对/usr/include/netinet/in.h文件的分析

在学习了计算机网络的理论基础知识之后，如果能够对Linux下的TCP/IP实现代码进行一定的分析，将十分有利于加深对计算机网络理论的理解，同时能够使我们对网络既有理性的理解，也具有感性的认识。下面我们对Linux ubuntu 2.6.38-8-generic中的/usr/include/netinet/in.h文件结合网络理论进行一些分析。

/* Standard well-defined IP protocols.  */
enum
{
    IPPROTO_IP = 0,       /* Dummy protocol for TCP.  */
#define IPPROTO_IP        IPPROTO_IP
    IPPROTO_HOPOPTS = 0,  /* IPv6 Hop-by-Hop options.  */
#define IPPROTO_HOPOPTS        IPPROTO_HOPOPTS
    IPPROTO_ICMP = 1,       /* Internet Control Message Protocol.  */
#define IPPROTO_ICMP        IPPROTO_ICMP
    IPPROTO_IGMP = 2,       /* Internet Group Management Protocol. */
#define IPPROTO_IGMP        IPPROTO_IGMP
    IPPROTO_IPIP = 4,       /* IPIP tunnels (older KA9Q tunnels use 94).  */
#define IPPROTO_IPIP        IPPROTO_IPIP
    IPPROTO_TCP = 6,      /* Transmission Control Protocol.  */
#define IPPROTO_TCP        IPPROTO_TCP
    IPPROTO_EGP = 8,      /* Exterior Gateway Protocol.  */
#define IPPROTO_EGP        IPPROTO_EGP
    IPPROTO_PUP = 12,       /* PUP protocol.  */
#define IPPROTO_PUP        IPPROTO_PUP
    IPPROTO_UDP = 17,       /* User Datagram Protocol.  */
#define IPPROTO_UDP        IPPROTO_UDP
    IPPROTO_IDP = 22,       /* XNS IDP protocol.  */
#define IPPROTO_IDP        IPPROTO_IDP
    IPPROTO_TP = 29,      /* SO Transport Protocol Class 4.  */
#define IPPROTO_TP        IPPROTO_TP
    IPPROTO_DCCP = 33,    /* Datagram Congestion Control Protocol.  */
#define IPPROTO_DCCP        IPPROTO_DCCP
    IPPROTO_IPV6 = 41,    /* IPv6 header.  */
#define IPPROTO_IPV6        IPPROTO_IPV6
    IPPROTO_ROUTING = 43,  /* IPv6 routing header.  */
#define IPPROTO_ROUTING        IPPROTO_ROUTING
    IPPROTO_FRAGMENT = 44, /* IPv6 fragmentation header.  */
#define IPPROTO_FRAGMENT    IPPROTO_FRAGMENT
    IPPROTO_RSVP = 46,    /* Reservation Protocol.  */
#define IPPROTO_RSVP        IPPROTO_RSVP
    IPPROTO_GRE = 47,       /* General Routing Encapsulation.  */
#define IPPROTO_GRE        IPPROTO_GRE
    IPPROTO_ESP = 50,     /* encapsulating security payload.  */
#define IPPROTO_ESP        IPPROTO_ESP
    IPPROTO_AH = 51,      /* authentication header.  */
#define IPPROTO_AH        IPPROTO_AH
    IPPROTO_ICMPV6 = 58,  /* ICMPv6.  */
#define IPPROTO_ICMPV6        IPPROTO_ICMPV6
    IPPROTO_NONE = 59,    /* IPv6 no next header.  */
#define IPPROTO_NONE        IPPROTO_NONE
    IPPROTO_DSTOPTS = 60,  /* IPv6 destination options.  */
#define IPPROTO_DSTOPTS        IPPROTO_DSTOPTS
    IPPROTO_MTP = 92,       /* Multicast Transport Protocol.  */
#define IPPROTO_MTP        IPPROTO_MTP
    IPPROTO_ENCAP = 98,   /* Encapsulation Header.  */
#define IPPROTO_ENCAP        IPPROTO_ENCAP
    IPPROTO_PIM = 103,    /* Protocol Independent Multicast.  */
#define IPPROTO_PIM        IPPROTO_PIM
    IPPROTO_COMP = 108,   /* Compression Header Protocol.  */
#define IPPROTO_COMP        IPPROTO_COMP
    IPPROTO_SCTP = 132,   /* Stream Control Transmission Protocol.  */
#define IPPROTO_SCTP        IPPROTO_SCTP
    IPPROTO_UDPLITE = 136, /* UDP-Lite protocol.  */
#define IPPROTO_UDPLITE        IPPROTO_UDPLITE
    IPPROTO_RAW = 255,       /* Raw IP packets.  */
#define IPPROTO_RAW        IPPROTO_RAW
    IPPROTO_MAX
};

