Merge pull request #16723 from benpicco/gnrc_sock_tcp-telnet

sys/net/application_layer: add telnet server module & example

examples/telnet_server/Makefile

@@ -0,0 +1,51 @@
+# name of your application
+APPLICATION = telnet_server
+
+# If no BOARD is found in the environment, use this default:
+BOARD ?= native
+
+# This has to be the absolute path to the RIOT base directory:
+RIOTBASE ?= $(CURDIR)/../..
+
+# Include packages that pull up and auto-init the link layer.
+# NOTE: 6LoWPAN will be included if IEEE802.15.4 devices are present
+USEMODULE += netdev_default
+USEMODULE += auto_init_gnrc_netif
+# Activate ICMPv6 error messages
+USEMODULE += gnrc_icmpv6_error
+# Specify the mandatory networking modules for IPv6
+USEMODULE += gnrc_ipv6_default
+# Additional networking modules that can be dropped if not needed
+USEMODULE += gnrc_icmpv6_echo
+USEMODULE += netutils
+# Add also the shell, some shell commands
+USEMODULE += shell
+USEMODULE += shell_commands
+USEMODULE += ps
+# Include the telnet server
+USEMODULE += stdio_telnet
+
+# Enable faster re-connects
+CFLAGS += -DCONFIG_GNRC_TCP_EXPERIMENTAL_DYN_MSL_EN=1
+
+# Acknowledge that telnet is bad
+I_UNDERSTAND_THAT_TELNET_IS_INSECURE = 1
+
+# enable debug output via UART
+FEATURES_OPTIONAL += periph_uart
+
+# Optionally include DNS support. This includes resolution of names at an
+# upstream DNS server and the handling of RDNSS options in Router Advertisements
+# to auto-configure that upstream DNS server.
+# USEMODULE += sock_dns              # include DNS client
+# USEMODULE += gnrc_ipv6_nib_dns     # include RDNSS option handling
+
+# Comment this out to disable code in RIOT that does safety checking
+# which is not needed in a production environment but helps in the
+# development process:
+DEVELHELP ?= 1
+
+include $(RIOTBASE)/Makefile.include
+
+# Set a custom channel if needed
+include $(RIOTMAKE)/default-radio-settings.inc.mk

examples/telnet_server/Makefile.ci

@@ -0,0 +1,37 @@
+BOARD_INSUFFICIENT_MEMORY := \
+    arduino-duemilanove \
+    arduino-leonardo \
+    arduino-mega2560 \
+    arduino-nano \
+    arduino-uno \
+    atmega1284p \
+    atmega328p \
+    atmega328p-xplained-mini \
+    atxmega-a3bu-xplained \
+    bluepill-stm32f030c8 \
+    derfmega128 \
+    i-nucleo-lrwan1 \
+    mega-xplained \
+    microduino-corerf \
+    msb-430 \
+    msb-430h \
+    nucleo-f030r8 \
+    nucleo-f031k6 \
+    nucleo-f042k6 \
+    nucleo-f303k8 \
+    nucleo-f334r8 \
+    nucleo-l011k4 \
+    nucleo-l031k6 \
+    nucleo-l053r8 \
+    samd10-xmini \
+    stk3200 \
+    slstk3400a \
+    stm32f030f4-demo \
+    stm32f0discovery \
+    stm32g0316-disco \
+    stm32l0538-disco \
+    telosb \
+    waspmote-pro \
+    z1 \
+    zigduino \
+    #

