ZyXEL Communications ZyWall USG20-VPN - Page 452

Chapter 28 Anti-Spam
USG20(W)-VPN Series User’s Guide
452
Figure 303 Conflicting DNSBL Replies Example
1 The USG receives an e-mail that was sent from IP address a.b.c.d and relayed by an e-mail server
at IP address w.x.y.z. The USG sends a separate query to each of its DNSBL domains for IP address
a.b.c.d. The USG sends another separate query to each of its DNSBL domains for IP address
w.x.y.z.
2 DNSBL A replies that IP address a.b.c.d does not match any entries in its list (not spam).
3 While waiting for a DNSBL reply about IP address w.x.y.z, the USG receives a reply from DNSBL B
saying IP address a.b.c.d is in its list.
4 The USG immediately classifies the e-mail as spam and takes the action for spam that you defined
in the anti-spam policy. In this example it was an SMTP mail and the defined action was to drop the
mail. The USG does not wait for any more DNSBL replies.
DNSBL A
DNSBL B
DNSBL C
IPs: a.b.c.d
w.x.y.z
1
2
a
.
b
.
c
.
d
N
o
t
s
p
a
m
3
4
a
.
b
.
c
.
d
?
w
.
x
.
y
.
z
?
a
.
b
.
c
.
d
?
w
.
x
.
y
.
z
?
a.b.c.d?
w.x.y.z?
a.b.c.d Spam!