How to Use tcpdump to Capture and Analyze Network Traffic

Have you ever hosted a server (game or web) on your home computer and shared your IP address with friends, but no one could connect? The issue lies in your router, which hides connected devices behind its own IP address. Everything within the router is a local network, while everything outside is a global network. However, there is no direct mediator between them, only a barrier preventing external connections. The solution is port forwarding, a technology that directs external requests to an internal device and vice versa. In Linux operating systems, the iptables utility is used for this purpose, which will be the focus of this article. The commands shown in this guide were executed on a Hostman cloud server running Ubuntu 22.04. What Is Port Forwarding? Port forwarding (also known as port mapping) redirects network traffic from one port to another, either through a router (hardware-level) or a firewall (software-level). With port forwarding, devices within a local network become accessible from the global network. Without it, external requests cannot reach internal devices. Common scenarios where port forwarding is needed: Connecting to a home server (game server, surveillance cameras, data storage). Hosting game servers or websites on a home PC. Accessing a remote desktop. Remote device management. For example, if a server in a local network operates on port 8080, port forwarding allows it to be accessed from the global network through port 80. Example Setup: A computer with IP 192.168.1.100 (internal/gray IP) runs a web server listening on port 8080. The computer is within a Wi-Fi router’s local network, which has an external IP 203.0.113.10 (public/white IP), listening on port 80. All global network requests to port 80 on the router are forwarded to port 8080 on the internal computer. This setup allows us to redirect incoming traffic from the global network to the local network. How Does Port Forwarding Work in Linux? Linux has built-in tools for handling incoming and outgoing traffic. These tools act as a packet filtering and modification pipeline. Port forwarding in Linux is based on NAT (Network Address Translation), configured using the iptables system utility. What Is NAT? NAT (Network Address Translation) is a technique that converts external requests from the global network into internal requests within the local network (and vice versa). Technically, NAT modifies IP addresses and ports in data packets. It is not a standalone utility but a concept or approach. There are two main types of NAT: SNAT (Source NAT) – Modifies the source IP address in outgoing packets. DNAT (Destination NAT) – Modifies the destination IP address in incoming packets. While NAT protects the local network from external access, it requires port forwarding for incoming connections. What Is Iptables and How Does It Work? Iptables is a Linux utility used to configure NAT (and more) by modifying tables with rule chains that control traffic. Iptables has five main rule chains: INPUT – Handles incoming packets. FORWARD – Handles forwarded packets. OUTPUT – Handles outgoing packets. PREROUTING – Handles packets before routing. POSTROUTING – Handles packets after routing. Iptables has five tables, each using specific rule chains: filter – Allows or blocks packets (INPUT, FORWARD, OUTPUT). nat – Modifies IP addresses and ports (OUTPUT, PREROUTING, POSTROUTING). mangle – Alters packet headers (INPUT, FORWARD, OUTPUT, PREROUTING, POSTROUTING). raw – Controls connection filtering (OUTPUT, PREROUTING). security – Applies additional security policies (INPUT, FORWARD, OUTPUT). The rule chains act as hooks in the packet processing pipeline, allowing iptables to implement port forwarding in Linux. How Port Forwarding Works in Iptables Port forwarding in iptables follows a standard packet processing flow based on three possible directions: Incoming (INPUT) – Packets sent to the local system. Outgoing (OUTPUT) – Packets sent from the local system. Forwarded (FORWARD) – Packets routed through the system. Incoming Packets (INPUT) Processing Order raw (PREROUTING) – Connection filtering. mangle (PREROUTING) – Packet modification. nat (PREROUTING) – Changes the destination address. If the packet is for this system, continue to INPUT processing. Otherwise, forward it. mangle (INPUT) – Final packet modification. filter (INPUT) – Packet filtering. security (INPUT) – Security policy enforcement. Outgoing Packets (OUTPUT) Processing Order raw (OUTPUT) – Connection filtering. mangle (OUTPUT) – Packet modification. nat (OUTPUT) – Changes the destination address. filter (OUTPUT) – Final packet filtering. security (OUTPUT) – Security policy enforcement. mangle (POSTROUTING) – Final packet modification. nat (POSTROUTING) – Changes the source address. Forwarded Packets (FORWARD) Processing Order raw (PREROUTING) – Connection filtering. mangle (PREROUTING) – Packet modification. nat (PREROUTING) – Changes the destination address. Forwarding decision is made. mangle (FORWARD) – Packet modification. filter (FORWARD) – Packet filtering. security (FORWARD) – Security policy enforcement. mangle (POSTROUTING) – Final packet modification. nat (POSTROUTING) – Changes the source address. General Processing Order of Tables: raw mangle nat filter security Types of Port Forwarding Common types of port forwarding include: Local Forwarding – Redirects traffic within the same machine. Example: An application on a local server sends a request to a specific port. Interface Forwarding – Redirects traffic between different network interfaces. Example: A packet from the global network arrives on one interface and is forwarded to another. Remote Host Forwarding – Redirects traffic from a remote server to a local host. Example: A request from a remote server is forwarded to a local machine. Each type of port forwarding is implemented using a specific set of rules in the iptables tables. Using the Iptables Command In most Linux distributions, the iptables utility is already installed. You can check this by querying its version: iptables --version If iptables is not installed, you need to install it manually. First, update the package list: sudo apt update Then, install it: sudo apt install iptables -y By default, Linux uses the ufw firewall, which automatically configures iptables. To avoid conflicts, you must stop the ufw service first: sudo systemctl stop ufw Then, disable it: sudo systemctl disable ufw Iptables Command Structure The basic syntax of the iptables command is as follows: iptables [TABLE] [COMMAND] [CHAIN] [NUMBER] [CONDITION] [ACTION] In each specific command, only some of these parameters are used: TABLE: The name of one of the five tables where the rule is added. COMMAND: The operation to perform on a specific rule or chain. CHAIN: The name of the chain where the operation is performed. NUMBER: The rule number to manipulate. CONDITION: The condition under which the rule applies. ACTION: The transformation to be applied to the packet. Selecting a Table The -t flag specifies the table to operate within: For filter: iptables -t filter For nat: iptables -t nat For mangle: iptables -t mangle For raw: iptables -t raw For security: iptables -t security If the -t flag is not specified, the default table is filter. The security table is rarely used. Manipulating Rules We can perform different operations on rules within each chain: Add a rule to the end of a chain (-A): iptables -A INPUT -s 192.168.123.132 -j DROP This rule blocks incoming connections from the specified IP address. Delete a rule by its number (-D): iptables -D OUTPUT 7 Insert a rule at a specific position (-I): iptables -I INPUT 5 -s 192.168.123.132 -j DROP Replace a rule (-R): iptables -R INPUT 5 -s 192.168.123.132 -j ACCEPT This replaces a previously added blocking rule with an allow rule. Flush all rules in a chain (-F): iptables -F INPUT Manipulating Chains We can also perform operations on entire chains: Create a new chain (-N): iptables -N SOMENAME Delete a chain (-X): iptables -X SOMENAME Rename a chain (-E): iptables -E SOMENAME NEWNAME Set default policy for a chain (-P): iptables -P INPUT DROP This blocks all incoming connections to the server. Reset statistics for a chain (-Z): iptables -Z INPUT Setting Conditions Each rule can have conditions for its execution: Specify the protocol (-p): iptables -A INPUT -p tcp -j ACCEPT This allows incoming connections using the TCP protocol. Specify the source address (-s): iptables -A INPUT -s 192.168.123.132 -j DROP Specify the destination address (-d): iptables -A OUTPUT -d 192.168.123.132 -j DROP Specify network interface for incoming traffic (-i): iptables -A INPUT -i eth2 -j DROP Specify network interface for outgoing traffic (-o): iptables -A OUTPUT -o eth3 -j ACCEPT Specify the destination port (--dport): iptables -A INPUT -p tcp --dport 80 -j ACCEPT Specify the source port (--sport): iptables -A INPUT -p tcp --sport 1023 -j DROP Negate a condition (!): iptables -A INPUT ! -s 192.168.123.132 -j DROP This blocks all incoming connections except from the specified IP address. Specifying Actions Each table supports different actions: For the filter table: ACCEPT – Allow the packet. DROP – Block the packet. REJECT – Block the packet and send a response. LOG – Log packet information. RETURN – Stop processing in the current chain. For the nat table: DNAT – Change the packet’s destination address. SNAT – Change the packet’s source address. MASQUERADE – Change the source address dynamically. REDIRECT – Redirect traffic to the local machine. Port Forwarding with Iptables Local Port Forwarding To redirect local traffic from one port to another: sudo iptables -t nat -A PREROUTING -p tcp --dport 8080 -j REDIRECT --to-port 80 To remove the rule: sudo iptables -t nat -D PREROUTING -p tcp --dport 8080 -j REDIRECT --to-port 80 Forwarding Between Interfaces To forward port 8080 from interface eth0 to port 80 on eth1: sudo iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 8080 -j DNAT --to-destination 10.0.0.100:80 Then, allow packet forwarding: sudo iptables -A FORWARD -p tcp -d 10.0.0.100 --dport 80 -j ACCEPT Forwarding to a Remote Host To forward incoming packets to a remote server: Enable packet forwarding in the system settings: echo 1 > /proc/sys/net/ipv4/ip_forward Add a port forwarding rule: sudo iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 8080 -j DNAT --to-destination 