LDAP服务器连接失败问题求助：PHP应用及ldapsearch均无法连接，openssl调试异常

各位大佬好，我遇到了一个LDAP连接的棘手问题，折腾好久都没解决，想请大家帮忙看看！

环境信息 #

• 运行环境：RedHat系统 + Apache服务器 + PHP应用（SELinux已禁用）

遇到的错误 #

1. PHP应用连接报错：Apache日志中出现以下错误

ldap_bind(): Unable to bind to server: Can't contact LDAP server

2. 命令行工具报错：手动执行ldapsearch命令时，同样报错

ldap_sasl_bind(SIMPLE): Can't contact LDAP server (-1)

调试操作及结果 #

我尝试用openssl命令排查SSL连接问题，执行了：

openssl s_client -connect xx.com:636 -debug

得到的输出结果如下：

CONNECTED(00000003)
write:errno=0
---
no peer certificate available
---
No client certificate CA names sent
---
SSL handshake has read 0 bytes and written 322 bytes
Verification: OK
---
New, (NONE), Cipher is (NONE)
Secure Renegotiation IS NOT supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
Early data was not sent
Verify return code: 0 (ok)
---
---
no peer certificate available
---
No client certificate CA names sent
---
SSL handshake has read 0 bytes and written 322 bytes
Verification: OK
---
New, (NONE), Cipher is (NONE)
Secure Renegotiation IS NOT supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
Early data was not sent
Verify return code: 0 (ok)

已排查的情况 #

• 防火墙：已经开放了636端口，排除防火墙拦截的可能
• LDAP配置文件：/etc/openldap/ldap.conf已正确配置，文件中指定的证书存在且权限设置正确

现在我有点摸不着头绪，有没有大佬遇到过类似的问题？希望能得到大家的帮助，谢谢！

备注：内容来源于stack exchange，提问作者el sparrow