Linux socket编程套接字选项教程

套接字选项概述

===============================

有很多方法来获取和设置套接字的选项, 以影响套接字行为:

getsockopt和setsocketopt;
fcntl;
ioctl;
- - - - -

getsockopt和setsockopt

=============================================

2个函数仅用于套接字, 分别用于获取和设置套接字选项

#include <sys/types.h>          /* See NOTES */
#include <sys/socket.h>

int getsockopt(int sockfd, int level, int optname, void *optval, socklen_t *optlen);
int setsockopt(int sockfd, int level, int optname, const void *optval, socklen_t optlen);

参数
sockfd 一个打开的套接字描述符
level 级别, 指定系统中解释选项的代码或为通用套接字代码, 或为某个特定于协议的代码(e.g. IPv4/6, TCP, STCP)
optval 指向某个变量(optval)的指针, setsockopt从optval中取得选项待设置的新值, getsockopt则把已获得的选项当前值存放到*optval中
optlen 对setsockopt是值参数, 对getsockopt是值-结果参数, 用于指明optval的大小

套接字选项粗分为2个基本类型: 启用或进制某个特性的二元选项, 称为标志选项; 取得并返回我们可以设置或检查的特定值的选项, 称为值选项.

套接字层和IP层的套接字选项汇总表:

