此代码使用变量“ ok”，但未定义

This code is functioning but I don't understand how.

In the code below hostProxy[host] may or may not contain a function. I don't understand how the variable "ok" is defined or how it gets its value. It is not defined before this line.

if fn, ok := hostProxy[host]; ok {
    fn.ServeHTTP(w, r)
    return
}

if target, ok := hostTarget[host]; ok {
    ....
}

This is covered in Spec: Index expressions:

An index expression on a map a of type map[K]V used in an assignment or initialization of the special form

v, ok = a[x]
v, ok := a[x]
var v, ok = a[x]
var v, ok T = a[x]

yields an additional untyped boolean value. The value of ok is true if the key x is present in the map, and false otherwise.

So for example this code:

if fn, ok := hostProxy[host]; ok {
    fn.ServeHTTP(w, r)
    return
}

Means to get the value associated with host key from the hostProxy map, create and store the value in the fn variable, and the result (whether the key was found in the map) in the ok variable. And this ok variable (which will be of type bool) is used as the condition of the if statement. So if the host key is in the hostProxy map, it goes ahead and uses it.

Yes ok isn't defined before, but in your example you have := which will define variables for you under the hood and will assign values obtained from map lookup.