nathelper_admin.xml

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<!-- Module User's Guide -->

<chapter>
	
	<title>&adminguide;</title>
	
	<section>
	<title>Overview</title>
	<para>
		This is a module to help with &nat; traversal and reuse
		of <acronym>TCP</acronym> connections. In particular, 
		it helps symmetric &ua;s that don't advertise they are symmetric 
		and are not able to determine their public address. 
	</para>
	<para>
		The function <function>fix_nated_contact()</function> rewrites the <quote>Contact</quote>
		header field with request's source address:port pair. The function
		<function>fix_nated_sdp()</function> adds the active direction indication 
		to &sdp; (flag 0x01) and updates the source &ip; address too (flag 0x02). The function
		<function>fix_nated_register()</function> exports exports the request's source
		address:port into an AVP to be used during <function>save()</function> and should
		be used for <quote>REGISTER</quote> requests.
	</para>
        <para>
		Note: <function>fix_nated_contact</function> changes the <quote>Contact</quote>
		header, thus it breaks the RFC. Although usually this is not an issue, it may
		cause problems with strict SIP clients.  An alternative is to use
		<function>add_contact_alias()</function> that together with
		the <function>handle_ruri_alias()</function> is standards conforming and also
		supports reuse of TCP/TLS connections.
	</para>
	</section>

	<section>
	<title>NAT pinging types</title>
	<para>
		Currently, the nathelper module supports two types of NAT pings:
	</para>
	<itemizedlist>
		<listitem>
			<para>
			<emphasis>UDP package</emphasis> - 4 bytes (zero filled) UDP 
			packages are sent to the contact address.
			</para>
			<itemizedlist>
				<listitem>
				<para><emphasis>Advantages:</emphasis> low bandwitdh traffic,
				easy to generate by &kamailio;;
				</para>
				</listitem>
				<listitem>
				<para><emphasis>Disadvantages:</emphasis> unidirectional 
				traffic through NAT (inbound - from outside to inside); As 
				many NATs do update the bind timeout only on outbound traffic,
				the bind may expire and closed.
				</para>
				</listitem>
			</itemizedlist>
		</listitem>
		<listitem>
			<para>
			<emphasis>SIP request</emphasis> - a stateless SIP request is 
			sent to the contact address.
			</para>
			<itemizedlist>
				<listitem>
				<para><emphasis>Advantages:</emphasis> bidirectional traffic
				through NAT, since each PING request from &kamailio; (inbound 
				traffic) will force the SIP client to generate a SIP reply 
				(outbound traffic) - the NAT bind will be surely kept open.
				</para>
				</listitem>
				<listitem>
				<para><emphasis>Disadvantages:</emphasis> higher bandwitdh 
				traffic, more expensive (as time) to generate by &kamailio;;
				</para>
				</listitem>
			</itemizedlist>
		</listitem>
	</itemizedlist>
	</section>

	<section>
	<title>Dependencies</title>
	<section>
		<title>&kamailio; Modules</title>
		<para>
		The following modules must be loaded before this module:
			<itemizedlist>
			<listitem>
			<para>
				<emphasis>usrloc</emphasis> module - only if the NATed 
				contacts are to be pinged.
			</para>
			</listitem>
			</itemizedlist>
		</para>
	</section>
	<section>
		<title>External Libraries or Applications</title>
		<para>
		The following libraries or applications must be installed before 
		running &kamailio; with this module loaded:
			<itemizedlist>
			<listitem>
			<para>
				<emphasis>None</emphasis>.
			</para>
			</listitem>
			</itemizedlist>
		</para>
	</section>
	</section>

