未公开的mustang核心秘密(四):jdk中实现的rawsocket

以前面试被问道这样一个问题，你用过rawsocket吗，我回答用过，做ICMP，实现非TCPIP协议以外的协议。
ICMP是非IP协议，java实现了TCPIP协议，没有实现ICMP协议，JDK1.5发生了变化，InetAddress提供boolean isReachable(int timeout)
测试是否可以达到该地址。实现尽最大努力试图到达主机，但防火墙和服务器配置可能阻塞请求，使其在某些特定的端口可以访问时处于不可到达状态。如果可以获得权限，则典型实现将使用 ICMP ECHO REQUEST；否则它将试图在目标主机的端口 7 (Echo) 上建立 TCP 连接。
超时值（以毫秒为单位）指示尝试应该使用的最大时间量。如果在获取应答前操作超时了，则视为主机不可到达。负值将导致抛出 IllegalArgumentException。
//测试是否可以到达主机
public boolean IsReach(String host){
boolean www=false;
boolean ww=true;
try {
InetAddress address = InetAddress.getByName(host);
for(int i=0;i<1000;i++){
www = address.isReachable(500);
if(!www) {ww=www;System.out.println(ww);continue;};

}
} catch (UnknownHostException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return ww;

}
jdk实现了ICMP协议，关注ipv4，v6就不管了。
Inet4AddressImpl.c文件
/*
* Class: java_net_Inet4AddressImpl
* Method: isReachable0
* Signature: ([bI[bI)Z
*/
JNIEXPORT jboolean JNICALL
Java_java_net_Inet4AddressImpl_isReachable0(JNIEnv *env, jobject this,
jbyteArray addrArray,
jint timeout,
jbyteArray ifArray,
jint ttl) {
jint addr;
jbyte caddr[4];
jint fd;
struct sockaddr_in him;
struct sockaddr_in* netif = NULL;
struct sockaddr_in inf;
int len = 0;
WSAEVENT hEvent;
int connect_rv = -1;
int sz;

/**
* Convert IP address from byte array to integer
*/
sz = (*env)->GetArrayLength(env, addrArray);
if (sz != 4) {
return JNI_FALSE;
}
memset((char *) &him, 0, sizeof(him));
(*env)->GetByteArrayRegion(env, addrArray, 0, 4, caddr);
addr = ((caddr[0]<<24) & 0xff000000);
addr |= ((caddr[1] <<16) & 0xff0000);
addr |= ((caddr[2] <<8) & 0xff00);
addr |= (caddr[3] & 0xff);
addr = htonl(addr);
/**
* Socket address
*/
him.sin_addr.s_addr = addr;
him.sin_family = AF_INET;
len = sizeof(him);

/**
* If a network interface was specified, let's convert its address
* as well.
*/
if (!(IS_NULL(ifArray))) {
(*env)->GetByteArrayRegion(env, ifArray, 0, 4, caddr);
addr = ((caddr[0]<<24) & 0xff000000);
addr |= ((caddr[1] <<16) & 0xff0000);
addr |= ((caddr[2] <<8) & 0xff00);
addr |= (caddr[3] & 0xff);
addr = htonl(addr);
inf.sin_addr.s_addr = addr;
inf.sin_family = AF_INET;
inf.sin_port = 0;
netif = &inf;
}

#if 0
/*
* Windows implementation of ICMP & RAW sockets is too unreliable for now.
* Therefore it's best not to try it at all and rely only on TCP
* We may revisit and enable this code in the future.
*/

/*
* Let's try to create a RAW socket to send ICMP packets
* This usually requires "root" privileges, so it's likely to fail.
*/
fd = NET_Socket(AF_INET, SOCK_RAW, IPPROTO_ICMP);
if (fd != -1) {
/*
* It didn't fail, so we can use ICMP_ECHO requests.
*/
return ping4(env, fd, &him, timeout, netif, ttl);
}
#endif

/*
* Can't create a raw socket, so let's try a TCP socket
*/
fd = NET_Socket(AF_INET, SOCK_STREAM, 0);
if (fd == JVM_IO_ERR) {
/* note: if you run out of fds, you may not be able to load
* the exception class, and get a NoClassDefFoundError
* instead.
*/
NET_ThrowNew(env, WSAGetLastError(), "Can't create socket");
return JNI_FALSE;
}
if (ttl > 0) {
setsockopt(fd, IPPROTO_IP, IP_TTL, (const char *)&ttl, sizeof(ttl));
}
/*
* A network interface was specified, so let's bind to it.
*/
if (netif != NULL) {
if (bind(fd, (struct sockaddr*)netif, sizeof(struct sockaddr_in)) < 0) {
NET_ThrowNew(env, WSAGetLastError(), "Can't bind socket");
closesocket(fd);
return JNI_FALSE;
}
}

/*
* Make the socket non blocking so we can use select/poll.
*/
hEvent = WSACreateEvent();
WSAEventSelect(fd, hEvent, FD_READ|FD_CONNECT|FD_CLOSE);

/* no need to use NET_Connect as non-blocking */
him.sin_port = htons(7); /* Echo */
connect_rv = connect(fd, (struct sockaddr *)&him, len);

