Send Email with STARTTLS - sockets

Question

I’m trying to send an email with the STARTTLS command. I have setup a test account in Gmail and set it up to only accept inbound email with a TLS connection.
For reasons I don’t want to go into I can’t use JavaMail or other email libraries.
I have been able to send emails to this test account with using openssl. So I know that the account has been setup properly.
Example that worked: openssl s_client -starttls smtp -crlf -connect aspmx.l.google.com:25
I also have been able to send emails to this email account using a .Net application incorporating TLS.
I know that my example (below) is not the proper way to send emails, because I’m not reacting on the server’s response, but I thought this is a good/short way to create an example to demonstrate the problem.
I have been trying for a while to get this to work. I have tried connecting with different ports (465, 587, 25) with similar results. The error that I get is on the command “AUTH LOGIN”, but I’m already not getting any response from the server at my previous command “EHLO aspmx.l.google.com”.
The error that I’m getting is: “Error: Software caused connection abort: socket write error”.
Am I on the right path to negotiating a TLS connection to transmit an email or am I missing something obvious?
Any help will be much appreciated.
Example:
import java.io.BufferedReader;
import java.io.DataOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.Socket;
import javax.net.ssl.SSLSocket;
import javax.net.ssl.SSLSocketFactory;
import javax.xml.bind.DatatypeConverter;
public class SendEmailWithTLSConnectionTest {
private static DataOutputStream dos;
private static BufferedReader out = null;
public static void main(String[] args) throws Exception
{
try
{
int delay = 1000;
String username = DatatypeConverter.printBase64Binary("leo#tls.calcium.co.nz".getBytes());
String password = DatatypeConverter.printBase64Binary("2wsxZAQ!".getBytes());
Socket sock = new Socket("aspmx.l.google.com", 25);
out = new BufferedReader(new InputStreamReader(sock.getInputStream()));
(new Thread(new Runnable()
{
public void run()
{
while(true)
{
try
{
if(out != null)
{
String line;
while((line = out.readLine()) != null)
{
System.out.println("SERVER: "+line);
}
}
}
catch (IOException e)
{
System.out.println("IOException SERVER! Error: " + e);
try {
Thread.sleep(1000 * 5);
} catch (InterruptedException e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
}
}
}
}
})).start();
dos = new DataOutputStream(sock.getOutputStream());
send("EHLO aspmx.l.google.com\r\n");
Thread.sleep(delay * 5);
send("STARTTLS\r\n");
Thread.sleep(delay * 5);
SSLSocket sslSocket = (SSLSocket) ((SSLSocketFactory) SSLSocketFactory.getDefault()).createSocket(
sock,
sock.getInetAddress().getHostAddress(),
587,
true);
sslSocket.setUseClientMode(true);
sslSocket.setEnableSessionCreation(true);
// Thread.sleep(delay * 5);
// sslSocket.startHandshake();
send("EHLO aspmx.l.google.com\r\n");
Thread.sleep(delay * 5);
send("AUTH LOGIN\r\n");
Thread.sleep(delay * 5);
send(username + "\r\n");
Thread.sleep(delay * 5);
send(password + "\r\n");
Thread.sleep(delay * 5);
send("MAIL FROM: <leo#tls.calcium.co.nz>\r\n");
Thread.sleep(delay * 5);
send("RCPT TO: <leo#tls.calcium.co.nz>\r\n");
Thread.sleep(delay * 5);
send("DATA\r\n");
Thread.sleep(delay * 5);
send("Test 1 2 3");
Thread.sleep(delay * 5);
send("\r\n.\r\n");
Thread.sleep(delay * 5);
send("QUIT\r\n");
}
catch(Exception ex)
{
System.out.println("Exception when sending out test. Error: " + ex.getMessage());
}
}
private static void send(String s) throws Exception
{
dos.writeBytes(s);
System.out.println("CLIENT: "+s);
}
}
Output:
SERVER: 220 mx.google.com ESMTP on10si24036122pac.132 - gsmtp
CLIENT: EHLO aspmx.l.google.com
SERVER: 250-mx.google.com at your service, [103.23.17.19]
SERVER: 250-SIZE 35882577
SERVER: 250-8BITMIME
SERVER: 250-STARTTLS
SERVER: 250-ENHANCEDSTATUSCODES
SERVER: 250-PIPELINING
SERVER: 250-CHUNKING
SERVER: 250 SMTPUTF8
CLIENT: STARTTLS
SERVER: 220 2.0.0 Ready to start TLS
CLIENT: EHLO aspmx.l.google.com
Exception when sending out test. Error: Software caused connection abort: socket write error

