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Annotation of embedaddon/ipsec-tools/src/racoon/doc/README.gssapi, revision 1.1.1.1

1.1       misho       1: The gss-api authentication mechanism implementation for racoon was
                      2: based on the ietf draft draft-ietf-ipsec-isakmp-gss-auth-06.txt.
                      3: 
                      4: The implementation uses the Heimdal gss-api library, i.e. gss-api
                      5: on top of Kerberos 5. The Heimdal gss-api library had to be modified
                      6: to meet the requirements of using gss-api in a daemon. More specifically,
                      7: the gss_acquire_cred() call did not work for other cases than
                      8: GSS_C_NO_CREDENTIAL ("use default creds"). Daemons are often started
                      9: as root, and have no Kerberos 5 credentials, so racoon explicitly
                     10: needs to acquire its credentials. The usual method (already used
                     11: by login authentication daemons) in these situations is to add
                     12: a set of special credentials to be used. For example, authentication
                     13: by daemons concerned with login credentials, uses 'host/fqdn' as
                     14: its credential, where fqdn is the hostname on the interface that
                     15: is being used. These special credentials need to be extracted into
                     16: a local keytab from the kdc. The default value used in racoon
                     17: is 'ike/fqdn', but it can be overridden in the racoon config file.
                     18: 
                     19: The modification to the Heimdal gss-api library implements the
                     20: mechanism above. If a credential other than GSS_C_NO_CREDENTIAL
                     21: is specified to gss_acquire_cred(), it first looks in the default
                     22: credential cache if it its principal matches the desired credential.
                     23: If not, it extracts it from the default keytab file, and stores
                     24: it in a memory-based credential cache, part of the gss credential
                     25: structure.
                     26: 
                     27: 
                     28: 
                     29: The modifcations to racoon itself are as follows:
                     30: 
                     31:        * The racoon.conf config file accepts a new keyword, "gssapi_id",
                     32:          to be used inside a proposal specification. It specifies
                     33:          a string (a Kerberos 5 principal in this case), specifying the
                     34:          credential that racoon will try to acquire. The default value
                     35:          is 'ike/fqdn', where fqdn is the hostname for the interface
                     36:          being used for the exchange. If the id is not specified, no
                     37:          GSS endpoint attribute will be specified in the first SA sent.
                     38:          However, if the initiator does specify a GSS endpoint attribute,
                     39:          racoon will always respond with its own GSS endpoint name
                     40:          in the SA (the default one if not specified by this option).
                     41: 
                     42:        * The racoon.conf file accepts "gssapi_krb" as authentication
                     43:          method inside a proposal specification. The number used
                     44:          for this method is 65001, which is a temporary number as
                     45:          specified in the draft.
                     46: 
                     47:        * The cftoken.l and cfparse.y source files were modified to
                     48:          pick up the configuration options. The original sources
                     49:          stored algorithms in bitmask, which unfortunately meant
                     50:          that the maximum value was 32, clearly not enough for 65001.
                     51:          After consulting with the author (sakane@kame.net), it turned
                     52:          out that method was a leftover, and no longer needed. I replaced
                     53:          it with plain integers.
                     54: 
                     55:        * The gss-api specific code was concentrated as much as possible
                     56:          in gssapi.c and gssapi.h. The code to call functions defined
                     57:          in these files is conditional on HAVE_GSSAPI, except for the
                     58:          config scan code. Specifying this flag on the compiler commandline
                     59:          is conditional on the --enable-gssapi option to the configure
                     60:          script.
                     61: 
                     62:        * Racoon seems to want to send accepted SA proposals back to
                     63:          the initiator in a verbatim fashion, leaving no room to
                     64:          insert the (variable-length) GSS endpoint name attribute.
                     65:          I worked around this by re-assembling the extracted SA
                     66:          into a new SA if the gssapi_krb method is used, and the
                     67:          initiator sent the name attribute. This scheme should
                     68:          possibly be re-examined by the racoon maintainers, storing
                     69:          the SAs (the transformations, to be more precise) in a different
                     70:          fashion to allow for variable-length attributes to be
                     71:          re-inserted would be a good change, but I considered it to be
                     72:          beyond the scope of this project.
                     73: 
                     74:        * The various state functions for aggressive and main mode
                     75:          (in isakmp_agg.c and isakmp_ident.c respectively) were
                     76:          changed to conditionally change their behavior if the
                     77:          gssapi_krb method is specified.
                     78: 
                     79: 
                     80: This implementation tried to follow the specification in the ietf draft
                     81: as close as possible. However, it has not been tested against other
                     82: IKE daemon implementations. The only other one I know of is Windows 2000,
                     83: and it has some caveats. I attempted to be Windows 2000 compatible.
                     84: Should racoon be tried against Windows 2000, the gssapi_id option in
                     85: the config file must be used, as Windows 2000 expects the GSS endpoint
                     86: name to be sent at all times. I have my doubts as to the W2K compatibility,
                     87: because the spec describes the GSS endpoint name sent by W2K as
                     88: an unicode string 'xxx@domain', which doesn't seem to match the
                     89: required standard for gss-api + kerberos 5 (i.e. I am fairly certain
                     90: that such a string will be rejected by the Heimdal gss-api library, as it
                     91: is not a valid Kerberos 5 principal).
                     92: 
                     93: With the Heimdal gss-api implementation, the gssapi_krb authentication
                     94: method will only work in main mode. Aggressive mode does not allow
                     95: for the extra round-trips needed by gss_init_sec_context and
                     96: gss_accept_sec_context when mutual authentication is requested.
                     97: The draft specifies that the a fallback should be done to main mode,
                     98: through the return of INVALID-EXCHANGE-TYPE if it turns out that
                     99: the gss-api mechanisms needs more roundtrips. This is implemented.
                    100: Unfortunately, racoon does not seem to properly fall back to
                    101: its next mode, and this is not specific to the gssapi_krb method.
                    102: So, to avoid problems, only specify main mode in the config file.
                    103: 
                    104: 
                    105:        -- Frank van der Linden <fvdl@wasabisystems.com>
                    106: