'\" te
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.TH KRB5_AUTH_RULES 7 "November 22, 2021"
.SH NAME
krb5_auth_rules \- overview of Kerberos V5 authorization
.SH DESCRIPTION
When kerberized versions of the \fBftp\fR, \fBrdist\fR, \fBrcp\fR,
\fBrlogin\fR, \fBrsh\fR, \fBtelnet\fR, or \fBssh\fR clients are used to connect
to a server, the identity of the originating user must be authenticated to the
Kerberos V5 authentication system. Account access can then be authorized if
appropriate entries exist in the \fB~/.k5login\fR file, the \fBgsscred\fR
table, or if the default GSS/Kerberos authentication rules successfully map the
Kerberos principal name to Unix login name.
.sp
.LP
To avoid security problems, the \fB~/.k5login\fR file must be owned by the
remote user on the server the client is attempting to access. The file should
contain a private authorization list comprised of Kerberos principal names of
the form \fIprincipal/instance\fR@\fIrealm\fR. The \fI/instance\fR variable is
optional in Kerberos principal names. For example, different principal names
such as \fBjdb@ENG.EXAMPLE.COM\fR and \fBjdb/happy.eng.example.com@ENG.EXAMPLE.COM\fR
would each be legal, though not equivalent, Kerberos principals. The client is
granted access if the \fB~/.k5login\fR file is located in the login directory
of the remote user account and if the originating user can be authenticated to
one of the principals named in the file. See \fBkadm5.acl\fR(5) for more
information on Kerberos principal names.
.sp
.LP
When no \fB~/.k5login\fR file is found in the remote user's login account, the
Kerberos V5 principal name associated with the originating user is checked
against the \fBgsscred\fR table. If a \fBgsscred\fR table exists and the
principal name is matched in the table, access is granted if the Unix user ID
listed in the table corresponds to the user account the client is attempting to
access. If the Unix user ID does not match, access is denied. See
\fBgsscred\fR(8).
.sp
.LP
For example, an originating user listed in the \fBgsscred\fR table with the
principal name \fBjdb@ENG.EXAMPLE.COM\fR and the \fBuid\fR \fB23154\fR is granted
access to the \fBjdb-user\fR account if \fB23154\fR is also the \fBuid\fR of
\fBjdb-user\fR listed in the user account database. See \fBpasswd\fR(5).
.sp
.LP
Finally, if there is no \fB~/.k5login\fR file and the Kerberos V5 identity of
the originating user is not in the \fBgsscred\fR table, or if the \fBgsscred\fR
table does not exist, the client is granted access to the account under the
following conditions (default GSS/Kerberos auth rules):
.RS +4
.TP
.ie t \(bu
.el o
The user part of the authenticated principal name is the same as the Unix
account name specified by the client.
.RE
.RS +4
.TP
.ie t \(bu
.el o
The realm part of the client and server are the same, unless the
\fBkrb5.conf\fR(5) \fIauth_to_local_realm\fR parameter is used to create
equivalence.
.RE
.RS +4
.TP
.ie t \(bu
.el o
The Unix account name exists on the server.
.RE
.sp
.LP
For example, if the originating user has the principal name
\fBjdb@ENG.EXAMPLE.COM\fR and if the server is in realm \fBSALES.EXAMPLE.COM\fR, the
client would be denied access even if \fBjdb\fR is a valid account name on the
server. This is because the realms \fBSALES.EXAMPLE.COM\fR and \fBENG.EXAMPLE.COM\fR
differ.
.sp
.LP
The \fBkrb5.conf\fR(5) \fIauth_to_local_realm\fR parameter also affects
authorization. Non-default realms can be equated with the default realm for
authenticated \fBname-to-local name\fR mapping.
.SH FILES
.ne 2
.na
\fB\fB~/.k5login\fR\fR
.ad
.RS 15n
Per user-account authorization file.
.RE

.sp
.ne 2
.na
\fB\fB/etc/passwd\fR\fR
.ad
.RS 15n
System account file. This information may also be in a directory service. See
\fBpasswd\fR(5).
.RE

.SH ATTRIBUTES
See \fBattributes\fR(7) for a description of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Evolving
.TE

.SH SEE ALSO
.BR ftp (1),
.BR rcp (1),
.BR rdist (1),
.BR rlogin (1),
.BR rsh (1),
.BR telnet (1),
.BR kadm5.acl (5),
.BR krb5.conf (5),
.BR passwd (5),
.BR attributes (7),
.BR gss_auth_rules (7),
.BR gsscred (8)