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rpc.gssd(8) System Manager’s Manual rpc.gssd(8)


rpc.gssd – RPCSEC_GSS daemon


rpc.gssd [-DfMnlvr] [-k keytab] [-p pipefsdir] [-d ccachedir] [-t time‐
out] [-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 ser‐

Security contexts are established using security credentials. A cre‐
dential 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 ker‐
beros(1) for more on principals).

For certain operations, a credential is required which represents no
user, is otherwise unprivileged, and is always available. This is
referred to as a machine credential.

Machine credentials are typically established using a service princi‐
pal, whose encrypted password, called its key, is stored in a file,
called a keytab, to avoid requiring a user prompt. A machine creden‐
tial effectively does not expire because the system can renew it as
needed without user intervention.

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 result‐
ing 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 cre‐
dential 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
A user credential is established by a user and is then shared with the
kernel and rpc.gssd. A machine credential is established by rpc.gssd
for the kernel when there is no user. Therefore rpc.gssd must already
have the materials on hand to establish this credential without requir‐
ing user intervention.

rpc.gssd searches the local system’s keytab for a principal and key to
use to establish the machine credential. By default, rpc.gssd assumes
the file /etc/krb5.keytab contains principals and keys that can be used
to obtain machine credentials.

rpc.gssd searches in the following order for a principal to use. The
first matching credential is used. For the search, and
are replaced with the local system’s hostname and Kerberos


The entries match on the service name and realm, but ignore
the hostname. These can be used if a principal matching the local
host’s name is not found.

Note that the first principal in the search order is a user principal
that enables Kerberized NFS when the local system is joined to an
Active Directory domain using Samba. A password for this principal
must be provided in the local system’s keytab.

You can specify another keytab by using the -k option if
/etc/krb5.keytab does not exist or does not provide one of these prin‐

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 sys‐
tem’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 opera‐
tions 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 DNS Reverse lookups are not used for determining the server
names pass to GSSAPI. This option will reverses that and forces
the use of DNS Reverse resolution of the server’s IP address to
retrieve the server name to use in GSAPI authentication.

-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 encryp‐
tion 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

-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

-r If the RPCSEC_GSS library supports setting debug level,
increases the verbosity of the output (can be specified multiple

-R realm
Kerberos tickets from this realm will be preferred when scanning
available credentials cache files to be used to create a con‐
text. By default, the default realm, as configured in the Ker‐
beros 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.

-e Return EACCESS instead of EKEYEXPIRED when a user’s credentials
expire. Returning EKEYEXPIRED was introduced around kernel
2.6.34 and causes all nfs4 I/O to block when a user’s creden‐
tials expire. This option reverts to old bevavior.


rpc.svcgssd(8), kerberos(1), kinit(1), krb5.conf(5)

Dug Song
Andy Adamson
Marius Aamodt Eriksen
J. Bruce Fields

20 Feb 2013 rpc.gssd(8)