examples/telnet_server/main.c

@@ -0,0 +1,104 @@
+/*
+ * Copyright (C) 2021 ML!PA Consulting GmbH
+ *
+ * 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     examples
+ * @{
+ *
+ * @file
+ * @brief       Example application for demonstrating the RIOT telnet server
+ *
+ * @author      Benjamin Valentin <benjamin.valentin@ml-pa.com>
+ *
+ * @}
+ */
+
+#include <stdio.h>
+
+#include "net/ipv6/addr.h"
+#include "net/gnrc.h"
+#include "net/gnrc/netif.h"
+#include "net/telnet.h"
+#include "shell.h"
+#include "msg.h"
+
+#define MAIN_QUEUE_SIZE     (8)
+static msg_t _main_msg_queue[MAIN_QUEUE_SIZE];
+
+static void _print_addr(void)
+{
+    gnrc_netif_t *netif = NULL;
+    while ((netif = gnrc_netif_iter(netif))) {
+        ipv6_addr_t ipv6_addrs[CONFIG_GNRC_NETIF_IPV6_ADDRS_NUMOF];
+        int res = gnrc_netapi_get(netif->pid, NETOPT_IPV6_ADDR, 0, ipv6_addrs,
+                                  sizeof(ipv6_addrs));
+
+        if (res < 0) {
+            continue;
+        }
+        for (unsigned i = 0; i < (unsigned)(res / sizeof(ipv6_addr_t)); i++) {
+            char ipv6_addr[IPV6_ADDR_MAX_STR_LEN];
+
+            ipv6_addr_to_str(ipv6_addr, &ipv6_addrs[i], IPV6_ADDR_MAX_STR_LEN);
+            printf("My address is %s\n", ipv6_addr);
+        }
+    }
+}
+
+static void _print_motd(void)
+{
+    puts("RIOT telnet example application");
+
+    puts("╔═══════════════════════════════════════════════════╗");
+    puts("║telnet is entirely unencrypted and unauthenticated.║");
+    puts("║Do not use this on public networks.                ║");
+    puts("╚═══════════════════════════════════════════════════╝");
+}
+
+void telnet_cb_pre_connected(sock_tcp_t *sock)
+{
+    sock_tcp_ep_t ep;
+    char addr_str[IPV6_ADDR_MAX_STR_LEN];
+
+    sock_tcp_get_local(sock, &ep);
+    ipv6_addr_to_str(addr_str, (ipv6_addr_t *)ep.addr.ipv6, sizeof(addr_str));
+
+    printf("%s connected\n", addr_str);
+}
+
+void telnet_cb_disconneced(void)
+{
+    puts("disconnected");
+}
+
+void telnet_cb_connected(sock_tcp_t *sock)
+{
+    (void)sock;
+    _print_motd();
+}
+
+int main(void)
+{
+    /* we need a message queue for the thread running the shell in order to
+     * receive potentially fast incoming networking packets */
+    msg_init_queue(_main_msg_queue, MAIN_QUEUE_SIZE);
+
+    _print_motd();
+
+    /* print address so we can connect to it */
+    _print_addr();
+
+    /* start shell */
+    printf("All up, awaiting connection on port %u\n", CONFIG_TELNET_PORT);
+    printf("Local shell disabled");
+    char line_buf[SHELL_DEFAULT_BUFSIZE];
+    shell_run(NULL, line_buf, SHELL_DEFAULT_BUFSIZE);
+
+    /* should be never reached */
+    return 0;
+}

makefiles/dependency_resolution.inc.mk

@@ -77,4 +77,17 @@ else
     $(shell $(COLOR_ECHO) "$(COLOR_RED)Deprecated modules are in use:$(COLOR_RESET)"\
                           "$(DEPRECATED_MODULES_USED)" 1>&2)
   endif
+
+  # Warn about telnet
+  ifneq (,$(filter auto_init_telnet,$(USEMODULE)))
+    ifneq (1,$(I_UNDERSTAND_THAT_TELNET_IS_INSECURE))
+      $(shell $(COLOR_ECHO) "$(COLOR_RED)Telnet will be started automatically, "\
+                            "make sure you understand why this almost certainly "\
+                            "is a REALLY BAD idea before proceeding!$(COLOR_RESET)" 1>&2)
+      $(error I_UNDERSTAND_THAT_TELNET_IS_INSECURE must be set to 1 to proceed)
+    else
+      $(shell $(COLOR_ECHO) "$(COLOR_YELLOW)Telnet will be started automatically,"\
+                            "don't run this on public networks!$(COLOR_RESET)" 1>&2)
+    endif
+  endif
 endif

makefiles/pseudomodules.inc.mk

@@ -210,6 +210,7 @@ PSEUDOMODULES += stdio_cdc_acm
 PSEUDOMODULES += stdio_ethos
 PSEUDOMODULES += stdio_nimble_debug
 PSEUDOMODULES += stdio_uart_rx
+PSEUDOMODULES += stdio_telnet
 PSEUDOMODULES += stm32_eth
 PSEUDOMODULES += stm32_eth_auto
 PSEUDOMODULES += stm32_eth_link_up

makefiles/stdio.inc.mk

@@ -7,6 +7,7 @@ STDIO_MODULES = \
   stdio_rtt \
   stdio_semihosting \
   stdio_uart \
+  stdio_telnet \
   #
 
 ifneq (,$(filter newlib picolibc,$(USEMODULE)))
@@ -63,6 +64,11 @@ ifneq (,$(filter stdio_semihosting,$(USEMODULE)))
   FEATURES_REQUIRED_ANY += cpu_core_cortexm|arch_riscv
 endif
 
