How to use tcpdump in Linux

I encountered a problem yesterday. after the project was deployed on the server, I found that a JSP file could not be accessed and reported a 500 error ~~~ At first, I thought it was a code problem. After optimizing the code, I found that the error 500 was still reported. then I copied a copy and changed the name to access it ~ _~! Unknown cause ~ I began to suspect network problems ~ According to a problem that my colleague encountered yesterday, after deploying the project to the server, I found that a JSP file could not be accessed and reported a 500 error ~~~
At first, I thought it was a code problem. After optimizing the code, I found that the error 500 was still reported. then I copied a copy and changed the name to access it ~ _~! Unknown cause ~
I began to suspect network problems ~
According to the help of colleagues and the guidance of linux experts, the following actions are completed:
Run the following command under the root user in linux:
Packet capture method:
Tcpdump-I bond0 host 172.29.10.56-x-s0-w ZTE. cap
172.29.10.56 is the IP address of the data stream.
ZTE. cap is the name of the packet capture file, which must be ". cap"
Capture the cap file and use WiresharkAnalyze captured data packets
The usage of tcpdump in linux is as follows:

Keyword about type
It mainly includes host, net, port, such as host 210.27.48.2, specifying 210.27.48.2 as a host, net 202.0.0.0 specifying 202.0.0.0 as a network address, and port 23 specifying port number as 23. If no type is specified, the default type is host.

Key words for determining the transmission direction
These keywords mainly include 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.

Protocol keyword,
It 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 the above three types of keywords, other important keywords are as follows: gateway, broadcast, less, greater, and there are three logical operations. The non-calculation 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 09000000
0e80 0000 902b 4695 0980 8701 00140002
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 ffffffff
0452 ffff 0000 e85b 6d85 40080002
0640 4d41 5354 4552 5f57 4542 00000000
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 or210.27.48.3 \)
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 80J 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
K to intercept the communication between the host 210.27.48.1 and the 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 or210.27.48.3 \)
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

Supplement:
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 port80 destination port is 80
Or
# Tcpdump? I eth0 host hostname and src port80 the source port is 80, which is generally a 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 and dstport 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 enteredpromiscuous mode
May 7 20:03:57 localhost kernel: device eth0 leftpromiscuous mode
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.