5 files changed, 200 insertions, 0 deletions
diff --git a/modules/nsid/.packaging/test.config b/modules/nsid/.packaging/test.config
new file mode 100644
index 0000000..de54cce
--- /dev/null
+++ b/modules/nsid/.packaging/test.config
@@ -0,0 +1,4 @@
+-- SPDX-License-Identifier: GPL-3.0-or-later
+modules.load('nsid')
+assert(nsid)
+quit()
diff --git a/modules/nsid/README.rst b/modules/nsid/README.rst
new file mode 100644
index 0000000..3d0bf4e
--- /dev/null
+++ b/modules/nsid/README.rst
@@ -0,0 +1,35 @@
+.. SPDX-License-Identifier: GPL-3.0-or-later
+
+.. _mod-nsid:
+
+Name Server Identifier (NSID)
+=============================
+
+Module ``nsid`` provides server-side support for :rfc:`5001`
+which allows DNS clients to request resolver to send back its NSID
+along with the reply to a DNS request.
+This is useful for debugging larger resolver farms
+(e.g. when using :ref:`systemd-multiple-instances`, anycast or load balancers).
+
+NSID value can be configured in the resolver's configuration file:
+
+.. code-block:: lua
+
+   modules.load('nsid')
+   nsid.name('instance 1')
+
+.. tip:: When dealing with Knot Resolver running in `multiple instances`
+        managed with systemd see :ref:`instance-specific-configuration`.
+
+You can also obtain configured NSID value:
+
+.. code-block:: lua
+
+   > nsid.name()
+   'instance 1'
+
+The module can be disabled at run-time:
+
+.. code-block:: lua
+
+   modules.unload('nsid')
diff --git a/modules/nsid/meson.build b/modules/nsid/meson.build
new file mode 100644
index 0000000..354e70b
--- /dev/null
+++ b/modules/nsid/meson.build
@@ -0,0 +1,25 @@
+# C module: nsid
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+nsid_src = files([
+  'nsid.c',
+])
+c_src_lint += nsid_src
+
+nsid_mod = shared_module(
+  'nsid',
+  nsid_src,
+  dependencies: [
+    libknot,
+    luajit,
+  ],
+  include_directories: mod_inc_dir,
+  name_prefix: '',
+  install: true,
+  install_dir: modules_dir,
+  link_with: kresd,
+)
+
+config_tests += [
+  ['nsid', files('nsid.test.lua')],
+]
diff --git a/modules/nsid/nsid.c b/modules/nsid/nsid.c
new file mode 100644
index 0000000..a67dcdb
--- /dev/null
+++ b/modules/nsid/nsid.c
@@ -0,0 +1,114 @@
+/* Copyright (C) Knot Resolver contributors
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * This module provides NSID support according to RFC 5001. */
+
+#include <libknot/packet/pkt.h>
+#include <contrib/cleanup.h>
+#include <ccan/json/json.h>
+#include <lauxlib.h>
+
+#include "daemon/engine.h"
+#include "lib/layer.h"
+
+struct nsid_config {
+	uint8_t *local_nsid;
+	size_t local_nsid_len;
+};
+
+static int nsid_finalize(kr_layer_t *ctx) {
+	const struct kr_module *module = ctx->api->data;
+	const struct nsid_config *config = module->data;
+	struct kr_request *req = ctx->req;
+
+	/* no local NSID configured, do nothing */
+	if (config->local_nsid == NULL)
+		return ctx->state;
+
+	const knot_rrset_t *src_opt = req->qsource.packet->opt_rr;
+	/* no EDNS in request, do nothing */
+	if (src_opt == NULL)
+		return ctx->state;
+
+	const uint8_t *req_nsid = knot_edns_get_option(src_opt, KNOT_EDNS_OPTION_NSID, NULL);
+	/* NSID option must be explicitly requested */
