Hi, I encounter a strange problem when using 2.6.30 kernel. There are two processes, one runs main1() and the other runs main2(). Two processes are running simultaneously on uni-processor system. Besides, eth0 is a proprietory ethernet driver. IFF_ALLMULTI can be set on eth0 successfully most of the time. The result looks like: main2: save_flags = 0x1043, flags = 0x1243 seldom, setting IFF_ALLMULTI on eth0 will not take effect and the result looks like: main2: save_flags = 0x1002, flags = 0x1043 main2: ret = 0 Please give me some suggestion about how to debug this issue. int ifUp(const char *ifName) { int sockfd = 0; struct ifreq ifr; int ret = 0; if (ifName == NULL) { ret = -1; } else { if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) { ret = -1; } else { strncpy(ifr.ifr_name, ifName, IFNAMSIZ); ifr.ifr_flags = (IFF_UP | IFF_RUNNING | IFF_MULTICAST); if (ioctl(sockfd, SIOCSIFFLAGS, &ifr) < 0) { ret = -1; } close(sockfd); } } return ret; } short get_interface_flags(char *ifname) { struct ifreq ifr; int sockfd, err; sockfd = socket(PF_INET, SOCK_DGRAM, 0); if (sockfd <= 0) return -1; strncpy(ifr.ifr_name, ifname, IFNAMSIZ); err = ioctl(sockfd, SIOCGIFFLAGS, (void*)&ifr); close(sockfd); if (err != 0) return -1; return ifr.ifr_flags; } short set_interface_flags(char *ifname, short flags) { struct ifreq ifr; int sockfd, err; sockfd = socket(PF_INET, SOCK_DGRAM, 0); if (sockfd <= 0) return -1; strncpy(ifr.ifr_name, ifname, IFNAMSIZ); ifr.ifr_flags = flags; err = ioctl(sockfd, SIOCSIFFLAGS, (void*)&ifr); close(sockfd); if (err != 0) return -1; return 0; } int main1() { ifUp("eth0"); } int main2() { short flags, save_flags; short ret; /* Set the interface flags to receive all multicast packets */ save_flags = get_interface_flags("eth0"); if (save_flags != -1) { ret = set_interface_flags("eth0", save_flags | IFF_ALLMULTI); flags = get_interface_flags("eth0"); printf("%s: save_flags = 0x%x, flags = 0x%x\n", __FUNCTION__, save_flags, flags); if (((flags & IFF_ALLMULTI) != IFF_ALLMULTI)) { printf("%s: ret = %d\n", __FUNCTION__, ret); } } } Thanks & Regards, GC