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(New page: * [http://www.hezmatt.org/~mpalmer/blog/general/severe_discomforts_the_joy_of_udp.html good explanation of problems with UDP on interfaces that have multiple IP addresses] and consequently...) |
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* [http://www.hezmatt.org/~mpalmer/blog/general/severe_discomforts_the_joy_of_udp.html 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 | * [http://www.hezmatt.org/~mpalmer/blog/general/severe_discomforts_the_joy_of_udp.html 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 | ||
| + | * [http://www.abc.se/~m6695/udp.html 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): | ||
| + | <pre> | ||
| + | #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; | ||
| + | } | ||
| + | </pre> | ||
| + | Chat with the "server" using socat: | ||
| + | socat - udp:localhost:9930 | ||
| + | (or substitute localhost for a remote machine on which it's running) | ||
Latest revision as of 12:04, 24 February 2009
- 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)