	<section>
	<title>Parameters</title>
	<section id="nathelper.p.force_socket">
		<title><varname>force_socket</varname> (string)</title>
		<para>
		Socket to be used when sending NAT pings for UDP communication.
		If no one specified, the OS will choose a socket.
		</para>
		<para>
		<emphasis>
			Default value is <quote>NULL</quote>.
		</emphasis>
		</para>
		<example>
		<title>Set <varname>force_socket</varname> parameter</title>
		<programlisting format="linespecific">
...
modparam("nathelper", "force_socket", "localhost:33333")
...
</programlisting>
		</example>
	</section>
	<section id="nathelper.p.natping_interval">
		<title><varname>natping_interval</varname> (integer)</title>
		<para>
		Period of time in seconds between sending the NAT pings to all 
		currently registered &ua;s to keep their &nat; bindings alive. 
		Value of 0 disables this functionality.
		</para>
		<note><para>
		Enabling the NAT pinging functionality will force the module to
		bind itself to USRLOC module.
		</para></note>
		<para>
		<emphasis>
			Default value is 0.
		</emphasis>
		</para>
		<example>
		<title>Set <varname>natping_interval</varname> parameter</title>
		<programlisting format="linespecific">
...
modparam("nathelper", "natping_interval", 10)
...
</programlisting>
		</example>
	</section>
	<section id="nathelper.p.ping_nated_only">
		<title><varname>ping_nated_only</varname> (integer)</title>
		<para>
		If this variable is set then only contacts that have 
		<quote>behind_NAT</quote> flag in user location database set will 
		get ping.
		</para>
		<para>
		<emphasis>
			Default value is 0.
		</emphasis>
		</para>
		<example>
		<title>Set <varname>ping_nated_only</varname> parameter</title>
		<programlisting format="linespecific">
...
modparam("nathelper", "ping_nated_only", 1)
...
</programlisting>
		</example>
	</section>
	<section id="nathelper.p.natping_processes">
		<title><varname>natping_processes</varname> (integer)</title>
		<para>
		How many timer processes should be created by the module for the
		exclusive task of sending the NAT pings.
		</para>
		<para>
		<emphasis>
			Default value is 1.
		</emphasis>
		</para>
		<example>
		<title>Set <varname>natping_processes</varname> parameter</title>
		<programlisting format="linespecific">
...
modparam("nathelper", "natping_processes", 3)
...
</programlisting>
		</example>
	</section>
	<section id="nathelper.p.natping_socket">
		<title><varname>natping_socket</varname> (string)</title>
		<para>
		Spoof the natping's source-ip to this address. Works only for IPv4.
		</para>
		<para>
		<emphasis>
			Default value is NULL.
		</emphasis>
		</para>
		<example>
		<title>Set <varname>natping_socket</varname> parameter</title>
		<programlisting format="linespecific">
...
modparam("nathelper", "natping_socket", "192.168.1.1:5006")
...
</programlisting>
		</example>
	</section>
	<section id="nathelper.p.received_avp">
		<title><varname>received_avp</varname> (str)</title>
		<para>
		The name of the Attribute-Value-Pair (AVP) used to store the URI 
		containing the received IP, port, and protocol. The URI is created 
		by fix_nated_register function of nathelper module and the attribute 
		is then used by the registrar to store the received parameters. Do 
		not forget to change the value of corresponding parameter in
		registrar module if you change the value of this parameter.
		</para>
		<note>
		<para>
		You must set this parameter if you use <function>fix_nated_register</function>. In such
		case you must set the parameter with same name in the <quote>registrar</quote>
		module to same value.
		</para>
		</note>
		<para>
		<emphasis>
			Default value is "NULL" (disabled).
		</emphasis>
		</para>
		<example>
		<title>Set <varname>received_avp</varname> parameter</title>
		<programlisting format="linespecific">
...
modparam("nathelper", "received_avp", "$avp(i:42)")
...
</programlisting>
		</example>
	</section>
	<section id="nathelper.p.sipping_bflag">
		<title><varname>sipping_bflag</varname> (integer)</title>
		<para>
		Which branch flag should be used by the module to identify NATed 
		contacts for which it should perform NAT ping via a SIP request 
		instead if dummy UDP package.
		</para>
		<para>
		<emphasis>
			Default value is -1 (disabled).
		</emphasis>
		</para>
		<example>
		<title>Set <varname>sipping_bflag</varname> parameter</title>
		<programlisting format="linespecific">
...
modparam("nathelper", "sipping_bflag", 7)
...
</programlisting>
		</example>
	</section>
	<section id="nathelper.p.sipping_from">
		<title><varname>sipping_from</varname> (string)</title>
