C unix domain sockets, recvfrom() doesn't set struct sockaddr* src_addr

Question

I'm writing an application that listens for UDP packets over a unix domain socket. Consider the following code block.

int sockfd;
struct sockaddr_un servaddr;

sockfd = socket(AF_LOCAL, SOCK_DGRAM, 0);

if(sockfd < 0)
{
    perror("socket() failed");
}

unlink(port);

bzero(&servaddr, sizeof(servaddr));
servaddr.sun_family = AF_LOCAL;
strcpy(servaddr.sun_path, port);    

if(bind(sockfd, (struct sockaddr *)&servaddr, sizeof(servaddr)) < 0)
{
    perror("bind() failed");
    close(sockfd);
}

int n;
struct sockaddr_un cliaddr;
socklen_t len = sizeof(cliaddr);
discovery_msgs client_message;


bzero(&client_message, sizeof(client_message));
// Wait for a message to be received
n = recvfrom(sock_fd, &client_message, sizeof(client_message), 0, 
    (struct sockaddr *) &cliaddr, &len);

// At this point n = 560, client_message is filled with the expected data 
//len = 0 and cliaddr has no information about the client that sent the data

Now the type of client_message isn't really important, I'm receiving a UDP packet and client_message contains all of the data I expect. The problem begins when I look at cliaddr and len after calling recvfrom. cliaddr is not modified by recvfrom like it normally is with normal network TCP/UDP and len is set to 0 after the call(which means recvfrom wrote no data to &cliaddr). I need the information in cliaddr to be populated with the unix domain path so I can send a response.

What am I doing wrong?

Answer 1

The solution is binding the socket on the client side when using Unix domain sockets. Otherwise the transient pathname created for sending the UDP packet immediately disappears after sendto(), which explains why the client's address information is not available on the server side.

See Stevens Network Programming page 419 or see this for an example client implementation that solves this issue: libpix.org/unp/unixdgcli01_8c_source.html

#include    "unp.h"

int
main(int argc, char **argv)
{
    int                 sockfd;
    struct sockaddr_un  cliaddr, servaddr;

    sockfd = Socket(AF_LOCAL, SOCK_DGRAM, 0);

    bzero(&cliaddr, sizeof(cliaddr));       /* bind an address for us */
    cliaddr.sun_family = AF_LOCAL;
    strcpy(cliaddr.sun_path, tmpnam(NULL));

    Bind(sockfd, (SA *) &cliaddr, sizeof(cliaddr));

    bzero(&servaddr, sizeof(servaddr)); /* fill in server's address */
    servaddr.sun_family = AF_LOCAL;
    strcpy(servaddr.sun_path, UNIXDG_PATH);

    dg_cli(stdin, sockfd, (SA *) &servaddr, sizeof(servaddr));

    exit(0);
}

Note: unp.h defines Bind() which is simply bind() with some error checking(commonly used throughout Stevens Network Programming). In the same manner, (SA *) is the equivalent to (struct sockaddr *).