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+.\" ========================================================================
+.\"
+.IX Title "xpans 1"
+.TH xpans 1 "July 23, 2013" "version 2.1.15" "SAORD Documentation"
+.\" For nroff, turn off justification. Always turn off hyphenation; it makes
+.\" way too many mistakes in technical documents.
+.if n .ad l
+.nh
+.SH "NAME"
+\&\fBxpans: the \s-1XPA\s0 Name Server\fR
+.SH "SYNOPSIS"
+.IX Header "SYNOPSIS"
+.Vb 1
+\& xpans [\-h] [\-e] [\-k sec] [\-p port] [\-l log] [\-s security log] [\-P n]
+.Ve
+.SH "OPTIONS"
+.IX Header "OPTIONS"
+.Vb 8
+\& \-h print help message
+\& \-e exit when there are no more XPA connections
+\& \-k send keepalive messages every n sec
+\& \-l log data base entries to specified file
+\& \-p listen for connections on specified port
+\& \-s log security info for each connection to specified file
+\& \-P accept proxy requests (P=1) using separate thread (P=2)
+\& \-\-version display version and exit
+.Ve
+.SH "DESCRIPTION"
+.IX Header "DESCRIPTION"
+The xpans name server is an XPA-enabled program that is used to
+manage the names and ports of \s-1XPA\s0 access points. It is started
+automatically when an \s-1XPA\s0 access point is registered. You can access
+the name server using xpaget to get a list of registered access points.
+.PP
+The \fIxpans\fR name server provides a crucial link between \s-1XPA\s0
+clients and servers. When an \s-1XPA\s0 server defines an access point using
+\&\fIXPANew()\fR, \fIXPACmdNew()\fR, or \fIXPAInfoNew()\fR, the name of the access point
+is registered in the name service, along with connection information.
+The name server then matches class:name templates passed to it by \s-1XPA\s0
+clients with these registered entries, so that the clients can
+communicate with the appropriate servers.
+.PP
+The socket connection between an XPA-enabled program and
+\&\fIxpans\fR is kept open until the former exits (or explicitly
+closes the connection). Apparently, some Internet equipment (e.g. \s-1DSL\s0
+modems) can cause such a connection to time-out after a period of
+inactivity. To prevent this from happening, you can use the \-k
+[sec] switch to send a short keep-alive message to each open
+connection after the specified time delay. (Note that this
+application level use of keep-alive is necessary only if you are
+serving XPA-enabled clients over the Internet and have to deal with
+long-term connections involving \s-1DSL\s0 or similar equipment. \s-1XPA\s0 uses
+the ordinary socket-level keep-alive, which works for all other cases.)
+\&\s-1NB\s0 (12/2/2009): Out-of-band (\s-1URG\s0) \s-1TCP\s0 data, used by xpans
+keep-alive, is changed by some Cisco routers into in-band data.
+Encountering such a router will break the keep-alive function and may
+break your \s-1XPA\s0 server as well. Proceed with caution!
+.PP
+The \fIxpans\fR program will be started automatically (assuming it
+can be found in the user's path) when the first \s-1XPA\s0 access point is
+registered. It therefore need not be started explicitly. However,
+when started automatically, the \fI\-e\fR switch is used, so that
+the name server will exit when there are no more \s-1XPA\s0 access points
+registered. If you wish to keep the name server running continually,
+simply start it manually without the \fI\-e\fR switch.
+.PP
+The name server will keep a log of registered access points if the
+\&\fI\-l [log]\fR switch is used on the command line (this is the
+case for automatic start-up). The log contains enough name and connection
+information to allow you to re-register all \s-1XPA\s0 access points in case
+the name server process is terminated prematurely. For example, after
+the ds9 access point is registered,the log will contain the entry:
+.PP
+.Vb 1
+\& add 838e2f67:1863 ds9 ds9 gs eric
+.Ve
+.PP
+If \fIxpans\fR is terminated but ds9 still is running, you
+can re-register both access points for the ds9 process by running:
+.PP
+.Vb 1
+\& xpaset \-p 838e2f67:1863 \-nsconnect
+.Ve
+.PP
+Notice that the ip:port specifier is used with \fIxpaset\fR to bypass
+the need for contacting the name server (which does not have the name
+registered yet!)
+.PP
+The name server will keep a log of security information if the \-s
+[security log] switch is used on the command line. For each
+accepted connection, (including connections via the \fIxpaget\fR
+command), information will be logged about the host issuing the
+command and the parameters passed into the program. This is most
+useful when \fIxpans\fR is accepting connections from untrusted
+machines.
+.PP
+When an \s-1XPA\s0 access point is removed by a server using \fI\fIXPAFree()\fI\fR,
+the access information is removed from the name server. If an
+XPA-enabled process is terminated, all names registered by that process
+will be removed automatically. The log file is always updated to
+reflect the currently registered access points.
