Ubuntu Manpage: rpc.gssd - RPCSEC

Provided by: nfs-common_1.3.4-2.5ubuntu3.7_amd64

NAME

       rpc.gssd - RPCSEC_GSS daemon

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.

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
A user credential is established by a user and is then shared with the kernel and
rpc.gssd. A machine credential is established by rpc.gssd for the kernel when there is no
user. Therefore rpc.gssd must already have the materials on hand to establish this
credential without requiring user intervention.

rpc.gssd searches the local system's keytab for a principal and key to use to establish
the machine credential. By default, rpc.gssd assumes the file /etc/krb5.keytab contains
principals and keys that can be used to obtain machine credentials.

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.

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.

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.

You can specify another 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.

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