doc: update installation document

diff --git a/doc/installation.adoc b/doc/installation.adoc
index 151674a08c7d6b4ccf869c4b82f835e4660eeb99..eea9088b6becf3e37eeecb4f7a3fb71e692f5c83 100644 (file)
--- a/doc/installation.adoc
+++ b/doc/installation.adoc
@@ -29,8 +29,8 @@ After unpacking the source code, change directory into it, and type
 ----
 
 This is a shell script that automatically determines the system type. There is
-a single optional parameter, `--prefix` which indicates the directory tree
-where the software should be installed. For example,
+an optional parameter `--prefix`, which indicates the directory tree where the
+software should be installed. For example,
 
 ----
 ./configure --prefix=/opt/free
@@ -40,11 +40,11 @@ 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:
+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 CFLAGS=-O ./configure --prefix=/opt/free
+CC=cc ./configure --prefix=/opt/free
 ----
 
 for Bourne-family shells, or
@@ -63,11 +63,26 @@ 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.
+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 https://www.lysator.liu.se/~nisse/nettle/[Nettle],
+https://developer.mozilla.org/en-US/docs/Mozilla/Projects/NSS[NSS], or
+http://www.libtom.net/LibTomCrypt/[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.
+specify the `--disable-readline` flag to `configure`.
 
 If a `timepps.h` header is available (e.g. from the
 http://linuxpps.org[LinuxPPS project]), `chronyd` will be built with PPS API
@@ -75,6 +90,9 @@ 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
 
 ----
@@ -122,6 +140,16 @@ 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
+https://github.com/seccomp/libseccomp[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
@@ -132,12 +160,12 @@ 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
+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:
+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...
@@ -161,12 +189,12 @@ normally searched by the compiler and linker, you need to use extra options:
 
 == 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 `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 install directory for the
-man pages. This overrides the `man` subdirectory of the argument to the
---prefix option.
+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
@@ -174,8 +202,8 @@ man pages. This overrides the `man` subdirectory of the argument to the
 
 to set both options together.
 
-The final option is the `DESTDIR` option to the make command. For example, you
-could use the commands
+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