LDAP uses user login instead of admin account

Description:

Since the last versions of Rocket.Chat, my instance requests LDAP with a normal filter by admin account and after that a new request, logging in by userdn directly. I am not completely confident, but my expectation is, the login should not be with a distinctive user. I my case, my LDAP wasn't allowing that either.

Steps to reproduce:

1. login with ldap user
2. check ldap debug log

Expected behavior:

Single login with configured ldap administration account should be found in logs. E.g.:

{
  "messageID": 2,
  "protocolOp": "SearchRequest",
  "baseObject": {
    "rdns": [
      {
        "o": "myOrganization"
      }
    ],
    "rdnSpaced": false,
    "length": 1
  },
  "scope": "sub",
  "derefAliases": 0,
  "sizeLimit": 100000,
  "timeLimit": 10 ,
  "typesOnly": false,
  "filter": "(&(&(objectclass=person))(|(cn=person@mail.de)(mail=person@mail.de)))",
  "attributes": [],
  "controls": []
}

Actual behavior:

This request can be found as well in the logs, in addition to the one above:

{
  "messageID": 4,
  "protocolOp": "SearchRequest",
  "baseObject": {
    "rdns": [
      {
        "cn": "myusername"
      },
      {
        "ou": "users"
      },
      {
        "o": "myOrganization"
      }
    ],
    "rdnSpaced": true,
    "length": 3
  },
  "scope": "sub",
  "derefAliases": 0,
  "sizeLimit": 0,
  "timeLimit": 10,
  "typesOnly": false,
  "filter": "(objectclass=*)",
  "attributes": [],
  "controls": []
}

Server Setup Information:

• Version of Rocket.Chat Server: 0.70.0

Additional context

I think that the Pull Request #11264 caused the problem. See https://github.com/RocketChat/Rocket.Chat/pull/11264#pullrequestreview-160474523

Start with the LDAP debug logs and compare the configured administration-account search with the additional user-DN search described in the issue. Review Pull Request #11264 and its discussion; done means logging in with an LDAP user performs only the expected search using the configured LDAP administration account.