API: Add endpoint to check custom domain resolution (#6110)

server/routes/api/urls/urls.ts

@@ -1,5 +1,6 @@
+import dns from "dns";
 import Router from "koa-router";
-import { parseDomain } from "@shared/utils/domains";
+import { getBaseDomain, parseDomain } from "@shared/utils/domains";
 import parseDocumentSlug from "@shared/utils/parseDocumentSlug";
 import parseMentionUrl from "@shared/utils/parseMentionUrl";
 import { isInternalUrl } from "@shared/utils/urls";
@@ -7,7 +8,7 @@ import { NotFoundError, ValidationError } from "@server/errors";
 import auth from "@server/middlewares/authentication";
 import { rateLimiter } from "@server/middlewares/rateLimiter";
 import validate from "@server/middlewares/validate";
-import { Document, User } from "@server/models";
+import { Document, Share, Team, User } from "@server/models";
 import { authorize } from "@server/policies";
 import { presentDocument, presentMention } from "@server/presenters/unfurls";
 import presentUnfurl from "@server/presenters/unfurls/unfurl";
@@ -84,4 +85,63 @@ router.post(
   }
 );
 
+router.post(
+  "urls.validateCustomDomain",
+  rateLimiter(RateLimiterStrategy.OneHundredPerHour),
+  auth(),
+  validate(T.UrlsCheckCnameSchema),
+  async (ctx: APIContext<T.UrlsCheckCnameReq>) => {
+    const { hostname } = ctx.input.body;
+
+    const [team, share] = await Promise.all([
+      Team.findOne({
+        where: {
+          domain: hostname,
+        },
+      }),
+      Share.findOne({
+        where: {
+          domain: hostname,
+        },
+      }),
+    ]);
+    if (team || share) {
+      throw ValidationError("Domain is already in use");
+    }
+
+    let addresses;
+    try {
+      addresses = await new Promise<string[]>((resolve, reject) => {
+        dns.resolveCname(hostname, (err, addresses) => {
+          if (err) {
+            return reject(err);
+          }
+          return resolve(addresses);
+        });
+      });
+    } catch (err) {
+      if (err.code === "ENOTFOUND") {
+        throw NotFoundError("No CNAME record found");
+      }
+
+      throw ValidationError("Invalid domain");
+    }
+
+    if (addresses.length === 0) {
+      throw ValidationError("No CNAME record found");
+    }
+
+    const address = addresses[0];
+    const likelyValid = address.endsWith(getBaseDomain());
+
+    if (!likelyValid) {
+      throw ValidationError("CNAME is not configured correctly");
+    }
+
+    ctx.body = {
+      success: true,
+    };
+  }
+);
+
 export default router;