* * If the proxy selector returns an empty list or null.
   *
   * Otherwise it lists the proxies in the order they will be attempted.
   *
   * @param url a URL with only the scheme, hostname, and port specified.
   */
  open fun proxySelectEnd(
    call: Call,
    url: HttpUrl,
    proxies: List<@JvmSuppressWildcards Proxy>,
  ) {
  }

  /**

				listenerStatus:        lds,
				routeStatus:           rds,
				endpointStatus:        eds,
				extensionconfigStatus: ecds,
			})
			if len(fullStatus) == 0 {
				return nil, nil, fmt.Errorf("no proxies found (checked %d istiods)", len(drs))
			}

			sort.Slice(fullStatus, func(i, j int) bool {
				return fullStatus[i].proxyID < fullStatus[j].proxyID
			})
		}
	}
	if len(mappedResp) > 0 {

 * connection the client has many options:
 *
 *  * **HTTP proxy:** a proxy server may be explicitly configured for the client. Otherwise, the
 *    [proxy selector][java.net.ProxySelector] is used. It may return multiple proxies to attempt.
 *  * **IP address:** whether connecting directly to an origin server or a proxy, opening a socket
 *    requires an IP address. The DNS server may return multiple IP addresses to attempt.
 *

//// tab | Python 3.8+

```Python hl_lines="10"
{!> ../../docs_src/header_params/tutorial002.py!}
```

////

/// warning | "Aviso"

Antes de definir `convert_underscores` como `False`, lembre-se de que alguns proxies e servidores HTTP não permitem o uso de cabeçalhos com sublinhados.

///

## Cabeçalhos duplicados

É possível receber cabeçalhos duplicados. Isso significa, o mesmo cabeçalho com vários valores.

    }

    @Override public void proxySelectStart(Call call, HttpUrl url) {
      printEvent("proxySelectStart");
    }

    @Override public void proxySelectEnd(Call call, HttpUrl url, List<Proxy> proxies) {
      printEvent("proxySelectEnd");
    }

    @Override public void dnsStart(Call call, String domainName) {
      printEvent("dnsStart");
    }

///

```Python hl_lines="10"
{!> ../../docs_src/header_params/tutorial002.py!}
```

////

/// warning

Before setting `convert_underscores` to `False`, bear in mind that some HTTP proxies and servers disallow the usage of headers with underscores.

///

## Duplicate headers

It is possible to receive duplicate headers. That means, the same header with multiple values.

    override val call: Call,
    val url: HttpUrl,
  ) : CallEvent()

  data class ProxySelectEnd(
    override val timestampNs: Long,
    override val call: Call,
    val url: HttpUrl,
    val proxies: List<Proxy>?,
  ) : CallEvent() {
    override fun closes(event: CallEvent): Boolean = event is ProxySelectStart && call == event.call && url == event.url
  }

  data class DnsStart(
    override val timestampNs: Long,

    }

    @Override public void proxySelectStart(Call call, HttpUrl url) {
      printEvent("proxySelectStart");
    }

    @Override public void proxySelectEnd(Call call, HttpUrl url, List<Proxy> proxies) {
      printEvent("proxySelectEnd");
    }

    @Override public void callStart(Call call) {
      printEvent("callStart");
    }

    @Override public void dnsStart(Call call, String domainName) {

  protocols: List<Protocol>,
  connectionSpecs: List<ConnectionSpec>,
  /**
   * Returns this address's proxy selector. Only used if the proxy is null. If none of this
   * selector's proxies are reachable, a direct connection will be attempted.
   */
  @get:JvmName("proxySelector") val proxySelector: ProxySelector,
) {
  /**

Istio has a variety of [controllers](https://kubernetes.io/docs/concepts/architecture/controller/), which basically watch some inputs and do something.
This can be reading from Kubernetes and writing other objects back, writing to proxies over XDS, etc.

Unfortunately, writing controllers is very error prone, even for seemingly simple cases.
To work around this, Istio has a variety of abstractions meant to make writing controllers easier.

## Clients