XFWP(1) X Version 11 (Release 6.4) XFWP(1)
NAME [Toc] [Back]
xfwp - X firewall proxy
SYNOPSIS [Toc] [Back]
xfwp [option ...]
COMMAND LINE OPTIONS [Toc] [Back]
The command line options that can be specified are:
-cdt num_secs
Used to override the default time-to-close (604800
seconds) for xfwp client data connections on which
there is no activity (connections over which X
protocol is already being relayed by xfwp)
-clt num_secs
Used to override the default time-to-close (86400
seconds) for xfwp client listen ports (ports on xfwp
to which X clients first connect when trying to
reach an X server)
-pdt num_secs
Used to override the default time-to-close (3600
seconds) for Proxy Manager connections on which
there is no activity
-config file_name
Used to specify the configuration the name of the
configuration file
-pmport port_number
Used to override the default port address (4444) for
proxy manager connections
-verify Used to display the configuration file rule that was
actually matched for each service request
-logfile file_name
Used to specify the name of a file where audit
information should be logged. The format of a
logged entry is: time of day; event code; source IP
address; destination IP address; and configuration
rule number. The event codes are: "0" for a
successful connection; "1" if a connection is denied
because of a configuration rule; and "2" if a
connection is denied because of an authorization
failure. If the event code is "1", and a
configuration file is used, the configuration rule
number is the line number of the configuration file
where the match was made (see the section
CONFIGURATION FILE for more information). If the
event code is not "1", or if no configuration file
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is used, the configuration rule number is "-1".
-loglevel {0,1}
Used to specify the amount of audit detail that
should be logged. If "1", all connections are
logged. If "0", only unsuccessful connections are
logged.
-max_pm_conns num_connections
Used to specify the maximum number of Proxy Manager
connections. The default is 10.
-max_server_conns num_connections
Used to specify the maximum number of X server
connections. The default is 100.
DESCRIPTION [Toc] [Back]
The X firewall proxy (xfwp) is an application layer gateway
proxy that may be run on a network firewall host to forward
X traffic across the firewall. Used in conjunction with the
X server Security extension and authorization checking, xfwp
constitutes a safe, simple, and reliable mechanism both to
hide the addresses of X servers located on the Intranet and
to enforce a server connection policy. Xfwp cannot protect
against mischief originating on the Intranet; however, when
properly configured it can guarantee that only trusted
clients originating on authorized external Internet hosts
will be allowed inbound access to local X servers.
To use xfwp there must be an X proxy manager running in the
local environment which has been configured at start-up to
know the location of the xfwp. [NOTE: There may be more
than one xfwp running in a local environment; see notes
below on load balancing for further discussion.] Using the
xfindproxy utility (which relays its requests through the
proxy manager) a user asks xfwp to allocate a client listen
port for a particular X server, which is internally
associated with all future connection requests for that
server. This client listen port address is returned by the
proxy manager through xfindproxy. The xfwp hostname and
port number is then passed out-of-band (i.e., via a Web
browser) to some remote X client, which will subsequently
connect to xfwp instead of to the target X server.
When an X client connection request appears on one of xfwp's
listen ports, xfwp connects to the X server associated with
this listen port and performs authorization checks against
the server as well as against its own configurable access
control list for requesting clients. If these checks fail,
or if the requested server does not support the X Security
Extension, the client connection is refused. Otherwise, the
connection is accepted and all ensuing data between client
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and server is relayed by xfwp until the client terminates
the connection or, in the case of an inactive client, until
a configured timeout period is exceeded. Xfwp is designed
to block while waiting for activity on its connections,
thereby minimizing demand for system cycles.
If xfwp is run without a configuration file and thus no
sitepolicy is defined, if xfwp is using an X server where
xhost + has been run to turn off host-based authorization
checks, when a client tries to connect to this X server via
xfwp, the X server will deny the connection. If xfwp does
not define a sitepolicy, host-based authorization must be
turned on for clients to connect to an X server via the
xfwp.
INTEROPERATION WITH IP PACKET-FILTERING ROUTERS [Toc] [Back]
The whole purpose of the xfwp is to provide reliable control
over access to Intranet X servers by clients originating
outside the firewall. At the present time, such access
control is typically achieved by firewall configurations
incorporating IP packet-filtering routers. Frequently, the
rules for such filters deny access to X server ports (range
6000 - 6xxx) for all Intranet host machines.
In order for xfwp to do its job, restrictions on access for
ports 6001 - 6xxx must be removed from the rule-base of the
IP packet-filtering router. [NOTE: xfwp only assigns ports
in the range beginning with 6001; access to port 6000 on all
Intranet hosts may continue to be denied.] This does not
mean the Intranet firewall will be opened for indiscriminate
entry by X clients. Instead, xfwp supports a fully
configurable rule-based access control system, similar to
that of the IP packet-filter router itself. Xfwp in effect
adds another level of packet-filtering control which is
fully configurable and applies specifically to X traffic.
