The XMLRPC Module

Jan Janak

   iptelorg GmbH

   Copyright © 2005 iptelorg GmbH
     __________________________________________________________________

   Table of Contents

   1. Admin Guide

        1. Parameters

              1.1. route (string)
              1.2. autoconversion (string)
              1.3. escape_cr (integer)
              1.4. double_lf_to_crlf (integer)
              1.5. mode (integer)
              1.6. url_skip (str)
              1.7. url_match (str)

        2. Functions

              2.1. dispatch_rpc()
              2.2. xmlrpc_reply(code, reason)

   List of Tables

   1. Data Type Conversion

   List of Examples

   1.
   1.1. Set route parameter
   1.2. Set the autoconversion parameter
   1.3. Set the escape_cr parameter
   1.4. Set the double_lf_to_crlf parameter
   1.5. Set the mode parameter
   1.6. Set url_skip parameter
   1.7. Set url_match parameter
   1.8. dispatch_rpc usage
   1.9. xmlrpc_reply usage

1. Design Goals

     * Implemented as a module.
     * API independent of transport protocols.
     * Reuse transports available in SER.
     * The possibility to encrypt all communication.
     * The possibility to authenticate clients.
     * Easy integration with existing languages and implementations.
     * Easy and straightforward implementation of management functions in
       SER modules.

2. Overview of Operation

   This module implements the XML-RPC transport and encoding interface for
   ser RPCs.

   The XML-RPC protocol encodes the name of the method to be called along
   with its parameter in an XML document which is then conveyed using HTTP
   (Hyper Text Transfer Protocol) to the server. The server will extract
   the name of the function to be called along with its parameters from
   the XML document, execute the function, and encode any data returned by
   the function into another XML document which is then returned to the
   client in the body of a 200 OK reply to the HTTP request.

   XML-RPC is similar to more popular SOAP (Simple Object Access
   Protocol), which is an XML-based messaging framework used in Web
   Services developed within the World Wide Web Consortium. Both protocols
   are using HTTP as the transport protocol for XML documents, but XML-RPC
   is much simpler and easier to implement than SOAP.

   Here is an example of single XML-RPC function call to determine current
   time:
POST /RPC2 HTTP/1.0
User-Agent: Radio UserLand/7.1b7 (WinNT)
Host: time.xmlrpc.com
Content-Type: text/xml
Content-length: 131

<?xml version="1.0"?>
<methodCall>
<methodName>currentTime.getCurrentTime</methodName>
<params>
</params>
</methodCall>

   And the response returned by the server:
HTTP/1.1 200 OK
Connection: close
Content-Length: 183
Content-Type: text/xml
Date: Wed, 03 Oct 2001 15:53:38 GMT
Server: UserLand Frontier/7.0.1-WinNT

<?xml version="1.0"?>
<methodResponse>
<params>

<value><dateTime.iso8601>20011003T08:53:38</dateTime.iso8601>
</value>

</params>
</methodResponse>

   XML-RPC specification spells HTTP as the official transport protocol
   for XML-RPC documents. SER does not directly support HTTP, it is a SIP
   server so SIP is the only protocol supported by SER. Because we would
   like to reuse all transport protocols available in SER, such as TCP and
   TLS, it would be natural to use modified version of XML-RPC which would
   run on top of SIP instead of HTTP. XML-RPC documents would be then
   encoded in the bodies of SIP requests and replies would be sent by the
   server in the bodies of SIP replies. This way we could reuse all
   transport protocols (including UDP) and message parsers available in
   SER.

   Although this approach seems to be the logical choice, there is one big
   drawback. No existing XML-RPC implementations support SIP as the
   transport protocol, and there are many existing implementations
   available for vast majority of existing languages. See the XML-RPC
   implementation page for more details. Extending existing
   implementations with SIP support would not be easy.