/**
* connection established or refused immediately, either way it means
* we were able to reach the host!
*/
if (connect_rv == 0 || WSAGetLastError() == WSAECONNREFUSED) {
WSACloseEvent(hEvent);
closesocket(fd);
return JNI_TRUE;
} else {
int optlen;

switch (WSAGetLastError()) {
case WSAEHOSTUNREACH: /* Host Unreachable */
case WSAENETUNREACH: /* Network Unreachable */
case WSAENETDOWN: /* Network is down */
case WSAEPFNOSUPPORT: /* Protocol Family unsupported */
WSACloseEvent(hEvent);
closesocket(fd);
return JNI_FALSE;
}

if (WSAGetLastError() != WSAEWOULDBLOCK) {
NET_ThrowByNameWithLastError(env, JNU_JAVANETPKG "ConnectException",
"connect failed");
WSACloseEvent(hEvent);
closesocket(fd);
return JNI_FALSE;
}

timeout = NET_Wait(env, fd, NET_WAIT_CONNECT, timeout);

/* has connection been established */

if (timeout >= 0) {
optlen = sizeof(connect_rv);
if (JVM_GetSockOpt(fd, SOL_SOCKET, SO_ERROR, (void*)&connect_rv,
&optlen) <0) {
connect_rv = WSAGetLastError();
}

if (connect_rv == 0 || connect_rv == WSAECONNREFUSED) {
WSACloseEvent(hEvent);
closesocket(fd);
return JNI_TRUE;
}
}
}
WSACloseEvent(hEvent);
closesocket(fd);
return JNI_FALSE;
}
注意中间的#if 0这是c程序写屏蔽代码的最好方法。也就是说，jdk根本没有使用ICMP协议来判断主机是否存在，这是可以理解，因为一个ping发过去，稍有经验的黑客，就可以判断出主机的操作系统，为以后扫描做准备。一般来说，很少有人在主机打开echo端口，java这个函数做的有些多余。
jdk没有调用的ping代码
/**
* ping implementation.
* Send a ICMP_ECHO_REQUEST packet every second until either the timeout
* expires or a answer is received.
* Returns true is an ECHO_REPLY is received, otherwise, false.
*/
static jboolean
ping4(JNIEnv *env, jint fd, struct sockaddr_in* him, jint timeout,
struct sockaddr_in* netif, jint ttl) {
jint size;
jint n, len, hlen1, icmplen;
char sendbuf[1500];
char recvbuf[1500];
struct icmp *icmp;
struct ip *ip;
WSAEVENT hEvent;
struct sockaddr sa_recv;
jint tmout2;
u_short pid, seq=1;
int read_rv = 0;

pid = (u_short) getpid();
size = 60*1024;
setsockopt(fd, SOL_SOCKET, SO_RCVBUF, (const char *) &size, sizeof(size));
/**
* A TTL was specified, let's set the socket option.
*/
if (ttl > 0) {
setsockopt(fd, IPPROTO_IP, IP_TTL, (const char *) &ttl, sizeof(ttl));
}

/**
* A network interface was specified, let's bind to it.
*/
if (netif != NULL) {
if (bind(fd, (struct sockaddr*)netif, sizeof(struct sockaddr_in)) < 0) {
NET_ThrowNew(env, WSAGetLastError(), "Can't bind socket");
closesocket(fd);
return JNI_FALSE;
}
}

/**
* Let's make the socket non blocking
*/
hEvent = WSACreateEvent();
WSAEventSelect(fd, hEvent, FD_READ|FD_CONNECT|FD_CLOSE);

/**
* send 1 ICMP REQUEST every second until either we get a valid reply
* or the timeout expired.
*/
do {
/**
* construct the ICMP header
*/
memset(sendbuf, 0, 1500);
icmp = (struct icmp *) sendbuf;
icmp->icmp_type = ICMP_ECHO;
icmp->icmp_code = 0;
icmp->icmp_id = htons(pid);
icmp->icmp_seq = htons(seq);
seq++;
/**
* checksum has to be set to zero before we can calculate the
* real checksum!
*/
icmp->icmp_cksum = 0;
icmp->icmp_cksum = in_cksum((u_short *)icmp, 64);
/**
* Ping!
*/
n = sendto(fd, sendbuf, 64, 0, (struct sockaddr *)him,
sizeof(struct sockaddr));
if (n < 0 && WSAGetLastError() != WSAEWOULDBLOCK) {
NET_ThrowNew(env, WSAGetLastError(), "Can't send ICMP packet");
closesocket(fd);
WSACloseEvent(hEvent);
return JNI_FALSE;
}

/*
* wait for 1 second at most
*/
tmout2 = timeout > 1000 ? 1000 : timeout;
do {
tmout2 = NET_Wait(env, fd, NET_WAIT_READ, tmout2);
if (tmout2 >= 0) {
len = sizeof(sa_recv);
n = recvfrom(fd, recvbuf, sizeof(recvbuf), 0, &sa_recv, &len);
ip = (struct ip*) recvbuf;
hlen1 = (ip->ip_hl) << 2;
icmp = (struct icmp *) (recvbuf + hlen1);
icmplen = n - hlen1;
/**
* Is that a proper ICMP reply?
*/
if (icmplen >= 8 && icmp->icmp_type == ICMP_ECHOREPLY &&
ntohs(icmp->icmp_id) == pid) {
closesocket(fd);
WSACloseEvent(hEvent);
return JNI_TRUE;
}
}
} while (tmout2 > 0);
timeout -= 1000;
} while (timeout > 0);
closesocket(fd);
WSACloseEvent(hEvent);
return JNI_FALSE;
}