Answer 1

Am I on the right path to negotiating a TLS connection to transmit an email or am I missing something obvious?
You are missing important steps.
Most SMTP servers implement STARTTLS only on port 587, though some servers also implement it on port 25 as well (Gmail does). You must parse the server's EHLO response to know whether STARTTLS is allowed or not.
After you receive a successful STARTTLS response, you must initiate and complete an SSL/TLS handshake before then sending any further SMTP commands. You are not doing that step (you commented out the call to SSLSocket.startHandshake()). The server is expecting a handshake hello from you, but you are sending a new EHLO command instead, which the server interprets as a bad handshake and closes the connection, which gets reported to you when you send the AUTH LOGIN command.
Also, you are connecting to port 25, but then telling SSLSocketFactory that you connected to port 587 instead. You need to be consistent.
Also, once you have established the SSL/TLS session, you can't use the original Socket for reading/sending anymore. You would be sending unencrypted data directly to the server, and reading back the server's raw encrypted data. You must use the SSLSocket instead, so it can encrypt whatever you send and decrypt whatever you read. So, you will have to reinitialize your input/output streams accordingly (and get rid of your reading thread altogether, as it does not belong in this code. SMTP is synchronous - send a command, read a response, send a command, read a response, etc).
You need something more along the lines of this:
import java.io.BufferedReader;
import java.io.DataOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.Socket;
import javax.net.ssl.SSLSocket;
import javax.net.ssl.SSLSocketFactory;
import javax.xml.bind.DatatypeConverter;
public class SendEmailWithTLSConnectionTest {
private static DataOutputStream dos;
private static BufferedReader out = null;
public static void main(String[] args) throws Exception
{
try
{
String username = DatatypeConverter.printBase64Binary("leo#tls.calcium.co.nz".getBytes());
String password = DatatypeConverter.printBase64Binary("2wsxZAQ!".getBytes());
Socket sock = new Socket("aspmx.l.google.com", 587);
out = new BufferedReader(new InputStreamReader(sock.getInputStream()));
dos = new DataOutputStream(sock.getOutputStream());
if (sendCmd("EHLO aspmx.l.google.com") == 250)
{
// TODO: parse response
if (true/*response contains STARTTLS capability*/)
{
sendCmd("STARTTLS", 220);
SSLSocket sslSocket = (SSLSocket) ((SSLSocketFactory) SSLSocketFactory.getDefault()).createSocket(
sock,
sock.getInetAddress().getHostAddress(),
sock.getPort(),
true);
sslSocket.setUseClientMode(true);
sslSocket.setEnableSessionCreation(true);
System.out.println("CLIENT: securing connection");
sslSocket.startHandshake();
// on an initial handshake, startHandshake() blocks the calling
// thread until the handshake is finished...
System.out.println("CLIENT: secured");
sock = sslSocket;
out = new BufferedReader(new InputStreamReader(sock.getInputStream()));
dos = new DataOutputStream(sock.getOutputStream());
sendCmd("EHLO aspmx.l.google.com", 250);
}
}
else
sendCmd("HELO aspmx.l.google.com", 250);
sendCmd("AUTH LOGIN", 334);
if (sendCmd(username, new int[]{235, 334}) == 334)
sendCmd(password, 235);
sendCmd("MAIL FROM: <leo#tls.calcium.co.nz>", 250);
sendCmd("RCPT TO: <leo#tls.calcium.co.nz>", new int[]{250, 251});
sendCmd("DATA", 354);
sendLine("From: <leo#tls.calcium.co.nz>");
sendLine("To: <leo#tls.calcium.co.nz>");
sendLine("Subject: test");
sendLine("");
sendLine("Test 1 2 3");
sendCmd(".", 250);
sendCmd("QUIT", 221);
}
catch(Exception ex)
{
System.out.println("Exception when sending out test. Error: " + ex.getMessage());
}
}
private static void sendLine(String s) throws Exception
{
dos.writeBytes(s + "\r\n");
System.out.println("CLIENT: " + s);
}
private static int sendCmd(String s) throws Exception
{
sendLine(s);
String line = out.readLine();
System.out.println("SERVER: " + line);
int respCode = Integer.parseInt(line.substring(0, 3));
while ((line.length() > 3) && (line.charAt(3) == '-'))
{
line = out.readLine();
System.out.println("SERVER: " + line);
}
return respCode;
}
private static int sendCmd(String s, int expectedRespCode) throws Exception
{
int respCode = sendCmd(s);
checkResponse(respCode, expectedRespCode);
return respCode;
}
private static int sendCmd(String s, int[] expectedRespCodes) throws Exception
{
int respCode = sendCmd(s);
checkResponse(respCode, expectedRespCodes);
return respCode;
}
private static void checkResponse(int actualRespCode, int expectedRespCode)
{
if (actualRespCode != expectedRespCode)
throw new Exception("command failed");
}
private static void checkResponse(int actualRespCode, int[] expectedRespCodes)
{
for (int i = 0; i < expectedRespCodes.length; ++i)
{
if (actualRespCode == expectedRespCode)
return;
}
throw new Exception("command failed");
}
}