   Because extending available XML-RPC implementation would be too
   expensive, we could also do it the other way around, keep existing
   XML-RPC implementations and extend SER to support HTTP. Extending SER
   with HTTP support is easier than it might seem at a first glance, due
   to the similarity between SIP requests and HTTP requests.

   SER already supports TCP, so existing HTTP implementations can send
   HTTP requests to it. HTTP requests are missing certain mandatory SIP
   header fields, such as Via, From, and CSeq. The contents of the header
   fields is mainly used for transaction matching. A SIP server could
   perform two basic operations when processing an HTTP request:
     * Terminate the request, execute the function and send a reply back.
     * Forward the request to another SIP server.

   Nothing special is needed on the SIP server terminating the request,
   except that it has to know where it should send the reply. Parsing of
   HTTP header field bodies would fail because we do not have parsers for
   them in SER, but that does not matter anyway because all the
   information is encoded in the body of the request. HTTP requests
   contain no Via header fields. Via header fields are used by SIP
   implementations to determine the destination (IP, transport protocol,
   and port number) for replies. When processing HTTP requests the SIP
   server needs to create a fake Via header field based on the source IP
   address and port number of the TCP connection. The SIP server will use
   this information when sending a reply back.

   Forwarding of HTTP requests by SIP proxies is a little bit more
   complicated and there are several limitations. First of all, we can
   only use stateless forwarding, no transactional forwarding, because
   HTTP requests do not contain all the header fields needed for
   transaction matching. Any attempt to call t_relay on an HTTP requests
   would fail. HTTP requests always use TCP and thus we could use
   stateless forwarding on the SIP server, provided that the request will
   be also forwarded over TCP. Stateless forwarding does not require the
   mandatory header fields (which are missing here) and it would work. In
   addition to that, the SIP server would also append fake Via header
   field to the request and change the contents of the Request-URI. The
   Request-URI of HTTP requests sent by XML-RPC implementations typically
   contain something like "/RPC2" and the first SIP server processing the
   request should rewrite the value with a valid SIP URI.

   Figure RPC Example shows a scenario which involves two SIP servers, one
   performs HTTP request "normalization" and forwarding, and the other
   terminates the request, executes corresponding function, and generates
   a reply.

                            [xmlrpc_example.png]

   Example RPC Scenario

   Step 1. An HTTP user agent sends an ordinary HTTP request to a SIP
   server. The user agent can either establish a connection directly to
   port 5060 or the SIP server can be configured to listen on port 80. The
   request contains standard HTTP headers and an XML-RPC document in the
   body:
POST / HTTP/1.0.
Host: localhost:5060
User-Agent: xmlrpclib.py/1.0.1 (by www.pythonware.com)
Content-Type: text/xml
Content-Length: 111

<?xml version='1.0'?>
<methodCall>
<methodName>usrloc.statistics</methodName>
<params>
</params>
</methodCall>

   This particular request calls method "statistics" from from usrloc
   module of SER. The method has no parameters.

   The outbound SIP server receives the HTTP request and performs a set of
   actions called "SIP-normalization". This includes creation of fake Via
   header field based on the source IP and port of the TCP connection,
   looking up of the target SIP server that should terminate and process
   the request, and rewriting of the Request-URI with the SIP URI of the
   target SIP server. Modified HTTP request will be then forwarded
   statelessly to the target SIP server.
POST sip:proxy01.sip-server.net HTTP/1.0
Via: SIP/2.0/TCP 127.0.0.1:3571
Host: localhost:5060
User-Agent: xmlrpclib.py/1.0.1 (by www.pythonware.com)
Content-Type: text/xml
Content-Length: 111

<?xml version='1.0'?>
<methodCall>
<methodName>usrloc.statistics</methodName>
<params>
</params>
</methodCall>

   Step 2. "normalized" HTTP request is statelessly forwarded to the
   target SIP server over TCP.

