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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 and 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 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.

Releases

Please be aware of the policy regarding releases, code stability, and security:

  • In git, the tip of the master branch should always build on Linux and FreeBSD, and is likely to be as stable as any other changeset. A careful review of patches is conducted before being pushed to github.
  • Periodically, a version tag will be pushed to github for an old(er) changeset--0.1, 0.2, etc. These tags mark a particular release of stud that has seen heavy testing and several weeks of production stability. Conservative users are advised to use a tag.
  • stud has an optional build that utilizes shared memory-based SSL contexts and UDP peer communication to keep a session cache between many child processes running on many machines. The use of this build can dramatically speed up SSL handshakes on many-core and/or clustered deployments. However, it's important to acknowledge the inevitable theoretical security tradeoffs associated with the use of this (substantially more complex) binary. Therefore, the deeply paranoid are advised to use only the standard stud binary at the cost of some performance.

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).

Installing

To install stud:

$ make
$ sudo make install

Usage

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

Detail about the entire set of options can be found by invoking stud -h:

Encryption Methods:
  --tls                    TLSv1 (default)
  --ssl                    SSLv3 (implies no TLSv1)
  -c CIPHER_SUITE          set allowed ciphers (default is OpenSSL defaults)
  -e ENGINE                set OpenSSL engine

Socket:
  -b HOST,PORT             backend [connect] (default is "127.0.0.1,8000")
  -f HOST,PORT             frontend [bind] (default is "*,8443")

Performance:
  -n CORES                 number of worker processes (default is 1)
  -B BACKLOG               set listen backlog size (default is 100)

Security:
  -r PATH                  chroot
  -u USERNAME              set gid/uid after binding the socket

Logging:
  -q                       be quiet; emit only error messages
  -s                       send log message to syslog in addition to stderr/stdout

Special:
  --write-ip               write 1 octet with the IP family followed by the IP
                           address in 4 (IPv4) or 16 (IPv6) octets little-endian
                           to backend before the actual data
  --write-proxy            write HaProxy's PROXY (IPv4 or IPv6) protocol line
                           before actual data

stud uses no configuration file.

Serving HTTPS

If you're using stud for HTTPS, please make sure to use the --ssl option!

Diffie–Hellman

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

Authors

stud was originally written by Jamie Turner (@jamwt) and is maintained by the Bump (http://bu.mp) server team. It currently (12/11) provides server-side TLS termination for over 65 million Bump users.

Contributors:

* Colin Percival @cperciva      -- early security 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,
                                   performance tweaks
* 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
* Joe Williams                  -- Syslog support
* Jason Cook                    -- SSL option tweaks (performance)
* Artur Bergman                 -- Socket tweaks (performance)