Packet capture parameters of the tcpdump command

The tcpdump command parameter uses the command line method. its command format is: tcpdump [-adeflnNOpqStvx] [-c quantity] [-F file name] [-I network interface] [-r file name] [-ssnaplen] [-T type] [-w file name] [expression] 1. introduction to tcpdump options-a Changes the network address and broadcast address

TcpdumpCommand parameters

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

-      Converts a network address and broadcast address into a name;

-D      The code that matches the information package is given in a compilation format that people can understand;

-Dd    The code that matches the information package is given in the format of the C language program segment;

-Ddd  The matching information package code is given in decimal format;

-E      Print 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      Do not convert the network address into a name;

-T      No timestamp is printed on each output line;

-V      Output a slightly detailed information. for example, the IP package can contain ttl and service type information;

-Vv    Outputs detailed message information;

-C      After receiving the specified number of packages, tcpdump stops;

-F            Reads expressions from the specified file and ignores other expressions;

-I     Network interface of the listener;

-R      Reads packages from a specified file (these packages are generally generated using the-w option );

-W     Directly write the package into the file, without analysis and printing;

-T            The packets monitored are directly interpreted as specified types of packets. 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 types of keywords, including host,

Net, port, for example, host 210.27.48.2, indicating that 210.27.48.2 is a host, and net 202.0.0.0 indicates

202.0.0.0 is a network address and 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,

These keywords 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, dst net 202.0.0.0 indicates that the destination network address is 202.0.0.0. If no direction keyword is specified

The default value is the src or dst keyword.

The third type is the protocol keyword, which mainly includes fddi, ip, arp, rarp, tcp, udp, and other types. Fddi indicates that

The specific network protocol on FDDI (distributed optical fiber data interface network) is actually the alias of "ether", fddi and e

Ther has a similar source address and destination address, so you can use the fddi protocol package as the ether package for processing and analysis.

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, there are three logical operations. The non-operation is 'not ''! ', And the operation is 'and',' & '; or the operation is 'o

R', '| ';

These keywords can be combined to form a powerful combination condition to meet people's needs. The following are several examples:

Description.

(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 using parentheses in the command line, be sure to use a backslash. for example:

# 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. 847509eth0 <8: 0: 20: 79: 5b: 46 0: 90: 27: 58: af: 1a ip 60: h219.33357> ice. telnet 0: 0 (0) ack 22535 win 8760 (DF)

   Analysis: 21: 50: 12 indicates the display time, 847509 indicates the ID number, and eth0 indicates that eth0 accepts the data packet from the network interface, eth0> indicates that the data packet is sent from the network interface device, 8: 0: 20: 79: 5b: 46 is the MAC address of the host H219. it indicates that it is a packet sent from the 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. ack22535 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, and F (F

IN), P (PUSH), R (RST) "." (not marked); data-seqno is the sequence number of data IN the data packet, and ack is

The sequence number expected next time. window indicates the size of the window that receives the cache. urgent indicates whether there is an emergency pointer in the data packet.

Options is an option.

(4) UDP packet output information

The general output information of the UDP packet captured with TCPDUMP is:

Route. port1> ice. port2: udp lenth

UDP is very simple. the output line above indicates a UDP packet sent from the port1 port of the host ROUTE to the port2 port of the host ICE. the type is UDP and the package length is lenth.