Thread: detect UDP server failure

I'm trying to write a udp client program that will detect if the server went down. This program is for a linux or BSD system. Here's a short test program:

Code:

#include <netinet/in.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>  /*sleep()*/

int main(int argc, char**argv)
{
    int srcsock;
    struct sockaddr_in server_addr;
    char sendst[1000];    
    int PORTNUM = 4000;
    int numrec, res, junk;

    srcsock=socket(AF_INET,SOCK_DGRAM,0);
    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr=inet_addr(argv[1]);
    server_addr.sin_port=htons(PORTNUM);

    connect(srcsock, (struct sockaddr *)&server_addr, sizeof(server_addr));
    sleep(1);
    strcpy(sendst,"This is just junk1");
    numrec = write(srcsock,sendst,strlen(sendst));
    printf("%d\n", numrec);
    sleep(1);
    strcpy(sendst,"This is just junk2");
    numrec = write(srcsock,sendst,strlen(sendst));
    printf("%d\n", numrec);
    sleep(1);
    strcpy(sendst,"This is just junk3");
    numrec = write(srcsock,sendst,strlen(sendst));
    printf("%d\n", numrec);
    sleep(1);
    strcpy(sendst,"This is just junk4");
    numrec = write(srcsock,sendst,strlen(sendst));
    printf("%d\n", numrec);
    sleep(1);
    strcpy(sendst,"This is just junk5");
    numrec = write(srcsock,sendst,strlen(sendst));
    printf("%d\n", numrec);

    close(srcsock);
}

The return from this is the correct string length for each string if the server is up, but when the server is down when I run the program, it returns:
19
-1
19
-1
19

Is there a reliable way to detect if the remote server died either during the message, or if the message could not be sent? I know it's UDP, but other programs seem to have this ability.

You may get more reliable results by using send instead of write...

Just sending a message to a UDP server will probably not tell you if the packet arrived or not, or even if it was possible to send it. You need to find a way that the other end is responding to the UDP packet to find out if it's working or not.

If you have control of the server-side software, you can introduce some sort of "If you receive this, send something back" type thingy. If that's not the case, then you need to figure some other way to send packets that get a response back (a server that simply swallows the packets is pretty useless, so one would expect that the server does have some sort of response to certain packets, right?)

--
Mats

Well, according to the docs: "Note that the successful completion of a send does not indicate that the data was successfully delivered.". You might check errno, though, it might be set if an error occured...

A couple more minor ideas. Specifiying SO_SNDTIMEO in your socket options (non-windows systems only) may work. Also, have you tried sendto? Sorry I'm not of much help - I'm flying blind here, never done any UDP myself...

With UDP you never know.

The way other programs "deal with this" is to wait a while. Then they give up.

Originally Posted by brewbuck

With UDP you never know.

The way other programs "deal with this" is to wait a while. Then they give up.

And I take it you mean "wait a while" means "wait for a response".

--
Mats