Sniffer tool in Linux: Introduction to tcpdump

Among today's many hacking technologies, sniffer is the most common and important technology. Anyone who has used the sniffer tool on windows (for example, netxray and snifferpro) may know that in a shared Lan, using the sniffer tool can provide a clear view of all the traffic in the network! Among today's many hacking technologies, Sniffer ( Sniffer) Is the most common and important technology. I have used Sniffer tool(For example, netxray and SnifferPro software) friends may know that in a shared Lan, using sniffer ToolsYou can see all the traffic in the network at a glance! The Sniffer tool is actually a packet capture tool on the network. It can also analyze captured packets. In a shared network, information packets are the network interfaces of all hosts in the network, but before the sniffer tool is used, the network device of the host determines whether the information packet should be received, so that it will discard the information packet that should not be received. the sniffer tool allows the network device of the host to receive all the information packets that have arrived, this achieves the effect of network listening. In fact, the sniffer tool can be used by hackers and is also beneficial to network administrators and network programmers. Network administrators can keep abreast of the actual network conditions by using the sniffer. when network performance drops sharply, you can use the sniffer tool to analyze the causes and find out the source of network congestion. For network programmers, the sniffer tool is used to debug programs.
Next we will introduce you LinuxExcellent sniffer ------- Tcpdump. (The operations below are all in redhat6.2. Linux2.2.14 the environment has been tested .)
　　
　　 1. TcpdumpInstall
In LinuxUnder tcpdump InstallIt is very simple. Generally, there are two installation methods. An rpm Package is used for installation. In addition, it is installed in the form of a source program.
1. rpm Package installation
This form of installation is the simplest installation method. The rpm package is to compile the software and package it into a binary format. you can directly install the rpm command without modifying anything. Use the following command to log on as a super user:
# Rpm-ivh tcpdump-3_4a5.rpm
In this way, tcpdump is successfully installed on your linux system. It's easy.
2. installation of the source program
Since the rpm Package is easy to install, why should we use a complex source program for installation? In fact, the biggest attraction of linux is that there are a lot of software on it that provides the source program. people can modify the source program to meet their special needs. Therefore, I especially recommend that you install the source program.
　　
The first step is to obtain the source program. in the installation method of the source program, we first need to obtain the tcpdump source program distribution package. this distribution package has two forms, one is the tarcompressed package (tcpdump-3_4a5.tar. z), the other is the rpm distribution package (tcpdump-3_4a5.src.rpm ). The two types of content are similar. different types of content are compressed. you can use the following command to uncompress A. tar compressed package:
# Tar xvfz tcpdump-3_4a5.tar.Z
Run the following command to install the rpm Package:
# Rpm-ivh tcpdump-3_4a5.src.rpm
In this way, extract the source code of tcpdump to the/usr/src/redhat/SOURCES Directory.
　　
Step 2 Prepare the source program for compilation
Before compiling the source program, you are advised to confirm that the library file libpcap has been installed. this library file is the library file required by tcpdump software. Similarly, you also need a standard C language compiler. In linux, the standard c language compiler is generally gcc. In the tcpdump source program directory. One file is Makefile. in. the configure command automatically generates Makefile files from the Makefile. in file. In the Makefile. in file, you can modify the BINDEST and MANDEST macro definitions according to the system configuration. the default value is
BINDEST = @ sbindir @
MANDEST = @ mandir @
The first macro value indicates the path name of the binary file where tcpdump is installed, and the second macro value indicates the path name of the tcpdump man help page. you can modify them to meet system requirements.
　　
Step 3 compile the source program
Use the configure script in the source program directory to read various required attributes from the system. According to the Makefile. in file, the Makefile file is generated automatically, so that the. make command can be used to compile the tcpdump source program according to the rules in the Makefile file. Run the make install command to install the compiled binary file tcpdump.
