75.1 TLS для SMTP: STARTTLS и неявный TLS (465)
- SmtpServer: STARTTLS (RFC 3207) — SslHandler ставится сразу после того, как ответ 220 ушёл открытым текстом; сессия начинается заново; строки, присланные открытым текстом вместе со STARTTLS, не выполняются, соединение закрывается (CVE-2011-0411); свежий декодер строк без остатков открытого текста - неявный TLS на SMTPS_PORT (RFC 8314); AUTH — только внутри TLS (или SMTP_AUTH_WITHOUT_TLS); Received: ESMTPS/ESMTPSA - SmtpTls: свой сертификат (MAIL_TLS_CERT/KEY) или самоподписанный, создаётся один раз в MAIL_TLS_DIR (в образе /app/data/tls); TLS 1.2/1.3 - тесты: STARTTLS + AUTH, подмешанный открытый текст, JavaMail со starttls.required, порт 465, переиспользование сертификата (38 всего) - проверено вживую: openssl s_client -starttls smtp и порт 465, TLSv1.3 α Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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@@ -1,13 +1,22 @@
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package com.mailserver.smtp;
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import io.netty.buffer.ByteBuf;
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import io.netty.buffer.ByteBufUtil;
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import io.netty.channel.ChannelHandlerContext;
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import io.netty.channel.ChannelPipeline;
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import io.netty.handler.codec.LineBasedFrameDecoder;
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import io.netty.handler.codec.MessageToMessageDecoder;
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import io.netty.handler.ssl.SslContext;
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import io.netty.handler.ssl.SslHandler;
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import io.netty.handler.timeout.ReadTimeoutHandler;
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import org.slf4j.Logger;
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import org.slf4j.LoggerFactory;
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import reactor.core.publisher.Flux;
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import reactor.core.publisher.Mono;
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import reactor.core.scheduler.Schedulers;
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import reactor.netty.Connection;
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import reactor.netty.DisposableServer;
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import reactor.netty.NettyOutbound;
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import reactor.netty.tcp.TcpServer;
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import java.io.ByteArrayOutputStream;
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@@ -32,7 +41,10 @@ import java.util.concurrent.TimeUnit;
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* <li>the message is stored or queued before the client gets 250 — on failure 451 and the client retries;</li>
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* <li>database and DNS work runs on a worker thread, not on the event loop.</li>
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* </ul>
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* No STARTTLS yet: AUTH is offered only when {@code authWithoutTls} is on (a closed network: inside a VM or a docker network).
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* TLS (RFC 3207 STARTTLS, or implicit TLS on a separate port, RFC 8314): with a {@code tls} context STARTTLS is
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* offered; AUTH only over TLS, or without it when {@code authWithoutTls} is on (a closed network: VM, docker network).
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* After STARTTLS the session starts over (new EHLO), and lines sent in plain text together with STARTTLS are not
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* executed — the connection is closed (command injection, CVE-2011-0411).
