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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. When used with the PROXY protocol, stud can also transparently pass an existing PROXY header to the cleartext stream. This is especially useful if a TCP proxy is used in front of stud. Using either of these techniques, 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.


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).


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. If multiple certificates are given, stud will attempt to perform SNI (Server Name Indication) on new connections, by comparing the indicated name with the names on each of the certificates, in order. The first certificate that matches will be used. If none of the certificates matches, the last certificate will be used as the default.

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


        --config=FILE      Load configuration from specified file.
        --default-config   Prints default configuration to stdout.


      --tls                   TLSv1 (default)
      --ssl                   SSLv3 (implies no TLSv1)
  -c  --ciphers=SUITE         Sets allowed ciphers (Default: "")
  -e  --ssl-engine=NAME       Sets OpenSSL engine (Default: "")
  -O  --prefer-server-ciphers Prefer server list order


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


  -n  --workers=NUM          Number of worker processes (Default: 1)
  -B  --backlog=NUM          Set listen backlog size (Default: 100)
  -k  --keepalive=SECS       TCP keepalive on client socket (Default: 3600)


  -r  --chroot=DIR           Sets chroot directory (Default: "")
  -u  --user=USER            Set uid/gid after binding the socket (Default: "")
  -g  --group=GROUP          Set gid after binding the socket (Default: "")

  -q  --quiet                Be quiet; emit only error messages
  -s  --syslog               Send log message to syslog in addition to stderr/stdout
  --syslog-facility=FACILITY Syslog facility to use (Default: "daemon")

      --daemon               Fork into background and become a daemon (Default: off)
      --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
                             (Default: off)
      --write-proxy          Write HaProxy's PROXY (IPv4 or IPv6) protocol line
                             before actual data
                             (Default: off)
      --proxy-proxy          Proxy HaProxy's PROXY (IPv4 or IPv6) protocol line
                             before actual data
                             (Default: off)

  -t  --test                 Test configuration and exit
  -V  --version              Print program version and exit
  -h  --help                 This help message

Configuration File

Stud can also use a configuration file that supports all the same options as the command-line arguments. You can use stud --default-config to generate the default configuration on stdout; then, customize your configuration and pass it to stud --config=FILE.

Serving HTTPS

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


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 (http://bu.mp) server team. It currently (12/11) provides server-side TLS termination for over 85 million Bump users.

Special thanks to Colin Percival (@cperciva) for an early security audit and code review.

Finally, thank you to all the stud contributors, who have taken the program from a good start to a solid project: