help!! why socket has no respone?

["wang rui" <wang_work2002 at hotmail dot com>]

Hi,friends

i have written a simple client-server app to test udp socket,but they do 
not work correctly! For the server has
no respone while request coming in.

the program is below:

Client.c:

#include <stdio.h>
#include <sys/types.h>
#include <ctype.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <unistd.h>
#include <arpa/inet.h>

int main(int argc,char* argv[]) {
 int sockfd;
 struct sockaddr_in their_addr;
 int addr_len;
 char sendbuf[8192]="\0";

 if (argc<2) { 
   printf("Usage : client HOSTADDR\n");
   exit(1);
 }

 if ((sockfd=socket(AF_INET,SOCK_DGRAM,0))==-1) {
   printf("error in socket!\n");
   exit(1);
 }

 their_addr.sin_family=AF_INET;
 their_addr.sin_port=htons(161);
 their_addr.sin_addr.s_addr=inet_addr(argv[1]);
 bzero(&(their_addr.sin_zero),8);
 addr_len=sizeof(their_addr);
 while (fgets(sendbuf,8192,stdin)!=NULL) {
   sendto(sockfd,sendbuf,strlen(sendbuf),0,(struct 
sockaddr*)&their_addr,addr_len);
 }

 close(sockfd);
 exit(1);
}
Server.c:

#include <stdio.h>
#include <sys/types.h>
#include <ctype.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <netdb.h>

void pro_udp_service(int sockfd);

int main(int argc,char* argv[]) {
 int sockfd;
 struct sockaddr_in my_addr;
 int addr_len;
 struct protoent *ppe;
 int htons_port;
 if ((ppe=getprotobyname("udp"))==0) {
   printf("error in getprotobyname!\n");
   exit(1);
 }
 if ((sockfd=socket(AF_INET,SOCK_DGRAM,ppe->p_proto))==-1) {
   printf("error in socket!\n");
   exit(1);
 }

 my_addr.sin_family=AF_INET;  
 my_addr.sin_port=htons(161);
 my_addr.sin_addr.s_addr=INADDR_ANY;
 if (bind(sockfd,(struct sockaddr*)&my_addr,sizeof(struct sockaddr))==-1) 
{
   printf("error in binding!\n");
   exit(1);
 }

 pro_udp_service(sockfd);
 close(sockfd);
 exit(1);
}


void pro_udp_service(int sockfd) {
 char recvbuf[8192];
 int n=0,len;
 struct sockaddr* addr;
 
 while (1) {
   len=sizeof(struct sockaddr_in);
   if ((n=recvfrom(sockfd,recvbuf,sizeof(recvbuf),0,addr,&len))==-1) {
     printf("error in recieving data!\n");
     exit(1);
   }
 }
}

with "tcpdump" i can see that the udp packet is truely sent from client to 
server,but the server doesn't work with the request.

19:35:27.487859 z.32770 > w.snmp:  [len6<asnlen49] (DF)
0x0000	 4500 0024 0000 4000 4011 a418 c0a8 0ab3	E..$..@.@.......
0x0010	 c0a8 0aad 8002 00a1 0010 23dc 3131 3131	..........#.1111
0x0020	 3131 310a 0000 0000 0000 0000 0000     	111...........

But,on the other hand,if do such change,it works fine:
Client.c:
their_addr.sin_port=htons(161); ==> their_addr.sin_port=htons(6161); /* 
portnum>1024 */

Server.c:
my_addr.sin_port=htons(161); ==> my_addr.sin_port=htons(6161);

can you tell me why this happens?It puzzles me for a long time.
and how can i write my server deamon with port number below 1024?
thanks a lot for your help.
wang

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