+To establish GSS security contexts using these credential files,
+the Linux kernel RPC client depends on a userspace daemon called
+.BR rpc.gssd .
+The
+.B rpc.gssd
+daemon uses the rpc_pipefs filesystem to communicate with the kernel.
+.SS User Credentials
+When a user authenticates using a command such as
+.BR kinit (1),
+the resulting credential is stored in a file with a well-known name
+constructed using the user's UID.
+.P
+To interact with an NFS server
+on behalf of a particular Kerberos-authenticated user,
+the Linux kernel RPC client requests that
+.B rpc.gssd
+initialize a security context with the credential
+in that user's credential file.
+.P
+Typically, credential files are placed in
+.IR /tmp .
+However,
+.B rpc.gssd
+can search for credential files in more than one directory.
+See the description of the
+.B -d
+option for details.
+.SS Machine Credentials
+A user credential is established by a user and
+is then shared with the kernel and
+.BR rpc.gssd .
+A machine credential is established by
+.B rpc.gssd
+for the kernel when there is no user.
+Therefore
+.B rpc.gssd
+must already have the materials on hand to establish this credential
+without requiring user intervention.
+.P
+.B rpc.gssd
+searches the local system's keytab for a principal and key to use
+to establish the machine credential.
+By default,
+.B rpc.gssd
+assumes the file
+.I /etc/krb5.keytab
+contains principals and keys that can be used to obtain machine credentials.
+.P
+.B 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 realm.
+.sp
+ <HOSTNAME>$@<REALM>
+.br
+ root/<hostname>@<REALM>
+.br
+ nfs/<hostname>@<REALM>
+.br
+ host/<hostname>@<REALM>
+.br
+ root/<anyname>@<REALM>
+.br
+ nfs/<anyname>@<REALM>
+.br
+ host/<anyname>@<REALM>
+.sp
+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.
+.P
+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.
+.P
+You can specify another keytab by using the
+.B -k
+option if
+.I /etc/krb5.keytab
+does not exist or does not provide one of these principals.
+.SS Credentials for UID 0
+UID 0 is a special case.
+By default
+.B 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.
+.P
+Specify the
+.B -n
+option when starting
+.B 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.
+.P
+When
+.B -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