为什么 libcurl 不为 IP 设置 SNI？

我刚刚注意到当我使用 IP 进行 HTTPS 调用时，libcurl 没有设置 SNI 字段。我找到了这个：

https://github.com/curl/curl/blame/master/lib/vtls/openssl.c

#ifdef SSL_CTRL_SET_TLSEXT_HOSTNAME
  if((0 == Curl_inet_pton(AF_INET, hostname, &addr)) &&
#ifdef ENABLE_IPV6
     (0 == Curl_inet_pton(AF_INET6, hostname, &addr)) &&
#endif
     sni &&
     !SSL_set_tlsext_host_name(BACKEND->handle, hostname))
    infof(data, "WARNING: failed to configure server name indication (SNI) "
          "TLS extension\n");
#endif

根据Curl_inet_pton文档：

 * inet_pton(af, src, dst)
 *      convert from presentation format (which usually means ASCII printable)
 *      to network format (which is usually some kind of binary format).
 * return:
 *      1 if the address was valid for the specified address family
 *      0 if the address wasn't valid (`dst' is untouched in this case)
 *      -1 if some other error occurred (`dst' is untouched in this case, too)

正如预期的那样，它返回了1对于 IP 地址（例如192.160.0.1），因此不会调用SSL_set_tlsext_host_name.

Why?

因为这是不允许的。

RFC 3546“传输层安全 (TLS) 扩展”第3.1节 https://www.rfc-editor.org/rfc/rfc3546#section-3.1写得很清楚：

“HostName”中不允许使用文字 IPv4 和 IPv6 地址。