   Step 3. The target SIP server receives the HTTP request and executes
   function called dispatch_rpc from xmlrpc SER module. This function will
   parse the XML-RPC document in the body of the request and lookup the
   function to be called among all RPC functions exported by the SER core
   and modules. Function dispatch_rpc will be called from the
   configuration file just like any other function:
if (method == "POST" || method == "GET") {
    dispatch_rpc();
    break;
};

   This particular configuration snippet executes the function whenever
   SER receives GET or POST requests. These two method names indicate
   HTTP.

   Step 4. The function dispatch_rpc scans through the list of all
   exported RPC functions searching for the statistics function of the
   usrloc module. The SER RPC Module API describes in detail how modules
   export RPC functions.

   Step 5. As the RPC function from usrloc module is running and gathering
   statistics, it calls functions of RPC interface to prepare the result
   for the caller.

   Step 6. Once the RPC function finishes, xmlrpc module will build the
   XML-RPC document from the data received from usrloc module and generate
   a reply which will be sent to the caller.

   Steps 7. and 8. HTTP reply is sent back to the caller and the remote
   procedure call finishes.
HTTP/1.0 200 OK
Via: SIP/2.0/TCP 127.0.0.1:3571
Server: Sip EXpress router (0.10.99-janakj_experimental (i386/linux))
Content-Length: 651
Warning: 392 127.0.0.1:5060 "Noisy feedback tells:  pid=9975 req_src_ip=127.0.0
1 req_src_port=3571 in_uri=/ out_uri=sip:proxy01.sip-server.net via_cnt==1"

<?xml version="1.0" encoding="UTF-8"?>
<methodResponse>
<params>
<value><array>
<value><struct>
<member><name>domain</name>
<value><string>aliases</string></value></member>
<member><name>users</name>
<value><i4>0</i4></value></member>
<member><name>expired</name>
<value><i4>0</i4></value></member>
</struct></value>
<value><struct>
<member><name>domain</name>
<value><string>location</string></value></member>
<member><name>users</name>
<value><i4>0</i4></value></member>
<member><name>expired</name>
<value><i4>0</i4></value></member>
</struct></value>
</array></value>
</params>
</methodResponse>

Note

   The scenario described on Figure RPC Example involves two SIP servers.
   This is just to demonstrate that in setups containing more SIP servers
   it is possible to forward HTTP requests from one SIP server to another
   and use standard SIP routing mechanisms to decide which SIP server
   should process the request. There is no need to have multiple SIP
   servers in simple setups, because one SIP server can both add fake Via
   header field and process the RPC at the same time. Modified
   configuration file snipped could then look like this:
if (method == "POST" || method == "GET") {
    dispatch_rpc(); # Process the request
    break;
};

3. XML-RPC Implementation

   3.1. Requests
   3.2. Replies
   3.3. Type Conversion
   3.4. Limitations
   3.5. Interoperability Problems

   The purpose of the functions of this module is to convert XML-RPC
   document carried in the body of HTTP requests into data returned by the
   RPC interface and back. The module also contains functions necessary to
   "normalize" HTTP requests. The module uses xmlrpc-c library to perform
   XML-RPC related functions.

   The module always returns 200 OK HTTP reply, it will never return any
   other HTTP reply. Failures are expressed in XML-RPC documents in the
   body of the reply. There is basic method introspection support in the
   module. Currently the module can list all functions exported by the
   server and for each function it can return the documentation string
   describing the function.

3.1. Requests

   Requests processed by the module are standard XML-RPC requests encoded
   in bodies of HTTP requests.
POST / HTTP/1.0
Host: localhost:5060
User-Agent: xmlrpclib.py/1.0.1 (by www.pythonware.com)
Content-Type: text/xml
Content-Length: 112

<?xml version='1.0'?>
<methodCall>
<methodName>system.listMethods</methodName>
<params>
</params>
</methodCall>

   The name of the method to be called in this example is "listMethods".
   This is one of the introspection methods. SER will call dispatch_rpc
   function of xmlrpc module to handle the request. The function will
   parse the XML-RPC document, lookup listMethods function in the list of
   all export RPC functions, prepare the context for the function and
   execute it.

