implemented initial support for tls-alpn-01 verification

docs/tls-alpn.md

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+# TLS-ALPN-01
+
+With `tls-alpn-01`-type verification Let's Encrypt (or the ACME-protocol in general) is checking if you are in control of a domain by accessing
+your webserver using a custom ALPN and expecting a specially crafted TLS certificate containing a verification token.
+It will do that for any (sub-)domain you want to sign a certificate for.
+
+Dehydrated generates the required verification certificates, but the delivery is out of its scope.
+
+### Example nginx config
+
+On an nginx tcp load-balancer you can use the `ssl_preread` module to map a different port for acme-tls
+requests than for e.g. HTTP/2 or HTTP/1.1 requests.
+
+Your config should look something like this:
+
+```nginx
+stream {
+  server {
+    map $ssl_preread_alpn_protocols $tls_port {
+      ~\bacme-tls/1\b 10443;
+      default 443;
+    }
+
+    server {
+      listen 443;
+      listen [::]:443;
+      proxy_pass 10.13.37.42:$tls_port;
+      ssl_preread on;
+    }
+  }
+}
+```
+
+That way https requests are forwarded to port 443 on the backend server, and acme-tls/1 requests are
+forwarded to port 10443.
+
+In the future nginx might support internal routing based on custom ALPNs, but for now you'll have to
+use a custom responder for the alpn verification certificates (see below).
+
+### Example responder
+
+I hacked together a simple responder in Python, it might not be the best, but it works for me:
+
+```python
+#!/usr/bin/env python3
+
+import ssl
+import socketserver
+import threading
+import re
+import os
+
+ALPNDIR="/etc/dehydrated/alpn-certs"
+PROXY_PROTOCOL=False
+
+FALLBACK_CERTIFICATE="/etc/ssl/certs/ssl-cert-snakeoil.pem"
+FALLBACK_KEY="/etc/ssl/private/ssl-cert-snakeoil.key"
+
+class ThreadedTCPServer(socketserver.ThreadingMixIn, socketserver.TCPServer):
+    pass
+
+class ThreadedTCPRequestHandler(socketserver.BaseRequestHandler):
+    def create_context(self, certfile, keyfile, first=False):
+        ssl_context = ssl.create_default_context(ssl.Purpose.CLIENT_AUTH)
+        ssl_context.set_ciphers('ECDHE+AESGCM')
+        ssl_context.set_alpn_protocols(["acme-tls/1"])
+        ssl_context.options |= ssl.OP_NO_TLSv1 | ssl.OP_NO_TLSv1_1
+        if first:
+            ssl_context.set_servername_callback(self.load_certificate)
+        ssl_context.load_cert_chain(certfile=certfile, keyfile=keyfile)
+        return ssl_context
+
+    def load_certificate(self, sslsocket, sni_name, sslcontext):
+        print("Got request for %s" % sni_name)
+        if not re.match(r'^(([a-zA-Z]{1})|([a-zA-Z]{1}[a-zA-Z]{1})|([a-zA-Z]{1}[0-9]{1})|([0-9]{1}[a-zA-Z]{1})|([a-zA-Z0-9][-_.a-zA-Z0-9]{0,61}[a-zA-Z0-9]))\.([a-zA-Z]{2,13}|[a-zA-Z0-9-]{2,30}.[a-zA-Z]{2,3})$', sni_name):
+            return
+
+        certfile = os.path.join(ALPNDIR, "%s.crt.pem" % sni_name)
+        keyfile = os.path.join(ALPNDIR, "%s.key.pem" % sni_name)
+
+        if not os.path.exists(certfile) or not os.path.exists(keyfile):
+            return
+
+        sslsocket.context = self.create_context(certfile, keyfile)
+
+    def handle(self):
+        if PROXY_PROTOCOL:
+            buf = b""
+            while b"\r\n" not in buf:
+                buf += self.request.recv(1)
+
+        ssl_context = self.create_context(FALLBACK_CERTIFICATE, FALLBACK_KEY, True)
+        newsock = ssl_context.wrap_socket(self.request, server_side=True)
+
+if __name__ == "__main__":
+    HOST, PORT = "0.0.0.0", 10443
+
+    server = ThreadedTCPServer((HOST, PORT), ThreadedTCPRequestHandler, bind_and_activate=False)
+    server.allow_reuse_address = True
+    try:
+        server.server_bind()
+        server.server_activate()
+        server.serve_forever()
+    except:
+        server.shutdown()
+```