gssd(8) — Linux manual page


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

NAME         top

       rpc.gssd - RPCSEC_GSS daemon

SYNOPSIS         top

       rpc.gssd [-DfMnlvrHC] [-k keytab] [-p pipefsdir] [-d ccachedir]
       [-t timeout] [-T timeout] [-U 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 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

       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.

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
       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.


       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

   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

       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.

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

       -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

       -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

       -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:

              Value which is equivalent to the number of -v.

              Value which is equivalent to the number of -r.

              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 -k.

              Equivalent to -d.

              Equivalent to -R.

              Equivalent to -U.

              Setting to true is equivalent to providing the -C flag.

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

       In addtion, the following value is recognized from the [general]

              Equivalent to -p.

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.
       Information about the project can be found at 
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       obtained from the project's upstream Git repository
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       2023-12-22.  (At that time, the date of the most recent commit
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                               20 Feb 2013                   rpc.gssd(8)

Pages that refer to this page: nfs(5)nfs.conf(5)svcgssd(8)