Hello,
While writing some client server application in c, we noticed a strange
behavior : if we try to connect endlessly to a given local port where
nobody is listening, and if the port is >= to 32768, after several
thousands tries ( Connection refused ) connect will return 0.
This behavior is not exhibited when port is < 32768.
We confirmed this behavior in kernel 2.6.17-10, 2.6.18-1, 2.6.8, on x86
and 2.4.21-32 on ia64, on several hardware configurations.
Distribution is debian or ubuntu.
Attached is a source file that demonstrate this behavior.
./a.out port_number
Sample execution :
./a.out 35489
Out port : 35489
connect try 1 failed : Connection refused
connect try 2 failed : Connection refused
connect try 3 failed : Connection refused
.....
connect try 6089 failed : Connection refused
connect try 6090 failed : Connection refused
Connection success : 6091 try
Connection closed
last error : Connection refused
Is this behavior to be expected?
Can it be disabled?
Thanks in advance.
Regards,
Brice Videau
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <string.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <errno.h>
int main(int argc, char **argv) {
unsigned short out_port = 35000;
char * endptr;
struct sockaddr_in server_address;
int out_socket;
int i=1;
if( argc > 1 ) {
out_port = (unsigned short) strtol(argv[1], &endptr, 10);
errno = 0;
if(errno != 0 || out_port == 0 || endptr == argv[1] ) {
perror("Invalid port number");
exit(1);
}
}
printf( "Out port : %hu\n", out_port );
fflush(NULL);
memset( &server_address, 0, sizeof(struct sockaddr_in) );
server_address.sin_family = AF_INET;
server_address.sin_port = htons(out_port);
if ( inet_pton( AF_INET, "127.0.0.1", &(server_address.sin_addr) ) <= 0 ) {
perror("inet_pton error");
exit(1);
}
if ( (out_socket = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
perror("socket error");
exit(1);
}
while (connect(out_socket, (struct sockaddr *) &server_address, sizeof(struct sockaddr_in)) < 0)
{
fprintf(stderr,"connect try %i failed : ",i);
perror("");
fflush(NULL);
i++;
}
printf("Connection success : %i try\n",i);
fflush(NULL);
if( close(out_socket) < 0 ) {
perror("close error");
exit(1);
}
printf("Connection closed\n");
fflush(NULL);
fprintf(stderr,"last error : ");
perror("");
return 0;
}
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