通过源码理解rarp协议（基于linux1.2.13）

本文主要是介绍通过源码理解rarp协议（基于linux1.2.13），希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

rarp是通过mac地址查询ip的协议，主要用于有mac的主机，但是没有ip的情况。我们先看看rarp协议的协议定义（来自网上的图）。

rarp协议的格式和arp协议是一样的，他们都是通过一种地址查询另外一种地址。操作系统内维护了一个转换表。定义如下。

struct rarp_table
{struct rarp_table  *next;             /* Linked entry list           */unsigned long      ip;                /* ip address of entry         */unsigned char      ha[MAX_ADDR_LEN];  /* Hardware address            */unsigned char      hlen;              /* Length of hardware address  */unsigned char      htype;             /* Type of hardware in use     */struct device      *dev;              /* Device the entry is tied to */
};

初始化的时候是空的，这个表格的数据来源于，用户通过操作系统提供的接口设置。我们看如何操作这个表。

int rarp_ioctl(unsigned int cmd, void *arg)
{struct arpreq r;struct sockaddr_in *si;int err;switch(cmd){case SIOCDRARP:if (!suser())return -EPERM;err = verify_area(VERIFY_READ, arg, sizeof(struct arpreq));if(err)return err;memcpy_fromfs(&r, arg, sizeof(r));if (r.arp_pa.sa_family != AF_INET)return -EPFNOSUPPORT;si = (struct sockaddr_in *) &r.arp_pa;rarp_destroy(si->sin_addr.s_addr);return 0;case SIOCGRARP:err = verify_area(VERIFY_WRITE, arg, sizeof(struct arpreq));if(err)return err;return rarp_req_get((struct arpreq *)arg);case SIOCSRARP:if (!suser())return -EPERM;err = verify_area(VERIFY_READ, arg, sizeof(struct arpreq));if(err)return err;return rarp_req_set((struct arpreq *)arg);default:return -EINVAL;}/*NOTREACHED*/return 0;
}

通过ioctl函数，我们可以对表格进行增删改查。我们只关注新增的逻辑。因为其他的是类似的。下面是arpreq 的定义

struct arpreq {struct sockaddr	arp_pa;		/* protocol address		*/struct sockaddr	arp_ha;		/* hardware address		*/int			arp_flags;	/* flags			*/struct sockaddr       arp_netmask;    /* netmask (only for proxy arps) */
};

static int rarp_req_set(struct arpreq *req)
{struct arpreq r;struct rarp_table *entry;struct sockaddr_in *si;int htype, hlen;unsigned long ip;struct rtable *rt;memcpy_fromfs(&r, req, sizeof(r));/**	We only understand about IP addresses... */if (r.arp_pa.sa_family != AF_INET)return -EPFNOSUPPORT;switch (r.arp_ha.sa_family) {case ARPHRD_ETHER:htype = ARPHRD_ETHER;hlen = ETH_ALEN;break;default:return -EPFNOSUPPORT;}si = (struct sockaddr_in *) &r.arp_pa;ip = si->sin_addr.s_addr;if (ip == 0){printk("RARP: SETRARP: requested PA is 0.0.0.0 !\n");return -EINVAL;}// rt = ip_rt_route(ip, NULL, NULL);if (rt == NULL)return -ENETUNREACH;/**	Is there an existing entry for this address?  Find out...*/cli();// 判断之前是不是已经存在for (entry = rarp_tables; entry != NULL; entry = entry->next)if (entry->ip == ip)break;/**	If no entry was found, create a new one.*/// 不存在则创建一个表项if (entry == NULL){entry = (struct rarp_table *) kmalloc(sizeof(struct rarp_table),GFP_ATOMIC);// 还没初始化则初始化if(initflag){rarp_init();initflag=0;}entry->next = rarp_tables;rarp_tables = entry;}entry->ip = ip;entry->hlen = hlen;entry->htype = htype;memcpy(&entry->ha, &r.arp_ha.sa_data, hlen);entry->dev = rt->rt_dev;sti();  return 0;
}

我们看到这里会往表里插入一个表项（如果不存在的话），还有另外一个逻辑是rarp_init。

static void rarp_init (void)
{/* Register the packet type */rarp_packet_type.type=htons(ETH_P_RARP);dev_add_pack(&rarp_packet_type);
}

