Envoy proxy 源代码解读 - original_dst cluster

original_dst cluster 定义如下：

   - name: cluster1
      type: ORIGINAL_DST                                                                                                                                  
      lb_policy: ORIGINAL_DST_LB

type 设置成 ORIGINAL_DST，同时 lb_policy 需要设置为 ORIGINAL_DST_LB，否则配置解析的时候会报错：

[critical][main] [external/envoy/source/server/server.cc:92] error initializing configuration 'echo2_server.yaml': cluster: cluster type 'original_dst' may only be used with LB type 'original_dst_lb'

orignal_dst cluster 获取 upstream 的 ip 信息时来源有两个，一个是由设置了 use_original_dst: true 的 listener 所初始化 Envoy::Extensions::ListenerFilters::OriginalDst::OriginalDstFilter 解析请求的 original_dst（通常是 iptable redirect 而来）：

// envoy/source/extensions/filters/listener/original_dst.cc
Network::FilterStatus OriginalDstFilter::onAccept(Network::ListenerFilterCallbacks& cb) {
  ENVOY_LOG(debug, "original_dst: New connection accepted");
  Network::ConnectionSocket& socket = cb.socket();
  const Network::Address::Instance& local_address = *socket.localAddress();

  if (local_address.type() == Network::Address::Type::Ip) {
    Network::Address::InstanceConstSharedPtr original_local_address =
        getOriginalDst(socket.ioHandle().fd());

    // A listener that has the use_original_dst flag set to true can still receive
    // connections that are NOT redirected using iptables. If a connection was not redirected,
    // the address returned by getOriginalDst() matches the local address of the new socket.
    // In this case the listener handles the connection directly and does not hand it off.
    if (original_local_address) {
      // Restore the local address to the original one.
      socket.restoreLocalAddress(original_local_address);
    }
  }

  return Network::FilterStatus::Continue;
}

另外一个是，如果 cluster 设置了 use_http_header: true：