Understanding the Ping and Traceroute Commands

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Written by yUmis(牛奶咖喱) a.k.a. 红头发
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Homepage http://www.show-tym.com
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Jun.27th.2004
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什么是ping?
ping是Packet Internet Groper的缩写,这个命令是最常见的用于检测网络设备可访问性的方法.它使用Internet Control Message Protocol(ICMP)的echo信息可以来决定:
1.远程设备是否可用
2.与远程主机通信的来回旅程(round-trip)的延迟(delay)
3.包(packet)的丢失情况

ping是如何工作的?
首先,ping发送echo request packet到某个地址,然后等待应答(reply),当echo request到达目标地址以后,在一个有效的时间内(timeout之前)返回echo reply packet给源地址.这样即说明能够ping通
(Cisco的router的timeout值默认为2秒)

下面是一个在启用了debug ip packet detail命令之后使用ping命令的输出内容的例子.(注意该debug命令将占用router大量的CPU资源甚至可能导致网络负载过大直至崩溃,记得在使用完毕以后no debug all)
Router1#debug ip packet detail
IP packet debugging is on (detailed)

Router1#ping 12.0.0.2
Type escape sequence to abort.
Sending 5, 100 byte ICMP Echos to 12.0.0.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/6/8 ms

Router1#
Jan 20 15:54:47.487: IP: s=12.0.0.1 (local), d=12.0.0.2 (Serial0), len 100,
sending
Jan 20 15:54:47.491: ICMP type=8, code=0
这是12.0.0.1发送到10.0.0.2的ICMP包,type=8说明是echo(详见后面的类型示意图)
Jan 20 15:54:47.523: IP: s=12.0.0.2 (Serial0), d=12.0.0.1 (Serial0), len 100,
rcvd 3
Jan 20 15:54:47.527: ICMP type=0, code=0
来自10.0.0.2的应答,type=0说明是echo reply(详见后面的类型示意图)

ICMP type & code示意图如下:

此主题相关图片如下：

一些返回的符号的含义如下:
1.叹号(!)代表成功收到响应
2.句号(.)代表在等待应答的时候超时间
3.U代表目标不可达(destination unreachable)或接收到错误的PDU
4.Q代表source quench,说明目标地址过于繁忙
5.M代表不能分片(fragment)
6.问号(?)代表未知的包的类型
7.符号&代表包的生存期(lifetime)超出

为什么我不能ping通?
来看一个例子.如下图:

此主题相关图片如下：

先看下4个router的配置,如下:
Router1#
!
!
interface Serial0
ip address 12.0.0.1 255.255.255.0
no fair-queue
clockrate 64000
!
!
Router2#
!
!
interface Serial0
ip address 23.0.0.2 255.255.255.0
no fair-queue
clockrate 64000
!
interface Serial1
ip address 12.0.0.2 255.255.255.0
!
!
Router3#
!
!
interface Serial0
ip address 34.0.0.3 255.255.255.0
no fair-queue
!
interface Serial1
ip address 23.0.0.3 255.255.255.0
!
!
Router4#
!
!
interface Serial0
ip address 34.0.0.4 255.255.255.0
no fair-queue
clockrate 64000
!
!

我们来试试在Router1上ping Router4:
Router1#ping 34.0.0.4
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 34.0.0.4, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
5个句号说明了结果失败

接下来启用debug ip packet detail实时监测,看下到底发生了什么:
Router1#debug ip packet

IP packet debugging is on

再来ping Router4:
Router1#ping 34.0.0.4
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 34.0.0.4, timeout is 2 seconds:
Jan 20 16:00:25.603: IP: s=12.0.0.1 (local), d=34.0.0.4, len 100, unroutable.
Jan 20 16:00:27.599: IP: s=12.0.0.1 (local), d=34.0.0.4, len 100, unroutable.
Jan 20 16:00:29.599: IP: s=12.0.0.1 (local), d=34.0.0.4, len 100, unroutable.
Jan 20 16:00:31.599: IP: s=12.0.0.1 (local), d=34.0.0.4, len 100, unroutable.
Jan 20 16:00:33.599: IP: s=12.0.0.1 (local), d=34.0.0.4, len 100, unroutable.
Success rate is 0 percent (0/5)
可以看到输出包含unroutable,说明包无法被路由,那是因为我们没启用任何路由协议

在Router1上配置一条路由:
Router1#config t
Enter configuration commands, one per line. End with CNTL/Z.
Router1(config)#ip route 0.0.0.0 0.0.0.0 Serial0

再次启用debug ip packet detail以后,ping Router4:
Router1#ping 34.0.0.4
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 34.0.0.4, timeout is 2 seconds:
U.U.U
Success rate is 0 percent (0/5)
Jan 20 16:05:30.659: IP: s=12.0.0.1 (local), d=34.0.0.4 (Serial0), len 100,
sending
Jan 20 16:05:30.663: ICMP type=8, code=0
Jan 20 16:05:30.691: IP: s=12.0.0.2 (Serial0), d=12.0.0.1 (Serial0), len 56,
rcvd 3
Jan 20 16:05:30.695: ICMP type=3, code=1
Jan 20 16:05:30.699: IP: s=12.0.0.1 (local), d=34.0.0.4 (Serial0), len 100,
sending
Jan 20 16:05:30.703: ICMP type=8, code=0
Jan 20 16:05:32.699: IP: s=12.0.0.1 (local), d=34.0.0.4 (Serial0), len 100,
sending
Jan 20 16:05:32.703: ICMP type=8, code=0
Jan 20 16:05:32.731: IP: s=12.0.0.2 (Serial0), d=12.0.0.1 (Serial0), len 56,
rcvd 3
Jan 20 16:05:32.735: ICMP type=3, code=1
Jan 20 16:05:32.739: IP: s=12.0.0.1 (local), d=34.0.0.4 (Serial0), len 100,
sending
Jan 20 16:05:32.743: ICMP type=8, code=0

