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Wireless network programming in Unix (question)


 
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# 1  
Old 08-21-2010
Wireless network programming in Unix (question)

Greetings everyone,

I've been using OpenWRT for some time primarly for research in the University. I've also started with some basic network programming (sending UDP packets for instance). But since most of the available tutorials on unix network programming are more related to ethernet programming, I'm not sure if using it for wireless interfaces would be the same. I would like to know if there's any difference between ethernet and wireless raw socket programming.

I would also like to do some raw packets sniffing on a wireless interface "eth1". What structures should I use in order to read the prism headers in monitor mode? I've taken a look here and I can see that it's ok to use these structures:

Code:
struct prism_value
{
  uint32 did;
  uint16 status;
  uint16 len;
  uint32 data;
};

struct prism_header
{
  uint32 msgcode;
  uint32 msglen;
  u8char devname[16];
  struct prism_value hosttime;
  struct prism_value mactime;
  struct prism_value channel;
  struct prism_value rssi;
  struct prism_value sq;
  struct prism_value signal;
  struct prism_value noise;
  struct prism_value rate;
  struct prism_value istx;
  struct prism_value frmlen;
};

Has anyone done some wireless network programming/scripting?

I'm running OpenWRT on a WRT54G-TM, Broadcom BCM5352 chip, Linux 2.4.35.4.

thank you
# 2  
Old 08-22-2010
Quote:
But since most of the available tutorials on unix network programming are more related to ethernet programming, I'm not sure if using it for wireless interfaces would be the same. I would like to know if there's any difference between ethernet and wireless raw socket programming.
If by "ethernet" programming you mean opening a file descriptor and binding it to a socket in order to establish a TCP connection, or send UDP (connectionless) datagrams, then whether the underlying datalink layer is ethernet (802.3) or wireless (802.11) makes no difference. This is commonly what is referred to as "socket programming."

Capturing 'raw' datalink layer packets using something like BPF (Berkeley Packet Filter) doesn't require any special knowledge about the underlying datalink layer as the BPF device interface takes care of the differences. You will need to know how to parse the packets that the filter delivers. I'm assuming that the library you referenced (libpcap) provides a similar interface and from the brief look at the documentation it provides the necessary header files to make the interpretation of the data straight forward. My experience is limited to using BPF on both 802.3 and 802.11, so I cannot speak to your question about libpcap structures; sorry.

Two links that might provide some good information:
http://en.wikipedia.org/wiki/OSI_model The OSI networking model
http://www.gsp.com/cgi-bin/man.cgi?section=9&topic=bpf BPF man page
# 3  
Old 08-23-2010
The computer gets to control which network it connects to, which is very different from ethernet over copper, but once it's actually connected it acts just like any network. UDP tutorials and the like all apply perfectly fine to wireless --as well it should, otherwise we'd all need special new wireless-enabled clients and servers for mundane things like browsing the web!

This is because it's done in layers. The only program on your computer that needs to care whether it's wireless ethernet or not is the one that picks which wireless network connection your card connects to. Other programs just treat it like any other ethernet connection.

Last edited by Corona688; 08-23-2010 at 03:40 PM..
# 4  
Old 08-23-2010
Thank you very much for replying.

So sending data over sockets is a very low level approach. Structures are used to organize the data (bits or hex values).

Now, what if I'm unable to "organize" or "cast" such incomming traffic (in the case of packet sniffing with raw sockets) to a structure, but instead having to manually interpret the hex values in C? How can I "cast" these values in such case? (ie. I get: 77 6c 30 00 00 1c 10 c4 2d 70 08 00 45..." in hex, how do I convert it to actual data?)

thank you
# 5  
Old 08-23-2010
You can convert a string to a byte like this:

Code:
const char *hex="5d";
unsigned char byte;

sscanf(hex, "%02hhx", &byte);

[COLOR="#738fbf"]

---------- Post updated at 03:21 PM ---------- Previous update was at 03:19 PM ----------

As for putting the card into monitor mode, your information's as good as mine.
 

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