how to read a tcpdump

Absolutely, here's the full and revised explanation with the example:

An example TCP dump looks like this: timestamp source IP > destination IP.protocol : flags [TCP flags], seq sequence numbers, ack acknowledgement number, win window size, options [TCP options], length payload length : payload

Here's an actual example: 12:14:35.783589 IP ip.your.machine > domain.com.http: Flags [P.], seq 1:74, ack 1, win 512, options [nop,nop,TS val 3302576859 ecr 3302576859], length 73: HTTP: GET / HTTP/1.1

TCP Flag codes include:

Flags [S] - SYN: Synchronization sequence numbers to initiate a connection

Flags [F] - FIN: Finish, used to close a connection

Flags [P] - PSH: Push function is utilized

Flags [R] - RST: Reset the connection

Flags [.] - ACK: Acknowledgment

Here's a breakdown of the example:

12:14:35.783589 IP ip.your.machine > domain.com.http: Flags [P.], seq 1:74, ack 1, win 512, options [nop,nop,TS val 3302576859 ecr 3302576859], length 73: HTTP: GET / HTTP/1.1

12:14:35.783589: The timestamp of when the packet was captured.

IP: The protocol of the packet. In this case, it's IP.

ip.your.machine: The source IP address.

> domain.com.http: The destination IP address and the protocol (HTTP in this case).

Flags [P.]: The TCP flags for this packet. "P." stands for PSH (Push) and ACK (Acknowledgment).

seq 1:74: The sequence number for this packet. This packet is sending bytes 1 through 74.

ack 1: The acknowledgement field. This is the next sequence number that the sender of the ACK is expecting. It's the sequence number plus the segment length received in the last packet.

win 512: The window size, indicating the number of bytes that can be received before needing to send an acknowledgment.

options [nop,nop,TS val 3302576859 ecr 3302576859]: The TCP options for this packet. It includes two No-Operation (nop) options and a Timestamp (TS) option with value (val) 3302576859 and echo reply (ecr) 3302576859.

length 73: The length of the payload (in bytes).

HTTP: GET / HTTP/1.1: The payload itself, which is an HTTP GET request in this case.

 

Regarding the options:

nop: No Operation. It's used for alignment purposes and doesn't carry any information.

TS val 3302576859: This is the Timestamp value. It's the value of the sender's timestamp clock when this segment was sent.

ecr 3302576859: This is the Echo Reply timestamp. It's the timestamp value that was received in the TSval field of the segment being acknowledged.