$ strace -e socket,ioctl ifconfig eth0
socket(PF_FILE, SOCK_DGRAM, 0) = 3
socket(PF_INET, SOCK_DGRAM, IPPROTO_IP) = 4
socket(PF_INET6, SOCK_DGRAM, IPPROTO_IP) = 5
ioctl(5, SIOCGIFFLAGS, {ifr_name="eth0", ifr_flags=IFF_UP|IFF_BROADCAST|IFF_RUNNING|IFF_MULTICAST}) = 0
ioctl(5, SIOCGIFHWADDR, {ifr_name="eth0", ifr_hwaddr=00:19:21:10:41:ef}) = 0
ioctl(5, SIOCGIFMETRIC, {ifr_name="eth0", ifr_metric=0}) = 0
ioctl(5, SIOCGIFMTU, {ifr_name="eth0", ifr_mtu=1500}) = 0
ioctl(5, SIOCGIFMAP, {ifr_name="eth0", ifr_map={mem_start=0, mem_end=0, base_addr=0, irq=20, dma=0, port=0}}) = 0
ioctl(5, SIOCGIFMAP, {ifr_name="eth0", ifr_map={mem_start=0, mem_end=0, base_addr=0, irq=20, dma=0, port=0}}) = 0
ioctl(5, SIOCGIFTXQLEN, {ifr_name="eth0", ifr_qlen=1000}) = 0
ioctl(4, SIOCGIFADDR, {ifr_name="eth0", ifr_addr={AF_INET, inet_addr("10.67.7.119")}}) = 0
ioctl(4, SIOCGIFDSTADDR, {ifr_name="eth0", ifr_dstaddr={AF_INET, inet_addr("10.67.7.119")}}) = 0
ioctl(4, SIOCGIFBRDADDR, {ifr_name="eth0", ifr_broadaddr={AF_INET, inet_addr("10.67.7.255")}}) = 0
ioctl(4, SIOCGIFNETMASK, {ifr_name="eth0", ifr_netmask={AF_INET, inet_addr("255.255.255.0")}}) = 0
实际上ifconfig命令就是通过socket ioctl 拿到网卡的信息，诸如ip地址，MAC MTU 之类的
MAC地址解析-广播地址、组播地址和单播地址的特征
紫梦芊 发表于: 三月 16th, 2011 | 阅读: | 评论数: (0)
MAC地址分成三类，分别是广播地址、组播地址和单播地址。首先，FF：FF：FF：FF：FF：FF毫无疑问是广播地址。每个网卡出厂时被分配唯一一个单播地址，头24位是设备制造厂商的编号，由IEEE(电气与电子工程师协会)分配，后24位是设备厂商为网卡制定的唯一编号。例如08：00：20：0A：8C：6D是单播地址的例子，其中08：00：20是著名的CPU厂商AMD的编号。单播地址的特征是头8位的最低位为0，于是单播地址共有47位的地址空间。
另一类地址就是组播地址。MAC组播地址的特征是头8位的最低位是1，于是MAC组播地址空间相当巨大——除去全1的47位地址空间。例如01：80：C2：00：00：00是一个组播地址，表示802.1d网桥多播组。网桥就是使用这个地址，相互之间交换配置信息，运行分布式生成树算法，消除网络拓扑结构中的环路。
IP层也有单播、组播和广播的概念。MAC组播地址中的01：00：5E：00：00：00到01：00：5E：7F：FF：FF共计23位对应于IP组播地址。给定一个IP组播地址，将其低23位与01：00：5E：00：00：00的低位取“与”运算，即可的到一个 MAC组播地址。但IP组播地址有28位地址空间，对应的MAC组播地址却只有23位，这样每32个IP组播地址被映射到同一MAC地址上。
MAC组播地址空间远远大于IP组播地址空间。如果网络层仅有IP协议，那么完全能将IP组播地址一对一地映射到MAC组播地址！但MAC地址是一个数据链路层的概念，其上层的网络层远远不只有IP。
用ifconfig修改mac地址：
用ifconfig eth3 hw ether F1:AC:6F:B8:C2:BF 设置mac地址时，会检查它的类型，如果是广播或组播地址或全0的MAC，就会判断它无效，并提示：
SIOCSIFHWADDR: Cannot assign requested address信息。
Cannot assign requested address是由strerror 函数打印出来的信息。
errno 是错误代码，在 errno.h头文件中；
perror是错误输出函数，输出格式为：msg:errno对应的错误信息(加上一个换行符)；
strerror 是通过参数 errnum （就是errno），返回对应的错误信息。它的提示信息见：http://www.freebsd.org/cgi/cvsweb.cgi/src/lib/libc/gen/errlst.c?rev=1.8.12.1;content-type=text%2Fplain中介绍，如：
const char *const sys_errlist[] = {
"Undefined error: 0", /* 0 - ENOERROR */
"Operation not permitted", /* 1 - EPERM */
"No such file or directory", /* 2 - ENOENT */
"No such process", /* 3 - ESRCH */
"Interrupted system call", /* 4 - EINTR */
"Input/output error", /* 5 - EIO */
"Device not configured", /* 6 - ENXIO */
"Argument list too long", /* 7 - E2BIG */
"Exec format error", /* 8 - ENOEXEC */
"Bad file descriptor", /* 9 - EBADF */
"No child processes", /* 10 - ECHILD */
"Resource deadlock avoided", /* 11 - EDEADLK */
"Cannot allocate memory", /* 12 - ENOMEM */
"Permission denied", /* 13 - EACCES */
"Bad address", /* 14 - EFAULT */
"Block device required", /* 15 - ENOTBLK */
"Device busy", /* 16 - EBUSY */
"File exists", /* 17 - EEXIST */
"Cross-device link", /* 18 - EXDEV */
"Operation not supported by device", /* 19 - ENODEV */
"Not a directory", /* 20 - ENOTDIR */
"Is a directory", /* 21 - EISDIR */
"Invalid argument", /* 22 - EINVAL */
"Too many open files in system", /* 23 - ENFILE */
"Too many open files", /* 24 - EMFILE */
"Inappropriate ioctl for device", /* 25 - ENOTTY */
"Text file busy", /* 26 - ETXTBSY */
