Use tcpdump in Linux to analyze network data packets

The linuxtcpdump command must be used as root. Example: capture data on port 8804 of the local machine and save the packet capture results in the test. cap file tcpdump-X-s0-wtest.capport8804tcpdump-X-s0-wtest.caphost220.194.51.1tcpdump-X-s0-wtest.capport880 linux TcpdumpThe command must be used as root. For example, capture the data on port 8804 of the local machine and save the packet capture results in the test. cap file.
Tcpdump-X-s 0-w test. cap port 8804
Tcpdump-X-s 0-w test. cap host 220.194.51.1
Tcpdump-X-s 0-w test. cap port 8804
-X Print each packet (minus its link level header) in hex andASCII.
-S snaplen Setting snaplen to 0 means use the required length tocatch whole packets.
-W file Write the raw packets to file rather than parsing andprinting them out.
Then, you can use the Ethereal graphical interface to view the detailed information of the captured package.
This is especially useful when using remote login to work on the server, because the terminal mode cannot use graphical tools to capture packets.
Tcpdump-X-s 0-w/home/qxt_new/test5.cap dst or src221.130.33.92
Tcpdump-X-s 0-w/home/qxt_new/test234.cap dst port 22
Tcpdump-X-s 0-w/home/qxt_new/test234.cap dst port 60000-ieth1
Tcpdump-X-s 0-w/home/qxt_new/test. cap dst or src221.130.33.92 and dst or src port 7890
Tcpdump usage (17:19:19) Category: Linux topics
The first type keyword mainly includes host, net, port, for example host 210.27.48.2. it indicates that 210.27.48.2 is a host, and net 202.0.0.0 indicates that 202.0.0.0 is a network address, port 23 indicates that the port number is 23. If no type is specified, the default type is host.
The second type is the key words for determining the transmission direction, including src, dst, dst or src, dst and src, which indicate the transmission direction. For example, src 210.27.48.2 indicates that the source address in the IP package is 210.27.48.2, and dstnet 202.0.0.0 indicates that the destination network address is 202.0.0.0. If no direction keyword is specified, the src ordst keyword is used by default.
The third type is the protocol keyword, which mainly includes fddi, ip, arp, rarp, tcp, udp, and other types. Fddi indicates a specific network protocol on FDDI (distributed optical fiber data interface network). In fact, it is an alias of "ether". fddi and ether have similar source and destination addresses, therefore, the fddi protocol package can be processed and analyzed as the ether package. The other keywords indicate the protocol content of the listener package. If no protocol is specified, tcpdump listens to the information packages of all protocols.
In addition to these three types of keywords, other important keywords include gateway, broadcast, less, greater, and three logical operations. The non-operation type is 'not ''! ', And the operation is 'and',' & '; or the operation is 'or',' │ '; these keywords can be combined to form a powerful combination condition to meet people's needs. The following are several examples.
Under normal circumstances, directly starting tcpdump will monitor all the data packets flowing through the first network interface.
# Tcpdump
Tcpdump: listening on fxp0
11:58:47. 873028 202.102.245.40.netbios-ns> 202.102.245.127.netbios-ns: udp 50
11:58:47. 974331 0: 10: 7b: 8: 3a: 56> 1: 80: c2: 0: 0: 0 802.1d ui/Clen = 43
0000 0000 0080 0000 1007 cf08 0900 0000
0e80 0000 902b 4695 0980 8701 0014
000f 0000 902b 4695 0008 00
11:58:48. 373134 0: 0: e8: 5b: 6d: 85> Broadcast sap e0 ui/Clen = 97
Ffff 0060 0004 ffff
0452 ffff 0000 e85b 6d85 4008 0002
0640 4d41 5354 4552 5f57 4542 0000 0000
00, 0000
Use the-I parameter to specify the network interface of the tcpdump listener, which is useful when the computer has multiple network interfaces,
Use the-c parameter to specify the number of data packets to be monitored,
Use the-w parameter to specify to write the listening data packet to the file and save it
A wants to intercept all packets received and sent by all 210.27.48.1 hosts:
# Tcpdump host 210.27.48.1
B. to intercept the communication between host 210.27.48.1 and host 210.27.48.2 or 210.27.48.3, run the following command: (when parentheses are applied in the command line, be sure
# Tcpdump host 210.27.48.1 and \ (210.27.48.2 or 210.27.48.3 \)
C if you want to obtain the IP package for all hosts except 210.27.48.1 and 210.27.48.2, run the following command:
