Выполнена работа №1

linux1/application/app.py

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+from flask import Flask, request, jsonify
+
+app = Flask(__name__)
+
+
+@app.route("/get", methods=["GET"])
+def handle_get():
+    return jsonify({"method": "GET", "message": "Hello from GET endpoint"}), 200
+
+
+@app.route("/post", methods=["POST"])
+def handle_post():
+    data = request.get_json(silent=True) or {}
+    return jsonify({"method": "POST", "message": "Hello from POST endpoint", "received": data}), 200
+
+
+@app.route("/put", methods=["PUT"])
+def handle_put():
+    data = request.get_json(silent=True) or {}
+    return jsonify({"method": "PUT", "message": "Hello from PUT endpoint", "received": data}), 200
+
+
+if __name__ == "__main__":
+    app.run(host="0.0.0.0", port=5000)

linux1/configs/linux_a/netplan/00-installer-config.yaml

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+network:
+  ethernets:
+    enp0s3:
+      # Bridge adapter — access from host machine
+      dhcp4: true
+    enp0s8:
+      # Internal network "servernet" — communication with VM B
+      dhcp4: no
+      addresses: [192.168.13.10/24]
+      gateway4: 192.168.13.1
+  version: 2

linux1/configs/linux_a/systemd/web-server.service

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+[Unit]
+Description=Flask Web Server
+After=network.target
+
+[Service]
+Type=simple
+User=kulakov_1
+WorkingDirectory=/home/kulakov_1/server/
+ExecStart=/usr/bin/python3 /home/kulakov_1/server/app.py
+Restart=on-failure
+RestartSec=3
+
+[Install]
+WantedBy=multi-user.target

linux1/configs/linux_b/iptables/setup_iptables.sh

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+#!/bin/bash
+# IP Forwarding — enable kernel packet forwarding between interfaces
+echo 1 > /proc/sys/net/ipv4/ip_forward
+sed -i 's/#net.ipv4.ip_forward=1/net.ipv4.ip_forward=1/' /etc/sysctl.conf
+
+# Allow NEW connections from clientnet (enp0s9) to servernet (enp0s8) only on port 5000
+iptables -A FORWARD -i enp0s9 -o enp0s8 -p tcp --syn --dport 5000 -m conntrack --ctstate NEW -j ACCEPT
+
+# Allow established/related packets in both directions
+iptables -A FORWARD -i enp0s9 -o enp0s8 -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
+iptables -A FORWARD -i enp0s8 -o enp0s9 -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
+
+# Drop everything else in FORWARD chain
+iptables -P FORWARD DROP
+
+# NAT: masquerade packets going from clientnet to servernet
+iptables -t nat -A POSTROUTING -o enp0s8 -j MASQUERADE
+
+# Save rules so they persist after reboot
+apt-get install -y iptables-persistent
+iptables-save > /etc/iptables/rules.v4
+ip6tables-save > /etc/iptables/rules.v6
+
+echo "iptables rules applied and saved."

linux1/configs/linux_b/netplan/00-installer-config.yaml

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+network:
+  ethernets:
+    enp0s3:
+      # Bridge adapter — access from host machine
+      dhcp4: true
+    enp0s8:
+      # Internal network "servernet" — gateway side facing VM A
+      dhcp4: no
+      addresses: [192.168.13.1/24]
+    enp0s9:
+      # Internal network "clientnet" — gateway side facing VM C
+      dhcp4: no
+      addresses: [192.168.8.1/24]
+  version: 2

linux1/configs/linux_c/netplan/00-installer-config.yaml

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+network:
+  ethernets:
+    enp0s3:
+      # Bridge adapter — access from host machine
+      dhcp4: true
+    enp0s8:
+      # Internal network "clientnet" — communication with VM B
+      dhcp4: no
+      addresses: [192.168.8.10/24]
+      gateway4: 192.168.8.1
+  version: 2

linux1/docker-compose.yml

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+services:
+  linux_a:
+    image: alpine:3.19
+    container_name: kulakov_server
+    hostname: kulakov_server
+    networks:
+      servernet:
+        ipv4_address: 192.168.13.10
+    privileged: true
+    tty: true
+    volumes:
+      - ./application:/app
+      - ./scripts/setup_linux_a.sh:/setup.sh
+    entrypoint: sh /setup.sh
+
+  linux_b:
+    image: alpine:3.19
+    container_name: kulakov_gateway
+    hostname: kulakov_gateway
+    networks:
+      servernet:
+        ipv4_address: 192.168.13.1
+      clientnet:
+        ipv4_address: 192.168.8.1
+    privileged: true
+    tty: true
+    volumes:
+      - ./scripts/setup_linux_b.sh:/setup.sh
+    entrypoint: sh /setup.sh
+
+  linux_c:
+    image: alpine:3.19
+    container_name: kulakov_client
+    hostname: kulakov_client
+    networks:
+      clientnet:
+        ipv4_address: 192.168.8.10
+    privileged: true
+    tty: true
+    volumes:
+      - ./scripts/setup_linux_c.sh:/setup.sh
+    entrypoint: sh /setup.sh
+    depends_on:
+      - linux_a
+      - linux_b
+
+networks:
+  servernet:
+    driver: bridge
+    ipam:
+      config:
+        - subnet: 192.168.13.0/24
+          gateway: 192.168.13.254
+  clientnet:
+    driver: bridge
+    ipam:
+      config:
+        - subnet: 192.168.8.0/24
+          gateway: 192.168.8.254