192.168.1.100:80 Allow forwarded packets to be sent out: sudo iptables -t nat -A POSTROUTING -p tcp -d 192.168.1.100 --dport 80 -j MASQUERADE Alternatives to iptables for Port Forwarding It should be noted that iptables is not the only tool for traffic management. There are several popular alternatives. nftables nftables is a more modern tool for managing traffic in Linux. Unlike iptables, it does not have predefined tables, and its syntax is more straightforward and concise. Additionally, this utility uses a single command, nft, to manage all types of traffic: IPv4, IPv6, ARP, and Ethernet. In contrast, iptables requires additional commands such as ip6tables, arptables, and ebtables for these tasks. firewalld firewalld is a more complex traffic management tool in Linux, built around the concept of zones and services. This allows network resources to be assigned different levels of security. The configuration of firewalld is broader and more flexible. For example, instead of manually defining rules for each port, we can specify specific services. Additionally, firewalld provides a more interactive command-line interface, allowing real-time traffic management. Conclusion While there are alternatives, iptables remains the primary tool for traffic control in Linux. It provides a structured way to filter, modify, and forward packets, making it a powerful solution for managing network traffic.

The OAuth 2 is an authorization protocol designed to restrict access to personal accounts created on HTTP resources. Typical implementations include DigitalOcean, GitHub, and Facebook. OAuth 2 works by delegating the authentication process to the server where the account is located. This approach allows third-party applications to request access to user data, such as for registration using pre-filled credentials. OAuth 2 can be used in cloud services, desktop computers, and mobile applications for smartphones and tablets. In this guide, we will explore how OAuth works and discuss the main practical applications of this tool. OAuth Roles OAuth 2 defines four main roles: the resource owner, the client, the resource server, and the authorization server. Let’s go through each role in detail. Resource Owner The resource owner is the user who is authenticated using their credentials to access their personal account. The accessible area may have restricted permissions—either read-only or with write and modification capabilities. Server (Resource Server and Authorization Server) The resource server stores user account data in a protected form. The authorization server verifies the authenticity of entered login credentials and generates authorization tokens for applications. These tokens allow applications to access user data. Both servers are logically combined into a unified system, which is perceived by external services through the API interface. Thus, we will refer to the resource and authorization servers collectively as the Service or API. Client (Application) The client refers to the application requesting access to the user’s account. Before activation, two conditions must be met: authentication within the application and a positive response from the API. Protocol Overview Having reviewed the OAuth 2 roles, let's look at the authorization protocol and the information exchange process. Below is a sequence of typical operations: The application prompts the user to enter authentication credentials. If the login and password are correct, an authorization grant is issued. The application sends a request to the API, including the authorization grant. If the application is authenticated, the server generates an access token for the specific instance, completing the authorization process. The application requests resources using the API. The resource server validates the token and provides data only upon confirmation. The sequence may vary depending on the developer and the software’s purpose, but the general scheme remains consistent. Registering an Application Before using OAuth, an application must be registered with the service. This is typically done under the Developer or API section on the website. The following details are required: Application name Website where the application is hosted Callback URL (Redirect URL) The Callback URL is a link to the page that the service will open in case of access denial or after successful authorization. Client Identification Upon registering the application, user credentials are created—these include the Client ID and Client Secret: Client ID is a public string used by the API to verify the application's legitimacy and generate URLs for authorized users. Client Secret is used for authentication within the API and is only visible to the API and the application. Authorization Grants OAuth 2 provides four types of authorization grants depending on the situation: Authorization Code – Used for server-side applications. Implicit Grant – Suitable for mobile applications, including web versions. Resource Owner Password Credentials – Used for trusted applications, such as those integrated with an online service. Client Credentials – Used for API-based operations. Now, let's examine each type in more detail. Authorization Code Grant This is the