See section entitled CONFIGURATION FILE, below, for further
details.
INSTALLATION, SETUP AND TROUBLESHOOTING
Xfwp is typically run as a background process on the
Intranet firewall host. It can be launched using any of the
command-line options described above. As noted above, xfwp
works only in conjunction with proxy manager and the
xfindproxy utility. It can also be configured to support a
user-defined X server site security policy, in which the X
server is required to indicate to xfwp whether or not it
supports the particular policy. Consult the X server man
pages for further information on these components. Xfwp
diagnostics can be turned on by compiling with the -DDEBUG
switch. Connection status can be recorded by using the
-logfile and -loglevel command line options.
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XFWP(1) X Version 11 (Release 6.4) XFWP(1)
On Silicon Graphics systems, xfwp is not installed by
default. The subsystem x_eoe.sw.xfwp must be installed.
When this subsystem is installed, scripts are also installed
that allow xfwp to be run as a daemon that begins executing
at system startup and is stopped at system shutdown. To
configure xfwp to run in this fashion, execute the command:
chkconfig xfwp on
To stop the xfwp daemon on a running system, execute the
command:
/etc/init.d/xfwp stop
To start the xfwp daemon on a running system, execute the
command:
/etc/init.d/xfwp start
The file /usr/lib/X11/xfwp/options can be used to provide
any of the command line options described above to the xfwp
daemon. For instance, it is recommended that a
configuration file (see below) be created to restrict access
to approved hosts. It is important to note that by default
the xfwp daemon is run without any options, which means that
ALL HOSTS ARE ALLOWED ACCESS.
PERFORMANCE, LOAD BALANCING AND RESOURCE MANAGEMENT
Xfwp manages four different kinds of connections: proxy
manager (PM) data, X client listen, X client data, and X
server. The sysadmin employing xfwp must understand how the
resources for each of these connection types are allocated
and reclaimed by xfwp in order to optimize the availability
of xfwp service.
Each connection-type has a default number of allocation
slots and a default timeout. The number of allocation slots
for PM connections and X server connections is configurable
via command line options. Connection timeouts are also
configurable via command line options. Each connection
timeout represents the period the connection will be allowed
to remain open in the absence of any activity on that
connection. Whenever there is activity on a connection, the
time-to-close is automatically reset. The default
distribution of total process connection slots across the
four connection types, as well as the choice of default
timeouts for the connection types, is governed by a number
of assumptions embedded in the xfwp use model.
The default number of PM connections is 10 and the default
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XFWP(1) X Version 11 (Release 6.4) XFWP(1)
duration for PM connections is 3,600 seconds (1 hour) for
each connection after time of last activity. At start-up,
xfwp listens for PM connection requests on any non-reserved
port (default of 4444 if not specified on the xfwp commandline).
The PM normally connects to xfwp only when a call is
made to the PM with xfindproxy. Thereafter, the PM remains
connected to xfwp, even after the messaging between them has
been completed, for the default connection duration period.
In some cases this may result in depletion of available PM
connection slots. If the sysadmin expects connections to a
single xfwp from many PM's, xfwp should be started using the
-pdt command line option, with a timeout value reflecting
the desired duration that inactive connections will be
permitted to remain open.
Xfwp client listeners are set up by a call to xfindproxy and
continue to listen for X client connection requests for a
default duration of 86,400 seconds (24 hours) from the point
of last activity. After this time they are automatically
closed and their fd's recovered for future allocation. In
addressing the question of how to choose some alternative
timeout value which will guarantee the availability of
client listen ports, sysadmins should take into
consideration the expected delay between the time when the
listener was allocated (using xfindproxy) and the time when
a client actually attempts to connect to xfwp, as well the
likelihood that client listeners will be re-used after the
initial client data connection is closed.
Each client connection is allocated a default lifetime of
604,800 seconds (7 * 24 hours) from the point when it last
saw activity. After this time it is automatically closed
and its fd's recovered for future allocation. Because
server connections are not actually established until a
connection request from a remote X client arrives at one of
the xfwp's client listen ports, the client data timeout
applies both to client-xfwp connections as well as to xfwpserver
connections. If the system administrator expects
many client data connections through xfwp, an overriding of
the default timeout should be considered.
CONFIGURATION FILE [Toc] [Back]
The xfwp configuration file resides on the xfwp host machine
and is used to determine whether X client data connection
requests will be permitted or denied. The path to the file
is specified at start-up time. If no configuration file is
specified, all X client data connection requests routed
through xfwp will be by default permitted, assuming that
other X server authorization checks are successful. If a
configuration file is supplied but none of its entries
matches the connection request then the connection is by
default denied.