+ifneq (,$(filter stdio_telnet,$(USEMODULE)))
+  DEFAULT_MODULE += auto_init_telnet
+  USEMODULE += telnet
+endif
+
 # enable stdout buffering for modules that benefit from sending out buffers in larger chunks
 ifneq (,$(filter picolibc,$(USEMODULE)))
   ifneq (,$(filter stdio_cdc_acm stdio_ethos slipdev_stdio stdio_semihosting,$(USEMODULE)))

sys/Makefile

@@ -11,6 +11,9 @@ endif
 ifneq (,$(filter cipher_modes,$(USEMODULE)))
   DIRS += crypto/modes
 endif
+ifneq (,$(filter telnet,$(USEMODULE)))
+  DIRS += net/application_layer/telnet
+endif
 ifneq (,$(filter constfs,$(USEMODULE)))
   DIRS += fs/constfs
 endif

sys/Makefile.dep

@@ -324,6 +324,11 @@ ifneq (,$(filter sema_inv,$(USEMODULE)))
   USEMODULE += atomic_utils
 endif
 
+ifneq (,$(filter telnet,$(USEMODULE)))
+  USEMODULE += pipe
+  USEMODULE += sock_tcp
+endif
+
 ifneq (,$(filter luid,$(USEMODULE)))
   FEATURES_OPTIONAL += periph_cpuid
 endif

sys/auto_init/auto_init.c

@@ -305,6 +305,12 @@ void auto_init(void)
         gnrc_ipv6_auto_subnets_init();
     }
 
+    if (IS_USED(MODULE_AUTO_INIT_TELNET)) {
+        LOG_DEBUG("auto_init TELNET server\n");
+        extern void telnet_server_start(void);
+        telnet_server_start();
+    }
+
     if (IS_USED(MODULE_AUTO_INIT_MULTIMEDIA)) {
         LOG_DEBUG("auto_init MULTIMEDIA\n");
         if (IS_USED(MODULE_DFPLAYER)) {

sys/include/net/telnet.h

@@ -0,0 +1,95 @@
+/*
+ * Copyright (C) 2021 ML!PA Consulting GmbH
+ *
+ * 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.
+ */
+
+/**
+ * @defgroup    net_telnet basic Telnet server implementation
+ * @ingroup     net_ipv6
+ * @brief       Telnet server functions
+ * @{
+ *
+ * @file
+ * @brief   minimal Telnet server ([RFC 854](https://tools.ietf.org/html/rfc854)) implementation
+ * @note    This implementation only supports a single client and no options.
+ *
+ * @warning Telnet is entirely unencrypted! Do not use it on public networks.
+ *          This is intended to aid debugging on networks that are isolated from the Internet.
+ *
+ * @author  Benjamin Valentin <benjamin.valentin@ml-pa.com>
+ */
+#ifndef NET_TELNET_H
+#define NET_TELNET_H
+
+#include "net/sock/tcp.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief   The port for the Telnet server to listen on
+ */
+#ifndef CONFIG_TELNET_PORT
+#define CONFIG_TELNET_PORT (23)
+#endif
+
+/**
+ * @brief   Start the Telnet server thread
+ *
+ * @return  0 on success, error otherwise
+ */
+int telnet_server_start(void);
+
+/**
+ * @brief   Write data to the telnet client
+ *
+ * @param[in] buffer    The buffer to send to the client
+ * @param[in] len       The length of the buffer
+ *
+ * @return  0 on success, error otherwise
+ */
+int telnet_server_write(const void* buffer, size_t len);
+
+/**
+ * @brief   Read data from the telnet client, will block until data is available.
+ *
+ * @param[out] buffer   The buffer to write data from the client
+ * @param[in]  count    Number of bytes to read
+ *
+ * @return  number of bytes read, error otherwise
+ */
+int telnet_server_read(void* buffer, size_t count);
+
+/**
+ * @brief   Callback function that gets called when a telnet client connects
+ *          but before stdio is redirected.
+ *
+ * @param[in] sock  Socket of the client that just connected
+ *                  only use with @ref sock_tcp_get_local
+ */
+void telnet_cb_pre_connected(sock_tcp_t *sock);
+
+/**
+ * @brief   Callback function that gets called when a telnet client connects
+ *          after stdio is redirected.
+ *
+ * @param[in] sock  Socket of the client that just connected
+ *                  only use with @ref sock_tcp_get_local
+ */
+void telnet_cb_connected(sock_tcp_t *sock);
+
+/**
+ * @brief   Callback function that gets called after a telnet client disconnected.
+ */
+void telnet_cb_disconneced(void);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* NET_TELNET_H */
+/** @} */

