ftp-server

Pseudo code

1. Loop through available fds
2. check the type of fd:
   • listen_sock_cntl: accept the connection and watch the conn_sock
   • conn_sock_cntl:
     • work on the previous request, until EAGAIN
     • read the request into buffer, until EAGAIN; on full buffer, clear it
       • when \r\n is detected, try to handle the request
   • listen_sock_data: accept the new PASV connection
   • conn_sock_data: no matter of PASV or PORT;
     • check for upload / download

Commands and implementation

Note: CMD sets last_fail according to status code, and goto CMD.

Part 1

• USER:
  • WORK_RESPONSE_0:
    • if not anonymous then set fail msg
  • RESPONSE_0:
    • try to send response; on fully written, goto CMD
    • if fail == true: goto CMD
• PASS:
  • WORK_RESPONSE_0:
    • reject if last_cmd is not a USER or last failed
  • RESPONSE_0:
    • try to send response; on fully written, goto CMD
• RNFR:
  • WORK_RESPONSE_0:
    • 450 on ENOENT, 350 otherwise
  • RESPONSE_0:
    • try to send response; on fully written, goto CMD
• RNTO:
  • WORK_RESPONSE_0:
    • reject with 503 if last is not RNFR or last failed
  • RESPONSE_0:
    • try to send response; on fully written, goto CMD
• CWD / MKD / RMD / DELE:
  • WORK_RESPONSE_0:
    • check: root + 'path' inside root & path exists
    • generate response
  • RESPONSE_0:
    • try to send response; on fully written, goto CMD
• SYST:
  • WORK_RESPONSE_0:
    • generate response directly: 215 UNIX Type: L8
  • RESPONSE_0:
    • try to send response; on fully written, goto CMD
• TYPE:
  • WORK_RESPONSE_0:
    • generate response directly:
      • 200 Type set to I. if TYPE I
      • 504 Unsupported type otherwise
  • RESPONSE_0:
    • try to send response; on fully written, goto CMD
• PWD:
  • WORK_RESPONSE_0:
    • generate response directly
  • RESPONSE_0:
    • try to send response; on fully written, goto CMD
• QUIT:
  • WORK_RESPONSE_0:
    • goto QUIT directly

Part 2

• PASV:
  • WORK_RESPONSE_0
    • close current data connection if any;
    • if current mode is pasv, unregister pasv listen fd and close it
    • then create a new socket
    • bind it, listen to it (with only 1 allowed client) and register it with epoll (EPOLLIN); set to nonblock for accept()
    • set mode to PASV
    • make response text
  • RESPONSE_0:
    • try to send response; on fully written, goto CMD
  • Note: try accepting a new data connection iff
    • mode is pasv
    • data connection filedes is -1
    • then epoll read on it
• PORT:
  • WORK_RESPONSE_0:
    • close current data connection if any;
    • if current mode is pasv, unregister pasv listen fd and close it
    • then create a new nonblocking socket, but do not connect
    • save the client sockaddr (remember htonl etc.) for further use
  • RESPONSE_0:
    • try to send response; on fully written, goto CMD
• LIST:
  • WORK_RESPONSE_0:
    • fstat on cwd (or arg if any)
    • generate cntl output
  • RESPONSE_0:
    • try to send cntl response; on fully written, goto WORK_DATA
  • WORK_DATA:
    • prepare DIR*
    • generate ls-like output, write to data buf
    • goto DATA_BUF
  • DATA_BUF:
    • try to send data from buf, while generating more data;
      • on fully written, close the connection(-1) and generate 226 write buf on cntl, then goto RESPONSE_1
  • RESPONSE_1:
    • try writing the buf of cntl; on fully written, goto CMD
• RETR:
  • WORK_RESPONSE_0:
    • check stat & generate message
  • RESPONSE_0:
    • send response and continue if all done
    • on all done: goto WORK_DATA
  • WORK_DATA:
    • open file fd
    • if mode is PORT
      • connect()
      • epoll -> wait on writable
      • goto WORK_DATA, and return
    • else
      • check if client is connected
      • epoll -> wait on writable
    • if fails, check close, make error response & goto RESPONSE_1
  • DATA_SENDFILE:
    • try sendfile(), until done
    • if done, goto RESPONSE_1
  • RESPONSE_1
    • close 2x data conn + unreg epoll & generate response &
    • send all response, on done, goto CMD
• STOR:
  • WORK_RESPONSE_0:
    • check stat & generate message
  • RESPONSE_0:
    • send response and continue if all done
    • on all done: goto WORK_DATA
  • WORK_DATA:
    • open file fd
    • if mode is PORT
      • connect()
      • epoll -> wait on writable
      • goto WORK_DATA, and return
    • else
      • check if client is connected
      • epoll -> wait on writable
    • if fails, check close, make error response & goto RESPONSE_1
  • DATA_BUF:
    • try read into buffer, write file, until done
    • if done, close 2x data conn + unreg epoll & generate response & goto RESPONSE_1
    • if error, close 2x data conn + unreg epoll, generate new response on cntl & goto RESPONSE_1
  • RESPONSE_1
    • send all response, on done, goto CMD

error handling

• epoll on HUP of control connection, and close data connection accordingly
• TELNET strings

use a state machine for each client

For each command or command sequence there are three possible
outcomes: success (S), failure (F), and error (E).  In the state
diagrams below we use the symbol B for "begin", and the symbol W for
"wait for reply".

           ------------------------------------
          |                                    |
  Begin   |                                    |
    |     V                                    |
    |   +---+  cmd   +---+ 2         +---+     |
     -->|   |------->|   |---------->|   |     |
        |   |        | W |           | S |-----|
     -->|   |     -->|   |-----      |   |     |
    |   +---+    |   +---+ 4,5 |     +---+     |
    |     |      |    | |      |               |
    |     |      |   1| |3     |     +---+     |
    |     |      |    | |      |     |   |     |
    |     |       ----  |       ---->| F |-----
    |     |             |            |   |
    |     |             |            +---+
     -------------------
          |
          |
          V
         End

Debugging

QUIT, ABOR, PASV, PWD, RETR, STOR, PORT, MKD, CWD, LIST, RMD, RNFR, RNTO, DELE,