When working with LDAP authentication on CentOS systems, you might encounter the frustrating error:

ldap_sasl_interactive_bind_s: Unknown authentication method (-6)
additional info: SASL(-4): no mechanism available:

This typically occurs when trying to use ldapsearch commands while other authentication methods (like SSH or NSLCD) work fine. The root cause usually lies in missing SASL mechanisms or incorrect configuration.

First, check your current LDAP configuration:

cat /etc/openldap/ldap.conf

A typical working configuration should include:

TLS_CACERTDIR /etc/openldap/cacerts
URI ldap://ldap1.example.com ldap://ldap2.example.com
BASE dc=example,dc=com

The error suggests missing SASL mechanisms. Install the necessary packages:

yum install cyrus-sasl cyrus-sasl-gssapi cyrus-sasl-plain

Try these alternative authentication methods:

Simple Bind Authentication

ldapsearch -x -D "cn=admin,dc=tst,dc=domain,dc=tld" -W -b "dc=tst,dc=domain,dc=tld"

Using TLS Certificate

ldapsearch -ZZ -x -H ldap://ldap1.tst.domain.tld -D "cn=admin,dc=tst,dc=domain,dc=tld" -W -b "dc=tst,dc=domain,dc=tld"

For more detailed debugging:

ldapsearch -d 1 -x -D "cn=admin,dc=tst,dc=domain,dc=tld" -W

Check SASL mechanisms with:

ldapsearch -LLL -Y EXTERNAL -H ldapi:/// -b "" -s base supportedSASLMechanisms

Create or modify /etc/sasl2/slapd.conf:

pwcheck_method: auxprop
auxprop_plugin: sasldb
mech_list: PLAIN LOGIN CRAM-MD5 DIGEST-MD5 NTLM

Then restart the SASL daemon:

systemctl restart saslauthd

For quick authentication tests:

ldapwhoami -x -D "cn=admin,dc=tst,dc=domain,dc=tld" -W

This can help isolate whether the issue is with authentication specifically or with search operations.

After making changes, verify your setup with:

ldapsearch -x -LLL -H ldap://ldap1.tst.domain.tld -D "cn=admin,dc=tst,dc=domain,dc=tld" -W -b "dc=tst,dc=domain,dc=tld" "(objectclass=*)"

This should return your LDAP directory contents without SASL errors.

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When working with LDAP authentication on CentOS systems, you might encounter the frustrating SASL(-4): no mechanism available error during ldapsearch operations. This typically indicates a mismatch between the authentication methods your client is attempting to use and what the LDAP server actually supports.

Before diving into fixes, let's examine a typical working configuration (as seen in the question):

TLS_CACERTDIR /etc/openldap/cacerts
URI ldap://ldap1.tst.domain.tld ldap://ldap2.tst.domain.tld
BASE dc=tst,dc=domain,dc=tld

The error suggests missing SASL mechanisms. This usually occurs because:

• Required SASL libraries aren't installed
• Certificate paths are misconfigured
• SSL/TLS versions mismatch between client and server
• Missing or incorrect PAM/NSS configuration

Here's how to properly configure ldapsearch authentication:

1. Install Required SASL Packages

yum install cyrus-sasl cyrus-sasl-gssapi cyrus-sasl-plain

2. Configure ldap.conf Properly

# /etc/openldap/ldap.conf
BASE dc=tst,dc=domain,dc=tld
URI ldaps://ldap1.tst.domain.tld ldaps://ldap2.tst.domain.tld
TLS_CACERT /etc/openldap/cacerts/cacert.pem
TLS_REQCERT allow
SASL_MECH GSSAPI

3. Test Simple Authentication

First try a simple bind without SASL:

ldapsearch -x -H ldap://ldap1.tst.domain.tld -b "dc=tst,dc=domain,dc=tld" \
-D "cn=admin,dc=tst,dc=domain,dc=tld" -W

4. Enable SASL Debugging

For detailed troubleshooting:

export SASL_PATH=/usr/lib/sasl2
export SASL_MECH=GSSAPI
ldapsearch -d 7 -Y GSSAPI -H ldap://ldap1.tst.domain.tld

For certificate-based authentication:

ldapsearch -ZZ -H ldaps://ldap1.tst.domain.tld \
-D "cn=admin,dc=tst,dc=domain,dc=tld" \
-W -b "dc=tst,dc=domain,dc=tld" \
-y /path/to/admin.password

Ensure your certificate directory is properly set up:

ls -l /etc/openldap/cacerts/
# Should show valid symlinks like:
# cf848aa4.0 -> cacert.pem