		<para>
		The parameter sets the SIP URI to be used in generating the SIP
		requests for NAT ping purposes. To enable the SIP request pinging
		feature, you have to set this parameter. The SIP request pinging 
		will be used only for requests marked so.
		</para>
		<para>
		<emphasis>
			Default value is <quote>NULL</quote>.
		</emphasis>
		</para>
		<example>
		<title>Set <varname>sipping_from</varname> parameter</title>
		<programlisting format="linespecific">
...
modparam("nathelper", "sipping_from", "sip:pinger@siphub.net")
...
</programlisting>
		</example>
	</section>
	<section id="nathelper.p.sipping_method">
		<title><varname>sipping_method</varname> (string)</title>
		<para>
		The parameter sets the SIP method to be used in generating the SIP
		requests for NAT ping purposes.
		</para>
		<para>
		<emphasis>
			Default value is <quote>OPTIONS</quote>.
		</emphasis>
		</para>
		<example>
		<title>Set <varname>sipping_method</varname> parameter</title>
		<programlisting format="linespecific">
...
modparam("nathelper", "sipping_method", "INFO")
...
</programlisting>
		</example>
	</section>
	<section id="nathelper.p.natping_disable_bflag">
		<title><varname>natping_disable_bflag</varname> (integer)</title>
		<para>
		What branch flag should be used by the module to disable NAT pings
		on a per-registration basis. If the given flag is set for a
		particular registration, then no NAT pings will be sent at all,
		regardless of any other conditions.
		</para>
		<para>
		<emphasis>
			Default value is -1 (disabled).
		</emphasis>
		</para>
		<example>
		<title>Set <varname>natping_disable_bflag</varname> parameter</title>
		<programlisting format="linespecific">
...
modparam("nathelper", "natping_disable_bflag", 8)
...
</programlisting>
		</example>
	</section>
	<section id="nathelper.p.nortpproxy_str">
		<title><varname>nortpproxy_str</varname> (string)</title>
		<para>
		The parameter sets the SDP attribute used by nathelper to mark
		the packet SDP that information have already been mangled.
		</para>
		<para>
		If empty string, no marker will be added or checked.
		</para>
		<note><para>
		The string must be a complete SDP line, including the EOH (\r\n).
		</para></note>
		<para>
		<emphasis>
			Default value is <quote>a=nortpproxy:yes\r\n</quote>.
		</emphasis>
		</para>
		<example>
		<title>Set <varname>nortpproxy_str</varname> parameter</title>
		<programlisting format="linespecific">
...
modparam("nathelper", "nortpproxy_str", "a=sdpmangled:yes\r\n")
...
</programlisting>
		</example>
	</section>
	<section id="nathelper.p.keepalive_timeout">
		<title><varname>keepalive_timeout</varname> (int)</title>
		<para>
		The parameter sets the interval in secods after which a natted
		contact is removed from location table if it does not reply to SIP
		keepalives (usually OPTIONS ping requests).
		</para>
		<para>
		The features is available only for UDP contacts that are stored in memory
		(not working for db only mode for usrloc module).
		</para>
		<para>
		Keepalives are sent stateless, not using TM module. The value of this
		parameter has to be few times higher than natping_interval.
		</para>
		<para>
		<emphasis>
			Default value is <quote>0</quote> (feature disabled).
		</emphasis>
		</para>
		<example>
		<title>Set <varname>keepalive_timeout</varname> parameter</title>
		<programlisting format="linespecific">
...
modparam("nathelper", "keepalive_timeout", 120)
...
</programlisting>
		</example>
	</section>
	<section id="nathelper.p.udpping_from_path">
		<title><varname>udpping_from_path</varname> (int)</title>
		<para>
		Enable sending UDP pings (keepalives) using raw socket from Path
		address.
		</para>
		<para>
		<emphasis>
			Default value is <quote>0</quote> (feature disabled).
		</emphasis>
		</para>
		<example>
		<title>Set <varname>udpping_from_path</varname> parameter</title>
		<programlisting format="linespecific">
...
modparam("nathelper", "udpping_from_path", 1)
...
</programlisting>
		</example>
	</section>
	<section id="nathelper.p.append_sdp_oldmediaip">
		<title><varname>append_sdp_oldmediaip</varname> (int)</title>
		<para>
		The parameter controls if oldmediaip field should be appended to the SDP.
		</para>
		<para>
		<emphasis>
			Default value is <quote>1</quote> (feature enabled).
		</emphasis>
		</para>
		<example>
		<title>Set <varname>append_sdp_oldmediaip</varname> parameter</title>
		<programlisting format="linespecific">
...
modparam("nathelper", "append_sdp_oldmediaip", 1)
...
</programlisting>
		</example>
	</section>
	</section>