很显然，上面的代码定义了所以基于IP协议的一些协议的枚举。

其中为我们大多数人熟知的协议如下：

1.    IPPROTO_ICMP = 1,   /* Internet Control Message Protocol.  */

      该协议简写为ICMP，即Internet控制报文协议，著名的ping命令就是利用该协议来实现的。

2.    IPPROTO_TCP = 6,   /* Transmission Control Protocol.  */

       TCP协议，即传输层控制协议。

3.    IPPROTO_UDP = 17,   /* User Datagram Protocol.  */

       UDP协议，即用户数据报协议。

4.    IPPROTO_MTP = 92,   /* Multicast Transport Protocol.  */

       多播传输协议。

还有它们对应的IPv6的协议。

/* Type to represent a port.  */
typedef uint16_t in_port_t;
/* Standard well-known ports.  */
enum
{
    IPPORT_ECHO = 7,           /* Echo service.  */
    IPPORT_DISCARD = 9,        /* Discard transmissions service.  */
    IPPORT_SYSTAT = 11,        /* System status service.  */
    IPPORT_DAYTIME = 13,       /* Time of day service.  */
    IPPORT_NETSTAT = 15,       /* Network status service.  */
    IPPORT_FTP = 21,           /* File Transfer Protocol.  */
    IPPORT_TELNET = 23,        /* Telnet protocol.  */
    IPPORT_SMTP = 25,          /* Simple Mail Transfer Protocol.  */
    IPPORT_TIMESERVER = 37,    /* Timeserver service.  */
    IPPORT_NAMESERVER = 42,    /* Domain Name Service.  */
    IPPORT_WHOIS = 43,         /* Internet Whois service.  */
    IPPORT_MTP = 57,

    IPPORT_TFTP = 69,          /* Trivial File Transfer Protocol.  */
    IPPORT_RJE = 77,
    IPPORT_FINGER = 79,        /* Finger service.  */
    IPPORT_TTYLINK = 87,
    IPPORT_SUPDUP = 95,        /* SUPDUP protocol.  */


    IPPORT_EXECSERVER = 512,    /* execd service.  */
    IPPORT_LOGINSERVER = 513,   /* rlogind service.  */
    IPPORT_CMDSERVER = 514,
    IPPORT_EFSSERVER = 520,

    /* UDP ports.  */
    IPPORT_BIFFUDP = 512,
    IPPORT_WHOSERVER = 513,
    IPPORT_ROUTESERVER = 520,

    /* Ports less than this value are reserved for privileged processes.  */
    IPPORT_RESERVED = 1024,

    /* Ports greater this value are reserved for (non-privileged) servers.  */
    IPPORT_USERRESERVED = 5000
};

typedef uint16_t in_port_t;定义了端口为一个16位的无符号整数。

接下来的enum定义了一些系统常用的端口号。

    IPPORT_NETSTAT = 15,        /* Network status service.  */  提供netstat命令行服务的端口号

    IPPORT_FTP = 21,        /* File Transfer Protocol.  */  FTP使用的端口号

    IPPORT_TELNET = 23,               /* Telnet protocol.  */  提供telent命令行服务的端口号

    IPPORT_SMTP = 25,        /* Simple Mail Transfer Protocol.  */ 简单邮件协议的端口号

    IPPORT_NAMESERVER = 42,     /* Domain Name Service.  */  DNS的端口号

    /* Ports less than this value are reserved for privileged processes.  */

    IPPORT_RESERVED = 1024,

所以小于1024的端口号是系统预用的端口号。

/* Internet address.  */
typedef uint32_t in_addr_t;
struct in_addr
{
    in_addr_t s_addr;
};