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If a line in the configuration file begins with the '#'
character or a new-line character, the line is ignored and
the evaluator will skip the line.
The configuration file supports two entirely independent
authorization checks: one which is performed by xfwp
itself, and a second which is the result of xfwp's querying
the target X server. For the first of these, the
configuration file employs a syntax and semantic similar to
that of IP packet-filtering routers. It contains zero or
more source-destination rules of the following form:
{permit | deny} <src> <src mask> [<dest> <dest mask>
[<operator> <service>]]
permit/deny the keywords ``permit'' or ``deny'' indicate
whether the rule will enable or disable access,
respectively
src the IP address against the host who originated
the connection request will be matched,
expressed in IP format (x.x.x.x)
src mask a subnet mask, also in IP format, for further
qualifying the source mask. Bits set in the
mask indicate bits of the incoming address to be
ignored when comparing to the specified src
dest the IP address against which the destination of
the incoming connection request (i.e. the host
IP of the X server to which the incoming client
is attempting to connect) will be matched
dest mask a subnet mask, also in IP format, for further
qualifying the destination mask. Bits set in
the mask indicate bits of the destination
address to be ignored when comparing to the
specified dest
operator always ``eq'' (if the service field is not NULL)
service one of the following three strings: ``pm'',
``fp'', or ``cd'', corresponding to proxy
manager, xfindproxy, or client data,
respectively
For the second type of authorization check, the
configuration file contains zero or more site policy rules
of the following form:
{require | disallow} sitepolicy <site_policy>
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XFWP(1) X Version 11 (Release 6.4) XFWP(1)
require specifies that the X server must be configured
with at least one of the corresponding site
policies, else it must refuse the connection.
disallow specifies that the X server must not be
configured with any of the corresponding site
policies, else it must refuse the connection.
sitepolicy a required keyword
<site_policy>
specifies the policy string. The string may
contain any combination of alphanumeric
characters subject only to interpretation by the
target X server
RULES FOR EVALUATING THE XFWP CONFIGURATION [Toc] [Back]
For the first type of configurable authorization checking,
access can be permitted or denied for each connection type
based upon source and, optionally, destination and service.
Each file entry must at a minimum specify the keyword
``permit'' or ``deny'' and the two source fields. The
destination and service fields can be used to provide
finer-grained access control if desired.
The algorithm for rule-matching is as follows:
while (more entries to check)
{
if ((<originator IP> AND (NOT <src mask>)) == src)
[if ((<dest X server IP> AND (NOT <dest mask>)) ==
dest)]
[if (service fields present and matching)]
do either permit or deny connection depending
on keyword
else
continue
}
if (no rule matches)
deny connection
If a permit or deny rule does not specify a service and
operation, then the rule applies to all services. If a
configuration file is specified and it contains at least one
valid deny or permit rule, then a host that is not
explicitly permitted will be denied a connection.
Site policy configuration checking constitutes a separate
(and X server only) authorization check on incoming
connection requests. Any number of require or disallow
rules may be specified, but all rules must be of the same
type; that is, a single rule file cannot have both
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XFWP(1) X Version 11 (Release 6.4) XFWP(1)
``require'' and ``disallow'' keywords. The algorithm for
this check is as follows:
if (X server recognizes any of the site policy strings)
if (keyword == require)
permit connection
else
deny connection
else
if (keyword == require)
deny connection
else
permit connection
The site policy check is performed by xfwp only if the
source-destination rules permit the connection.
EXAMPLES
# if and only if server supports one of these policies then authorize
# connections, but still subject to applicable rule matches
#
require sitepolicy policy1
require sitepolicy policy2
#
# deny pm connections originating on 8.7.6.5 [NOTE: If pm service
# is explicitly qualified, line must include destination fields as
# shown.]
#
deny 8.7.6.5 0.0.0.0 0.0.0.0 255.255.255.255 eq pm
#
# permit xfindproxy X server connects to anywhere [NOTE: If
# fp service is explicitly qualified, line must include source fields
# as shown.]
#
permit 0.0.0.0 255.255.255.255 0.0.0.0 255.255.255.255 eq fp
#
# permit all connection types originating from the 192.0.0.0
# IP domain only
#
permit 192.0.0.0 0.255.255.255
Care should be taken that source-destination rules are
written in the correct order, as the first matching rule
will be applied. In addition to parser syntax checking, a
special command-line switch (-verify) has been provided to
assist the sysadmin in determining which rule was actually
matched.
BUGS [Toc] [Back]
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XFWP(1) X Version 11 (Release 6.4) XFWP(1)
Xfwp should check server site policy and security extension
before allocating a listen port.
SEE ALSO [Toc] [Back]
xfindproxy (1), Proxy Management Protocol spec V1.0,
proxymngr(1), Xserver(1)
AUTHOR [Toc] [Back]
Reed Augliere, consulting to X Consortium, Inc.
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