</variablelist>
<para>If one or both security gateways are doing forwarding firewalling
(possibly including masquerading),
and this is specified using the firewall parameters,
tunnels established with IPsec are exempted from it
so that packets can flow unchanged through the tunnels.
(This means that all subnets connected in this manner must have
distinct, non-overlapping subnet address blocks.)
This is done by the default
<emphasis remap='I'>updown</emphasis>
script (see
<citerefentry><refentrytitle>ipsec_pluto</refentrytitle><manvolnum>8</manvolnum></citerefentry>).</para>

<para>The implementation of this makes certain assumptions about firewall setup,
and the availability of the 
<emphasis remap='I'>Linux Advanced Routing</emphasis> tools.
In situations calling for more control,
it may be preferable for the user to supply his own
<emphasis remap='I'>updown</emphasis>
script,
which makes the appropriate adjustments for his system.</para>
</refsect2>

