Altostratus
MVPThis 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