"File too large", /* 27 - EFBIG */
"No space left on device", /* 28 - ENOSPC */
"Illegal seek", /* 29 - ESPIPE */
"Read-only file system", /* 30 - EROFS */
"Too many links", /* 31 - EMLINK */
"Broken pipe", /* 32 - EPIPE */
/* math software */
"Numerical argument out of domain", /* 33 - EDOM */
"Result too large", /* 34 - ERANGE */
/* non-blocking and interrupt i/o */
"Resource temporarily unavailable", /* 35 - EAGAIN */
/* 35 - EWOULDBLOCK */
"Operation now in progress", /* 36 - EINPROGRESS */
"Operation already in progress", /* 37 - EALREADY */
/* ipc/network software -- argument errors */
"Socket operation on non-socket", /* 38 - ENOTSOCK */
"Destination address required", /* 39 - EDESTADDRREQ */
"Message too long", /* 40 - EMSGSIZE */
"Protocol wrong type for socket", /* 41 - EPROTOTYPE */
"Protocol not available", /* 42 - ENOPROTOOPT */
"Protocol not supported", /* 43 - EPROTONOSUPPORT */
"Socket type not supported", /* 44 - ESOCKTNOSUPPORT */
"Operation not supported", /* 45 - EOPNOTSUPP */
"Protocol family not supported", /* 46 - EPFNOSUPPORT */
/* 47 - EAFNOSUPPORT */
"Address family not supported by protocol family",
"Address already in use", /* 48 - EADDRINUSE */
"Can't assign requested address", /* 49 - EADDRNOTAVAIL */ ->就是上面例子中的提示
/* ipc/network software -- operational errors */
"Network is down", /* 50 - ENETDOWN */
"Network is unreachable", /* 51 - ENETUNREACH */
"Network dropped connection on reset", /* 52 - ENETRESET */
"Software caused connection abort", /* 53 - ECONNABORTED */
"Connection reset by peer", /* 54 - ECONNRESET */
"No buffer space available", /* 55 - ENOBUFS */
"Socket is already connected", /* 56 - EISCONN */
"Socket is not connected", /* 57 - ENOTCONN */
"Can't send after socket shutdown", /* 58 - ESHUTDOWN */
"Too many references: can't splice", /* 59 - ETOOMANYREFS */
"Operation timed out", /* 60 - ETIMEDOUT */
"Connection refused", /* 61 - ECONNREFUSED */
"Too many levels of symbolic links", /* 62 - ELOOP */
"File name too long", /* 63 - ENAMETOOLONG */
/* should be rearranged */
"Host is down", /* 64 - EHOSTDOWN */
"No route to host", /* 65 - EHOSTUNREACH */
"Directory not empty", /* 66 - ENOTEMPTY */
/* quotas & mush */
"Too many processes", /* 67 - EPROCLIM */
"Too many users", /* 68 - EUSERS */
"Disc quota exceeded", /* 69 - EDQUOT */
/* Network File System */
"Stale NFS file handle", /* 70 - ESTALE */
"Too many levels of remote in path", /* 71 - EREMOTE */
"RPC struct is bad", /* 72 - EBADRPC */
"RPC version wrong", /* 73 - ERPCMISMATCH */
"RPC prog. not avail", /* 74 - EPROGUNAVAIL */
"Program version wrong", /* 75 - EPROGMISMATCH */
"Bad procedure for program", /* 76 - EPROCUNAVAIL */
"No locks available", /* 77 - ENOLCK */
"Function not implemented", /* 78 - ENOSYS */
"Inappropriate file type or format", /* 79 - EFTYPE */
"Authentication error", /* 80 - EAUTH */
"Need authenticator", /* 81 - ENEEDAUTH */
"Identifier removed", /* 82 - EIDRM */
"No message of desired type", /* 83 - ENOMSG */
"Value too large to be stored in data type", /* 84 - EOVERFLOW */
"Operation canceled", /* 85 - ECANCELED */
"Illegal byte sequence", /* 86 - EILSEQ */
"Attribute not found", /* 87 - ENOATTR */
/* General */
"Programming error", /* 88 - EDOOFUS */
"Bad message", /* 89 - EBADMSG */
"Multihop attempted", /* 90 - EMULTIHOP */
"Link has been severed", /* 91 - ENOLINK */
"Protocol error", /* 92 - EPROTO */
};
const int sys_nerr = sizeof(sys_errlist) / sizeof(sys_errlist[0]);
在linux系统中，它的定义在 include/asm-generic/errno.h中