sys/net/application_layer/Kconfig

@@ -21,3 +21,9 @@ rsource "asymcute/Kconfig"
 rsource "emcute/Kconfig"
 
 endmenu # MQTT-SN
+
+menu "Telnet"
+
+rsource "telnet/Kconfig"
+
+endmenu # Telnet

sys/net/application_layer/telnet/Kconfig

@@ -0,0 +1,28 @@
+# Copyright (c) 2021 ML!PA Consulting GmbH
+#
+# 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.
+#
+menuconfig KCONFIG_USEMODULE_TELNET
+    bool "Configure telnet server"
+    depends on USEMODULE_TELNET
+    help
+        Configure telnet module using Kconfig. If not set default values and
+        CFLAGS will be used.
+
+if KCONFIG_USEMODULE_TELNET
+
+config TELNET_PORT
+    int "Server port"
+    default 23
+    help
+        Server port, the default is the one specified in RFC 855.
+
+config TELNET_TCP_QUEUE_SIZE
+    int "TCP Queue Size"
+    default 1
+    help
+        Maximum number of incoming TCP connections.
+
+endif # KCONFIG_USEMODULE_TELNET

sys/net/application_layer/telnet/Makefile

@@ -0,0 +1 @@
+include $(RIOTBASE)/Makefile.base

sys/net/application_layer/telnet/stdio_telnet.c

@@ -0,0 +1,86 @@
+/*
+ * Copyright (C) 2021 ML!PA Consulting GmbH
+ *
+ * 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     sys
+ * @{
+ *
+ * @file
+ * @brief       STDIO over Telnet implementation
+ *
+ * This file implements STDIO via a Telnet server with fallback UART output
+ *
+ * @author      Benjamin Valentin <benjamin.valentin@ml-pa.com>
+ *
+ * @}
+ */
+
+#include <errno.h>
+
+#include "board.h"
+#include "kernel_defines.h"
+#include "net/telnet.h"
+#if IS_USED(MODULE_PERIPH_UART)
+#include "stdio_uart.h"
+#include "periph/uart.h"
+#endif
+#if IS_USED(MODULE_VFS)
+#include "vfs.h"
+#endif
+#ifdef CPU_NATIVE
+#include "native_internal.h"
+#endif
+
+#define ENABLE_DEBUG 0
+#include "debug.h"
+
+static inline void _init_fallback(void)
+{
+#if defined(STDIO_UART_DEV) && IS_USED(MODULE_PERIPH_UART)
+    uart_init(STDIO_UART_DEV, STDIO_UART_BAUDRATE, NULL, NULL);
+#endif
+}
+
+static inline int _write_fallback(const void* buffer, size_t len)
+{
+#if defined(CPU_NATIVE)
+    real_write(STDOUT_FILENO, buffer, len);
+#elif defined(STDIO_UART_DEV) && IS_USED(MODULE_PERIPH_UART)
+    uart_write(STDIO_UART_DEV, buffer, len);
+#else
+    (void)buffer;
+#endif
+
+    return len;
+}
+
+void stdio_init(void)
+{
+    _init_fallback();
+
+#if IS_USED(MODULE_VFS)
+    vfs_bind_stdio();
+#endif
+}
+
+ssize_t stdio_read(void* buffer, size_t count)
+{
+    return telnet_server_read(buffer, count);
+}
+
+ssize_t stdio_write(const void* buffer, size_t len)
+{
+    int res = telnet_server_write(buffer, len);
+
+    /* write to UART if no client is connected */
+    if (res == -ENOTCONN) {
+        return _write_fallback(buffer, len);
+    }
+
+    return res;
+}

