tools: add a systemd unit for static rulesets

There is a customer request (bugreport) for wanting to trivially load a ruleset
from a well-known location on boot, forwarded to me by M. Gerstner. A systemd
service unit is hereby added to provide that functionality. This is based on
various distributions attempting to do same, for example,

https://src.fedoraproject.org/rpms/nftables/tree/rawhide
https://gitlab.alpinelinux.org/alpine/aports/-/blob/master/main/nftables/nftables.initd
https://gitlab.archlinux.org/archlinux/packaging/packages/nftables
Acked-by: Eric Garver <eric@garver.life>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

INSTALL

@@ -42,6 +42,12 @@ Installation instructions for nftables
 	The base directory for arch-independent files. Defaults to
 	$prefix/share.
 
+ --with-unitdir=
+
+	Directory for systemd unit files. Defaults to the value obtained from
+	pkg-config for systemd.pc, and ${prefix}/lib/systemd/system as a
+	fallback.
+
  --disable-debug
 
 	Disable debugging

Makefile.am

@@ -377,18 +377,19 @@ dist_pkgdata_DATA = \
 	files/nftables/netdev-ingress.nft \
 	$(NULL)
 
-pkgdocdir = ${docdir}/examples
+exampledir = ${docdir}/examples
 
-dist_pkgdoc_SCRIPTS = \
+dist_example_SCRIPTS = \
 	files/examples/ct_helpers.nft \
 	files/examples/load_balancing.nft \
 	files/examples/secmark.nft \
 	files/examples/sets_and_maps.nft \
 	$(NULL)
 
-pkgsysconfdir = ${sysconfdir}/nftables/osf
+pkgsysconfdir = ${sysconfdir}/${PACKAGE}
+osfdir = ${pkgsysconfdir}/osf
 
-dist_pkgsysconf_DATA = \
+dist_osf_DATA = \
 	files/osf/pf.os \
 	$(NULL)
 
@@ -412,3 +413,10 @@ EXTRA_DIST += \
 
 pkgconfigdir = $(libdir)/pkgconfig
 pkgconfig_DATA = libnftables.pc
+unit_DATA = tools/nftables.service
+man_MANS = tools/nftables.service.8
+doc_DATA = files/nftables/main.nft
+
+tools/nftables.service: tools/nftables.service.in ${top_builddir}/config.status
+	${AM_V_GEN}${MKDIR_P} tools
+	${AM_V_at}sed -e 's|@''sbindir''@|${sbindir}|g;s|@''pkgsysconfdir''@|${pkgsysconfdir}|g' <${srcdir}/tools/nftables.service.in >$@

configure.ac

@@ -114,6 +114,16 @@ AC_CHECK_DECLS([getprotobyname_r, getprotobynumber_r, getservbyport_r], [], [],
 #include <netdb.h>
 ]])
 
+AC_ARG_WITH([unitdir],
+	[AS_HELP_STRING([--with-unitdir=PATH], [Path to systemd service unit directory])],
+	[unitdir="$withval"],
+	[
+		unitdir=$("$PKG_CONFIG" systemd --variable systemdsystemunitdir 2>/dev/null)
+		AS_IF([test -z "$unitdir"], [unitdir='${prefix}/lib/systemd/system'])
+	])
+AC_SUBST([unitdir])
+
+
 AC_CONFIG_FILES([					\
 		Makefile				\
 		libnftables.pc				\
@@ -127,4 +137,5 @@ nft configuration:
   use mini-gmp:			${with_mini_gmp}
   enable man page:              ${enable_man_doc}
   libxtables support:		${with_xtables}
-  json output support:          ${with_json}"
+  json output support:          ${with_json}
+  systemd unit:			${unitdir}"

files/nftables/main.nft

@@ -0,0 +1,22 @@
+#!/usr/sbin/nft -f
+
+# template static firewall configuration file
+#
+# copy this over to /etc/nftables/rules/main.nft as a starting point for
+# configuring a rule set which will be loaded by nftables.service.
+
+table inet filter {
+	chain input {
+		type filter hook input priority filter;
+	}
+	chain forward {
+		type filter hook forward priority filter;
+	}
+	chain output {
+		type filter hook output priority filter;
+	}
+}
+
+# this can be used to split the rule set into multiple smaller files concerned
+# with specific topics, like forwarding rules
+#include "/etc/nftables/rules/forwarding.nft"

tools/nftables.service.8

@@ -0,0 +1,17 @@
+.TH nftables.service 8 "" "nftables" "nftables admin reference"
+.SH Name
+nftables.service \(em Static Firewall Configuration with nftables.service
+.SH Description
+An nftables systemd service is provided which allows to setup static firewall
+rulesets based on a configuration file.
+.PP
+To use this service, you need to create the main configuration file in
+/etc/nftables/rules/main.nft. A template for this can be copied from
+/usr/share/doc/nftables/main.nft. Alternatively, `nft list ruleset >main.nft`
+could be used to save the active configuration (if any) to the file.
+.PP
+Once the desired static firewall configuration is in place, it can be tested by
+running `systemctl start nftables.service`. To enable the service at boot time,
+run `systemctl enable nftables.service`.
+.SH See also
+\fBnft\fP(8)

tools/nftables.service.in

@@ -0,0 +1,21 @@
+[Unit]
+Description=nftables static rule set
+Documentation=man:nftables.service(8)
+Wants=network-pre.target
+Before=network-pre.target shutdown.target
+Conflicts=shutdown.target
+DefaultDependencies=no
+ConditionPathExists=@pkgsysconfdir@/rules/main.nft
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+StandardInput=null
+ProtectSystem=full
+ProtectHome=true
+ExecStart=@sbindir@/nft 'flush ruleset; include "@pkgsysconfdir@/rules/main.nft"'
+ExecReload=@sbindir@/nft 'flush ruleset; include "@pkgsysconfdir@/rules/main.nft"'
+ExecStop=@sbindir@/nft flush ruleset
+
+[Install]
+WantedBy=sysinit.target