need to redirect http://abc.com to www.abc.com

when HTTP_REQUEST {
    if { [HTTP::host] eq "abc.com" } { HTTP::redirect http://www.abc.com }
}

when HTTP_REQUEST {
    if { [HTTP::host] eq "abc.com" } { HTTP::redirect http://www.abc.com }
}

We use an iRule on multiple VIPs, so we went with this instead:

when HTTP_REQUEST {
  if {not ([HTTP::host] starts_with "www")} {
    HTTP::redirect "http://www.[HTTP::host][HTTP::uri]"
  }
}