1rpc.gssd(8) System Manager's Manual rpc.gssd(8)
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6 rpc.gssd - RPCSEC_GSS daemon
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9 rpc.gssd [-DfMnlvr] [-k keytab] [-p pipefsdir] [-d ccachedir] [-t time‐
10 out] [-R realm]
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13 The RPCSEC_GSS protocol, defined in RFC 5403, is used to provide strong
14 security for RPC-based protocols such as NFS.
15 Before exchanging RPC requests using RPCSEC_GSS, an RPC client must
17 establish a GSS security context. A security context is shared state
18 on each end of a network transport that enables GSS-API security ser‐
19 vices.
20 Security contexts are established using security credentials. A cre‐
22 dential grants temporary access to a secure network service, much as a
23 railway ticket grants temporary access to use a rail service.
24 A user typically obtains a credential by providing a password to the
26 kinit(1) command, or via a PAM library at login time. A credential
27 acquired with a user principal is known as a user credential (see ker‐
28 beros(1) for more on principals).
29 For certain operations, a credential is required which represents no
31 user, is otherwise unprivileged, and is always available. This is
32 referred to as a machine credential.
33 Machine credentials are typically established using a service princi‐
35 pal, whose encrypted password, called its key, is stored in a file,
36 called a keytab, to avoid requiring a user prompt. A machine creden‐
37 tial effectively does not expire because the system can renew it as
38 needed without user intervention.
39 Once obtained, credentials are typically stored in local temporary
41 files with well-known pathnames.
42
44 To establish GSS security contexts using these credential files, the
45 Linux kernel RPC client depends on a userspace daemon called rpc.gssd.
46 The rpc.gssd daemon uses the rpc_pipefs filesystem to communicate with
47 the kernel.
48 User Credentials
50 When a user authenticates using a command such as kinit(1), the result‐
51 ing credential is stored in a file with a well-known name constructed
52 using the user's UID.
53 To interact with an NFS server on behalf of a particular Kerberos-
55 authenticated user, the Linux kernel RPC client requests that rpc.gssd
56 initialize a security context with the credential in that user's cre‐
57 dential file.
58 Typically, credential files are placed in /tmp. However, rpc.gssd can
60 search for credential files in more than one directory. See the
61 description of the -d option for details.
62 Machine Credentials
64 A user credential is established by a user and is then shared with the
65 kernel and rpc.gssd. A machine credential is established by rpc.gssd
66 for the kernel when there is no user. Therefore rpc.gssd must already
67 have the materials on hand to establish this credential without requir‐
68 ing user intervention.
69 rpc.gssd searches the local system's keytab for a principal and key to
71 use to establish the machine credential. By default, rpc.gssd assumes
72 the file /etc/krb5.keytab contains principals and keys that can be used
73 to obtain machine credentials.
74 rpc.gssd searches in the following order for a principal to use. The
76 first matching credential is used. For the search, <hostname> and
77 <REALM> are replaced with the local system's hostname and Kerberos
78 realm.
79 <HOSTNAME>$@<REALM>
81 root/<hostname>@<REALM>
82 nfs/<hostname>@<REALM>
83 host/<hostname>@<REALM>
84 root/<anyname>@<REALM>
85 nfs/<anyname>@<REALM>
86 host/<anyname>@<REALM>
87 The <anyname> entries match on the service name and realm, but ignore
89 the hostname. These can be used if a principal matching the local
90 host's name is not found.
91 Note that the first principal in the search order is a user principal
93 that enables Kerberized NFS when the local system is joined to an
94 Active Directory domain using Samba. A password for this principal
95 must be provided in the local system's keytab.
96 You can specify another keytab by using the -k option if
98 /etc/krb5.keytab does not exist or does not provide one of these prin‐
99 cipals.
100 Credentials for UID 0
102 UID 0 is a special case. By default rpc.gssd uses the system's machine
103 credentials for UID 0 accesses that require GSS authentication. This
104 limits the privileges of the root user when accessing network resources
105 that require authentication.
106 Specify the -n option when starting rpc.gssd if you'd like to force the
108 root user to obtain a user credential rather than use the local sys‐
109 tem's machine credential.
110 When -n is specified, the kernel continues to request a GSS context
112 established with a machine credential for NFSv4 operations, such as
113 SETCLIENTID or RENEW, that manage state. If rpc.gssd cannot obtain a
114 machine credential (say, the local system has no keytab), NFSv4 opera‐
115 tions that require machine credentials will fail.
116 Encryption types
118 A realm administrator can choose to add keys encoded in a number of
119 different encryption types to the local system's keytab. For instance,
120 a host/ principal might have keys for the aes256-cts-hmac-sha1-96,
121 aes128-cts-hmac-sha1-96, des3-cbc-sha1, and arcfour-hmac encryption
122 types. This permits rpc.gssd to choose an appropriate encryption type
123 that the target NFS server supports.
