Ubuntu Manpage: rpc.gssd - RPCSEC

Provided by: nfs-common_2.6.4-4ubuntu1_amd64

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 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 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 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 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 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 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 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.

-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.

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 include:

       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.

AUTHORS

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

                                                   20 Feb 2013                                       rpc.gssd(8)