FTP Explicit 在蜂窝数据上失败

Question

这个问题是this original question 的后续问题。在与 FTP 隐式解决了一段时间后，我再次决定研究这个问题。这一次，我用纯 Java（非安卓）编写了一个简单的 FTP 客户端，并在我通过手机的热点连接到互联网时分析了 SSL/TLS 调试消息。

问题发生在发送 ClientHello 后的 TLS 握手期间。就 FTP 而言，这对应于服务器成功接受 AUTH TLS 之后。交换失败表示如下：

*** ClientHello, TLSv1.2
RandomCookie:  GMT: 1416712655 bytes = { <binary data> }
Session ID:  {}
Cipher Suites: [TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256, TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256, TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA, TLS_DHE_RSA_WITH_AES_128_CBC_SHA256, TLS_DHE_RSA_WITH_AES_128_GCM_SHA256, TLS_DHE_RSA_WITH_AES_128_CBC_SHA, TLS_RSA_WITH_AES_128_CBC_SHA256, TLS_RSA_WITH_AES_128_CBC_SHA]
Compression Methods:  { 0 }
Extension elliptic_curves, curve names: {secp256r1, sect163k1, sect163r2, secp192r1, secp224r1, sect233k1, sect233r1, sect283k1, sect283r1, secp384r1, sect409k1, sect409r1, secp521r1, sect571k1, sect571r1, secp160k1, secp160r1, secp160r2, sect163r1, secp192k1, sect193r1, sect193r2, secp224k1, sect239k1, secp256k1}
Extension ec_point_formats, formats: [uncompressed]
Extension signature_algorithms, signature_algorithms: SHA512withECDSA, SHA512withRSA, SHA384withECDSA, SHA384withRSA, SHA256withECDSA, SHA256withRSA, SHA224withECDSA, SHA224withRSA, SHA1withECDSA, SHA1withRSA, SHA1withDSA, MD5withRSA
Extension server_name, server_name: [type=host_name (0), value=<server>]
Extension renegotiation_info, renegotiated_connection: <empty>
***
main, WRITE: TLSv1.2 Handshake, length = 188
main, handling exception: java.net.SocketTimeoutException: Read timed out
Exception in thread "main" java.net.SocketTimeoutException: Read timed out
    at java.net.SocketInputStream.socketRead0(Native Method)
    at java.net.SocketInputStream.read(SocketInputStream.java:150)
    at java.net.SocketInputStream.read(SocketInputStream.java:121)
    at sun.security.ssl.InputRecord.readFully(InputRecord.java:465)
main, called close()

作为参考，成功兑换如下：

*** ClientHello, TLSv1.2
RandomCookie:  GMT: 1416713014 bytes = { <binary data> }
Session ID:  {}
Cipher Suites: [TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256, TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256, TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA, TLS_DHE_RSA_WITH_AES_128_CBC_SHA256, TLS_DHE_RSA_WITH_AES_128_GCM_SHA256, TLS_DHE_RSA_WITH_AES_128_CBC_SHA, TLS_RSA_WITH_AES_128_CBC_SHA256, TLS_RSA_WITH_AES_128_CBC_SHA]
Compression Methods:  { 0 }
Extension elliptic_curves, curve names: {secp256r1, sect163k1, sect163r2, secp192r1, secp224r1, sect233k1, sect233r1, sect283k1, sect283r1, secp384r1, sect409k1, sect409r1, secp521r1, sect571k1, sect571r1, secp160k1, secp160r1, secp160r2, sect163r1, secp192k1, sect193r1, sect193r2, secp224k1, sect239k1, secp256k1}
Extension ec_point_formats, formats: [uncompressed]
Extension signature_algorithms, signature_algorithms: SHA512withECDSA, SHA512withRSA, SHA384withECDSA, SHA384withRSA, SHA256withECDSA, SHA256withRSA, SHA224withECDSA, SHA224withRSA, SHA1withECDSA, SHA1withRSA, SHA1withDSA, MD5withRSA
Extension server_name, server_name: [type=host_name (0), value=<server>]
Extension renegotiation_info, renegotiated_connection: <empty>
***
main, WRITE: TLSv1.2 Handshake, length = 188
main, READ: TLSv1.2 Handshake, length = 81
*** ServerHello, TLSv1.2
RandomCookie:  GMT: 142326665 bytes = { <binary data> }
Session ID:  { <session id data> }
Cipher Suite: TLS_RSA_WITH_AES_128_CBC_SHA256
Compression Method: 0
Extension renegotiation_info, renegotiated_connection: <empty>
***

我尝试过使用 TLSv1.2、TLSv1.1 和 TLSv1，都得到了相同的结果，即套接字超时并且服务器关闭了连接。 FTP 服务器（FileZilla Server 0.9.48 beta）没有提供该问题的详细日志。

非常感谢您对此问题的任何见解。

Answer 1

我建议您的提供商的流量“优化”会导致问题。用于“优化”流量的深度数据包检测对于蜂窝网络非常常见，通常用于改善流量，有时也用于阻止不需要的流量。

这与您的问题有何关系： 在蜂窝网络中，通常需要 NAT，即您没有公共 IP 地址。但是，至少 FTP 中的活动模式需要一个公共 IP，因此在 FTP 控制连接中使用为 PORT/EPRT 命令转换 IP 和端口的帮助程序并不罕见。这些帮助程序无法使用加密连接，这意味着它们要么故意阻止此类连接，要么可能会因为不再了解流量而意外阻止它们。