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NAME | SYNOPSIS | INTRODUCTION | DESCRIPTION | OPTIONS | CONFIGURATION FILE | SEE ALSO | AUTHORS | COLOPHON |
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rpc.gssd(8) System Manager's Manual rpc.gssd(8)
rpc.gssd - RPCSEC_GSS daemon
rpc.gssd [-DfMnlvrHC] [-k keytab] [-p pipefsdir] [-d ccachedir]
[-t timeout] [-T timeout] [-U timeout] [-R realm]
The RPCSEC_GSS protocol, defined in RFC 5403, is used to provide
strong security for RPC-based protocols such as NFS.
Before exchanging RPC requests using RPCSEC_GSS, an RPC client
must establish a GSS security context. A security context is
shared state on each end of a network transport that enables GSS-
API security services.
Security contexts are established using security credentials. A
credential grants temporary access to a secure network service,
much as a railway ticket grants temporary access to use a rail
service.
A user typically obtains a credential by providing a password to
the kinit(1) command, or via a PAM library at login time. A
credential acquired with a user principal is known as a user
credential (see kerberos(1) for more on principals).
Certain operations require a credential that represents no
particular user or represents the host itself. This kind of
credential is called a machine credential.
A host establishes its machine credential using a service
principal whose encrypted password is stored in a local file
known as a keytab. A machine credential remains effective
without user intervention as long as the host can renew it.
Once obtained, credentials are typically stored in local
temporary files with well-known pathnames.
To establish GSS security contexts using these credential files,
the Linux kernel RPC client depends on a userspace daemon called
rpc.gssd. The rpc.gssd daemon uses the rpc_pipefs filesystem to
communicate with the kernel.
User Credentials
When a user authenticates using a command such as kinit(1), the
resulting credential is stored in a file with a well-known name
constructed using the user's UID.
To interact with an NFS server on behalf of a particular
Kerberos-authenticated user, the Linux kernel RPC client requests
that rpc.gssd initialize a security context with the credential
in that user's credential file.
Typically, credential files are placed in /tmp. However,
rpc.gssd can search for credential files in more than one
directory. See the description of the -d option for details.
Machine Credentials
rpc.gssd searches the default keytab, /etc/krb5.keytab, in the
following order for a principal and password to use when
establishing the machine credential. For the search, rpc.gssd
replaces <hostname> and <REALM> with the local system's hostname
and Kerberos realm.
<HOSTNAME>$@<REALM>
root/<hostname>@<REALM>
nfs/<hostname>@<REALM>
host/<hostname>@<REALM>
root/<anyname>@<REALM>
nfs/<anyname>@<REALM>
host/<anyname>@<REALM>
rpc.gssd selects one of the <anyname> entries if it does not find
a service principal matching the local hostname, e.g. if DHCP
assigns the local hostname dynamically. The <anyname> facility
enables the use of the same keytab on multiple systems. However,
using the same service principal to establish a machine
credential on multiple hosts can create unwanted security
exposures and is therefore not recommended.
Note that <HOSTNAME>$@<REALM> is a user principal that enables
Kerberized NFS when the local system is joined to an Active
Directory domain using Samba. The keytab provides the password
for this principal.
You can specify a different keytab by using the -k option if
/etc/krb5.keytab does not exist or does not provide one of these
principals.
Credentials for UID 0
UID 0 is a special case. By default rpc.gssd uses the system's
machine credentials for UID 0 accesses that require GSS
authentication. This limits the privileges of the root user when
accessing network resources that require authentication.
Specify the -n option when starting rpc.gssd if you'd like to
force the root user to obtain a user credential rather than use
the local system's machine credential.
When -n is specified, the kernel continues to request a GSS
context established with a machine credential for NFSv4
operations, such as SETCLIENTID or RENEW, that manage state. If
rpc.gssd cannot obtain a machine credential (say, the local
system has no keytab), NFSv4 operations that require machine
credentials will fail.
Encryption types
A realm administrator can choose to add keys encoded in a number
of different encryption types to the local system's keytab. For
instance, a host/ principal might have keys for the aes256-cts-
hmac-sha1-96, aes128-cts-hmac-sha1-96, des3-cbc-sha1, and
arcfour-hmac encryption types. This permits rpc.gssd to choose
an appropriate encryption type that the target NFS server
supports.
These encryption types are stronger than legacy single-DES
encryption types. To interoperate in environments where servers
support only weak encryption types, you can restrict your client
to use only single-DES encryption types by specifying the -l
option when starting rpc.gssd.
-D The server name passed to GSSAPI for authentication is
normally the name exactly as requested. e.g. for NFS it
is the server name in the "servername:/path" mount
request. Only if this servername appears to be an IP
address (IPv4 or IPv6) or an unqualified name (no dots)
will a reverse DNS lookup will be performed to get the
canoncial server name.
If -D is present, a reverse DNS lookup will always be
used, even if the server name looks like a canonical name.
So it is needed if partially qualified, or non canonical
names are regularly used.
Using -D can introduce a security vulnerability, so it is
recommended that -D not be used, and that canonical names
always be used when requesting services.
