我是靠谱客的博主 清爽大象，这篇文章主要介绍linux路由选择流程1 ip_rcv_finish2 ip_route_input_slow3 ip_mkroute_input4 整体函数流程，现在分享给大家，希望可以做个参考。
1 ip_rcv_finish
ip_rcv这个函数主要是对数据包做各种正确性验证，然后调用掉网络过滤子系统的在PRE_ROUTEING链上的回调函数，经过网络子系统的处理在调用ip_rec_finish，ip_rcv_finish主要的工作：确定数据包是前送还是在本机协议栈上传，如果是前送要确保输出网络设备和下一个接受栈的地址。
我们看一下ip_rcv_finish的源码，首先判断skb->dst是否为NULL，如果是表明数据包的去向还没有定，那么需要调用ip_route_input决定数据包的去向
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14static int ip_rcv_finish(struct sk_buff *skb) { const struct iphdr *iph = ip_hdr(skb); struct rtable *rt; /* * Initialise the virtual path cache for the packet. It describes * how the packet travels inside Linux networking. */ if (skb->dst == NULL) { //目的地址是空 int err = ip_route_input(skb, iph->daddr, iph->saddr, iph->tos, skb->dev); } ..... }
ip_route_input函数主要是调用rt_hash根据目的地址得到路由哈希值，然后调用ip_route_input_slow继续进一步处理
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8int ip_route_input(struct sk_buff *skb, __be32 daddr, __be32 saddr, u8 tos, struct net_device *dev) { struct rtable * rth; unsigned hash; ..... return ip_route_input_slow(skb, daddr, saddr, tos, dev); //下一步进行路由判断 }
2 ip_route_input_slow
ip_route_inout_slow最重要的是调用fib_lookup根据目的地址查找路由表得到路由结果struct fib_result *res，决定数据包去向，如果res.type == RTN_LOCAL那么就是本地接受的数据，那么设置数据包下一步处理函数为ip_local_deliver，如果是转发数据包那么进一步调用ip_mkroute_input
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65static int ip_route_input_slow(struct sk_buff *skb, __be32 daddr, __be32 saddr, u8 tos, struct net_device *dev) { struct fib_result res; ...... if ((err = fib_lookup(net, &fl, &res)) != 0) { //这个函数很重要，查询路由表，将路由结果保存在struct fib_result *res， 决定数据包的走向 if (!IN_DEV_FORWARD(in_dev)) goto e_hostunreach; goto no_route; } ...... if (res.type == RTN_LOCAL) { //本地接受的数据包， int result; result = fib_validate_source(saddr, daddr, tos, net->loopback_dev->ifindex, dev, &spec_dst, &itag); if (result < 0) goto martian_source; if (result) flags |= RTCF_DIRECTSRC; spec_dst = daddr; goto local_input; } ...... err = ip_mkroute_input(skb, &res, &fl, in_dev, daddr, saddr, tos); //路由转发处理 brd_input: if (skb->protocol != htons(ETH_P_IP)) goto e_inval; if (ipv4_is_zeronet(saddr)) spec_dst = inet_select_addr(dev, 0, RT_SCOPE_LINK); else { err = fib_validate_source(saddr, 0, tos, 0, dev, &spec_dst, &itag); if (err < 0) goto martian_source; if (err) flags |= RTCF_DIRECTSRC; } flags |= RTCF_BROADCAST; res.type = RTN_BROADCAST; RT_CACHE_STAT_INC(in_brd); local_input: //本地接受处理，做一些初始化 rth = dst_alloc(&ipv4_dst_ops); if (!rth) rth->rt_iif = rth->fl.iif = dev->ifindex; rth->u.dst.dev = net->loopback_dev; dev_hold(rth->u.dst.dev); rth->idev = in_dev_get(rth->u.dst.dev); rth->rt_gateway = daddr; rth->rt_spec_dst= spec_dst; rth->u.dst.input= ip_local_deliver; //设置本地结束处理函数 rth->rt_flags = flags|RTCF_LOCAL; ...... }
2.1 fib_lookup