Summary:
# Tar xvfz tcpdump-3_4a5.tar.Z
# Vi Makefile. in
#./Configure
# Make
# Make install
　　
　　 II. Use of Tcpdump
Tcpdump adopts the command line method. its command format is:
Tcpdump [-adeflnNOpqStvx] [-c quantity] [-F file name]
[-I network interface] [-r file name] [-s snaplen]
[-T type] [-w file name] [expression]
　　
1. Introduction to tcpdump options
-A converts a network address and broadcast address into a name;
-D. give the code that matches the information package in an assembly format that people can understand;
-Dd provides the code that matches the information package in the format of the C program segment;
-Ddd provides the matching information package code in decimal format;
-E prints the header information of the data link layer in the output line;
-F print the Internet address in numbers;
-L changes the standard output to the buffer row format;
-N does not convert the network address into a name;
-T no timestamp is printed on each output line;
-V outputs a slightly detailed information. for example, the IP package can contain ttl and service type information;
-Vv: output detailed message information;
-C. after receiving the specified number of packages, tcpdump stops;
-F read the expression from the specified file and ignore other expressions;
-I indicates the network interface of the listener;
-R reads packets from a specified file (these packets are generally generated using the-w option );
-W directly writes the package into the file and does not analyze or print it out;
-T directly interpret the listening packet as a specified type of message. Common types include rpc (remote process call) and snmp (Simple Network Management Protocol ;)
　　
2. Introduction to tcpdump expressions
The expression is a regular expression. tcpdump uses it as a condition for filtering packets. if a packet meets the expression conditions, the packet will be captured. If no conditions are provided, all information packets on the network will be intercepted.
In an expression, the following types of keywords are generally used. one is about the type of keywords, including host, net, and port. for example, host210.27.48.2 indicates that 210.27.48.2 is a host, 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 orsrc, 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 dst net 202.0.0.0 indicates that the destination network address is 202.0.0.0. If no direction keyword is specified, the src or dst 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 the three types of keywords, other important keywords are as follows: gateway, broadcast, less, greater, and there are three logical operations. The non-operation 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.
(1) all packets received and sent by all hosts 210.27.48.1 are to be intercepted:
# Tcpdump host 210.27.48.1
(2) 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 \)
(3) 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
(4) 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
　　
3. Introduction to output results of tcpdump
Below we will introduce the output information of several typical tcpdump commands.
(1) data link layer header information
Run the command # tcpdump -- e host ice
Ice is a linux host. her MAC address is 0: 90: 27: 58: AF: 1A.
H219 is a SUN workstation with SOLARIC. its MAC address is 8: 0: 20: 79: 5B: 46. the output result of the previous command is as follows:
21:50:12. 847509 eth0 ice. telne
T 0: 0 (0) ack 22535 win 8760 (DF)
Analysis: 21: 50: 12 indicates the display time, 847509 indicates the ID number, eth0 indicates to send data packets from the network interface device, 8: 0: 20: 79: 5b: 46 is the MAC address of host H219, which indicates that it is a packet sent from source address H219. 0: 90: 27: 58: af: 1a is the MAC address of the host ICE, indicating that the destination address of the data packet is ICE. ip indicates that the data packet is an IP data packet, and 60 indicates the length of the data packet, h219.33357> ice. telnet indicates that the packet is the TELNET (23) Port sent from Port 33357 of host H219 to host ICE. ack 22535 indicates to respond to a packet whose serial number is 222535. win 8760 indicates that the size of the sending window is 8760.
　　
(2) TCPDUMP output information of ARP packets
Run the command # tcpdump arp
The output result is:
22:32:42. 802509 eth0> arp who-has route tell ice (0: 90: 27: 58: af: 1a)
22:32:42. 802902 eth0 analysis: 22:32:42 is the timestamp, 802509 is the ID number, eth0> indicates that the packet is sent from the host, arp indicates that the packet is an ARP Request packet, who-has route tellice indicates that it is the MAC address of the host ROUTE requested by the host ICE. 0: 90: 27: 58: af: 1a is the MAC address of the host ICE.
　　
(3) TCP packet output information
The common output information of TCP packets captured with TCPDUMP is:
Src> dst: flags data-seqno ack window urgent options
Src> dst: Indicates from the source address to the destination address. flags indicates the flag information in the TCP packet, S indicates the SYN mark, F (FIN), P (PUSH), R (RST )". "(not marked); data-seqno is the sequence number of the data in the data packet, and ack is expected next time