recvfrom() returning wrong address

[Hamish Moffatt]

I'm using recvfrom() to receive UDP datagrams sent to the network
broadcast address, and then using from the from address to send a reply
directly to the originator.

Occasionally recvfrom() sets the from address to be my local interface
address instead though. Very weird.

Here's what I mean. I have three systems on my subnet, 192.168.1.118,
119 and .120. 118 is sending broadcasts to 192.168.1.255.

tcpdump shows (on either .119 or .120):
03:27:26.377061 IP 192.168.1.118.1025 > 192.168.1.255.65022: UDP, length 22

All are configured with a broadcast address of 192.168.1.255.

I wrote a little program to call recvfrom() and dump the reported
address the packets come from (source below); here's the output on .119:

# ./recvfrom 
returned from recvfrom
received from: 192.168.1.118

However when I run it on .120,

# ./recvfrom 
returned from recvfrom
received from: 192.168.1.120

How can this be?

Here's my recvfrom.c test program.

I'm running uClibc 0.9.28, but a quick glance at SVN shows that most of
the networking code hasn't changed in 18 months.

Thanks
Hamish



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

int main(void) {
    int sock;
    struct sockaddr_in sa;
    struct sockaddr_in from;
    int fromlen;
    char buf[4096];

    sock = socket(AF_INET, SOCK_DGRAM, 0);
    if (sock < 0)
        perror("sock");

    sa.sin_addr.s_addr = INADDR_ANY;
    sa.sin_port = htons(65022);
    sa.sin_family = AF_INET;

    if (bind(sock, (struct sockaddr*)&sa, sizeof(sa)) < 0)
        perror("bind");

    fromlen = sizeof(from);
    if (recvfrom(sock, buf, 4096, 0, (struct sockaddr*)&sa, &fromlen) < 0)
        perror("recvfrom");

    printf("returned from recvfrom\n");
    printf("received from: %s\n", inet_ntoa(sa.sin_addr));

    return 0;
}
-- 
Hamish Moffatt VK3SB <hamish at debian.org> <hamish at cloud.net.au>