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author | William Joye <wjoye@cfa.harvard.edu> | 2017-10-26 16:45:12 (GMT) |
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committer | William Joye <wjoye@cfa.harvard.edu> | 2017-10-26 16:45:12 (GMT) |
commit | 6054f36bb658916bd231b9002efb94444e2871c8 (patch) | |
tree | a815500ec9bf0e100f4075a825213605c382def0 /doc/pod/xpans.pod | |
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Squashed 'xpa/' content from commit d6a54301
git-subtree-dir: xpa
git-subtree-split: d6a543018b46c5a6c47b5ff91e0e131549595ca6
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diff --git a/doc/pod/xpans.pod b/doc/pod/xpans.pod new file mode 100644 index 0000000..071dd68 --- /dev/null +++ b/doc/pod/xpans.pod @@ -0,0 +1,226 @@ +=pod + +=head1 NAME + + + +B<xpans: the XPA Name Server> + + + +=head1 SYNOPSIS + + + + + + xpans [-h] [-e] [-k sec] [-p port] [-l log] [-s security log] [-P n] + + + + +=head1 OPTIONS + + + + + + -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 + + + + +=head1 DESCRIPTION + + + + +The xpans name server is an XPA-enabled program that is used to +manage the names and ports of XPA access points. It is started +automatically when an XPA access point is registered. You can access +the name server using xpaget to get a list of registered access points. + +The I<xpans> name server provides a crucial link between XPA +clients and servers. When an XPA server defines an access point using +XPANew(), XPACmdNew(), or XPAInfoNew(), 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 XPA +clients with these registered entries, so that the clients can +communicate with the appropriate servers. + + +The socket connection between an XPA-enabled program and +I<xpans> is kept open until the former exits (or explicitly +closes the connection). Apparently, some Internet equipment (e.g. DSL +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 DSL or similar equipment. XPA uses +the ordinary socket-level keep-alive, which works for all other cases.) +NB (12/2/2009): Out-of-band (URG) TCP 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 XPA server as well. Proceed with caution! + + +The I<xpans> program will be started automatically (assuming it +can be found in the user's path) when the first XPA access point is +registered. It therefore need not be started explicitly. However, +when started automatically, the I<-e> switch is used, so that +the name server will exit when there are no more XPA access points +registered. If you wish to keep the name server running continually, +simply start it manually without the I<-e> switch. + + +The name server will keep a log of registered access points if the +I<-l [log]> 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 XPA 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: + + add 838e2f67:1863 ds9 ds9 gs eric + +If I<xpans> is terminated but ds9 still is running, you +can re-register both access points for the ds9 process by running: + + xpaset -p 838e2f67:1863 -nsconnect + +Notice that the ip:port specifier is used with I<xpaset> to bypass +the need for contacting the name server (which does not have the name +registered yet!) + + +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 I<xpaget> +command), information will be logged about the host issuing the +command and the parameters passed into the program. This is most +useful when I<xpans> is accepting connections from untrusted +machines. + + +When an XPA access point is removed by a server using I<XPAFree()>, +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. + + +The name server itself has an XPA access point names I<xpans> +registered through which you can find out information about currently +registered access points (assuming you have access to the name server; +see XPA Access Control for more information). +For each registered access point, the following information is returned: + + 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 + + + +For example, to display all currently registered access points, simply execute: + + xpaget xpans + +Continuing the example of ds9 above, this will return: + + DS9 ds9 gs 838e2f67:1863 eric + +If the same program has been started with different XPA access names, +you can look up only names matching a specified template. For example, +assume that ds9 has been started up using: + + ds9 & + ds9 -title ds9-1-eric & + ds9 -title ds9-2-eric & + +To lookup all ds9 access points which end in ".eric" and which can +be accessed using I<xpaset>, use: + + xpaget xpans "DS9:*.eric" "s" "*" + +This will return: + + DS9 ds9-2-eric gs 838e29d3:42102 eric + DS9 ds9-1-eric gs 838e29d3:42105 eric + +The third argument "*" 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. + + +The name server uses the I<XPA_METHOD> environment variable +to determine whether it should listen for requests on INET or LOCAL +sockets. Since XPA access points also use this environment variable, +the choice of socket method will be consistent. Note that, when INET +sockets are used, a local server can be accessed from remote machines +if the I<XPA_NSINET> environment variable is set to point to +the local machine. See +XPA Environment Variables +for more information. + + +An experimental feature of xpans is its ability to act as a proxy to +XPA servers behind firewalls that want to communicate with external +processes. The basic idea is the following: an XPA server (call it +"foo") on host1, possibly behind a firewall, makes a remote connection +to a proxy-enabled xpans program on host2 (specifying host2's XPA method). +For example: + + xpaset -p foo -remote 'host2:28571' + -proxy # on host1 + +When this is done, host2 can use xpaset, xpaget, and xpainfo calls to +communicate with the XPA 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 XPASet() or XPAGet() +call to foo, passing commands and data back and forth between the two +programs. + + +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 XPA_IOCALLSXPA environment variable, so +that multiple proxy requests can be handled at the same time, instead of +serially. + + +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. + + + +=head1 SEE ALSO + + + +See xpa(n) for a list of XPA help pages + + + +=cut |