# Installation

The software is distributed as source code which has to be compiled. The source code is supplied in the form of a gzipped tar file, which unpacks to a subdirectory identifying the name and version of the program.

The following programs and libraries with their development files are needed to build `chrony`:

- C compiler (gcc or clang recommended)

- GNU Make

- Nettle, NSS, or LibTomCrypt (optional)

- Editline (optional)

- libcap (Linux only, optional)

- libseccomp (Linux only, optional)

- timepps.h header (optional)

- Asciidoctor (for HTML documentation)

- Bash (for testing)

After unpacking the source code, change directory into it, and type

    ./configure

This is a shell script that automatically determines the system type. There is an optional parameter `--prefix`, which indicates the directory tree where the software should be installed. For example,

    ./configure --prefix=/opt/free

will install the `chronyd` daemon into `/opt/free/sbin` and the `chronyc` control program into `/opt/free/bin`. The default value for the prefix is `/usr/local`.

The `configure` script assumes you want to use `gcc` as your compiler. If you want to use a different compiler, you can configure this way:

    CC=cc ./configure --prefix=/opt/free

for Bourne-family shells, or

    setenv CC cc
    setenv CFLAGS -O
    ./configure --prefix=/opt/free

for C-family shells.

If the software cannot (yet) be built on your system, an error message will be shown. Otherwise, `Makefile` will be generated.

On Linux, if development files for the libcap library are available, `chronyd` will be built with support for dropping root privileges. On other systems no extra library is needed. The default user which `chronyd` should run as can be specified with the `--with-user` option of the `configure` script.

If development files for the POSIX threads library are available, `chronyd` will be built with support for asynchronous resolving of hostnames specified in the `server`, `peer`, and `pool` directives. This allows `chronyd` operating as a server to respond to client requests when resolving a hostname. If you don’t want to enable the support, specify the `--disable-asyncdns` flag to `configure`.

If development files for the Nettle, NSS, or libtomcrypt library are available, `chronyd` will be built with support for other cryptographic hash functions than MD5, which can be used for NTP authentication with a symmetric key. If you don’t want to enable the support, specify the `--disable-sechash` flag to `configure`.

If development files for the editline or readline library are available, `chronyc` will be built with line editing support. If you don’t want this, specify the `--disable-readline` flag to `configure`.

If a `timepps.h` header is available (e.g. from the LinuxPPS project), `chronyd` will be built with PPS API reference clock driver. If the header is installed in a location that isn’t normally searched by the compiler, you can add it to the searched locations by setting the `CPPFLAGS` variable to `-I/path/to/timepps`.

The `--help` option can be specified to `configure` to print all options supported by the script.

Now type

    make

to build the programs.

If you want to build the manual in HTML, type

    make docs

Once the programs have been successfully compiled, they need to be installed in their target locations. This step normally needs to be performed by the superuser, and requires the following command to be entered.

    make install

This will install the binaries and man pages.

To install the HTML version of the manual, enter the command

    make install-docs

Now that the software is successfully installed, the next step is to set up a configuration file. The default location of the file is /etc/chrony.conf. Several examples of configuration with comments are included in the examples directory. Suppose you want to use public NTP servers from the pool.ntp.org project as your time reference. A minimal useful configuration file could be

    pool pool.ntp.org iburst
    makestep 1.0 3
    rtcsync

Then, `chronyd` can be run. For security reasons, it’s recommended to create an unprivileged user for `chronyd` and specify it with the `-u` command-line option or the `user` directive in the configuration file, or set the default user with the `--with-user` configure option before building.

## Support for system call filtering

`chronyd` can be built with support for the Linux secure computing (seccomp) facility. This requires development files for the libseccomp library and the `--enable-scfilter` option specified to `configure`. The `-F` option of `chronyd` will enable a system call filter, which should significantly reduce the kernel attack surface and possibly prevent kernel exploits from `chronyd` if it is compromised.

## Support for line editing libraries

`chronyc` can be built with support for line editing, this allows you to use the cursor keys to replay and edit old commands. Two libraries are supported which provide such functionality, editline and GNU readline.

Please note that readline since version 6.0 is licensed under GPLv3+ which is incompatible with chrony’s license GPLv2. You should use editline instead if you don’t want to use older readline versions.

The `configure` script will automatically enable the line editing support if one of the supported libraries is available. If they are both available, the editline library will be used.

If you don’t want to use it (in which case `chronyc` will use a minimal command line interface), invoke `configure` like this:

    ./configure --disable-readline other-options...

If you have editline, readline or ncurses installed in locations that aren’t normally searched by the compiler and linker, you need to use extra options:

`--with-readline-includes=directory_name`

  This defines the name of the directory above the one where `readline.h` is. `readline.h` is assumed to be in `editline` or `readline` subdirectory of the named directory.

`--with-readline-library=directory_name`

  This defines the directory containing the `libedit.a` or `libedit.so` file, or `libreadline.a` or `libreadline.so` file.

`--with-ncurses-library=directory_name`

  This defines the directory containing the `libncurses.a` or `libncurses.so` file.

## Extra options for package builders

The `configure` and `make` procedures have some extra options that may be useful if you are building a distribution package for `chrony`.

The `--mandir=DIR` option to `configure` specifies an installation directory for the man pages. This overrides the `man` subdirectory of the argument to the `--prefix` option.

    ./configure --prefix=/usr --mandir=/usr/share/man

to set both options together.

The final option is the `DESTDIR` option to the `make` command. For example, you could use the commands

    ./configure --prefix=/usr --mandir=/usr/share/man
    make all docs
    make install DESTDIR=./tmp
    cd tmp
    tar cvf - . | gzip -9 > chrony.tar.gz

to build a package. When untarred within the root directory, this will install the files to the intended final locations.