UNP_22_高级UDP套接字编程

// 该结构体用来返回最后两项
struct unp_in_pktinfo{
    struct in_addr ipi_addr;
    int            ipi_ifindex;
};
// 前三个参数类似于 recvfrom，后面的参数用来返回上面提到的项
ssize_t recvfrom_flags(int fd, void *ptr, size_t nbytes, int *flagsp,
			   SA *sa, socklen_t *salenptr, struct unp_in_pktinfo *pktp)
{
	struct msghdr	msg;
	struct iovec	iov[1];
	ssize_t			n;
// 不同的实现差异
#ifdef	HAVE_MSGHDR_MSG_CONTROL
	struct cmsghdr	*cmptr;
	union {
	  struct cmsghdr	cm;
	  char				control[CMSG_SPACE(sizeof(struct in_addr)) +
								CMSG_SPACE(sizeof(struct unp_in_pktinfo))];
	} control_un;

	msg.msg_control = control_un.control;
	msg.msg_controllen = sizeof(control_un.control);
	msg.msg_flags = 0;
#else
	bzero(&msg, sizeof(msg));	/* make certain msg_accrightslen = 0 */
#endif
	// 填写一个 msghdr 结构并用来调用 recvmsg
	msg.msg_name = sa;
	msg.msg_namelen = *salenptr;
	iov[0].iov_base = ptr;
	iov[0].iov_len = nbytes;
	msg.msg_iov = iov;
	msg.msg_iovlen = 1;

	if ( (n = recvmsg(fd, &msg, *flagsp)) < 0)
		return(n);

	*salenptr = msg.msg_namelen;	/* pass back results */
	if (pktp)
		bzero(pktp, sizeof(struct unp_in_pktinfo));	/* 0.0.0.0, i/f = 0 */

#ifndef	HAVE_MSGHDR_MSG_CONTROL
	*flagsp = 0;					/* pass back results */
	return(n);
#else
	// 返回 msg_flags 的成员值
	*flagsp = msg.msg_flags;
	if (msg.msg_controllen < sizeof(struct cmsghdr) ||
		(msg.msg_flags & MSG_CTRUNC) || pktp == NULL)
		return(n);
	// 处理辅助数据
	for (cmptr = CMSG_FIRSTHDR(&msg); cmptr != NULL;
		 cmptr = CMSG_NXTHDR(&msg, cmptr)) {

#ifdef	IP_RECVDSTADDR
		// 如果目的 IP 地址作为控制信息返回，就返回给调用者
		if (cmptr->cmsg_level == IPPROTO_IP &&
			cmptr->cmsg_type == IP_RECVDSTADDR) {

			memcpy(&pktp->ipi_addr, CMSG_DATA(cmptr),
				   sizeof(struct in_addr));
			continue;
		}
#endif

#ifdef	IP_RECVIF
		if (cmptr->cmsg_level == IPPROTO_IP &&
			cmptr->cmsg_type == IP_RECVIF) {
			struct sockaddr_dl	*sdl;

			sdl = (struct sockaddr_dl *) CMSG_DATA(cmptr);
			pktp->ipi_ifindex = sdl->sdl_index;
			continue;
		}
#endif
		err_quit("unknown ancillary data, len = %d, level = %d, type = %d",
				 cmptr->cmsg_len, cmptr->cmsg_level, cmptr->cmsg_type);
	}
	return(n);
#endif	/* HAVE_MSGHDR_MSG_CONTROL */
}

ssize_t Recvfrom_flags(int fd, void *ptr, size_t nbytes, int *flagsp,
			   SA *sa, socklen_t *salenptr, struct unp_in_pktinfo *pktp)
{
	ssize_t		n;

	n = recvfrom_flags(fd, ptr, nbytes, flagsp, sa, salenptr, pktp);
	if (n < 0)
		err_quit("recvfrom_flags error");

	return(n);
}

#undef	MAXLINE
// 测试更大的数据报，用来测试截断标志
#define	MAXLINE	20		/* to see datagram truncation */

void
dg_echo(int sockfd, SA *pcliaddr, socklen_t clilen)
{
	int						flags;
	const int				on = 1;
	socklen_t				len;
	ssize_t					n;
	char					mesg[MAXLINE], str[INET6_ADDRSTRLEN],
							ifname[IFNAMSIZ];
	struct in_addr			in_zero;
	struct unp_in_pktinfo	pktinfo;