124 These encryption types are stronger than legacy single-DES encryption
126 types. To interoperate in environments where servers support only weak
127 encryption types, you can restrict your client to use only single-DES
128 encryption types by specifying the -l option when starting rpc.gssd.
129
131 -D The server name passed to GSSAPI for authentication is normally
132 the name exactly as requested. e.g. for NFS it is the server
133 name in the "servername:/path" mount request. Only if this
134 servername appears to be an IP address (IPv4 or IPv6) or an
135 unqualified name (no dots) will a reverse DNS lookup will be
136 performed to get the canoncial server name.
137 If -D is present, a reverse DNS lookup will always be used, even
139 if the server name looks like a canonical name. So it is needed
140 if partially qualified, or non canonical names are regularly
141 used.
142 Using -D can introduce a security vulnerability, so it is recom‐
144 mended that -D not be used, and that canonical names always be
145 used when requesting services.
146 -f Runs rpc.gssd in the foreground and sends output to stderr (as
148 opposed to syslogd)
149 -n When specified, UID 0 is forced to obtain user credentials which
151 are used instead of the local system's machine credentials.
152 -k keytab
154 Tells rpc.gssd to use the keys found in keytab to obtain machine
155 credentials. The default value is /etc/krb5.keytab.
156 -l When specified, restricts rpc.gssd to sessions to weak encryp‐
158 tion types such as des-cbc-crc. This option is available only
159 when the local system's Kerberos library supports settable
160 encryption types.
161 -p path
163 Tells rpc.gssd where to look for the rpc_pipefs filesystem. The
164 default value is /var/lib/nfs/rpc_pipefs.
165 -d search-path
167 This option specifies a colon separated list of directories that
168 rpc.gssd searches for credential files. The default value is
169 /tmp:/run/user/%U. The literal sequence "%U" can be specified
170 to substitue the UID of the user for whom credentials are being
171 searched.
172 -M By default, machine credentials are stored in files in the first
174 directory in the credential directory search path (see the -d
175 option). When -M is set, rpc.gssd stores machine credentials in
176 memory instead.
177 -v Increases the verbosity of the output (can be specified multiple
179 times).
180 -r If the RPCSEC_GSS library supports setting debug level,
182 increases the verbosity of the output (can be specified multiple
183 times).
184 -R realm
186 Kerberos tickets from this realm will be preferred when scanning
187 available credentials cache files to be used to create a con‐
188 text. By default, the default realm, as configured in the Ker‐
189 beros configuration file, is preferred.
190 -t timeout
192 Timeout, in seconds, for kernel GSS contexts. This option allows
193 you to force new kernel contexts to be negotiated after timeout
194 seconds, which allows changing Kerberos tickets and identities
195 frequently. The default is no explicit timeout, which means the
196 kernel context will live the lifetime of the Kerberos service
197 ticket used in its creation.
198 -T timeout
200 Timeout, in seconds, to create an RPC connection with a server
201 while establishing an authenticated gss context for a user. The
202 default timeout is set to 5 seconds. If you get messages like
203 "WARNING: can't create tcp rpc_clnt to server %servername% for
204 user with uid %uid%: RPC: Remote system error - Connection timed
205 out", you should consider an increase of this timeout.
206
208 Many of the options that can be set on the command line can also be
209 controlled through values set in the [gssd] section of the
210 /etc/nfs.conf configuration file. Values recognized include:
211 verbosity
213 Value which is equivalent to the number of -v.
214 rpc-verbosity
216 Value which is equivalent to the number of -r.
217 use-memcache
219 A Boolean flag equivalent to -M.
220 use-machine-creds
222 A Boolean flag. Setting to false is equivalent to giving the -n
223 flag.
224 avoid-dns
226 Setting to false is equivalent to providing the -D flag.
227 limit-to-legacy-enctypes
229 Equivalent to -l.
230 context-timeout
232 Equivalent to -T.
233 rpc-timeout
235 Equivalent to -t.
236 keytab-file
238 Equivalent to -k.
239 cred-cache-directory
241 Equivalent to -d.
242 preferred-realm
244 Equivalent to -R.
245 In addtion, the following value is recognized from the [general] sec‐
247 tion:
248 pipefs-directory
250 Equivalent to -p.
251
254 rpc.svcgssd(8), kerberos(1), kinit(1), krb5.conf(5)
255
257 Dug Song <dugsong@umich.edu>
258 Andy Adamson <andros@umich.edu>
259 Marius Aamodt Eriksen <marius@umich.edu>
260 J. Bruce Fields <bfields@umich.edu>
261 20 Feb 2013 rpc.gssd(8)