Linux Networking TLS

This project was built to deeply understand how TCP and TLS operate over a routed Layer-3 network.

Instead of using physical hardware, Linux network namespaces were used to simulate:

• Isolated network nodes (client, router, server)
• Two different IP subnets
• IP forwarding across a software router
• Real packet-level protocol behavior

The objective was to analyze:

• TCP 3-way handshake
• TLS 1.2 handshake workflow
• Certificate exchange in plaintext
• Encryption boundary after ChangeCipherSpec
• TTL decrement across a router (Layer-3 proof)

This project was later extended to include Mutual TLS (mTLS) to demonstrate two-way authentication, where both client and server verify each other's identity using certificates.

This project provides a hands-on, packet-level understanding of secure communication mechanisms used in modern distributed systems and zero-trust architectures.

Topology

• Default gateway (red): 10.0.1.1
• Default gateway (blue): 10.0.2.1
• IP forwarding enabled in router namespace

• Ubuntu 22.04 (or compatible Linux system)
• iproute2
• OpenSSL
• tcpdump
• Wireshark (host machine)

./generate_certs.sh

sudo ./setup.sh

sudo ip netns exec red ping 10.0.2.2

Expected observation:

• Successful ping replies
• TTL decreases from 64 → 63 (proof of router traversal)

sudo ./cleanup.sh

This section demonstrates capturing and analyzing the TLS handshake at packet level in a clean, linear workflow.

sudo ip netns exec router tcpdump -i any port 4433 -w tls_capture.pcap

Keep this running.

(Open a new terminal)

sudo ip netns exec blue openssl s_server \
 -accept 4433 \
 -cert /certs/server.crt \
 -key /certs/server.key \
 -tls1_2

Expected output:

ACCEPT

(Open another terminal)

sudo ip netns exec red openssl s_client \
 -connect 10.0.2.2:4433 \
 -CAfile /certs/ca.crt \
 -tls1_2

This initiates the TLS handshake.

Press:

Ctrl + C

in the tcpdump terminal.

Open the generated .pcap file in Wireshark.

tls
tcp.port == 4433

• TCP 3-way handshake occurs before TLS begins
• TLS handshake messages visible:
  • ClientHello
  • ServerHello
  • Certificate
  • ServerHelloDone
  • ClientKeyExchange
• Encryption starts after ChangeCipherSpec
• Subsequent packets appear as:

  TLS Application Data
  

This section demonstrates mutual authentication where both client and server verify each other using certificates.

sudo ip netns exec router tcpdump -i any port 4433 -w mtls_capture.pcap

Keep this running.

sudo ip netns exec blue openssl s_server \
 -accept 4433 \
 -cert /certs/server.crt \
 -key /certs/server.key \
 -CAfile /certs/ca.crt \
 -Verify 1 \
 -tls1_2

sudo ip netns exec red openssl s_client \
 -connect 10.0.2.2:4433 \
 -cert /certs/client.crt \
 -key /certs/client.key \
 -CAfile /certs/ca.crt \
 -tls1_2

Expected result:

• Handshake succeeds
• Client and server authenticate each other

sudo ip netns exec red openssl s_client \
 -connect 10.0.2.2:4433 \
 -CAfile /certs/ca.crt

Expected result:

• Handshake fails
• Server rejects client

Press:

Ctrl + C

in the tcpdump terminal.

Open the generated .pcap file in Wireshark.

tls
tcp.port == 4433

• Server sends Certificate Request
• Client responds with its Certificate
• Client proves identity using Certificate Verify
• Mutual authentication is established
• Without client certificate → handshake failure

After ChangeCipherSpec, Wireshark shows:

TLS Application Data

This applies to both TLS and mTLS, indicating the transition from asymmetric to symmetric encryption.

Initial TTL: 64
Observed TTL: 63

network_namespaces/
│
├── blue_namespace/
│ ├── server.crt
│ ├── server.key
│ └── ca.crt
│
├── red_namespace/
│ ├── client.crt
│ ├── client.key
│ └── ca.crt
│
├── ca/
│ ├── ca.crt
│ └── ca.key
│
├── diagrams/
│ └── topology.png
│
├── screenshots/
│
├── report/
│ └── report.pdf
│
├── setup.sh
├── cleanup.sh
├── generate_certs.sh
├── README.md
├── tls_capture.pcap
└── mtls_capture.pcap

Self-signed certificates are used for educational purposes only.

• Layer-3 routing using namespaces
• TCP handshake understanding
• TLS 1.2 internals
• Mutual TLS (mTLS) authentication
• Certificate Authority (CA) concepts
• Packet-level analysis with Wireshark

https://nitkeduin-my.sharepoint.com/:v:/g/personal/tahiliani_nitk_edu_in/EZsxo6VafiBIn3ybNUNOYPYBJ9Oe7nvBMFc81vTTC-FhtQ?e=b16yGn

Samyak Gedam
National Institute of Technology Surathkal, Karnataka
Built as part of first task in mini-project course during my 2nd Semester.