3.2. Replies

   The module will always generate 200 OK. Other response codes and
   classes are reserved for SER. The status code of the XML-RPC reply,
   response code, and additional data will be encoded in the body of the
   reply. Failure replies do not contain any data, just the response code
   and reason phrase:
HTTP/1.0 200 OK
Via: SIP/2.0/TCP 127.0.0.1:2464
Server: Sip EXpress router (0.10.99-janakj_experimental (i386/linux))
Content-Length: 301

<?xml version="1.0" encoding="UTF-8"?>
<methodResponse>

<fault>
<value><struct>
<member><name>faultCode</name>
<value><i4>501</i4></value></member>
<member><name>faultString</name>
<value><string>Method Not Implemented</string></value></member>
</struct></value>
</fault>

</methodResponse>

   This particular reply indicates that there is no such RPC method
   available on the server.

   Success replies always contain at least one return value. In our case
   the simplest success replies contain single boolean with value 1:
HTTP/1.0 200 OK
Via: SIP/2.0/TCP 127.0.0.1:4626
Server: Sip EXpress router (0.10.99-janakj_experimental (i386/linux))
Content-Length: 150

<?xml version="1.0" encoding="UTF-8"?>
<methodResponse>
<params>
<value><boolean>1</boolean></value>
</params>
</methodResponse>

   This is exactly how the reply looks like when an RPC function does not
   add any data to the reply set.

   If an RPC function adds just a single item (it calls add once with just
   one character in the formatting string) then the data will be converted
   to XML-RPC representation according to the rules described in SER RPC
   Type Conversion and the reply will contain just the single value:
HTTP/1.0 200 OK
Via: SIP/2.0/TCP 127.0.0.1:3793
Server: Sip EXpress router (0.10.99-janakj_experimental (i386/linux))
Content-Length: 216

<?xml version="1.0" encoding="UTF-8"?>
<methodResponse>
<params>
<value><string>Server: Sip EXpress router (0.10.99-janakj_experimental (i
386/linux))</string></value>
</params>
</methodResponse>

   If an RPC function adds more than one data items to the result set then
   the module will return an array containing all the data items:
HTTP/1.0 200 OK
Via: SIP/2.0/TCP 127.0.0.1:2932
Server: Sip EXpress router (0.10.99-janakj_experimental (i386/linux))
Content-Length: 276

<?xml version="1.0" encoding="UTF-8"?>
<methodResponse>
<params>
<value><array>
<value><string>./ser</string></value>
<value><string>-f</string></value>
<value><string>ser.cfg</string></value>
</array></value>
</params>
</methodResponse>

   This is probably the most common scenario.

3.3. Type Conversion

   The data types of the RPC API are converted to the data types of
   XML-RPC and vice versa. Table 1, "Data Type Conversion" shows for each
   RPC API data type corresponding XML-RPC data type.

   Table 1. Data Type Conversion
   RPC API XML-RPC RPC Example XML-RPC Example
   Integer <i4></i4> rpc->add("d", 42) <i4>42</i4>
   Float <double></double> rpc->add("f", -12.214) <double>-12.214</double>
   String <string></string> rpc->add("s","Don't panic") <string>Don't
   panic</string>
   Struct <struct></struct>
   rpc->struct_add(handle,"sd","param1",42,"param2",-12.214)
<struct>
  <member>
    <name>param1</name>
    <value>
      <i4>42</i4>
    </value>
  </member>
  <member>
    <name>param2</name>
    <value>
      <double>-12.214</i4>
    </value>
  </member>
</struct>

3.4. Limitations

   SER xmlrpc modules does not implement all data types allowed in
   XML-RPC. As well it does not implement arrays and nested structures.
   This simplification is a feature, not bug. In our case the XML-RPC
   interface will be used mainly for management purposes and we do not
   need all the bells and whistles of XML-RPC. Parsing and interpreting
   nested structures is complex and we try to avoid it.

