gssd

Synopsis

rpc.gssd [-DfMnlvr] [-k keytab] [-p pipefsdir] [-d ccachedir] [-t 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).

For certain operations, a credential is required which represents no user, is otherwise unprivileged, and is always available. This is referred to as a machine credential.

Machine credentials are typically established using a service principal, whose encrypted password, called its key, is stored in a file, called a keytab, to avoid requiring a user prompt. A machine credential effectively does not expire because the system can renew it as needed without user intervention.

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.

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.

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.

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

Referenced By

gssd_selinux(8), nfs(5), nfs.conf(5).

The man page rpc.gssd(8) is an alias of gssd(8).