Network & Security Manager 2012.2R9, coming June, 2014, will require 32 bit RedHat 5.7+.

Juniper will be updating the version of JAVA used in NSM to JAVA 7.
JAVA 7 is not compatible with RedHat 4 and must have RedHat 5.5 or greater.
Juniper has tested JAVA 7 with RedHat 5.7 and this will be the official minimum OS.

NSM is only compatible with 32 bit Linux. Please keep this in mind when upgrading the OS.
(NSM is not compatible with 64 bit Linux)

If it is necessary to stay on a version of RedHat earlier than RedHat 5.7, please do not upgrade NSM to 2012.2R9.

The NSM Hardware Appliances have an upgrade path to Centos 5.7 which is equivalent to RedHat 5.7.
Please use the Centos 5.7 upgrade procedure to update NSM appliances prior to upgrading to NSM 2012.2R9.

NSM Appliance Centos Upgrade Guide