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> static DEFINE_PER_CPU(struct net_device_stats, loopback_stats); #define LOOPBACK_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 */ static void emulate_large_send_offload(struct sk_buff *skb) { struct iphdr *iph = skb->nh.iph; struct tcphdr *th = (struct tcphdr*)(skb->nh.raw + (iph->ihl * 4)); unsigned int doffset = (iph->ihl + th->doff) * 4; unsigned int mtu = skb_shinfo(skb)->tso_size + doffset; unsigned int offset = 0; u32 seq = ntohl(th->seq); u16 id = ntohs(iph->id); while (offset + doffset < skb->len) { unsigned int frag_size = min(mtu, skb->len - offset) - doffset; struct sk_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 = (struct tcphdr*)(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((unsigned char *) 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 */ static int loopback_xmit(struct sk_buff *skb, struct net_device *dev) { struct net_device_stats *lb_stats; skb_orphan(skb); skb->protocol=eth_type_trans(skb,dev); skb->dev=dev; #ifndef LOOPBACK_MUST_CHECKSUM skb->ip_summed = CHECKSUM_UNNECESSARY; #endif if (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); return 0; } 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); } static struct net_device_stats *get_stats(struct net_device *dev) { struct net_device_stats *stats = dev->priv; int i; if (!stats) { return NULL; } memset(stats, 0, sizeof(struct net_device_stats)); for (i=0; i < NR_CPUS; i++) { struct net_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; } return stats; } u32 loopback_get_link(struct net_device *dev) { return 1; } static struct ethtool_ops loopback_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。 */ struct net_device loopback_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 __init loopback_init(void) { struct net_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(struct net_device_stats), GFP_KERNEL); if (stats) { memset(stats, 0, sizeof(struct net_device_stats)); loopback_dev.priv = stats; loopback_dev.get_stats = &get_stats; } /* 注册网络设备 */ return register_netdev(&loopback_dev); }; EXPORT_SYMBOL(loopback_dev);
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