Irule SNAT based on destination IP address.

I believe you would need IP::local_addr in this instance for the destination, and I think you have the irule in the right place.

I'm running some similar SNAT's at the moment, but I had some odd issues.

Here's the same thing (there may be more efficient ways of doing this...), but using datagroups and an snat pool instead of a single snat. I had some issues using a single snat in my config, but it worked fine using an snatpool consisting of a single snat IP.

This will match any source IP in datagroup webserver_datagroup, where the destination is in external_servers, and snat it to the address(es) in snat_pool_1

 
 when CLIENT_ACCEPTED { 
 set failed 0 
 if {[matchclass [IP::client_addr] equals $::webserver_datagroup] \ 
              and [matchclass [IP::local_addr] equals $::external_servers]}{ 
 use snatpool snat_pool_1 
 } 
 }