERR;

    /* RFC5953 Section 5.1.2 step 1:

     * 4)  The TLS Transport Model passes the transportDomain,

       transportAddress, incomingMessage, and incomingMessageLength to

       the Dispatcher using the receiveMessage ASI:

    */

    /* In our implementation, this is done simply by returning */

    return rc;

}

static int

netsnmp_tlstcp_send(netsnmp_transport *t, const void *buf, int size,

                    void **opaque, int *olength)

{

    int rc = -1;

    const netsnmp_tmStateReference *tmStateRef = NULL;

    _netsnmpTLSBaseData *tlsdata;

    DEBUGTRACETOK("tlstcp");

    /* RFC5953 section 5.2: 

      1)  If tmStateReference does not refer to a cache containing values

      for tmTransportDomain, tmTransportAddress, tmSecurityName,

      tmRequestedSecurityLevel, and tmSameSecurity, then increment the

      snmpTlstmSessionInvalidCaches counter, discard the message, and

      return the error indication in the statusInformation.  Processing

      of this message stops.

    */

    /* Implementation Notes: the tmStateReference is stored in the opaque ptr */

    if (opaque != NULL && *opaque != NULL &&

        *olength == sizeof(netsnmp_tmStateReference)) {

        tmStateRef = (const netsnmp_tmStateReference *) *opaque;

    } else {

        snmp_log(LOG_ERR, "TLSTCP was called with an invalid state; possibly the wrong security model is in use.  It should be 'tsm'.\n");

        snmp_increment_statistic(STAT_TLSTM_SNMPTLSTMSESSIONINVALIDCACHES);

        return SNMPERR_GENERR;

    }

    /* RFC5953 section 5.2: 

       2)  Extract the tmSessionID, tmTransportDomain, tmTransportAddress,

       tmSecurityName, tmRequestedSecurityLevel, and tmSameSecurity

       values from the tmStateReference.  Note: The tmSessionID value

       may be undefined if no session exists yet over which the message

       can be sent.

    */

    /* Implementation Notes:

       - Our session will always exist by now as it's created when the

         transport object is created. Auto-session creation is handled

         higher in the stack.

       - We don't "extract" per say since we just leave the data in

         the structure.

       - The sessionID is stored in the t->data memory pointer.

    */

    /* RFC5953 section 5.2: 

       3)  If tmSameSecurity is true and either tmSessionID is undefined or

           refers to a session that is no longer open then increment the

           snmpTlstmSessionNoSessions counter, discard the message and

           return the error indication in the statusInformation.  Processing

           of this message stops.

    */

    /* Implementation Notes:

       - We would never get here if the sessionID was either undefined

         or different.  We tie packets directly to the transport

         object and it could never be sent back over a different

         transport, which is what the above text is trying to prevent.

     */

    /* RFC5953 section 5.2: 

       4)  If tmSameSecurity is false and tmSessionID refers to a session

           that is no longer available then an implementation SHOULD open a

           new session using the openSession() ASI (described in greater

           detail in step 5b).  Instead of opening a new session an

           implementation MAY return a snmpTlstmSessionNoSessions error to

           the calling module and stop processing of the message.

    */

    /* Implementation Notes:

       - We would never get here if the sessionID was either undefined

         or different.  We tie packets directly to the transport

         object and it could never be sent back over a different

         transport, which is what the above text is trying to prevent.

       - Auto-connections are handled higher in the Net-SNMP library stack

     */

    /* RFC5953 section 5.2: 

       5)  If tmSessionID is undefined, then use tmTransportDomain,

           tmTransportAddress, tmSecurityName and tmRequestedSecurityLevel

           to see if there is a corresponding entry in the LCD suitable to

           send the message over.

           5a)  If there is a corresponding LCD entry, then this session

                will be used to send the message.

