Date: Mon, 2 Feb 2009 17:24:14 -0500 From: Derek Tattersall <dlt@mebtel.net> To: Robert Watson <rwatson@FreeBSD.org> Cc: freebsd-net@FreeBSD.org Subject: Re: Multicast source address in recvfrom() Message-ID: <20090202222414.GA59860@oriental.arm.org> In-Reply-To: <alpine.BSF.2.00.0902021827470.77103@fledge.watson.org> References: <20090201183057.GA47405@oriental.arm.org> <alpine.BSF.2.00.0902021827470.77103@fledge.watson.org>
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* Robert Watson <rwatson@FreeBSD.org> [090202 17:06]:
> On Sun, 1 Feb 2009, Derek Tattersall wrote:
>
> >In order to become familiar with multicast implementation using FreeBSD, I
> >found via Google a pair of test programs which multicast sent a simple
> >text message and received the text message. I added some code to report
> >the source address, because none of the references that I looked at
> >specified the source IP address in the frame.
> >
> >I ran the sender on A -current system, AMD64 vintage last week. The
> >receiver was on a -current system I386 vintage last week. TCPDUMP shows
> >the source IP address in the frame as (correctly) 192.168.0.15. The
> >receiver reports the source IP address as 200.231.191.191. I have also
> >run the same test with an OpenBSD 4.4 Release I386 system as the receiver.
> >The openBSD system reports the sender as 192.168.0.15. A Fedora 10 system
> >reported the source IP address as 0.0.0.0.
> >
> >Googling the RFCs and other information and referring to Comer's and
> >Stevens' books on TCPIP I can't determine what should be reported. Does
> >anybody have clue for me?
>
> Hi Derek:
>
> It might depend on how you're querying the source address. Could you post
> a code excerpt?
>
> Robert N M Watson
> Computer Laboratory
> University of Cambridge
It's a straightforward test program.
/**********************************************************************
*
* MulticastReceiver.c: Receive a multicast message
*
* Copyright (C) 2009 Derek Tattersall.
*
* See the COPYING file for license information.
*
* Author: Derek Tattersall
*
* Latest Revision: Sat Jan 31 07:21:04 2009
*
**********************************************************************/
/* Includes***********************************************************/
#include <stdio.h> /* for printf() and fprintf() */
#include <sys/socket.h> /* for socket(), connect(), sendto(), and recvfrom() */
#include <arpa/inet.h> /* for sockaddr_in and inet_addr() */
#include <netinet/in.h> /* for IPPROTO_UDP */
#include <stdlib.h> /* for atoi() and exit() */
#include <string.h> /* for memset() */
#include <unistd.h> /* for close() */
/* Defines ***********************************************************/
#define MAXRECVSTRING 255 /* Longest string to receive */
/* Type Definitions **************************************************/
/* Function Declarations *********************************************/
void DieWithError(char *errorMessage); /* External error handling function */
/* Global Variables **************************************************/
/* Function Implementations ******************************************/
int main(int argc, char *argv[])
{
int sock; /* Socket */
struct sockaddr_in multicastAddr; /* Multicast Address */
char *multicastIP; /* IP Multicast Address */
unsigned short multicastPort; /* Port */
struct sockaddr_in senderAddr; /* Sender Address */
int senderLen; /* length of sender address socket */
char senderAddrBuf[INET_ADDRSTRLEN]; /* conversion buffer */
char recvString[MAXRECVSTRING+1]; /* Buffer for received string */
int recvStringLen; /* Length of received string */
struct ip_mreq multicastRequest; /* Multicast address join structure */
if (argc != 3) /* Test for correct number of arguments */
{
fprintf(stderr,"Usage: %s <Multicast IP> <Multicast Port>\n", argv[0]);
exit(1);
}
multicastIP = argv[1]; /* First arg: Multicast IP address (dotted quad) */
multicastPort = atoi(argv[2]);/* Second arg: Multicast port */
/* Create a best-effort datagram socket using UDP */
if ((sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0)
DieWithError("socket() failed");
/* Construct bind structure */
memset(&multicastAddr, 0, sizeof(multicastAddr)); /* Zero out structure */
multicastAddr.sin_family = AF_INET; /* Internet address family */
multicastAddr.sin_addr.s_addr = htonl(INADDR_ANY); /* Any incoming interface */
multicastAddr.sin_port = htons(multicastPort); /* Multicast port */
/* Bind to the multicast port */
if (bind(sock, (struct sockaddr *) &multicastAddr, sizeof(multicastAddr)) < 0)
DieWithError("bind() failed");
/* Specify the multicast group */
multicastRequest.imr_multiaddr.s_addr = inet_addr(multicastIP);
/* Accept multicast from any interface */
multicastRequest.imr_interface.s_addr = htonl(INADDR_ANY);
/* Join the multicast address */
if (setsockopt(sock, IPPROTO_IP, IP_ADD_MEMBERSHIP, (void *) &multicastRequest,
sizeof(multicastRequest)) < 0)
DieWithError("setsockopt() failed");
/* Receive a single datagram from the server */
if ((recvStringLen = recvfrom(sock, recvString, MAXRECVSTRING, 0, (struct sockaddr *)&senderAddr, (socklen_t *)&senderLen)) < 0)
DieWithError("recvfrom() failed");
if ((inet_ntop(AF_INET, &senderAddr.sin_addr.s_addr, senderAddrBuf, INET_ADDRSTRLEN)) == 0)
senderAddrBuf[0] = '\0';
recvString[recvStringLen] = '\0';
printf("Received from %s: %s\n", senderAddrBuf, recvString); /* Print the received string */
close(sock);
exit(0);
}
/* vim: set sts=0 sw=8 ts=8: *****************************************/
Note that the sender's address is taken from the recvfrom call per the
man page and printed via inet_ntop call. The DieWithError routine is a
simple print error message and exit with a 1. I have used similar code
with UDP recieving apps before.
--
Best regards,
Derek Tattersall
dlt@mebtel.net dlt666@yahoo.com dtatters@gmail.com
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