tests: add sock dns client test application

tests/gnrc_sock_dns/Makefile

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+APPLICATION = gnrc_sock_dns
+include ../Makefile.tests_common
+
+RIOTBASE ?= $(CURDIR)/../..
+
+BOARD_INSUFFICIENT_MEMORY := chronos telosb nucleo32-f042 nucleo32-f031 nucleo-f030 nucleo-l053 nucleo32-l031 stm32f0discovery
+
+USEMODULE += sock_dns
+USEMODULE += gnrc_sock_udp
+USEMODULE += gnrc_ipv6_default
+USEMODULE += gnrc_netdev_default
+USEMODULE += auto_init_gnrc_netif
+
+USEMODULE += shell_commands
+
+USEMODULE += posix
+
+CFLAGS += -DDEVELHELP
+
+LOW_MEMORY_BOARDS := nucleo-f334 msb-430 msb-430h weio
+
+ifeq ($(BOARD),$(filter $(BOARD),$(LOW_MEMORY_BOARDS)))
+    CFLAGS += -DGNRC_PKTBUF_SIZE=512 -DGNRC_IPV6_NETIF_ADDR_NUMOF=4 -DGNRC_IPV6_NC_SIZE=1
+endif
+
+include $(RIOTBASE)/Makefile.include

tests/gnrc_sock_dns/README.md

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+# Overview
+
+This folder contains a test application for RIOT's sock-based DNS client.
+
+# How to test with native
+
+Setup up a tap interface:
+
+    $ sudo ip tuntap add dev tap0 mode tap user $(id -u -n)
+    $ sudo ip a a 2001:db8::1/64 dev tap0
+    $ sudo ip link set up dev tap0
+
+Start dnsmasq (in another console):
+
+    $ sudo dnsmasq -d -2 -z -i tap0 -q --no-resolv \
+        --dhcp-range=::1,constructor:tap0,ra-only \
+        --listen-address 2001:db8::1 \
+        --host-record=example.org,10.0.0.1,2001:db8::1
+
+(NetworkManager is known to start an interfering dnsmasq instance. It needs to
+be stopped before this test.)
+
+Then run the test application
+
+    $ make term
+
+The application will take a little while to auto-configure it's IP address.
+Then you should see something like
+
+    example.org resolves to 2001:db8::1

tests/gnrc_sock_dns/main.c

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+/*
+ * Copyright (C) 2017 Kaspar Schleiser <kaspar@schleiser.de>
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser
+ * General Public License v2.1. See the file LICENSE in the top level
+ * directory for more details.
+ */
+
+/**
+ * @ingroup     tests
+ * @{
+ *
+ * @file
+ * @brief       sock DNS client test application
+ *
+ * @author      Kaspar Schleiser <kaspar@schleiser.de>
+ *
+ * @}
+ */
+
+#include <stddef.h>
+#include <stdio.h>
+
+#include <arpa/inet.h>
+
+#include "net/sock/dns.h"
+#include "net/sock/util.h"
+#include "xtimer.h"
+
+#ifndef TEST_NAME
+#define TEST_NAME   "example.org"
+#endif
+
+#ifndef DNS_SERVER
+#define DNS_SERVER  "[2001:db8::1]:53"
+#endif
+
+/* global DNS server UDP endpoint */
+sock_udp_ep_t sock_dns_server;
+
+/* import "ifconfig" shell command, used for printing addresses */
+
+extern int _netif_config(int argc, char **argv);
+
+int main(void)
+{
+    uint8_t addr[16] = {0};
+
+    sock_udp_str2ep(&sock_dns_server, DNS_SERVER);
+
+    puts("waiting for router advertisement...");
+    xtimer_usleep(1U*1000000);
+
+    /* print network addresses */
+    puts("Configured network interfaces:");
+    _netif_config(0, NULL);
+
+    int res = sock_dns_query(TEST_NAME, addr, AF_UNSPEC);
+    if (res > 0) {
+        char addrstr[INET6_ADDRSTRLEN];
+        inet_ntop(res == 4 ? AF_INET : AF_INET6, addr, addrstr, sizeof(addrstr));
+        printf("%s resolves to %s\n", TEST_NAME, addrstr);
+    }
+    else {
+        printf("error resolving %s\n", TEST_NAME);
+    }
+
+    return 0;
+}