xfwp
NAME
xfwp - X firewall proxy
SYNOPSIS
xfwp [-pdt <#secs>] [-clt <#secs>] [-cdt <#secs>] [-pmport
<port#>] [-config <path>] [-verify]
COMMAND LINE OPTIONS
The command line options that can be specified to xfwp
are:
-cdt 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 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)
-config Used to specify the configuration file path/name
-pdt Used to override the default time-to-close (3600
seconds) for proxy manager connections on which
there is no activity
-pmport 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
DESCRIPTION
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 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.
INTEROPERATION WITH IP PACKET-FILTERING ROUTERS
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 man pages for
further information on these components. Xfwp diagnostics
can be turned on by recompiling with the -DDEBUG switch.
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 (effectively file descriptors) for each connection
type is coded as a constant in the xfwp source. Default
connection timeouts are also coded as constants in the
xfwp source. 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.
Xfwp allows a maximum of 10 PM connections. This constant
is defined in the xfwp source, along with a default
duration of 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 command-
line). 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 currently allows a maximum of 100 client listen
connections, each one of which is associated with a
different Intranet X server. 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.
Xfwp allows a maximum of 4096 X client data connections.
Each of these connections 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 xfwp-server connections. If the system administrator
expects many client data connections through xfwp, an
overriding of the default timeout should be considered.
CONFIGURATION FILE
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.
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>]]
# comment delimiter; evaluator will skip these
lines
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>
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>
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 FILE ENTRIES
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
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
``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.
EXAMPLE
S
# if and only if server supports one of these policies then authorize
# connections, but still subject to applicable rule matches
#
require policy1
require 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
Xfwp should support audit/logging.
The constants which define how many connections of each
type xfwp can keep open may be too large for certain
platforms.
If an X server disconnects from xfwp before a client can
connect to xfwp, the client will hang.
Xfwp should check server site policy and security
extension before allocating a listen port.
SEE ALSO
xfindproxy (1), Proxy Management Protocol spec V1.0,
proxymngr(1), Xserver(1)
AUTHOR
Reed Augliere, consulting to X Consortium, Inc.
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