sys/net/application_layer/telnet/telnet_server.c

@@ -0,0 +1,303 @@
+/*
+ * Copyright (C) 2021 ML!PA Consulting GmbH
+ *
+ * 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.
+ */
+
+/**
+ * @{
+ *
+ * @file
+ * @brief       Telnet server implementation
+ *
+ * @author      Benjamin Valentin <benjamin.valentin@ml-pa.com>
+ *
+ * @}
+ */
+
+#include <string.h>
+#include <fcntl.h>
+#include "net/sock/tcp.h"
+#include "net/telnet.h"
+#include "pipe.h"
+
+#define ENABLE_DEBUG 0
+#include "debug.h"
+
+#ifndef CONFIG_TELNET_TCP_QUEUE_SIZE
+#define CONFIG_TELNET_TCP_QUEUE_SIZE (1)
+#endif
+
+static char _stdin_pipe_buf[16];
+static ringbuffer_t _stdin_ringbuffer;
+static pipe_t _stdin_pipe;
+
+static sock_tcp_queue_t sock_queue;
+static sock_tcp_t socks[CONFIG_TELNET_TCP_QUEUE_SIZE];
+static sock_tcp_t *client;
+
+static char telnet_stack[THREAD_STACKSIZE_DEFAULT];
+
+#define SOCK_TCP_TIMEOUT_MS 50
+
+enum {
+    TELNET_CMD_EOF  = 236,
+    TELNET_CMD_SE   = 240,
+    TELNET_CMD_NOP,
+    TELNET_CMD_DATA_MARK,
+    TELNET_CMD_BRK,
+    TELNET_CMD_IP,
+    TELNET_CMD_AO,
+    TELNET_CMD_AYT,
+    TELNET_CMD_EC,
+    TELNET_CMD_EL,
+    TELNET_CMD_GA,
+    TELNET_CMD_SB   = 250,
+    TELNET_CMD_WILL,
+    TELNET_CMD_WONT,
+    TELNET_CMD_DO,
+    TELNET_CMD_DONT,
+    TELNET_CMD_IAC
+};
+
+enum {
+    TELNET_OPT_BINARY   = 0,
+    TELNET_OPT_ECHO     = 1,
+    TELNET_OPT_SUP_GO_AHEAD = 3,
+    TELNET_OPT_STATUS   = 5,
+};
+
+static bool connected;
+static mutex_t connected_mutex = MUTEX_INIT_LOCKED;
+static mutex_t sock_mutex;
+
+__attribute__((weak))
+void telnet_cb_pre_connected(sock_tcp_t *sock)
+{
+    (void)sock;
+}
+
+__attribute__((weak))
+void telnet_cb_connected(sock_tcp_t *sock)
+{
+    (void)sock;
+}
+
+__attribute__((weak))
+void telnet_cb_disconneced(void)
+{
+}
+
+static void _acquire(void)
+{
+    mutex_lock(&sock_mutex);
+}
+
+static void _release(void)
+{
+    mutex_unlock(&sock_mutex);
+}
+
+static void _connected(void)
+{
+    telnet_cb_pre_connected(client);
+
+    connected = true;
+    mutex_unlock(&connected_mutex);
+
+    telnet_cb_connected(client);
+}
+
+static void _disconnect(void)
+{
+    mutex_trylock(&connected_mutex);
+    connected = false;
+
+    telnet_cb_disconneced();
+}
+
+static int _write_buffer(const void* buffer, size_t len)
+{
+    int res = 0;
+    const char *buf = buffer;
+    _acquire();
+
+    while (len) {
+        /* telnet expects \r\n line endings */
+        /* https://datatracker.ietf.org/doc/html/rfc5198#appendix-C */
+        const char *nl = memchr(buf, '\n', len);
+        if (nl) {
+            const char cr = '\r';
+            size_t before_nl = nl - buf;
+
+            /* write string before \n */
+            res = sock_tcp_write(client, buf, before_nl);
+            if (res < 0) {
+                break;
+            }
+
+            /* insert \r */
+            res = sock_tcp_write(client, &cr, 1);
+            if (res < 0) {
+                break;
+            }
+
+            buf = nl;
+            len -= before_nl;
+        }
+
+        res = sock_tcp_write(client, buf, len);
+        if (res < 0) {
+            break;
+        }
+        len -= res;
+        buf += res;
+    }
+    _release();
+
+    return res < 0 ? res : 0;
+}
+
+static uint8_t _will(uint8_t option)
+{
+    switch (option) {
+    /* agree to suppress go-ahead packets */
+    /* see RFC 858 */
+    case TELNET_OPT_SUP_GO_AHEAD: return TELNET_CMD_DO;
+    }
+
+    return TELNET_CMD_WONT;
+}
+
+static void _process_cmd(uint8_t cmd, uint8_t option)
