码为uip_arpc中的void uip_arp_arpin(void)函数.doc

ARP应答部分代码为uip_arp.c中的void uip_arp_arpin(void)函数.这个函数是在设备接收到ARP包时,由驱动程序调用的.如果收到是ARP包是一个对本地主机上次发送的ARP请求的应答,那么就从包中取得自己想要的主机的MAC地址,加入自己的ARP缓存表中.如果收到是一个ARP请求,那就把自己的MAC地址打包成一个ARP应答,发送给请求的主机.看代码uip_arp.c的254行:1. /*-*/2. /*3. * ARP processing for incoming ARP packets.4. *对传入的ARP包的处理.5. * This function should be called by the device driver when an ARP6. * packet has been received. The function will act differently7. * depending on the ARP packet type: if it is a reply for a request8. * that we previously sent out, the ARP cache will be filled in with9. * the values from the ARP reply. If the incoming ARP packet is an ARP10. * request for our IP address, an ARP reply packet is created and put11. * into the uip_buf buffer.12. *此函数在收到ARP包时由设备驱动调用,函数行为会因包类型而有不同.如果收到的是一个对前先发送的请求的应答13. *则根据应答的值填充缓存.如果传入的包是对我们的IP的请求,则创建一个ARP应答,并放入uip_buf中.14. * When the function returns, the value of the global variable uip_len15. * indicates whether the device driver should send out a packet or16. * not. If uip_len is zero, no packet should be sent. If uip_len is17. * non-zero, it contains the length of the outbound packet that is18. * present in the uip_buf buffer.19. *函数返回时,全局变量uip_len的值指明了设备驱动要不要发送包.若uip_len为0,则不需发送,若uip_len不是0,20. * 则其值是uip_buf中包含的要传出的包的大小.21. * This function expects an ARP packet with a prepended Ethernet22. * header in the uip_buf buffer, and the length of the packet in the23. * global variable uip_len.此函数预期中的uip_buf中有一个带以太网头的ARP包.其长度存为uip_len中.24. */25. /*-*/26. void27. uip_arp_arpin(void)28. 29.30. if(uip_len opcode) 37. case HTONS(ARP_REQUEST):38. /* ARP request. If it asked for our address, we send out a39. reply. 如果是一个ARP请求,则发送应答.*/40. if(uip_ipaddr_cmp(BUF-dipaddr, uip_hostaddr) 41. /* First, we register the one who made the request in our ARP42. table, since it is likely that we will do more communication43. with this host in the future.首先,我们将发送请求的主机注册到ARP缓存表中,因为我们很可能要跟它要有更多的交流 */44. uip_arp_update(BUF-sipaddr, &BUF-shwaddr);45.46. /* The reply opcode is 2. 应答的操作码为2*/47. BUF-opcode = HTONS(2);48.49. memcpy(BUF-dhwaddr.addr, BUF-shwaddr.addr, 6);50. memcpy(BUF-shwaddr.addr, uip_ethaddr.addr, 6);51. memcpy(BUF-ethhdr.src.addr, uip_ethaddr.addr, 6);52. memcpy(BUF-ethhdr.dest.addr, BUF-dhwaddr.addr, 6);53.54. BUF-dipaddr0 = BUF-sipaddr0;55. BUF-dipaddr1 = BUF-sipaddr1;56. BUF-sipaddr0 = uip_hostaddr0;57. BUF-sipaddr1 = uip_hostaddr1;58.59. BUF-ethhdr.type = HTONS(UIP_ETHTYPE_ARP);60. uip_len = sizeof(struct arp_hdr);61. 62. break;63. case HTONS(ARP_REPLY):64. /* ARP reply. We insert or update the ARP table if it was meant65. for us. 如果收到的是一个ARP应答,而且也是我们所要的应答的话,就插件并更新ARP缓存表*/66. if(uip_ipaddr_cmp(BUF-dipaddr, uip_hostaddr) 67. uip_arp_update(BUF-sipaddr, &BUF-shwaddr);68. 