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

NAME         top

       rpc.gssd - RPCSEC_GSS daemon

SYNOPSIS         top

       rpc.gssd [-DfMnlvr] [-k keytab] [-p pipefsdir] [-d ccachedir] [-t
       timeout] [-R realm]

INTRODUCTION         top

       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

       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).

       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
       principal, whose encrypted password, called its key, is stored in a
       file, called a keytab, to avoid requiring a user prompt.  A machine
       credential 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.

DESCRIPTION         top

       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
       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 requiring 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, <hostname> and
       <REALM> are replaced with the local system's hostname and Kerberos


       The <anyname> 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

   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

OPTIONS         top

       -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

       -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.


       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:

              A Boolean flag equivalent to -M.

              A Boolean flag. Setting to false is equivalent to giving the
              -n flag.

              Setting to false is equivalent to providing the -D flag.

              Equivalent to -l.

              Equivalent to -T.

              Equivalent to -t.

              Equivalent to -p.

              Equivalent to -k.

              Equivalent to -d.

              Equivalent to -R.

SEE ALSO         top

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

AUTHORS         top

       Dug Song <>
       Andy Adamson <>
       Marius Aamodt Eriksen <>
       J. Bruce Fields <>

COLOPHON         top

       This page is part of the nfs-utils (NFS utilities) project.
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                                 20 Feb 2013                     rpc.gssd(8)