By default, slapd(8) will listen on both the IPv4 and IPv6 "any" addresses. It is often desirable to have slapd listen on select address/port pairs. For example, listening only on the IPv4 address 127.0.0.1 will disallow remote access to the directory server. E.g.:
slapd -h ldap://127.0.0.1
While the server can be configured to listen on a particular interface address, this doesn't necessarily restrict access to the server to only those networks accessible via that interface. To selective restrict remote access, it is recommend that an IP Firewall be used to restrict access.
See Command-line Options and slapd(8) for more information.
IP firewall capabilities of the server system can be used to restrict access based upon the client's IP address and/or network interface used to communicate with the client.
Generally, slapd(8) listens on port 389/tcp for ldap:// sessions and port 636/tcp for ldaps://) sessions. slapd(8) may be configured to listen on other ports.
As specifics of how to configure IP firewall are dependent on the particular kind of IP firewall used, no examples are provided here. See the document associated with your IP firewall.
slapd(8) supports TCP Wrappers. TCP Wrappers provide a rule-based access control system for controlling TCP/IP access to the server. For example, the host_options(5) rule:
slapd: 10.0.0.0/255.0.0.0 127.0.0.1 : ALLOW
slapd: ALL : DENY
allows only incoming connections from the private network 10.0.0.0 and localhost (127.0.0.1) to access the directory service. Note that IP addresses are used as slapd(8) is not normally configured to perform reverse lookups.
It is noted that TCP wrappers require the connection to be accepted. As significant processing is required just to deny a connection, it is generally advised that IP firewall protection be used instead of TCP wrappers.
See hosts_access(5) for more information on TCP wrapper rules.