Allow traffic from subnet

main.go

package main

import (
	"fmt"
	"log"
	"net"
	"net/http"
	"strings"
)

func main() {
	s := &http.Server{
		Handler: http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			fmt.Fprint(w, "OK")
		}),
	}

	l, err := net.Listen("tcp4", ":8080")
	if err != nil {
		log.Fatal(err)
	}

	l = wrapWithAllowCIDR(l, "192.168.0.0/24")

	if err := s.Serve(l); err != nil {
		log.Fatal(err)
	}
}

type StubListener struct {
	AcceptFn func() (net.Conn, error)
	CloseFn  func() error
	AddrFn   func() net.Addr
}

var _ net.Listener = &StubListener{}

func (l *StubListener) Accept() (net.Conn, error) {
	return l.AcceptFn()
}

func (l *StubListener) Close() error {
	return l.CloseFn()
}

func (l *StubListener) Addr() net.Addr {
	return l.AddrFn()
}

func wrapWithAllowCIDR(l net.Listener, cidr string) net.Listener {
	_, ipn, err := net.ParseCIDR(cidr)
	if err != nil {
		panic(fmt.Sprintf("invalid cidr: %s", cidr))
	}

	return &StubListener{
		AcceptFn: func() (net.Conn, error) {
			c, err := l.Accept()
			if err != nil {
				return c, err
			}

			addrParts := strings.Split(c.RemoteAddr().String(), ":")
			ip := net.ParseIP(addrParts[0])

			if !ipn.Contains(ip) {
				return c, c.Close()
			}

			return c, nil
		},
		CloseFn: l.Close,
		AddrFn:  l.Addr,
	}
}