Ubuntu Manpage: rpc.gssd - RPCSEC

Provided by: nfs-common_2.6.1-2ubuntu4_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)