69. break;70. 71.72. return;73. 复制代码这里有一件事是很有意思的,就是说如果某个主机请求得到我们的MAC的地址,我们先把它的MAC地址加入到自己的表中.就好比社交网络中,别人请求加我们为好友,如果我们接收的话,也自动加对方为好友一样.既然对方找上我们了,肯定是要做进一步的交流,互加MAC地址也很自然的.有了上一篇文章,这里似乎不必做过多的解释了.但还是说一下吧,看看我们怎么做应答的.如果收到了一个请求,我们要做应答:1. /* The reply opcode is 2. */2. BUF-opcode = HTONS(2);3.4. memcpy(BUF-dhwaddr.addr, BUF-shwaddr.addr, 6);5. memcpy(BUF-shwaddr.addr, uip_ethaddr.addr, 6);6. memcpy(BUF-ethhdr.src.addr, uip_ethaddr.addr, 6);7. memcpy(BUF-ethhdr.dest.addr, BUF-dhwaddr.addr, 6);8.9. BUF-dipaddr0 = BUF-sipaddr0;10. BUF-dipaddr1 = BUF-sipaddr1;11. BUF-sipaddr0 = uip_hostaddr0;12. BUF-sipaddr1 = uip_hostaddr1;13.14. BUF-ethhdr.type = HTONS(UIP_ETHTYPE_ARP);15. uip_len = sizeof(struct arp_hdr);复制代码由于请求和应答包很多地方是相同的,如上文中的绿色部分.我们只需将收到的请求稍加修改就可以发送回去了.首先,要改一下MAC地址,四个地方.然后要将目标主机IP设为设为请求包的源主机IP,目的主机IP设为我们的IP.就可以了.另外说下对ARP缓存表的设置:1. 这个函数是集插入,更新一体的,有两个参数,IP地址,MAC地址.2. static void3. uip_arp_update(u16_t *ipaddr, struct uip_eth_addr *ethaddr)4. 5. register struct arp_entry *tabptr;6. /* Walk through the ARP mapping table and try to find an entry to7. update. If none is found, the IP - MAC address mapping is8. inserted in the ARP table. 遍历ARP映射表,看看有没有需要更新的表项,如果没有,就将新的表项插入.*/9. for(i = 0; i ipaddr0 != 0 &14. tabptr-ipaddr1 != 0) 15.16. /* Check if the source IP address of the incoming packet matches17. the IP address in this ARP table entry. 看看传入的IP有没有匹配项.*/18. if(ipaddr0 = tabptr-ipaddr0 &19. ipaddr1 = tabptr-ipaddr1) 20.21. /* An old entry found, update this and return. 如果有己存的匹配项,修改该项的MAC地址和更新时间.*/22. memcpy(tabptr-ethaddr.addr, ethaddr-addr, 6);23. tabptr-time = arptime;24.25. return;26. 27. 28. 29.30. /* If we get here, no existing ARP table entry was found, so we31. create one. 如果运行到这里,说明没有己存的表项,则创建一个.*/32.33. /* First, we try to find an unused entry in the ARP table. 先看看有没有空表项可用.*/34. for(i = 0; i ipaddr0 = 0 &37. tabptr-ipaddr1 = 0) 38. break;39. 40. 41.42. /* If no unused entry is found, we try to find the oldest entry and43. throw it away. 如果没空表项,就找到一个最旧的表项,扔掉它,换成我们的.*/44. if(i = UIP_ARPTAB_SIZE) 45. tmpage = 0;46. c = 0;47. for(i = 0; i time tmpage) 50. tmpage = arptime - tabptr-time;51. c = i;52. 53. 54. i = c;55. tabptr = &arp_tablei;56. 57. /* Now, i is the ARP table entry which we will fill with the new58. information. 现在i就是我们最终得到的表项,我们把新的信息插入到这里.*/59. memcpy(tabptr-ipaddr, ipaddr, 4);60. memcpy(tabptr-ethaddr.addr, ethaddr-addr, 6);61. tabptr-time = arptime;复制代码OK,uip_arp.c中还有两个函数,楼下继续.1.看代码:1. /*-*/2. /*3. * Periodic ARP processing function.4. *ARP周期性处理函数.5. * This function performs periodic timer processing in the ARP module6. * and should be called at regular intervals. The recommended interval7. * is 10 seconds between the calls.8. *此函数在ARP模块中施行周期性处理,它应该每隔一段时间就调用一次.推荐为10秒.9. */