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In nsswitch.conf file I have an entry "passwd files sss". Also I have the evidence that the server is having LDAP authentication. Since the "files" for passwd is enabled and it points the local authentication , which one take the precedence?

2 Answers 2

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The file /etc/nsswitch.conf defines the order of NSS modules used to do lookups in NSS maps. Strictly speaking lookups in the passwd map are not used for authentication, only for checking the existence of a user trying to login.

Password authentication is done via the PAM stack configured in /etc/pam.d for earch service using PAM.

  • For local users the PAM module pam_unix.so does a NSS lookup in the shadow map to get the hashed password and check the user's password against this.
  • For remote LDAP users the shadow map should not be used. For example sssd sends a LDAP simple bind request to let the LDAP server check the user's password remotely. It does not query password hashes.

So to find out the order of PAM authc modules used you have to examime the files in directory /etc/pam.d. Most Linux systems today have common include files they use for every service such as /etc/pam.d/common-auth, most times a symlink to common-auth-pc created by tool pam-config.

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See man 5 nsswitch.conf (bold added):

The first column is the database name. The remaining columns specify:

  • One or more service specifications, for example, "files", "db", or "nis". The order of the services on the line determines the order in which those services will be queried, in turn, until a result is found.

And your case, files takes precedence, which is as it should be - usually you don't want an LDAP entry for say, root, to override local entries.

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