How Spark determines the default IP to use for the Spark driver

Spark::findLocalIpAddress

  /**
   * Get the local host's IP address in dotted-quad format (e.g. 1.2.3.4).
   * Note, this is typically not used from within core spark.
   */
  lazy val localIpAddress: String = findLocalIpAddress()
  lazy val localIpAddressHostname: String = getAddressHostName(localIpAddress)

  private def findLocalIpAddress(): String = {
    val defaultIpOverride = System.getenv("SPARK_LOCAL_IP")
    if (defaultIpOverride != null) {
      defaultIpOverride
    } else {
      val address = InetAddress.getLocalHost
      if (address.isLoopbackAddress) {
        // Address resolves to something like 127.0.1.1, which happens on Debian; try to find
        // a better address using the local network interfaces
        // getNetworkInterfaces returns ifs in reverse order compared to ifconfig output order
        // on unix-like system. On windows, it returns in index order.
        // It's more proper to pick ip address following system output order.
        val activeNetworkIFs = NetworkInterface.getNetworkInterfaces.toList
        val reOrderedNetworkIFs = if (isWindows) activeNetworkIFs else activeNetworkIFs.reverse
        for (ni <- reOrderedNetworkIFs) {
          for (addr <- ni.getInetAddresses if !addr.isLinkLocalAddress &&
               !addr.isLoopbackAddress && addr.isInstanceOf[Inet4Address]) {
            // We've found an address that looks reasonable!
            logWarning("Your hostname, " + InetAddress.getLocalHost.getHostName + " resolves to" +
              " a loopback address: " + address.getHostAddress + "; using " + addr.getHostAddress +
              " instead (on interface " + ni.getName + ")")
            logWarning("Set SPARK_LOCAL_IP if you need to bind to another address")
            return addr.getHostAddress
          }
        }
        logWarning("Your hostname, " + InetAddress.getLocalHost.getHostName + " resolves to" +
          " a loopback address: " + address.getHostAddress + ", but we couldn't find any" +
          " external IP address!")
        logWarning("Set SPARK_LOCAL_IP if you need to bind to another address")
      }
      address.getHostAddress
    }
  }