Initial commit

checker/starttls_smtp.go

@@ -0,0 +1,86 @@
+package checker
+
+import (
+	"bufio"
+	"fmt"
+	"net"
+	"strings"
+)
+
+func init() {
+	registerStartTLS("smtp", starttlsSMTP)
+	registerStartTLS("submission", starttlsSMTP)
+}
+
+// starttlsSMTP implements ESMTP EHLO + STARTTLS (RFC 3207).
+func starttlsSMTP(conn net.Conn, sni string) error {
+	rw := bufio.NewReadWriter(bufio.NewReader(conn), bufio.NewWriter(conn))
+
+	if err := readSMTPGreeting(rw.Reader); err != nil {
+		return fmt.Errorf("read greeting: %w", err)
+	}
+
+	if _, err := rw.WriteString("EHLO checker.happydomain.org\r\n"); err != nil {
+		return fmt.Errorf("write ehlo: %w", err)
+	}
+	if err := rw.Flush(); err != nil {
+		return fmt.Errorf("flush ehlo: %w", err)
+	}
+	lines, err := readSMTPResponse(rw.Reader)
+	if err != nil {
+		return fmt.Errorf("read ehlo: %w", err)
+	}
+	if !hasSTARTTLSExt(lines) {
+		return fmt.Errorf("%w: EHLO did not advertise STARTTLS", errStartTLSNotOffered)
+	}
+
+	if _, err := rw.WriteString("STARTTLS\r\n"); err != nil {
+		return fmt.Errorf("write starttls: %w", err)
+	}
+	if err := rw.Flush(); err != nil {
+		return fmt.Errorf("flush starttls: %w", err)
+	}
+	resp, err := readSMTPResponse(rw.Reader)
+	if err != nil {
+		return fmt.Errorf("read starttls: %w", err)
+	}
+	if len(resp) == 0 || !strings.HasPrefix(resp[0], "220") {
+		return fmt.Errorf("server refused STARTTLS: %s", strings.Join(resp, " / "))
+	}
+	return nil
+}
+
+func readSMTPGreeting(r *bufio.Reader) error {
+	_, err := readSMTPResponse(r)
+	return err
+}
+
+// readSMTPResponse reads one multi-line SMTP response (lines with "NNN-" are
+// continuation, "NNN " terminates).
+func readSMTPResponse(r *bufio.Reader) ([]string, error) {
+	var out []string
+	for {
+		line, err := r.ReadString('\n')
+		if err != nil {
+			return out, err
+		}
+		line = strings.TrimRight(line, "\r\n")
+		out = append(out, line)
+		if len(line) < 4 || line[3] == ' ' {
+			return out, nil
+		}
+	}
+}
+
+func hasSTARTTLSExt(lines []string) bool {
+	for _, l := range lines {
+		if len(l) < 4 {
+			continue
+		}
+		rest := strings.ToUpper(strings.TrimSpace(l[4:]))
+		if rest == "STARTTLS" || strings.HasPrefix(rest, "STARTTLS ") {
+			return true
+		}
+	}
+	return false
+}