val request = Request(server1.url("/"))

    server1.enqueue(refusedStream)
    server2.enqueue(bodyResponse)

    dns[server1.hostName] = listOf(ipv6, ipv4)
    socketFactory[ipv6] = server1.socketAddress
    socketFactory[ipv4] = server2.socketAddress

    client =
      client
        .newBuilder()
        .fastFallback(false)
        .apply {
          retryOnConnectionFailure = false

    InetAddress ipv4 = InetAddresses.forString("1.2.3.4");
    assertThat(InetAddresses.getEmbeddedIPv4ClientAddress(testIp)).isEqualTo(ipv4);

    // Test 6to4 address.
    testIp = (Inet6Address) InetAddresses.forString("2002:0102:0304::1");
    assertTrue(InetAddresses.hasEmbeddedIPv4ClientAddress(testIp));
    ipv4 = InetAddresses.forString("1.2.3.4");

 *   <dt>An IPv6 "IPv4 mapped" address, {@code "::ffff:192.168.0.1"}.
 *   <dd>{@code 00 00 00 00 00 00 00 00 00 00 ff ff c0 a8 00 01}
 * </dl>
 *
 * <p>A few notes about IPv6 "IPv4 mapped" addresses and their observed use in Java.
 *
 * <p>"IPv4 mapped" addresses were originally a representation of IPv4 addresses for use on an IPv6
 * socket that could receive both IPv4 and IPv6 connections (by disabling the {@code IPV6_V6ONLY}

    Collections.synchronizedMap(mutableMapOf())
  private val requestedHosts: MutableList<String> = Collections.synchronizedList(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
  }

    # Docker on Windows doesn't support the `host` networking mode, and so
    # port-forwarding is required for the container to detect it's running on GCE.
    export IP_ADDR=$(powershell -command "(Get-NetIPAddress -AddressFamily IPv4 -InterfaceAlias 'vEthernet (nat)').IPAddress")
    netsh interface portproxy add v4tov4 listenaddress=$IP_ADDR listenport=80 connectaddress=169.254.169.254 connectport=80
    # A local firewall rule for the container is added in

   * Generate an error if the host might be a non-bracketed IPv6 literal.
   *
   * <p>URI formatting requires that IPv6 literals be surrounded by brackets, like "[2001:db8::1]".
   * Chain this call after {@link #fromString(String)} to increase the strictness of the parser, and
   * disallow IPv6 literals that don't contain these brackets.
   *
   * <p>Note that this parser identifies IPv6 literals solely based on the presence of a colon. To

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

# proxy
if [ "x$FESS_PROXY_HOST" != "x" ]; then

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
)

REM proxy
if NOT "%FESS_PROXY_HOST%" == "" (

 * 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.

_2022-02-01_

**This release introduces fast fallback to better support mixed IPv4+IPv6 networks.** Fast fallback
is what we're calling our implementation of Happy Eyeballs, [RFC 8305][rfc_8305]. With this
feature OkHttp will attempt both IPv6 and IPv4 connections concurrently, keeping whichever connects
first. Fast fallback gives IPv6 connections a 250 ms head start so IPv6 is preferred on networks
where it's available.