diff --git a/Knot.files b/Knot.files
index e8cc427c72a635eae244f181edda1138b86d3fe3..6dc06cdc9494139f7dc618bcc3fc093c2eb98748 100644
--- a/Knot.files
+++ b/Knot.files
@@ -23,6 +23,8 @@ src/contrib/files.c
src/contrib/files.h
src/contrib/getline.c
src/contrib/getline.h
+src/contrib/json.c
+src/contrib/json.h
src/contrib/libbpf/bpf/bpf.c
src/contrib/libbpf/bpf/bpf.h
src/contrib/libbpf/bpf/bpf_core_read.h
diff --git a/src/contrib/Makefile.inc b/src/contrib/Makefile.inc
index 32fb30750c5c602ea088c141907b768f1f1641ef..eb5a545f4cee5f6de1ec61005cae558df3176123 100644
--- a/src/contrib/Makefile.inc
+++ b/src/contrib/Makefile.inc
@@ -37,6 +37,8 @@ libcontrib_la_SOURCES = \
contrib/files.h \
contrib/getline.c \
contrib/getline.h \
+ contrib/json.c \
+ contrib/json.h \
contrib/macros.h \
contrib/mempattern.c \
contrib/mempattern.h \
diff --git a/src/contrib/json.c b/src/contrib/json.c
new file mode 100644
index 0000000000000000000000000000000000000000..814c87d044c0f46372663a043a86e10646bc32ad
--- /dev/null
+++ b/src/contrib/json.c
@@ -0,0 +1,215 @@
+/* Copyright (C) 2022 CZ.NIC, z.s.p.o. <knot-dns@labs.nic.cz>
+
+ This program is free software: you can redistribute it and/or modify
+ it under the terms of the GNU General Public License as published by
+ the Free Software Foundation, either version 3 of the License, or
+ (at your option) any later version.
+
+ This program is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ GNU General Public License for more details.
+
+ You should have received a copy of the GNU General Public License
+ along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <assert.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "contrib/json.h"
+
+#define MAX_DEPTH 8
+
+enum {
+ BLOCK_INVALID = 0,
+ BLOCK_OBJECT,
+ BLOCK_LIST,
+};
+
+/*! One indented block of JSON. */
+struct block {
+ /*! Block type. */
+ int type;
+ /*! Number of elements written. */
+ int count;
+};
+
+struct jsonw {
+ /*! Output file stream. */
+ FILE *out;
+ /*! Indentaiton string. */
+ const char *indent;
+ /*! List to be used as a stack of blocks in progress. */
+ struct block stack[MAX_DEPTH];
+ /*! Index pointing to the top of the stack. */
+ int top;
+};
+
+static const char *DEFAULT_INDENT = "\t";
+
+static void start_block(jsonw_t *w, int type)
+{
+ assert(w->top > 0);
+
+ struct block b = {
+ .type = type,
+ .count = 0,
+ };
+
+ w->top -= 1;
+ w->stack[w->top] = b;
+}
+
+static struct block *cur_block(jsonw_t *w)
+{
+ if (w && w->top < MAX_DEPTH) {
+ return &w->stack[w->top];
+ }
+ return NULL;
+}
+
+/*! Insert new line and indent for the next write. */
+static void wrap(jsonw_t *w)
+{
+ fputc('\n', w->out);
+ int level = MAX_DEPTH - w->top;
+ for (int i = 0; i < level; i++) {
+ fprintf(w->out, "%s", w->indent);
+ }
+}
+
+static void end_block(jsonw_t *w)
+{
+ assert(w->top < MAX_DEPTH);
+
+ w->top += 1;
+}
+
+static void escaped_print(jsonw_t *w, const char *str)
+{
+ fputc('"', w->out);
+ for (const char *pos = str; *pos != '\0'; pos++) {
+ char c = *pos;
+ if (c == '\\' || c == '\"') {
+ fputc('\\', w->out);
+ }
+ fputc(c, w->out);
+ }
+ fputc('"', w->out);
+}
+
+static void align_key(jsonw_t *w, const char *key)
+{
+ struct block *top = cur_block(w);
+ if (top && top->count++) {
+ fputc(',', w->out);
+ }
+
+ wrap(w);
+
+ if (key && key[0]) {
+ escaped_print(w, key);
+ fprintf(w->out, ": ");
+ }
+}
+
+jsonw_t *jsonw_new(FILE *out, const char *indent)
+{
+ assert(out);
+
+ jsonw_t *w = calloc(1, sizeof(*w));
+ if (w == NULL) {
+ return w;
+ }
+