           5b)  If there is not a corresponding LCD entry, then open a

                session using the openSession() ASI (discussed further in

                Section 5.3.1).  Implementations MAY wish to offer message

                buffering to prevent redundant openSession() calls for the

                same cache entry.  If an error is returned from

                openSession(), then discard the message, discard the

                tmStateReference, increment the snmpTlstmSessionOpenErrors,

                return an error indication to the calling module and stop

                processing of the message.

    */

    /* Implementation Notes:

       - We would never get here if the sessionID was either undefined

         or different.  We tie packets directly to the transport

         object and it could never be sent back over a different

         transport, which is what the above text is trying to prevent.

       - Auto-connections are handled higher in the Net-SNMP library stack

     */

    /* our session pointer is functionally t->data */

    if (NULL == t->data) {

        snmp_log(LOG_ERR, "netsnmp_tlstcp_send received no incoming data\n");

        return -1;

    }

    tlsdata = t->data;

    if (tlsdata->ssl == NULL) {

        snmp_log(LOG_ERR, "tlstcp_send was called without a SSL connection.\n");

        return SNMPERR_GENERR;

    }

    /* If the first packet and we have no secname, then copy the

       important securityName data into the longer-lived session

       reference information. */

    if ((tlsdata->flags & NETSNMP_TLSBASE_IS_CLIENT) &&

        !tlsdata->securityName && tmStateRef && tmStateRef->securityNameLen > 0)

        tlsdata->securityName = strdup(tmStateRef->securityName);

    /* RFC5953 section 5.2: 

       6)  Using either the session indicated by the tmSessionID if there

           was one or the session resulting from a previous step (4 or 5),

           pass the outgoingMessage to (D)TLS for encapsulation and

           transmission.

    */

    rc = SSL_write(tlsdata->ssl, buf, size);

    DEBUGMSGTL(("tlstcp", "wrote %d bytes\n", size));

    if (rc < 0) {

        _openssl_log_error(rc, tlsdata->ssl, "SSL_write");

    }

    return rc;

}

static int

netsnmp_tlstcp_close(netsnmp_transport *t)

{

    _netsnmpTLSBaseData *tlsdata;

    if (NULL == t || NULL == t->data)

        return -1;

    /* RFC5953 Section 5.4.  Closing a Session

       1)  Increment either the snmpTlstmSessionClientCloses or the

           snmpTlstmSessionServerCloses counter as appropriate.

    */

    if (t->flags & NETSNMP_TLSBASE_IS_CLIENT)

        snmp_increment_statistic(STAT_TLSTM_SNMPTLSTMSESSIONCLIENTCLOSES);

    else 

        snmp_increment_statistic(STAT_TLSTM_SNMPTLSTMSESSIONSERVERCLOSES);

    /* RFC5953 Section 5.4.  Closing a Session

       2)  Look up the session using the tmSessionID.

    */

    tlsdata = (_netsnmpTLSBaseData *) t->data;

    /* RFC5953 Section 5.4.  Closing a Session

       3)  If there is no open session associated with the tmSessionID, then

           closeSession processing is completed.

    */

    /* Implementation notes: if we have a non-zero tlsdata then it's

       always true */

    /* RFC5953 Section 5.3.1: Establishing a Session as a Client

       4)  Have (D)TLS close the specified connection.  This SHOULD include

           sending a close_notify TLS Alert to inform the other side that

           session cleanup may be performed.

    */

    DEBUGMSGTL(("tlstcp", "Shutting down SSL connection\n"));

    if (tlsdata->ssl) {

        SSL_shutdown(tlsdata->ssl);

    }

    netsnmp_tlsbase_free_tlsdata(tlsdata);

    t->data = NULL;

    return netsnmp_socketbase_close(t);

}

static int

netsnmp_tlstcp_accept(netsnmp_transport *t)

{

    BIO            *accepted_bio;

    int             rc;

    SSL_CTX *ctx;

    SSL     *ssl;

    _netsnmpTLSBaseData *tlsdata = NULL;

