From Electron Cloud
- good explanation of problems with UDP on interfaces that have multiple IP addresses and consequently why it's a good idea to bind separately to each interface's IP address
- trivial UDP server example which binds to a port, receives packets and prints them out
See what ports are bound:
netstat -lun
Watch UDP traffic:
tcpdump -A -u port 9877 -i iface
Modified version of the UDP server linked above, which sends replies (and compiles without warnings):
#include <arpa/inet.h> #include <netinet/in.h> #include <stdio.h> #include <sys/types.h> #include <sys/socket.h> #include <unistd.h> #include <stdlib.h> #include <string.h> #define BUFLEN 512 #define NPACK 10 #define PORT 9930 void diep(char *s) { perror(s); exit(1); } int main(void) { struct sockaddr_in si_me, si_other; int s, i, slen=sizeof(si_other); char buf[BUFLEN]; if ((s=socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP))==-1) diep("socket"); memset((char *) &si_me, 0, sizeof(si_me)); si_me.sin_family = AF_INET; si_me.sin_port = htons(PORT); si_me.sin_addr.s_addr = htonl(INADDR_ANY); if (bind(s, (struct sockaddr*)&si_me, sizeof(si_me))==-1) diep("bind"); for (i=0; i<NPACK; i++) { ssize_t len; if ((len = recvfrom(s, buf, BUFLEN, 0, (struct sockaddr*)&si_other, &slen)) < 0) diep("recvfrom()"); buf[len] = '\0'; printf("Received packet (%d bytes) from %s:%d\nData: %s\n\n", len, inet_ntoa(si_other.sin_addr), ntohs(si_other.sin_port), buf); char reply[256]; int rlen = snprintf(reply, 255, "I'll see your '%s' and send you 'this'\n", buf); sendto(s, reply, rlen, 0, (struct sockaddr*)&si_other, slen); } close(s); return 0; }
Chat with the "server" using socat:
socat - udp:localhost:9930
(or substitute localhost for a remote machine on which it's running)