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*/
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public class SmtpServer {
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@@ -42,9 +54,32 @@ public class SmtpServer {
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static final int MAX_LINE = 64 * 1024;
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static final int MAX_RECIPIENTS = 100;
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private static final int MAX_BAD_COMMANDS = 20;
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// Compared by identity: TLS starts right after exactly this reply is written
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private static final String STARTTLS_READY = new String("220 2.0.0 Ready to start TLS\r\n");
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/**
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* @param tls certificate for STARTTLS / implicit TLS; null — no TLS
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* @param implicitTls TLS from the first byte (port 465), no STARTTLS
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*/
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public record Settings(String host, int port, String hostname, long maxMessageBytes, boolean authWithoutTls,
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Duration idleTimeout) {
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Duration idleTimeout, SslContext tls, boolean implicitTls) {
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public Settings(String host, int port, String hostname, long maxMessageBytes, boolean authWithoutTls,
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Duration idleTimeout) {
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this(host, port, hostname, maxMessageBytes, authWithoutTls, idleTimeout, null, false);
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}
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}
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/** A line of input and whether it came over TLS. */
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record Line(byte[] bytes, boolean tls) {
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}
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/** Marks each line: decoded behind an SslHandler (encrypted) or not. */
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private static final class LineTagger extends MessageToMessageDecoder<ByteBuf> {
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@Override
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protected void decode(ChannelHandlerContext ctx, ByteBuf frame, List<Object> out) {
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out.add(new Line(ByteBufUtil.getBytes(frame), ctx.pipeline().get(SslHandler.class) != null));
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}
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}
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private final Settings settings;
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@@ -57,32 +92,70 @@ public class SmtpServer {
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}
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public void start() {
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server = TcpServer.create()
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if (settings.implicitTls() && settings.tls() == null) {
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throw new IllegalStateException("Implicit TLS needs a certificate");
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}
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TcpServer tcp = TcpServer.create()
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.host(settings.host())
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.port(settings.port())
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.doOnConnection(connection -> connection
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.addHandlerFirst("smtp-idle", new ReadTimeoutHandler(settings.idleTimeout().toSeconds(), TimeUnit.SECONDS))
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.addHandlerLast("smtp-lines", new LineBasedFrameDecoder(MAX_LINE, true, false)))
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.addHandlerLast("smtp-lines", new LineBasedFrameDecoder(MAX_LINE, true, false))
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.addHandlerLast("smtp-tag", new LineTagger()));
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if (settings.implicitTls()) {
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tcp = tcp.secure(spec -> spec.sslContext(settings.tls()));
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}
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server = tcp
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.handle((inbound, outbound) -> {
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Connection[] connection = {null};
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String[] remoteIp = {"unknown"};
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inbound.withConnection(c -> {
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connection[0] = c;
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if (c.address() instanceof InetSocketAddress address && address.getAddress() != null) {
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remoteIp[0] = address.getAddress().getHostAddress();
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}
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});
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Session session = new Session(remoteIp[0]);
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Flux<String> replies = inbound.receive().asByteArray()
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Session session = new Session(remoteIp[0], settings.implicitTls());
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Flux<String> replies = inbound.receiveObject().cast(Line.class)
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.concatMap(session::onLine)
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.onErrorResume(e -> {
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log.debug("SMTP {}: connection closed: {}", session.remoteIp, e.toString());
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return Mono.just("421 4.4.2 " + settings.hostname() + " closing connection\r\n");
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})
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.takeUntil(reply -> session.closing);
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return outbound.sendString(Flux.concat(
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Mono.just("220 " + settings.hostname() + " ESMTP ready\r\n"), replies), StandardCharsets.US_ASCII);
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// One reply at a time, each written before the next line is handled: after "220 Ready to start TLS"
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// the TLS handler goes in only when the reply has left in plain text
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return Flux.concat(Mono.just("220 " + settings.hostname() + " ESMTP ready\r\n"), replies)
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.concatMap(reply -> send(outbound, reply)
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.then(Mono.defer(() -> reply == STARTTLS_READY ? startTls(connection[0], session) : Mono.<Void>empty())))
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.then();
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})
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.bindNow(Duration.ofSeconds(10));
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log.info("SMTP server listening on {}:{} ({})", settings.host(), port(), settings.hostname());
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log.info("SMTP server listening on {}:{} ({}{})", settings.host(), port(), settings.hostname(),
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settings.implicitTls() ? ", implicit TLS" : settings.tls() != null ? ", STARTTLS" : ", no TLS");
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}
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private static Mono<Void> send(NettyOutbound outbound, String reply) {
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return outbound.sendString(Mono.just(reply), StandardCharsets.US_ASCII).then();
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}
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/** TLS handler in front, fresh line decoder (no plain-text leftovers mixed into encrypted input). */
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private Mono<Void> startTls(Connection connection, Session session) {
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ChannelPipeline pipeline = connection.channel().pipeline();