3.5. Interoperability Problems

   Due to a bug in Python xmlrpclib there is an interoperability problem
   with basic clients using it: by default an xmlrpclib client expects the
   server to immediately close the connection after answering and if the
   server does not close the connections the xmlrpclib client will wait
   forever.

   There are 2 ways to work around this problem: write a "fixed" Transport
   class and initialize xmlpclib using it (recommended) or make ser close
   the tcp connection after each request.

   The examples/xmlrpc_test.py provides a very simple example of using
   xmlrpclib with a Transport class that works.

   For the second solution (force closing tcp connections after answering)
   the XMLRPC route should have a set_reply_close() command before
   dispatch_rpc(). set_reply_no_connect() is also recommended (avoid
   trying to open tcp connection to xmlrpc clients that closed it).
   Alternatively ending the XMLRPC route with return -1, exit -1 or drop
   -1 can also be used, but note that this will not work for async rpcs
   (it will close the connection immeidately and not on the async
   response).

   Example 1.
route[XMLRPC]{
        # close connection only for xmlrpclib user agents
        if search("^User-Agent:.*xmlrpclib"))
                set_reply_close();
        set_reply_no_connect(); # optional
        dispatch_rpc();
}

   Another common problem is CRLF handling. According to the xml spec CR
   ('\r') must be escaped (to &#xD;) or they will be "normalized" when
   parsing the xml document. However some xmlrpc clients do not follow
   this rule (e.g. clients based on the python or php xmlrpclib) and send
   CRLF unescaped. A possible workaround is to enable automatic LFLF to
   CRLF conversion (using the double_lf_to_crlf modules parameter) and
   replace CRLF with LFLF in the client queries.

4. Client Examples

     * examples/xmlrpc_test.pl (basic perl application that builds and
       sends an XMLRPC request from its commandline parameters).
     * examples/xmlrpc_test.py (basic python application that builds and
       sends an XMLRPC request from its commandline parameters).
     * ser_ctl (complex python application that uses the XML-RPC interface
       implemented by the xmlrpc module).
     * serweb (php application that can use the XML-RPC interface to call
       ser functions).

Chapter 1. Admin Guide

   Table of Contents

   1. Parameters

        1.1. route (string)
        1.2. autoconversion (string)
        1.3. escape_cr (integer)
        1.4. double_lf_to_crlf (integer)
        1.5. mode (integer)
        1.6. url_skip (str)
        1.7. url_match (str)

   2. Functions

        2.1. dispatch_rpc()
        2.2. xmlrpc_reply(code, reason)

1. Parameters

   1.1. route (string)
   1.2. autoconversion (string)
   1.3. escape_cr (integer)
   1.4. double_lf_to_crlf (integer)
   1.5. mode (integer)
   1.6. url_skip (str)
   1.7. url_match (str)

1.1. route (string)

   Name of the route called for XMLRPC messages.

   This route will be called only for HTTP messages whose method is either
   GET or POST. The message visible inside the route will be a HTTP
   request converted to SIP (the uri will be fixed and a fake via will be
   added).

   The route should perform additional security checks to ensure the
   client is authorized to execute management/RPC functions and then it
   should call the dispatch_rpc().

   Default: the main route is used.

   Example 1.1. Set route parameter
modparam("xmlrpc", "route", "route_for_xmlrpcs")

1.2. autoconversion (string)

   Enable or disable automatic parameter type conversion globally, for all
   the methods parameters. If on, a type mismatch in a method parameter
   will not cause a fault if it is possible to automatically convert it to
   the type expected by the method.

   Default: off.

   It is recommended to leave this parameter to its default off value and
   fix instead the client application (which should use the proper types)
   or to modify the target rpc to accept any type (see the rpc scan '.'
   modifier).

