path: root/mta_recv.go

package main

import (
	"bufio"
	"bytes"
	"slices"
	"strings"
)

type server_state_t uint

const (
	server_state_begin server_state_t = iota
	server_state_helo
	server_state_mail
	server_state_rcpt
)

func handle_incoming_server_connection(reader *bufio.Reader, writer *bufio.Writer) error {
	_, _ = writer.WriteString("220 " + config.Server_name + " " + VERSION + "\r\n")
	_ = writer.Flush()
	server_state := server_state_begin
	var remote_server_name string
	var current_mail_from string
	var current_rcpt_to []string
	for {
		line, err := reader.ReadString('\n')
		if err != nil {
			return err
		}
		line = strings.TrimSuffix(line, "\n")
		line = strings.TrimSuffix(line, "\r")
		cmd_end := strings.IndexByte(line, ' ')
		var param_start int
		if cmd_end == -1 {
			cmd_end = len(line)
			param_start = len(line)
		} else {
			param_start = cmd_end + 1
		}
		cmd := strings.ToUpper(line[:cmd_end])
		param := line[param_start:]
	switch_cmd:
		switch cmd {
		case "HELO":
			if param == "" { // TODO: actually validate the hostname
				_, _ = writer.WriteString("501 Syntax: HELO hostname\r\n")
				_ = writer.Flush()
				break
			}
			remote_server_name = param
			_ = remote_server_name // TODO
			server_state = server_state_helo
			_, _ = writer.WriteString("250 " + config.Server_name + "\r\n")
			_ = writer.Flush()
		case "MAIL":
			switch server_state {
			case server_state_begin:
				_, _ = writer.WriteString("503 5.5.1 Error: send HELO/EHLO first\r\n")
				_ = writer.Flush()
				break switch_cmd
			case server_state_helo:
				break
			case server_state_mail:
				_, _ = writer.WriteString("503 5.5.1 Error: nested MAIL command\r\n")
				_ = writer.Flush()
				break switch_cmd
			}
			if len(param) <= len("FROM:") || strings.ToUpper(param[:len("FROM:")]) != "FROM:" {
				_, _ = writer.WriteString("501 5.5.4 Syntax: MAIL FROM:<address>\r\n")
				_ = writer.Flush()
				break
			}
			current_mail_from = param[len("FROM:"):]
			current_rcpt_to = []string{}
			server_state = server_state_mail
			_, _ = writer.WriteString("250 2.1.0 Ok\r\n")
			_ = writer.Flush()
			// TODO: Address validation
		case "RCPT":
			if server_state != server_state_mail && server_state != server_state_rcpt {
				_, _ = writer.WriteString("503 5.5.1 Error: need MAIL command\r\n")
				_ = writer.Flush()
				break
			}
			if len(param) <= len("TO:") || strings.ToUpper(param[:len("TO:")]) != "TO:" {
				_, _ = writer.WriteString("501 5.5.4 Syntax: RCPT TO:<address>\r\n")
				_ = writer.Flush()
				break
			}
			current_rcpt_to = append(current_rcpt_to, param[len("TO:"):])
			server_state = server_state_rcpt
			_, _ = writer.WriteString("250 2.1.5 Ok\r\n")
			_ = writer.Flush()
		case "DATA":
			if server_state != server_state_rcpt {
				_, _ = writer.WriteString("503 5.5.1 Error: need RCPT command\r\n")
				_ = writer.Flush()
				break
			}
			_, _ = writer.WriteString("354 End data with <CR><LF>.<CR><LF>\r\n")
			_ = writer.Flush()
			var current_data []byte
			for {
				tmp, err := reader.ReadSlice('\r')
				if err != nil {
					return err
				}

				// reader.ReadSlice returns an internal buffer that gets
				// overwritten on the next reader operation. So we must
				// make a copy; also we have to allocate data_part to
				// the correct length because [[builtin.copy]] copies
				// min(len(dst), len(src)) items.
				data_part := make([]byte, len(tmp))
				copy(data_part, tmp)

				next_four, err := reader.Peek(4)
				if err != nil {
					return err
				}
				if bytes.Equal(next_four, []byte{'\n', '.', '\r', '\n'}) {
					current_data = slices.Concat(current_data, data_part[:len(data_part)-1])
					break
				}
				current_data = slices.Concat(current_data, data_part)
			}
			_, err := reader.Discard(4)
			if err != nil {
				return err
			}
			err = deliver_incoming(current_mail_from, current_rcpt_to, current_data)
			if err == nil {
				_, _ = writer.WriteString("250 2.0.0 Ok: Accepted\r\n")
			} else {
				_, _ = writer.WriteString("500 2.0.0 Error: "+err.Error()+"\r\n")
			}
			_ = writer.Flush()
			server_state = server_state_helo
		case "QUIT":
			_, _ = writer.WriteString("221 2.0.0 Bye\r\n")
			_ = writer.Flush()
			return nil
		case "NOOP":
			_, _ = writer.WriteString("250 2.0.0 Ok\r\n")
			_ = writer.Flush()
		case "RSET":
			if server_state != server_state_begin {
				server_state = server_state_helo
			}
			_, _ = writer.WriteString("250 2.0.0 Ok\r\n")
			_ = writer.Flush()
		default:
			_, _ = writer.WriteString("500 5.5.2 Error: command not recognized\r\n")
			_ = writer.Flush()
		}
	}
}