leve级别optname选项名getset说明标志数据类型SOL\_SOCKETSO\_BROADCAST··允许发送广播数据报·intSOL\_SOCKETSO\_DEBUG··开启调试跟踪·intSOL\_SOCKETSO\_DONTROUTE··绕过外出路由表查询·intSOL\_SOCKETSO\_ERROR·获取待处理错误并清除intSOL\_SOCKETSO\_KEEPALIVE··周期性测试连接是否仍存活·intSOL\_SOCKETSO\_LINEGER··若有数据待发送, 则延迟关闭linger{}SOL\_SOCKETSO\_OOBINLINE··让接收到的带外数据继续在线留存·intSOL\_SOCKETSO\_RCVBUF··接收缓冲区大小intSOL\_SOCKETSO\_SNDBUF··发送缓冲区大小intSOL\_SOCKETSO\_RCVLOWAT··接收缓冲区低水位标记intSOL\_SOCKETSO\_SNDLOWAT··发送缓冲区低水位标记intSOL\_SOCKETSO\_RCVTIMEO··接收超时timeval{}SOL\_SOCKETSO\_SNDTIMEO··发送超时timeval{}SOL\_SOCKETSO\_REUSEADDR··允许重用本地地址·intSOL\_SOCKETSO\_REUSEPORT··允许重用本地端口·intSOL\_SOCKETSO\_TYPE·取得套接字类型intSOL\_SOCKETSO\_USELOOPBACK··路由套接字取得所发送数据的副本·intIPPROTO\_IPIP\_HDRINCL··随数据包含的IP首部·intIPPROTO\_IPIP\_OPTIONS··IP首部选项IPPROTO\_IPIP\_RECVDSTADDR··返回目的IP地址·intIPPROTO\_IPIP\_RECVIF··返回接收接口索引·intIPPROTO\_IPIP\_TOS··服务类型和优先权intIPPROTO\_IPIP\_TTL··存活时间intIPPROTO\_IPIP\_MULTICAST\_IP··指定外出接口in\_addr{}IPPROTO\_IPIP\_MULTICAST\_TTL··指定外出TTLu\_charIPPROTO\_IPIP\_MULTICAST\_LOOP··指定是否环回u\_charIPPROTO\_IPIP\_ADD\_MEMBERSHIP·加入多播组ip\_mreq{}IPPROTO\_IPIP\_DROP\_MEMBERSHIP·离开多播组ip\_mreq{}IPPROTO\_IPIP\_BLOCK\_SOURCE·阻塞多播源ip\_mreq\_source{}IPPROTO\_IPIP\_UNBLOCK\_SOURCE·开通多播源ip\_mreq\_source{}IPPROTO\_IPIP\_ADD\_SOURCE\_MEMBERSHIP·加入源特定多播组ip\_mreq\_source{}IPPROTO\_IPIP\_DROP\_SOURCE\_MEMBERSHIP·离开源特定多播组ip\_mreq\_source{}IPPROTO\_ICMPV6ICMP6\_FILTER··指定待传递的ICMPv6消息类型icmp6\_filter{}IPPROTO\_IPV6IPV6\_CHECKSUM··用于原始套接字的校验和字段偏移intIPPROTO\_IPV6IPV6\_DONTFRAG··丢弃大的分组而非将其分片·intIPPROTO\_IPV6IPV6\_NEXTHOP··指定下一跳地址sockaddr\_in6{}IPPROTO\_IPV6IPV6\_PATHMTU·获取当前路径MTUip6\_mtuinfo{}IPPROTO\_IPV6IPV6\_RECVDSTOPTS··接收目的地选项·intIPPROTO\_IPV6IPV6\_RECVHOPLIMIT··接收单播跳限·intIPPROTO\_IPV6IPV6\_RECVHOPOPTS··接收步跳选项·intIPPROTO\_IPV6IPV6\_RECVPATHMTU··接收路径MTU·intIPPROTO\_IPV6IPV6\_RECVPKTINFO··接收分组信息·intIPPROTO\_IPV6IPV6\_RECVRTHDR··接收源路径·intIPPROTO\_IPV6IPV6\_RECVTCLASS··接收流通类别·intIPPROTO\_IPV6IPV6\_UNICAST\_HOPS··默认单播跳限intIPPROTO\_IPV6IPV6\_USE\_MIN\_MTU··使用最小MTU·intIPPROTO\_IPV6IPV6\_V6ONLY··禁止v4兼容·intIPPROTO\_IPV6IPV6\_XXX··黏附性辅助数据IPPROTO\_IPV6IPV6\_MULTICAST\_IF··指定外出接口u\_intIPPROTO\_IPV6IPV6\_MULTICAST\_HOPS··指定外出跳限intIPPROTO\_IPV6IPV6\_MULTICAST\_LOOP··指定是否环回·u\_intIPPROTO\_IPV6IPV6\_JOIN\_GROUP·加入多播组ipv6\_mreq{}IPPROTO\_IPV6IPV6\_LEAVE\_GROUP·离开多播组ipv6\_mreq{}IPPROTO\_IP或
IPPROTO\_IPV6MCAST\_JOIN\_GROUP·加入多播组group\_req{}IPPROTO\_IP或
IPPROTO\_IPV6MCAST\_LEAVE\_GROUP·离开多播组group\_source\_req{}IPPROTO\_IP或
IPPROTO\_IPV6MCAST\_BLOCK\_GROUP·阻塞多播组group\_source\_req{}IPPROTO\_IP或
IPPROTO\_IPV6MCAST\_UNBLOCK\_GROUP·开通多播组group\_source\_req{}IPPROTO\_IP或
IPPROTO\_IPV6MCAST\_JOIN\_SOURCE\_GROUP·加入源特定多播组group\_source\_req{}IPPROTO\_IP或
IPPROTO\_IPV6MCAST\_LEAVE\_SOURCE\_GROUP·离开源特定多播组group\_source\_req{}IPPROTO\_TCPTCP\_MAXSEG··TCP最大分节大小intIPPROTO\_TCPTCP\_NODELAY··禁止Nagle算法·intIPPROTO\_SCTPSCTP\_ADAPTION\_LAYER··适配层指示stcp\_setadaption{}IPPROTO\_SCTPSCTP\_ASSOCINFO·检查并设置关联信息stcp\_assocparams{}IPPROTO\_SCTPSCTP\_AUTOCLOSE··自动关闭操作intIPPROTO\_SCTPSCTP\_DEFAULT\_SEND\_PARAM··默认发送参数stcp\_sndrcvinfo{}IPPROTO\_SCTPSCTP\_DISABLE\_FRAGMENTS··SCTP分片·intIPPROTO\_SCTPSCTP\_EVENTS··感兴趣事件的通知sctp\_event\_subscribe{}IPPROTO\_SCTPSCTP\_GET\_PEER\_ADDR\_INFO获取对端地址状态sctp\_paddrinfo{}IPPROTO\_SCTPSCTP\_I\_WANT\_MAPPED\_V4\_ADDR··映射的v4地址·intIPPROTO\_SCTPSCTP\_INTTMSG··默认的INIT参数sctp\_initmsg{}IPPROTO\_SCTPSCTP\_MAXBURST··最大猝发大小intIPPROTO\_SCTPSCTP\_MAXSEG··最大分片大小intIPPROTO\_SCTPSCTP\_NODELAY··禁止Nagle算法·intIPPROTO\_SCTPSCTP\_PEER\_ADDR\_PARAMS·对端地址参数sctp\_paddrparams{}IPPROTO\_SCTPSCTP\_PRIMARY\_ADDR·主目的地址sctp\_setprim{}IPPROTO\_SCTPSCTP\_RTOINFO·RTO信息sctp\_rtoinfo{}IPPROTO\_SCTPSCTP\_SET\_PEER\_PRIMARY\_ADDR·对端的主目的地址sctp\_setpeerprim{}IPPROTO\_SCTPSCTP\_STATUS·获取关联状态sctp\_status{}

