How to know if a DNS result is cache-able?

Products

• ProxySG Software - SGOS
• SWG VA-100

Issue/Introduction

You want to know if a result resolved from DNS cache-able.

Resolution

1. If you have a Linux machine, you may use the "DIG" to perform a DNS resolve.
2. Notice from below example on the highlighted in "Yellow" are actually showing if a DNS result is cache-able.
3. Below example www.example.com.xx shows that it's result is not cache-able, hence the DNS Cache TTL is 0
4. While example.com.xx shows that the DNS result are cache-able, hence DNS Cache TTL shows some numbers.

example from Linux: 

# dig @x.x.x.x +nocmd www.example.com.xx<http://www.example.com.xx> +noall +answer
 www.example.com.xx<http://www.example.com.xx>.    0       IN      A       xxx.xx.xxx.xx 

 

# dig @x.x.x.x +nocmd example.com.xx +noall +answer
example.com.xx.        6886    IN      A       xxxx.xx.xx.xx

2. If not, you may use ProxySG to perform a test dns resolve.

example Blue Coat ProxySG:

Blue Coat SG210 Series#test dns www.example.com.xx
Performing DNS lookup for: www.example.com.xx

Sending A query for www.example.com.xx to xxxx.xx.xx.xx.

DNS Response data:
Official Host Name: www.example.com.xx
Resolved Addresses:  xxx.xx.xxx.xx
Cache TTL: 0, cache MISS
DNS Resolver Response: Success

Blue Coat SG210 Series#test dns.example.com.xx

Performing DNS lookup for: example.com.xx

 

Sending A query for example.com.xx to xxx.xx.xxx.xx 

 

DNS Response data:

Official Host Name: example.com.xx

Resolved Addresses:  xxx.xx.xxx.xx

Cache TTL: 7200, cache MISS

DNS Resolver Response: Success