how to disable nginx internal dns cache?

[nevernet]

my network structure is: 
proxy server 1(nginx)-->proxy server2(nginx)-->web server

proxy server2 has dynamic ip address.  the ip address will be changed
untime. so i use one domain for it: p2.domain.com.

but proxy server1(nginx) has been cached the ip of domain: p2.domain.com.  
so when the new ip was assigned to p2.domain.com, but proxy server1(nginx)
still has old ip address.  

i have searched on google, somebody said that nginx will cache the dns entry
for 300 seconds(5minutes),  but after 5 minutes ,event after 5 hours , the
proxy server1(nginx) still has old ip address, 

please help me, any ideas will be appreciated.

thank you

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,238835,238835#msg-238835