Page 68 - DCAP106_OPERATING_SYSTEM_TOOLS

Unit 3: Connecting to the Internet

Self Assessment Notes

Fill in the blanks:
1. ....................................... is a machine that matches IP addresses with hostnames.

2. The tab “.......................................” is used to associate a physical device with a network
connection.
3. When you specify an ISDN modem as the device type, the ....................................... Tool
presents a list of supported ISDN modems.
4. Specify the ....................................... of your network or specify Auto if you want to connect
to any available network.
5. The DNS tab of the ....................................... Tool lets you configure DNS.
6. Translating a hostname to an IP address is called ........................................
7. Translating an IP address to a hostname is called .......................................
8. Using the tab, you can specify the hostname of your system, and the ......................................
of primary, secondary, and tertiary DNS servers.

3.2 Managing Multiple ISPs and Connections

There are various places where multiple Internet connections can be managed by a linux based
router/masquerading device. Here, we will discuss some of the more common setups concerning
multiple Internet connections and how to manage them with iptables, ipchains, and iproute2. One
of the first distinctions you can make when planning how to use multiple Internet connections
is what inbound services you expect to host and how you want to split traffic over the multiple
links.

Now we will discuss the issues involved with two separate uplinks to two different providers.
Let us assume the following:

z z You are not using BGP, and you do not have your own AS. If you are using BGP and have
your own AS, you have a different set of problems than the problems described here.
z z You have two netblocks from two different ISPs.

z z You are funneling your internal network through this routing device, which is performing
masquerading/NAT to the Internet.

3.2.1 Outbound Traffic Using Multiple Connections to the Internet

There are two major uses for multiple Internet links connected to the same internal network.
Selecting an outbound link based on the type of outbound service is the one common use. The
other is to split traffic arbitrarily across multiple ISPs for reasons like failover and to accommodate
greater aggregate bandwidth than would be available on a single uplink.
Here, we will discuss how to classify traffic for different ISPs, how to handle the packet filtering
for this sort of classification scheme, and how to create routing tables appropriate for the task at
hand.
The simplest way to split Internet access into two separate groups is by source IP of the outbound
packet. This can be done most simply with ip rule and a second routing table.

Example: Assume that masq-gw in the example network gets a second, low cost network
connection through a DSL vendor.

LOVELY PROFESSIONAL UNIVERSITY 61