Dockerhost detection

dockerhost.sh

#!/usr/bin/env sh

# The default hostname to set
DEFAULT_DOCKER_HOSTNAME=${DEFAULT_DOCKER_HOSTNAME-dockerhost.internal}

# A comma-delimited list of extra hostnames to add as aliases
EXTRA_DOCKER_HOSTNAMES=${EXTRA_DOCKER_HOSTNAMES-}

# Whether to export variables
EXPORT_DOCKERHOST=${EXPORT_DOCKERHOST-1}

# Whether to output to /etc/hosts
UPDATE_HOSTFILE_DOCKERHOST=${UPDATE_HOSTFILE_DOCKERHOST-1}

# Whether to echo to stdout
ECHO_DOCKERHOST=${ECHO_DOCKERHOST-1}

# Default host type
DOCKERHOST_OS="unknown"

# Default IP for Linux hosts
DOCKERHOST_IP=""

# Test Windows name
WINDOWS_HOSTNAME="docker.for.win.localhost"

# Test Mac name
MACOS_HOSTNAME="docker.for.mac.localhost"

################################################################
# These are functions to allow this script to be easily modified
# into other forms. Specifically get_dockerhost_address can be 
# used to fill in DNS or used with NetworkManager.

# Get the base command we use to look up the remote
# If timeout exists, get the right timeout type
get_base_command() {
  # If we can't ping, we can't test.
  if ! command -v ping > /dev/null 2>&1; then
    return 1
  fi  
  
  # A command string we'll be building
  COMMAND=""

  # Check for the ability to limit the timeout
  if command -v timeout >/dev/null 2>&1; then
    # We still have a problem - there are two widely used incompatible
    # variants of timeout. Let's see which one we have.
    
    # env should not be a builtin, doesn't require a pipe,
    # has a consistent command line test, and is unlikely to fail
    if timeout -t1 env >/dev/null 2>&1; then
      COMMAND="timeout -t1 "
    else
      COMMAND="timeout 1s "
    fi
  fi

  # Final version of command
  COMMAND="$COMMAND ping -c1 "
  echo "$COMMAND"
  return 0
}

# Get the remote IP address of a host
get_remote_ip() {
  TEST_HOST=$1
  REMOTE_EXISTS=1
  IP=""
  
  # This allows us to trim the "terminated" BusyBox signal output text
  IP=$(sh -c "$BASE_COMMAND $TEST_HOST" 2>&1)
  
  # Exit codes: 0 - found the host, 1/2 failed to find, 124/143 terminated
  REMOTE_EXISTS="$?"

  # If we got 127.0.0.1 or ::1, we're not inside a container
  if echo "$IP" | grep  -q -m1 -F '127.0.0.1' >/dev/null 2>&1; then
    return 1
  elif echo "$IP" | grep  -q -m1 -F '(::1)' >/dev/null 2>&1; then
    return 1
  fi

  if [ "$REMOTE_EXISTS" != "0" ]; then
    return 1
  fi

  return 0  

}

# Perform the lookups
get_dockerhost_address() {
  # If we can't test, let's get out of here
  if ! BASE_COMMAND=$(get_base_command); then
    # This image doesn't support testing via ping
    return 1
  fi

  if DOCKERHOST_IP=$(get_remote_ip "$WINDOWS_HOSTNAME"); then
    DOCKERHOST_OS="windows"
  elif DOCKERHOST_IP=$(get_remote_ip "$MACOS_HOSTNAME"); then
    DOCKERHOST_OS="macosx"
  else
    DOCKERHOST_OS="linux"
    DOCKERHOST_IP=$(ip r | grep ^default | cut -d" " -f3)
  fi

  DOCKERHOST_IP=$(echo "$DOCKERHOST_IP" | grep -m1 -oE '[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}' )
  echo "$DOCKERHOST_IP" "$DOCKERHOST_OS"
  return 0
}

update_dockerhost_dns() {
# Use this format to allow additional info to be returned
# We need to use <<END instead of <<< to be ash-compatible
  read DOCKERHOST_IP DOCKERHOST_OS <<END
$(get_dockerhost_address)
END
  
  if [ "$EXPORT_DOCKERHOST" = "1" ]; then
    export DOCKERHOST_IP
    export DOCKERHOST_OS
  fi

  if [ "$ECHO_DOCKERHOST" = "1" ]; then
    echo "$DOCKERHOST_IP"
  fi

  if [ "$EXPORT_DOCKERHOST" != "1" ]; then
    return 0
  fi

  DOCKER_HOSTNAME_LIST="$DEFAULT_DOCKER_HOSTNAME"

  if [ ! -z "$EXTRA_DOCKER_HOSTNAMES" ]; then
    DOCKER_HOSTNAME_LIST="$EXTRA_DOCKER_HOSTNAMES,$DOCKER_HOSTNAME_LIST"
  fi

  IFS=',';

  for name in $DOCKER_HOSTNAME_LIST; do
    echo "$DOCKERHOST_IP $name" >> /etc/hosts
  done

  unset IFS

  return 0
}

# If you just want to use this as output...

update_dockerhost_dns

#### 
# To use this as an init script, uncomment the below and replace with your executable
# Otherwise, source/exec this file.

# exec "/usr/sbin/apachectl" "-DFOREGROUND"