udhcp源码详解（四） 之租赁IP的管理

Server端对于租赁出去的IP的管理是基于结构体dhcpOfferedAddr的，该结构体的定义是在leases.c文件里：（结构体的成员介绍说明见详解之数据结构）

   1:  struct dhcpOfferedAddr {

   2:      uint8_t chaddr[16];

   3:      uint32_t yiaddr;    /* network order */

   4:      uint32_t expires;    /* host order */

   5:  };

在dhcpd.c文件里用该结构体定义指向该结构数组的指针leases；

/* dhcpd.c */

#include <leases.h>

struct dhcpOfferedAddr *leases;

在读取完配置信息server_config后，就可以依据配置信息的内存为该结构数组申请内存空间了：

   1:      /* 

   2:       * strut dhcpOfferedAddr {

   3:       *    uint8_t        chaddr[16];

   4:       *    uint32_t    yiaddr;            //network order

   5:       *    uint32_t    expires;        //host order

   6:       * };

   7:       *

   8:       * 该结构数组leases是记录租赁出去的IP,    yiaddr

   9:       * 租赁IP客户机的MAC, charddr

  10:       * 租赁的到期时间,expires(从1970年1月1日00:00:00到租赁到期的总共的秒数)

  11:       *

  12:       * dhcp server启动后(可能是异常重启)

  13:       * 所以要充server_config.file里读取上次为哪些客户机分配了IP

  14:       * 并把读取到的内容添加到leases数组里

  15:       */

  16:      leases = xzalloc(server_config.max_leases * sizeof(struct dhcpOfferedAddr));

  17:      read_leases(server_config.lease_file); 

  18:          //server_config.lease_file = "/var/lib/misc/udhcpd.leases"

  19:   

 

2). 查找租赁最早到期的租约：

   1:  /* 

   2:   * add a lease into the table, clearing out any old ones

   3:   * add_lease是把MAC:chaddr, IP : yiaddr, expires: lease

   4:   * 租赁信息添加到leases数组里

   5:   * 函数首先调用clear_lease把数组里旧的删除掉(与chaddr or yiaddr相同的)

   6:   * 从leases数组里找到最早到期的,再添加

   7:   *

   8:   * 返回oldest (NULL 表示未添加成功) 

   9:   */

  10:  struct dhcpOfferedAddr *add_lease(uint8_t *chaddr, uint32_t yiaddr, unsigned long lease)

  11:  {

  12:      struct dhcpOfferedAddr *oldest;

  13:   

  14:      /* clean out any old ones */

  15:      clear_lease(chaddr, yiaddr);

  16:   

  17:      /* 从leases数组里找到一个最早到期的oldest,以便添加新的 */

  18:      oldest = oldest_expired_lease();

  19:   

  20:      if (oldest) //oldest != NULL ，Found...

  21:      {

  22:          memcpy(oldest->chaddr, chaddr, 16);

  23:          oldest->yiaddr = yiaddr;

  24:   

  25:          /* 

  26:           * 函数的第三个参数,即lease,表示的是租赁的剩余时间

  27:           * 在动态租赁数组leases里记录的租赁到期时间

  28:           *

  29:           * 所以 oldest->expires = time(0) +　lease;

  30:           */

  31:          oldest->expires = time(0) + lease;

  32:      }

  33:   

  34:      return oldest;

  35:  }

 

3). 通过MAC或IP查找租约：

   1:  /* Find the first lease that matches chaddr, NULL if no match */

   2:  struct dhcpOfferedAddr *find_lease_by_chaddr(uint8_t *chaddr)

   3:  {

   4:      unsigned int i;

   5:   

   6:      for (i = 0; i < server_config.max_leases; i++)

   7:          if (!memcmp(leases[i].chaddr, chaddr, 16)) return &(leases[i]);

   8:   

   9:      return NULL;

  10:  }

  11:   

  12:  /* Find the first lease that matches yiaddr, NULL is no match */

  13:  struct dhcpOfferedAddr *find_lease_by_yiaddr(uint32_t yiaddr)

  14:  {

  15:      unsigned int i;

  16:   

  17:      for (i = 0; i < server_config.max_leases; i++)

  18:          if (leases[i].yiaddr == yiaddr) return &(leases[i]);

  19:   

  20:      return NULL;

  21:  }

 

4). 检查IP addr是否被网络中主机所使用，若被使用的话添加到leases数组里：

   1:  /*

   2:   * check is an IP is taken, if it is, add it to the lease table

   3:   *

   4:   * 检查addr is a free IP(网络中是否有主机使用addr)

   5:   * if addr is a used IP, 把addr添加到租赁数组里

   6:   *

   7:   *    1. 返回0 表示addr 是a free IP

   8:   *     2. 返回1表示addr已被网络中的某台主机使用了

   9:   *        并且把使用addr的主机添加到leases数组里

  10:   *        其中MAC: blank_chaddr, IP: addr, expires: server_config.confict_time(def: 3600sec)

