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:
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