allowed through if this happens.Setup SummaryTo set up DNSBL SPAM filtering in the SMTP ALG, the following list summarizes the steps:• Specify which DNSBL servers are to be used. There can be multiple and they can act both asbackups to each other as well as confirmation of a sender's status.• Specify a weight for each server which will determine how important it is in deciding if email isSPAM or not in the calculation of a weighted sum.• Specify the threshold for designating an email as SPAM. If the weighted sum is equal or greaterthan this then an email will be considered to be SPAM.• Specify a textual tag to prefix to the Subject field of email designated as SPAM.• Specify the Drop threshold. If the weighted sum is equal or greater than this then an email willbe dropped entirely. This threshold should be greater or equal to the SPAM threshold. If they areequal then the Drop threshold will have precedence so that all email will be dropped when thatthreshold is reached.• Optionally specify an email address to which dropped email will be sent (as an alternative tosimply discarding it). Optionally specify that the TXT messages sent by the DNSBL servers thatfailed are inserted into the header of these emails.Caching Addresses for PerformanceTo speed processing NetDefendOS maintains a cache of the most recently looked-up senderaddresses in local memory. If the cache becomes full then the oldest entry is written over first.The Address Timeout value for the cache can be changed by the administrator. This determines howlong any address will be valid for once it is saved in the cache. After this period of time has expired,a new query for a cached sender address must be sent to the DNSBL servers.The cache is emptied at startup or reconfiguration and its size of this cache can be controlled by theadministrator.For the DNSBL subsystem overall:• Number of emails checked.• Number of emails SPAM tagged.• Number of dropped emails.For each DNSBL server accessed:• Number of positive (is SPAM) responses from each configured DNSBL server.• Number of queries sent to each configured DNSBL server.• Number of failed queries (without replies) for each configured DNSBL server.The dnsbl CLI Command6.2.5. The SMTP ALG Chapter 6. Security Mechanisms214
