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[历史归档]本文原发布于 cstriker1407.info 个人博客，内容为历史存档，仅供参考。
发布时间：2016-01-21｜ 标题：LinuxLoopBack简单笔记｜分类：操作系统 / linux / kernel ｜标签：linux·kernel·loopback

在Linux网络设备中，有一个网络回环设备，代码非常简单，这里笔记下:

/* * INET An implementation of the TCP/IP protocol suite for the LINUX * operating system. INET is implemented using the BSD Socket * interface as the means of communication with the user level. * * Pseudo-driver for the loopback interface. * * Version: @(#)loopback.c 1.0.4b 08/16/93 * * Authors: Ross Biro, <bir7@leland.Stanford.Edu> * Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG> * Donald Becker, <becker@scyld.com> * * Alan Cox : Fixed oddments for NET3.014 * Alan Cox : Rejig for NET3.029 snap #3 * Alan Cox : Fixed NET3.029 bugs and sped up * Larry McVoy : Tiny tweak to double performance * Alan Cox : Backed out LMV's tweak - the linux mm * can't take it... * Michael Griffith: Don't bother computing the checksums * on packets received on the loopback * interface. * Alexey Kuznetsov: Potential hang under some extreme * cases removed. * * This program is free software; you can redistribute it and/or * modify it under the terms of the GNU General Public License * as published by the Free Software Foundation; either version * 2 of the License, or (at your option) any later version. */#include<linux/kernel.h>#include<linux/jiffies.h>#include<linux/module.h>#include<linux/interrupt.h>#include<linux/fs.h>#include<linux/types.h>#include<linux/string.h>#include<linux/socket.h>#include<linux/errno.h>#include<linux/fcntl.h>#include<linux/in.h>#include<linux/init.h>#include<asm/system.h>#include<asm/uaccess.h>#include<asm/io.h>#include<linux/inet.h>#include<linux/netdevice.h>#include<linux/etherdevice.h>#include<linux/skbuff.h>#include<linux/ethtool.h>#include<net/sock.h>#include<net/checksum.h>#include<linux/if_ether.h>/* For the statistics structure. */#include<linux/if_arp.h>/* For ARPHRD_ETHER */#include<linux/ip.h>#include<linux/tcp.h>#include<linux/percpu.h>staticDEFINE_PER_CPU(structnet_device_stats,loopback_stats);#defineLOOPBACK_OVERHEAD(128+MAX_HEADER+16+16)/* KISS: just allocate small chunks and copy bits. * * So, in fact, this is documentation, explaining what we expect * of largesending device modulo TCP checksum, which is ignored for loopback. *//* 如果包很大，就拆分成多个skb 调用收包接口 */staticvoidemulate_large_send_offload(structsk_buff*skb){structiphdr*iph=skb->nh.iph;structtcphdr*th=(structtcphdr*)(skb->nh.raw+(iph->ihl*4));unsignedintdoffset=(iph->ihl+th->doff)*4;unsignedintmtu=skb_shinfo(skb)->tso_size+doffset;unsignedintoffset=0;u32 seq=ntohl(th->seq);u16 id=ntohs(iph->id);while(offset+doffset<skb->len){unsignedintfrag_size=min(mtu,skb->len-offset)-doffset;structsk_buff*nskb=alloc_skb(mtu+32,GFP_ATOMIC);/* 新的skb */if(!nskb)break;skb_reserve(nskb,32);nskb->mac.raw=nskb->data-14;nskb->nh.raw=nskb->data;iph=nskb->nh.iph;memcpy(nskb->data,skb->nh.raw,doffset);if(skb_copy_bits(skb,doffset+offset,nskb->data+doffset,frag_size))/* 