ESPHome YAML to configure an ESP32 for UART PitM duties, both with local relay and external relay using socat.

external_pitm.yaml

esphome:
  name: serial-pitm
  friendly_name: Serial Person in the Middle

esp32:
  board: esp32dev
  framework:
    type: arduino

# Enable logging
logger:

# Enable Home Assistant API
api:
  reboot_timeout: 0s

ota:

wifi:
  ssid: !secret wifi_ssid
  password: !secret wifi_password

  # Enable fallback hotspot (captive portal) in case wifi connection fails
  ap:
    ssid: "Serial Pitm Hotspot"

captive_portal:

web_server:

external_components:
  - source: github://oxan/esphome-stream-server
 
uart:
  - id: uart_bus1
    rx_pin: GPIO25
    tx_pin: GPIO26
    baud_rate: 9600
    debug:
      direction: BOTH
      dummy_receiver: false

  - id: uart_bus2
    rx_pin: GPIO14
    tx_pin: GPIO27
    baud_rate: 9600
#    debug:
#      direction: RX
#      dummy_receiver: false

# Provides a mechanism on the ESP32 to change the baud rate without
# having to recompile and flash new firmware. Can be accessed via the
# web interface
select:
  - id: change_baud_rate
    name: Baud rate
    platform: template
    options:
      - "2400"
      - "9600"
      - "38400"
      - "57600"
      - "115200"
      - "256000"
      - "512000"
      - "921600"
    initial_option: "9600"
    optimistic: true
    restore_value: True
    internal: false
    entity_category: config
    icon: mdi:swap-horizontal
    set_action:
      - lambda: |-
          auto u1 = id(uart_bus1);
          auto u2 = id(uart_bus2);
          u1->flush();
          u2->flush();
          uint32_t new_baud_rate = stoi(x);
          ESP_LOGD("change_baud_rate", "Changing baud rate from %i to %i", u1->get_baud_rate(), new_baud_rate);
          if (u1->get_baud_rate() != new_baud_rate ||
              u2->get_baud_rate() != new_baud_rate) {
            u1->set_baud_rate(new_baud_rate);
            u2->set_baud_rate(new_baud_rate);
            u1->load_settings();
            u2->load_settings();
          }
          
stream_server:
  - uart_id: uart_bus1
    port: 2001

  - uart_id: uart_bus2
    port: 2002

onboard_pitm.yaml

esphome:
  name: serial-pitm
  friendly_name: Serial Person in the Middle

esp32:
  board: esp32dev
  framework:
    type: arduino

# Enable logging
logger:

# Enable Home Assistant API
api:
  reboot_timeout: 0s

ota:

wifi:
  ssid: !secret wifi_ssid
  password: !secret wifi_password

  # Enable fallback hotspot (captive portal) in case wifi connection fails
  ap:
    ssid: "Serial Pitm Hotspot"

captive_portal:

web_server:

external_components:
  - source: github://ssieb/esphome_components
    components: [ uart_mitm ]
 
uart:
  - id: uart_bus1
    rx_pin: GPIO25
    tx_pin: GPIO26
    baud_rate: 9600
    debug:
      direction: BOTH
      dummy_receiver: false

  - id: uart_bus2
    rx_pin: GPIO14
    tx_pin: GPIO27
    baud_rate: 9600
#    debug:
#      direction: RX
#      dummy_receiver: false

# Provides a mechanism on the ESP32 to change the baud rate without
# having to recompile and flash new firmware. Can be accessed via the
# web interface
select:
  - id: change_baud_rate
    name: Baud rate
    platform: template
    options:
      - "2400"
      - "9600"
      - "38400"
      - "57600"
      - "115200"
      - "256000"
      - "512000"
      - "921600"
    initial_option: "9600"
    optimistic: true
    restore_value: True
    internal: false
    entity_category: config
    icon: mdi:swap-horizontal
    set_action:
      - lambda: |-
          auto u1 = id(uart_bus1);
          auto u2 = id(uart_bus2);
          u1->flush();
          u2->flush();
          uint32_t new_baud_rate = stoi(x);
          ESP_LOGD("change_baud_rate", "Changing baud rate from %i to %i", u1->get_baud_rate(), new_baud_rate);
          if (u1->get_baud_rate() != new_baud_rate ||
              u2->get_baud_rate() != new_baud_rate) {
            u1->set_baud_rate(new_baud_rate);
            u2->set_baud_rate(new_baud_rate);
            u1->load_settings();
            u2->load_settings();
          }

uart_mitm:
  uart1: uart_bus1
  uart2: uart_bus2

socat-mallet.sh

#!/bin/sh

# Relays traffic from on UART stream server on the ESP32 through
# Mallet listening as a SOCKS server on localhost:1080, relayed
# from Mallet to the other stream server on the ESP32

socat TCP:serial-pitm:2001 SOCKS:127.0.0.1:serial-pitm:2002,socksport=1080

socat-relay.sh

#!/bin/sh

# Relays traffic from one UART stream server on the ESP32 to 
# the other one, dumping a hex trace of the traffic in the process

socat -x TCP:serial-pitm:2001 TCP:serial-pitm:2002