source: rtems-libbsd/ipsec-tools/src/racoon/doc/README.gssapi @ ff36f5e

5-freebsd-12
Last change on this file since ff36f5e was ff36f5e, checked in by Christian Mauderer <christian.mauderer@…>, on May 30, 2018 at 12:27:35 PM

Import ipsec-tools 0.8.2.

Import unchanged ipsec-tools sources in the release version 0.8.2. The
homepage of ipsec-tools is http://ipsec-tools.sourceforge.net/. The
sources can be obtained from there.

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File size: 5.4 KB
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1The gss-api authentication mechanism implementation for racoon was
2based on the ietf draft draft-ietf-ipsec-isakmp-gss-auth-06.txt.
3
4The implementation uses the Heimdal gss-api library, i.e. gss-api
5on top of Kerberos 5. The Heimdal gss-api library had to be modified
6to meet the requirements of using gss-api in a daemon. More specifically,
7the gss_acquire_cred() call did not work for other cases than
8GSS_C_NO_CREDENTIAL ("use default creds"). Daemons are often started
9as root, and have no Kerberos 5 credentials, so racoon explicitly
10needs to acquire its credentials. The usual method (already used
11by login authentication daemons) in these situations is to add
12a set of special credentials to be used. For example, authentication
13by daemons concerned with login credentials, uses 'host/fqdn' as
14its credential, where fqdn is the hostname on the interface that
15is being used. These special credentials need to be extracted into
16a local keytab from the kdc. The default value used in racoon
17is 'ike/fqdn', but it can be overridden in the racoon config file.
18
19The modification to the Heimdal gss-api library implements the
20mechanism above. If a credential other than GSS_C_NO_CREDENTIAL
21is specified to gss_acquire_cred(), it first looks in the default
22credential cache if it its principal matches the desired credential.
23If not, it extracts it from the default keytab file, and stores
24it in a memory-based credential cache, part of the gss credential
25structure.
26
27
28
29The 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
80This implementation tried to follow the specification in the ietf draft
81as close as possible. However, it has not been tested against other
82IKE daemon implementations. The only other one I know of is Windows 2000,
83and it has some caveats. I attempted to be Windows 2000 compatible.
84Should racoon be tried against Windows 2000, the gssapi_id option in
85the config file must be used, as Windows 2000 expects the GSS endpoint
86name to be sent at all times. I have my doubts as to the W2K compatibility,
87because the spec describes the GSS endpoint name sent by W2K as
88an unicode string 'xxx@domain', which doesn't seem to match the
89required standard for gss-api + kerberos 5 (i.e. I am fairly certain
90that such a string will be rejected by the Heimdal gss-api library, as it
91is not a valid Kerberos 5 principal).
92
93With the Heimdal gss-api implementation, the gssapi_krb authentication
94method will only work in main mode. Aggressive mode does not allow
95for the extra round-trips needed by gss_init_sec_context and
96gss_accept_sec_context when mutual authentication is requested.
97The draft specifies that the a fallback should be done to main mode,
98through the return of INVALID-EXCHANGE-TYPE if it turns out that
99the gss-api mechanisms needs more roundtrips. This is implemented.
100Unfortunately, racoon does not seem to properly fall back to
101its next mode, and this is not specific to the gssapi_krb method.
102So, 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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