拷贝新的skb 内容 */BUG();skb_put(nskb,doffset+frag_size);nskb->ip_summed=CHECKSUM_UNNECESSARY;nskb->dev=skb->dev;nskb->priority=skb->priority;nskb->protocol=skb->protocol;nskb->dst=dst_clone(skb->dst);memcpy(nskb->cb,skb->cb,sizeof(skb->cb));nskb->pkt_type=skb->pkt_type;th=(structtcphdr*)(nskb->nh.raw+iph->ihl*4);iph->tot_len=htons(frag_size+doffset);iph->id=htons(id);iph->check=0;iph->check=ip_fast_csum((unsignedchar*)iph,iph->ihl);th->seq=htonl(seq);if(offset+doffset+frag_size<skb->len)th->fin=th->psh=0;netif_rx(nskb);/* 上送 */offset+=frag_size;seq+=frag_size;id++;}dev_kfree_skb(skb);}/* * The higher levels take care of making this non-reentrant (it's * called with bh's disabled). *//* 发送回调函数 */staticintloopback_xmit(structsk_buff*skb,structnet_device*dev){structnet_device_stats*lb_stats;skb_orphan(skb);skb->protocol=eth_type_trans(skb,dev);skb->dev=dev;#ifndefLOOPBACK_MUST_CHECKSUMskb->ip_summed=CHECKSUM_UNNECESSARY;#endifif(skb_shinfo(skb)->tso_size){BUG_ON(skb->protocol!=htons(ETH_P_IP));BUG_ON(skb->nh.iph->protocol!=IPPROTO_TCP);emulate_large_send_offload(skb);return0;}dev->last_rx=jiffies;lb_stats=&per_cpu(loopback_stats,get_cpu());lb_stats->rx_bytes+=skb->len;lb_stats->tx_bytes+=skb->len;lb_stats->rx_packets++;lb_stats->tx_packets++;put_cpu();/* 当拿到要发送的包之后，直接调用收包即可。 */netif_rx(skb);return(0);}staticstructnet_device_stats*get_stats(structnet_device*dev){structnet_device_stats*stats=dev->priv;inti;if(!stats){returnNULL;}memset(stats,0,sizeof(structnet_device_stats));for(i=0;i<NR_CPUS;i++){structnet_device_stats*lb_stats;if(!cpu_possible(i))continue;lb_stats=&per_cpu(loopback_stats,i);stats->rx_bytes+=lb_stats->rx_bytes;stats->tx_bytes+=lb_stats->tx_bytes;stats->rx_packets+=lb_stats->rx_packets;stats->tx_packets+=lb_stats->tx_packets;}returnstats;}u32loopback_get_link(structnet_device*dev){return1;}staticstructethtool_opsloopback_ethtool_ops={.get_link=loopback_get_link,.get_tso=ethtool_op_get_tso,.set_tso=ethtool_op_set_tso,};/* 由于loopback是在本机内循环，因此非常简单，只需要重写发送函数即可。 */structnet_deviceloopback_dev={.name="lo",.mtu=(16*1024)+20+20+12,.hard_start_xmit=loopback_xmit,/* 发送回调函数 */.hard_header=eth_header,.hard_header_cache=eth_header_cache,.header_cache_update=eth_header_cache_update,.hard_header_len=ETH_HLEN,/* 14 */.addr_len=ETH_ALEN,/* 6 */.tx_queue_len=0,.type=ARPHRD_LOOPBACK,/* 0x0001*/.rebuild_header=eth_rebuild_header,.flags=IFF_LOOPBACK,.features=NETIF_F_SG|NETIF_F_FRAGLIST|NETIF_F_NO_CSUM|NETIF_F_HIGHDMA|NETIF_F_LLTX,.ethtool_ops=&loopback_ethtool_ops,};/* loopback 模块初始化 *//* Setup and register the of the LOOPBACK device. */int__initloopback_init(void){structnet_device_stats*stats;/* loopback模块非常简单，只需要额外申请内存来统计收发包个数即可。 *//* Can survive without statistics */stats=kmalloc(sizeof(structnet_device_stats),GFP_KERNEL);if(stats){memset(stats,0,sizeof(structnet_device_stats));loopback_dev.priv=stats;loopback_dev.get_stats=&get_stats;}/* 注册网络设备 */returnregister_netdev(&loopback_dev);};EXPORT_SYMBOL(loopback_dev);