    DEBUGMSGTL(("tlstcp", "netsnmp_tlstcp_accept called\n"));

    tlsdata = (_netsnmpTLSBaseData *) t->data;

    rc = BIO_do_accept(tlsdata->accept_bio);

    if (rc <= 0) {

        snmp_log(LOG_ERR, "BIO_do_accept failed\n");

        _openssl_log_error(rc, NULL, "BIO_do_accept");

        /* XXX: need to close the listening connection here? */

        return -1;

    }

    tlsdata->accepted_bio = accepted_bio = BIO_pop(tlsdata->accept_bio);

    if (!accepted_bio) {

        snmp_log(LOG_ERR, "Failed to pop an accepted bio off the bio stack\n");

        /* XXX: need to close the listening connection here? */

        return -1;

    }

    /* create the OpenSSL TLS context */

    ctx = tlsdata->ssl_context;

    /* create the server's main SSL bio */

    ssl = tlsdata->ssl = SSL_new(ctx);

    if (!tlsdata->ssl) {

        snmp_log(LOG_ERR, "TLSTCP: Failed to create a SSL BIO\n");

        BIO_free(accepted_bio);

        tlsdata->accepted_bio = NULL;

        return -1;

    }

    SSL_set_bio(ssl, accepted_bio, accepted_bio);

    if ((rc = SSL_accept(ssl)) <= 0) {

        snmp_log(LOG_ERR, "TLSTCP: Failed SSL_accept\n");

        _openssl_log_error(rc, ssl, "SSL_accept");

        SSL_shutdown(tlsdata->ssl);

        SSL_free(tlsdata->ssl);

        tlsdata->accepted_bio = NULL; /* freed by SSL_free */

        tlsdata->ssl = NULL;

        return -1;

    }   

    /*

     * currently netsnmp_tlsbase_wrapup_recv is where we check for

     * algorithm compliance, but for tls we know the algorithms

     * at this point, so we could bail earlier...

     */

#if 0 /* moved checks to netsnmp_tlsbase_wrapup_recv */

    netsnmp_openssl_null_checks(tlsdata->ssl, &no_auth, NULL);

    if (no_auth != 0) { /* null/unknown authentication */

        /* xxx-rks: snmp_increment_statistic(STAT_???); */

        snmp_log(LOG_ERR, "tlstcp: connection with NULL authentication\n");

        SSL_shutdown(tlsdata->ssl);

        SSL_free(tlsdata->ssl);

        tlsdata->accepted_bio = NULL; /* freed by SSL_free */

        tlsdata->ssl = NULL;

        return -1;

    }

#endif

    /* RFC5953 Section 5.3.2: Accepting a Session as a Server

       A (D)TLS server should accept new session connections from any client

       that it is able to verify the client's credentials for.  This is done

       by authenticating the client's presented certificate through a

       certificate path validation process (e.g.  [RFC5280]) or through

       certificate fingerprint verification using fingerprints configured in

       the snmpTlstmCertToTSNTable.  Afterward the server will determine the

       identity of the remote entity using the following procedures.

       The (D)TLS server identifies the authenticated identity from the

       (D)TLS client's principal certificate using configuration information

       from the snmpTlstmCertToTSNTable mapping table.  The (D)TLS server

       MUST request and expect a certificate from the client and MUST NOT

       accept SNMP messages over the (D)TLS connection until the client has

       sent a certificate and it has been authenticated.  The resulting

       derived tmSecurityName is recorded in the tmStateReference cache as

       tmSecurityName.  The details of the lookup process are fully

       described in the DESCRIPTION clause of the snmpTlstmCertToTSNTable

       MIB object.  If any verification fails in any way (for example

       because of failures in cryptographic verification or because of the

       lack of an appropriate row in the snmpTlstmCertToTSNTable) then the

       session establishment MUST fail, and the

       snmpTlstmSessionInvalidClientCertificates object is incremented.  If

       the session can not be opened for any reason at all, including

       cryptographic verification failures, then the

       snmpTlstmSessionOpenErrors counter is incremented and processing

       stops.

