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 [-DfMnlvrH] [-k keytab] [-p pipefsdir] [-d ccachedir] [-t
10 timeout] [-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 es‐
17 tablish a GSS security context. A security context is shared state on
18 each end of a network transport that enables GSS-API security services.
19 Security contexts are established using security credentials. A cre‐
21 dential grants temporary access to a secure network service, much as a
22 railway ticket grants temporary access to use a rail service.
23 A user typically obtains a credential by providing a password to the
25 kinit(1) command, or via a PAM library at login time. A credential ac‐
26 quired with a user principal is known as a user credential (see ker‐
27 beros(1) for more on principals).
28 Certain operations require a credential that represents no particular
30 user or represents the host itself. This kind of credential is called
31 a machine credential.
32 A host establishes its machine credential using a service principal
34 whose encrypted password is stored in a local file known as a keytab.
35 A machine credential remains effective without user intervention as
36 long as the host can renew it.
37 Once obtained, credentials are typically stored in local temporary
39 files with well-known pathnames.
40
42 To establish GSS security contexts using these credential files, the
43 Linux kernel RPC client depends on a userspace daemon called rpc.gssd.
44 The rpc.gssd daemon uses the rpc_pipefs filesystem to communicate with
45 the kernel.
46 User Credentials
48 When a user authenticates using a command such as kinit(1), the result‐
49 ing credential is stored in a file with a well-known name constructed
50 using the user's UID.
51 To interact with an NFS server on behalf of a particular Kerberos-au‐
53 thenticated user, the Linux kernel RPC client requests that rpc.gssd
54 initialize a security context with the credential in that user's cre‐
55 dential file.
56 Typically, credential files are placed in /tmp. However, rpc.gssd can
58 search for credential files in more than one directory. See the de‐
59 scription of the -d option for details.
60 Machine Credentials
62 rpc.gssd searches the default keytab, /etc/krb5.keytab, in the follow‐
63 ing order for a principal and password to use when establishing the ma‐
64 chine credential. For the search, rpc.gssd replaces <hostname> and
65 <REALM> with the local system's hostname and Kerberos realm.
66 <HOSTNAME>$@<REALM>
68 root/<hostname>@<REALM>
69 nfs/<hostname>@<REALM>
70 host/<hostname>@<REALM>
71 root/<anyname>@<REALM>
72 nfs/<anyname>@<REALM>
73 host/<anyname>@<REALM>
74 rpc.gssd selects one of the <anyname> entries if it does not find a
76 service principal matching the local hostname, e.g. if DHCP assigns the
77 local hostname dynamically. The <anyname> facility enables the use of
78 the same keytab on multiple systems. However, using the same service
79 principal to establish a machine credential on multiple hosts can cre‐
80 ate unwanted security exposures and is therefore not recommended.
81 Note that <HOSTNAME>$@<REALM> is a user principal that enables Kerber‐
83 ized NFS when the local system is joined to an Active Directory domain
84 using Samba. The keytab provides the password for this principal.
85 You can specify a different keytab by using the -k option if
87 /etc/krb5.keytab does not exist or does not provide one of these prin‐
88 cipals.
89 Credentials for UID 0
91 UID 0 is a special case. By default rpc.gssd uses the system's machine
92 credentials for UID 0 accesses that require GSS authentication. This
93 limits the privileges of the root user when accessing network resources
94 that require authentication.
95 Specify the -n option when starting rpc.gssd if you'd like to force the
97 root user to obtain a user credential rather than use the local sys‐
98 tem's machine credential.
99 When -n is specified, the kernel continues to request a GSS context es‐
101 tablished with a machine credential for NFSv4 operations, such as SET‐
102 CLIENTID or RENEW, that manage state. If rpc.gssd cannot obtain a ma‐
103 chine credential (say, the local system has no keytab), NFSv4 opera‐
104 tions that require machine credentials will fail.
105 Encryption types
107 A realm administrator can choose to add keys encoded in a number of
108 different encryption types to the local system's keytab. For instance,
109 a host/ principal might have keys for the aes256-cts-hmac-sha1-96,
110 aes128-cts-hmac-sha1-96, des3-cbc-sha1, and arcfour-hmac encryption
111 types. This permits rpc.gssd to choose an appropriate encryption type
112 that the target NFS server supports.
113 These encryption types are stronger than legacy single-DES encryption
115 types. To interoperate in environments where servers support only weak
116 encryption types, you can restrict your client to use only single-DES
117 encryption types by specifying the -l option when starting rpc.gssd.