说明:

数据类型形如linger{}, 表示struct linger;
标有"标志"的列指出一个选项是否为标志选项. 当给这些标志选项调用getsockopt函数时, *optval是一个整数. *optval中返回的值为0, 表示相应的选项被禁止, 不为0表示相应的选项被启用;
setsockopt函数需要一个不为0 的optval值来启用选项, 为0 的optval来禁止选项;
如果"标志"列不含"·", 那么相应选项用于在用户进程与系统之间传递所指定数据类型的值;

- - - - -

检查选项是否受支持并获取默认值

=======================================

思路: 利用getsockopt 获取每个level对应的选项名, 如果没有出错, 则证明支持该选项; 如果出错, 则说明不支持. 如果读取出来的值大小, 跟选项汇总表不一致, 则表明可能有哪里出问题, 打印出来.
一个sockopt的判断伪代码

sockfd = socket();

if (getsockopt(fd, opt_level, opt_name, &val, &len) < 0) {
    // getsockopt error
    ...
}
else {
// no getsockopt error
// print opt_val_str
}

如果IPV6\_DONTFRAG, SCTP\_AUTOCLOSE等宏定义无法识别, 请确认安装好libsctp-dev, lksctp-tools库后, 然后#include <netinet/sctp.h>
安装命令, 详见 Unix下基于SCTP socket的通信：一对一场景 | 简书

$ sudo apt-get install libsctp-dev lksctp-tools

源代码:

#include <sys/types.h> 
#include <sys/socket.h>
#include <stdio.h>
#include <stdlib.h>
#include <netinet/in.h>
#include <netinet/tcp.h>
#include <netinet/sctp.h>

/* UNP官方代码 */
#include "ourhdr.h" 

union val {
    int i_val;
    long l_val;
    struct linger linger_val;
    struct timeval timeval_val;
} val;

int Socket(int domain, int type, int protocol);
static char *sock_str_flag(union val *, int);
static char *sock_str_int(union val *, int);
static char *sock_str_linger(union val *, int);
static char *sock_str_timeval(union val *, int);

