Question

I am a beginner to networking and I have a few questions regarding networking.
1)How can a process execute code that is sent from a different computer on the network. Generally a process’s code segment cannot be changed once its loaded to ensure protection. (Also I can execute some arbitrary code to corrupt the process’s memory)
2)Also can a process hear to multiple ports ? And multiple processes can hear to a same port ? For example two https associated with port 80. How to distinguish between the processes and how to ensure protection ?
3)Also I would like to know how listen is implemented in sockets. Are they implemented as software interrupts ?

Any good book recommendations are very much appreciated.

Thanks & Regards,

Mousey.

Answer 1

Q: How can a process execute code sent from another machine?
A: Generally, this is a bad idea as the security concerns are difficult to fully explore. However, this can be done by saving the network-delivered code to a separate executable and then launching this new program. This can also be done on most systems by just treating the raw bytes received as code; load the bytes into the heap (not the stack!), cast the address to a function pointer, and call it. Again though, this is almost certainly a bad idea.

Q: Can a process listen on multiple ports simultaneously?
A: Yes. By the way, HTTPS is port 443. HTTP is port 80.

Q: Can multiple processes listen on the same port (with the same protocol, on the same address)?
A: No. Other processes might be able to eavesdrop and also receive the packets, but they’re not directly bound to the port. In general, only one process can be bound to a given protocol/port/address 3-tuple.

Q: How is blocking while listening on a socket implemented?
A: By the operating system, in its own fashion. Generally a thread is moved into the “blocking” state when it calls accept, read, or poll/select on a non-ready socket, and will not receive CPU time until some data have arrived.