   Example 1.2. Set the autoconversion parameter
modparam("xmlrpc", "autoconversion", 1)

1.3. escape_cr (integer)

   Enable CR ('\r') escaping in replies. If enabled each '\r' in the
   xmlrpc reply will be replaced with "&#xD;", according to the xml spec.

   It should be turned off only if you suspect interoperability problems
   with older clients.

   Default: on.

   Example 1.3. Set the escape_cr parameter
modparam("xmlrpc", "escape_cr", 1)

1.4. double_lf_to_crlf (integer)

   When enabled double LFs ('\n\n') in the input xmlrpc strings will be
   replaced with CR LF ('\r\n'). This makes LF LF behave like an escape
   character for CR LF and is needed for compatibility with kamailio tools
   and to work around buggy xmlrpc clients that don't escape the CR in CR
   LF ('\r' should be escaped to "&#xD;" otherwise according to the xml
   spec "\r\n" will be transformed to '\n'), but need to send CR LF in the
   strings (e.g. they use tm.t_uac_wait).

   Note: when this option is turned on, there is no way to send a double
   LF ('\n\n'), it will always be transformed in CR LF ('\r\n').

   Default: off.

   Example 1.4. Set the double_lf_to_crlf parameter
modparam("xmlrpc", "double_lf_to_crlf", 1)

1.5. mode (integer)

   When set to 1, xmlrpc module does not register to core the callback
   functions for non-SIP messages. Useful when other module register a
   callback for HTTP request, being the decision of admin when to call the
   XMLRPC route (or functions).

   Default: 0.

   Example 1.5. Set the mode parameter
modparam("xmlrpc", "mode", 1)

1.6. url_skip (str)

   Regular expression to match the HTPP URL. If there is match, then
   xmlrpc route is not executed.

   Default value is null (don't skip).

   Example 1.6. Set url_skip parameter
...
modparam("xmlrpc", "url_skip", "^/sip")
...

1.7. url_match (str)

   Regular expression to match the HTTP URL. If there is no match, then
   xmlrpc route is not executed. This check is done after url_skip, so if
   both url_skip and url_match would match then the xmlrpc route is not
   executed (url_skip has higher priority).

   Default value is null (match everything).

   Example 1.7. Set url_match parameter
...
modparam("xmlrpc", "url_match", "^/RPC2")
...

2. Functions

   2.1. dispatch_rpc()
   2.2. xmlrpc_reply(code, reason)

2.1.  dispatch_rpc()

   This function processes an XMLRPC request, found in the body of the
   request.

   It should be used only in a route specified using the "route" module
   parameter or if the request method is GET or POST (using it for other
   request methods will not have adverse side-effects, but it will
   probably not work).

   dispatch_rpc() extracts the XML-RPC document from the body of the
   request to determine the name of the RPC method to be called and then
   it searches through the list of all the RPC functions to find a
   function with matching name. If such a function is found then
   dispatch_rpc() will pass control to the function to handle the request.

   Example 1.8. dispatch_rpc usage
#...
modparam("xmlrpc", "route", "XMLRPC");
#...
route[XMLRPC]{
        if search("^User-Agent:.*xmlrpclib"))
                set_reply_close();
        set_reply_no_connect(); # optional
        dispatch_rpc();
}

2.2.  xmlrpc_reply(code, reason)

   This function can be called from the config script to directly generate
   an XML-RPC reply.

   Example 1.9. xmlrpc_reply usage
#...
modparam("xmlrpc", "route", "XMLRPC");
#...
route[XMLRPC]{
        # allow XMLRPC requests only on TLS and only if the client
        # certificate is valid
        if (proto!=TLS){
                xmlrpc_reply("400", "xmlrpc allowed only over TLS");
                return;
        }
        if (@tls.peer.verified!=""){
                xmlrpc_reply("400", "Unauthorized");
                return;
        }
        if search("^User-Agent:.*xmlrpclib"))
                set_reply_close();
        set_reply_no_connect(); # optional
        dispatch_rpc();
}
