在linux系统下利用PC机实现路由转发的具体步骤

1、网络拓扑

　　网络拓扑如下所示，我们在这里用到了三台机子做实验，分别是①、④、⑦号机，使用①号机ping⑦号机，④号机作为路由转发。

　　

2、错误的路由配置

　　首先我们使用如下的配置方法，配置这三台机子的路由表：

　　1）在①号机种配置如下，让目的网段是10.0.4.0/24的从eth1端口出去

route  add -net 10.0.4.0/24 dev eth1

　　在①号机的查看路由表输入如下命令：

route -n

　　①号机的路由表的结果如下：

 　　2）在⑦号机使用同样方法配置路由，结果如下：

 　　3）在4号机配置路由转发功能，即将/etc/sysctl.conf文件里面的net.ipv4.ip_forward的值置1：

 　　4）所有的配置已经完成，我们在①号机ping④号机

ping  10.0.4.3

 　　结果如下，即ping 不通：

ping 10.0.4.3 (10.0.4.3) 56(84) bytes of data.from 10.0.1.3 icmp_seq=2 destination host unreachablefrom 10.0.1.3 icmp_seq=3 destination host unreachablefrom 10.0.1.3 icmp_seq=4 destination host unreachablefrom 10.0.1.3 icmp_seq=6 destination host unreachablefrom 10.0.1.3 icmp_seq=7 destination host unreachablefrom 10.0.1.3 icmp_seq=8 destination host unreachable

　　这里为了方便研究，把①号机的eth1配置放出来

eth1      link encap:ethernet  hwaddr 00:16:ec:af:cb:cb            inet addr:10.0.1.3  bcast:10.255.255.255  mask:255.255.255.0          inet6 addr: fe80::216:ecff:feaf:cbcb/64 scope:link          up broadcast running multicast  mtu:1500  metric:1          rx packets:4564 errors:0 dropped:0 overruns:0 frame:0          tx packets:6688 errors:0 dropped:0 overruns:0 carrier:0          collisions:0 txqueuelen:1000           rx bytes:459463 (448.6 kib)  tx bytes:546633 (533.8 kib)          interrupt:23 base address:0x6000

　　在①号机ping 的同时，我在④号机抓eth1包，结果如下：

[root@h4~]# tcpdump -i eth1 -enntcpdump: verbose output suppressed, use -v or -vv for full protocol decodelistening on eth1, link-type en10mb (ethernet), capture size 65535 bytes15:26:44.388614 00:16:ec:af:cb:cb > ff:ff:ff:ff:ff:ff, ethertype arp (0x0806), length 60: request who-has 10.0.4.3 tell 10.0.1.3, length 4615:26:45.391014 00:16:ec:af:cb:cb > ff:ff:ff:ff:ff:ff, ethertype arp (0x0806), length 60: request who-has 10.0.4.3 tell 10.0.1.3, length 4615:26:47.387821 00:16:ec:af:cb:cb > ff:ff:ff:ff:ff:ff, ethertype arp (0x0806), length 60: request who-has 10.0.4.3 tell 10.0.1.3, length 4615:26:48.391220 00:16:ec:af:cb:cb > ff:ff:ff:ff:ff:ff, ethertype arp (0x0806), length 60: request who-has 10.0.4.3 tell 10.0.1.3, length 4615:26:49.392621 00:16:ec:af:cb:cb > ff:ff:ff:ff:ff:ff, ethertype arp (0x0806), length 60: request who-has 10.0.4.3 tell 10.0.1.3, length 46

 　　可见①号一直在寻找配有10.0.4.3 ip的机子的mac地址，即一直在发arp包。但是路由器（④号机）默认是不转发arp报文的，所有①号机永远也ping不通⑦号机。

