src/connectivity/network/mdns/service/mdns_interface_transceiver.cc - fuchsia - Git at Google

 // Copyright 2017 The Fuchsia Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.

 #include "src/connectivity/network/mdns/service/mdns_interface_transceiver.h"

 #include <arpa/inet.h>
 #include <errno.h>
 #include <lib/async/cpp/task.h>
 #include <lib/async/default.h>
 #include <lib/syslog/cpp/macros.h>
 #include <net/if.h>
 #include <poll.h>
 #include <sys/socket.h>

 #include <algorithm>
 #include <iostream>

 #include <fbl/unique_fd.h>

 #include "lib/fostr/hex_dump.h"
 #include "src/connectivity/network/mdns/service/dns_formatting.h"
 #include "src/connectivity/network/mdns/service/dns_reading.h"
 #include "src/connectivity/network/mdns/service/dns_writing.h"
 #include "src/connectivity/network/mdns/service/mdns_addresses.h"
 #include "src/connectivity/network/mdns/service/mdns_interface_transceiver_v4.h"
 #include "src/connectivity/network/mdns/service/mdns_interface_transceiver_v6.h"

 namespace mdns {

 // static
 std::unique_ptr<MdnsInterfaceTransceiver> MdnsInterfaceTransceiver::Create(inet::IpAddress address,
                                                                            const std::string& name,
                                                                            uint32_t index,
                                                                            Media media) {
   if (address.is_v4()) {
     return std::make_unique<MdnsInterfaceTransceiverV4>(address, name, index, media);
   } else {
     return std::make_unique<MdnsInterfaceTransceiverV6>(address, name, index, media);
   }
 }

 MdnsInterfaceTransceiver::MdnsInterfaceTransceiver(inet::IpAddress address, const std::string& name,
                                                    uint32_t index, Media media)
     : address_(address),
       name_(name),
       index_(index),
       media_(media),
       inbound_buffer_(kMaxPacketSize),
       outbound_buffer_(kMaxPacketSize) {
   FX_DCHECK(media_ == Media::kWired || media_ == Media::kWireless);
 }

 MdnsInterfaceTransceiver::~MdnsInterfaceTransceiver() {}

 bool MdnsInterfaceTransceiver::Start(const MdnsAddresses& addresses,
                                      InboundMessageCallback callback) {
   FX_DCHECK(callback);
   FX_DCHECK(!socket_fd_.is_valid()) << "Start called when already started.";

   addresses_ = &addresses;

   std::cout << "Starting mDNS on interface " << name_ << " " << address_ << " using port "
             << addresses.port() << "\n";

   socket_fd_ = fbl::unique_fd(socket(address_.family(), SOCK_DGRAM, 0));

   if (!socket_fd_.is_valid()) {
     FX_LOGS(ERROR) << "Failed to open socket, " << strerror(errno);
     return false;
   }

   // Set socket options and bind.
   if (SetOptionSharePort() != 0 || SetOptionDisableMulticastLoop() != 0 ||
       SetOptionJoinMulticastGroup() != 0 || SetOptionOutboundInterface() != 0 ||
       SetOptionUnicastTtl() != 0 || SetOptionMulticastTtl() != 0 ||
       SetOptionFamilySpecific() != 0 || SetOptionBindToDevice() != 0 || Bind() != 0) {
     socket_fd_.reset();
     return false;
   }

   inbound_message_callback_ = std::move(callback);

   WaitForInbound();
   return true;
 }

 void MdnsInterfaceTransceiver::Stop() {
   FX_DCHECK(socket_fd_.is_valid()) << "Stop called when stopped.";
   fd_waiter_.Cancel();
   socket_fd_.reset();
 }

 void MdnsInterfaceTransceiver::SetAlternateAddress(const inet::IpAddress& alternate_address) {
   FX_DCHECK(alternate_address.family() != address_.family());

   alternate_address_ = alternate_address;
 }

 void MdnsInterfaceTransceiver::SendMessage(DnsMessage* message,
                                            const inet::SocketAddress& address) {
   FX_DCHECK(message);
   FX_DCHECK(address.is_valid());
   FX_DCHECK(address.family() == address_.family() || address == addresses_->v4_multicast());

   FixUpAddresses(&message->answers_);
   FixUpAddresses(&message->authorities_);
   FixUpAddresses(&message->additionals_);
   message->UpdateCounts();

   PacketWriter writer(std::move(outbound_buffer_));
   writer << *message;
   size_t packet_size = writer.position();
   outbound_buffer_ = writer.GetPacket();

   ssize_t result = SendTo(outbound_buffer_.data(), packet_size, address);

