feat: add target check

Part of https://www.w3.org/TR/webmention/#request-verification

1 files changed, 22 insertions, 2 deletions

diff --git a/main.go b/main.go
index 565f23c..d80ba6a 100644
--- a/main.go
+++ b/main.go
@@ -3,15 +3,18 @@ package main
 import (
 	"net/http"
 	"net/url"
+	"path"
+	"strings"
 )
 
 const (
-	errSrcInvalid = "source is not a parsable URL"
+	errSrcInvalid     = "source is not a parsable URL"
+	errTgtNotAccepted = "can not process webmentions for this target"
 )
 
 // endpoint is a webmention receiver.
 type endpoint struct {
-	allowPrefix string
+	allowPrefix string // host (or host:port) and path prefix for the targets served by this endpoint
 }
 
 // ServeHTTP is http.Handler implementation.
@@ -21,4 +24,21 @@ func (ep endpoint) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		w.WriteHeader(http.StatusBadRequest)
 		w.Write([]byte(errSrcInvalid))
 	}
+	target, err := url.Parse(r.PostFormValue("target"))
+	if err != nil || !ep.targetAllowed(target) {
+		w.WriteHeader(http.StatusBadRequest)
+		w.Write([]byte(errTgtNotAccepted))
+	}
+}
+
+// targetAllowed shows whether ep can accept a webmention for the target.
+func (ep endpoint) targetAllowed(target *url.URL) bool {
+	if !strings.HasSuffix(ep.allowPrefix, "/") {
+		ep.allowPrefix = ep.allowPrefix + "/"
+	}
+	tgt := path.Join(target.Host, target.Path)
+	if !strings.HasSuffix(tgt, "/") {
+		tgt = tgt + "/"
+	}
+	return strings.HasPrefix(tgt, ep.allowPrefix)
 }