3、正确的配置

　　在①号机种配置路由，命令如下：

route add -net 10.0.4.0/24 gw 10.0.1.2

　　这时候①号机的路由表：

[root@h1 ~]# [root@h1 ~]# route -nkernel ip routing tabledestination     gateway         genmask         flags metric ref    use iface10.0.4.0        10.0.1.2        255.255.255.0   ug    0      0        0 eth110.0.5.0        0.0.0.0         255.255.255.0   u     0      0        0 eth210.0.1.0        0.0.0.0         255.255.255.0   u     0      0        0 eth1192.168.99.0    0.0.0.0         255.255.255.0   u     1      0        0 eth00.0.0.0         192.168.99.1    0.0.0.0         ug    0      0        0 eth0

　　同样的方法配置⑦号机的路由表

root@h7:~# route -n内核 ip 路由表目标            网关            子网掩码        标志  跃点   引用  使用 接口0.0.0.0         192.168.99.1    0.0.0.0         ug    0      0        0 eth010.0.1.0        10.0.4.2        255.255.255.0   ug    0      0        0 eth110.0.4.0        0.0.0.0         255.255.255.0   u     1      0        0 eth110.0.7.0        0.0.0.0         255.255.255.0   u     1      0        0 eth2192.168.99.0    0.0.0.0         255.255.255.0   u     1      0        0 eth0

　　下面再进行ping测试，在①号机ping⑦号机，结果能够ping通。在这里我们问了方便分析,首先列出各网卡的mac地址

①号机 eth1:hwaddr 00:16:ec:af:cb:cb④号机 eth1:hwaddr 40:61:86:32:8f:0b ④号机 eth4:hwaddr 40:61:86:32:8f:0e⑦号机 eth1:hwaddr 00:25:90:93:40:79

　　④号机eth1抓包如下：

[root@h4 ~]# tcpdump -i eth1 -enntcpdump: verbose output suppressed, use -v or -vv for full protocol decodelistening on eth1, link-type en10mb (ethernet), capture size 65535 bytes16:02:26.809445 00:16:ec:af:cb:cb > 40:61:86:32:8f:0b, ethertype ipv4 (0x0800), length 98: 10.0.1.3 > 10.0.4.3: icmp echo request, id 8079, seq 1, length 6416:02:26.810723 40:61:86:32:8f:0b > 00:16:ec:af:cb:cb, ethertype ipv4 (0x0800), length 98: 10.0.4.3 > 10.0.1.3: icmp echo reply, id 8079, seq 1, length 6416:02:27.811847 00:16:ec:af:cb:cb > 40:61:86:32:8f:0b, ethertype ipv4 (0x0800), length 98: 10.0.1.3 > 10.0.4.3: icmp echo request, id 8079, seq 2, length 6416:02:27.813136 40:61:86:32:8f:0b > 00:16:ec:af:cb:cb, ethertype ipv4 (0x0800), length 98: 10.0.4.3 > 10.0.1.3: icmp echo reply, id 8079, seq 2, length 6416:02:28.813248 00:16:ec:af:cb:cb > 40:61:86:32:8f:0b, ethertype ipv4 (0x0800), length 98: 10.0.1.3 > 10.0.4.3: icmp echo request, id 8079, seq 3, length 6416:02:28.814551 40:61:86:32:8f:0b > 00:16:ec:af:cb:cb, ethertype ipv4 (0x0800), length 98: 10.0.4.3 > 10.0.1.3: icmp echo reply, id 8079, seq 3, length 6416:02:29.814648 00:16:ec:af:cb:cb > 40:61:86:32:8f:0b, ethertype ipv4 (0x0800), length 98: 10.0.1.3 > 10.0.4.3: icmp echo request, id 8079, seq 4, length 64