   ++messages_sent_;
   bytes_sent_ += packet_size;

   // Host down errors are expected. See fxbug.dev/62074.
   if (result < 0 && errno != EHOSTDOWN && errno != ENETUNREACH) {
     FX_LOGS(ERROR) << "Failed to sendto " << address << " from " << name_ << " (" << address_
                    << "), size " << packet_size << ", " << strerror(errno);
   }
 }

 void MdnsInterfaceTransceiver::SendAddress(const std::string& host_full_name) {
   DnsMessage message;
   message.answers_.push_back(GetAddressResource(host_full_name));

   SendMessage(&message, addresses_->v4_multicast());
 }

 void MdnsInterfaceTransceiver::SendAddressGoodbye(const std::string& host_full_name) {
   DnsMessage message;
   // Not using |GetAddressResource| here, because we want to modify the ttl.
   message.answers_.push_back(MakeAddressResource(host_full_name, address_));
   message.answers_.back()->time_to_live_ = 0;

   SendMessage(&message, addresses_->v4_multicast());
 }

 void MdnsInterfaceTransceiver::LogTraffic() {
   std::cout << "interface " << name_ << " " << address_ << "\n";
   std::cout << "    messages received:  " << messages_received_ << "\n";
   std::cout << "    bytes received:     " << bytes_received_ << "\n";
   std::cout << "    messages sent:      " << messages_sent_ << "\n";
   std::cout << "    bytes sent:         " << bytes_sent_ << "\n";
 }

 int MdnsInterfaceTransceiver::SetOptionBindToDevice() {
   char ifname[IF_NAMESIZE];
   uint32_t index = this->index();
   if (if_indextoname(index, ifname) == nullptr) {
     FX_LOGS(ERROR) << "Failed to look up interface name with index=" << index << ", error "
                    << strerror(errno);
   }
   int result = setsockopt(socket_fd_.get(), SOL_SOCKET, SO_BINDTODEVICE, &ifname,
                           strnlen(ifname, IF_NAMESIZE));
   if (result < 0) {
     FX_LOGS(ERROR) << "Failed to set socket option SO_BINDTODEVICE with ifname=" << ifname
                    << ", error" << strerror(errno);
   }
   return result;
 }

 int MdnsInterfaceTransceiver::SetOptionSharePort() {
   int param = 1;
   int result = setsockopt(socket_fd_.get(), SOL_SOCKET, SO_REUSEADDR, &param, sizeof(param));
   if (result < 0) {
     FX_LOGS(ERROR) << "Failed to set socket option SO_REUSEADDR, " << strerror(errno);
     return result;
   }

   param = 1;
   result = setsockopt(socket_fd_.get(), SOL_SOCKET, SO_REUSEPORT, &param, sizeof(param));
   if (result < 0) {
     FX_LOGS(ERROR) << "Failed to set socket option SO_REUSEPORT, " << strerror(errno);
   }

   return result;
 }

 void MdnsInterfaceTransceiver::WaitForInbound() {
   fd_waiter_.Wait([this](zx_status_t status, uint32_t events) { InboundReady(status, events); },
                   socket_fd_.get(), POLLIN);
 }

 void MdnsInterfaceTransceiver::InboundReady(zx_status_t status, uint32_t events) {
   sockaddr_storage source_address_storage;
   socklen_t source_address_length = address_.is_v4() ? sizeof(sockaddr_in) : sizeof(sockaddr_in6);
   ssize_t result =
       recvfrom(socket_fd_.get(), inbound_buffer_.data(), inbound_buffer_.size(), 0,
                reinterpret_cast<sockaddr*>(&source_address_storage), &source_address_length);
   if (result < 0) {
     FX_LOGS(ERROR) << "Failed to recvfrom, " << strerror(errno);
     // Wait a bit before trying again to avoid spamming the log.
     async::PostDelayedTask(
         async_get_default_dispatcher(), [this]() { WaitForInbound(); }, zx::sec(10));
     return;
   }

   ++messages_received_;
   bytes_received_ += result;

   ReplyAddress reply_address(source_address_storage, address_, media_);

   if (reply_address.socket_address().address() == address_) {
     // This is an outgoing message that's bounced back to us. Drop it.
     WaitForInbound();
     return;
   }

