User Guide

40-20
AsyncOS 9.1.2 for Cisco Email Security Appliances User Guide
Chapter 40 Testing and Troubleshooting
Troubleshooting the Network
• Use the nslookup command to check the DNS functionality.
The
nslookup command can confirm that the appliance is able to reach and resolve hostnames and
IP addresses from a working DNS (domain name service) server.
• Use the tophosts command via the CLI or the GUI, and sort by Active Recipients.
mail3.example.com> nslookup
Please enter the host or IP to resolve.
[]> example.com
Choose the query type:
1. A
2. CNAME
3. MX
4. NS
5. PTR
6. SOA
7. TXT
[1]>
A=192.0.34.166 TTL=2d
Table 40-3 Checking DNS Functionality: Query Types
Query Type Description
A the host's Internet address
CNAME the canonical name for an alias
MX the mail exchanger
NS the name server for the named zone
PTR the hostname if the query is an Internet address, otherwise the pointer to other
information
SOA the domain's “start-of-authority” information
TXT the text information