　　④号机eth4抓包如下：

root@h4 ~]# tcpdump -i eth4 -enntcpdump: verbose output suppressed, use -v or -vv for full protocol decodelistening on eth4, link-type en10mb (ethernet), capture size 65535 bytes16:02:26.809460 40:61:86:32:8f:0e > 00:25:90:93:40:79, ethertype ipv4 (0x0800), length 98: 10.0.1.3 > 10.0.4.3: icmp echo request, id 8079, seq 1, length 6416:02:26.810715 00:25:90:93:40:79 > 40:61:86:32:8f:0e, ethertype ipv4 (0x0800), length 98: 10.0.4.3 > 10.0.1.3: icmp echo reply, id 8079, seq 1, length 6416:02:27.811853 40:61:86:32:8f:0e > 00:25:90:93:40:79, ethertype ipv4 (0x0800), length 98: 10.0.1.3 > 10.0.4.3: icmp echo request, id 8079, seq 2, length 6416:02:27.813130 00:25:90:93:40:79 > 40:61:86:32:8f:0e, ethertype ipv4 (0x0800), length 98: 10.0.4.3 > 10.0.1.3: icmp echo reply, id 8079, seq 2, length 6416:02:28.813255 40:61:86:32:8f:0e > 00:25:90:93:40:79, ethertype ipv4 (0x0800), length 98: 10.0.1.3 > 10.0.4.3: icmp echo request, id 8079, seq 3, length 6416:02:28.814545 00:25:90:93:40:79 > 40:61:86:32:8f:0e, ethertype ipv4 (0x0800), length 98: 10.0.4.3 > 10.0.1.3: icmp echo reply, id 8079, seq 3, length 64

　　⑦号机eth1抓包如下：

root@h7:~# tcpdump -i eth1 -enntcpdump: verbose output suppressed, use -v or -vv for full protocol decodelistening on eth1, link-type en10mb (ethernet), capture size 65535 bytes16:02:27.222853 40:61:86:32:8f:0e > 00:25:90:93:40:79, ethertype ipv4 (0x0800), length 98: 10.0.1.3 > 10.0.4.3: icmp echo request, id 8079, seq 1, length 6416:02:27.222867 00:25:90:93:40:79 > 40:61:86:32:8f:0e, ethertype ipv4 (0x0800), length 98: 10.0.4.3 > 10.0.1.3: icmp echo reply, id 8079, seq 1, length 6416:02:28.225226 40:61:86:32:8f:0e > 00:25:90:93:40:79, ethertype ipv4 (0x0800), length 98: 10.0.1.3 > 10.0.4.3: icmp echo request, id 8079, seq 2, length 6416:02:28.225237 00:25:90:93:40:79 > 40:61:86:32:8f:0e, ethertype ipv4 (0x0800), length 98: 10.0.4.3 > 10.0.1.3: icmp echo reply, id 8079, seq 2, length 6416:02:29.226638 40:61:86:32:8f:0e > 00:25:90:93:40:79, ethertype ipv4 (0x0800), length 98: 10.0.1.3 > 10.0.4.3: icmp echo request, id 8079, seq 3, length 6416:02:29.226649 00:25:90:93:40:79 > 40:61:86:32:8f:0e, ethertype ipv4 (0x0800), length 98: 10.0.4.3 > 10.0.1.3: icmp echo reply, id 8079, seq 3, length 6416:02:30.228059 40:61:86:32:8f:0e > 00:25:90:93:40:79, ethertype ipv4 (0x0800), length 98: 10.0.1.3 > 10.0.4.3: icmp echo request, id 8079, seq 4, length 64

 

　　从抓取的包中我们不难看出，①号机在ping ⑦号机时，由于其中路由表配置了通过四号机的eth1（10.0.1.2）地址，这个地址对应的mac①号机已经缓存了，所有没有进行arp广播就直接开 始发送icmp包，并且目的ip是⑦号机，目的mac是④号机的eth1的，之后在④号机路由中又将目的mac变成了④号机的eth4的，目的ip不变， 回来的过程相仿。

4、结论

　　由 于linux路由器默认不转发arp报文到,所有若像”错误的配置“那样配置路由，①号机一直处在询问目的mac的阶段而无法让路由器④号机转发数据包， 所有我们可以通过”正确的配置“那样配置路由让①号机使用④号机eth1的mac出去，然后再一步一步转发。或者通过”错误的配置“那样配置路由，然后在 ④号机中使用arp代理，从而让①号机获得⑦号机的mac，从而从发送arp报文阶段到发送icmp包阶段。谢谢阅读，希望能帮到大家，请继续关注，我们会努力分享更多优秀的文章。