Add denyHosts flag to deny URLs for certain hosts

For example, when running in a Docker swarm cluster we dont want it to
have access to our internal services available under *.weave.local

Closes #85

README.md

@@ -183,19 +183,28 @@ Reload the [codercat URL][], and you should now get an error message.  You can
 specify multiple hosts as a comma separated list, or prefix a host value with
 `*.` to allow all sub-domains as well.
 
-### Allowed Hosts List ###
+### Allowed and Denied Hosts List ###
 
 You can limit the remote hosts that the proxy will fetch images from using the
-`allowHosts` flag.  This is useful, for example, for locking the proxy down to
-your own hosts to prevent others from abusing it.  Of course if you want to
-support fetching from any host, leave off the allowHosts flag.  Try it out by
-running:
+`allowHosts` and `denyHosts` flags.  This is useful, for example, for locking
+the proxy down to your own hosts to prevent others from abusing it.  Of course
+if you want to support fetching from any host, leave off these flags.
+
+Try it out by running:
 
     imageproxy -allowHosts example.com
 
-Reload the [codercat URL][], and you should now get an error message.  You can
-specify multiple hosts as a comma separated list, or prefix a host value with
-`*.` to allow all sub-domains as well.
+Reload the [codercat URL][], and you should now get an error message.
+Alternately, try running:
+
+    imageproxy -denyHosts octodex.github.com
+
+Reloading the [codercat URL][] will still return an error message.
+
+You can specify multiple hosts as a comma separated list to either flag, or
+prefix a host value with `*.` to allow or deny all sub-domains as well.
+
+If a host matches both an allowed an a denied host, the request will be denied.
 
 ### Allowed Content-Type List ###
 

cmd/imageproxy/main.go

@@ -44,6 +44,7 @@ const defaultMemorySize = 100
 var addr = flag.String("addr", "localhost:8080", "TCP address to listen on")
 var allowHosts = flag.String("allowHosts", "", "comma separated list of allowed remote hosts")
 var whitelist = flag.String("whitelist", "", "deprecated. use 'allowHosts' instead")
+var denyHosts = flag.String("denyHosts", "", "comma separated list of denied remote hosts")
 var referrers = flag.String("referrers", "", "comma separated list of allowed referring hosts")
 var baseURL = flag.String("baseURL", "", "default base URL for relative remote URLs")
 var cache tieredCache
@@ -70,6 +71,9 @@ func main() {
 	if *allowHosts != "" {
 		p.AllowHosts = strings.Split(*allowHosts, ",")
 	}
+	if *denyHosts != "" {
+		p.DenyHosts = strings.Split(*denyHosts, ",")
+	}
 	if *referrers != "" {
 		p.Referrers = strings.Split(*referrers, ",")
 	}

imageproxy.go

@@ -49,6 +49,10 @@ type Proxy struct {
 	// Whitelist should no longer be used. Use "AllowHosts" instead.
 	Whitelist []string
 
+	// DenyHosts specifies a list of remote hosts that images cannot be
+	// proxied from.
+	DenyHosts []string
+
 	// Referrers, when given, requires that requests to the image
 	// proxy come from a referring host. An empty list means all
 	// hosts are allowed.
@@ -225,6 +229,10 @@ func (p *Proxy) allowed(r *Request) error {
 		return fmt.Errorf("request does not contain an allowed referrer: %v", r)
 	}
 
+	if validHost(p.DenyHosts, r.URL) {
+		return fmt.Errorf("request contains a denied host %v", r)
+	}
+
 	if len(p.AllowHosts) == 0 && len(p.SignatureKey) == 0 {
 		return nil // no allowed hosts or signature key, all requests accepted
 	}