+ w->out = out;
+ w->indent = indent ? indent : DEFAULT_INDENT;
+ w->top = MAX_DEPTH;
+
+ return w;
+}
+
+void jsonw_free(jsonw_t **w)
+{
+ if (w == NULL) {
+ return;
+ }
+
+ free(*w);
+ *w = NULL;
+}
+
+void jsonw_object(jsonw_t *w, const char *key)
+{
+ assert(w);
+
+ align_key(w, key);
+ fprintf(w->out, "{");
+ start_block(w, BLOCK_OBJECT);
+}
+
+void jsonw_list(jsonw_t *w, const char *key)
+{
+ assert(w);
+
+ align_key(w, key);
+ fprintf(w->out, "[");
+ start_block(w, BLOCK_LIST);
+}
+
+void jsonw_str(jsonw_t *w, const char *key, const char *value)
+{
+ assert(w);
+
+ align_key(w, key);
+ escaped_print(w, value);
+}
+
+void jsonw_ulong(jsonw_t *w, const char *key, unsigned long value)
+{
+ assert(w);
+
+ align_key(w, key);
+ fprintf(w->out, "%lu", value);
+}
+
+void jsonw_int(jsonw_t *w, const char *key, int value)
+{
+ assert(w);
+
+ align_key(w, key);
+ fprintf(w->out, "%d", value);
+}
+
+
+void jsonw_bool(jsonw_t *w, const char *key, bool value)
+{
+ assert(w);
+
+ align_key(w, key);
+ fprintf(w->out, "%s", value ? "true" : "false");
+}
+
+void jsonw_end(jsonw_t *w)
+{
+ assert(w);
+
+ struct block *top = cur_block(w);
+ if (top == NULL) {
+ return;
+ }
+
+ end_block(w);
+ wrap(w);
+
+ switch (top->type) {
+ case BLOCK_OBJECT:
+ fprintf(w->out, "}");
+ break;
+ case BLOCK_LIST:
+ fprintf(w->out, "]");
+ break;
+ }
+}
diff --git a/src/contrib/json.h b/src/contrib/json.h
new file mode 100644
index 0000000000000000000000000000000000000000..d57b5976f1ab72239252eacfbf1d9a2e7f548195
--- /dev/null
+++ b/src/contrib/json.h
@@ -0,0 +1,81 @@
+/* Copyright (C) 2022 CZ.NIC, z.s.p.o. <knot-dns@labs.nic.cz>
+
+ This program is free software: you can redistribute it and/or modify
+ it under the terms of the GNU General Public License as published by
+ the Free Software Foundation, either version 3 of the License, or
+ (at your option) any later version.
+
+ This program is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ GNU General Public License for more details.
+
+ You should have received a copy of the GNU General Public License
+ along with this program. If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#pragma once
+
+#include <stdio.h>
+#include <stdbool.h>
+
+/*!
+ * Simple pretty JSON writer.
+ */
+struct jsonw;
+typedef struct jsonw jsonw_t;
+
+/*!
+ * Create new JSON writer.
+ *
+ * @param out Output file stream.
+ * @param indent Indentation string.
+ *
+ * @return JSON writer or NULL for allocation error.
+ */
+jsonw_t *jsonw_new(FILE *out, const char *indent);
+
+/*!
+ * Free JSON writer created with jsonw_new.
+ */
+void jsonw_free(jsonw_t **w);
+
+/*!
+ * Start writing a new object.
+ *
+ * The following writes will represent key and value pairs respectively until
+ * jsonw_end is called.
+ */
+void jsonw_object(jsonw_t *w, const char *key);
+
+/*!
+ * Start writing a new list.
+ *
+ * The following writes will represent values until jsonw_end is called.
+ */
+void jsonw_list(jsonw_t *w, const char *key);
+
+/*!
+ * Write string as JSON. The string will be escaped properly.
+ */
+void jsonw_str(jsonw_t *w, const char *key, const char *value);
+
+/*!
+ * Write unsigned long value as JSON.
+ */
+void jsonw_ulong(jsonw_t *w, const char *key, unsigned long value);
+
+/*!
+ * Write integer as JSON.
+ */
+void jsonw_int(jsonw_t *w, const char *key, int value);
+
+/*!
+ * Write boolean value as JSON.
+ */
+void jsonw_bool(jsonw_t *w, const char *key, bool value);
+
+/*!
+ * Terminate in-progress object or list.
+ */
+void jsonw_end(jsonw_t *w);