Cisco Cisco Email Security Appliance C170 Guia Do Utilizador
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Cisco AsyncOS 8.0.1 for Email User Guide
Chapter 29 System Administration
Changing Network Settings
For example, suppose you configure four DNS servers, with two of them at priority 0, one at priority 1,
and one at priority 2:
and one at priority 2:
AsyncOS will randomly choose between the two servers at priority 0. If one of the priority 0 servers is
down, the other will be used. If both of the priority 0 servers are down, the priority 1 server (1.2.3.6) is
used, and then, finally, the priority 2 (1.2.3.7) server.
down, the other will be used. If both of the priority 0 servers are down, the priority 1 server (1.2.3.6) is
used, and then, finally, the priority 2 (1.2.3.7) server.
The timeout period is the same for both priority 0 servers, longer for the priority 1 server, and longer still
for the priority 2 server.
for the priority 2 server.
Using the Internet Root Servers
The Cisco AsyncOS DNS resolver is designed to accommodate the large number of simultaneous DNS
connections required for high-performance email delivery.
connections required for high-performance email delivery.
Note
If you choose to set the default DNS server to something other than the Internet root servers, that server
must be able to recursively resolve queries for domains for which it is not an authoritative server.
must be able to recursively resolve queries for domains for which it is not an authoritative server.
Reverse DNS Lookup Timeout
The Cisco appliance attempts to perform a “double DNS lookup” on all remote hosts connecting to a
listener for the purposes of sending or receiving email. [That is: the system acquires and verifies the
validity of the remote host's IP address by performing a double DNS lookup. This consists of a reverse
DNS (PTR) lookup on the IP address of the connecting host, followed by a forward DNS (A) lookup on
the results of the PTR lookup. The system then checks that the results of the A lookup match the results
of the PTR lookup. If the results do not match, or if an A record does not exist, the system only uses the
IP address to match entries in the Host Access Table (HAT).] This particular timeout period applies only
to this lookup and is not related to the general DNS timeout discussed in
listener for the purposes of sending or receiving email. [That is: the system acquires and verifies the
validity of the remote host's IP address by performing a double DNS lookup. This consists of a reverse
DNS (PTR) lookup on the IP address of the connecting host, followed by a forward DNS (A) lookup on
the results of the PTR lookup. The system then checks that the results of the A lookup match the results
of the PTR lookup. If the results do not match, or if an A record does not exist, the system only uses the
IP address to match entries in the Host Access Table (HAT).] This particular timeout period applies only
to this lookup and is not related to the general DNS timeout discussed in
The default value is 20 seconds. You can disable the reverse DNS lookup timeout globally across all
listeners by entering ‘0’ as the number of seconds.
listeners by entering ‘0’ as the number of seconds.
If the value is set to 0 seconds, the reverse DNS lookup is not attempted, and instead the standard timeout
response is returned immediately. This also prevents the appliance from delivering mail to domains that
require TLS-verified connections if the receiving host’s certificate has a common name (CN) that maps
to the host’s IP lookup.
response is returned immediately. This also prevents the appliance from delivering mail to domains that
require TLS-verified connections if the receiving host’s certificate has a common name (CN) that maps
to the host’s IP lookup.
Table 29-11
Example of DNS Servers, Priorities, and Timeout Intervals
Priority
Server(s)
Timeout (seconds)
0
1.2.3.4, 1.2.3.5
5, 5
1
1.2.3.6
10
2
1.2.3.7
45