How to check a username/password combination?

LDAP supports a compare of userPassword. You send the password, the server does the compare and returns true or false. This is the not-requiring a login way to authenticate users.

Look into the WhoAmI Extended Operation (RFC 4532).

WhoAmI serves one purpose really - validate submitted bind credentials. It should not affect nor provoke any "login restrictions" (that I know of).

WhoAmI can be done using a dedicated binary (such as "ldapwhoami"), or it can be done using Net::LDAP::Extension::WhoAmI (Perl) or some other such language that supports LDAP operations. Do note that "testing a password" using some "Search" function is an ill-advised test method.

For example, if my DN is "uid=max,ou=users,dc=company,dc=com" and my password is "@secret", one could do this via the dedicated binary on a Linux box (note -ZZ is used for TLS confidentiality, which is possibly unsupported or optional in your environment):

ldapwhoami -x -w "@secret" -D uid=max,ou=users,dc=company,dc=com -ZZ -H ldap://address.of.your.ldapserver/

If the user/pass combination is correct, the answer returned is:

dn:uid=max,ou=users,dc=company,dc=com

If the user/pass combination is NOT correct, the answer returned is (usually):

(49) Invalid Credentials

This could mean, as I said, the password and/or username is wrong, the user does not exist, or the LDAP server's ACLs are broken in such a way that authentication is not possible. More often than not, its the user/pass combo being mistyped, or the user not existing.

In closing, the LDAPWhoAmI operation is a very lightweight and simple method of validating credentials. It also works via other mechanisms too (e.g: Kerberos Single Sign-On, Digest-MD5, etc etc).

Watch out using bind for checking username/password, on some systems it will count as a login, and with login restrictions it might fail.

Using compare is a better option for just checking the password.