xfwp man page

xfwp — X firewall proxy


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


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 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 Manager connections.  The default is 10.

-max_pm_conns num_connections

Used to specify the maximum number of X server connections.  The default is 100.


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.

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 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.

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 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.   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 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.

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>]]


the keywords “permit” or “deny” indicate whether the  rule will enable or disable access, respectively


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


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


always “eq” (if the service field is not NULL)


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>


specifies that the X server must be configured with at least one of the corresponding site policies, else it must refuse the connection.


specifies that the X server must not be configured with any of the corresponding site policies, else it must refuse the connection.


a required keyword


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
 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 “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
     deny connection
   if (keyword == require)
     deny connection
     permit connection

The site policy check is performed by xfwp only if the source-destination rules permit the connection.


# 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 [NOTE:  If pm service
# is explicitly qualified, line must include destination fields as
# shown.]
deny  eq  pm
# permit xfindproxy X server connects to anywhere [NOTE:  If
# fp service is explicitly qualified, line must include source fields
# as shown.]
permit  eq  fp 
# permit all connection types originating from the 
# IP domain only 

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.  


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)


Reed Augliere, consulting to X Consortium, Inc.

Referenced By

nxagent(1), proxymngr(1), X(7), Xserver(1).

xfwp 1.0.2 X Version 11