#ifdef	IP_RECVDSTADDR
	// 开启 IP_RECVDSTADDR 和 IP_RECVIF 套接字选项
	if (setsockopt(sockfd, IPPROTO_IP, IP_RECVDSTADDR, &on, sizeof(on)) < 0)
		err_ret("setsockopt of IP_RECVDSTADDR");
#endif
#ifdef	IP_RECVIF
	if (setsockopt(sockfd, IPPROTO_IP, IP_RECVIF, &on, sizeof(on)) < 0)
		err_ret("setsockopt of IP_RECVIF");
#endif
	bzero(&in_zero, sizeof(struct in_addr));	/* all 0 IPv4 address */

	for ( ; ; ) {
		len = clilen;
		flags = 0;
		n = Recvfrom_flags(sockfd, mesg, MAXLINE, &flags,
						   pcliaddr, &len, &pktinfo);
		printf("%d-byte datagram from %s", n, Sock_ntop(pcliaddr, len));
		// 返回的 IP 地址不为 0，调用 Inet_ntop 转换表达形式
		if (memcmp(&pktinfo.ipi_addr, &in_zero, sizeof(in_zero)) != 0)
			printf(", to %s", Inet_ntop(AF_INET, &pktinfo.ipi_addr,
										str, sizeof(str)));
		// 返回接口索引不为 0，调用 If_indextoname 获取接口名字并打印
		if (pktinfo.ipi_ifindex > 0)
			printf(", recv i/f = %s",
				   If_indextoname(pktinfo.ipi_ifindex, ifname));
		// 这四个标志位，分别测试是否打开，打开则打印
#ifdef	MSG_TRUNC
		if (flags & MSG_TRUNC)	printf(" (datagram truncated)");
#endif
#ifdef	MSG_CTRUNC
		if (flags & MSG_CTRUNC)	printf(" (control info truncated)");
#endif
#ifdef	MSG_BCAST
		if (flags & MSG_BCAST)	printf(" (broadcast)");
#endif
#ifdef	MSG_MCAST
		if (flags & MSG_MCAST)	printf(" (multicast)");
#endif
		printf("\n");

		Sendto(sockfd, mesg, n, 0, pcliaddr, len);
	}
}

void	mydg_echo(int, SA *, socklen_t, SA *);

int main(int argc, char **argv) {
	int					sockfd;
	const int			on = 1;
	pid_t				pid;
	struct ifi_info		*ifi, *ifihead;
	struct sockaddr_in	*sa, cliaddr, wildaddr;

	for (ifihead = ifi = Get_ifi_info(AF_INET, 1);
		 ifi != NULL; ifi = ifi->ifi_next) {

			/*4bind unicast address */
		sockfd = Socket(AF_INET, SOCK_DGRAM, 0);

		Setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &on, sizeof(on));

		sa = (struct sockaddr_in *) ifi->ifi_addr;
		sa->sin_family = AF_INET;
		sa->sin_port = htons(SERV_PORT);
		Bind(sockfd, (SA *) sa, sizeof(*sa));
		printf("bound %s\n", Sock_ntop((SA *) sa, sizeof(*sa)));

		if ( (pid = Fork()) == 0) {		/* child */
			mydg_echo(sockfd, (SA *) &cliaddr, sizeof(cliaddr), (SA *) sa);
			exit(0);		/* never executed */
		}

		if (ifi->ifi_flags & IFF_BROADCAST) {
				/* 4try to bind broadcast address */
			sockfd = Socket(AF_INET, SOCK_DGRAM, 0);
			Setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &on, sizeof(on));

			sa = (struct sockaddr_in *) ifi->ifi_brdaddr;
			sa->sin_family = AF_INET;
			sa->sin_port = htons(SERV_PORT);
			if (bind(sockfd, (SA *) sa, sizeof(*sa)) < 0) {
				if (errno == EADDRINUSE) {
					printf("EADDRINUSE: %s\n",
						   Sock_ntop((SA *) sa, sizeof(*sa)));
					Close(sockfd);
					continue;
				} else
					err_sys("bind error for %s",
							Sock_ntop((SA *) sa, sizeof(*sa)));
			}
			printf("bound %s\n", Sock_ntop((SA *) sa, sizeof(*sa)));

			if ( (pid = Fork()) == 0) {		/* child */
				mydg_echo(sockfd, (SA *) &cliaddr, sizeof(cliaddr),
						  (SA *) sa);
				exit(0);		/* never executed */
			}
		}
	}

		/* 4bind wildcard address */
	sockfd = Socket(AF_INET, SOCK_DGRAM, 0);
	Setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &on, sizeof(on));

	bzero(&wildaddr, sizeof(wildaddr));
	wildaddr.sin_family = AF_INET;
	wildaddr.sin_addr.s_addr = htonl(INADDR_ANY);
	wildaddr.sin_port = htons(SERV_PORT);
	Bind(sockfd, (SA *) &wildaddr, sizeof(wildaddr));
	printf("bound %s\n", Sock_ntop((SA *) &wildaddr, sizeof(wildaddr)));

	if ( (pid = Fork()) == 0) {		/* child */
		mydg_echo(sockfd, (SA *) &cliaddr, sizeof(cliaddr), (SA *) sa);
		exit(0);		/* never executed */
	}
	exit(0);
}

void
mydg_echo(int sockfd, SA *pcliaddr, socklen_t clilen, SA *myaddr)
{
	int			n;
	char		mesg[MAXLINE];
	socklen_t	len;

	for ( ; ; ) {
		len = clilen;
		n = Recvfrom(sockfd, mesg, MAXLINE, 0, pcliaddr, &len);
		printf("child %d, datagram from %s", getpid(),
			   Sock_ntop(pcliaddr, len));
		printf(", to %s\n", Sock_ntop(myaddr, clilen));

		Sendto(sockfd, mesg, n, 0, pcliaddr, len);
	}
}