我正在尝试下载 CA 证书以与我的 cURL PHP 程序一起使用，以便我可以在本地验证服务器。我正在使用以下命令，但我得到的证书列表太多，无法确定哪一个是正确的。
openssl s_client -connect maps.google.com:443 -showcerts
我从该命令获得的输出如下：
CONNECTED(00000003)
depth=2 /C=US/O=GeoTrust Inc./CN=GeoTrust Global CA
verify error:num=20:unable to get local issuer certificate
verify return:0
---
Certificate chain
0 s:/C=US/ST=California/L=Mountain View/O=Google Inc/CN=google.com
i:/C=US/O=Google Inc/CN=Google Internet Authority G2
-----BEGIN CERTIFICATE-----
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-----END CERTIFICATE-----
1 s:/C=US/O=Google Inc/CN=Google Internet Authority G2
i:/C=US/O=GeoTrust Inc./CN=GeoTrust Global CA
-----BEGIN CERTIFICATE-----
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-----END CERTIFICATE-----
2 s:/C=US/O=GeoTrust Inc./CN=GeoTrust Global CA
i:/C=US/O=Equifax/OU=Equifax Secure Certificate Authority
-----BEGIN CERTIFICATE-----
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---
Server certificate
subject=/C=US/ST=California/L=Mountain View/O=Google Inc/CN=google.com
issuer=/C=US/O=Google Inc/CN=Google Internet Authority G2
---
No client certificate CA names sent
---
SSL handshake has read 10364 bytes and written 456 bytes
---
New, TLSv1/SSLv3, Cipher is AES128-SHA
Server public key is 2048 bit
Secure Renegotiation IS supported
Compression: NONE
Expansion: NONE
SSL-Session:
Protocol : TLSv1
Cipher : AES128-SHA
Session-ID: 4083BF08C73AE573B7A4E9D8FE1A56169B84EDF192A956D34035785106593C53
Session-ID-ctx:
Master-Key: 7366BCF9D518917DDBC4444DF52DA1E6BA6DBB75AC05C2BEC07B8230F16815F7176A4BC2B72E85A9A07874CD2AD36C79
Key-Arg : None
Start Time: 1478635083
Timeout : 300 (sec)
Verify return code: 0 (ok)
---
read:errno=0
我正在使用的教程：
http://unitstep.net/blog/2009/05/05/using-curl-in-php-to-access-https-ssltls-protected-sites/
答案1
都不是。通常 TLS 服务器只发送中间 CA；它们不要发送根 CA，因为他们可以安全地假设：
- 它已经存在于客户端的信任锚（受信任的 CA）列表中，因此没有必要
- 如果不存在，则意味着客户端没有相信它，因此包括它不会改变任何事情。
一般来说，最可靠的选择是使用Mozilla CA 证书包，其拥有可用于验证大多数网站的 CA 证书。
如果由于某种原因你不能使用它，那么首先看看最后的证书链中的证书（Equifax 颁发的 GeoTrust），然后找到其发行人（Equifax）添加到系统的 CA 列表中，并提取那保存到一个文件。
（虽然从技术上讲中间 CA 也可以充当信任锚，但旧版 OpenSSL 仅接受根 CA。）