  11:   *

  12:   */

  13:  static int check_ip(uint32_t addr)

  14:  {

  15:      struct in_addr temp;

  16:   

  17:      /* arpping()返回0表示addr used */

  18:      if (arpping(addr, server_config.server, server_config.arp, server_config.interface) == 0) {

  19:          temp.s_addr = addr;

  20:          LOG(LOG_INFO, "%s belongs to someone, reserving it for %ld seconds",

  21:              inet_ntoa(temp), server_config.conflict_time);

  22:   

  23:          /*

  24:           * 因为刚在未在leases数组里找到这个已被used 的IP

  25:           * 所以要把这个used IP添加到leases数组里

  26:           * 租赁客户机MAC 为blank_chaddr(黑户)

  27:           * 租赁的IP即used IP(addr)

  28:           * 租赁时间:server_config.conflict_time

  29:           */

  30:          add_lease(blank_chaddr, addr, server_config.conflict_time);

  31:          return 1;

  32:      } else return 0;

  33:  }

 

5） 从可用IP地址池里找到一个可用的IP（a Free IP）

uint32_t find_address(int check_expire);

该函数的调用是在，server端接收到DHCPDISOCVER的报文的时候，会为client提供一个IP地址：

a) server首先利用client的MAC地址在leases数组里查找该client以前是否在这里租赁过IP，租赁过的话，把以前的IP提供给client

b) 第一种情况不满足的话，server会检查DHCPDISCOVER报文的选项字段，client是否有请求的IP（该选项信息的CODE ：DHCP_REQUESTED_IP），有的话检查该IP是否为Free，可以的话把Request IP提供给client。

c) 上面两种情况都不满足的话，就调用find_address这个函数了。

   1:  /*

   2:   * find an assignable address, it check_expired is true,

   3:   * we check all the expired leases as well.

   4:   * Maybe this should try expired leases by age...

   5:   *

   6:   * 找到一个可分配的IP,如果check_expired is true,

   7:   * 会检查所有到期租约

   8:   * find_address()函数找到free IP 返回IP的值(network order)

   9:   * 未找到返回0

  10:   * 

  11:   */

  12:  uint32_t find_address(int check_expired)

  13:  {

  14:      uint32_t addr, ret;

  15:      struct dhcpOfferedAddr *lease = NULL;

  16:   

  17:      /*

  18:       * 在端上进行计算比较的时候使用 host order

  19:       * 一般存储的时候采用 network order

  20:       *

  21:       * 遍历整个可分配地址server_config.start -- server_config.end

  22:       */

  23:           /* addr is in host order here */

  24:      addr = ntohl(server_config.start);

  25:      for (;addr <= ntohl(server_config.end); addr++) 

  26:      {

  27:          /* ie, 192.168.55.0 */

  28:          if (!(addr & 0xFF)) continue;

  29:   

  30:          /* ie, 192.168.55.255 */

  31:          if ((addr & 0xFF) == 0xFF) continue;

  32:   

  33:          /* 

  34:           * Only do if it isn't 

  35:                   * an assigned as a static lease

  36:           * rexervedIP()检查addr是否在静态租赁链表里

  37:           * addr在静态链表的话,函数返回1,反之返回0

  38:           */

  39:          if(!reservedIp(server_config.static_leases, htonl(addr)))

  40:          {

  41:              /* lease is not taken */

  42:              ret = htonl(addr);

  43:   

  44:              /* !(lease = find_lease_by_yiaddr(ret)) 地址addr是否已被租赁出去了 */

  45:              if ((!(lease = find_lease_by_yiaddr(ret)) ||

  46:   

  47:                       /* 

  48:                        * or it expired and we are checking for expired leases

  49:                   * or 租赁出去的话,

  50:                   * addr是否是a free IP还取决与

  51:                   * lease_expired(lease)是否已到期和check_expired

  52:                   * 

  53:                        */

  54:                       (check_expired  && lease_expired(lease))) &&

  55:   

  56:                       /* 

  57:                        * and it isn't on the network

  58:                        * 如果addr满足上面两个条件的其中一个

  59:                        * 1.!(lease = find_lease_by_yiaddr(ret)) 没有被租赁出去                      * 2.(check_expired && lease_expired(lease)) 已被租赁出去了,

  60:                        * 但租赁过期了(check_expired == ture)

  61:                        *

  62:                        * addr 满足上面条件之一,还要检查addr是否被网络上其他主机使用

  63:                        * 

  64:                        * check_ip() 发送arp Request包确认addr是否被使用

  65:                        * 返回0, addr is a free IP

  66:                        */

  67:                       !check_ip(ret)) 

  68:              {

  69:              return ret;

  70:              break;

  71:              }

  72:          }

  73:      }

  74:      return 0;

  75:  }