internal fun reorderForHappyEyeballs(addresses: List<InetAddress>): List<InetAddress> {
  if (addresses.size < 2) {
    return addresses
  }

  val (ipv6, ipv4) = addresses.partition { it is Inet6Address }

  return if (ipv6.isEmpty() || ipv4.isEmpty()) {
    addresses
  } else {
    interleave(ipv6, ipv4)
  }

import java.net.InetAddress;
import java.text.ParseException;
import org.jspecify.annotations.Nullable;

/**
 * A syntactically valid host specifier, suitable for use in a URI. This may be either a numeric IP
 * address in IPv4 or IPv6 notation, or a domain name.
 *
 * <p>Because this class is intended to represent host specifiers which can reasonably be used in a

  private val hostAddresses: MutableMap<String, List<InetAddress>> = mutableMapOf()
  private val requestedHosts: MutableList<String> = mutableListOf()
  private var nextAddress = 0xff000064L // 255.0.0.100 in IPv4; ::ff00:64 in IPv6.

  /** Sets the results for `hostname`.  */
  operator fun set(
    hostname: String,
    addresses: List<InetAddress>,
  ): FakeDns {
    hostAddresses[hostname] = addresses
    return this
  }

  private fun decodeIpv6(input: String): ByteArray? = decodeIpv6(input, 0, input.length)

  @Test
  fun testToCanonicalHost() {
    // IPv4
    assertThat("127.0.0.1".toCanonicalHost()).isEqualTo("127.0.0.1")
    assertThat("1.2.3.4".toCanonicalHost()).isEqualTo("1.2.3.4")

    // IPv6
    assertThat("::1".toCanonicalHost()).isEqualTo("::1")
    assertThat("2001:db8::1".toCanonicalHost()).isEqualTo("2001:db8::1")

    }
  }

  /** No SNI for literal IPv6 addresses. */
  @Test
  fun ipv6() {
    val recordedRequest = requestToHostnameViaProxy("2607:f8b0:400b:804::200e")
    assertThat(recordedRequest.url.host).isEqualTo("2607:f8b0:400b:804::200e")
    assertThat(recordedRequest.handshakeServerNames).isEmpty()
  }

  /** No SNI for literal IPv4 addresses. */
  @Test
  fun ipv4() {

 * [system DNS service][SYSTEM], which is the default. Some applications may provide their own
 * implementation to use a different DNS server, to prefer IPv6 addresses, to prefer IPv4 addresses,
 * or to force a specific known IP address.
 *
 * Implementations of this interface must be safe for concurrent use.
 */
fun interface Dns {
  /**

FESS_JAVA_OPTS="$FESS_JAVA_OPTS -Djava.awt.headless=true"

# maximum # keep-alive connections to maintain at once
FESS_JAVA_OPTS="$FESS_JAVA_OPTS -Dhttp.maxConnections=20"

# Force the JVM to use IPv4 stack
if [ "x$FESS_USE_IPV4" != "x" ]; then
  FESS_JAVA_OPTS="$FESS_JAVA_OPTS -Djava.net.preferIPv4Stack=true"
fi

FESS_JAVA_OPTS="$FESS_JAVA_OPTS -Djna.nosys=true"

set FESS_JAVA_OPTS=%FESS_JAVA_OPTS% -Djava.awt.headless=true

REM maximum # keep-alive connections to maintain at once
set FESS_JAVA_OPTS=%FESS_JAVA_OPTS% -Dhttp.maxConnections=20

REM Force the JVM to use IPv4 stack
if NOT "%FESS_USE_IPV4%" == "" (
set FESS_JAVA_OPTS=%FESS_JAVA_OPTS% -Djava.net.preferIPv4Stack=true
)

set FESS_JAVA_OPTS=%FESS_JAVA_OPTS% -Djna.nosys=true

          }

          connectionUser.dnsEnd(socketHost, result)
          result
        }

      // Try each address for best behavior in mixed IPv4/IPv6 environments.
      val orderedAddresses =
        when {
          fastFallback -> reorderForHappyEyeballs(addresses)
          else -> addresses
        }

      for (inetAddress in orderedAddresses) {

 * Prefer to alternate IP addresses from different address families, (IPv6 / IPv4), starting with IPv6.
 * Don't start a new attempt until 250 ms after the most recent attempt was started.
 * Keep whichever TCP connection succeeds first and cancel all the others.