netroy · August 20, 2018 12:47 · Aug 20, 2018
diff --git a/lambda.js b/lambda.js
@@ -0,0 +1,39 @@
+"use strict";
+
+const { promisify } = require('util');
+const { resolveCname } = require('dns');
+const resolveCnameAsync = promisify(resolveCname);
+
+let resultCache = null;
+const deferClearCache = () => setTimeout(() => { resultCache = null }, 1000);
+
+const resolve = async () => {
+  if (resultCache) {
+    return resultCache;
+  }
+
+  const result = await resolveCnameAsync('blahblah.host.com');
+  if (result[0].includes('us-east-1')) {
+    resultCache = ['us-east-1', 'east-bucket'];
+  } else {
+    resultCache = ['us-west-2', 'west-bucket'];           
+  }
+  deferClearCache();
+  return resultCache;
+};
+
+exports.handler = async event => {
+  const request = event.Records[0].cf.request;
+  const [region, bucket] = await resolve();
+  const domainName = `${bucket}.s3.${region}.amazonaws.com`;
+  request.origin = {
+    s3: {
+      domainName,
+      region,
+      authMethod: 'none',
+      path: '',
+    }
+  };
+  request.headers['host'] = [{ key: 'host', value: domainName }];
+  return request;
+};