tcp packet arrivat at the application?

0
votes

In the client-server environment, when client sends a packet (with source ip / dest ip / ports ... etc) requesting "GET /index.php ... etc",
at the server application (daemon) arrives the whole packet (the whole bits of data) including mac, IPs, ports, tcp flags, payload ? Or just the payload ?

Because I don;t understand how the scripts can read remote address (like echo $_SERVER['REMOTE_ADDR']; )

1
tcpclient-servertcp-ip

1 Answers

1
votes

The server machine gets the whole packet. Its kernel and TCP/IP stack receives and processes it. The application server is using a socket to talk to the kernel, which is a higher-layer interface than raw packets; therefore it has a different view. Assuming we are talking about TCP, you will find among other things:

  • Information from the physical or datalink layer (such as source and destination MAC addresses) is not available on the socket (unless you do very fancy and probably non-portable things).
  • Some information from the IP & TCP layer is made available so the application can retrieve it using special system calls such as getsockname() and getpeername(). This includes the IP addresses and ports.
  • The application is not concerned with most of the rest of the information from the IP & TCP layers and it is not made available on the socket. For example, options, window size, checksum, fragment offset.
  • The application sends and receives data on the socket as though it was a continuous stream of bytes. It does not know or care how the datastream is broken up into small packets each containing a piece of the data.

for the specific case of $_SERVER['REMOTE_ADDR']; which you highlight, this information comes from the aforementioned getpeername() system call. PHP calls this for you and makes the information available.