Allow listening on a specific address

The address is read from the ADDR environment variable. Defaults to
every address.

2 files changed, 7 insertions, 3 deletions

diff --git a/README.md b/README.md
index 1fe7219..ca9464e 100644
--- a/README.md
+++ b/README.md
@@ -11,5 +11,9 @@ You need to set up the following environment variables for it to work:
   e.g. `golang.org/x/` for `golang.org/x/image`, and
 * `VCS` for the type of VCS you are using, e.g. `git`.
 
-Additionally, the version under `gcp/` will bind to the port passed in
-the `PORT` environment variable, or 8080 if that variable is not set.
+Additionally, the version under `gcp/` will use:
+
+* `ADDR` for the address to listen on, defaulting to every address, and
+* `PORT` for the port to listen on, defaulting to 8080.
+
+See https://golang.org/pkg/net/#Dial for details of the address format.
diff --git a/gcp/main.go b/gcp/main.go
index c5c8466..bb792cc 100644
--- a/gcp/main.go
+++ b/gcp/main.go
@@ -31,5 +31,5 @@ func main() {
 		port = "8080"
 	}
 	http.HandleFunc("/", redirect)
-	log.Fatal(http.ListenAndServe(":"+port, nil))
+	log.Fatal(http.ListenAndServe(os.Getenv("ADDR")+":"+port, nil))
 }