xfwp
XFWP(1) XFWP(1)
NAME
xfwp - X firewall proxy
SYNOPSIS
xfwp [option ...]
COMMAND LINE OPTIONS
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 configura
tion 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
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failure. If the event code is "1", and a configu
ration 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 is used, the
configuration rule number is "-1".
-loglevel {0,1}
Used to specify the amount of audit detail that
should be logged. If "0", all connections are
logged. If "1", only unsuccessful connections are
logged.
-max_pm_conns num_connections
Used to specify the maximum number of Proxy Man
ager connections. The default is 10.
-max_pm_conns num_connections
Used to specify the maximum number of X server
connections. The default is 100.
DESCRIPTION
The X firewall proxy (xfwp) is an application layer gate
way proxy that may be run on a network firewall host to
forward X traffic across the firewall. Used in conjunc
tion with the X server Security extension and authoriza
tion checking, xfwp constitutes a safe, simple, and reli
able 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 autho
rized 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
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X server.
When an X client connection request appears on one of
xfwp's listen ports, xfwp connects to the X server associ
ated with this listen port and performs authorization
checks against the server as well as against its own con
figurable 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.
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
The whole purpose of the xfwp is to provide reliable con
trol over access to Intranet X servers by clients origi
nating outside the firewall. At the present time, such
access control is typically achieved by firewall configu
rations incorporating IP packet-filtering routers. Fre
quently, 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 con
trol 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
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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.
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 allo
cated 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 con
figurable via command line options. Connection timeouts
are also configurable via command line options. Each con
nection 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 con
nection, 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
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 command-line). The PM normally connects to xfwp only
when a call is made to the PM with xfindproxy. There
after, 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 dura
tion that inactive connections will be permitted to remain
open.
Xfwp client listeners are set up by a call to xfindproxy
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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
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 configu
ration 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 suc
cessful. If a configuration file is supplied but none of
its entries matches the connection request then the con
nection is by default denied.
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 query
ing the target X server. For the first of these, the con
figuration 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>]]
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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 man
ager, xfindproxy, or client data, respectively
For the second type of authorization check, the configura
tion file contains zero or more site policy rules of the
following form:
{require | disallow} sitepolicy <site_policy>
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 con
figured 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 char
acters subject only to interpretation by the
target X server
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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 ser
vice. Each file entry must at a minimum specify the key
word ``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 con
nection 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.
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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
Xfwp should check server site policy and security exten
sion before allocating a listen port.
SEE ALSO
xfindproxy (1), Proxy Management Protocol spec V1.0, prox
ymngr(1), Xserver(1)
AUTHOR
Reed Augliere, consulting to X Consortium, Inc.
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