在linux系統下利用PC機實現路由轉發的具體步驟

　　網絡拓撲如下所示，我們在這裡用到了三台機子做實驗，分別是①、④、⑦號機，使用①號機ping⑦號機，④號機作為路由轉發。

　　首先我們使用如下的配置方法，配置這三台機子的路由表：

　　1）在①號機種配置如下，讓目的網段是10.0.4.0/24的從eth1端口出去

route  add -net 10.0.4.0/24 dev eth1

　　在①號機的查看路由表輸入如下命令：

route -n

　　①號機的路由表的結果如下：

 　　2）在⑦號機使用同樣方法配置路由，結果如下：

 　　3）在4號機配置路由轉發功能，即將/etc/sysctl.conf文件裡面的net.ipv4.ip_forward的值置1：

 　　4）所有的配置已經完成，我們在①號機ping④號機

ping  10.0.4.3

 　　結果如下，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

　　這裡為了方便研究，把①號機的eth1配置放出來

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號機ping 的同時，我在④號機抓eth1包，結果如下：

[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

 　　可見①號一直在尋找配有10.0.4.3 IP的機子的mac地址，即一直在發arp包。但是路由器（④號機）默認是不轉發arp報文的，所有①號機永遠也ping不通⑦號機。

　　在①號機種配置路由，命令如下：

route add -net 10.0.4.0/24 gw 10.0.1.2

　　這時候①號機的路由表：

[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

　　同樣的方法配置⑦號機的路由表

root@h7:~# route -n內核 IP 路由表目標            網關            子網掩碼        標志  躍點   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

　　下面再進行ping測試，在①號機ping⑦號機，結果能夠ping通。在這裡我們問了方便分析,首先列出各網卡的MAC地址

①號機 eth1:HWaddr 00:16:EC:AF:CB:CB④號機 eth1:HWaddr 40:61:86:32:8F:0B ④號機 eth4:HWaddr 40:61:86:32:8F:0E⑦號機 eth1:HWaddr 00:25:90:93:40:79

　　④號機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號機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號機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

　　從抓取的包中我們不難看出，①號機在ping ⑦號機時，由於其中路由表配置了通過四號機的eth1（10.0.1.2）地址，這個地址對應的mac①號機已經緩存了，所有沒有進行arp廣播就直接開 始發送ICMP包，並且目的ip是⑦號機，目的MAC是④號機的eth1的，之後在④號機路由中又將目的MAC變成了④號機的eth4的，目的ip不變， 回來的過程相仿。

　　由 於linux路由器默認不轉發arp報文到,所有若像”錯誤的配置“那樣配置路由，①號機一直處在詢問目的MAC的階段而無法讓路由器④號機轉發數據包， 所有我們可以通過”正確的配置“那樣配置路由讓①號機使用④號機eth1的MAC出去，然後再一步一步轉發。或者通過”錯誤的配置“那樣配置路由，然後在 ④號機中使用arp代理，從而讓①號機獲得⑦號機的MAC，從而從發送arp報文階段到發送ICMP包階段。謝謝閱讀，希望能幫到大家，請繼續關注腳本之家，我們會努力分享更多優秀的文章。