       Servers that wish to support multiple principals at a particular port

       SHOULD make use of a (D)TLS extension that allows server-side

       principal selection like the Server Name Indication extension defined

       in Section 3.1 of [RFC4366].  Supporting this will allow, for

       example, sending notifications to a specific principal at a given TCP

       or UDP port.

    */

    /* Implementation notes:

       - we expect fingerprints to be stored in the transport config

       - we do not currently support mulitple principals and only offer one

    */

    if ((rc = netsnmp_tlsbase_verify_client_cert(ssl, tlsdata))

        != SNMPERR_SUCCESS) {

        /* XXX: free needed memory */

        snmp_log(LOG_ERR, "TLSTCP: Falied checking client certificate\n");

        snmp_increment_statistic(STAT_TLSTM_SNMPTLSTMSESSIONINVALIDCLIENTCERTIFICATES);

        SSL_shutdown(tlsdata->ssl);

        SSL_free(tlsdata->ssl);

        tlsdata->accepted_bio = NULL; /* freed by SSL_free */

        tlsdata->ssl = NULL;

        return -1;

    }

    /* XXX: check acceptance criteria here */

    DEBUGMSGTL(("tlstcp", "accept succeeded on sock %d\n", t->sock));

    /* RFC5953 Section 5.1.2 step 1, part2::

     * If this is the first message received through this session and

     the session does not have an assigned tlstmSessionID yet then the

     snmpTlstmSessionAccepts counter is incremented and a

     tlstmSessionID for the session is created.  This will only happen

     on the server side of a connection because a client would have

     already assigned a tlstmSessionID during the openSession()

     invocation.  Implementations may have performed the procedures

     described in Section 5.3.2 prior to this point or they may

     perform them now, but the procedures described in Section 5.3.2

     MUST be performed before continuing beyond this point.

    */

    /* We're taking option 2 and incrementing the session accepts here

       rather than upon receiving the first packet */

    snmp_increment_statistic(STAT_TLSTM_SNMPTLSTMSESSIONACCEPTS);

    /* XXX: check that it returns something so we can free stuff? */

    return BIO_get_fd(tlsdata->accepted_bio, NULL);

}

netsnmp_transport *

netsnmp_tlstcp_open(netsnmp_transport *t)

{

    _netsnmpTLSBaseData *tlsdata;

    BIO *bio;

    SSL_CTX *ctx;

    SSL *ssl;

    int rc = 0;

    _netsnmp_verify_info *verify_info;

    netsnmp_assert_or_return(t != NULL, NULL);

    netsnmp_assert_or_return(t->data != NULL, NULL);

    netsnmp_assert_or_return(sizeof(_netsnmpTLSBaseData) == t->data_length,

                             NULL);

    tlsdata = t->data;

    if (tlsdata->flags & NETSNMP_TLSBASE_IS_CLIENT) {

        /* Is the client */

        /* RFC5953 Section 5.3.1:  Establishing a Session as a Client

         *    1)  The snmpTlstmSessionOpens counter is incremented.

         */

        snmp_increment_statistic(STAT_TLSTM_SNMPTLSTMSESSIONOPENS);

        /* RFC5953 Section 5.3.1:  Establishing a Session as a Client

          2)  The client selects the appropriate certificate and cipher_suites

              for the key agreement based on the tmSecurityName and the

              tmRequestedSecurityLevel for the session.  For sessions being

              established as a result of a SNMP-TARGET-MIB based operation, the

              certificate will potentially have been identified via the

              snmpTlstmParamsTable mapping and the cipher_suites will have to

              be taken from system-wide or implementation-specific

              configuration.  If no row in the snmpTlstmParamsTable exists then

              implementations MAY choose to establish the connection using a

              default client certificate available to the application.

