BIG-IP : irule not matching uri as expected

Are you trying to log the 404 error in the response packet? try using when HTTP_RESPONSE function and see if you can log 404 error.

does the logged /404 uri match exactly /404 as per your irule or does it start with or contain /404 i.e. /404/

cheers

not sure what's going on then - I'm using 11.6 VE, prehaps you can run the same commands and post your results

ltm rule test404 {
  when HTTP_REQUEST {
    if {[HTTP::uri] != "/404"} {
      log local0. "uri = [HTTP::uri]"
    }
  }
}

[root@f5lab] config  curl http://10.12.12.145/402
[root@f5lab] config  curl http://10.12.12.145/403
[root@f5lab] config  curl http://10.12.12.145/404
[root@f5lab] config  curl http://10.12.12.145/405

Sep 18 16:43:00 f5lab info tmm[17994]: Rule /Common/test404 : uri = /402
Sep 18 16:43:05 f5lab info tmm[17994]: Rule /Common/test404 : uri = /403
Sep 18 16:43:11 f5lab info tmm[17994]: Rule /Common/test404 : uri = /405

Try using ne instead of !=. You may be having an issue with data type conversion. Using ne will ensure you are doing a string comparison. I'm not positive that is the issue, but it's an easy change to test.