most common authorization method, as it is secure and suitable for applications where the source code and client secret are stored on a protected server. Step 1: Create an Authorization Link The application presents the user with the following link: https://cloud.example.com/v1/oauth/authorize?response_type=code&client_id=CLIENT_ID&redirect_uri=CALLBACK_URL&scope=read Components: authorize endpoint: Used for OAuth authentication via API. client_id: Identifies the requesting application. redirect_uri: Redirects the user after authorization. response_type=code: Requests an authorization code. scope=read: Specifies read-only access. Step 2: User Authentication When the user clicks the link, they are prompted to log in. Upon successful authentication, the application is either authorized or denied access. Step 3: Authorization Code is Sent to the Application If authorization is granted, the system redirects the browser to: https://example.com/callback?code=AUTHORIZATION_CODE Step 4: Request an Access Token The application sends the authorization code to the server along with authentication details: https://cloud.example.com/v1/oauth/token?client_id=CLIENT_ID&client_secret=CLIENT_SECRET&grant_type=authorization_code&code=AUTHORIZATION_CODE&redirect_uri=CALLBACK_URL Step 5: Server Returns an Access Token If authentication is successful, the API returns an access token: { "access_token":"ACCESS_TOKEN", "token_type":"bearer", "expires_in":2592000, "refresh_token":"REFRESH_TOKEN", "scope":"read", "uid":100101, "info":{ "name":"User", "email":"info@example.com" } } The application is now connected to the server. Implicit Grant This method is used in mobile applications and web browsers. Unlike the Authorization Code Grant, it does not ensure the confidentiality of the client secret. Step 1: Generate an Authorization Link The authorization service provides a link: https://cloud.example.com/v1/oauth/authorize?response_type=token&client_id=CLIENT_ID&redirect_uri=CALLBACK_URL&scope=read Step 2: Authenticate the User Upon successful authentication, the server returns an access token directly to the browser: https://example.com/callback#token=ACCESS_TOKEN Step 3: Extract and Use the Token The application extracts the token from the URL and uses it for API requests. Resource Owner Password Credentials Grant This method involves the user entering credentials directly into the service. It is used when other methods are not viable, such as system-integrated applications. https://oauth.example.com/token?grant_type=password&username=USERNAME&password=PASSWORD&client_id=CLIENT_ID Upon successful authentication, the server returns an access token. Client Credentials Grant This method allows access to the service’s own resources: https://oauth.example.com/token?grant_type=client_credentials&client_id=CLIENT_ID&client_secret=CLIENT_SECRET If authentication is successful, an access token is returned. Refreshing an Access Token To refresh an access token before it expires: https://cloud.example.com/v1/oauth/token?grant_type=refresh_token&client_id=CLIENT_ID&client_secret=CLIENT_SECRET&refresh_token=REFRESH_TOKEN Conclusion We have explored the different ways to use OAuth in applications, depending on their requirements. Now, users understand how access to remote services is managed and what factors to consider when choosing an authorization method.

When working with networks in Linux, you may need to open or close a network port. Port management is essential for security — the fewer open ports in a system, the fewer potential attack vectors it has. Furthermore, if a port is closed, an attacker cannot gather information about the service running on that specific port. This guide will explain how to open or close ports as well as how to check open ports in Linux distributions such as Ubuntu/Debian and CentOS/RHEL using firewalls like ufw, firewalld, and iptables. It will also  We will demonstrate this process on two Linux distributions: Ubuntu 22.04 and CentOS 9, run on Hostman VPS. All commands provided here will work on any Debian-based or RHEL-based distributions. What is a Network Port? Ports are used to access specific applications and protocols. For example, a server can host both a web server and a database—ports direct traffic to the appropriate service. Technically, a network port is a non-negative integer ranging from 0 to 65535. Reserved Ports (0-1023): Used by popular protocols and network services like SSH (port 22), FTP (port 21), HTTP (port 80), and HTTPS (port 443). Registered Ports (1024-49151): These ports can be used by specific applications for communication. Dynamic Ports (49151-65535): These are used for temporary connections and can be dynamically assigned to applications. How to Open Ports in Debian-Based Linux Distributions On Debian-based systems (Ubuntu, Debian, Linux Mint, etc.), you can use ufw (Uncomplicated Firewall). ufw comes pre-installed on most popular APT-based distributions. To check if ufw is installed, run: ufw version If the version is displayed, ufw is installed. Otherwise, install it with: apt update && apt -y install ufw By default, ufw is inactive, meaning all ports are open. You can check its status with: ufw status To activate it, use: ufw enable You will need to confirm by entering y. Note that enabling ufw may interrupt current SSH connections. By default, ufw blocks all incoming traffic and allows all outgoing traffic. To check the default policy, use: cat /etc/default/ufw Opening Ports in ufw To open a port, use the command: ufw allow <port_number> For example, to open port 22 for SSH, run: ufw allow 22 You can list multiple port numbers separated by commas, followed by the protocol (tcp or udp): ufw allow 80,443,8081,8443/tcpufw allow 80,443,8081,8443/udp Instead of specifying port numbers, you can use the service name as defined in /etc/services. For example, to open the Telnet service, which uses port 23 by default: ufw allow telnet Note: You cannot specify multiple service names at once; ufw will return an error: To open a port range, use the following syntax: ufw allow <start_port>:<end_port>/<protocol> Example: ufw allow 8000:8080/tcp Closing Ports in ufw To close a port using ufw, use the command: ufw deny <port_number> For example, to close port 80, run: ufw deny 80 You can also use the service name instead of the port number. For example, to close port 21 used by the FTP protocol: ufw deny ftp Checking Open Ports in ufw To list all open and closed ports in the Linux system, use: ufw status Another option to view open ports in Linux is: ufw status verbose How to Open a Port in RHEL-Based Linux Distributions Linux RHEL-based distributions (CentOS 7+, RHEL 7+, Fedora 18+, OpenSUSE 15+) use firewalld by default. Opening Ports in firewalld To check if firewalld is installed, run: firewall-offline-cmd -V If the version is displayed, firewalld is installed. Otherwise, install it manually: dnf install firewalld By default, firewalld is disabled. Check its status with: firewall-cmd --state To enable firewalld, run: systemctl start firewalld To open port 8080 for the TCP protocol, use: firewall-cmd --zone=public --add-port=8080/tcp --permanent --zone=public: Specifies the zone for the rule. --add-port=8080/tcp: Specifies the port and protocol (TCP or UDP). --permanent: Saves the rule to persist after a system reboot. Without this parameter, the change will only last until the next reboot. Alternatively, you can open a port in Linux by specifying a service name instead of a port number. For example, to open the HTTP (port 80) protocol: firewall-cmd --zone=public --add-service=http --permanent Reload firewalld to apply the changes: firewall-cmd --reload Closing Ports in firewalld You can close a port using either its number or service name. To close a port using its number, run: firewall-cmd --zone=public --remove-port=8080/tcp --permanent To close a port using the service name, run: firewall-cmd --zone=public --remove-service=http --permanent After opening or closing a port, always reload firewalld to apply the changes: firewall-cmd --reload Listing Open Ports in firewalld To list all open ports in your Linux system, you can use: firewall-cmd --list-ports Managing Ports in iptables Unlike ufw and firewalld, iptables comes pre-installed in many Linux distributions, including Ubuntu, Debian, RHEL, Rocky Linux, and AlmaLinux. Opening Ports in iptables To open port 8182 for incoming connections, use: iptables -A INPUT -p tcp --dport 8182 -j ACCEPT -A INPUT: The -A flag is used to add one or more rules. INPUT specifies the chain to which the rule will be added (in this case, incoming connections). -p tcp: Specifies the protocol. Supported values include tcp, udp, udplite, icmp, esp, ah, and sctp. --dport 8182: Specifies the port to be opened or closed. -j ACCEPT: Defines the action for the port. ACCEPT allows traffic through the port. To open a port for outgoing connections, use the OUTPUT chain instead: iptables -A OUTPUT -p tcp --dport 8182 -j ACCEPT To open a range of ports, use the --match multiport option: iptables -A INPUT -p tcp --match multiport --dports 1024:2000 -j ACCEPT Closing Ports in iptables To close a port, use the -D option and set the action to DROP. For example, to close port 8182 for incoming connections: iptables -A INPUT -p tcp --dport 8182 -j DROP To close a range of ports, use the same syntax as for opening a range, but replace ACCEPT with DROP: iptables -A INPUT -p tcp --match multiport --dports 1024:2000 -j DROP Saving iptables Rules By default, iptables rules are only effective until you restart the server. To save the rules permanently, install the iptables-persistent utility. For APT-based distributions: apt update && apt -y install iptables-persistent For DNF-based distributions: dnf -y install iptables-persistent To save the current rules, run: iptables-save After the next server reboot, the rules will be automatically reloaded. Viewing Open Ports in iptables To list all current rules and opened ports on the Linux machine, use: iptables -L -v -n To list rules specifically for IPv4, use: iptables -S To list rules for IPv6, use: ip6tables -S Conclusion In this guide, we demonstrated how to open and close network ports in Linux and check currently open ports using three different utilities: ufw, firewalld, and iptables. Proper port management reduces the risk of potential network attacks and helps obscure information about the services using those ports.