socket-examples - network socket example programs

This is a collection of network socket example programs. They range from trivial echo programs to more complex packet manipulation programs.

The simplest "hello, world" socket program is the echo client/server. The client connects to the server and any input it sends is echoed back.

(terminal 1)
./server
Listening on port 5123

(terminal 2)
./client server.com 5123
abcd
abcd
(input is echoed back, server is working)

Examples are given for TCP, UDP and UNIX domain sockets.

The calc client sends a mathematical expression to the `calc server which then calculates the answer and returns the result.

(terminal 1)
$ ./calc_server 5001

(terminal 2)
$ ./calc_client localhost 5001
Enter expression:1+1
Answer: 2
...

The oneftp program transfers one file from the client to the server. It shows how to properly detect the end of a large transfer.

(terminal 1)
$ ./oneftp_server out.dat

(terminal 2)
$ ./oneftp_client localhost in.dat

Automatic repeat request (ARQ) is the general idea of resending a packet that has been lost. The arq program uses a socket library which is unreliable. It then employs a stop and wait ARQ scheme to overcome these errors and reliably perform transfers.

(terminal 1)
./snw-server
Port: 16245

(terminal 2)
./snw-client localhost 16245 data
(output will be in data.out)

md5sum data.*
(should be identical)

The packets program uses the libpcap library, like Wireshark does, to monitor the packets being transferred on an interface. It then displays information about the packets such as the addresses and type.

$ sudo ./packets wlan0
Capturing on interface 'wlan0'
48:5b:39:5b:6b:87 -> 1:80:c2:0:0:0 [len:46] 
48:5b:39:5b:6b:87 -> 1:80:c2:0:0:0 [len:46] 
0:1a:70:5a:6e:9 -> 8c:70:5a:83:2b:64 [ARP] 192.168.2.1 requests 192.168.2.113 
8c:70:5a:83:2b:64 -> 0:1a:70:5a:6e:9 [ARP] 192.168.2.113 at 8c:70:5a:83:2b:64 
48:5b:39:5b:6b:87 -> 1:80:c2:0:0:0 [len:46] 
48:5b:39:5b:6b:87 -> 1:80:c2:0:0:0 [len:46] 
8c:70:5a:83:2b:64 -> 0:1a:70:5a:6e:9 [IPv4] 192.168.2.113 -> 8.8.8.8 
0:1a:70:5a:6e:9 -> 8c:70:5a:83:2b:64 [IPv4] 8.8.8.8 -> 192.168.2.113 
8c:70:5a:83:2b:64 -> 0:1a:70:5a:6e:9 [IPv4] 192.168.2.113 -> 149.20.4.69 
0:1a:70:5a:6e:9 -> 8c:70:5a:83:2b:64 [IPv4] 149.20.4.69 -> 192.168.2.113 
8c:70:5a:83:2b:64 -> 0:1a:70:5a:6e:9 [IPv4] 192.168.2.113 -> 8.8.8.8 
0:1a:70:5a:6e:9 -> 8c:70:5a:83:2b:64 [IPv4] 8.8.8.8 -> 192.168.2.113 

The arp_resolver program takes an IP address as input and tries to resolve the MAC address by sending out ARP packets.

$ sudo ./arp_resolver wlan0
Enter next IP address: 192.168.2.1
MAC: AB:CD:EF:00:12:34
Enter next IP address: 8.8.8.8
MAC: Lookup failed
Enter next IP address:
$

IMPORTANT Do not run this program on a public network. This could be seen as an attack and you could be prosecuted by the authorities.

When an ARP request is sent asking for the MAC address for a given IP it should only receive one response. The arp_responder will maliciously create a response for addresses it is not necessarily authoritative for.

Given a file of IP address and MAC addresses

192.168.2.189  C0:04:AB:43:22:FF
192.168.2.190  D4:DE:AD:BE:EF:FF

this program will respond to any requests for these entries.

$ sudo ./arp_responder eth0 addresses.txt
^C (quit)
$

See the README included with arp_responder for more information.

Copyright © 2014, Jeremiah Mahler. All Rights Reserved.
This project is free software and released under the GNU General Public License.