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// Copyright (c) 2012 The Chromium 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 <arpa/inet.h>
#include <errno.h>
#include <netinet/in.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>
#include <string>
#include "base/basictypes.h"
#include "base/command_line.h"
#include "base/eintr_wrapper.h"
#include "base/logging.h"
#include "net/base/big_endian.h"
#include "net/base/net_util.h"
#include "net/dns/dns_protocol.h"
#include "tools/android/common/daemon.h"
#include "tools/android/common/net.h"
namespace {
// Mininum request size: 1 question containing 1 QNAME, 1 TYPE and 1 CLASS.
const size_t kMinRequestSize = sizeof(net::dns_protocol::Header) + 6;
// The name reference in the answer pointing to the name in the query.
// Its format is: highest two bits set to 1, then the offset of the name
// which just follows the header.
const uint16 kPointerToQueryName =
static_cast<uint16>(0xc000 | sizeof(net::dns_protocol::Header));
const uint32 kTTL = 86400; // One day.
void SendRefusedResponse(int sock, const sockaddr_in& client_addr, uint16 id) {
net::dns_protocol::Header response;
response.id = htons(id);
response.flags = htons(net::dns_protocol::kFlagResponse |
net::dns_protocol::kFlagAA |
net::dns_protocol::kFlagRD |
net::dns_protocol::kFlagRA |
net::dns_protocol::kRcodeREFUSED);
response.qdcount = 0;
response.ancount = 0;
response.nscount = 0;
response.arcount = 0;
HANDLE_EINTR(sendto(sock, &response, sizeof(response), 0,
reinterpret_cast<const sockaddr*>(&client_addr),
sizeof(client_addr)));
}
void SendResponse(int sock, const sockaddr_in& client_addr, uint16 id,
uint16 qtype, const char* question, size_t question_length) {
net::dns_protocol::Header header;
header.id = htons(id);
header.flags = htons(net::dns_protocol::kFlagResponse |
net::dns_protocol::kFlagAA |
net::dns_protocol::kFlagRD |
net::dns_protocol::kFlagRA |
net::dns_protocol::kRcodeNOERROR);
header.qdcount = htons(1);
header.ancount = htons(1);
header.nscount = 0;
header.arcount = 0;
// Size of RDATA which is a IPv4 or IPv6 address.
size_t rdata_size = qtype == net::dns_protocol::kTypeA ?
net::kIPv4AddressSize : net::kIPv6AddressSize;
// Size of the whole response which contains the header, the question and
// the answer. 12 is the sum of sizes of the compressed name reference, TYPE,
// CLASS, TTL and RDLENGTH.
size_t response_size = sizeof(header) + question_length + 12 + rdata_size;
if (response_size > net::dns_protocol::kMaxUDPSize) {
LOG(ERROR) << "Response is too large: " << response_size;
SendRefusedResponse(sock, client_addr, id);
return;
}
char response[net::dns_protocol::kMaxUDPSize];
net::BigEndianWriter writer(response, arraysize(response));
writer.WriteBytes(&header, sizeof(header));
// Repeat the question in the response. Some clients (e.g. ping) needs this.
writer.WriteBytes(question, question_length);
// Construct the answer.
writer.WriteU16(kPointerToQueryName);
writer.WriteU16(qtype);
writer.WriteU16(net::dns_protocol::kClassIN);
writer.WriteU32(kTTL);
writer.WriteU16(rdata_size);
if (qtype == net::dns_protocol::kTypeA)
writer.WriteU32(INADDR_LOOPBACK);
else
writer.WriteBytes(&in6addr_loopback, sizeof(in6_addr));
DCHECK(writer.ptr() - response == response_size);
HANDLE_EINTR(sendto(sock, response, response_size, 0,
reinterpret_cast<const sockaddr*>(&client_addr),
sizeof(client_addr)));
}
void HandleRequest(int sock, const char* request, size_t size,
const sockaddr_in& client_addr) {
if (size < kMinRequestSize) {
LOG(ERROR) << "Request is too small " << size
<< "\n" << tools::DumpBinary(request, size);
return;
}
net::BigEndianReader reader(request, size);
net::dns_protocol::Header header;
reader.ReadBytes(&header, sizeof(header));
uint16 id = ntohs(header.id);
uint16 flags = ntohs(header.flags);
uint16 qdcount = ntohs(header.qdcount);
uint16 ancount = ntohs(header.ancount);
uint16 nscount = ntohs(header.nscount);
uint16 arcount = ntohs(header.arcount);
const uint16 kAllowedFlags = 0x07ff;
if ((flags & ~kAllowedFlags) ||
qdcount != 1 || ancount || nscount || arcount) {
LOG(ERROR) << "Unsupported request: FLAGS=" << flags
<< " QDCOUNT=" << qdcount
<< " ANCOUNT=" << ancount
<< " NSCOUNT=" << nscount
<< " ARCOUNT=" << arcount
<< "\n" << tools::DumpBinary(request, size);
SendRefusedResponse(sock, client_addr, id);
return;
}
// request[size - 5] should be the end of the QNAME (a zero byte).
// We don't care about the validity of QNAME because we don't parse it.
const char* qname_end = &request[size - 5];
if (*qname_end) {
LOG(ERROR) << "Error parsing QNAME\n" << tools::DumpBinary(request, size);
SendRefusedResponse(sock, client_addr, id);
return;
}
reader.Skip(qname_end - reader.ptr() + 1);
uint16 qtype;
uint16 qclass;
reader.ReadU16(&qtype);
reader.ReadU16(&qclass);
if ((qtype != net::dns_protocol::kTypeA &&
qtype != net::dns_protocol::kTypeAAAA) ||
qclass != net::dns_protocol::kClassIN) {
LOG(ERROR) << "Unsupported query: QTYPE=" << qtype << " QCLASS=" << qclass
<< "\n" << tools::DumpBinary(request, size);
SendRefusedResponse(sock, client_addr, id);
return;
}
SendResponse(sock, client_addr, id, qtype,
request + sizeof(header), size - sizeof(header));
}
} // namespace
int main(int argc, char** argv) {
printf("Fake DNS server\n");
CommandLine command_line(argc, argv);
if (tools::HasHelpSwitch(command_line) || command_line.GetArgs().size()) {
tools::ShowHelp(argv[0], "", "");
return 0;
}
int sock = socket(AF_INET, SOCK_DGRAM, 0);
if (sock < 0) {
perror("create socket");
return 1;
}
sockaddr_in addr;
memset(&addr, 0, sizeof(addr));
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
addr.sin_port = htons(53);
int reuse_addr = 1;
if (HANDLE_EINTR(bind(sock, reinterpret_cast<sockaddr*>(&addr),
sizeof(addr))) < 0) {
perror("server bind");
HANDLE_EINTR(close(sock));
return 1;
}
if (!tools::HasNoSpawnDaemonSwitch(command_line))
tools::SpawnDaemon(0);
while (true) {
sockaddr_in client_addr;
socklen_t client_addr_len = sizeof(client_addr);
char request[net::dns_protocol::kMaxUDPSize];
int size = HANDLE_EINTR(recvfrom(sock, request, sizeof(request),
MSG_WAITALL,
reinterpret_cast<sockaddr*>(&client_addr),
&client_addr_len));
if (size < 0) {
// Unrecoverable error, can only exit.
LOG(ERROR) << "Failed to receive a request: " << strerror(errno);
HANDLE_EINTR(close(sock));
return 1;
}
if (size > 0)
HandleRequest(sock, request, size, client_addr);
}
}
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