	<section>
	<title>Functions</title>
	<section id="nathelper.f.fix_nated_contact">
		<title>
		<function moreinfo="none">fix_nated_contact()</function>
		</title>
		<para>
		Rewrites the <quote>Contact</quote> header to contain the request's source 
		address:port.
		</para>
		<para>
		This function can be used from REQUEST_ROUTE, ONREPLY_ROUTE, BRANCH_ROUTE.
		</para>
		<example>
		<title><function>fix_nated_contact</function> usage</title>
		<programlisting format="linespecific">
...
if (search("User-Agent: Cisco ATA.*") {fix_nated_contact();};
...
</programlisting>
		</example>
	</section>
	<section id="nathelper.f.fix_nated_sdp">
		<title>
		<function moreinfo="none">fix_nated_sdp(flags [, ip_address])</function>
		</title>
		<para>
		Alters the SDP information in orer to facilitate NAT traversal. What
		changes to be performed may be controled via the 
		<quote>flags</quote> parameter. Return value is -1 if error occurred,
		1 if ip's were replaced, 2 if no ip's were replaced.
		</para>
		<para>Meaning of the parameters is as follows:</para>
		<itemizedlist>
			<listitem><para>
			<emphasis>flags</emphasis> - the value may be a bitwise OR of 
			the following flags:
			</para>
			<itemizedlist>
				<listitem>
					<para><emphasis>0x01</emphasis> - adds 
					<quote>a=direction:active</quote> SDP line;
					</para>
				</listitem>
				<listitem>
					<para><emphasis>0x02</emphasis> - rewrite media
					&ip; address (c=) with source address of the message
					or the provided IP address (the provide IP address take
					precedence over the source address).</para>
				</listitem>
				<listitem>
					<para><emphasis>0x04</emphasis> - adds 
						<quote>a=nortpproxy:yes</quote> SDP line;</para>
				</listitem>
				<listitem>
					<para><emphasis>0x08</emphasis> - rewrite IP from
					origin description (o=) with source address of the message
					or the provided IP address (the provide IP address take
					precedence over the source address).</para>
				</listitem>
			</itemizedlist>
			</listitem>
			<listitem><para>
			<emphasis>ip_address</emphasis> - IP to be used for rewritting SDP.
			If not specified, the received signalling IP will be used. The
			parameter allows pseudo-variables usage. NOTE: For the IP to be
			used, you need to use 0x02 or 0x08 flags, otherwise it will have
			no effect.
			</para>
			</listitem>
		</itemizedlist>
		<para>
		This function can be used from REQUEST_ROUTE, ONREPLY_ROUTE, 
		FAILURE_ROUTE, BRANCH_ROUTE.
		</para>
		<example>
		<title><function>fix_nated_sdp</function> usage</title>
		<programlisting format="linespecific">
...
if (search("User-Agent: Cisco ATA.*") {fix_nated_sdp("3");};
...
</programlisting>
		</example>
	</section>
	<section id="nathelper.f.add_rcv_param">
		<title>
		<function moreinfo="none">add_rcv_param([flag])</function>,
		</title>
		<para>
		Add a received parameter to the <quote>Contact</quote> header fields
		or the Contact URI. The parameter will contain the URI created from the
		source IP, port, and protocol of the packet containing the SIP message.
		The parameter can be then processed by another registrar. This is useful,
		for example, when replicating register messages using <function>t_replicate</function>
		function to another registrar.
		</para>
		<para>Meaning of the parameters is as follows:</para>
		<itemizedlist>
			<listitem><para>
			<emphasis>flag</emphasis> - flags to indicate if the parameter
			should be added to Contact URI or Contact header. If the flag is
			non-zero, the parameter will be added to the Contact URI. If not
			used or equal to zero, the parameter will go to the Contact 
			header.
			</para></listitem>
		</itemizedlist>
		<para>
		This function can be used from REQUEST_ROUTE.
		</para>
		<example>
		<title><function>add_rcv_paramer</function> usage</title>
		<programlisting format="linespecific">
...
add_rcv_param(); # add the parameter to the Contact header
....
add_rcv_param("1"); # add the parameter to the Contact URI
...
</programlisting>
		</example>
	</section>
	<section id="nathelper.f.fix_nated_register">
		<title>
		<function moreinfo="none">fix_nated_register()</function>
		</title>
		<para>
		The function creates a URI consisting of the source IP, port, and 
		protocol and stores the URI in an Attribute-Value-Pair. The URI will 
		be appended as "received" parameter to Contact in 200 OK and 
		registrar will store it in the received cloumn in the location table.
		</para>
		<para>
		Note: You have to set the <quote>received_avp</quote> parameter of the
		nathelper module and the registrar module (both module parameters must
		have the same value) to use this function.
		</para>
		<para>
		This function can be used from REQUEST_ROUTE.
		</para>
		<example>
		<title><function>fix_nated_register</function> usage</title>
		<programlisting format="linespecific">
...
fix_nated_register();
...
</programlisting>
		</example>
	</section>
	<section id="nathelper.f.nat_uac_test">
		<title>
			<function>nat_uac_test(flags)</function>
		</title>
		<para>
			Tries to guess if client's request originated behind a nat.
			The parameter determines what heuristics is used.
		</para>
		<para>Meaning of the flags is as follows:</para>
		<itemizedlist>
			<listitem><para>
			<emphasis>1</emphasis> -  The <quote>Contact</quote> header field is searched 
			for occurrence of RFC1918 or RFC6598 addresses.
			</para></listitem>
			<listitem><para>
			<emphasis>2</emphasis> -  the "received" test is used: address
			in the <quote>Via</quote> header is compared against source
			IP address of signaling
			</para></listitem>
			<listitem><para>
			<emphasis>4</emphasis> -  The Top Most <quote>Via</quote> is searched 
			for occurrence of RFC1918 or RFC6598 addresses
			</para></listitem>
			<listitem><para>
			<emphasis>8</emphasis> -  The SDP is searched for occurrence of 
			RFC1918 or RFC6598 addresses
			</para></listitem>
			<listitem><para>
			<emphasis>16</emphasis> -  Test if the source port is different
			from the port in the <quote>Via</quote> header
			</para></listitem>
			<listitem><para>
			<emphasis>32</emphasis> -  Test if the source IP address of
			signaling is a RFC1918 or RFC6598 address
			</para></listitem>
			<listitem><para>
			<emphasis>64</emphasis> -  Test if the source connection of
			signaling is a WebSocket
			</para></listitem>
			<listitem><para>
			<emphasis>128</emphasis> -  Test if the <quote>Contact</quote> header
			URI port differs from the source port of the request (Warning: this is
			might be legal or even intended combination in non NATted scenarios)
			</para></listitem>
			</itemizedlist>
		<para>
		All flags can be bitwise combined, the test returns true if any of 
		the tests identified a NAT.
		</para>
		<para>
		This function can be used from REQUEST_ROUTE, ONREPLY_ROUTE, FAILURE_ROUTE, BRANCH_ROUTE.
		</para>
		<example>
		<title><function>nat_uac_test</function> usage</title>
		<programlisting format="linespecific">
...
if(nat_uac_test("19")) {
    rtpproxy_manage("co");
}
...
</programlisting>
		</example>
	</section>

