[sip-router] / src / modules / nathelper / doc / 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 packet</emphasis> - 4 bytes (zero filled) UDP 
                        packets are sent to the contact address.
                        </para>
                        <itemizedlist>
                                <listitem>
                                <para><emphasis>Advantages:</emphasis> low bandwidth 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 UDP 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 bandwidth 
                                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", "127.0.0.1:5060")
...
</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 parameter is set to 1 then only contacts that have
                <quote>behind_NAT</quote> flag in user location database set will
                get ping.
                </para>
                <para>
                If it is 0 and sipping_flag is not set, then the 4-bytes UDP ping is
                sent to all contacts. If it is 0 and sipping_flag parameter is set, then
                SIP-request-based pinging is sent to all contacts.
                </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 of dummy UDP packet.
                </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 seconds 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 id="nathelper.p.filter_server_id">
                <title><varname>filter_server_id</varname> (int)</title>
                <para>
                Filter contacts by <quote>server_id</quote> core parameter.
                Use this parameter to limit pinging. When set to <quote>1</quote>,
                only proxy instances which send packets are those where core server_id
                matches server_id saved in usrloc.
                <emphasis>
                        Default value is <quote>0</quote> (disabled).
                </emphasis>
                </para>
                <example>
                <title>Set <varname>filter_server_id</varname> parameter</title>
                <programlisting format="linespecific">
...
modparam("nathelper", "filter_server_id", 1)
...
</programlisting>
                </example>
        </section>

        <section id="nathelper.p.nat_addr_mode">
                <title><varname>nat_addr_mode</varname> (int)</title>
                <para>
                        If set to 0, only default private net addresses are checked by
                        nat_uac_test(). If set to 1, other reserved net addresses are
                        checked by nat_uac_test() as well.
                </para>
                <para>Default private net addresses are:</para>
                <itemizedlist>
                        <listitem>
                                <para>10.0.0.0/8</para>
                        <listitem>
                        <listitem>
                                <para>172.16.0.0/12</para>
                        <listitem>
                        <listitem>
                                <para>192.168.0.0/16</para>
                        <listitem>
                        <listitem>
                                <para>100.64.0.0/10 - RFC6598 - Carrier Grade NAT</para>
                        <listitem>
                        <listitem>
                                <para>192.0.0.0/29 - RFC7335 - IPv4 Service Continuity Prefix</para>
                        <listitem>
                <itemizedlist>
                <para>Reserved net addresses are:</para>
                <itemizedlist>
                        <listitem>
                                <para>192.0.0.0/24 - RFC7335 - IETF Protocol Assignments</para>
                        <listitem>
                <itemizedlist>
                <para>
                <emphasis>
                        Default value is <quote>1</quote>.
                </emphasis>
                </para>
                <example>
                <title>Set <varname>nat_addr_mode</varname> parameter</title>
                <programlisting format="linespecific">
...
modparam("nathelper", "nat_addr_mode", 0)
...
</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 order to facilitate NAT traversal. What
                changes to be performed may be controlled 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 rewriting 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
                (available for all transports) or to the Contact URI (available only
                for UDP traffic).
                </para>
                <para>
                The parameter will contain the URI created from the
                source IP, port, and protocol (if different than UDP) 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 column 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. If the <quote>Via</quote> header contains
                        no port, it uses the default SIP port 5060
                        </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. If the <quote>Via</quote> header contains
                        no port, it uses the default SIP port 5060
                        </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>
                        <listitem><para>
                        <emphasis>256</emphasis> -  Test if the SDP connection address is different
                        from source IP address. It will work also with multiple connection address
                        lines.
                        </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, port, or transport protocol differs
        from that 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 $ct\n");
            send_reply("400", "Bad request");
            exit;
        };
    };
...
                </programlisting>
                </example>
        </section>
                <section id="nathelper.set_alias_to_pv">
                <title>
                <function moreinfo="none">set_alias_to_pv(target_avp)</function>
                </title>
                <para>
                Reads <quote>;alias=ip~port~transport</quote> from Contact header then
                writes to target pseudo-variable as a sip uri.
                </para>
                <para>
                This function can be used from
                REQUEST_ROUTE, ONREPLY_ROUTE, BRANCH_ROUTE, and FAILURE_ROUTE.
                </para>
                <example>
                <title><function>set_alias_to_pv</function> usage</title>
                <programlisting format="linespecific">
...
                set_alias_to_pv("$avp(aliasuri)");
...
                </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>RPC Commands</title>
                <section id="nathelper.r.enable_ping">
                        <title><function moreinfo="none">nathelper.enable_ping</function></title>
                        <para>
                        Enables natping if parameter value different 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">nathelper.enable_ping</function> usage</title>
                        <programlisting format="linespecific">
...
$ &kamcmd; nathelper.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>