-f Runs rpc.gssd in the foreground and sends output to stderr
(as opposed to syslogd)
-n When specified, UID 0 is forced to obtain user credentials
which are used instead of the local system's machine
credentials.
-k keytab
Tells rpc.gssd to use the keys found in keytab to obtain
machine credentials. The default value is
/etc/krb5.keytab.
-l When specified, restricts rpc.gssd to sessions to weak
encryption types such as des-cbc-crc. This option is
available only when the local system's Kerberos library
supports settable encryption types.
-p path
Tells rpc.gssd where to look for the rpc_pipefs
filesystem. The default value is /var/lib/nfs/rpc_pipefs.
-d search-path
This option specifies a colon separated list of
directories that rpc.gssd searches for credential files.
The default value is /tmp:/run/user/%U. The literal
sequence "%U" can be specified to substitue the UID of the
user for whom credentials are being searched.
-M By default, machine credentials are stored in files in the
first directory in the credential directory search path
(see the -d option). When -M is set, rpc.gssd stores
machine credentials in memory instead.
-v Increases the verbosity of the output (can be specified
multiple times).
-r If the RPCSEC_GSS library supports setting debug level,
increases the verbosity of the output (can be specified
multiple times).
-R realm
Kerberos tickets from this realm will be preferred when
scanning available credentials cache files to be used to
create a context. By default, the default realm, as
configured in the Kerberos configuration file, is
preferred.
-t timeout
Timeout, in seconds, for kernel GSS contexts. This option
allows you to force new kernel contexts to be negotiated
after timeout seconds, which allows changing Kerberos
tickets and identities frequently. The default is no
explicit timeout, which means the kernel context will live
the lifetime of the Kerberos service ticket used in its
creation.
-T timeout
Timeout, in seconds, to create an RPC connection with a
server while establishing an authenticated gss context for
a user. The default timeout is set to 5 seconds. If you
get messages like "WARNING: can't create tcp rpc_clnt to
server %servername% for user with uid %uid%: RPC: Remote
system error - Connection timed out", you should consider
an increase of this timeout.
-U timeout
Timeout, in seconds, for upcall threads. Threads
executing longer than timeout seconds will cause an error
message to be logged. The default timeout is 30 seconds.
The minimum is 5 seconds. The maximum is 600 seconds.
-C In addition to logging an error message for threads that
have timed out, the thread will be canceled and an error
of -ETIMEDOUT will be reported to the kernel.
-H Avoids setting $HOME to "/". This allows rpc.gssd to read
per user k5identity files versus trying to read
/.k5identity for each user.
If -H is not set, rpc.gssd will use the first match found
in /var/kerberos/krb5/user/$EUID/client.keytab and will
not use a principal based on host and/or service
parameters listed in $HOME/.k5identity.
Many of the options that can be set on the command line can also
be controlled through values set in the [gssd] section of the
/etc/nfs.conf configuration file. Values recognized include:
verbosity
Value which is equivalent to the number of -v.
rpc-verbosity
Value which is equivalent to the number of -r.
use-memcache
A Boolean flag equivalent to -M.
use-machine-creds
A Boolean flag. Setting to false is equivalent to giving
the -n flag.
avoid-dns
Setting to false is equivalent to providing the -D flag.
limit-to-legacy-enctypes
Equivalent to -l.
allowed-enctypes
Allows you to restrict rpc.gssd to using a subset of the
encryption types permitted by the kernel and the krb5
libraries. This is useful if you need to interoperate
with an NFS server that does not have support for the
newer SHA2 and Camellia encryption types, for example.
This configuration file option does not have an equivalent
command-line option.
context-timeout
Equivalent to -t.
rpc-timeout
Equivalent to -T.
keytab-file
Equivalent to -k.
cred-cache-directory
Equivalent to -d.
preferred-realm
Equivalent to -R.
upcall-timeout
Equivalent to -U.
cancel-timed-out-upcalls
Setting to true is equivalent to providing the -C flag.
set-home
Setting to false is equivalent to providing the -H flag.
In addtion, the following value is recognized from the [general]
section:
pipefs-directory
Equivalent to -p.
rpc.svcgssd(8), kerberos(1), kinit(1), krb5.conf(5)
Dug Song <[email protected]>
Andy Adamson <[email protected]>
Marius Aamodt Eriksen <[email protected]>
J. Bruce Fields <[email protected]>
This page is part of the nfs-utils (NFS utilities) project.
Information about the project can be found at
⟨http://linux-nfs.org/wiki/index.php/Main_Page⟩. If you have a
bug report for this manual page, see
⟨http://linux-nfs.org/wiki/index.php/Main_Page⟩. This page was
obtained from the project's upstream Git repository
⟨git://git.linux-nfs.org/projects/steved/nfs-utils.git⟩ on
2024-06-14. (At that time, the date of the most recent commit
that was found in the repository was 2024-05-10.) If you
discover any rendering problems in this HTML version of the page,
or you believe there is a better or more up-to-date source for
the page, or you have corrections or improvements to the
information in this COLOPHON (which is not part of the original
manual page), send a mail to [email protected]
20 Feb 2013 rpc.gssd(8)
Pages that refer to this page: nfs(5), nfs.conf(5), svcgssd(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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