51 lines
3.3 KiB
Markdown
51 lines
3.3 KiB
Markdown
# TCP Flags and Lengths
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| Flag Combination | Flags Set (Hex) | Purpose | IP+TCP Payload Size | Ethernet Frame Size | XDP Filtering Logic |
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|------------------|-----------------|---------|---------------------|---------------------|---------------------|
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| SYN | 0x02 | Initiates connection | 40–60 bytes | 64–78 bytes | Rate limit SYN packets per source IP to prevent floods. Drop if rate exceeds threshold. Use SynProxy. |
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| SYN-ACK | 0x12 | Acknowledges SYN | 40–60 bytes | 64–78 bytes | Validate against recent SYN requests using eBPF hash maps. Drop if no matching SYN. |
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| ACK | 0x10 | Acknowledges data | 40 bytes | 64 bytes | Allow for established connections. Rate limit to prevent ACK floods. |
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| PSH-ACK | 0x18 | Pushes data | 40–1500 bytes | 64–1518 bytes | Allow for data transfer. Rate limit large packets to prevent floods. Basic Connection tracking |
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| FIN-ACK | 0x11 | Closes connection | 40 bytes | 64 bytes | Allow for connection closure. Drop if part of a FIN scan (no prior SYN). |
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| RST | 0x04 | Resets connection | 40 bytes | 64 bytes | Allow for error handling. Drop if part of a reset attack (high rate from single IP). Basic Connection tracking |
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| NULL | 0x00 | Invalid (no flags) | 40 bytes | 64 bytes | Drop immediately as invalid/malicious. |
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| XMAS | 0x29 (FIN+PSH+URG) | Probing attack | 40 bytes | 64 bytes | Drop immediately as invalid/malicious. |
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| SYN-FIN | 0x03 | Invalid combination | 40 bytes | 64 bytes | Drop immediately as invalid. |
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| URG-ACK | 0x30 | Urgent data (rare) | 40 bytes | 64 bytes | Allow if rare, but monitor for anomalies. Note: URG is rarely used in modern applications. |
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| ACK-PSH-URG | 0x38 | Data with urgent flag | 40–1500 bytes | 64–1518 bytes | Allow for specific use cases, but rate limit to prevent abuse. |
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| SYN-RST | 0x06 | Invalid combination | 40 bytes | 64 bytes | Drop immediately as invalid. |
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**Notes**:
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- **IP+TCP Payload Size**: Includes 20 bytes IPv4 header + 20–40 bytes TCP header (with options like MSS or Window Scaling for SYN/SYN-ACK) + 0–1460 bytes data.
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- **Ethernet Frame Size**: Includes 14 bytes Ethernet header + 4 bytes FCS. Minimum frame size is 64 bytes, maximum is 1518 bytes (without VLAN tagging).
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- **URG Flag**: The URG flag is rarely used in modern protocols and should be closely monitored for potential misuse.
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# TCP 3-Way Handshake in Normal case
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```mermaid
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sequenceDiagram
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participant Client
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participant Server
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Client->>Server: SYN
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Note right of Server: Server receives SYN
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Server-->>Client: SYN-ACK
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Note left of Client: Client receives SYN-ACK
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Client->>Server: ACK
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Note right of Server: Connection established
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```
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# TCP 3-Way Handshake with Mitigation
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```mermaid
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sequenceDiagram
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participant Client
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participant Server
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Client->>Server: SYN (seq = x)
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Note right of Server: Server receives SYN, generates SYN cookie<br>Cookie = hash(source IP, source port, dest IP, dest port, seq, timestamp)
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Server-->>Client: SYN-ACK (seq = y, ack = x+1, cookie in seq)
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Note left of Client: Client receives SYN-ACK
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Client->>Server: ACK (seq = x+1, ack = y+1)
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Note right of Server: Server validates cookie<br>If valid, reconstructs state and establishes connection and save the IP for auto whitelisting
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Note right of Server: Connection established
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``` |