+.PP
+The name server itself has an \s-1XPA\s0 access point names \fIxpans\fR
+registered through which you can find out information about currently
+registered access points (assuming you have access to the name server;
+see \s-1XPA\s0 Access Control for more information).
+For each registered access point, the following information is returned:
+.PP
+.Vb 5
+\& class # class of the access point
+\& name # name of the access point
+\& access # allowed access (g=xpaget,s=xpaset,i=xpainfo)
+\& id # socket access method (host:port for inet, file for local/unix)
+\& user # user name of access point owner
+.Ve
+.PP
+For example, to display all currently registered access points, simply execute:
+.PP
+.Vb 1
+\& xpaget xpans
+.Ve
+.PP
+Continuing the example of ds9 above, this will return:
+.PP
+.Vb 1
+\& DS9 ds9 gs 838e2f67:1863 eric
+.Ve
+.PP
+If the same program has been started with different \s-1XPA\s0 access names,
+you can look up only names matching a specified template. For example,
+assume that ds9 has been started up using:
+.PP
+.Vb 3
+\& ds9 &
+\& ds9 \-title ds9\-1\-eric &
+\& ds9 \-title ds9\-2\-eric &
+.Ve
+.PP
+To lookup all ds9 access points which end in \*(L".eric\*(R" and which can
+be accessed using \fIxpaset\fR, use:
+.PP
+.Vb 1
+\& xpaget xpans "DS9:*.eric" "s" "*"
+.Ve
+.PP
+This will return:
+.PP
+.Vb 2
+\& DS9 ds9\-2\-eric gs 838e29d3:42102 eric
+\& DS9 ds9\-1\-eric gs 838e29d3:42105 eric
+.Ve
+.PP
+The third argument \*(L"*\*(R" requests all access points from all users.
+You also can specify a specific user name and only access points
+registered by that user will be returned.
+.PP
+The name server uses the \fI\s-1XPA_METHOD\s0\fR environment variable
+to determine whether it should listen for requests on \s-1INET\s0 or \s-1LOCAL\s0
+sockets. Since \s-1XPA\s0 access points also use this environment variable,
+the choice of socket method will be consistent. Note that, when \s-1INET\s0
+sockets are used, a local server can be accessed from remote machines
+if the \fI\s-1XPA_NSINET\s0\fR environment variable is set to point to
+the local machine. See
+\&\s-1XPA\s0 Environment Variables
+for more information.
+.PP
+An experimental feature of xpans is its ability to act as a proxy to
+\&\s-1XPA\s0 servers behind firewalls that want to communicate with external
+processes. The basic idea is the following: an \s-1XPA\s0 server (call it
+\&\*(L"foo\*(R") on host1, possibly behind a firewall, makes a remote connection
+to a proxy-enabled xpans program on host2 (specifying host2's \s-1XPA\s0 method).
+For example:
+.PP
+.Vb 1
+\& xpaset \-p foo \-remote \*(Aqhost2:28571\*(Aq + \-proxy # on host1
+.Ve
+.PP
+When this is done, host2 can use xpaset, xpaget, and xpainfo calls to
+communicate with the \s-1XPA\s0 server foo. All command communication is
+performed via the xpans socket connection between foo on host1 and
+xpans on host2 (which was initiated by foo from inside the firewall).
+Data communication is similarly performed using a socket connection
+initiated on host1 (usually with a port value two greater than the
+port value of the main xpans socket connection). An xpaset or xpaget
+call on host2 contacts xpans, which performs an \fIXPASet()\fR or \fIXPAGet()\fR
+call to foo, passing commands and data back and forth between the two
+programs.
+.PP
+By default, proxy connections are not allowed by xpans. If the \-P switch is
+specified with a value of 1, proxy connection are allowed, but all proxy
+communication is performed in the same thread as xpans processing. If
+a value of 2 is specified, the proxy processing is performed in a
+separate thread (assuming pthreads are supported on your system).
+Because xpa callback processing of any type can take a long time and
+therefore can interfere with normal xpans processing, threaded proxy
+connections (\-P 2) are recommended. When using proxy connections, it
+might also be useful to set the \s-1XPA_IOCALLSXPA\s0 environment variable, so
+that multiple proxy requests can be handled at the same time, instead of
+serially.
+.PP
+Note that this proxy interface to xpans is experimental. It is used
+to provide remote data analysis capabilities on the Chandra-Ed system
+using ds9. (See http://chandra\-ed.cfa.harvard.edu and
+http://hea\-www.harvard.edu/saord/ds9 for more details). As always, please
+contact us if you have problems or questions.
+.SH "SEE ALSO"
+.IX Header "SEE ALSO"
+See xpa(n) for a list of \s-1XPA\s0 help pages