+	if (req_nsid == NULL)
+		return ctx->state;
+
+	/* Check violation of https://tools.ietf.org/html/rfc5001#section-2.1:
+	 * The resolver MUST NOT include any NSID payload data in the query */
+	if (knot_edns_opt_get_length(req_nsid) != 0)
+		kr_log_debug(NSID, "[%05u.  ] FORMERR: NSID option in query "
+			       "must not contain payload, continuing\n", req->uid);
+		/* FIXME: actually change RCODE in answer to FORMERR? */
+
+	/* Sanity check, answer should have EDNS as well but who knows ... */
+	if (kr_fails_assert(req->answer->opt_rr))
+		return ctx->state;
+
+	if (knot_edns_add_option(req->answer->opt_rr, KNOT_EDNS_OPTION_NSID,
+				 config->local_nsid_len, config->local_nsid,
+				 &req->pool) != KNOT_EOK) {
+		/* something went wrong and there is no way to salvage content of OPT RRset */
+		kr_log_debug(NSID, "[%05u.  ] unable to add NSID option\n", req->uid);
+		knot_rrset_clear(req->answer->opt_rr, &req->pool);
+	}
+
+	return ctx->state;
+}
+
+static char* nsid_name(void *env, struct kr_module *module, const char *args)
+{
+	struct engine *engine = env;
+	struct nsid_config *config = module->data;
+	if (args) {  /* set */
+		/* API is not binary safe, we need to fix this one day */
+		uint8_t *arg_copy = (uint8_t *)strdup(args);
+		if (arg_copy == NULL)
+			luaL_error(engine->L, "[nsid] error while allocating new NSID value\n");
+		free(config->local_nsid);
+		config->local_nsid = arg_copy;
+		config->local_nsid_len = strlen(args);
+	}
+
+	/* get */
+	if (config->local_nsid != NULL)
+		return json_encode_string((char *)config->local_nsid);
+	else
+		return NULL;
+}
+
+KR_EXPORT
+int nsid_init(struct kr_module *module) {
+	static kr_layer_api_t layer = {
+		.answer_finalize = &nsid_finalize,
+	};
+	layer.data = module;
+	module->layer = &layer;
+
+	static const struct kr_prop props[] = {
+	    { &nsid_name, "name", "Get or set local NSID value" },
+	    { NULL, NULL, NULL }
+	};
+	module->props = props;
+
+	struct nsid_config *config = calloc(1, sizeof(struct nsid_config));
+	if (config == NULL)
+		return kr_error(ENOMEM);
+
+	module->data = config;
+	return kr_ok();
+}
+
+KR_EXPORT
+int nsid_deinit(struct kr_module *module) {
+	struct nsid_config *config = module->data;
+	if (config != NULL) {
+		free(config->local_nsid);
+		free(config);
+		module->data = NULL;
+	}
+	return kr_ok();
+}
+
+KR_MODULE_EXPORT(nsid)
diff --git a/modules/nsid/nsid.test.lua b/modules/nsid/nsid.test.lua
new file mode 100644
index 0000000..8e5d4f9
--- /dev/null
+++ b/modules/nsid/nsid.test.lua
@@ -0,0 +1,22 @@
+-- SPDX-License-Identifier: GPL-3.0-or-later
+-- disable networking so we can get SERVFAIL immediately
+net.ipv4 = false
+net.ipv6 = false
+
+-- test for nsid.name() interface
+local function test_nsid_name()
+	if nsid then
+		modules.unload('nsid')
+	end
+	modules.load('nsid')
+	same(nsid.name(), nil, 'NSID modes not provide default NSID value')
+	same(nsid.name('123456'), '123456', 'NSID value can be changed')
+	same(nsid.name(), '123456', 'NSID module remembers configured NSID value')
+	modules.unload('nsid')
+	modules.load('nsid')
+	same(nsid.name(), nil, 'NSID module reload removes configured value')
+end
+
+return {
+	test_nsid_name,
+}