middleware/proxy: config syntax cleanups (#435)

* middleware/proxy: config syntax cleanups

Allow port numbers to be used in the transfer statements and clean
up the proxy stanza parsing. Also allow, when specifying an upstream,
/etc/resolv.conf (or any other file) to be used for getting the upstream
nameserver.

Add tests and fix the documentation to make clear what is allowed.

* Fix the other upstream parse as well

middleware/pkg/dnsutil/host_test.go

@@ -0,0 +1,85 @@
+package dnsutil
+
+import (
+	"io/ioutil"
+	"os"
+	"testing"
+)
+
+func TestParseHostPortOrFile(t *testing.T) {
+	tests := []struct {
+		in        string
+		expected  string
+		shouldErr bool
+	}{
+		{
+			"8.8.8.8",
+			"8.8.8.8:53",
+			false,
+		},
+		{
+			"8.8.8.8:153",
+			"8.8.8.8:153",
+			false,
+		},
+		{
+			"/etc/resolv.conf:53",
+			"",
+			true,
+		},
+		{
+			"resolv.conf",
+			"127.0.0.1:53",
+			false,
+		},
+	}
+
+	err := ioutil.WriteFile("resolv.conf", []byte("nameserver 127.0.0.1\n"), 0600)
+	if err != nil {
+		t.Fatalf("Failed to write test resolv.conf")
+	}
+	defer os.Remove("resolv.conf")
+
+	for i, tc := range tests {
+		got, err := ParseHostPortOrFile(tc.in)
+		if err == nil && tc.shouldErr {
+			t.Errorf("Test %d, expected error, got nil", i)
+			continue
+		}
+		if err != nil && tc.shouldErr {
+			continue
+		}
+		if got[0] != tc.expected {
+			t.Errorf("Test %d, expected %q, got %q", i, tc.expected, got[0])
+		}
+	}
+}
+
+func TestParseHostPort(t *testing.T) {
+	tests := []struct {
+		in        string
+		expected  string
+		shouldErr bool
+	}{
+		{"8.8.8.8:53", "8.8.8.8:53", false},
+		{"a.a.a.a:153", "", true},
+		{"8.8.8.8", "8.8.8.8:53", false},
+		{"8.8.8.8:", "8.8.8.8:53", false},
+		{"8.8.8.8::53", "", true},
+		{"resolv.conf", "", true},
+	}
+
+	for i, tc := range tests {
+		got, err := ParseHostPort(tc.in, "53")
+		if err == nil && tc.shouldErr {
+			t.Errorf("Test %d, expected error, got nil", i)
+			continue
+		}
+		if err != nil && !tc.shouldErr {
+			t.Errorf("Test %d, expected no error, got %q", i, err)
+		}
+		if got != tc.expected {
+			t.Errorf("Test %d, expected %q, got %q", i, tc.expected, got)
+		}
+	}
+}