Laboratory III

¿ Qué comandos serían necesarios ejecutar para que un sistema Linux pudiese sustituir el encaminador R2 mostrado en el diagrama ? Asume todos aquellos datos que necesites para realizar el ejercicio (nombre de interfaces, gateway, etc)

Network Diagram
Click for larger view

Previous Steps

Enable IP Forwarding


echo 1 > /proc/sys/net/ipv4/ip_forward


vim /etc/sysctl.conf

changue the value of  net.ipv4.ip_forward = 1

sysctl -p /etc/sysctl.conf # enable  changues

Configuring network intefaces


ifconfig eth0 down

ifconfig eth0 netmask up
ifconfig eth1 down

ifconfig eth1 netmask up


on debian: edit /etc/network/interfaces like this

auto lo

iface lo inet loopback

iface eth0 inet static

iface eth1 inet static

red hat and derivates: edit /etc/sysconfig/network-scripts/ifcfg-<interface name>

Device eth0 file /etc/sysconfig/network-scripts/ifcfg-eth0


Device eth1 file /etc/sysconfig/network-scripts/ifcfg-eth1



Option 1: Using Static Routing


#from network 2 to network 3 assumed not necessary

#from network 3 to network 2 assumed not necessary

#from network 3 to network 1

ip route add via dev eth0


on Debian

edit /etc/network/interfaces

write this at after the interfaces setup

up route add via dev eth0

on Fedora

edit /etc/sysconfig/network-scripts/route-<device>


if you want to add more routes increment the numbers next to GATEWAY, for example: GATEWAY1= NETMASK1= ADDRESS1=


  • No extra processing and added resources as in the case of dynamic routing protocols
  • No extra bandwidth requirement caused by the transmission of excessive packets for the routing table update process
  • Extra security by manually admitting or rejecting routing to certain networks


  • Network Administrators need to know the complete network topology very well in order to configure routes correctly
  • Topology changes need manual adjustment to all routers something which is very time consuming

Option 2: Using NAT

Basically NAT works like static routing but changes the output ip maintaining a internal


# delete old configuration, if any
#Flush all the rules in filter and nat tables
iptables –flush
iptables –table nat –flush

# delete all chains that are not in default filter and nat table, if any
iptables –delete-chain
iptables –table nat –delete-chain

# Set up IP FORWARDing and Masquerading (NAT)
iptables –table nat –append POSTROUTING –out-interface eth0 -j MASQUERADE
iptables –append FORWARD –in-interface eth1 -j ACCEPT


store the rules into the ip tables into a rules set


same that static plus

It also benefits in a security sense as attackants can’t target a computer directly, they have to first get past the router.


  • Network Address Translation does not allow a true end-to-end connectivity that is required by some real time applications. A number of real-time applications require the creation of a logical tunnel to exchange the data packets quickly in real-time. It requires a fast and seamless connectivity devoid of any intermediaries such as a proxy server that tends to complicate and slow down the communications process.
  • NAT creates complications in the functioning of Tunneling protocols. Any communication that is routed through a Proxy server tends to be comparatively slow and prone to disruptions. Certain critical applications offer no room for such inadequacies. Examples include telemedicine and teleconferencing. Such applications find the process of network address translation as a bottleneck in the communication network creating avoidable distortions in the end-to-end connectivity.
  • NAT acts as a redundant channel in the online communication over the Internet. The twin reasons for the widespread popularity and subsequent adoption of the network address translation process were a shortage of IPv4 address space and the security concerns. Both these issues have been fully addressed in the IPv6 protocol. As the IPv6 slowly replaces the IPv4 protocol, the network address translation process will become redundant and useless while consuming the scarce network resources for providing services that will be no longer required over the IPv6 networks.

Option 3: Using RIP

Rip is a distance routing protocol, is more flexible that using static routers and necessary if the number of subnets grows. Do you want to fight against hundred of rules? or assume the risk of downtime’s created by a router malfunction?

install zebra


edit the /etc/zebra/ripd.conf file

redistribute connected

version 2

ip rip authentication string “max 16 characters”

router rip


  • Easy to configure and use
  • V2 supports VLSM and CIDR


  • Converges slowly on large networks
  • Doesn’t recognize bandwidth of links
  • Doesn’t support multiple paths for the same route
  • Routing updates can require significant bandwidth because the entire routing table is sent
  • Prone to routing loops

Option 4: Using OSPF (Open Shortest Path First)

OSPF is a routing protocol that uses the Dijkstra algorithm for get the quickest way. into a set of subnets where the routers are connected at different speeds could work better than R.I.P.

install zebra

add the necessary VTY in  /etc/services

zebrasrv        2600/tcp             # zebra service
zebra           2601/tcp              # zebra vty
ospfd           2604/tcp              # OSPFd vty
ospf6d          2606/tcp              # OSPF6d vty

edit zebra.conf file

hostname R2
password zebra
enable password z3bRa
log file /var/log/zebra/zebra.log
interface eth0
description Network 2
ip address
interface eth1
description Network 3
ip address

start zebra service

/usr/sbin/zebra –dk
/usr/sbin/ospfd –d

Telnet to port 2604 on the local machine to begin the OSPF configuration and type enable in order to get privileged mode

the next step will be announce the networks that we want  to publicity  in out networks

R2:~# telnet 0 2604
Connected to 0.
Escape character is ‘^]’.

Hello, this is zebra (version 0.84b)
Copyright 1996-2000 Kunihiro Ishiguro

User Access Verification

ospfd> enable
ospfd# configure terminal
ospfd(config)# router ospf
ospfd(config-router)# network area 0
ospfd(config-router)# passive-interface eth0

ospfd(config-router)# network area 0
ospfd(config-router)# passive-interface eth1
ospfd(config-router)# end
ospfd# write file
Configuration saved to /etc/zebra/ospfd.conf


  • Scalability – OSPF is specifically designed to operate with larger networks.
  • Full subnetting support – OSPF can fully support subnetting
  • Hello packets – OSPF uses small hello packets to verify link operation with out transferring large tables
  • TOS routing – OSPF can route packets by different criterion based on their type of service field
  • Tagged routes – Routes can be tagged with arbitrary values, easing interoperation.


  • very intensive processor
  • maintaining multiple copies of routing information, increasing the amount of memory needed
  • OSPF can be logically segmented by using areasnot as easy to learn as some other protocols
  • if an entire network is running OSPF, and one link within it is “bouncing” every few seconds, then OSPF updates would dominate the network by informing every other router every time the link changed state.