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SslHandler ssl = settings.tls().newHandler(connection.channel().alloc());
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pipeline.replace("smtp-lines", "smtp-lines", new LineBasedFrameDecoder(MAX_LINE, true, false));
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pipeline.addFirst("smtp-tls", ssl);
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return Mono.create(sink -> ssl.handshakeFuture().addListener(future -> {
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if (future.isSuccess()) {
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log.debug("SMTP {}: TLS {}", session.remoteIp, ssl.engine().getSession().getProtocol());
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sink.success();
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} else {
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log.info("SMTP {}: TLS handshake failed: {}", session.remoteIp, future.cause().toString());
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session.closing = true;
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connection.dispose();
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sink.success();
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}
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}));
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}
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public int port() {
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@@ -99,6 +172,7 @@ public class SmtpServer {
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private final class Session {
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final String remoteIp;
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boolean tls;
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State state = State.COMMAND;
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String helo;
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boolean extended;
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@@ -111,12 +185,17 @@ public class SmtpServer {
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int badCommands;
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volatile boolean closing;
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Session(String remoteIp) {
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Session(String remoteIp, boolean tls) {
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this.remoteIp = remoteIp;
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this.tls = tls;
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}
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Mono<String> onLine(byte[] raw) {
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byte[] line = stripCr(raw);
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Mono<String> onLine(Line input) {
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if (tls && !input.tls()) {
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// Sent in plain text after STARTTLS, before the handshake: never execute it
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return Mono.error(new IllegalStateException("plain-text input after STARTTLS"));
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}
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byte[] line = stripCr(input.bytes());
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return switch (state) {
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case DATA -> onDataLine(line);
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case AUTH_PLAIN -> authPlain(ascii(line));
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@@ -142,7 +221,7 @@ public class SmtpServer {
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}
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case "NOOP" -> reply("250 2.0.0 OK");
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case "VRFY" -> reply("252 2.5.0 Cannot VRFY user, but will accept message");
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case "STARTTLS" -> reply("454 4.7.0 TLS not available");
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case "STARTTLS" -> startTls(arg);
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case "QUIT" -> {
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closing = true;
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yield reply("221 2.0.0 " + settings.hostname() + " Bye");
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@@ -167,15 +246,41 @@ public class SmtpServer {
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sb.append("250-SIZE ").append(settings.maxMessageBytes()).append("\r\n");
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sb.append("250-8BITMIME\r\n");
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sb.append("250-PIPELINING\r\n");
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if (settings.authWithoutTls()) {
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if (settings.tls() != null && !tls) {
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sb.append("250-STARTTLS\r\n");
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}
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if (authAllowed()) {
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sb.append("250-AUTH PLAIN LOGIN\r\n");
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}
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sb.append("250 ENHANCEDSTATUSCODES\r\n");
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return Mono.just(sb.toString());
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}
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private boolean authAllowed() {
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return tls || settings.authWithoutTls();
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}
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private Mono<String> startTls(String arg) {
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if (settings.tls() == null) {
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return reply("454 4.7.0 TLS not available");
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}
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if (tls) {
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return bad("503 5.5.1 TLS already active");
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}
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if (!arg.isEmpty()) {
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return reply("501 5.5.4 Syntax: STARTTLS");
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}
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// RFC 3207 4.2: everything known about the client is forgotten, a new EHLO is expected
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tls = true;
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helo = null;
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extended = false;
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authenticatedUser = null;
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resetTransaction();
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return Mono.just(STARTTLS_READY);
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}
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private Mono<String> auth(String arg) {
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if (!settings.authWithoutTls()) {
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if (!authAllowed()) {
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return reply("538 5.7.11 Encryption required for requested authentication mechanism");
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}
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if (helo == null || !extended) {
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@@ -369,7 +474,7 @@ public class SmtpServer {
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private byte[] withReceivedHeader(byte[] message) {
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String received = "Received: from " + helo + " ([" + remoteIp + "])\r\n"
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+ "\tby " + settings.hostname() + " with " + (authenticatedUser != null ? "ESMTPA" : "ESMTP") + ";\r\n"
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+ "\tby " + settings.hostname() + " with " + (tls ? "ESMTPS" : "ESMTP") + (authenticatedUser != null ? "A" : "") + ";\r\n"
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+ "\t" + DateTimeFormatter.RFC_1123_DATE_TIME.format(ZonedDateTime.now()) + "\r\n";
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byte[] header = received.getBytes(StandardCharsets.US_ASCII);
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byte[] all = new byte[header.length + message.length];
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