              Otherwise, the certificate and appropriate cipher_suites will

              need to be passed to the openSession() ASI as supplemental

              information or configured through an implementation-dependent

              mechanism.  It is also implementation-dependent and possibly

              policy-dependent how tmRequestedSecurityLevel will be used to

              influence the security capabilities provided by the (D)TLS

              connection.  However this is done, the security capabilities

              provided by (D)TLS MUST be at least as high as the level of

              security indicated by the tmRequestedSecurityLevel parameter.

              The actual security level of the session is reported in the

              tmStateReference cache as tmSecurityLevel.  For (D)TLS to provide

              strong authentication, each principal acting as a command

              generator SHOULD have its own certificate.

        */

        /*

          Implementation notes: we do most of this in the

          sslctx_client_setup The transport should have been

          f_config()ed with the proper fingerprints to use (which is

          stored in tlsdata), or we'll use the default identity

          fingerprint if that can be found.

        */

        /* XXX: check securityLevel and ensure no NULL fingerprints are used */

        /* set up the needed SSL context */

        tlsdata->ssl_context = ctx = sslctx_client_setup(TLS_method(), tlsdata);

        if (!ctx) {

            snmp_log(LOG_ERR, "failed to create TLS context\n");

            return NULL;

        }

        /* RFC5953 Section 5.3.1:  Establishing a Session as a Client

           3)  Using the destTransportDomain and destTransportAddress values,

               the client will initiate the (D)TLS handshake protocol to

               establish session keys for message integrity and encryption.

        */

        /* Implementation note:

           The transport domain and address are pre-processed by this point

        */

        /* Create a BIO connection for it */

        DEBUGMSGTL(("tlstcp", "connecting to tlstcp %s\n",

		    tlsdata->addr_string));

        t->remote = (void *) strdup(tlsdata->addr_string);

        t->remote_length = strlen(tlsdata->addr_string) + 1;

        bio = BIO_new_connect(tlsdata->addr_string);

        /* RFC5953 Section 5.3.1:  Establishing a Session as a Client

           3) continued:

              If the attempt to establish a session is unsuccessful, then

              snmpTlstmSessionOpenErrors is incremented, an error indication is

              returned, and processing stops.

        */

        if (NULL == bio) {

            snmp_increment_statistic(STAT_TLSTM_SNMPTLSTMSESSIONOPENERRORS);

            snmp_log(LOG_ERR, "tlstcp: failed to create bio\n");

            _openssl_log_error(rc, NULL, "BIO creation");

            return NULL;

        }

        /* Tell the BIO to actually do the connection */

        if ((rc = BIO_do_connect(bio)) <= 0) {

            snmp_increment_statistic(STAT_TLSTM_SNMPTLSTMSESSIONOPENERRORS);

            snmp_log(LOG_ERR, "tlstcp: failed to connect to %s\n",

		     tlsdata->addr_string);

            _openssl_log_error(rc, NULL, "BIO_do_connect");

            BIO_free(bio);

            return NULL;

        }

        /* Create the SSL layer on top of the socket bio */

        ssl = tlsdata->ssl = SSL_new(ctx);

        if (NULL == ssl) {

            snmp_increment_statistic(STAT_TLSTM_SNMPTLSTMSESSIONOPENERRORS);

            snmp_log(LOG_ERR, "tlstcp: failed to create a SSL connection\n");

            BIO_free(bio);

            return NULL;

        }

        /* Bind the SSL layer to the BIO */ 

        SSL_set_bio(ssl, bio, bio);

        SSL_set_mode(ssl, SSL_MODE_AUTO_RETRY);

        verify_info = SNMP_MALLOC_TYPEDEF(_netsnmp_verify_info);

        if (NULL == verify_info) {

            snmp_increment_statistic(STAT_TLSTM_SNMPTLSTMSESSIONOPENERRORS);

            snmp_log(LOG_ERR, "tlstcp: failed to create a SSL connection\n");

            SSL_shutdown(ssl);

