A Complete Guide to the nslookup Command in Linux and Windows

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.

The iptables utility allows you to manage the network firewall in Linux distributions. iptables is a popular command-line utility for interacting with the built-in Linux kernel firewall called Netfilter, which has been included in the Linux kernel since version 2.4.  In this article, we will examine how iptables works and go through practical usage examples. Installing iptables As mentioned, iptables is included in nearly all Linux distributions, from the most common (Ubuntu, Debian, RHEL) to distributions like openSUSE, Arch Linux, Gentoo, and others. First, let's check if iptables is already installed on your cloud server by displaying its version with the command: iptables --version If this command returns a version number, iptables is already installed on the system. However, if you see the message iptables: command not found, you’ll need to install it manually. Below are instructions for installing iptables using package managers across various Linux distributions. Alternatively, you can compile and install iptables from the source code. APT For APT-based distributions (Ubuntu/Debian/Linux Mint/Kali Linux), use the command: apt -y install iptables RPM For RPM-based distributions (CentOS, Fedora, Red Hat Enterprise Linux, ALT Linux), use one of the following commands: For the YUM package manager: yum -y install iptables For the DNF package manager: dnf -y install iptables Pacman For Pacman-based distributions (Arch Linux, ArchLabs, Manjaro), use the command: pacman -S iptables All commands must be run as the root user or as a regular user with sudo privileges. How iptables Works iptables operates using a system of rules. These rules control incoming and outgoing traffic, organized into chains that either allow or block traffic. A more detailed breakdown of how iptables works is as follows: Network packets pass through one or more chains. As a network packet moves through a chain, each rule in that chain is applied to it. During this process, the packet is checked against specified criteria. If it does not meet a criterion, a specific action is applied to it. These actions can include allowing or blocking traffic, among other operations. Key iptables Terminology While working with iptables, you may encounter the following terms: Chain: A sequence or set of rules that determine how traffic will be handled. Rules: Defined actions that contain criteria and a target or goal. Module: An added feature that provides extra options for iptables, allowing for more extensive and complex traffic filtering rules. Table: An abstraction in iptables that stores chains of rules. iptables includes the following tables: Security, Raw, NAT, Filter, and Mangle. Each table has a specific function, described below. iptables Tables Filter Table The Filter table is the default table, using three chains: OUTPUT, FORWARD, and INPUT. INPUT: Controls incoming connections. For instance, this might manage incoming SSH connections. FORWARD: Manages incoming connections not directed to the local device, typically used on a router. OUTPUT: Controls outgoing connections, such as navigating to a website using a browser. NAT Table The NAT (Network Address Translation) table includes three chains: PREROUTING, POSTROUTING, and OUTPUT. PREROUTING: Determines the destination IP address of a packet. POSTROUTING: Alters the source IP address. OUTPUT: Changes the target address of outgoing packets. Mangle Table The Mangle table is used to modify packet IP headers. Raw Table The Raw table provides a mechanism for marking packets to bypass connection tracking. Security Table The Security table enables interaction with various OS security mechanisms, such as SELinux. iptables Rules The rules in iptables are designed to control incoming and outgoing network traffic. Rules can also be used to configure port forwarding and create protocol-specific rules. Each rule is made up of criteria and a target. The criteria of a rule are matched, and the specified actions are applied to the target object. If a packet doesn’t match a rule’s criteria, the next rule is processed. The decisions made by iptables are called actions. Below is a list of key actions for handling connections: ACCEPT: Opens (allows) the connection. DROP: Closes the connection without sending a response to the client. QUEUE: Sends the packet to a queue for further processing by an external application. RETURN: Returns the packet to the previous rule, stopping the processing of the current rule. REJECT: Blocks the connection and sends an error message in response. DENY: Drops the incoming connection without sending a