ZyXEL Communications USG-300 - V2.20 ED 2, Manual

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Chapter 38 Anti-Spam
ZyWALL USG 300 User’s Guide
704
38.7 Anti-Spam Technical Reference
Here is more detailed anti-spam information.
DNSBL
• The ZyWALL checks only public sender and relay IP addresses, it does not check
private IP addresses.
• The ZyWALL sends a separate query (DNS lookup) for each sender or relay IP
address in the e-mail’s header to each of the ZyWALL’s DNSBL domains at the
same time.
• The DNSBL servers send replies as to whether or not each IP address matches
an entry in their list. Each IP address has a separate reply.
• As long as the replies are indicating the IP addresses do not match entries on
the DNSBL lists, the ZyWALL waits until it receives at least one reply for each IP
address.
• If the ZyWALL receives a DNSBL reply that one of the IP addresses is in the
DNSBL list, the ZyWALL immediately classifies the e-mail as spam and takes the
anti-spam policy’s configured action for spam. The ZyWALL does not wait for
any more DNSBL replies.
• If the ZyWALL receives at least one non-spam reply for each of an e-mail’s
routing IP addresses, the ZyWALL immediately classifies the e-mail as
legitimate and forwards it.
• Any further DNSBL replies that come after the ZyWALL classifies an e-mail as
spam or legitimate have no effect.
• The ZyWALL records DNSBL responses for IP addresses in a cache for up to 72
hours. The ZyWALL checks an e-mail’s sender and relay IP addresses against
the cache first and only sends DNSBL queries for IP addresses that are not in
the cache.
Edit Select an entry and click this to be able to modify it.
Remove Select an entry and click this to delete it.
Activate To turn on an entry, select it and click Activate .
Inactivate To turn off an entry, select it and click Inactivate .
Status The activate (light bulb) icon is lit when the entry is active and
dimmed when the entry is inactive.
# This is the entry’s index number in the list.
DNSBL Domain This is the name of a domain that maintains DNSBL servers. Enter the
domain that is maintaining a DNSBL.
Apply Click Apply to save your changes back to the ZyWALL.
Reset Click Reset to return the screen to its last-saved settings.
Table 187 Configuration > Anti-X > Anti-Spam > DNSBL (continued)
LABEL DESCRIPTION