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: }