118
120 -D The server name passed to GSSAPI for authentication is normally
121 the name exactly as requested. e.g. for NFS it is the server
122 name in the "servername:/path" mount request. Only if this
123 servername appears to be an IP address (IPv4 or IPv6) or an un‐
124 qualified name (no dots) will a reverse DNS lookup will be per‐
125 formed to get the canoncial server name.
126 If -D is present, a reverse DNS lookup will always be used, even
128 if the server name looks like a canonical name. So it is needed
129 if partially qualified, or non canonical names are regularly
130 used.
131 Using -D can introduce a security vulnerability, so it is recom‐
133 mended that -D not be used, and that canonical names always be
134 used when requesting services.
135 -f Runs rpc.gssd in the foreground and sends output to stderr (as
137 opposed to syslogd)
138 -n When specified, UID 0 is forced to obtain user credentials which
140 are used instead of the local system's machine credentials.
141 -k keytab
143 Tells rpc.gssd to use the keys found in keytab to obtain machine
144 credentials. The default value is /etc/krb5.keytab.
145 -l When specified, restricts rpc.gssd to sessions to weak encryp‐
147 tion types such as des-cbc-crc. This option is available only
148 when the local system's Kerberos library supports settable en‐
149 cryption types.
150 -p path
152 Tells rpc.gssd where to look for the rpc_pipefs filesystem. The
153 default value is /var/lib/nfs/rpc_pipefs.
154 -d search-path
156 This option specifies a colon separated list of directories that
157 rpc.gssd searches for credential files. The default value is
158 /tmp:/run/user/%U. The literal sequence "%U" can be specified
159 to substitue the UID of the user for whom credentials are being
160 searched.
161 -M By default, machine credentials are stored in files in the first
163 directory in the credential directory search path (see the -d
164 option). When -M is set, rpc.gssd stores machine credentials in
165 memory instead.
166 -v Increases the verbosity of the output (can be specified multiple
168 times).
169 -r If the RPCSEC_GSS library supports setting debug level, in‐
171 creases the verbosity of the output (can be specified multiple
172 times).
173 -R realm
175 Kerberos tickets from this realm will be preferred when scanning
176 available credentials cache files to be used to create a con‐
177 text. By default, the default realm, as configured in the Ker‐
178 beros configuration file, is preferred.
179 -t timeout
181 Timeout, in seconds, for kernel GSS contexts. This option allows
182 you to force new kernel contexts to be negotiated after timeout
183 seconds, which allows changing Kerberos tickets and identities
184 frequently. The default is no explicit timeout, which means the
185 kernel context will live the lifetime of the Kerberos service
186 ticket used in its creation.
187 -T timeout
189 Timeout, in seconds, to create an RPC connection with a server
190 while establishing an authenticated gss context for a user. The
191 default timeout is set to 5 seconds. If you get messages like
192 "WARNING: can't create tcp rpc_clnt to server %servername% for
193 user with uid %uid%: RPC: Remote system error - Connection timed
194 out", you should consider an increase of this timeout.
195 -H Avoids setting $HOME to "/". This allows rpc.gssd to read per
197 user k5identity files versus trying to read /.k5identity for
198 each user.
199 If -H is not set, rpc.gssd will use the first match found in
201 /var/kerberos/krb5/user/$EUID/client.keytab and will not use a
202 principal based on host and/or service parameters listed in
203 $HOME/.k5identity.
204
206 Many of the options that can be set on the command line can also be
207 controlled through values set in the [gssd] section of the
208 /etc/nfs.conf configuration file. Values recognized include:
209 verbosity
211 Value which is equivalent to the number of -v.
212 rpc-verbosity
214 Value which is equivalent to the number of -r.
215 use-memcache
217 A Boolean flag equivalent to -M.
218 use-machine-creds
220 A Boolean flag. Setting to false is equivalent to giving the -n
221 flag.
222 avoid-dns
224 Setting to false is equivalent to providing the -D flag.
225 limit-to-legacy-enctypes
227 Equivalent to -l.
228 context-timeout
230 Equivalent to -t.
231 rpc-timeout
233 Equivalent to -T.
234 keytab-file
236 Equivalent to -k.
237 cred-cache-directory
239 Equivalent to -d.
240 preferred-realm
242 Equivalent to -R.
243 set-home
245 Setting to false is equivalent to providing the -H flag.
246 In addtion, the following value is recognized from the [general] sec‐
248 tion:
249 pipefs-directory
251 Equivalent to -p.
252
255 rpc.svcgssd(8), kerberos(1), kinit(1), krb5.conf(5)
256
258 Dug Song <dugsong@umich.edu>
259 Andy Adamson <andros@umich.edu>
260 Marius Aamodt Eriksen <marius@umich.edu>
261 J. Bruce Fields <bfields@umich.edu>
262 20 Feb 2013 rpc.gssd(8)