Configuring Anti-Spam Real-Time Black List Filtering

The Policies > Anti-Spam > RBL Filter page only allows configuration of Real-Time Black List filtering if the Anti-Spam Service is not enabled.

SMTP Real-Time Black List (RBL) is a mechanism for publishing the IP addresses of SMTP spammers use. There are a number of organizations that compile this information both for free: http://www.spamhaus.org, and for profit: http://www.mail-abuse.com. A well-maintained list of RBL services and their efficacy can be found at:
http://www.sdsc.edu/~jeff/spam/cbc.html

NOTE: SMTP RBL is an aggressive spam filtering technique that can be prone to false-positives because it is based on lists compiled from reported spam activity. The SonicOS implementation of SMTP RBL filtering provides a number of fine-tuning mechanisms to help ensure filtering accuracy.

RBL list providers publish their lists using DNS. Blacklisted IP addresses appear in the database of the list provider's DNS domain using inverted IP notation of the SMTP server in question as a prefix to the domain name. A response code from 127.0.0.2 to 127.0.0.9 indicates some type of undesirability:

•	127.0.0.2 - Open Relay

•	127.0.0.3 - Dialup Spam Source

•	127.0.0.4 - Spam Source

•	127.0.0.5 - Smart Host

•	127.0.0.6 - Spamware Site

•	127.0.0.7 - Bad List Server

•	127.0.0.8 - Insecure Script

•	127.0.0.9 - Open Proxy Server

For example, if an SMTP server with IP address 1.2.3.4 has been blacklisted by RBL list provider sbl-xbl.spamhaus.org, then a DNS query to 4.3.2.1.sbl-xbl.spamhaus.org provides a 127.0.0.4 response, indicating that the server is a known source of spam, and the connection is dropped.

NOTE: Most spam today is known to be sent from hijacked or zombie machines running a thin SMTP server implementation.Unlike legitimate SMTP servers, these zombie machines rarely attempt to retry failed delivery attempts. After the delivery attempt is blocked by the SonicWALL RBL filter, no subsequent delivery attempts for that same piece of spam are made.

When Enable Real-time Black List Blocking is enabled on the Anti-Spam > RBL Filter page, inbound connections from hosts on the WAN, or outbound connections to hosts on the WAN are checked against each enabled RBL service with a DNS request to the DNS servers configured under RBL DNS Servers.

The RBL DNS Servers menu allows you to specify the DNS servers. You can choose Inherit Settings from WAN Zone or Specify DNS Servers Manually. If you select Specify DNS Servers Manually, enter the DNS server addresses in the DNS Server fields.

The DNS responses are collected and cached. If any of the queries result in a blacklisted response, the server is filtered. Responses are cached using TTL values, and non-blacklisted responses are assigned a cache TTL of two hours. If the cache fills up, then cache entries are discarded in a FIFO (first-in-first-out) fashion.

The IP address check uses the cache to determine if a connection should be dropped. Initially, IP addresses are not in the cache and a DNS request must be made. In this case the IP address is assumed innocent until proven guilty, and the check results in the allowing of the connection. A DNS request is made and results are cached in a separate task. When subsequent packets from this IP address are checked, if the IP address is blacklisted, the connection is dropped.