Answer 2

I have adjusted the answer from above to a working version.
I insert this below so it might be helpful to somebody else. Thank you to Remy Lebeau for your guidance.
package com.mailprimer.smtp.sender;
import java.io.BufferedReader;
import java.io.DataOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.Socket;
import javax.net.ssl.SSLSocket;
import javax.net.ssl.SSLSocketFactory;
import javax.xml.bind.DatatypeConverter;
public class SendEmailWithTLSConnectionTest {
private static DataOutputStream dos;
private static BufferedReader out = null;
public static void main(String[] args) throws Exception
{
try
{
String username = DatatypeConverter.printBase64Binary("leo#tls.calcium.co.nz".getBytes());
String password = DatatypeConverter.printBase64Binary("XXXXXXXXXX".getBytes());
Socket sock = new Socket("aspmx.l.google.com", 25);
out = new BufferedReader(new InputStreamReader(sock.getInputStream()));
dos = new DataOutputStream(sock.getOutputStream());
int responseCode = sendCommand("EHLO aspmx.l.google.com", 250);
if ( responseCode == 250)
{
// TODO: parse response
if (true/*response contains STARTTLS capability*/)
{
sendCmd("STARTTLS", 220);
SSLSocket sslSocket = (SSLSocket) ((SSLSocketFactory) SSLSocketFactory.getDefault()).createSocket(
sock,
sock.getInetAddress().getHostAddress(),
sock.getPort(),
true);
sslSocket.setUseClientMode(true);
sslSocket.setEnableSessionCreation(true);
System.out.println("CLIENT: securing connection");
sslSocket.startHandshake();
// on an initial handshake, startHandshake() blocks the calling
// thread until the handshake is finished...
System.out.println("CLIENT: secured");
sock = sslSocket;
out = new BufferedReader(new InputStreamReader(sock.getInputStream()));
dos = new DataOutputStream(sock.getOutputStream());
sendCmd("EHLO aspmx.l.google.com", 250);
}
}
else
{
sendCmd("HELO aspmx.l.google.com", 250);
}
sendCmd("MAIL FROM: <leo#tls.calcium.co.nz>", 250);
sendCmd("RCPT TO: <leo#tls.calcium.co.nz>", new int[]{250, 251});
sendCmd("DATA", 354);
sendLine("From: <leo#tls.calcium.co.nz>");
sendLine("To: <leo#tls.calcium.co.nz>");
sendLine("Subject: test");
sendLine("");
sendLine("Test 1 2 3");
sendCmd(".", 250);
sendCmd("QUIT", 221);
}
catch(Exception ex)
{
System.out.println("Exception when sending out test. Error: " + ex.getMessage());
}
}
private static void sendLine(String s) throws Exception
{
dos.writeBytes(s + "\r\n");
System.out.println("CLIENT: " + s);
}
private static int sendCommand(String s, int expectedRespCode) throws Exception
{
sendLine(s);
String line = out.readLine();
System.out.println("SERVER: " + line);
// Need to wait a little longer until the other response is finished.
Thread.sleep(100);
int respCode = Integer.parseInt(line.substring(0, 3));
if(expectedRespCode > 0)
{
while ((line.length() > 3) && ((line.charAt(3) == '-') || respCode != expectedRespCode))
{
line = out.readLine();
System.out.println("SERVER: " + line);
respCode = Integer.parseInt(line.substring(0, 3));
// Need to wait a little longer until the other response is finished.
Thread.sleep(100);
}
}
return respCode;
}
private static int sendCmd(String s, int expectedRespCode) throws Exception
{
int respCode = sendCommand(s, expectedRespCode);
checkResponse(respCode, expectedRespCode);
return respCode;
}
private static int sendCmd(String s, int[] expectedRespCodes) throws Exception
{
int respCode = sendCommand(s, 0);
checkResponse(respCode, expectedRespCodes);
return respCode;
}
private static void checkResponse(int actualRespCode, int expectedRespCode) throws Exception
{
if (actualRespCode != expectedRespCode)
throw new Exception("command failed");
}
private static void checkResponse(int actualRespCode, int[] expectedRespCodes) throws Exception
{
for (int i = 0; i < expectedRespCodes.length; ++i)
{
int expectedRespCode = expectedRespCodes[i];
if (actualRespCode == expectedRespCode)
return;
}
throw new Exception("command failed");
}
}