XFWP(1)

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.

EXAMPLES

# 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.