[原创]MTU的一个笔记

Cyg07

#虽然说的的对象是对MTU,但实际讲的是ICMP报文,IP数据报,PPP数据帧的首部作加减计算
#所有协议格式均参照《TCP/IP 详解》
#菜鸟学习笔记,叙述有很多问题,望指正

1.虚拟网关的检测

[root@www ~]# netstat -in
Kernel Interface table
Iface       MTU Met    RX-OK RX-ERR RX-DRP RX-OVR    TX-OK TX-ERR TX-DRP TX-OVRFlg
eth0       1500   0     1113      0      0      0      844      0      0      0BMRU
lo        16436   0     2205      0      0      0     2205      0      0      0LRU
#netstat -in 获取网卡(eth0)的MTU -- 1500
#lo是 环回IP 的代号

[root@www ~]# ping -c 4 -M do -s 1500 192.168.137.1
PING 192.168.137.1 (192.168.137.1) 1500(1528) bytes of data.
From 192.168.137.128 icmp_seq=0 Frag needed and DF set (mtu = 1500)
From 192.168.137.128 icmp_seq=0 Frag needed and DF set (mtu = 1500)
From 192.168.137.128 icmp_seq=0 Frag needed and DF set (mtu = 1500)
From 192.168.137.128 icmp_seq=0 Frag needed and DF set (mtu = 1500)

--- 192.168.137.1 ping statistics ---
0 packets transmitted, 0 received, +4 errors
#192.168.137.1 该虚拟机的网关
#ping -s 命令发送出的IP数据报都是在IP首部设置不分片(DF)标志比特(包含的是ICMP报文)
#反馈的信息是1500字节无法被发送
#减去IP首部20字节和ICMP首部8字节得到1472进行测试

[root@www ~]# ping -c 4 -M do -s 1472 192.168.137.1
PING 192.168.137.1 (192.168.137.1) 1472(1500) bytes of data.
1480 bytes from 192.168.137.1: icmp_seq=0 ttl=128 time=3.12 ms
1480 bytes from 192.168.137.1: icmp_seq=1 ttl=128 time=1.00 ms
1480 bytes from 192.168.137.1: icmp_seq=2 ttl=128 time=0.524 ms
1480 bytes from 192.168.137.1: icmp_seq=3 ttl=128 time=0.621 ms

--- 192.168.137.1 ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 3011ms
rtt min/avg/max/mdev = 0.524/1.318/3.120/1.056 ms, pipe 2

# :）
#超过1472字节的都无法发送

2.PPP设备的检测

[root@www ~]# traceroute [url]www.baidu.com[/url]
traceroute: Warning: [url]www.baidu.com[/url] has multiple addresses; using 220.181.37.4
traceroute to [url]www.a.shifen.com[/url] (220.181.37.4), 30 hops max, 38 byte packets
1  www (192.168.137.2)  2.101 ms  2.008 ms  2.349 ms
2  * * *
3  * * *
#汗!虚拟机上测试出问题了,只检测到虚拟网关
#我换在Windows2003测试下

C:\>tracert [url]www.baidu.com[/url]

Tracing route to [url]www.a.shifen.com[/url] [220.181.37.4]
over a maximum of 30 hops:

1    23 ms    17 ms    16 ms  1.42.4.122.broad.jn.sd.dynamic.163data.com.cn [1
22.4.42.1]
2    16 ms    15 ms    15 ms  222.173.1.145
3    18 ms    18 ms    18 ms  222.173.1.253
4    18 ms    19 ms    27 ms  222.173.1.77
5    17 ms    17 ms    16 ms  222.173.1.13
6    17 ms    18 ms    21 ms  219.146.19.1
7    32 ms    32 ms    42 ms  202.97.39.5
8    55 ms    55 ms    54 ms  202.97.34.45
9    54 ms    54 ms    56 ms  202.97.37.10
10    55 ms    54 ms    58 ms  220.181.16.149
11    63 ms    64 ms    70 ms  220.181.16.14
12    56 ms    56 ms    56 ms  220.181.17.94
13    55 ms    54 ms    55 ms  220.181.37.4

Trace complete.
#我们只截取拨号设备出去后的第一个目标地址122.4.42.1

C:\>ping -f -l 1472 122.4.42.1

Pinging 122.4.42.1 with 1472 bytes of data:

Request timed out.
Request timed out.
Request timed out.
Request timed out.

Ping statistics for 122.4.42.1:
   Packets: Sent = 4, Received = 0, Lost = 4 (100% loss)
#-f命令 发送在IP首部设置不分片(DF)标志比特的包含ICMP报文的IP数据报
# :( 失败!
#再减去PPP首部的8个字节进行测试

C:\>ping -f -l 1464 122.4.42.1

Pinging 122.4.42.1 with 1464 bytes of data:

Reply from 122.4.42.1: bytes=1464 time=43ms TTL=255
Reply from 122.4.42.1: bytes=1464 time=49ms TTL=255
Reply from 122.4.42.1: bytes=1464 time=41ms TTL=255
Reply from 122.4.42.1: bytes=1464 time=46ms TTL=255

Ping statistics for 122.4.42.1:
   Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
   Minimum = 41ms, Maximum = 49ms, Average = 44ms
#:)
#1464以上都无法发送!
#over.

ssffzz1

愣没看懂。

laodongmo

支持一下顶了