response. ESTABLISHED: Marks an already established connection, as the session has already received at least one packet Practical Application of iptables Let's look at using iptables in practice. All the commands below will work on any Linux distribution. iptables commands must be run as the root user or a regular user with sudo privileges. To display the current iptables configuration (including all existing rules), use the command: iptables --list For a more detailed output, which includes the number and size of processed packets in the INPUT, FORWARD, and OUTPUT chains, along with IP addresses and port numbers in numeric format, use: iptables --line-numbers -L -v -n You can also specify a specific chain to display rules for just that chain, such as: iptables -L INPUTiptables -L FORWARDiptables -L OUTPUT Initially, iptables does not create or store any rule chains, so the output of these commands may be empty. Blocking IP Addresses To block a specific IP address, add a rule to the INPUT chain and specify the appropriate table. In the command below, the table is explicitly set. If the -t option is omitted, the rule is added to the default Filter table. For example, to block the IP address 10.0.36.126: iptables -t filter -A INPUT -s 10.0.36.126 -j REJECT This command uses the following options: -t: Specifies the table for the rule. -A: Adds the rule to the specified chain, in this case, the INPUT chain. -s: Specifies the source IP address to which the action applies. -j: Specifies the action to take; here, traffic is rejected (action REJECT). To block an entire subnet, specify it with the -s option: iptables -A INPUT -s 10.0.36.0/24 -j REJECT Or, you can specify the subnet mask in full format: iptables -A INPUT -s 10.0.36.0/255.255.255.0 -j REJECT To block outgoing traffic to a specific IP address, use the OUTPUT chain and the -d option: iptables -A OUTPUT -d 10.0.36.126 -j REJECT Blocking Ports Ports can be blocked by specifying them directly. This is done with the --dport option, which designates the port of the service. Instead of a port number, you can use the service name. You must specify the protocol as well. For example, to block SSH connections from host 10.0.36.126 using the TCP protocol: iptables -A INPUT -p tcp --dport ssh -s 10.0.36.126 -j REJECT For the UDP protocol, use: iptables -A INPUT -p udp --dport ssh -s 10.0.36.126 -j REJECT Alternatively, to block SSH connections from 10.0.36.126 using the SSH service port (22), use: iptables -A INPUT -p tcp --dport 22 -s 10.0.36.126 -j REJECT To block SSH connections from any IP address over TCP: iptables -A INPUT -p tcp --dport ssh -j DROP Allowing an IP Address To allow traffic from a specific IP address, use the ACCEPT action. In the example below, all traffic from the IP address 10.0.36.126 is allowed: iptables -A INPUT -s 10.0.36.126 -j ACCEPT To allow traffic from a specific range of IP addresses, for example, from 10.0.36.126 to 10.0.36.156, use the iprange module and the --src-range option: iptables -A INPUT -m iprange --src-range 10.0.36.126-10.0.36.156 -j ACCEPT Here: iprange: A module for working with IP address ranges. --src-range: Specifies the source IP address range. To perform the reverse operation (allowing all traffic from the server to a specific IP range from 10.0.36.126 to 10.0.36.156), use the --dst-range option: iptables -A OUTPUT -m iprange --dst-range 10.0.36.126-10.0.36.156 -j ACCEPT --dst-range: Specifies the destination IP address range. Opening Ports To open a port, specify the protocol using the -p option. Supported protocols include tcp, udp, etc. A full list of supported protocols can be found in /etc/protocols: cat /etc/protocols Specify the port using the --dport option. You can use either numeric values or service names. The ACCEPT action is used to open ports. To open port 22 for TCP traffic from IP address 10.0.36.126: iptables -A INPUT -p tcp --dport 22 -s 10.0.36.126 -j ACCEPT To open multiple ports at once, use the multiport module and the --dports option, listing the ports separated by commas. For example, to open ports 22, 80, and 443 over TCP from IP address 10.0.36.126: iptables -A INPUT -p tcp -m multiport --dports 22,80,443 -s 10.0.36.126 -j ACCEPT multiport: A module for managing multiple ports simultaneously. --dports: Specifies multiple ports, unlike --dport, which supports only a single port. Blocking ICMP Traffic One commonly used feature in iptables is blocking ICMP traffic, often generated by the ping utility. To block incoming ICMP traffic, use the following command: iptables -A INPUT -j DROP -p icmp --icmp-type echo-request This command will prevent the ping command from receiving a response without displaying an error message. If you want