iptables 分析(三)

接上篇文章，find_target查到目标并加载成功，返回一个xtables_target型对象保存在target,分配空间，拷贝目标，初始化target对象。

if (target) { //成功

                size_t size;
                size = IPT_ALIGN(sizeof(struct ipt_entry_target))
                    + target->size;

                target->t = fw_calloc(1, size);
                target->t->u.target_size = size;//空间大小
                strcpy(target->t->u.user.name, jumpto); //保存目标名到xptables_targets

                set_revision(target->t->u.user.name,
                     target->revision); //设置版本

                if (target->init != NULL)
                    target->init(target->t); //初始化xptables_targets                    

                opts = merge_options(opts,
                         target->extra_opts,
                         &target->option_offset);//将target 的参数选项与旧的参数选项连接在一起由opts 返回，这样下一个循环可以分析target 的参数选项，一般在“default:”中进行分析

                if (opts == NULL)
                    exit_error(OTHER_PROBLEM,
                         "can't alloc memory!");
            }

接下来看个比较重要的函数iptc_init来从内核获取表的规则信息,tables就是是-t保存的表名

/*为了用户能够对内核态的规则进行操作，需要将内核态的规则信息读取到用户空间，对用户空间的规则进行修改后，再根据用户态的规则信息设置内核态的规则信息*/
    if (!*handle)
        *handle = iptc_init(*table); //调用 iptc_init获取表的规则信息，调用list_entries函数显示规则


    /* 加载此模块再获取 */
    if (!*handle && load_xtables_ko(modprobe_program, 0) != -1)
        *handle = iptc_init(*table);

    if (!*handle)
        exit_error(VERSION_PROBLEM,
             "can't initialize iptables table `%s': %s",
             *table, iptc_strerror(errno));

进入iptc_init:

iptc_handle_t iptc_init(const char *tablename);
#define TC_INIT        iptc_init //宏定义
TC_HANDLE_T  TC_INIT(const char *tablename) //获取表信息

{
    TC_HANDLE_T h;
    STRUCT_GETINFO info;
    unsigned int tmp;
    socklen_t s;

    iptc_fn = TC_INIT;

    if (strlen(tablename) >= TABLE_MAXNAMELEN) {
        errno = EINVAL;
        return NULL;
    }

 /*为获取信息打开一个套接字接口*/
    if (sockfd_use == 0) {
        sockfd = socket(TC_AF, SOCK_RAW, IPPROTO_RAW);
        if (sockfd < 0)
            return NULL;
    }
    sockfd_use++;
retry:
    s = sizeof(info);

    strcpy(info.name, tablename);
/*获取表基本信息,保存在info*/    
    if (getsockopt(sockfd, TC_IPPROTO, SO_GET_INFO, &info, &s) < 0) {
        if (--sockfd_use == 0) {
            close(sockfd);
            sockfd = -1;
        }
        return NULL;
    }

    DEBUGP("valid_hooks=0x%08x, num_entries=%u, size=%u\n",
        info.valid_hooks, info.num_entries, info.size);

    if ((h = alloc_handle(info.name, info.size, info.num_entries))
     == NULL) {
        if (--sockfd_use == 0) {
            close(sockfd);
            sockfd = -1;
        }
        return NULL;
    }

    /* Initialize current state */
    h->info = info;

    h->entries->size = h->info.size;

    tmp = sizeof(STRUCT_GET_ENTRIES) + h->info.size;

    if (getsockopt(sockfd, TC_IPPROTO, SO_GET_ENTRIES, h->entries,
         &tmp) < 0)//最后规则信息存入h->entries
        goto error;

#ifdef IPTC_DEBUG2
    {
        int fd = open("/tmp/libiptc-so_get_entries.blob",
                O_CREAT|O_WRONLY);
        if (fd >= 0) {
            write(fd, h->entries, tmp);
            close(fd);
        }
    }
#endif

    if (parse_table(h) < 0)
        goto error;

    CHECK(h);
    return h;
error:
    TC_FREE(&h);
    /* A different process changed the ruleset size, retry */
    if (errno == EAGAIN)
        goto retry;
    return NULL;
}

iptables与内核的交互，都是使用setsockopt函数(设置与某个套接字关联的选 项)来实现的,创建socket,通过getsockopt函数是获取与对应的套接字关联的选项,其标志位是SO_GET_INFO，获取保存在STRUCT_GETINFO类型info里，STRUCT_GETINFO结构也就是#define STRUCT_GETINFO　struct ipt_getinfo,最后getsockopt返回规则信息是ipt_getinfo型而TC_INIT函数返回一个具体的规则表信息iptc_handle型.

未完...