上面的结构定义了IP地址，为一个32位的无符号整数。

/* Definitions of the bits in an Internet address integer.
   On subnets, host and network parts are found according to
   the subnet mask, not these masks.
*/
#define    IN_CLASSA(a)        ((((in_addr_t)(a)) & 0x80000000) == 0)
#define    IN_CLASSA_NET        0xff000000
#define    IN_CLASSA_NSHIFT    24
#define    IN_CLASSA_HOST        (0xffffffff & ~IN_CLASSA_NET)
#define    IN_CLASSA_MAX        128

#define    IN_CLASSB(a)        ((((in_addr_t)(a)) & 0xc0000000) == 0x80000000)
#define    IN_CLASSB_NET        0xffff0000
#define    IN_CLASSB_NSHIFT    16
#define    IN_CLASSB_HOST        (0xffffffff & ~IN_CLASSB_NET)
#define    IN_CLASSB_MAX        65536

#define    IN_CLASSC(a)        ((((in_addr_t)(a)) & 0xe0000000) == 0xc0000000)
#define    IN_CLASSC_NET        0xffffff00
#define    IN_CLASSC_NSHIFT    8
#define    IN_CLASSC_HOST        (0xffffffff & ~IN_CLASSC_NET)

#define    IN_CLASSD(a)        ((((in_addr_t)(a)) & 0xf0000000) == 0xe0000000)
#define    IN_MULTICAST(a)        IN_CLASSD(a)

#define    IN_EXPERIMENTAL(a)    ((((in_addr_t)(a)) & 0xe0000000) == 0xe0000000)
#define    IN_BADCLASS(a)        ((((in_addr_t)(a)) & 0xf0000000) == 0xf0000000)

/* Address to accept any incoming messages.  */
#define    INADDR_ANY        ((in_addr_t) 0x00000000)
/* Address to send to all hosts.  */
#define    INADDR_BROADCAST    ((in_addr_t) 0xffffffff)
/* Address indicating an error return.  */
#define    INADDR_NONE        ((in_addr_t) 0xffffffff)

/* Network number for local host loopback.  */
#define    IN_LOOPBACKNET        127
/* Address to loopback in software to local host.  */
#ifndef INADDR_LOOPBACK
# define INADDR_LOOPBACK    ((in_addr_t) 0x7f000001) /* Inet 127.0.0.1.  */
#endif

/* Defines for Multicast INADDR.  */
#define INADDR_UNSPEC_GROUP    ((in_addr_t) 0xe0000000) /* 224.0.0.0 */
#define INADDR_ALLHOSTS_GROUP    ((in_addr_t) 0xe0000001) /* 224.0.0.1 */
#define INADDR_ALLRTRS_GROUP    ((in_addr_t) 0xe0000002) /* 224.0.0.2 */
#define INADDR_MAX_LOCAL_GROUP  ((in_addr_t) 0xe00000ff) /* 224.0.0.255 */

#define IN_CLASSA(a)            ((((in_addr_t)(a)) & 0x80000000) == 0)

#define IN_CLASSB(a) ((((in_addr_t)(a)) & 0xc0000000) == 0x80000000)

#define IN_CLASSC(a) ((((in_addr_t)(a)) & 0xe0000000) == 0xc0000000)

#define IN_CLASSD(a) ((((in_addr_t)(a)) & 0xf0000000) == 0xe0000000)

#define IN_EXPERIMENTAL(a) ((((in_addr_t)(a)) & 0xe0000000) == 0xe0000000)

#define IN_BADCLASS(a) ((((in_addr_t)(a)) & 0xf0000000) == 0xf0000000)

