*/
  @Throws(UnknownHostException::class)
  fun lookup(hostname: String): List<InetAddress>

  companion object {
    /**
     * A DNS that uses [InetAddress.getAllByName] to ask the underlying operating system to
     * lookup IP addresses. Most custom [Dns] implementations should delegate to this instance.
     */
    @JvmField
    val SYSTEM: Dns = DnsSystem()

    private class DnsSystem : Dns {

 * [okhttp-stats](https://github.com/flipkart-incubator/okhttp-stats): Get stats like average network speed.
 * [okhttp-system-keystore](https://github.com/charleskorn/okhttp-system-keystore): Use trusted certificates from the operating system keystore (Keychain on macOS, Certificate Store on Windows).
 * ⬜️ [Okio](https://github.com/square/okio/): A modern I/O API for Java.

#       and GRADLE_OPTS) rely on word-splitting, this is performed explicitly;
#       see the in-line comments for details.
#
#       There are tweaks for specific operating systems such as AIX, CygWin,
#       Darwin, MinGW, and NonStop.
#
#   (3) This script is generated from the Groovy template

  val referer: String? by option("-e", "--referer", help = "Referer URL")

  val verbose: Boolean by option("-v", "--verbose", help = "Makes $NAME verbose during the operation").flag()

  val sslDebug: Boolean by option(help = "Output SSL Debug").flag()

  val url: String? by argument(name = "url", help = "Remote resource URL")

  var client: Call.Factory? = null

        return new MetricsEventListener(call);
      } else {
        return EventListener.NONE;
      }
    }
  };

  ...
}
```

### Events with Failures

   * doesn't include framing overhead. If compression is enabled, uncompressed messages size
   * is used to calculate this value. It also doesn't include any bytes buffered by the operating
   * system or network intermediaries. This method returns 0 if no messages are waiting in the
   * queue. If may return a nonzero value after the web socket has been canceled; this indicates

  override fun checkClientTrusted(
    chain: Array<out X509Certificate>,
    authType: String?,
  ) = throw CertificateException("Unsupported operation")

  override fun checkServerTrusted(
    chain: Array<out X509Certificate>,
    authType: String,
  ) = throw CertificateException("Unsupported operation")

This lock guards internal state of each connection. This lock is never held for blocking operations. That means that we acquire the lock, read or write a few fields and release the lock. No I/O and no application-layer callbacks.

#### Http2Stream

  ) = throw CertificateException("Unsupported operation")

  override fun checkClientTrusted(
    chain: Array<out X509Certificate>,
    authType: String?,
  ) = throw CertificateException("Unsupported operation")

  override fun checkClientTrusted(
    chain: Array<out X509Certificate>,
    authType: String,
    engine: SSLEngine?,
  ) = throw CertificateException("Unsupported operation")

  override fun checkClientTrusted(

  }

  /**
   * Either writes this request to [sink] or measures its content length. We have one method
   * do double-duty to make sure the counting and content are consistent, particularly when it comes
   * to awkward operations like measuring the encoded length of header strings, or the
   * length-in-digits of an encoded integer.
   */
  private fun writeOrCountBytes(
    sink: BufferedSink?,
    countBytes: Boolean,
  ): Long {