Exam F5CAB3 Topic 1 Question 34 Discussion

A 400 Bad Request response from an HTTP/1.1 web server is a definitive indicator that the HTTP request sent by the health monitor is missing a required Host header . In HTTP/1.1, the Host header is mandatory per RFC 7230. Web servers - particularly those hosting named virtual hosts such as www.f5.com - will reject any HTTP/1.1 request that omits the Host header, returning a 400 error, which the BIG-IP monitor interprets as a failed health check, marking all members DOWN.
The correct Send string must include:
* The GET request line with HTTP/1.1 protocol declaration
* A \r\n (carriage return + line feed) after the request line
* The Host: www.f5.com header to satisfy HTTP/1.1 requirements
* A \r\n after the Host header
* Connection: Close header to instruct the server to close the connection after responding
* A final \r\n\r\n to properly terminate the HTTP request headers
Only option C satisfies all these requirements with the correct syntax and Host header inclusion.
Option D (the current configured string) omits the Host header entirely - the root cause of the 400 error.
Options A and B are structurally incomplete or syntactically malformed.
Reference: BIG-IP Administration - Data Plane Configuration, Module: Health Monitors - HTTP Monitor Send String Configuration and HTTP/1.1 Host Header Requirements.