   PacketReader reader(inbound_buffer_);
   reader.SetBytesRemaining(static_cast<size_t>(result));
   std::unique_ptr<DnsMessage> message = std::make_unique<DnsMessage>();
   reader >> *message.get();

   if (reader.complete()) {
     FX_DCHECK(inbound_message_callback_);
     inbound_message_callback_(std::move(message), reply_address);
   } else {
 #ifdef MDNS_TRACE
     FX_LOGS(WARNING) << "Couldn't parse message from " << reply_address << ", " << result
                      << " bytes: " << fostr::HexDump(inbound_buffer_.data(), result, 0);
 #else
     FX_LOGS(WARNING) << "Couldn't parse message from " << reply_address << ", " << result
                      << " bytes";
 #endif  // MDNS_TRACE
   }

   WaitForInbound();
 }

 std::shared_ptr<DnsResource> MdnsInterfaceTransceiver::GetAddressResource(
     const std::string& host_full_name) {
   FX_DCHECK(address_.is_valid());

   if (!address_resource_ || address_resource_->name_.dotted_string_ != host_full_name) {
     address_resource_ = MakeAddressResource(host_full_name, address_);
   }

   return address_resource_;
 }

 std::shared_ptr<DnsResource> MdnsInterfaceTransceiver::GetAlternateAddressResource(
     const std::string& host_full_name) {
   FX_DCHECK(alternate_address_.is_valid());

   if (!alternate_address_resource_ ||
       alternate_address_resource_->name_.dotted_string_ != host_full_name) {
     alternate_address_resource_ = MakeAddressResource(host_full_name, alternate_address_);
   }

   return alternate_address_resource_;
 }

 std::shared_ptr<DnsResource> MdnsInterfaceTransceiver::MakeAddressResource(
     const std::string& host_full_name, const inet::IpAddress& address) {
   std::shared_ptr<DnsResource> resource;

   if (address.is_v4()) {
     resource = std::make_shared<DnsResource>(host_full_name, DnsType::kA);
     resource->a_.address_.address_ = address;
   } else {
     resource = std::make_shared<DnsResource>(host_full_name, DnsType::kAaaa);
     resource->aaaa_.address_.address_ = address;
   }

   return resource;
 }

 void MdnsInterfaceTransceiver::FixUpAddresses(
     std::vector<std::shared_ptr<DnsResource>>* resources) {
   FX_DCHECK(resources);

   // This method is called from |SendMessage| to 'fix up' address resources in
   // a DNS message. |SendMessage| calls this method once for each of the three
   // resource lists in a message.
   //
   // If the resource list passed to this method contains no A or AAAA resources,
   // this method returns without making any modifications to the list.
   //
   // If the resource list does contain A or AAAA resources, those resources are
   // replaced with one or two new A or AAAA resources. The first of those new
   // resources contains the address bound by this instance (A if the address is
   // v4, AAAA if the address is v6). This address resource is returned by
   // |GetAddressResource|. If there is an 'alternate' address, a second new
   // address resource is added for that alternate address. That address resource
   // is returned by |GetAlternateAddressResource|.
   //
   // A/AAAA addresses in the original message come from two sources:
   // 1) The agent that sent the message may insert a placeholder A message that
   //    contains an invalid address.
   // 2) A different transceiver that sent the message previously may have
   //    inserted its own A/AAAA message(s). Because the mutated message is
   //    reused, we have to allow for this.

   std::string name;

   // Move A/AAAA resources to the end of the vector.
   auto iter = std::remove_if(
       resources->begin(), resources->end(), [&name](const std::shared_ptr<DnsResource>& resource) {
         if (resource->type_ != DnsType::kA && resource->type_ != DnsType::kAaaa) {
           return false;
         }

         name = resource->name_.dotted_string_;
         return true;
       });

   if (iter == resources->end()) {
     // No address resources found/moved.
     return;
   }

   FX_DCHECK(!name.empty());

   // There is at least one open slot. Fill it with the first A/AAAA resource
   // with the address resource for this interface.
   *iter++ = GetAddressResource(name);

   if (!alternate_address_.is_valid()) {
     // No alternate address. Clean up the remainder of the vector, and we're
     // done.
     resources->erase(iter, resources->end());
     return;
   }

   if (iter == resources->end()) {
     // We're at the end of the vector. Push the alternate address resource to
     // the back of the vector, and we're done.
     resources->push_back(GetAlternateAddressResource(name));
     return;
   }

   // Replace the second A/AAAA resource with the alternate address resource.
   *iter++ = GetAlternateAddressResource(name);

   // Clean up the remainder of the vector, and we're done.
   resources->erase(iter, resources->end());
 }

 }  // namespace mdns
	// Copyright 2017 The Fuchsia Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style license that can be
	// found in the LICENSE file.

