.. _internetRouter:

Internet Router configuration
=============================

The Internet Router is the gateway between the Synthetic Internet (SI)
and the internal network (or, network under test).

The Internet Router includes some configuration beyond
packet-forwarding, so that it decrements IP packets' time-to-live
(TTL) field, giving the resulting traffic the illusion of more routing
hops than are actually present in the SI.  It adjusts packets' TTLs
using firewall rules, which can be examined using the command

>>> iptables -t mangle -n -L

The firewall rules are assigned in the first place by associating a
TTL decrement value with each IP subnet that contains one or more
IP addresses in the SI.

In order to do this, the Internet Router needs to know the IP subnets
for which it will change TTL values.  These are the subnets for the
`Synthetic Internet's IP addresses <../si>`_, which are enumerated on
that host in ``/usr/local/tg/etc/ipfakes``.  While these are the
actual IP addresses on the SI, the size of the subnet containing them
is defined on the *Internet Router*, in
``/usr/local/tg/etc/sysconfig/si-ttldec`` (the ``MASKLEN`` setting).
The location of the SI's IP addresses on the *Internet Router* is also
defined in the same file (the ``IPFAKES`` settings).

To configure TTL decrementing:

#. Get the configuration file locations

   >>> source /usr/local/tg/etc/sysconfig/si-ttldec

#. Copy the SI's ``ipfakes/`` directory onto the Internet Router

   >>> scp -R si:/usr/local/tg/etc/ipfakes $IPFAKES

#. Start the TTL decrementing

   >>> systemctl start SkaionTTLDecrement.service

#. Reload the TTL decrementing configuration

   >>> systemctl restart SkaionTTLDecrement.service

#. Stop the TTL decrementing

   >>> systemctl stop SkaionTTLDecrement.service