man > gssd(8)

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

NAME
       rpc.gssd - RPCSEC_GSS daemon

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

INTRODUCTION
       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.

DESCRIPTION
       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 creden‐
       tial 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 princi‐
       pal 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 match‐
       ing 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 se‐
       curity 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 ma‐
       chine 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 op‐
       erations 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.

OPTIONS
       -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 re‐
              quest.  Only if this servername appears to be an IP address (IPv4 or IPv6) or  an  un‐
              qualified name (no dots) will a reverse DNS lookup will be performed to get the canon‐
              cial 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 de‐
              fault 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 sup‐
              ports 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 cre‐
              dential  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 config‐
              ured in the Kerberos configuration file, is preferred.

       -t timeout
              Timeout, in seconds, for kernel GSS contexts. This option allows you to force new ker‐
              nel 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 con‐
              sider an increase of this timeout.

       -U timeout
              Timeout, in seconds, for upcall threads.  Threads executing longer than  timeout  sec‐
              onds  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/ker‐
              beros/krb5/user/$EUID/client.keytab  and will not use a principal based on host and/or
              service parameters listed in $HOME/.k5identity.

CONFIGURATION FILE
       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  in‐
       clude:

       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.

       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.


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

AUTHORS
       Dug Song <dugsong AT umich.edu>
       Andy Adamson <andros AT umich.edu>
       Marius Aamodt Eriksen <marius AT umich.edu>
       J. Bruce Fields <bfields AT umich.edu>

                                             20 Feb 2013                                 rpc.gssd(8)