这个函数是往底层注册一个节点，当mac底层收到一个ETH_P_RARP类型的数据包的时候（在mac协议头里定义），就会执行rarp_packet_type中定义的函数。下面是该rarp_packet_type的定义

static struct packet_type rarp_packet_type =
{0, 0,                /* copy */rarp_rcv,NULL,NULL
};

rarp_rcv函数就是收到一个rarp请求的时候（来自其他主机），执行的函数。

int rarp_rcv(struct sk_buff *skb, struct device *dev, struct packet_type *pt)
{
/**	We shouldn't use this type conversion. Check later.*/// rarp协议报文struct arphdr *rarp = (struct arphdr *)skb->h.raw;// rarp协议数据部分unsigned char *rarp_ptr = (unsigned char *)(rarp+1);struct rarp_table *entry;long sip,tip;unsigned char *sha,*tha;            /* s for "source", t for "target" */// 硬件地址长度或类型不一致则忽略if (rarp->ar_hln != dev->addr_len || dev->type != ntohs(rarp->ar_hrd) || dev->flags&IFF_NOARP){kfree_skb(skb, FREE_READ);return 0;}/**	If it's not a RARP request, delete it.*/// 不是请求报文则忽略if (rarp->ar_op != htons(ARPOP_RREQUEST)){kfree_skb(skb, FREE_READ);return 0;}/**	Extract variable width fields*/// rarp协议首地址sha=rarp_ptr;// 发送端mac地址长度rarp_ptr+=dev->addr_len;// 拿到发送端ip，存到sipmemcpy(&sip,rarp_ptr,4);// 跳过4字节rarp_ptr+=4;// 目的mac地址tha=rarp_ptr;// 跳过mac地址长度rarp_ptr+=dev->addr_len;// 目的ip地址memcpy(&tip,rarp_ptr,4);/**	Process entry. Use tha for table lookup according to RFC903.*/cli();for (entry = rarp_tables; entry != NULL; entry = entry->next)// 判断mac地址是否相等if (!memcmp(entry->ha, tha, rarp->ar_hln))break;// 非空则说明找到if (entry != NULL){	// 拿到对应的ipsip=entry->ip;sti();// 回复，类似是响应ARPOP_RREPLYarp_send(ARPOP_RREPLY, ETH_P_RARP, sip, dev, dev->pa_addr, sha, dev->dev_addr);}elsesti();kfree_skb(skb, FREE_READ);return 0;
}

我们看到这个函数很长，不过逻辑比较简单，就是解析收到的rarp请求中的数据，然后根据其他主机请求的mac地址，从维护的表格中找到对应的ip（如果有的话），然后调用arp_send函数发送回包。下面列一下该函数的代码。

void arp_send(int type, int ptype, unsigned long dest_ip, struct device *dev, unsigned long src_ip, unsigned char *dest_hw, unsigned char *src_hw)
{struct sk_buff *skb;struct arphdr *arp;unsigned char *arp_ptr;/**	No arp on this interface.*/if(dev->flags&IFF_NOARP)return;/**	Allocate a buffer*/// 分配一个skb存储数据包skb = alloc_skb(sizeof(struct arphdr)+ 2*(dev->addr_len+4)+ dev->hard_header_len, GFP_ATOMIC);// 构造arp协议数据包skb->len = sizeof(struct arphdr) + dev->hard_header_len + 2*(dev->addr_len+4);skb->arp = 1;skb->dev = dev;// 不存在缓存，发完可以销毁skb->free = 1;// 构造mac头dev->hard_header(skb->data,dev,ptype,dest_hw?dest_hw:dev->broadcast,src_hw?src_hw:NULL,skb->len,skb);/* Fill out the arp protocol part. */arp = (struct arphdr *) (skb->data + dev->hard_header_len);arp->ar_hrd = htons(dev->type);arp->ar_pro = htons(ETH_P_IP);arp->ar_hln = dev->addr_len;arp->ar_pln = 4;arp->ar_op = htons(type);arp_ptr=(unsigned char *)(arp+1);memcpy(arp_ptr, src_hw, dev->addr_len);arp_ptr+=dev->addr_len;memcpy(arp_ptr, &src_ip,4);arp_ptr+=4;if (dest_hw != NULL)memcpy(arp_ptr, dest_hw, dev->addr_len);elsememset(arp_ptr, 0, dev->addr_len);arp_ptr+=dev->addr_len;memcpy(arp_ptr, &dest_ip, 4);// 调用mac头发送函数发送出去dev_queue_xmit(skb, dev, 0);
}