fib_lookup查询路由表，将路由结果保存在struct fib_result *res， 决定数据包的走向,先查找本地路由表,再查找main路由表
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14static inline int fib_lookup(struct net *net, const struct flowi *flp, struct fib_result *res) { struct fib_table *table; table = fib_get_table(net, RT_TABLE_LOCAL); if (!table->tb_lookup(table, flp, res)) //先查找本地路由表 return 0; table = fib_get_table(net, RT_TABLE_MAIN); //再查找main路由表 if (!table->tb_lookup(table, flp, res)) return 0; return -ENETUNREACH; }
3 ip_mkroute_input
ip_mkroute_input实际调用的是__mkroute_input，__mkroute_input根据路由结果确定数据包的发包网卡，设置数据包转发函数ip_forward。设置出口处理函数ip_output
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96static int __mkroute_input(struct sk_buff *skb, struct fib_result *res, struct in_device *in_dev, __be32 daddr, __be32 saddr, u32 tos, struct rtable **result) { struct rtable *rth; int err; struct in_device *out_dev; unsigned flags = 0; __be32 spec_dst; u32 itag; /* get a working reference to the output device */ out_dev = in_dev_get(FIB_RES_DEV(*res)); //根据路由表结果得到出口数据包dev if (out_dev == NULL) { if (net_ratelimit()) printk(KERN_CRIT "Bug in ip_route_input" "_slow(). Please, reportn"); return -EINVAL; } err = fib_validate_source(saddr, daddr, tos, FIB_RES_OIF(*res), //根据原地址，目的地址验证路由转发的可靠性 in_dev->dev, &spec_dst, &itag); if (err < 0) { ip_handle_martian_source(in_dev->dev, in_dev, skb, daddr, saddr); err = -EINVAL; goto cleanup; } if (err) flags |= RTCF_DIRECTSRC; if (out_dev == in_dev && err && (IN_DEV_SHARED_MEDIA(out_dev) || inet_addr_onlink(out_dev, saddr, FIB_RES_GW(*res)))) flags |= RTCF_DOREDIRECT; if (skb->protocol != htons(ETH_P_IP)) { /* Not IP (i.e. ARP). Do not create route, if it is * invalid for proxy arp. DNAT routes are always valid. */ if (out_dev == in_dev) { err = -EINVAL; goto cleanup; } } rth = dst_alloc(&ipv4_dst_ops); if (!rth) { err = -ENOBUFS; goto cleanup; } atomic_set(&rth->u.dst.__refcnt, 1); rth->u.dst.flags= DST_HOST; if (IN_DEV_CONF_GET(in_dev, NOPOLICY)) rth->u.dst.flags |= DST_NOPOLICY; if (IN_DEV_CONF_GET(out_dev, NOXFRM)) rth->u.dst.flags |= DST_NOXFRM; rth->fl.fl4_dst = daddr; rth->rt_dst = daddr; rth->fl.fl4_tos = tos; rth->fl.mark = skb->mark; rth->fl.fl4_src = saddr; rth->rt_src = saddr; rth->rt_gateway = daddr; rth->rt_iif = rth->fl.iif = in_dev->dev->ifindex; rth->u.dst.dev = (out_dev)->dev; //设置出口dev dev_hold(rth->u.dst.dev); rth->idev = in_dev_get(rth->u.dst.dev); rth->fl.oif = 0; rth->rt_spec_dst= spec_dst; rth->u.dst.input = ip_forward; //设置转发函数 rth->u.dst.output = ip_output; //设置出口函数 rth->rt_genid = rt_genid(dev_net(rth->u.dst.dev)); rt_set_nexthop(rth, res, itag); rth->rt_flags = flags; *result = rth; err = 0; cleanup: /* release the working reference to the output device */ in_dev_put(out_dev); return err; }
4 整体函数流程
最后
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