Strip route domain from ip::remote_addr

There are a number of ways of doing that, but here is one (though untested!) that should work:

when HTTP_REQUEST {
    HTTP::header insert X-Some-Header "[getfield [IP::client_addr] % 1]"
}

For more information on

getfield

• https://devcentral.f5.com/wiki/iRules.getfield.ashx

I also encourage, for header insertion, that you consider using Local Traffic Policies, if possible:

• SOL15085: Overview of the Local Traffic Policies feature