to display an error message like "Destination Port Unreachable," replace the DROP action with REJECT: iptables -A INPUT -j REJECT -p icmp --icmp-type echo-request Allowing ICMP Traffic To allow previously blocked ICMP traffic, run the following command: iptables -A INPUT -p icmp --icmp-type echo-request -j ACCEPT However, it’s important to note that if ICMP traffic was previously blocked with this command: iptables -A INPUT -j DROP -p icmp --icmp-type echo-request and then allowed with: iptables -A INPUT -p icmp --icmp-type echo-request -j ACCEPT the ICMP traffic will still be blocked, as the drop rule will be the first rule in the INPUT chain. Blocking Traffic by MAC Address In addition to IP addresses, traffic can be blocked based on the device’s MAC address. Below is an example to block traffic from a device with the MAC address 00:0c:29:ed:a9:60: iptables -A INPUT -m mac --mac-source 00:0c:29:ed:a9:60 -j DROP mac: A module for working with device MAC addresses. mac-source: Specifies the MAC address of the device. Allowing Traffic by MAC Address To allow traffic from a specific MAC address, use this command: iptables -A INPUT -m mac --mac-source 00:0c:29:ed:a9:60 -j ACCEPT Blocking traffic by MAC address with iptables will only work if the devices are on the same network segment. For broader use cases, blocking traffic by IP address is generally more effective. Allowing Traffic on the Loopback Interface Traffic on the loopback interface can also be controlled. To allow incoming traffic on the loopback interface, use: iptables -A INPUT -i lo -j ACCEPT For outgoing traffic on the loopback interface, the command is: iptables -A OUTPUT -o lo -j ACCEPT Restricting Network Access by Schedule One of the useful features of iptables is the ability to temporarily allow or restrict traffic to specific services or ports based on a schedule. For example, let’s say we want to allow incoming SSH access only on weekdays, Monday through Friday, from 9 AM to 6 PM. The command would look like this: iptables -A INPUT -p tcp --dport 22 -m time --timestart 09:00 --timestop 18:00 --weekdays Mon,Tue,Wed,Thu,Fri -j ACCEPT time: Module for working with time-based rules. timestart: Specifies the start time for the rule. timestop: Specifies the end time for the rule. weekdays: Specifies the days of the week when the rule will be active, separated by commas. Supported values are: Mon, Tue, Wed, Thu, Fri, Sat, Sun, or numbers 1 to 7. Saving iptables Rules By default, user-created iptables rules are not saved automatically. This means that the rules are cleared after a server reboot or shutdown. To save the rules, install the iptables-persistent package with the following command: apt -y install iptables-persistent During the installation, two dialog boxes will appear, allowing you to save the current rules to /etc/iptables/rules.v4 for IPv4 and /etc/iptables/rules.v6 for IPv6. To manually save all rules for the IPv4 protocol, use: iptables-save > /etc/iptables/rules.v4 For IPv6 rules, use: ip6tables-save > /etc/iptables/rules.v6 This method has a significant advantage: saved rules can be restored from the file, which is helpful, for example, when transferring rules to another host. To restore previously saved rules, run: iptables-restore < /etc/iptables/rules.v4 If executing this command on a different host, transfer the rule file first and then execute the restore command. Deleting Rules in iptables You can delete rules in iptables using several methods. Deleting a Specific Rule One way to delete a rule is to target a specific rule in a chain using its line number. To display the rule numbers, use: iptables -L --line-numbers For example, in the INPUT chain, we might see two rules that open ports 80 and 443 over TCP for IP addresses 10.0.36.126 (rule number 1) and 10.0.36.127 (rule number 2). To delete rule number 2, use: iptables -D INPUT 2 Then, display the list of all current rules to verify: iptables -L --line-numbers Rule number 2 should now be removed successfully. Deleting All Rules in a Specific Chain You can also delete all rules in a specific chain at once. For example, to clear all rules in the OUTPUT chain: iptables -F OUTPUT Deleting All Rules To delete all rules across all chains, simply run: iptables -F Use caution with this command, as it will remove all existing rules, including potentially essential ones. Conclusion In summary, iptables is a powerful tool for managing the built-in firewall in Linux-based operating systems. Its extensive features and modular support allow flexible configuration for controlling network traffic. For more detailed information on iptables, consult the official documentation or use the man iptables command in Linux-based systems.

