middleware/hosts for /etc/hosts parsing (#695)

* add hosts middleware

* forgot pointer receiver

* add appropriately modified hostsfile tests from golang repo

* remove test artifacts, separate hostsfile parsing from caching and opening, remove unused metrics references, move middleware up the chain

* refactored the logic for creating records and filtering ip address versions. also got PTR lookups working

* Add README.md. Modify config to be more concise. Add zones list to config. Filter PTR responses based on zones list.

* add Fallthrough and return correct dns response code otherwise

* Simplified Hostsfile to only store hosts in the zones we care about, and by ip version. Added handler tests and improved other tests.

* oops, goimports loaded a package from a different repo

middleware/hosts/setup_test.go

@@ -0,0 +1,86 @@
+package hosts
+
+import (
+	"testing"
+
+	"github.com/mholt/caddy"
+)
+
+func TestHostsParse(t *testing.T) {
+	tests := []struct {
+		inputFileRules      string
+		shouldErr           bool
+		expectedPath        string
+		expectedOrigins     []string
+		expectedFallthrough bool
+	}{
+		{
+			`hosts
+`,
+			false, "/etc/hosts", nil, false,
+		},
+		{
+			`hosts /tmp`,
+			false, "/tmp", nil, false,
+		},
+		{
+			`hosts /etc/hosts miek.nl.`,
+			false, "/etc/hosts", []string{"miek.nl."}, false,
+		},
+		{
+			`hosts /etc/hosts miek.nl. pun.gent.`,
+			false, "/etc/hosts", []string{"miek.nl.", "pun.gent."}, false,
+		},
+		{
+			`hosts {
+				fallthrough
+			}`,
+			false, "/etc/hosts", nil, true,
+		},
+		{
+			`hosts /tmp {
+				fallthrough
+			}`,
+			false, "/tmp", nil, true,
+		},
+		{
+			`hosts /etc/hosts miek.nl. {
+				fallthrough
+			}`,
+			false, "/etc/hosts", []string{"miek.nl."}, true,
+		},
+		{
+			`hosts /etc/hosts miek.nl. pun.gent. {
+				fallthrough
+			}`,
+			false, "/etc/hosts", []string{"miek.nl.", "pun.gent."}, true,
+		},
+	}
+
+	for i, test := range tests {
+		c := caddy.NewTestController("dns", test.inputFileRules)
+		h, err := hostsParse(c)
+
+		if err == nil && test.shouldErr {
+			t.Fatalf("Test %d expected errors, but got no error", i)
+		} else if err != nil && !test.shouldErr {
+			t.Fatalf("Test %d expected no errors, but got '%v'", i, err)
+		} else if !test.shouldErr {
+			if h.path != test.expectedPath {
+				t.Fatalf("Test %d expected %v, got %v", i, test.expectedPath, h.path)
+			}
+		} else {
+			if h.Fallthrough != test.expectedFallthrough {
+				t.Fatalf("Test %d expected fallthrough of %v, got %v", i, test.expectedFallthrough, h.Fallthrough)
+			}
+			if len(h.Origins) != len(test.expectedOrigins) {
+				t.Fatalf("Test %d expected %v, got %v", i, test.expectedOrigins, h.Origins)
+			}
+			for j, name := range test.expectedOrigins {
+				if h.Origins[j] != name {
+					t.Fatalf("Test %d expected %v for %d th zone, got %v", i, name, j, h.Origins[j])
+				}
+			}
+		}
+	}
+}