/**
* 自定义结构sock_opts, 包含了获得或输出套接字选项的所有信息
*/
struct sock_opts {
    const char *opt_str; /// 字符名称
    int opt_level; /// 级别
    int opt_name;  /// 名称
    char *(*opt_val_str)(union val *, int); /// 函数指针, 用于输出
} sock_opts[] = {
    {"SO_BROADCAST",  SOL_SOCKET,  SO_BROADCAST,     sock_str_flag},
    {"SO_DEBUG",      SOL_SOCKET,  SO_DEBUG,         sock_str_flag},
    {"SO_DONTROUTE",  SOL_SOCKET,  SO_DONTROUTE,     sock_str_flag},
    {"SO_ERROR",      SOL_SOCKET,  SO_ERROR,         sock_str_int },
    {"SO_KEEPALIVE",  SOL_SOCKET,  SO_KEEPALIVE,     sock_str_flag},
    {"SO_LINGER",     SOL_SOCKET,  SO_LINGER,        sock_str_linger},
    {"SO_OOBINLINE",  SOL_SOCKET,  SO_OOBINLINE,     sock_str_flag},
    {"SO_RCVBUF",     SOL_SOCKET,  SO_RCVBUF,        sock_str_int },
    {"SO_SNDBUF",     SOL_SOCKET,  SO_SNDBUF,        sock_str_int },
    {"SO_RCVLOWAT",   SOL_SOCKET,  SO_RCVLOWAT,      sock_str_int },
    {"SO_SNDLOWAT",   SOL_SOCKET,  SO_SNDLOWAT,      sock_str_int },
    {"SO_RCVTIMEO",   SOL_SOCKET,  SO_RCVTIMEO,      sock_str_timeval },
    {"SO_SNDTIMEO",   SOL_SOCKET,  SO_SNDTIMEO,      sock_str_timeval },

    {"SO_REUSEADDR",  SOL_SOCKET,  SO_REUSEADDR,     sock_str_flag },
#ifdef SO_REUSEPORT
    {"SO_REUSEPORT",  SOL_SOCKET,  SO_REUSEPORT,     sock_str_flag},
#else
    {"SO_REUSEPORT",  0,             0,                  NULL},
#endif

    {"SO_TYPE",        SOL_SOCKET,  SO_TYPE,            sock_str_int },
#ifdef SO_USELOOPBACK
    {"SO_USELOOPBACK",SOL_SOCKET,  SO_USELOOPBACK,   sock_str_flag},
#else
    {"SO_USELOOPBACK",  0,             0,                  NULL},
#endif

    {"IP_TOS",        IPPROTO_IP,  IP_TOS,              sock_str_int },
    {"IP_TTL",        IPPROTO_IP,  IP_TTL,           sock_str_int },

    {"IPV6_DONTFRAG",         IPPROTO_IPV6, IPV6_DONTFRAG,         sock_str_flag},
    {"IPV6_UNICAST_HOPS",      IPPROTO_IPV6, IPV6_UNICAST_HOPS,     sock_str_int },
    {"IPV6_V6ONLY",          IPPROTO_IPV6, IPV6_V6ONLY,              sock_str_flag},
    
    {"TCP_MAXSEG",   IPPROTO_TCP,  TCP_MAXSEG,        sock_str_int },
    {"TCP_NODELAY",  IPPROTO_TCP,  TCP_NODELAY,      sock_str_flag},

    {"SCTP_AUTOCLOSE",  IPPROTO_SCTP,  SCTP_AUTOCLOSE,     sock_str_int },
#ifdef SCTP_MAXBURST
    {"SCTP_MAXBURST",   IPPROTO_SCTP,  SCTP_MAXBURST,      sock_str_int },
#else
    {"SCTP_MAXBURST",  0,             0,                  NULL},
#endif

    {"SCTP_MAXSEG",      IPPROTO_SCTP,  SCTP_MAXSEG,           sock_str_int },
    {"SCTP_NODELAY",    IPPROTO_SCTP,  SCTP_NODELAY,       sock_str_flag},

    {NULL,                 0,                0,                    NULL}
};

int main()
{
    int fd;
    socklen_t len;
    struct sock_opts *ptr;

    for (ptr = sock_opts; ptr->opt_str != NULL; ++ptr) {
        printf("%s: ", ptr->opt_str);
        if (ptr->opt_val_str == NULL) {
            printf("(undefined)\n");
        }
        else {
            switch(ptr->opt_level) {
            case SOL_SOCKET:
            case IPPROTO_IP:
            case IPPROTO_TCP:
                fd = Socket(AF_INET, SOCK_STREAM, 0);
                break;