	#include "src/connectivity/network/mdns/service/mdns_interface_transceiver.h"

	#include <arpa/inet.h>
	#include <errno.h>
	#include <lib/async/cpp/task.h>
	#include <lib/async/default.h>
	#include <lib/syslog/cpp/macros.h>
	#include <net/if.h>
	#include <poll.h>
	#include <sys/socket.h>

	#include <algorithm>
	#include <iostream>

	#include <fbl/unique_fd.h>

	#include "lib/fostr/hex_dump.h"
	#include "src/connectivity/network/mdns/service/dns_formatting.h"
	#include "src/connectivity/network/mdns/service/dns_reading.h"
	#include "src/connectivity/network/mdns/service/dns_writing.h"
	#include "src/connectivity/network/mdns/service/mdns_addresses.h"
	#include "src/connectivity/network/mdns/service/mdns_interface_transceiver_v4.h"
	#include "src/connectivity/network/mdns/service/mdns_interface_transceiver_v6.h"

	namespace mdns {

	// static
	std::unique_ptr<MdnsInterfaceTransceiver> MdnsInterfaceTransceiver::Create(inet::IpAddress address,
	const std::string& name,
	uint32_t index,
	Media media) {
	if (address.is_v4()) {
	return std::make_unique<MdnsInterfaceTransceiverV4>(address, name, index, media);
	} else {
	return std::make_unique<MdnsInterfaceTransceiverV6>(address, name, index, media);
	}
	}

	MdnsInterfaceTransceiver::MdnsInterfaceTransceiver(inet::IpAddress address, const std::string& name,
	uint32_t index, Media media)
	: address_(address),
	name_(name),
	index_(index),
	media_(media),
	inbound_buffer_(kMaxPacketSize),
	outbound_buffer_(kMaxPacketSize) {
	FX_DCHECK(media_ == Media::kWired \|\| media_ == Media::kWireless);
	}

	MdnsInterfaceTransceiver::~MdnsInterfaceTransceiver() {}

	bool MdnsInterfaceTransceiver::Start(const MdnsAddresses& addresses,
	InboundMessageCallback callback) {
	FX_DCHECK(callback);
	FX_DCHECK(!socket_fd_.is_valid()) << "Start called when already started.";

	addresses_ = &addresses;

	std::cout << "Starting mDNS on interface " << name_ << " " << address_ << " using port "
	<< addresses.port() << "\n";

	socket_fd_ = fbl::unique_fd(socket(address_.family(), SOCK_DGRAM, 0));

	if (!socket_fd_.is_valid()) {
	FX_LOGS(ERROR) << "Failed to open socket, " << strerror(errno);
	return false;
	}

	// Set socket options and bind.
	if (SetOptionSharePort() != 0 \|\| SetOptionDisableMulticastLoop() != 0 \|\|
	SetOptionJoinMulticastGroup() != 0 \|\| SetOptionOutboundInterface() != 0 \|\|
	SetOptionUnicastTtl() != 0 \|\| SetOptionMulticastTtl() != 0 \|\|
	SetOptionFamilySpecific() != 0 \|\| SetOptionBindToDevice() != 0 \|\| Bind() != 0) {
	socket_fd_.reset();
	return false;
	}

	inbound_message_callback_ = std::move(callback);

	WaitForInbound();
	return true;
	}

	void MdnsInterfaceTransceiver::Stop() {
	FX_DCHECK(socket_fd_.is_valid()) << "Stop called when stopped.";
	fd_waiter_.Cancel();
	socket_fd_.reset();
	}

	void MdnsInterfaceTransceiver::SetAlternateAddress(const inet::IpAddress& alternate_address) {
	FX_DCHECK(alternate_address.family() != address_.family());

	alternate_address_ = alternate_address;
	}

	void MdnsInterfaceTransceiver::SendMessage(DnsMessage* message,
	const inet::SocketAddress& address) {
	FX_DCHECK(message);
	FX_DCHECK(address.is_valid());
	FX_DCHECK(address.family() == address_.family() \|\| address == addresses_->v4_multicast());

	FixUpAddresses(&message->answers_);
	FixUpAddresses(&message->authorities_);
	FixUpAddresses(&message->additionals_);
	message->UpdateCounts();

	PacketWriter writer(std::move(outbound_buffer_));
	writer << *message;
	size_t packet_size = writer.position();
	outbound_buffer_ = writer.GetPacket();

	ssize_t result = SendTo(outbound_buffer_.data(), packet_size, address);

