Application server java.security configuration

I have questions for following notes in java.security file

# default value is forever (FOREVER). For security reasons, this

# caching is made forever when a security manager is set. When a security

# manager is not set, the default behavior in this implementation

# is to cache for 30 seconds.

#

# NOTE: setting this to anything other than the default value can have

#    serious security implications. Do not set it unless

#    you are sure you are not exposed to DNS spoofing attack.

#

#networkaddress.cache.ttl=-1

 

Questions:

1. When set networkaddress.cache.ttl=0 - no cache, what security implications are?

2. Our application servers are behind firewall, are we still exposed to DNS spoofing if we set it to no cache?