Make LocalTunnel URL a constant

Add comments to explain process.

src/app.js

@@ -17,6 +17,9 @@ const channels = require('./channels')
 
 const app = express(feathers())
 
+// Hardcoded LocalTunnel URL for testing federation.
+const localDomain = 'iiuoizjrsz.localtunnel.me'
+
 // Load app configuration
 app.configure(configuration())
 // Enable CORS, security, compression, favicon and body parsing
@@ -33,11 +36,11 @@ app.use('/.well-known/webfinger', function (req, res, next) {
 
   res.set('Content-Type', 'application/jrd+json; charset=utf-8')
   res.send(JSON.parse(`{
-      "subject": "person@iiuoizjrsz.localtunnel.me",
+      "subject": `person@${localDomain}`,
       "links": [{
         "rel": "self",
         "type": "application/ld+json; profile='https://www.w3.org/ns/activitystreams'",
-        "href": "https://iiuoizjrsz.localtunnel.me/"
+        "href": "https://${localDomain}/"
       }]
     }`)
   )
@@ -58,8 +61,20 @@ app.use('/', function (req, res, next) {
 
     'application/ld+json; profile="https://www.w3.org/ns/activitystreams"': function () {
       res.set('Content-Type', 'application/activity+json; charset=utf-8')
-      res.set('Link', '<https://iiuoizjrsz.localtunnel.me/.well-known/webfinger?resource=acct%3Aperson%40iiuoizjrsz.localtunnel.me>; rel="lrdd"; type="application/xrd+xml", <https://iiuoizjrsz.localtunnel.me/>; rel="alternate"; type="application/activity+json"')
 
+      // WebFinger support is not a requirement in the ActivityPub spec
+      // but it is required for federation with Mastodon.
+      // https://github.com/tootsuite/mastodon/issues/4906#issuecomment-328844846
+
+      res.set('Link', `<https://${localDomain}/.well-known/webfinger?resource=acct%3Aperson%40${localDomain}>; rel="lrdd"; type="application/xrd+xml", <https://${localDomain}/>; rel="alternate"; type="application/activity+json"`)
+
+      // The structure of this reponse is modelled on the Mastodon response
+      // to an Actor request. This full response should not be necessary
+      // to successfully interoperate with Mastodon. According to the same
+      // issue thread, above, the only requirements according to Eugen are
+      // the presence of an inbox endpoint and a link header pointing to
+      // the WebFinger endpoint (and the implementation of the webfinger
+      // endpoint itself.)
       const sampleActorActivityStreamObject = JSON.parse(`{
         "@context": [
           "https://www.w3.org/ns/activitystreams",
@@ -77,15 +92,15 @@ app.use('/', function (req, res, next) {
           }
         ],
         "type": "Person",
-        "id": "https://iiuoizjrsz.localtunnel.me/",
-        "following": "https://iiuoizjrsz.localtunnel.me/following",
-        "followers": "https://iiuoizjrsz.localtunnel.me/followers",
-        "inbox": "https://iiuoizjrsz.localtunnel.me/inbox",
-        "outbox": "https://iiuoizjrsz.localtunnel.me/outbox",
+        "id": "https://${localDomain}",
+        "following": "https://${localDomain}/following",
+        "followers": "https://${localDomain}/followers",
+        "inbox": "https://${localDomain}/inbox",
+        "outbox": "https://${localDomain}/outbox",
         "preferredUsername": "person",
         "name": "Aral Balkan",
         "summary": "Just a guy.",
-        "url": "https://iiuoizjrsz.localtunnel.me/",
+        "url": "https://${localDomain}/",
         "icon": {
           "type": "Image",
           "mediaType": "image/jpeg",
@@ -97,11 +112,11 @@ app.use('/', function (req, res, next) {
           "url": "https://ar.al/images/aral-432.jpg"
         },
         "endpoints": {
-          "sharedInbox": "https://iiuoizjrsz.localtunnel.me/inbox"
+          "sharedInbox": "https://${localDomain}/inbox"
         },
         "publicKey": {
-          "owner": "https://iiuoizjrsz.localtunnel.me/",
-          "id": "https://iiuoizjrsz.localtunnel.me/#main-key",
+          "owner": "https://${localDomain}/",
+          "id": "https://${localDomain}/#main-key",
           "publicKeyPem": "-----BEGIN PUBLIC KEY-----MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA8NY/rAs24sgBWrTFiE0ovxSbv9ekts8NM109W6WM3s30SpAAmK/dPVzmMLeZxrsHaJVOFOCuSe2X2vVHUkYySMDokdIUWfHfGf+hpied8QPVJopoZq8cv3zz6HC8j7RFUaQYQMYi5JpdF7z9y0ZpI8bvRxWH2TPchJQe0uDk8Jvdlcqm/FfQIC6rQyLgLnX2/kRs0e7TeYTmMlXvtbmUqbtBd9FHIA0Kz9xyPm310N2E0Ca/pbDRYEylw5roRN1FI3teov3dR3Jxoy02iwHTeI9FOUF4K/MaS+ebZGjlI7ArJa4zHQ0gqslFlNLbi+4KdOI6CQKyjBZqRNBGlXBD8QIDAQAB-----END PUBLIC KEY-----"
         }
       }