Add sample 'findgw' scripts to the Multi-ISP doc

Signed-off-by: Tom Eastep <teastep@shorewall.net>

docs/MultiISP.xml

@@ -342,6 +342,33 @@
             may try to use those stale files to determine the gateway
             address.</para>
 
+            <para>If Shorewall is unable to detect the gateway, it is likely
+            because you are using a DHCP client that Shorewall doesn't
+            natively support. You can work around that issue by using the
+            <emphasis role="bold">findgw</emphasis> <ulink
+            url="shorewall_extension_scripts.htm">extension
+            script.</ulink></para>
+
+            <para>For example, these examples from Mika Ilmaranta, work with
+            RHEL7-based systems with nmcli:</para>
+
+            <programlisting>nmcli --terse --fields IP6.GATEWAY device show ${1} | cut -f2- -d':' # IPv6
+
+nmcli --terse --fields IP4.GATEWAY device show ${1} | cut -f2- -d':' #IPv4
+</programlisting>
+
+            <para>This one from PGNd works on OpenSuSE running wicked:</para>
+
+            <programlisting>svc_status=$( systemctl is-active wickedd-dhcp4.service )
+
+if [ $svc_status == 'active' ]; then
+  data="/var/lib/wicked/lease-${1}-dhcp-ipv4.xml"
+  if [ -f $data ]; then
+    gateway=$( xml_grep 'gateway' $data --text_only )
+    echo $gateway
+  fi
+fi</programlisting>
+
             <para>The GATEWAY may be omitted (enter '-') for point-to-point
             links.</para>
           </listitem>