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Verify SMTP in PHP
<?php
$server = "smtp.gmail.com"; // Who I connect to
$myself = "my_server.example.com"; // Who I am
$cabundle = '/etc/ssl/cacert.pem'; // Where my root certificates are
// Verify server. There's not much we can do, if we suppose that an attacker
// has taken control of the DNS. The most we can hope for is that there will
// be discrepancies between the expected responses to the following code and
// the answers from the subverted DNS server.
// To detect these discrepancies though, implies we knew the proper response
// and saved it in the code. At that point we might as well save the IP, and
// decouple from the DNS altogether.
$match1 = false;
$addrs = gethostbynamel($server);
foreach($addrs as $addr)
{
$name = gethostbyaddr($addr);
if ($name == $server)
{
$match1 = true;
break;
}
}
// Here we must decide what to do if $match1 is false.
// Which may happen often and for legitimate reasons.
print "Test 1: " . ($match1 ? "PASSED" : "FAILED") . "\n";
$match2 = false;
$domain = explode('.', $server);
array_shift($domain);
$domain = implode('.', $domain);
getmxrr($domain, $mxhosts);
foreach($mxhosts as $mxhost)
{
$tests = gethostbynamel($mxhost);
if (0 != count(array_intersect($addrs, $tests)))
{
// One of the instances of $server is a MX for its domain
$match2 = true;
break;
}
}
// Again here we must decide what to do if $match2 is false.
// Most small ISP pass test 2; very large ISPs and Google fail.
print "Test 2: " . ($match2 ? "PASSED" : "FAILED") . "\n";
// On the other hand, if you have a PASS on a server you use,
// it's unlikely to become a FAIL anytime soon.
// End of maybe-they-help-maybe-they-don't checks.
// Establish the connection
$smtp = fsockopen( "tcp://$server", 25, $errno, $errstr );
fread( $smtp, 512 );
// Here you can check the usual banner from $server (or in general,
// check whether it contains $server's domain name, or whether the
// domain it advertises has $server among its MX's.
// But yet again, Google fails both these tests.
fwrite($smtp,"HELO $myself\r\n");
fread($smtp, 512);
// Switch to TLS
fwrite($smtp,"STARTTLS\r\n");
fread($smtp, 512);
stream_set_blocking($smtp, true);
stream_context_set_option($smtp, 'ssl', 'verify_peer', true);
stream_context_set_option($smtp, 'ssl', 'allow_self_signed', false);
stream_context_set_option($smtp, 'ssl', 'capture_peer_cert', true);
stream_context_set_option($smtp, 'ssl', 'cafile', $cabundle);
$secure = stream_socket_enable_crypto($smtp, true, STREAM_CRYPTO_METHOD_TLS_CLIENT);
stream_set_blocking($smtp, false);
$opts = stream_context_get_options($smtp);
if (!isset($opts["ssl"]["peer_certificate"]))
$secure = false;
else
{
$cert = openssl_x509_parse($opts["ssl"]["peer_certificate"]);
$names = '';
if ('' != $cert)
{
if (isset($cert['extensions']))
$names = $cert['extensions']['subjectAltName'];
elseif (isset($cert['subject']))
{
if (isset($cert['subject']['CN']))
$names = 'DNS:' . $cert['subject']['CN'];
else
$secure = false; // No exts, subject without CN
}
else
$secure = false; // No exts, no subject
}
$checks = explode(',', $names);
// At least one $check must match $server
$tmp = explode('.', $server);
$fles = array_reverse($tmp);
$okay = false;
foreach($checks as $check)
{
$tmp = explode(':', $check);
if ('DNS' != $tmp[0]) continue; // candidates must start with DNS:
if (!isset($tmp[1])) continue; // and have something afterwards
$tmp = explode('.', $tmp[1]);
if (count($tmp) < 3) continue; // "*.com" is not a valid match
$cand = array_reverse($tmp);
$okay = true;
foreach($cand as $i => $item)
{
if (!isset($fles[$i]))
{
// We connected to www.example.com and certificate is for *.www.example.com -- bad.
$okay = false;
break;
}
if ($fles[$i] == $item)
continue;
if ($item == '*')
break;
}
if ($okay)
break;
}
if (!$okay)
$secure = false; // No hosts matched our server.
}
if (!$secure)
die("failed to connect securely\n");
print "Success!\n";
// Continue with connection...
?>