From 789c969491f765a08edc8398ecf5ca8700d13248 Mon Sep 17 00:00:00 2001
From: Robert Sesek <rsesek@bluestatic.org>
Date: Mon, 12 Dec 2016 00:57:40 -0500
Subject: [PATCH] A basic SMTP server.

---
 config.go      |   6 +-
 smtp.go        |   4 +
 smtp/conn.go   | 200 ++++++++++++++++++++++++++++++++++++++++++++++++-
 smtp/server.go |   1 +
 4 files changed, 204 insertions(+), 7 deletions(-)

diff --git a/config.go b/config.go
index 8f3c734..c50104b 100644
--- a/config.go
+++ b/config.go
@@ -4,6 +4,9 @@ type Config struct {
 	SMTPPort int
 	POP3Port int
 
+	// Hostname is the name of the MX server that is running.
+	Hostname string
+
 	Servers []Server
 }
 
@@ -11,9 +14,6 @@ type Server struct {
 	// Domain is the second component of a mail address: <local-part@domain.com>.
 	Domain string
 
-	// Hostname is the name of the MX server that is running.
-	Hostname string
-
 	TLSKeyPath  string
 	TLSCertPath string
 
diff --git a/smtp.go b/smtp.go
index b7131d5..92ac2e5 100644
--- a/smtp.go
+++ b/smtp.go
@@ -39,6 +39,10 @@ func (server *smtpServer) run() {
 	}
 }
 
+func (server *smtpServer) Name() string {
+	return server.config.Hostname
+}
+
 func (server *smtpServer) OnEHLO() error {
 	return nil
 }
diff --git a/smtp/conn.go b/smtp/conn.go
index e4734ae..f07afe1 100644
--- a/smtp/conn.go
+++ b/smtp/conn.go
@@ -1,11 +1,203 @@
 package smtp
 
 import (
-	"io"
+	"fmt"
 	"net"
+	"net/mail"
+	"net/textproto"
 )
 
-func AcceptConnection(conn net.Conn, server Server) error {
-	conn.Close()
-	return nil
+type state int
+
+const (
+	stateNew state = iota // Before EHOL.
+	stateInitial
+	stateMail
+	stateRecipient
+	stateData
+)
+
+type connection struct {
+	tp         *textproto.Conn
+	remoteAddr net.Addr
+
+	state
+	line string
+
+	ehlo     string
+	mailFrom *mail.Address
+	rcptTo   []mail.Address
+}
+
+func AcceptConnection(netConn net.Conn, server Server) error {
+	conn := connection{
+		tp:         textproto.NewConn(netConn),
+		remoteAddr: netConn.RemoteAddr(),
+		state:      stateNew,
+	}
+
+	var err error
+
+	conn.writeReply(250, fmt.Sprintf("%s ESMTP [%s] mailpopbox", server.Name(), netConn.LocalAddr().String()))
+
+	for {
+		conn.line, err = conn.tp.ReadLine()
+		if err != nil {
+			conn.writeReply(500, "line too long")
+			continue
+		}
+
+		var cmd string
+		if _, err = fmt.Sscanf(conn.line, "%s", &cmd); err != nil {
+			conn.writeBadSyntax()
+			continue
+		}
+
+		switch cmd {
+		case "QUIT":
+			conn.tp.Close()
+			break
+		case "HELO":
+			fallthrough
+		case "EHLO":
+			conn.doEHLO()
+		case "MAIL":
+			conn.doMAIL()
+		case "RCPT":
+			conn.doRCPT()
+		case "DATA":
+			conn.doDATA()
+		case "RSET":
+			conn.doRSET()
+		case "VRFY":
+			conn.doVRFY()
+		case "EXPN":
+			conn.writeReply(550, "access denied")
+		case "NOOP":
+			conn.writeOK()
+		case "HELP":
+			conn.writeReply(250, "https://tools.ietf.org/html/rfc5321")
+		default:
+			conn.writeReply(500, "unrecognized command")
+		}
+	}
+
+	return err
+}
+
+func (conn *connection) writeReply(code int, msg string) {
+	if len(msg) > 0 {
+		conn.tp.PrintfLine("%d %s", code, msg)
+	} else {
+		conn.tp.PrintfLine("%d", code)
+	}
+}
+
+func (conn *connection) writeOK() {
+	conn.writeReply(250, "OK")
+}
+
+func (conn *connection) writeBadSyntax() {
+	conn.writeReply(501, "syntax error")
+}
+
+func (conn *connection) writeBadSequence() {
+	conn.writeReply(503, "bad sequence of commands")
+}
+
+func (conn *connection) doEHLO() {
+	conn.resetBuffers()
+
+	var cmd string
+	_, err := fmt.Sscanf(conn.line, "%s %s", &cmd, &conn.ehlo)
+	if err != nil {
+		conn.writeBadSyntax()
+		return
+	}
+
+	conn.writeReply(250, fmt.Sprintf("Hello %s, I am glad to meet you", conn.ehlo))
+
+	conn.state = stateInitial
+}
+
+func (conn *connection) doMAIL() {
+	if conn.state != stateInitial {
+		conn.writeBadSequence()
+		return
+	}
+
+	var mailFrom string
+	_, err := fmt.Sscanf(conn.line, "MAIL FROM:%s", &mailFrom)
+	if err != nil {
+		conn.writeBadSyntax()
+		return
+	}
+
+	conn.mailFrom, err = mail.ParseAddress(mailFrom)
+	if err != nil {
+		conn.writeBadSyntax()
+		return
+	}
+
+	conn.state = stateMail
+	conn.writeOK()
+}
+
+func (conn *connection) doRCPT() {
+	if conn.state != stateMail && conn.state != stateRecipient {
+		conn.writeBadSequence()
+		return
+	}
+
+	var rcptTo string
+	_, err := fmt.Sscanf(conn.line, "RCPT TO:%s", &rcptTo)
+	if err != nil {
+		conn.writeBadSyntax()
+		return
+	}
+
+	address, err := mail.ParseAddress(rcptTo)
+	if err != nil {
+		conn.writeBadSyntax()
+	}
+
+	conn.rcptTo = append(conn.rcptTo, *address)
+
+	conn.state = stateRecipient
+	conn.writeOK()
+}
+
+func (conn *connection) doDATA() {
+	if conn.state != stateRecipient {
+		conn.writeBadSequence()
+		return
+	}
+
+	conn.writeReply(354, "Start mail input; end with <CRLF>.<CRLF>")
+
+	data, err := conn.tp.ReadDotBytes()
+	if err != nil {
+		// TODO: log error
+		conn.writeReply(552, "transaction failed")
+		return
+	}
+
+	fmt.Println(string(data))
+
+	conn.state = stateInitial
+	conn.writeOK()
+}
+
+func (conn *connection) doVRFY() {
+}
+
+func (conn *connection) doRSET() {
+	conn.state = stateInitial
+	conn.resetBuffers()
+	conn.writeOK()
+}
+
+func (conn *connection) resetBuffers() {
+	conn.mailFrom = nil
+	conn.rcptTo = make([]mail.Address, 0)
 }
diff --git a/smtp/server.go b/smtp/server.go
index e9539e8..fe935ef 100644
--- a/smtp/server.go
+++ b/smtp/server.go
@@ -1,6 +1,7 @@
 package smtp
 
 type Server interface {
+	Name() string
 	OnEHLO() error
 	OnMessageDelivered() error
 }
-- 
2.22.5