The Scalable TLS Unwrapping Daemon
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stud - The Scalable TLS Unwrapping Daemon

stud is a network proxy that terminates TLS/SSL connections and forwards the unencrypted traffic to some backend. It's designed to handle 10s of thousands of connections efficiently on multicore machines.

It follows a process-per-core model; a parent process spawns N children who each accept() on a common socket to distribute connected clients among them. Within each child, asynchronous socket I/O is conducted across the local connections using libev and OpenSSL's nonblocking API. By default, stud has an overhead of ~200KB per connection--it preallocates some buffer space for data in flight between frontend in backend.

stud has very few features--it's designed to be paired with an intelligent backend like haproxy or nginx. It maintains a strict 1:1 connection pattern with this backend handler so that the backend can dictate throttling behavior, maxmium connection behavior, availability of service, etc.

stud has one "cool trick"--it will optionally write the client IP address as the first few octets (depending on IPv4 or IPv6) to the backend--or provide that information using HAProxy's PROXY protocol. In this way, backends who care about the client IP can still access it even though stud itself appears to be the connected client.

Thanks to a contribution from Emeric at Exceliance (the folks behind HAProxy), a special build of stud can be made that utilitizes shared memory to use a common session cache between all child processes. This can speed up large stud deployments by avoiding client renegotiation.

Requirements and Limitations

stud requires:

libev >= 4
openssl (recent, >=1.0.0 recommended)

Stud currently works on Linux, OpenBSD, FreeBSD, and MacOSX. It has been tested the most heavily on Linux/x86_64.

While porting it to other POSIX platforms is likely trivial, it hasn't be done yet. Patches welcome!

If you're handling a large number of connections, you'll probably want to raise ulimit -n before running stud. It's very strongly recommended to not run stud as root; ideally, it would be run as a user ("stud", perhaps) that does nothing but run stud. Stud will setuid (using -u) after binding if you need to bind to a low port (< 1024).


To install stud:

$ make
$ sudo make install


The only required argument is a path to a PEM file that contains the certificate (or a chain of certificates) and private key.

The entire set of arguments can be invoked with stud -h:

Encryption Methods:
  --tls                    (TLSv1, default)
  --ssl                    (SSLv3)
  -c CIPHER_SUITE          (set allowed ciphers)

  -b HOST,PORT             (backend [connect], default ",8000")
  -f HOST,PORT             (frontend [bind], default "*,8443")

  -n CORES                 (number of worker processes, default 1)

  -r PATH                  (chroot)
  -u USERNAME              (set gid/uid after binding the socket)

  --write-ip               (write 1 octet with the IP family followed by
                            4 (IPv4) or 16 (IPv6) octets little-endian
                            to backend before the actual data)
  --write-proxy            (write HaProxy's PROXY protocol line before actual data:
                            "PROXY TCP4 <source-ip> <dest-ip> <source-port> <dest-port>\r\n"
                            Note, that dest-ip and dest-port are initialized once after the socket
                            is bound. It means that you will get as dest-ip instead of 
                            actual IP if that what the listening socket was bound to)

stud uses no configuration file.


To use DH with stud, you will need to add some bytes to your pem file:

% openssl dhparam -rand - 1024 >> PEMFILE

Be sure to set your cipher suite appropriately: -c DHE-RSA-AES256-SHA


stud was originally written by Jamie Turner (@jamwt) and is maintained by the Bump ( server team. It currently (6/11) provides server-side TLS termination for over 40 million Bump users.


* Colin Percival @cperciva      -- early audit and code review
* Frank DENIS @jedisct1         -- port to BSD, IPv6 support, various fixes
* Denis Bilenko                 -- HAProxy PROXY protocol support, chroot/setuid
* Joe Damato                    -- Diffie-Hellman parameter loading
* Benjamin Pineau               -- Chained cert loading, various fixes
* Carl Perry/Dreamhost          -- IPv6 PROXY support
* Emeric Brun/Exceliance        -- Session resumption and shared-memory
                                   session cache
* Vladimir Dronnikov            -- Logging cleanup
* James Golick/BitLove Inc.     -- SIGPIPE fixes and child-reaping