ZyXEL Communications ZyWall USG20-VPN - Page 451

Chapter 28 Anti-Spam
USG20(W)-VPN Series User’s Guide
451
Figure 302 DNSBL Legitimate E-mail Detection Example
1 The USG receives an e-mail that was sent from IP address c.c.c.c and relayed by an e-mail server
at IP address d.d.d.d. The USG sends a separate query to each of its DNSBL domains for IP address
c.c.c.c. The USG sends another separate query to each of its DNSBL domains for IP address
d.d.d.d.
2 DNSBL B replies that IP address d.d.d.d does not match any entries in its list (not spam).
3 DNSBL C replies that IP address c.c.c.c does not match any entries in its list (not spam).
4 Now that the USG has received at least one non-spam reply for each of the e-mail’s routing IP
addresses, the USG immediately classifies the e-mail as legitimate and forwards it. The USG does
not wait for any more DNSBL replies.
If the USG receives conflicting DNSBL replies for an e-mail routing IP address, the USG classifies
the e-mail as spam. Here is an example.
DNSBL A
DNSBL B
DNSBL C
IPs: c.c.c.c
d.d.d.d
1
c
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c
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c
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c
N
o
t
s
p
a
m
2
4
c
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c
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c
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c
?
d
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d
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d
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d
?
c
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c
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c
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c
?
d
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d
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d
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d
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c.c.c.c?
d.d.d.d?
d.d.d.d Not spam
3