Change HTTP header for request

This is fairly confusing.  Using host files - will only make things more confusing and near impossible to troubleshoot down the road.   You should only modify a host file as a quick test -and put it back - and us a proper DNS entry.

That being said - can we assume the "systems " host file entry is the client side - and not either the f5- or the backed server?   This would result in the client side computer making calls to abc.com -which should translate to 20.10.20.30.   from the client with modified host file   ping 20.10.20.30    <---will NOT do a nslookup,  it will see the entry in the host file and use that.

The next step is to ping dvwa.com and make sure that comes back to the same ip ( or what ever you are expecting).

Lasty - i believe you a look for either a url rewrite - or a header re-write.

https://community.f5.com/t5/technical-forum/how-to-do-http-header-replace-with-irule/td-p/118808
or if its more complex - and you want to detect the IP - and make the change from there
https://my.f5.com/manage/s/article/K000130245