	<section id="nathelper.f.is_rfc1918">
		<title>
		<function>is_rfc1918(ip_address)</function>
		</title>
		<para>
			Determines if the address in the parameter is an rfc1918 or rfc6598 address.
			The parameter allows pseudo-variables usage.
		</para>
		<para>
		This function can be used from ANY_ROUTE.
		</para>
		<example>
		<title><function>is_rfc1918</function> usage</title>
		<programlisting format="linespecific">
...
if(is_rfc1918("$rd")) {
    # domain in r-uri is private address
}
...
</programlisting>
		</example>
	</section>

	<section id="nathelper.f.add_contact_alias">
		<title>
		<function moreinfo="none">add_contact_alias([ip_addr, port, proto])</function>
		</title>
		<para>
		Adds an <quote>;alias=ip~port~transport</quote> parameter to
		the contact URI containing either received ip, port, and
		transport protocol or those given as parameters.  If called
		without parameters, <quote>;alias</quote> parameter is
		only added if received ip and port differ from those in
		contact URI.
		</para>
		<para>
		This function can be used from
		REQUEST_ROUTE, ONREPLY_ROUTE, BRANCH_ROUTE, and LOCAL_ROUTE.
		</para>
		<example>
		<title><function>add_contact_alias</function> usage</title>
		<programlisting format="linespecific">
...
    if (!is_present_hf("Record-Route")) {
        if (!add_contact_alias("$var(src_ip)", "$Rp", "tcp")) {
            xlog("L_ERR", "Error in aliasing contact $ct\n");
            send_reply("400", "Bad request");
            exit;
        };
    };
...
		</programlisting>
		</example>
	</section>

