Theory

In this episode I decided to take a slight diversion into networking fundamentals. As before, if you want to learn more about installing the ONICS tool suite, go back and listen to HPR 2882.

There are three key concepts to understand about modern networks. They are:

digital - the networks carry bits and bytes (binary digits)

packet switched - devices break data into blobs of data called "packets" and take turns sending and receiving those packets to/from other devices attached to the network

internetworked -- machines communicate using a protocol that allows traffic to traverse across multiple, independently-managed networks in a uniform way

My Setup

2 laptops connected to a home wifi network that has Internet connectivity.

Practicing sending data from a source machine to a destination machine. Both are running Linux.

Source machine:

Wifi interface: wlan0

Ethernet address: 00:22:fa:a7:69:90

IP address: 192.168.0.4

Destination machine

Wifi interface: wlo1

Ethernet address: 6c:88:14:7c:2e:14

IP address: 192.168.0.248

Internet Router:

Ethernet address: 00:0d:b9:23:f2:51

IP address: 192.168.0.1

More Terminology

Address - a number that identifies a machine's interface in a network

Packet - a blob of binary data sent as a unit over a network

Route - a rule that specifies how to forward traffic to a given address

Router / Gateway - a machine that uses the IP protocol and forwards traffic between multiple networks that it connects to

Network Protocol - a set of rules and data formats for exchanging information over a network

Standard UNIX Commands

ifconfig (no arguments or '-a')

list interfaces on a machine

ifconfig IFNAME

list properites about a given interface

ping -c 1 IPADDRESS

send an echo request to machine IPADDRESS

arp -na

Dump the Ethernet addresses of known nearby machines

netstat -nr

Dump the routes in a system

netstat -nr | grep "^0.0.0.0"

Find the route (and thus IP address) of the default gateway

ONICS Commands in this Episode

rawpkt - take a blob of data and wrap it in an XPKT format (so other ONICS tools can understand what it is)

ethwrap - take an XPKT and prepend an Ethernet header to it

ipwrap - take an XPKT and prepend an IP header to it

pktin - read a stream of packets from a network interface

pflt - filter a stream of packets so that only those matching a pattern get through

pktout - send a stream of packets to a network interface

x2hpkt - convert XPKTs into a hex dump

xpktdump - like x2hpkt, but send the output to a pager like 'less' for easy reading

Sending an Ethernet Packet to the Destination

On the receiver:

$ sudo pktin wlo1 |

pflt "not ip and eth.dst == 6c:88:14:7c:2e:14" |

x2hpkt

On the sender:

$ echo "hello world" |

rawpkt |

ethwrap "eth.dst = 6c:88:14:7c:2e:14; "

"eth.src = 00:22:fa:a7:69:90; "

"eth.ethtype = 12;" |