# Tcpdump ip host 210.27.48.1 and! 210.27.48.2
D. to obtain the telnet packet received or sent by the host 210.27.48.1, run the following command:
# Tcpdump tcp port 23 host 210.27.48.1
E. monitor the udp port 123 of the local machine. Port 123 is the ntp service port.
# Tcpdump udp port 123
The F system only monitors the communication data packets of the host named hostname. The host name can be a local host or any computer on the network. The following command reads all data sent by the host hostname:
# Tcpdump-I eth0 src host hostname
The command below G can monitor all data packets sent to the host hostname:
# Tcpdump-I eth0 dst host hostname
H we can also monitor the data packets through the specified Gateway:
# Tcpdump-I eth0 gateway Gatewayname
If you want to monitor the TCP or UDP data packets destined for the specified port, run the following command:
# Tcpdump-I eth0 host hostname and port 80
J. if you want to obtain an IP packet for all hosts except 210.27.48.1 and 210.27.48.2
, Run the following command:
# Tcpdump ip host 210.27.48.1 and! 210.27.48.2
K. to intercept the communication between host 210.27.48.1 and host 210.27.48.2 or 210.27.48.3, run the following command:
(When using parentheses in the command line, be sure
# Tcpdump host 210.27.48.1 and \ (210.27.48.2 or 210.27.48.3 \)
L If you want to obtain an IP packet for all hosts except 210.27.48.1 and 210.27.48.2, run the following command:
# Tcpdump ip host 210.27.48.1 and! 210.27.48.2
M to obtain the telnet packet received or sent by the host 210.27.48.1, run the following command:
# Tcpdump tcp port 23 host 210.27.48.1
The third type is the protocol keyword, which mainly includes fddi, ip, arp, rarp, tcp, udp, and other types.
In addition to the three types of keywords, other important keywords are as follows: gateway, broadcast, less,
Greater, there are three logical operations. The non-operation is 'not ''! ', And the operation is 'and',' & '; or the operation is 'o
R', '| ';
The second type is the key words for determining the transmission direction, including src, dst, dst or src, dst and src,
If we only need to list the data packets sent to port 80, use dst port; if we only want to see the data packets returned to port 80, use src port.
# Tcpdump? I eth0 host hostname and dst port 80 the destination port is 80
Or
# Tcpdump? I eth0 host hostname and src port 80 the source port is 80. generally, it is the host that provides http services.
If there are many conditions, add and or not before the conditions.
# Tcpdump-I eth0 host! 211.161.223.70 and! 211.161.223.71 anddst port 80
Logs in the ethernet mixed mode system are recorded.
May 7 20:03:46 localhost kernel: eth0: Promiscuous modeenabled.
May 7 20:03:46 localhost kernel: device eth0 entered promiscuousmode
May 7 20:03:57 localhost kernel: device eth0 left promiscuousmode
Tcpdump does not thoroughly decode the intercepted data. most of the content in the data packet is printed in hexadecimal format. Obviously, this is not conducive to the analysis of network faults. The common solution is to first use tcpdump with The-w parameter to capture data and save it to the file, and then use other programs for decoding and analysis. Of course, filter rules should also be defined to prevent the captured data packets from filling the entire hard disk.
In addition to the filter statement, there is also a very important parameter. that is to say, if this parameter is not set correctly, packet data will be lost!
It is the-s parameter, snaplen, that is, the interception length of the data packet. man will understand it carefully! The intercepted length is 60 bytes by default, but generally ethernetMTU is 1500 bytes. Therefore, when you want to capture packets larger than 60 bytes, using the default parameters will cause packet data loss!
You only need to use-s 0 to capture data by the package length!