• If a client:port1 sends a SYN packet to server:port2, and an ACK is sent fromclient:port1 to server:port2 after that SYN packet, then this connection isestablished.If a client:port1 sends a SYN to server:port2, and then there is no ACKbelonging to this session from client:port1 to server:port2, then thisconnection is failed.Then this engineer uses these two rules to identify failed connections and use thenumber of failed connections to detect scanning behaviors.If an attacker knows these two rules, how can he/she perform effective scanningattacks and meanwhile evade these two detection rules? Three-way handshake is used by a TCP client and a TCP server to establish aconnection, as illustrated below:1st: client:port1 -> server:port2, SYN2nd: server:port2 -> client:port1, SYNACK3rd: client:port1 -> server:port2, ACKWhen this client is performing scanning attacks, it will generated a large number offailed connections. In each failed connection, the three-way handshake fails tocomplete. People commonly use SYN together with the absence of its correspondingSYNACK in this same TCP session to identify whether this connection is failed.By investigating the failed connections, an engineer finds that in legitimate/benigncases, if the server does not return SYNACK to the client, the client will not send theACK packet after SYNACK (e.g., the 3rd packet above). Therefore, this engineersuggests that we can count the failed connections based on the following ruleswithout considering SYNACK:

Answer: We need to explain the when attacker send the data packer with and without ACK then what…

• If a client:port1 sends a SYN packet to server:port2, and an ACK is sent from client:port1 to server:port2 after that SYN packet, then this connection is established. If a client:port1 sends a SYN to server:port2, and then there is no ACK belonging to this session from client:port1 to server:port2, then this connection is failed. Then this engineer uses these two rules to identify failed connections and use the number of failed connections to detect scanning behaviors. If an attacker knows these two rules, how can he/she perform effective scanning attacks and meanwhile evade these two detection rules?

• If a client:port1 sends a SYN packet to server:port2, and an ACK is sent from client:port1 to server:port2 after that SYN packet, then this connection is established. If a client:port1 sends a SYN to server:port2, and then there is no ACK belonging to this session from client:port1 to server:port2, then this connection is failed. Then this engineer uses these two rules to identify failed connections and use the number of failed connections to detect scanning behaviors. If an attacker knows these two rules, how can he/she perform effective scanning attacks and meanwhile evade these two detection rules?

Database System Concepts

7th Edition

ISBN:9780078022159

Author:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan

Publisher:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan

Chapter1: Introduction

Section: Chapter Questions

Problem 1PE

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Question

• If a client:port1 sends a SYN packet to server:port2, and an ACK is sent from
client:port1 to server:port2 after that SYN packet, then this connection is
established.
If a client:port1 sends a SYN to server:port2, and then there is no ACK
belonging to this session from client:port1 to server:port2, then this
connection is failed.
Then this engineer uses these two rules to identify failed connections and use the
number of failed connections to detect scanning behaviors.
If an attacker knows these two rules, how can he/she perform effective scanning
attacks and meanwhile evade these two detection rules?

Three-way handshake is used by a TCP client and a TCP server to establish a
connection, as illustrated below:
1st: client:port1 -> server:port2, SYN
2nd: server:port2 -> client:port1, SYNACK
3rd: client:port1 -> server:port2, ACK
When this client is performing scanning attacks, it will generated a large number of
failed connections. In each failed connection, the three-way handshake fails to
complete. People commonly use SYN together with the absence of its corresponding
SYNACK in this same TCP session to identify whether this connection is failed.
By investigating the failed connections, an engineer finds that in legitimate/benign
cases, if the server does not return SYNACK to the client, the client will not send the
ACK packet after SYNACK (e.g., the 3rd packet above). Therefore, this engineer
suggests that we can count the failed connections based on the following rules
without considering SYNACK:

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