resolve: Make path to resolvconf(8) configurable

Prefer the configured command over finding it at the default location
over installing in the configured file.

References strongswan/strongswan#744

conf/plugins/resolve.opt

@@ -1,5 +1,5 @@
 charon.plugins.resolve.file = /etc/resolv.conf
-	File where to add DNS server entries.
+	File where to add DNS server entries if not using resolvconf(8).
 
 charon.plugins.resolve.resolvconf.iface_prefix = lo.inet.ipsec.
 	Prefix used for interface names sent to resolvconf(8).
@@ -9,3 +9,13 @@ charon.plugins.resolve.resolvconf.iface_prefix = lo.inet.ipsec.
 	a valid interface name according to the rules defined by resolvconf.  Also,
 	it should have a high priority according to the order defined in
 	**interface-order**(5).
+
+charon.plugins.resolve.resolvconf.path = /sbin/resolvconf
+	Path/command for resolvconf(8).
+
+	Path/command for **resolvconf**(8). The command is executed by a shell, so
+	"resolvconf" will work if it's in $PATH of the daemon.
+
+	If not configured, **resolvconf**(8) will be used if found at the default
+	location.  Otherwise, the file in _charon.plugins.resolve.file_ will be
+	modfied directly.

src/libcharon/plugins/resolve/resolve_handler.c

@@ -50,9 +50,9 @@ struct private_resolve_handler_t {
 	char *file;
 
 	/**
-	 * Use resolvconf instead of writing directly to resolv.conf
+	 * Path/command for resolvconf(8)
 	 */
-	bool use_resolvconf;
+	char *resolvconf;
 
 	/**
 	 * Prefix to be used for interface names sent to resolvconf
@@ -196,7 +196,7 @@ static bool invoke_resolvconf(private_resolve_handler_t *this, host_t *addr,
 	/* we use the nameserver's IP address as part of the interface name to
 	 * make them unique */
 	process = process_start_shell(NULL, install ? &in : NULL, &out, NULL,
-							"2>&1 %s %s %s%H", RESOLVCONF_EXEC,
+							"2>&1 %s %s %s%H", this->resolvconf,
 							install ? "-a" : "-d", this->iface_prefix, addr);
 
 	if (!process)
@@ -295,7 +295,7 @@ METHOD(attribute_handler_t, handle, bool,
 	this->mutex->lock(this->mutex);
 	if (array_bsearch(this->servers, addr, dns_server_find, &found) == -1)
 	{
-		if (this->use_resolvconf)
+		if (this->resolvconf)
 		{
 			handled = invoke_resolvconf(this, addr, TRUE);
 		}
@@ -362,7 +362,7 @@ METHOD(attribute_handler_t, release, void,
 		}
 		else
 		{
-			if (this->use_resolvconf)
+			if (this->resolvconf)
 			{
 				invoke_resolvconf(this, addr, FALSE);
 			}
@@ -483,17 +483,28 @@ resolve_handler_t *resolve_handler_create()
 			.destroy = _destroy,
 		},
 		.mutex = mutex_create(MUTEX_TYPE_DEFAULT),
-		.file = lib->settings->get_str(lib->settings, "%s.plugins.resolve.file",
+		.file = lib->settings->get_str(lib->settings,
-									   RESOLV_CONF, lib->ns),
+								"%s.plugins.resolve.file", RESOLV_CONF, lib->ns),
+		.resolvconf = lib->settings->get_str(lib->settings,
+								"%s.plugins.resolve.resolvconf.path",
+								NULL, lib->ns),
+		.iface_prefix = lib->settings->get_str(lib->settings,
+								"%s.plugins.resolve.resolvconf.iface_prefix",
+								RESOLVCONF_PREFIX, lib->ns),
 	);
 
-	if (stat(RESOLVCONF_EXEC, &st) == 0)
+	if (!this->resolvconf && stat(RESOLVCONF_EXEC, &st) == 0)
 	{
-		this->use_resolvconf = TRUE;
+		this->resolvconf = RESOLVCONF_EXEC;
-		this->iface_prefix = lib->settings->get_str(lib->settings,
-								"%s.plugins.resolve.resolvconf.iface_prefix",
-								RESOLVCONF_PREFIX, lib->ns);
 	}
 
+	if (this->resolvconf)
+	{
+		DBG1(DBG_CFG, "using '%s' to install DNS servers", this->resolvconf);
+	}
+	else
+	{
+		DBG1(DBG_CFG, "install DNS servers in '%s'", this->file);
+	}
 	return &this->public;
 }