The rbl-filter dns-server command specifies the DNS server for initiating RBL query requests.
The undo rbl-filter dns-server command deletes the DNS server for initiating RBL query requests.
| Parameter | Description | Value |
|---|---|---|
| ip-address1 | Specifies the IP address of the primary DNS server. | The value is in dotted decimal notation. |
| ip-address2 | Specifies the IP address of the secondary DNS server. | The value is in dotted decimal notation. |
RBL remote query uses the DNS protocol. The FW extracts the source IP address of the sender's SMTP server, includes this IP address and the RBL service name specified by a third-party RBL server in a message, and sends the message to the DNS server for resolution. After receiving the message, the DNS server retrieves the RBL service name, resolves the RBL server name to an IP address, and forwards the query request to the RBL server. The RBL server returns the result to the DNS server in reply code. The DNS server then forwards the reply code to the FW. The FW then determines whether the email is a junk mail based on the reply code.
You can deploy two DNS servers (a primary DNS server and a secondary DNS server) for improved availability.
By default, no DNS server is configured.
You can run the undo rbl-filter dns-server ip-address command to delete an RBL server. ip-address can be the address of the primary DNS server or the secondary DNS server.