	++messages_sent_;
	bytes_sent_ += packet_size;

	// Host down errors are expected. See fxbug.dev/62074.
	if (result < 0 && errno != EHOSTDOWN && errno != ENETUNREACH) {
	FX_LOGS(ERROR) << "Failed to sendto " << address << " from " << name_ << " (" << address_
	<< "), size " << packet_size << ", " << strerror(errno);
	}
	}

	void MdnsInterfaceTransceiver::SendAddress(const std::string& host_full_name) {
	DnsMessage message;
	message.answers_.push_back(GetAddressResource(host_full_name));

	SendMessage(&message, addresses_->v4_multicast());
	}

	void MdnsInterfaceTransceiver::SendAddressGoodbye(const std::string& host_full_name) {
	DnsMessage message;
	// Not using \|GetAddressResource\| here, because we want to modify the ttl.
	message.answers_.push_back(MakeAddressResource(host_full_name, address_));
	message.answers_.back()->time_to_live_ = 0;

	SendMessage(&message, addresses_->v4_multicast());
	}

	void MdnsInterfaceTransceiver::LogTraffic() {
	std::cout << "interface " << name_ << " " << address_ << "\n";
	std::cout << " messages received: " << messages_received_ << "\n";
	std::cout << " bytes received: " << bytes_received_ << "\n";
	std::cout << " messages sent: " << messages_sent_ << "\n";
	std::cout << " bytes sent: " << bytes_sent_ << "\n";
	}

	int MdnsInterfaceTransceiver::SetOptionBindToDevice() {
	char ifname[IF_NAMESIZE];
	uint32_t index = this->index();
	if (if_indextoname(index, ifname) == nullptr) {
	FX_LOGS(ERROR) << "Failed to look up interface name with index=" << index << ", error "
	<< strerror(errno);
	}
	int result = setsockopt(socket_fd_.get(), SOL_SOCKET, SO_BINDTODEVICE, &ifname,
	strnlen(ifname, IF_NAMESIZE));
	if (result < 0) {
	FX_LOGS(ERROR) << "Failed to set socket option SO_BINDTODEVICE with ifname=" << ifname
	<< ", error" << strerror(errno);
	}
	return result;
	}

	int MdnsInterfaceTransceiver::SetOptionSharePort() {
	int param = 1;
	int result = setsockopt(socket_fd_.get(), SOL_SOCKET, SO_REUSEADDR, &param, sizeof(param));
	if (result < 0) {
	FX_LOGS(ERROR) << "Failed to set socket option SO_REUSEADDR, " << strerror(errno);
	return result;
	}

	param = 1;
	result = setsockopt(socket_fd_.get(), SOL_SOCKET, SO_REUSEPORT, &param, sizeof(param));
	if (result < 0) {
	FX_LOGS(ERROR) << "Failed to set socket option SO_REUSEPORT, " << strerror(errno);
	}

	return result;
	}

	void MdnsInterfaceTransceiver::WaitForInbound() {
	fd_waiter_.Wait([this](zx_status_t status, uint32_t events) { InboundReady(status, events); },
	socket_fd_.get(), POLLIN);
	}

	void MdnsInterfaceTransceiver::InboundReady(zx_status_t status, uint32_t events) {
	sockaddr_storage source_address_storage;
	socklen_t source_address_length = address_.is_v4() ? sizeof(sockaddr_in) : sizeof(sockaddr_in6);
	ssize_t result =
	recvfrom(socket_fd_.get(), inbound_buffer_.data(), inbound_buffer_.size(), 0,
	reinterpret_cast<sockaddr*>(&source_address_storage), &source_address_length);
	if (result < 0) {
	FX_LOGS(ERROR) << "Failed to recvfrom, " << strerror(errno);
	// Wait a bit before trying again to avoid spamming the log.
	async::PostDelayedTask(
	async_get_default_dispatcher(), [this]() { WaitForInbound(); }, zx::sec(10));
	return;
	}

	++messages_received_;
	bytes_received_ += result;

	ReplyAddress reply_address(source_address_storage, address_, media_);

	if (reply_address.socket_address().address() == address_) {
	// This is an outgoing message that's bounced back to us. Drop it.
	WaitForInbound();
	return;
	}