	<section id="nathelper.f.handle_ruri_alias">
		<title>
		<function moreinfo="none">handle_ruri_alias()</function>
		</title>
		<para>
		Checks if the Request URI has an <quote>alias</quote>
		parameter and if so, removes it and sets the <quote>$du</quote> based
		on its value.  Note that this means that routing of request is based on 
		<quote>;alias</quote> parameter value of the Request URI rather than 
		the Request URI itself. If you call <function>handle_ruri_alias()</function>
		on a request, make sure that you screen the alias parameter value of
		Request URI the same way as you would screen the
		Request URI itself.
		</para>
		<para>
		Returns <emphasis>1</emphasis> if <quote>;alias</quote> parameter was
		found and <quote>$du</quote> was set and the <quote>$ru</quote> rewritten,
		<emphasis>2</emphasis> if the alias parameter was not found and
		nothing was done, or <emphasis>-1</emphasis> in case of error.
		</para>
		<para>
		This function can be used from
		REQUEST_ROUTE, BRANCH_ROUTE, and LOCAL_ROUTE.
		</para>
		<example>
		<title><function>handle_ruri_alias</function> usage</title>
		<programlisting format="linespecific">
...
    if ($du == "") {
        handle_ruri_alias();
        switch ($rc) {
        case -1:
            xlog("L_ERR", "Failed to handle alias of R-URI $ru\n");
            send_reply("400", "Bad request");
            exit;
        case 1:
            xlog("L_INFO", "Routing in-dialog $rm from $fu to $du\n");
            break;
        case 2:
            xlog("L_INFO", "Routing in-dialog $rm from $fu to $ru\n");
            break;
         };
    };
...
		</programlisting>
		</example>
	</section>

	<section id="nathelper.set_contact_alias">
		<title>
		<function moreinfo="none">set_contact_alias()</function>
		</title>
		<para>
		Adds an <quote>;alias=ip~port~transport</quote> parameter to the
		contact URI containing the received ip, port, and transport protocol.
		The new contact URI is immediately visible to other modules in the
		way the <function>fix_nated_contact()</function> does it.
		</para>
		<para>
		This function can be used from
		REQUEST_ROUTE, ONREPLY_ROUTE, BRANCH_ROUTE, and FAILURE_ROUTE.
		</para>
		<example>
		<title><function>set_contact_alias</function> usage</title>
		<programlisting format="linespecific">
...
    if (!is_present_hf("Record-Route")) {
        if (!set_contact_alias()) {
            xlog("L_ERR", "Error in aliasing contact $ct\n");
            send_reply("400", "Bad request");
            exit;
        };
    };
...
		</programlisting>
		</example>
	</section>

	</section>

	<section>
		<title>Exported Pseudo Variables</title>
		<section>
			<title><function moreinfo="none">$rr_count</function></title>
			<para>
			Number of Record Routes in received SIP request
			or reply.
			</para>
		<example>
		<title>$rr_count usage</title>
		<programlisting format="linespecific">
...
    $avp(rr_count) = $rr_count;
...
		</programlisting>
		</example>
	        </section>

		<section>
			<title><function moreinfo="none">$rr_top_count</function></title>
			<para>
			If topmost Record Route in received SIP request
			or reply is a double Record Route, value of
			$rr_top_count is 2.  If it a single Record
			Route, value of $rr_top_count 
			is 1.  If there is no Record Route(s), value of
			$rr_top_count is 0.
			</para>
		<example>
		<title>$rr_top_count usage</title>
		<programlisting format="linespecific">
...
    if ($rr_count == $avp(rr_count) + $rr_top_count) {
        route(ADD_CONTACT_ALIAS);
    };
...
		</programlisting>
		</example>
	        </section>

	</section>

	<section>
		<title><acronym>MI</acronym> Commands</title>
		<section id="nathelper.m.nh_enable_ping">
			<title><function moreinfo="none">nh_enable_ping</function></title>
			<para>
			Enables natping if parameter value greater than 0.
			Disables natping if parameter value is 0.
			</para>
			<para>
			The function takes only one parameter - a number in decimal format.
			</para>
			<example>
			<title><function moreinfo="none">nh_enable_ping</function> usage</title>
			<programlisting format="linespecific">
...
$ &ctltool; fifo nh_enable_ping 1
...
			</programlisting>
			</example>
		</section>

	</section>

	<section>
		<title>Selects</title>

		<section id="nathelper.s.rewrite_contact">
		<title>@nathelper.rewrite_contact[n]</title>
		<para>
			Get n-th Contact value with IP:Port rewritten to source ip:port. N is counted from 1.
		    Only IP:port is rewritten, remaining part are left unchanged. Full nameaddr is supported.
		</para>
		<example>
			<title>@nathelper.rewrite_contact usage</title>
			<programlisting>
...
$c = @nathelper.rewrite_contact[1];
...
$c2 = @nathelper.rewrite_contact[1].nameaddr.uri;
			</programlisting>
		</example>
		</section>

	</section>

</chapter>