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A SLIP to WiFi router

This is an implementation of a SLIP (Serial Line IP - RFC1055) router on the ESP8266. It can be used as simple (and slow) network interface to get WiFi connectivity. The ESP can act as STA or as AP. It transparently forwards any IP traffic through it. As it uses NAT no routing entries are required on the other side.

UPDATE: If you are experiencing DNS-issues or if you are looking for a dockerized build-environment have a look here:

Usage as STA

In this mode the ESP connects to the internet via an AP with ssid, password and offers at UART0 a SLIP interface with IP address This default can be changed in the file user_config.h.

To connect a Linux-based host, start the firmware on the ESP, connect it via serial to USB, and use the following commands on the host:

sudo slattach -L -p slip -s 115200 /dev/ttyUSB0&
sudo ifconfig sl0 pointopoint up mtu 1500


telnet 7777

gives you terminal access to the esp as router. On the ESP you then enter:

CMD>set ssid <your_ssid> 
CMD>set password <your_pw> 
CMD>set use_ap 0

To get full internet access you will need aditionally a route:

sudo route add default gw

and a DNS server - add an appropriate entry (e.g. public DNS server) in /etc/resolv.conf, eg. by (as root):

echo "nameserver" > /etc/resolv.conf

A script may help to automize this process.

The status LED (default: GPIO2) indicates:

  • permanently on: not connected (initial state after boot)
  • permanently off: connected (or SoftAP active), no traffic
  • rapidly blinking: in- or outgoing traffic

The default config of the router can be overwritten and persistenly saved to flash by using a console interface. This console is available via tcp port 7777 (e.g. from the host attached to the serial line - see above).

The console understands the following command:

  • help: prints a short help message
  • show [stats]: prints the current config and status
  • set ssid|pasword [value]: changes the named config parameter
  • set addr [ip-addr]: sets the IP address of the SLIP interface (default:
  • set speed [80|160]: sets the CPU clock frequency (default: 160)
  • set bitrate [bitrate]: sets the serial bitrate to a new value
  • portmap add [TCP|UDP] external_port internal_ip internal_port: adds a port forwarding (works in STA mode)
  • portmap remove [TCP|UDP] external_port: deletes a port forwarding
  • save: saves the current parameters to flash
  • quit: terminates a remote session
  • reset [factory]: resets the esp and applies the config, optionally resets WiFi params to default values
  • lock: locks the current config, changes are not allowed
  • unlock [password]: unlocks the config, requires password of the network AP
  • scan: does a scan for APs

If you want to enter non-ASCII or special characters you can use HTTP-style hex encoding (e.g. "My%20AccessPoint") or, only on the CLI, as shortcut C-style quotes with backslash (e.g. "My\ AccessPoint"). Both methods will result in a string "My AccessPoint".

Usage as AP

You can also turn the sides and make the ESP to work as AP - useful e.g. if you want to connect other devices to a RasPi that has no WiFi interface:

With a Linux-based host start the firmware on the esp, connect it via serial to USB, and use the following commands on the host:

sudo slattach -L -p slip -s 115200 /dev/ttyUSB0&
sudo ifconfig sl0 pointopoint up mtu 1500
sudo route add -net gw


telnet 7777

gives you terminal access to the ESP as router. On the ESP you then enter:

CMD>set ap_ssid <your_ssid> 
CMD>set ap_password <your_pw> 
CMD>set use_ap 1

Now STAs of this AP get IP adresses in the network and can reach the Linux machine as "". The AP interface is NATed, i.e. you cannot set up connections from the Linux machine to the STAs, but from STAs to Linux works. Useful e.g. if you want to reach an MQTT server on the Linux.

The console understands the following command for the AP mode:

  • set use_ap [0|1]: selects, whether the esp_slip_router uses an STA interface (use_ap = 0, default) or an AP interface (use_ap = 1)
  • set [ap_ssid|ap_password] value: changes the settings for the soft-AP of the ESP (for your stations)
  • set ap_channel [1-13]: sets the channel of the SoftAP (default 1)
  • set ap_open [0|1]: selects, whether the soft-AP uses WPA2-PSK security (ap_open=0, automatic, if an ap_password is set) or open (ap_open=1)
  • set ssid_hidden [0|1]: selects, whether the SSID of the soft-AP is hidden (ssid_hidden=1) or visible (ssid_hidden=0, default)
  • set max_clients [1-8]: sets the number of STAs that can connct to the SoftAP (limit of the ESP's SoftAP implementation is 8, default)
  • set addr_peer [ip-addr]: sets the IP address of the peer of the SLIP interface that is also the default gateway (default:
  • set dns [ip-addr]: sets the IP address of the DNS server that is distributed via DHCP (default:

Building and Flashing

To build this binary you download and install the esp-open-sdk ( The software was developed and tested usinfg NONOS SDK v2.2. Make sure, you can compile and download the included "blinky" example.

Then download this source tree in a separate directory and adjust the BUILD_AREA variable in the Makefile and any desired options in user/user_config.h. Build the esp_wifi_repeater firmware with "make". "make flash" flashes it onto an esp8266.

The source tree includes a binary version of the liblwip_open plus the required additional includes from my fork of esp-open-lwip. No additional install action is required for that. Only if you don't want to use the precompiled library, checkout the sources from . Use it to replace the directory "esp-open-lwip" in the esp-open-sdk tree. "make clean" in the esp_open_lwip dir and once again a "make" in the upper esp_open_sdk directory. This will compile a liblwip_open.a that contains the NAT-features. Replace liblwip_open_napt.a with that binary.

If you want to use the precompiled binaries you can flash them with " --port /dev/ttyUSB0 write_flash -fs 32m 0x00000 firmware/0x00000.bin 0x10000 firmware/0x10000.bin" (use -fs 8m for an ESP-01)

Softuart UART

As UART0, the HW UART of the esp8266 is busy with the SLIP protocoll, it cannot be used simultaniuosly as debugging output. This is highly uncomfortable especially during development. If you define DEBUG_SOFTUART in user_config.h, a second UART will be simulated in software (Rx GPIO 14, Tx GPIO 12, 19200 baud). All debug output (os_printf) will then be redirectd to this port.

Known Issues

  • Speed: 115200 is the max baudrate on many USB ports and the current standard speed. This is SLOW compared to the typical WiFi speeds. This means connectivity via the serial line works, even basic web browsing, but the speed is what you can expect from about 100kB/s... But IoT applications typically use much less bandwidth, also terminal access is fine.
  • A configuration to enable hardware flow control (RTS/CTS) is available in include/driver/uart.h. Sassa0 recompiled with UART_HW_RTS 1 and UART_HW_CTS 1. In that case slattach does not work anymore with it, even with removing the -L argument, however it does work in the Amiga with hardware flow control enabled (known to work with up to 57600 bauds). See issue #16.
  • If you are just interested in the SLIP interface as a basis for you own projects, you might have a look into the user_simple directory. It contains a minimal version of the router with no config console.


A SLIP to WiFi router








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