该宏用于判断一个IP地址是否是一个A类地址，因为我们知道：

A类地址的最高位为：0

B类地址的最高位为：10

C类地址的最高位为：110

D类地址的最高位为：1110

这几个宏分别：

提起IP地址的最高1位，如果最高位是0，则是A类IP地址

提起IP地址的最高2位，如果最高2位是10，则是B类IP地址

提起IP地址的最高3位，如果最高3位是110，则是C类IP地址

提起IP地址的最高4位，如果最高4位是1110，则是D类IP地址

#define IN_CLASSA_NET 0xff000000

用于提起A类IP地址的网络地址部分。

#define IN_CLASSA_NSHIFT 24

定义了A类IP地址的网络地址部分开始的偏移为24.（24--31位）

#define IN_CLASSA_HOST (0xffffffff & ~IN_CLASSA_NET)

用于提起A类IP地址的主机地址部分。

#define IN_CLASSA_MAX 128

定义了A类IP地址允许有128个子网（2的7次方等于128）。

其他关于B、C、D类地址的相关定义类似。

/* Address to accept any incoming messages.  */

#define INADDR_ANY ((in_addr_t) 0x00000000)

该宏定义的IP地址可以接收所有发送过来的数据包。

/* Address to send to all hosts.  */

#define INADDR_BROADCAST ((in_addr_t) 0xffffffff)

该宏定义了“有限广播地址”或者称为“本地网广播地址”，它用于在本地子网中进行广播。因为它的32位全部为1，所以它不可能去表示一个IP地址的网络地址部分，所以它只能用于本地子网进行广播。我们还有另外一种广播形式成为“定向广播”，它由某个Internate子网的网络部分加上全1的主机地址组成，可以用于定向向该Internate子网中的所有的主机进行广播，因为它可以指定向某个子网进行广播，所以叫“定向广播”。

/* Network number for local host loopback.  */

#define IN_LOOPBACKNET 127

定义了本地回网的网络地址部分。

/* Address to loopback in software to local host.  */

#ifndef INADDR_LOOPBACK

# define INADDR_LOOPBACK ((in_addr_t) 0x7f000001) /* Inet 127.0.0.1.  */

#endif

定义了本地回网的IP地址。

/* Defines for Multicast INADDR.  */

#define INADDR_UNSPEC_GROUP ((in_addr_t) 0xe0000000) /* 224.0.0.0 */

#define INADDR_ALLHOSTS_GROUP ((in_addr_t) 0xe0000001) /* 224.0.0.1 */

#define INADDR_ALLRTRS_GROUP    ((in_addr_t) 0xe0000002) /* 224.0.0.2 */

#define INADDR_MAX_LOCAL_GROUP  ((in_addr_t) 0xe00000ff) /* 224.0.0.255 */

这些宏定义了相关的组播地址。

#define INET_ADDRSTRLEN 16

#define INET6_ADDRSTRLEN 46

定义了IPv4的IP地址的长度为16个字节，IPv6的IP地址长度为46字节。

/* Structure describing an Internet socket address.  */
struct sockaddr_in
{
    __SOCKADDR_COMMON (sin_);
    in_port_t sin_port;             /* Port number.  */
    struct in_addr sin_addr;        /* Internet address.  */

    /* Pad to size of `struct sockaddr'.  */
    unsigned char sin_zero[sizeof (struct sockaddr) -
               __SOCKADDR_COMMON_SIZE -
               sizeof (in_port_t) -
               sizeof (struct in_addr)];
};

定义了表示IP地址的结构体。显然该结构体的大小和struct sockaddr的大小一样。

/* IPv4 multicast request.  */
struct ip_mreq
{
    /* IP multicast address of group.  */
    struct in_addr imr_multiaddr;

    /* Local IP address of interface.  */
    struct in_addr imr_interface;
};

struct ip_mreq_source
{
    /* IP multicast address of group.  */
    struct in_addr imr_multiaddr;

    /* IP address of source.  */
    struct in_addr imr_interface;

    /* IP address of interface.  */
    struct in_addr imr_sourceaddr;
};

定义了组播相关的结构体。