Laying the groundwork for passed-in configuration

The massive 80+ line `Customizations` variable is a hard-coded
monstrosity, and I've fallen out of love with it.

I'd like the customizations to be passed in from the caller, in this
case, `main.go`.

To that end, I've created a `default.json`, which should contain all the
customizations with the exception of the key-value functionality, which
I don't have a good way to deal with just yet.

src/sslip.io-dns-server/default.json

@@ -0,0 +1,85 @@
+[
+  {
+    "sslip.io.": {
+      "A": [
+        "78.46.204.247"
+      ],
+      "AAAA": [
+        "2a01:4f8:c17:b8f::2"
+      ],
+      "MX": [
+        {
+          "Pref": 10,
+          "MX": "mail.protonmail.ch."
+        },
+        {
+          "Pref": 10,
+          "MX": "mailsec.protonmail.ch."
+        }
+      ],
+      "TXT": [
+        "TXTSslipIoSPF"
+      ]
+    },
+    "k-v.io.": {
+      "A": [
+        "104.155.144.4"
+      ]
+    },
+    "_acme-challenge.k-v.io.": {
+      "TXT": []
+    },
+    "ns.sslip.io.": {
+      "A": [
+        "52.0.56.137",
+        "52.187.42.158",
+        "104.155.144.4"
+      ],
+      "AAAA": [
+        "2600:1f18:aaf:6900::a"
+      ]
+    },
+    "ns-aws.sslip.io.": {
+      "A": [
+        "52.0.56.137"
+      ],
+      "AAAA": [
+        "2600:1f18:aaf:6900::a"
+      ]
+    },
+    "ns-azure.sslip.io.": {
+      "A": [
+        "52.187.42.158"
+      ]
+    },
+    "ns-gce.sslip.io.": {
+      "A": [
+        "104.155.144.4"
+      ]
+    },
+    "protonmail._domainkey.sslip.io.": {
+      "CNAME": "protonmail.domainkey.dw4gykv5i2brtkjglrf34wf6kbxpa5hgtmg2xqopinhgxn5axo73a.domains.proton.ch."
+    },
+    "protonmail2._domainkey.sslip.io.": {
+      "CNAME": "protonmail2.domainkey.dw4gykv5i2brtkjglrf34wf6kbxpa5hgtmg2xqopinhgxn5axo73a.domains.proton.ch."
+    },
+    "protonmail3._domainkey.sslip.io.": {
+      "CNAME": "protonmail3.domainkey.dw4gykv5i2brtkjglrf34wf6kbxpa5hgtmg2xqopinhgxn5axo73a.domains.proton.ch."
+    },
+    "ip.sslip.io.": {
+      "TXT": [
+        "TXTIp"
+      ]
+    },
+    "version.status.sslip.io.": {
+      "TXT": [
+        "TXTVersion"
+      ]
+    },
+    "metrics.status.sslip.io.": {
+      "TXT": [
+        "TXTMetrics"
+      ]
+    }
+  }
+]

src/sslip.io-dns-server/xip/xip.go

@@ -153,15 +153,7 @@ var (
 					MX:   mx2,
 				},
 			},
-			TXT: func(_ *Xip, _ net.IP) ([]dnsmessage.TXTResource, error) {
+			TXT: TXTSslipIoSPF,
-				// Although multiple TXT records with multiple strings are allowed, we're sticking
-				// with a multiple TXT records with a single string apiece because that's what ProtonMail requires
-				// and that's what google.com does.
-				return []dnsmessage.TXTResource{
-					{TXT: []string{"protonmail-verification=ce0ca3f5010aa7a2cf8bcc693778338ffde73e26"}}, // ProtonMail verification; don't delete
-					{TXT: []string{"v=spf1 include:_spf.protonmail.ch mx ~all"}},
-				}, nil // Sender Policy Framework
-			},
 		},
 		"k-v.io.": {
 			A: []dnsmessage.AResource{
@@ -208,7 +200,7 @@ var (
 		},
 		// Special-purpose TXT records
 		"ip.sslip.io.": {
-			TXT: ipSslipIo,
+			TXT: TXTIp,
 		},
 		"version.status.sslip.io.": {
 			TXT: func(x *Xip, _ net.IP) ([]dnsmessage.TXTResource, error) {
@@ -221,7 +213,7 @@ var (
 			},
 		},
 		"metrics.status.sslip.io.": {
-			TXT: metricsSslipIo,
+			TXT: TXTMetrics,
 		},
 	}
 )
@@ -900,14 +892,25 @@ func (x *Xip) PTRResource(fqdn []byte) *dnsmessage.PTRResource {
 	return nil
 }
 
+// SFP records for sslio.io
+func TXTSslipIoSPF(_ *Xip, _ net.IP) ([]dnsmessage.TXTResource, error) {
+	// Although multiple TXT records with multiple strings are allowed, we're sticking
+	// with a multiple TXT records with a single string apiece because that's what ProtonMail requires
+	// and that's what google.com does.
+	return []dnsmessage.TXTResource{
+		{TXT: []string{"protonmail-verification=ce0ca3f5010aa7a2cf8bcc693778338ffde73e26"}}, // ProtonMail verification; don't delete
+		{TXT: []string{"v=spf1 include:_spf.protonmail.ch mx ~all"}},
+	}, nil // Sender Policy Framework
+}
+
 // when TXT for "ip.sslip.io" is queried, return the IP address of the querier
-func ipSslipIo(x *Xip, srcAddr net.IP) ([]dnsmessage.TXTResource, error) {
+func TXTIp(x *Xip, srcAddr net.IP) ([]dnsmessage.TXTResource, error) {
 	x.Metrics.AnsweredTXTSrcIPQueries++
 	return []dnsmessage.TXTResource{{TXT: []string{srcAddr.String()}}}, nil
 }
 
 // when TXT for "metrics.sslip.io" is queried, return the cumulative metrics
-func metricsSslipIo(x *Xip, _ net.IP) (txtResources []dnsmessage.TXTResource, err error) {
+func TXTMetrics(x *Xip, _ net.IP) (txtResources []dnsmessage.TXTResource, err error) {
 	<-x.DnsAmplificationAttackDelay
 	var metrics []string
 	uptime := time.Since(x.Metrics.Start)