+{
+    DEBUG("cmd: %u, option: %u\n", cmd, option);
+    switch (cmd) {
+    case TELNET_CMD_WILL:
+        {
+        uint8_t reply[] = {TELNET_CMD_IAC, _will(option), option};
+        _write_buffer(reply, sizeof(reply));
+        break;
+        }
+    }
+}
+
+static void *telnet_thread(void *arg)
+{
+    (void)arg;
+
+    static uint8_t rx_buf[64];
+
+    while (1) {
+        ssize_t res = sock_tcp_accept(&sock_queue, &client, SOCK_NO_TIMEOUT);
+        if (res < 0) {
+            DEBUG("accept error: %s\n", strerror(-res));
+            continue;
+        }
+
+        DEBUG("connected\n");
+        _connected();
+
+        bool is_cmd = false;
+        uint8_t is_option = 0;
+        while (1) {
+            _acquire();
+            res = sock_tcp_read(client, rx_buf, sizeof(rx_buf), SOCK_TCP_TIMEOUT_MS);
+            _release();
+            if (res == -ETIMEDOUT) {
+                continue;
+            }
+
+            if (res < 0) {
+                break;
+            }
+
+            for (int i = 0; i < res; ++i) {
+                uint8_t c = rx_buf[i];
+
+                if (is_cmd) {
+                    is_cmd = false;
+                    switch (c) {
+                    case TELNET_CMD_IAC:
+                        goto write;
+                    case TELNET_CMD_EOF:
+                        goto disco;
+                    default:
+                        is_option = c;
+                    }
+                    continue;
+                }
+
+                if (is_option) {
+                    _process_cmd(is_option, c);
+                    is_option = 0;
+                    continue;
+                }
+
+                if (c == TELNET_CMD_IAC) {
+                    is_cmd = true;
+                    continue;
+                }
+
+                if (c == 0) {
+                    continue;
+                }
+write:
+                pipe_write(&_stdin_pipe, &c, 1);
+            }
+        }
+disco:
+        _disconnect();
+        sock_tcp_disconnect(client);
+
+        if (res < 0) {
+            DEBUG("telnet: read: %s\n", strerror(res));
+        }
+    }
+
+    return NULL;
+}
+
+int telnet_server_write(const void* buffer, size_t len)
+{
+    if (connected) {
+        int res = _write_buffer(buffer, len);
+        return res ? res : (int)len;
+    }
+    return -ENOTCONN;
+}
+
+int telnet_server_read(void* buffer, size_t count)
+{
+    /* block until a connection is established */
+    mutex_lock(&connected_mutex);
+    int res = pipe_read(&_stdin_pipe, buffer, count);
+    if (connected) {
+        mutex_unlock(&connected_mutex);
+    }
+    return res;
+}
+
+int telnet_server_start(void)
+{
+    sock_tcp_ep_t ep = SOCK_IPV6_EP_ANY;
+    ep.port = CONFIG_TELNET_PORT;
+
+    int res = sock_tcp_listen(&sock_queue, &ep, socks, ARRAY_SIZE(socks), 0);
+    if (res) {
+        return res;
+    }
+
+    /* init RX ringbuffer */
+    ringbuffer_init(&_stdin_ringbuffer, _stdin_pipe_buf, sizeof(_stdin_pipe_buf));
+    pipe_init(&_stdin_pipe, &_stdin_ringbuffer, NULL);
+
+    /* initiate telnet server */
+    thread_create(telnet_stack, sizeof(telnet_stack),
+                  THREAD_PRIORITY_MAIN - 1, THREAD_CREATE_STACKTEST,
+                  telnet_thread, NULL, "telnet");
+
+    return 0;
+}

sys/net/gnrc/sock/include/sock_types.h

@@ -27,6 +27,8 @@
 
 #include "mbox.h"
 #include "net/af.h"
+#include "net/gnrc.h"
+#include "net/gnrc/tcp.h"
 #include "net/gnrc/netreg.h"
 #ifdef SOCK_HAS_ASYNC
 #include "net/sock/async/types.h"