Tcpdump Tutorial With Examples

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Tcpdump is packet sniffer for everyday use. There is lot of packet sniffers but tcpdump differs with his general availability and ease of use. Tcpdump use libcap library which is the core library used for packet sniffing. Here we will look general usage examples of packet sniffing. Be aware that to use tcpdump, tcpdump should have enough privilege and security mechanisms like selinux, apparmor should give permission. Captured data is generally written into file with pcap extension. Pcap files can be read and parsed with popular GUI based network tool Wireshark.

Man Tcpdump

More detailed information can be get from tcpdump man page like below.

Man Tcpdump

Man Tcpdump

List Interfaces

On a standard Linux system there is a lot of interfaces. I means not just network interface also USB interfaces exists. Tcpdump can listen USB protocol from USB interfaces and other special Kernel devices. Tcpdump numbers interfaces as optionally usage. First to select interface list interfaces tcpdump can sniff and capture.

List Interfaces

List Interfaces

As we see from screenshot tcpdump can sniff 10 interfaces on this Ubuntu system. Tcpdump also gives some information about interfaces status like up/down and connection type. There is a special interfaces type numbered 2 which named any where all interfaces traffic can be sniffed from this interfaces

Select Interface

To use one of listed interfaces interface name or index can be used. Here is a special interface named any which captures all interfaces.

OR

Capturing Specified Port with Tcpdump

Here we listen simply for http port

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Capturing Specified Port with Tcpdump

Capturing Specified Port with Tcpdump

Verbose Output

While capturing packets only IP header information is shown. To get more details about packets verbose option should be specified.

Verbose Output

Verbose Output

Increase Verbosity

To see more details than current details verbosity level can be increased with the -vvv by addind new to the option like below.

Increase Verbosity

Increase Verbosity

Decrement Verbosity

It can be decreased the default verbosity to get more elegant output for the packet sniffing with option -q

Decrement Verbosity

Decrement Verbosity

Display Captured Packets ASCII or Text

While capturing packets we may want to show them as ASCII or text. For example in HTTP traffic we can ouput HTTP headers directly to the console.

Print Captured Packets ASCII or Text

Print Captured Packets ASCII or Text

Display Captured Packets In Hex

Another way to display captured files is Hex format.

Display Captured Packets In Hex

Display Captured Packets In Hex

Display Port, Hosts As Number

By default port and ip numbers are displayed as text for example port 80 will shown as http or an ip address will shown as host name if it can be resolved. These information can be shown as numeric like below.

Display Port, Hosts As Number

Display Port, Hosts As Number

Set Packet Count To Capture

In some situations we want to limit captured packet count for example to identify a network traffic we generally need start of the TCP session. Specify count for captured packets here we set it to 5

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Set Packet Count To Capture

Set Packet Count To Capture

Set Packet Size To Capture

Another similar option to limiting capture is setting total capture size for each packet. Each packet can have different sizes this options will set standard size for each packet captured. Set size for captured packet , we set here 100 byte this is useful when redirecting captures to the file.

Set Packet Size To Capture

Set Packet Size To Capture

Capture Only Tcp Packets

Normally all network layers protocols are saved from Ethernet protocol to Application protocol. But only specific layer can be captured by specifying a protocol like TCP like below.

Capture Only Udp Packets

Like Tcp packet only Udp packets can be captured like below.

Capture Only Icmp Packets

This command will capture and sniff only Icmp packets.

Capture Only Arp Packets

Tcpdump have the ability to only capture Arp packets like below.

Capture Only IP Packets

As shown previous example we can capture specific protocol. In this example we will capture only IP packets.

Capture According To Destination IP

Another useful filter for network traffic while using tcpdump is filtering according to destination IP address. Destination IP address can be specified with dst  like below.

Capture According To Destination IP

Capture According To Destination IP

Capture According To Network

We have previously filtered captured packets according to host name. We can also use network filters to filter according to specified networks.

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Capture According To Network

Capture According To Network

Capture According To Port Range

Specifying multiple ports for filtering is not a practical solution. Tcpdump provides options to easily define port ranges with portrange option.

Capture According To Port Range

Capture According To Port Range

Saving Captured Packets To Pcap File

Captured packets information is printed to the console which is by default command line. If we want to save the to a file following command.Save captured files to the file named deneme.pcap . Actually there is a lot of supported capture file format but pcap is common usage. File

Saving Captured Packets To Pcap File

Saving Captured Packets To Pcap File

Reading Pcap Capture File

After saving capture file tcpdump can read the file and show output to the terminal with the following command.

Reading Pcap Capture File

Reading Pcap Capture File

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