package sslconf

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An OCaml version of Openssl's NCONF library


Dune Dependency





sslconf — an OCaml version of Openssl's NCONF library


sslconf is a reimplementation of the Openssl NCONF library in OCaml.

NCONF reads Openssl config files. It delivers a data structure and a query API. Under the data structure are hash tables with strings and name-value stacks as values. The query API hides details of implementation.

sslconf has only OCaml code, so it can be used in a unikernel.

sslconf is distributed under the ISC license.


Openssl Config File Features

Openssl NCONF documentation is here.

Features of interest in config files:

  • namespace support. Names are mapped to values within namespace-like sections.

  • the NCONF query ("get value", "get section") API. Values mapped to names in sections can be queried. Also, sections as a whole can be queried and returned as a stack of name-value pairs.

  • default names. A default section lets one define names which are valid in any section (if not redefined).

  • name references inside values. References to names in the same section ("unqualified" names) or in other sections ("qualified" names) can be embedded in values. Forward references are not supported.

  • environment variable support. Environment variables can be referenced in values as qualified names with section "ENV".

  • comments. Comments start with a hash character (#), and extend to the end of a line.

  • escapes. An escape character (\) can denote a whitespace control character (if followed by 'n', 'r', 'h', or 't'), or it can force inclusion of the character which follows it. If at the end of a line, an escape character requests a line continuation, i.e., to join the next line to the current line.

  • quote-wrapped parts. Double quotes or single quotes in values can surround substrings. Variable expansion is not applied to these substrings. Instances of the other quote also do not get interpreted.

In addition to NCONF features, this implementation adds serialization of NCONF structures to OCaml s-expressions.

Why NCONF? Why Openssl config files? Why config files?

Openssl config files are often recommended for SSL/TLS applications.

Values in Openssl configs are open to different (Unicode or other) encodings.

It is better to put secrets or sensitive data in a config file, than to expose them via command line arguments or environment variables. Process status command outputs can show command lines and environment variables to anyone (and may be transferred to centralized monitoring), whereas access to a config file can be limited to selected users.

Features of Openssl config files (see above) may be useful.


sslconf can be installed with opam:

opam install sslconf

If you don't use opam consult the opam file for build instructions.

Future Applications (or, what this library does not do)

This library does one thing well. It parses Openssl config files and converts them to a type isomorphic to Openssl CONF structs.

Openssl applications, and the Openssl crypto library, use CONF structs in many contexts.

Analogous applications are not implemented here. Hopefully, this work will lead to some.

An application of NCONF in Openssl is here.

Another application of NCONF is here.


Openssl NCONF documentation is here.

Our documentation and API reference is generated from source interfaces. It can be consulted online or via odig doc sslconf.

Example Programs

Directory examples has code for executables.

  • sslconf_show_config shows function Sslconf.conf_load_file.

  • sslconf_show_section shows function Sslconf.conf_get_section.

  • sslconf_show_value shows function Sslconf.conf_get_value.

Test Coverage

The library has a test suite with near-100% coverage.

Go here for a current coverage report.

A few cases are explicitly ignored. These cases satisfy the type checker, but can never happen.

Building, Testing, and Documentation from Source

To build:

cd lib
make build

To run tests:

cd lib
make runtest

To test with bisect_ppx test coverage:

(add "bisect_ppx -conditional" to the preprocess line in lib/jbuild)
cd lib
make coverage

A coverage report is copied to the _coverage directory.

To generate documentation:

cd lib
make doc

Generated documentation is copied to the doc directory.

To clean up,

cd lib
make clean

Support Code and Files

sslconf_dumpcases dumps test cases to config files in a directory, which must be empty or will be created.

sslconf_test creates and writes out a file cases.out in the directory it is run in. Usually, this directory is _build/default/test.

File cases.out rewrites the Testcase module, with expect strings set to the result of running sslconf_test.

If test/ is replaced by cases.out, and sslconf is rebuilt, all tests should run successfully.

A directory c holds a C language program dump_config.c which calls Openssl to dump config information. It can be used to compare Openssl output to outputs from this implementation.


Thanks to the implementers of the Astring and Bisect_ppx packages, and to the implementers of the Jbuilder and Topkg packages which made structuring and building this a pleasure. Also, thanks to the implementers of the more general packages Sexplib, Ppx_sexp_conv, and OUnit2. A final thanks to the sponsors and maintainers of Travis-CI, Github, and OPAM.


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