About UDP receiving ICMP port unreachable

This article is divided into 3 part

1： Message format

2： Cause of occurrence

3：linux How the protocol stack handles

4： How to get

1：

The message is as follows ,10.30.13.1 Go to 10.30.16.10 Of 80 The port sent a UDP message ,80 The port actually listens to TCP.

The server replied to an unreachable port ICMP,ICMP The data part is the request UDP ip Layer and above .

2： reason

    The first reason is to receive udp The port corresponding to the message server is not opened UDP The server . Notice the description here , It's not that the port is not open , But it didn't open UDP service , If it's on TCP service , Still will return port unreachable.

3：Linux Kernel pair UDP Handle ：

（1）： As a server, I received a UDP request ：

First , As a server , When a message passes through the check route , Purpose ip It's time to deliver this machine , after netfilter After the verdict ,

call ip_local_deliver_finish, It is based on ip Protocol type in header (TCP/UDP/ICMP/......), Call different 4 Layer interface function for processing . So I said before , Even if it's turned on TCP service , Established by the server socket Of hash and udp Super search socket Of hash atypism , It also returns that the port is unreachable .

about udp for ,handler yes udp_rcv, It directly calls __udp4_lib_rcv, Find the corresponding sock,

If sk non-existent if(sk != NULL), Just reply icmp destination unreachable（ This means that the server does not have a corresponding port to accept UDP Process flow ）, The function is very simple

    So as a server , A message that the destination port is not listening is received , The direct reply port is unreachable .

So as a client , How to handle the reply from the server Port unreachable The message ？

At first, the idea was very simple , In my submission , There is no interference between different agreements , namely TCP and UDP There is no direct interference .

Not to mention this nondescript icmp？ Then I was wrong .

（2） Received as a client ICMP Port unreachable reply ：

    As a client , Port unreachable message entry ip_local_deliver_finish, It calls icmp_rcv function , To deal with .（ In fact, this is what I thought at the beginning udp There will be no corresponding reason for port unreachable data , because udp The process is udp_rcv）.

    

    actually icmp_rcv The most important thing about functions is It calls for ：icmp_pointers[icmph->type].handler(skb);

handler = icmp_unreach

icmp_unreach The final step of the function , It's the last step ：

Is it very similar ip_local_deliver_finish？

It's very much like , It's just ip_local_deliver_finish in , Called ipprot->handler, And here you call ipprot->err_handler

about udp,err_handler = udp_err = __udp4_lib_err

In this function , Only enter the following process , The application will react ：

__udp4_lib_err First, according to skb->data in dip and sip, lookup socket,skb->data yes icmp The load of

So call... First __udp4_lib_lookup lookup socket, Reference time ,sip and dip It needs to be reversed .

__udp4_lib_err：

The prerequisite is inet->recverr For the wrong 0, perhaps inet->recverr by 0 however udp be in TCP_ESTABLISHED state .

Otherwise, the application cannot expect to receive data that is unreachable to the port , The application is waiting read Time out . So , In order to obtain udp Port unreachable condition

Yes 2 Methods ：

（1）：

int val = 1;

setsockopt(fd, IPPROTO_IP, IP_RECVERR , &val,sizeof(int));

（2）：

Yes udp Conduct connect operation , And will sendto Change to send

4：

udp Learn the source program that the port is unreachable （ Method 1： Set up Socket Options ; Method 2： Yes UDP Conduct Connect）

Be careful , In case of congestion ,recvfrom It will block , Even if a port unreachable message is received , It will also block . But after Method 1 and Method 2 after ,recvfrom Returns the , The return value is -1, then Judge errno Whether it is ECONNREFUSED To determine whether the port unreachable message is received .

#include <stdio.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <string.h>
#include <errno.h>
unsigned char revc_buf[1024];
 
int main()
{
	int fd,ret,recv_len,size=1024;
	struct sockaddr_in server_addr,addr;
	int val = 1;
	server_addr.sin_family = AF_INET;
	server_addr.sin_addr.s_addr = inet_addr("192.168.2.254");
	server_addr.sin_port = htons(77);
	
	fd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
	if(fd < 0)
	{	
		perror("socket fail ");
		return -1;
	}
	
	printf("socket sucess\n");
 
        // Method 1
	#if 1
	setsockopt(fd, IPPROTO_IP, IP_RECVERR , &val,sizeof(int));
	if(sendto(fd, "nihao", strlen("nihao"), 0, (const struct sockaddr *)&(server_addr), sizeof(struct sockaddr_in))<0)
	{
		perror("sendto fail ");
		return -1;
	}
	printf("sendto sucess\n");
	recv_len = recvfrom(fd, revc_buf, sizeof(revc_buf), 0, (struct sockaddr *)&addr, (int *)&size);
	if (recv_len == -1)
	{
		if (errno == ECONNREFUSED)
		{
			printf("Recv port unreachable\n");
		}
	}
	// Method 2
	#elif 0
	ret = connect(fd, (const struct sockaddr *) &(server_addr), sizeof (struct sockaddr_in));
	if(ret < 0)
	{
		printf("connect fail\n");
		return -1;
	}
	
	ret = send(fd, "ni hao", strlen("nihao"),0);
	if(ret < 0)
	{
		printf("write fail\n");
		return -1;
	}
	
	ret = recvfrom(fd, revc_buf, sizeof(revc_buf), 0, (struct sockaddr *)&addr, (int *)&size);
	if (ret == -1) {
		if (errno == ECONNREFUSED)
		{
			printf("Recv port unreachable\n");
		}
	}
 
	#endif
	close(fd);
	
	return 0;
}

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