Packet Sniffing

Hi everyone,

I need some professional input from folks that have experience with Internet security, especially packet sniffing and IP spoofing. I know how packet sniffing works and I can read and decipher the TCP/IP packet for information I want. My question is, can you packet sniff on someone else gateway without actually being on their WAN or LAN?

Secondly, I have some experience on how you can change your host file on most Unix machine and add fake routes but what I do want to know is how do spammers inject fake IP addresses in an SMTP message or how do people send fake IP headers to others. I am asking this question for educational purposes only and I will appreciate real world examples and answers.

Thanks

Of course you can packet sniff on a network other than yours, Its called GRE sniffing or something like that, you know about subnet masks , It comes into play here. You can get a video sample at http://www.hackingdefined.com/index.php/Demos. You sound like someone who wants to learn about computers in a different way,
Thats quite good.
 IP spoofing is as a result of creating raw packets. Raw packets are created by creating asocket and specifying it as a raw socket. Instead of SOCK_STREAM, you type in SOCK_RAW as the parameter in yoursocket declaration. Hey, you get to fill out the stuff yourself and this requires a basic understanding of an IP  packet  structure. You might do a google on raw sockets or GRE sniffing.
A good example of GRE sniffing was given in the book 'Stealing the continent, How to own the box' where Hex a female hackese breaks into a school network ( in short sha, she messes with the subnet mask, and changes the default gateway to point to her own router, then instructs her own router to send back replies to the schools LAN. I think so, )