#ifdef IPV6
            case IPPROTO_IPV6:
                fd = Socket(AF_INET6, SOCK_STREAM, 0);
                break;
#endif
#ifdef IPPROTO_SCTP
            case IPPROTO_SCTP:
                fd = Socket(AF_INET, SOCK_SEQPACKET, IPPROTO_SCTP);
                break;
#endif
            default:
                // err_quit("Can't create fd for level %d\n", ptr->opt_level);
                fprintf(stderr, "Can't create fd for level %d\n", ptr->opt_level);
            }

            len = sizeof(val);
            
            // if (fd < 0) continue;

            if (getsockopt(fd, ptr->opt_level, ptr->opt_name, &val, &len) == -1) {
                err_ret("getsockopt error");
            }
            else {
                printf("default = %s\n", (*ptr->opt_val_str)(&val, len));
            }

            close(fd);
        }
    }
    
    return 0;
}

int Socket(int domain, int type, int protocol)
{
    int fd = socket(domain, type, protocol);

    if (fd < 0) {
        perror("socket error");
        return -1;
    }

    return fd;
}


static char strres[128];
static char *sock_str_flag(union val *ptr, int len) 
{
    if (len != sizeof(int)) 
        snprintf(strres, sizeof(strres), "size (%d) not sizeof(int)", len);
    else 
        snprintf(strres, sizeof(strres), "%s", (ptr->i_val == 0) ? "off": "on");

    return strres;
}

static char *sock_str_int(union val *ptr, int len)
{
    if (len != sizeof(int)) 
        snprintf(strres, sizeof(strres), "size (%d) not sizeof(int)", len);
    else 
        snprintf(strres, sizeof(strres), "%d\t", ptr->i_val);

    return strres;
}

static char *sock_str_linger(union val *ptr, int len)
{
    if (len != sizeof(struct linger)) 
        snprintf(strres, sizeof(strres), "size (%d) not sizeof(struct linger)", len);
    else 
        snprintf(strres, sizeof(strres), "l_onoff = %d, l_linger = %d\t", ptr->linger_val.l_onoff, ptr->linger_val.l_linger);

    return strres;
}

static char *sock_str_timeval(union val *ptr, int len)
{
    if (len != sizeof(struct timeval)) 
        snprintf(strres, sizeof(strres), "size (%d) not sizeof(struct timeval)", len);
    else 
        snprintf(strres, sizeof(strres), "%ld sec, %ldusec\t", ptr->timeval_val.tv_sec, ptr->timeval_val.tv_usec);

    return strres;
}

运行结果:

SO_BROADCAST: default = off
SO_DEBUG: default = off
SO_DONTROUTE: default = off
SO_ERROR: default = 0    
SO_KEEPALIVE: default = off
SO_LINGER: default = l_onoff = 0, l_linger = 0    
SO_OOBINLINE: default = off
SO_RCVBUF: default = 131072    
SO_SNDBUF: default = 16384    
SO_RCVLOWAT: default = 1    
SO_SNDLOWAT: default = 1    
SO_RCVTIMEO: default = 0 sec, 0usec    
SO_SNDTIMEO: default = 0 sec, 0usec    
SO_REUSEADDR: default = off
SO_REUSEPORT: default = off
SO_TYPE: default = 1    
SO_USELOOPBACK: (undefined)
IP_TOS: default = 0    
IP_TTL: default = 64    
Can't create fd for level 41
IPV6_DONTFRAG: getsockopt error: Bad file descriptor
Can't create fd for level 41
IPV6_UNICAST_HOPS: getsockopt error: Bad file descriptor
Can't create fd for level 41
IPV6_V6ONLY: getsockopt error: Bad file descriptor
TCP_MAXSEG: default = 536    
TCP_NODELAY: default = off
SCTP_AUTOCLOSE: default = 0    
SCTP_MAXBURST: (undefined)
SCTP_MAXSEG: default = size (8) not sizeof(int)
SCTP_NODELAY: default = off

Linux socket编程套接字选项教程

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