	PacketReader reader(inbound_buffer_);
	reader.SetBytesRemaining(static_cast<size_t>(result));
	std::unique_ptr<DnsMessage> message = std::make_unique<DnsMessage>();
	reader >> *message.get();

	if (reader.complete()) {
	FX_DCHECK(inbound_message_callback_);
	inbound_message_callback_(std::move(message), reply_address);
	} else {
	#ifdef MDNS_TRACE
	FX_LOGS(WARNING) << "Couldn't parse message from " << reply_address << ", " << result
	<< " bytes: " << fostr::HexDump(inbound_buffer_.data(), result, 0);
	#else
	FX_LOGS(WARNING) << "Couldn't parse message from " << reply_address << ", " << result
	<< " bytes";
	#endif // MDNS_TRACE
	}

	WaitForInbound();
	}

	std::shared_ptr<DnsResource> MdnsInterfaceTransceiver::GetAddressResource(
	const std::string& host_full_name) {
	FX_DCHECK(address_.is_valid());

	if (!address_resource_ \|\| address_resource_->name_.dotted_string_ != host_full_name) {
	address_resource_ = MakeAddressResource(host_full_name, address_);
	}

	return address_resource_;
	}

	std::shared_ptr<DnsResource> MdnsInterfaceTransceiver::GetAlternateAddressResource(
	const std::string& host_full_name) {
	FX_DCHECK(alternate_address_.is_valid());

	if (!alternate_address_resource_ \|\|
	alternate_address_resource_->name_.dotted_string_ != host_full_name) {
	alternate_address_resource_ = MakeAddressResource(host_full_name, alternate_address_);
	}

	return alternate_address_resource_;
	}

	std::shared_ptr<DnsResource> MdnsInterfaceTransceiver::MakeAddressResource(
	const std::string& host_full_name, const inet::IpAddress& address) {
	std::shared_ptr<DnsResource> resource;

	if (address.is_v4()) {
	resource = std::make_shared<DnsResource>(host_full_name, DnsType::kA);
	resource->a_.address_.address_ = address;
	} else {
	resource = std::make_shared<DnsResource>(host_full_name, DnsType::kAaaa);
	resource->aaaa_.address_.address_ = address;
	}

	return resource;
	}

	void MdnsInterfaceTransceiver::FixUpAddresses(
	std::vector<std::shared_ptr<DnsResource>>* resources) {
	FX_DCHECK(resources);

	// This method is called from \|SendMessage\| to 'fix up' address resources in
	// a DNS message. \|SendMessage\| calls this method once for each of the three
	// resource lists in a message.
	//
	// If the resource list passed to this method contains no A or AAAA resources,
	// this method returns without making any modifications to the list.
	//
	// If the resource list does contain A or AAAA resources, those resources are
	// replaced with one or two new A or AAAA resources. The first of those new
	// resources contains the address bound by this instance (A if the address is
	// v4, AAAA if the address is v6). This address resource is returned by
	// \|GetAddressResource\|. If there is an 'alternate' address, a second new
	// address resource is added for that alternate address. That address resource
	// is returned by \|GetAlternateAddressResource\|.
	//
	// A/AAAA addresses in the original message come from two sources:
	// 1) The agent that sent the message may insert a placeholder A message that
	// contains an invalid address.
	// 2) A different transceiver that sent the message previously may have
	// inserted its own A/AAAA message(s). Because the mutated message is
	// reused, we have to allow for this.

	std::string name;

	// Move A/AAAA resources to the end of the vector.
	auto iter = std::remove_if(
	resources->begin(), resources->end(), [&name](const std::shared_ptr<DnsResource>& resource) {
	if (resource->type_ != DnsType::kA && resource->type_ != DnsType::kAaaa) {
	return false;
	}

	name = resource->name_.dotted_string_;
	return true;
	});

	if (iter == resources->end()) {
	// No address resources found/moved.
	return;
	}

	FX_DCHECK(!name.empty());

	// There is at least one open slot. Fill it with the first A/AAAA resource
	// with the address resource for this interface.
	*iter++ = GetAddressResource(name);

	if (!alternate_address_.is_valid()) {
	// No alternate address. Clean up the remainder of the vector, and we're
	// done.
	resources->erase(iter, resources->end());
	return;
	}

	if (iter == resources->end()) {
	// We're at the end of the vector. Push the alternate address resource to
	// the back of the vector, and we're done.
	resources->push_back(GetAlternateAddressResource(name));
	return;
	}

	// Replace the second A/AAAA resource with the alternate address resource.
	*iter++ = GetAlternateAddressResource(name);

	// Clean up the remainder of the vector, and we're done.
	resources->erase(iter, resources->end());
	}

	} // namespace mdns