Anti-Spam Technical Reference - ZyXEL Communications USG40 User Manual

Table 269 Configuration > UTM Profile > Anti-Spam > DNSBL (continued)
LABEL
Activate
Inactivate
Status
#
DNSBL Domain
Apply
Reset

39.8 Anti-Spam Technical Reference

Here is more detailed anti-spam information.
DNSBL
• The ZyWALL/USG checks only public sender and relay IP addresses, it does not check private IP
addresses.
• The ZyWALL/USG sends a separate query (DNS lookup) for each sender or relay IP address in the
e-mail's header to each of the ZyWALL/USG'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/USG waits until it receives at least one reply for each IP address.
• If the ZyWALL/USG receives a DNSBL reply that one of the IP addresses is in the DNSBL list, the
ZyWALL/USG immediately classifies the e-mail as spam and takes the anti-spam policy's
configured action for spam. The ZyWALL/USG does not wait for any more DNSBL replies.
• If the ZyWALL/USG receives at least one non-spam reply for each of an e-mail's routing IP
addresses, the ZyWALL/USG immediately classifies the e-mail as legitimate and forwards it.
• Any further DNSBL replies that come after the ZyWALL/USG classifies an e-mail as spam or
legitimate have no effect.
• The ZyWALL/USG records DNSBL responses for IP addresses in a cache for up to 72 hours. The
ZyWALL/USG 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.
Here is an example of an e-mail classified as spam based on DNSBL replies.
Chapter 39 Anti-Spam
DESCRIPTION
To turn on an entry, select it and click Activate.
To turn off an entry, select it and click Inactivate.
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.
This is the name of a domain that maintains DNSBL servers. Enter the domain that is
maintaining a DNSBL.
Click Apply to save your changes back to the ZyWALL/USG.
Click Reset to return the screen to its last-saved settings.
ZyWALL/USG Series User's Guide
694