Router2上出了什么问题?
Router2#debug ip packet detail
IP packet debugging is on (detailed)

Router2#
Jan 20 16:10:41.907: IP: s=12.0.0.1 (Serial1), d=34.0.0.4, len 100, unroutable
Jan 20 16:10:41.911: ICMP type=8, code=0
Jan 20 16:10:41.915: IP: s=12.0.0.2 (local), d=12.0.0.1 (Serial1), len 56, sending
Jan 20 16:10:41.919: ICMP type=3, code=1
Jan 20 16:10:41.947: IP: s=12.0.0.1 (Serial1), d=34.0.0.4, len 100, unroutable
Jan 20 16:10:41.951: ICMP type=8, code=0
Jan 20 16:10:43.943: IP: s=12.0.0.1 (Serial1), d=34.0.0.4, len 100, unroutable
Jan 20 16:10:43.947: ICMP type=8, code=0
Jan 20 16:10:43.951: IP: s=12.0.0.2 (local), d=12.0.0.1 (Serial1), len 56, sending
Jan 20 16:10:43.955: ICMP type=3, code=1
Jan 20 16:10:43.983: IP: s=12.0.0.1 (Serial1), d=34.0.0.4, len 100, unroutable
Jan 20 16:10:43.987: ICMP type=8, code=0
Jan 20 16:10:45.979: IP: s=12.0.0.1 (Serial1), d=34.0.0.4, len 100, unroutable
Jan 20 16:10:45.983: ICMP type=8, code=0
Jan 20 16:10:45.987: IP: s=12.0.0.2 (local), d=12.0.0.1 (Serial1), len 56, sending
Jan 20 16:10:45.991: ICMP type=3, code=1
Router1把包正确的发给了Router2,但是笨蛋Router2却不知道如何访问Router4(34.0.0.4),它回送unreachable ICMP信息给Router1

well,在Router2和3上启用RIP吧:
Router2#router rip
Router2#network 12.0.0.0
Router2#network 23.0.0.0

Router3#router rip
Router3#network 23.0.0.0
Router3#network 34.0.0.0

这下再来看能不能ping通:
Router1#debug ip packet
IP packet debugging is on

Router1#ping 34.0.0.4
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 34.0.0.4, timeout is 2 seconds:
Jan 20 16:16:13.367: IP: s=12.0.0.1 (local), d=34.0.0.4 (Serial0), len 100, sending.
Jan 20 16:16:15.363: IP: s=12.0.0.1 (local), d=34.0.0.4 (Serial0), len 100, sending.
Jan 20 16:16:17.363: IP: s=12.0.0.1 (local), d=34.0.0.4 (Serial0), len 100, sending.
Jan 20 16:16:19.363: IP: s=12.0.0.1 (local), d=34.0.0.4 (Serial0), len 100, sending.
Jan 20 16:16:21.363: IP: s=12.0.0.1 (local), d=34.0.0.4 (Serial0), len 100, sending.
Success rate is 0 percent (0/5)
shit!还是不行,没有得到任何应答,看下Router4上到底发生了什么:
Router4#debug ip packet
IP packet debugging is on

Router4#
Jan 20 16:18:45.903: IP: s=12.0.0.1 (Serial0), d=34.0.0.4 (Serial0), len 100,
rcvd 3
Jan 20 16:18:45.911: IP: s=34.0.0.4 (local), d=12.0.0.1, len 100, unroutable
Jan 20 16:18:47.903: IP: s=12.0.0.1 (Serial0), d=34.0.0.4 (Serial0), len 100,
rcvd 3
Jan 20 16:18:47.907: IP: s=34.0.0.4 (local), d=12.0.0.1, len 100, unroutable
Jan 20 16:18:49.903: IP: s=12.0.0.1 (Serial0), d=34.0.0.4 (Serial0), len 100,
rcvd 3
Jan 20 16:18:49.907: IP: s=34.0.0.4 (local), d=12.0.0.1, len 100, unroutable
Jan 20 16:18:51.903: IP: s=12.0.0.1 (Serial0), d=34.0.0.4 (Serial0), len 100,
rcvd 3
Jan 20 16:18:51.907: IP: s=34.0.0.4 (local), d=12.0.0.1, len 100, unroutable
Jan 20 16:18:53.903: IP: s=12.0.0.1 (Serial0), d=34.0.0.4 (Serial0), len 100,
rcvd 3
Jan 20 16:18:53.907: IP: s=34.0.0.4 (local), d=12.0.0.1, len 100, unroutable
原来是这样,Router4收到了来自Router1的包,但是这个笨蛋却不知道如何应答.好吧,我给你加条静态路由:
Router4(config)#ip route 0.0.0.0 0.0.0.0 Serial0

应该没问题了吧?let me try:
Router1#ping 34.0.0.4
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 34.0.0.4, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 32/35/36 ms
Bingo!终于成功了嘎嘎

上面这个例子里,ping不成功,是由于4个傻瓜不知道如何路由packet.下面再来看一个例子:

此主题相关图片如下：

Router1#ping 34.0.0.4
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 34.0.0.4, timeout is 2 seconds:
U.U.U
Success rate is 0 percent (0/5)
damn!又不成功!之前配置了路由协议了,应该不会是4个傻瓜找不到路的原因.那么,我们来1步步的排错吧:
Router1#ping 12.0.0.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 12.0.0.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/4/4 ms
ping Router2成功了,是不是Router3的问题呢??看看就知道了:
Router3#show ip interface brief
Serial0   34.0.0.3   YES   manual   up                &

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