php - GMail 421 4.7.0 稍后再试，关闭连接

Question

我试图找出为什么它无法使用 GMail 从我的服务器发送邮件。为此，我使用 SwiftMailer，但我可以将问题附在以下隔离代码中:

<?php

$sock = stream_socket_client('tcp://smtp.gmail.com:587', $errno, $errstr);
if (false == $sock) {
    die('error');
}
$server_response = fread($sock, 4096);
echo $server_response . PHP_EOL;
fclose($sock);

当我在本地计算机(Windows 10、XAMPP、PHP 5.6.24)上运行此代码时，输​​出为:

220 smtp.gmail.com ESMTP qp16sm1358626ejb.89 - gsmtp

So 220 means everything is fine.

But when I run the exact same code on my server (Debian Jessie, PHP 5.6.38), the output is:

421 4.7.0 Try again later, closing connection.

GMail's error reference https://support.google.com/a/answer/3726730 doesn't give any advice for this error message.

I also tried this on the server console without PHP and it works fine too:

> nc smtp.gmail.com 587
$ 220 smtp.gmail.com ESMTP n11sm3188821wmi.15 - gsmtp

So I can delimit the problem

• It has nothing to do with any security settings of my GMail account (because no authentification is sent at this point)
• My server is not blacklisted on GMail (because the nc command works fine)

Are there any settings of the php.ini that can cause this error?

UPDATE

Just for fun I set up a simple TCP server on port 587 on another machine and display the IP address of any connected client. There is no different if I use the above PHP client code or connecting via nc command. I don't know what could be the different for the GMail's SMTP server while conneting via the client PHP code and the nc command.

UPDATE 2

While enabling verbose mode in the nc command, could DNS fwd/rev mismatch be a problem?

> nc -v smtp.gmail.com 587
$ DNS fwd/rev mismatch: smtp.gmail.com != wr-in-f109.1e100.net
$ smtp.gmail.com [108.177.15.109] 587 (submission) open
$ 220 smtp.gmail.com ESMTP z15sm4348737wrn.89 - gsmtp

UPDATE 3

Run the above code with

$sock = stream_socket_client('tcp://wr-in-f109.1e100.net:587', $errno, $errstr);

它有效，它以 220 响应。但问题是，SwiftMailer 在使用 stream_socket_enable_crypto() 后尝试建立 TLS 安全性。这失败了

Peer certificate CN=`smtp.gmail.com' did not match expected CN=`wr-in-f109.1e100.net'

UPDATE 4

It's getting me crazy. When I use Python's TCP sockets to connect to smtp.gmail.com, GMail responses with 220:

import socket

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(('smtp.gmail.com', 587))
data = s.recv(4096)
s.close()

print data

输出:

220 smtp.gmail.com ESMTP p6sm8392243wmg.0 - gsmtp

What is the difference between Python's sockets and PHP's stream_socket_client?

Edit: The same behavior with PHP's non-streaming socket function:

$socket = socket_create(AF_INET, SOCK_STREAM, SOL_TCP);
if(!is_resource($socket)) onSocketFailure("Failed to create socket");
socket_connect($socket, "smtp.gmail.com", 587)
        or onSocketFailure("Failed to connect", $socket);

$line = socket_read($socket, 1024, PHP_NORMAL_READ);

var_dump($line);
socket_close($socket);

更新 5
现在我认为这是IPv6的问题。我不是一个大 C 程序员，但我试图理解 PHP 的实现。
看一下

xp_socket.c函数php_tcp_sockop_connect用途 php_network_connect_socket_to_host

network.c在这里实现

它使用 php_network_getaddresses

而这个函数使用 getaddrinfo

php_network_connect_socket_to_host迭代主机地址列表并尝试连接到它。如果成功，它将停止循环。
我没有用 C 尝试过，但我认为下面的 Python 代码是一样的:

import socket
print socket.getaddrinfo("smtp.gmail.com", 587)

哪个打印

[
 (10, 1, 6, '', ('2a00:1450:400c:c0c::6d', 587, 0, 0)),
 (10, 2, 17, '', ('2a00:1450:400c:c0c::6d', 587, 0, 0)),
 (10, 3, 0, '', ('2a00:1450:400c:c0c::6d', 587, 0, 0)),
 (2, 1, 6, '', ('108.177.15.109', 587)),
 (2, 2, 17, '', ('108.177.15.109', 587)),
 (2, 3, 0, '', ('108.177.15.109', 587))
]

(I formatted the line for better reading)

As you can see, there are three IPv6 addresses on the top of the list and php_network_connect_socket_to_host try these IPv6 adresses first before it comes to the IPv4 addresses.

So what happens when I try to connect directly to the first IPv6 address?

<?php

$sock = stream_socket_client('tcp://[2a00:1450:400c:c02::6c]:587', $errno, $errstr);
if (false == $sock) {
    die('error');
}
$server_response = fread($sock, 4096);
echo $server_response . PHP_EOL;
fclose($sock);

输出是:
421 4.7.0 稍后重试，关闭连接。

这个SO问题与这个观察有关:Why I can't send emails using smtp.gmail.com:587 on ipv6?

Answer 1

发现后，当使用 IPv6 地址时会出现错误消息，黑客更喜欢 IPv4 地址。
根据https://serverfault.com/questions/511237/linux-block-ipv6-for-certain-applications-hostnames#answer-514397最简单的方法是将以下行添加到 etc/hosts :

108.177.15.109 smtp.gmail.com

此解决方案的缺点是静态 IP 地址。 smtp.gmail.com可能会更改 IP 地址。这就是为什么我不将此答案标记为已接受的原因。
不知道能不能配置 /etc/gai.conf 更喜欢 IPv4 而不是 IPv6 smtp.gmail.com .
这篇文章可能很有趣，详细了解 getaddrinfo 的工作原理:https://jameshfisher.com/2018/02/03/what-does-getaddrinfo-do/