RESTBase is currently acting as a proxy for the Mobile-Content-Service. This works fine when a client issues requests against https://{domain}/api/rest_v1/, but when https://rest.wikimedia.org/{domain}/v1/ is used, a 503 is received.
$ curl https://en.wikipedia.org/api/rest_v1/page/mobile-html/Foobar -v > GET /api/rest_v1/page/mobile-html/Foobar HTTP/1.1 > User-Agent: curl/7.35.0 > Host: en.wikipedia.org > Accept: */* > < HTTP/1.1 200 OK < Server: nginx/1.9.4 < Date: Mon, 21 Sep 2015 11:22:25 GMT < Content-Type: text/html; charset=utf-8 < Transfer-Encoding: chunked < Connection: keep-alive < access-control-allow-origin: * < access-control-allow-headers: Accept, X-Requested-With, Content-Type < x-xss-protection: 1; mode=block < x-content-type-options: nosniff < x-frame-options: SAMEORIGIN < content-security-policy: default-src 'self'; object-src 'none'; media-src *; img-src *; style-src *; frame-ancestors 'self' < x-content-security-policy: default-src 'self'; object-src 'none'; media-src *; img-src *; style-src *; frame-ancestors 'self' < x-webkit-csp: default-src 'self'; object-src 'none'; media-src *; img-src *; style-src *; frame-ancestors 'self' < vary: Accept-Encoding < cache-control: private, max-age=0, s-maxage=0, must-revalidate < Access-Control-Allow-Origin: * < Access-Control-Allow-Methods: GET < Access-Control-Allow-Headers: accept, content-type < X-Content-Type-Options: nosniff < X-Frame-Options: SAMEORIGIN < X-XSS-Protection: 1; mode=block < Content-Security-Policy: default-src 'none'; media-src *; img-src *; style-src *; frame-ancestors 'self' < X-Content-Security-Policy: default-src 'none'; media-src *; img-src *; style-src *; frame-ancestors 'self' < X-WebKit-CSP: default-src 'none'; media-src *; img-src *; style-src *; frame-ancestors 'self' < X-Request-Id: 0908603d-6053-11e5-9d2c-b6c4e9e9329d < X-Varnish: 615462701, 4132442282, 1437068073 < Via: 1.1 varnish, 1.1 varnish, 1.1 varnish < Age: 0 < X-Cache: cp1065 miss (0), cp3030 miss (0), cp3013 frontend miss (0) < Strict-Transport-Security: max-age=31536000; includeSubDomains; preload
$ curl http://rest.wikimedia.org/en.wikipedia.org/v1/page/mobile-html/Foobar -v > GET /en.wikipedia.org/v1/page/mobile-html/Foobar HTTP/1.1 > User-Agent: curl/7.35.0 > Host: rest.wikimedia.org > Accept: */* > < HTTP/1.1 503 Service Unavailable < Proxy-Connection: Keep-Alive < Connection: Keep-Alive < Content-Length: 419 < Via: 1.1 varnish, 1.1 ISAROVINJ < Age: 0 < Date: Mon, 21 Sep 2015 11:23:47 GMT < Content-Type: text/html; charset=utf-8 < Server: Varnish < Retry-After: 5 < Accept-Ranges: bytes < X-Varnish: 1072778244 < X-Cache: cp1058 frontend miss (0)
The obvious blame candidate are CSP headers received by Varnish.
In spite of the fact that we plan to get rid of rest.wm.org, we should ensure equivalent functionality while it's still around.