使用Linux系统(PC机)做路由转发

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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。

　　

开启Linux 路由转发功能  http://www.linuxidc.com/Linux/2010-06/26654.htm

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

  1. 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：

  1. 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

 
  1. ping 10.0.4.3
 

 　　结果如下，即ping 不通：

  1. PING 10.0.4.3(10.0.4.3)56(84) bytes of data.
  2. From10.0.1.3 icmp_seq=2DestinationHostUnreachable
  3. From10.0.1.3 icmp_seq=3DestinationHostUnreachable
  4. From10.0.1.3 icmp_seq=4DestinationHostUnreachable
  5. From10.0.1.3 icmp_seq=6DestinationHostUnreachable
  6. From10.0.1.3 icmp_seq=7DestinationHostUnreachable
  7. From10.0.1.3 icmp_seq=8DestinationHostUnreachable

　　这里为了方便研究，把①号机的eth1配置放出来

  1. eth1 Link encap:EthernetHWaddr00:16:EC:AF:CB:CB
  2. inet addr:10.0.1.3Bcast:10.255.255.255Mask:255.255.255.0
  3. inet6 addr: fe80::216:ecff:feaf:cbcb/64Scope:Link
  4. UP BROADCAST RUNNING MULTICAST MTU:1500Metric:1
  5. RX packets:4564 errors:0 dropped:0 overruns:0 frame:0
  6. TX packets:6688 errors:0 dropped:0 overruns:0 carrier:0
  7. collisions:0 txqueuelen:1000
  8. RX bytes:459463(448.6KiB) TX bytes:546633(533.8KiB)
  9. Interrupt:23Base address:0x6000

　　在①号机ping 的同时，我在④号机抓eth1包，结果如下：

  1. [root@h4~]# tcpdump -i eth1 -enn
  2. tcpdump: verbose output suppressed,use-v or-vv for full protocol decode
  3. listening on eth1, link-type EN10MB (Ethernet), capture size 65535 bytes
  4. 15:26:44.38861400:16: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
  5. 15:26:45.39101400:16: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
  6. 15:26:47.38782100:16: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
  7. 15:26:48.39122000:16: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
  8. 15:26:49.39262100:16: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：

  1. route add -net 10.0.4.0/24 gw 10.0.1.2

　　这时候①号机的路由表：

  1. [root@h1 ~]#
  2. [root@h1 ~]# route -n
  3. Kernel IP routing table
  4. DestinationGatewayGenmaskFlagsMetricRefUseIface
  5. 10.0.4.010.0.1.2255.255.255.0 UG 000 eth1
  6. 10.0.5.00.0.0.0255.255.255.0 U 000 eth2
  7. 10.0.1.00.0.0.0255.255.255.0 U 000 eth1
  8. 192.168.99.00.0.0.0255.255.255.0 U 100 eth0
  9. 0.0.0.0192.168.99.10.0.0.0 UG 000 eth0

　　同样的方法配置⑦号机的路由表

  1. root@h7:~# route -n
  2. 内核 IP 路由表
  3. 目标网关子网掩码标志跃点引用使用接口
  4. 0.0.0.0192.168.99.10.0.0.0 UG 000 eth0
  5. 10.0.1.010.0.4.2255.255.255.0 UG 000 eth1
  6. 10.0.4.00.0.0.0255.255.255.0 U 100 eth1
  7. 10.0.7.00.0.0.0255.255.255.0 U 100 eth2
  8. 192.168.99.00.0.0.0255.255.255.0 U 100 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. ①号机 eth1:HWaddr00:16:EC:AF:CB:CB
  2. ④号机 eth1:HWaddr40:61:86:32:8F:0B
  3. ④号机 eth4:HWaddr40:61:86:32:8F:0E
  4. ⑦号机 eth1:HWaddr00:25:90:93:40:79

　　④号机eth1抓包如下：

  1. [root@h4 ~]# tcpdump -i eth1 -enn
  2. tcpdump: verbose output suppressed,use-v or-vv for full protocol decode
  3. listening on eth1, link-type EN10MB (Ethernet), capture size 65535 bytes
  4. 16:02:26.80944500:16: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 64
  5. 16:02:26.81072340:61:86:32:8f: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 64
  6. 16:02:27.81184700:16: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 64
  7. 16:02:27.81313640:61:86:32:8f: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 64
  8. 16:02:28.81324800:16: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 64
  9. 16:02:28.81455140:61:86:32:8f: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 64
  10. 16:02:29.81464800:16: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：

  1. root@h4 ~]# tcpdump -i eth4 -enn
  2. tcpdump: verbose output suppressed,use-v or-vv for full protocol decode
  3. listening on eth4, link-type EN10MB (Ethernet), capture size 65535 bytes
  4. 16:02:26.80946040:61:86:32:8f: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 64
  5. 16:02:26.81071500:25:90:93:40:79>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 64
  6. 16:02:27.81185340:61:86:32:8f: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 64
  7. 16:02:27.81313000:25:90:93:40:79>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 64
  8. 16:02:28.81325540:61:86:32:8f: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 64
  9. 16:02:28.81454500:25:90:93:40:79>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：

  1. root@h7:~# tcpdump -i eth1 -enn
  2. tcpdump: verbose output suppressed,use-v or-vv for full protocol decode
  3. listening on eth1, link-type EN10MB (Ethernet), capture size 65535 bytes
  4. 16:02:27.22285340:61:86:32:8f: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 64
  5. 16:02:27.22286700:25:90:93:40:79>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 64
  6. 16:02:28.22522640:61:86:32:8f: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 64
  7. 16:02:28.22523700:25:90:93:40:79>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 64
  8. 16:02:29.22663840:61:86:32:8f: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 64
  9. 16:02:29.22664900:25:90:93:40:79>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
  10. 16:02:30.22805940:61:86:32:8f: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

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  • 本文由 发表于 2020年8月22日 20:17:40
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