I had the need to set a custom HTTP request header in haproxy. For versions up to 1.4.x, the way to do this is :
reqadd X-Custom-Header:\ some_string
However, some_string is just a static string, and I could see no way of interpolating a variable in the string. Googling around, this is possible in haproxy 1.5.x with this method:
http-request set-header X-Custom-Header %[dst_port]
where dst_port is the variable we want to interpolate and %[variable] is the syntax for interpolation.
Other examples of variables available for you in haproxy.cfg are in Section 7.3 "Fetching samples" in the haproxy 1.5 configuration manual.