            BIO_free(bio);

            return NULL;

        }

        SSL_set_ex_data(ssl, tls_get_verify_info_index(), verify_info);

        /* Then have SSL do it's connection over the BIO */

        if ((rc = SSL_connect(ssl)) <= 0) {

            snmp_increment_statistic(STAT_TLSTM_SNMPTLSTMSESSIONOPENERRORS);

            snmp_log(LOG_ERR, "tlstcp: failed to ssl_connect\n");

            BIO_free(bio);

            return NULL;

        }

        /* RFC5953 Section 5.3.1: Establishing a Session as a Client

           3) continued:

              If the session failed to open because the presented

              server certificate was unknown or invalid then the

              snmpTlstmSessionUnknownServerCertificate or

              snmpTlstmSessionInvalidServerCertificates MUST be

              incremented and a snmpTlstmServerCertificateUnknown or

              snmpTlstmServerInvalidCertificate notification SHOULD be

              sent as appropriate.  Reasons for server certificate

              invalidation includes, but is not limited to,

              cryptographic validation failures and an unexpected

              presented certificate identity.

        */

        /* RFC5953 Section 5.3.1: Establishing a Session as a Client

           4)  The (D)TLS client MUST then verify that the (D)TLS server's

               presented certificate is the expected certificate.  The (D)TLS

               client MUST NOT transmit SNMP messages until the server

               certificate has been authenticated, the client certificate has

               been transmitted and the TLS connection has been fully

               established.

               If the connection is being established from configuration based

               on SNMP-TARGET-MIB configuration, then the snmpTlstmAddrTable

               DESCRIPTION clause describes how the verification is done (using

               either a certificate fingerprint, or an identity authenticated

               via certification path validation).

               If the connection is being established for reasons other than

               configuration found in the SNMP-TARGET-MIB then configuration and

               procedures outside the scope of this document should be followed.

               Configuration mechanisms SHOULD be similar in nature to those

               defined in the snmpTlstmAddrTable to ensure consistency across

               management configuration systems.  For example, a command-line

               tool for generating SNMP GETs might support specifying either the

               server's certificate fingerprint or the expected host name as a

               command line argument.

        */

        /* Implementation notes:

           - All remote certificate fingerprints are expected to be

             stored in the transport's config information.  This is

             true both for CLI clients and TARGET-MIB sessions.

           - netsnmp_tlsbase_verify_server_cert implements these checks

        */

        if (netsnmp_tlsbase_verify_server_cert(ssl, tlsdata) != SNMPERR_SUCCESS) {

            /* XXX: unknown vs invalid; two counters */

            snmp_increment_statistic(STAT_TLSTM_SNMPTLSTMSESSIONUNKNOWNSERVERCERTIFICATE);

            snmp_log(LOG_ERR, "tlstcp: failed to verify ssl certificate\n");

            SSL_shutdown(ssl);

            BIO_free(bio);

            return NULL;

        }

        /* RFC5953 Section 5.3.1: Establishing a Session as a Client

           5)  (D)TLS provides assurance that the authenticated identity has

               been signed by a trusted configured certification authority.  If

               verification of the server's certificate fails in any way (for

               example because of failures in cryptographic verification or the

               presented identity did not match the expected named entity) then

               the session establishment MUST fail, the

               snmpTlstmSessionInvalidServerCertificates object is incremented.

               If the session can not be opened for any reason at all, including

               cryptographic verification failures, then the

               snmpTlstmSessionOpenErrors counter is incremented and processing

               stops.

        */

        /* XXX: add snmpTlstmSessionInvalidServerCertificates on

           crypto failure */

        /* RFC5953 Section 5.3.1: Establishing a Session as a Client

           6)  The TLSTM-specific session identifier (tlstmSessionID) is set in

           the tmSessionID of the tmStateReference passed to the TLS

           Transport Model to indicate that the session has been established

           successfully and to point to a specific (D)TLS connection for