Sometimes, troubleshooting network issues requires capturing network traffic. tcpdump is a network traffic analyzer, or "sniffer," that allows you to intercept and analyze network traffic passing through the utility. This tool provides a rich set of options and various filters, making it versatile for different purposes. tcpdump is entirely console-based, meaning it doesn’t have a graphical interface, so it can be run on servers without GUI support. The first version of tcpdump was released back in 1988. Since then, it has been actively maintained, with new versions released every year. This article will cover various scenarios for using tcpdump. Prerequisites To follow this tutorial, you will need:  A cloud server or virtual machine with a Linux OS installed. Any Linux distribution will work. Access to the root user or a user with sudo privileges. Installing tcpdump We will install tcpdump on Ubuntu 22.04. The tcpdump package is available in the OS’s official repository. First, update the package index: sudo apt update Next, install the utility: sudo apt -y install tcpdump Confirm that the installation was successful by checking the tcpdump version: tcpdump --version Note that further use of the utility requires running it as the root user or a user with sudo privileges. Running tcpdump Without Parameters If you run tcpdump without any parameters, it will start capturing all traffic on all available interfaces in the system and display the data on the screen (stdout): tcpdump To stop the program, press Ctrl + C. After each run, tcpdump provides the following information: packets captured — shows the number of packets captured (packets that were received and processed by tcpdump). packets received by filter — shows the number of packets captured using filters. packets dropped by kernel — shows the number of packets dropped by the OS kernel. By default, tcpdump does not save its output. We will discuss saving the output to a file later in the article. tcpdump Output Format Let's analyze the output of a captured packet using the TCP protocol as an example. By default, tcpdump displays the following data for each capture: 09:33:57.063196 IP nexus-test.com.ssh > 192.168.111.1.50653: Flags [P.], seq 27376:27440, ack 321, win 521, length 64 The parameter descriptions are provided in the table below. Parameter Description 09:33:57.063196 Timestamp. Uses the format hours:minutes:seconds.fraction, where "fraction" represents seconds from midnight. In this example, the packet was captured at 9:33:57.063196. IP Protocol used. nexus-test.com.ssh Domain name (or IP address) and port of the source host. Here, ssh is shown instead of port number 22. To display addresses and protocols as numbers, run tcpdump with the -n option. 192.168.111.1.50653 Domain name (or IP address) and port of the destination host. Flags [P.] ACK flag(s) used to indicate the connection state. Multiple values are possible. In this example, P is used, indicating the PUSH flag for processing packets immediately rather than buffering them. seq 27376:27440 Sequence number of data in the packet. Shows the data range as bytes 27376 through 27440 in the packet. ack 321 Acknowledgment of the received packet. win 521 Window size in bytes, showing the available buffer space for receiving data. length 64 Packet length in bytes, indicating the payload size as the difference between the first and last sequence bytes. Practical Use of tcpdump Let’s move on to practical applications of tcpdump with examples. Displaying a List of Network Interfaces To list all network interfaces available in the system for traffic capture, use: tcpdump -D Capturing Traffic from a Specific Network Interface By default, tcpdump captures traffic from all available interfaces. To capture traffic from a specific network interface (e.g., ens33), use: tcpdump -i ens33 Disabling IP Address to Hostname Resolution By default, tcpdump converts IP addresses to hostnames and replaces port numbers with service names. To prevent tcpdump from converting IP addresses to hostnames, add the -n option: tcpdump -n To disable both IP-to-hostname and port-to-service name conversions, use the -nn option: tcpdump -nn Capturing a Specific Number of Packets By default, tcpdump captures an unlimited number of packets. To capture a specified number of packets, for example, 4 packets, use the -c option: tcpdump -c 4 Adding Date Information tcpdump does not display the date of packet capture by default. To include the date in the output, use the -tttt option. The date will appear at the beginning of each line in the format year:month:day: tcpdump -tttt Packet Filtering in tcpdump tcpdump has extensive filters that allow capturing only the desired packets. Here are some key filters. Filtering by Port To capture traffic on a specific port, use the port option. For example, to capture traffic on port 80 directed towards the destination, you can specify dst: tcpdump dst -n port 80 You can also specify a range of ports: tcpdump -n portrange 80-443 Filtering by Protocol tcpdump supports filtering by protocols. Supported protocol values include: ether, fddi, tr, wlan, ppp, slip, link, ip, arp, rarp, tcp, udp, icmp, and ipv6. Examples for capturing specific protocols are: tcpdump icmp tcpdump tcp tcpdump arp tcpdump udp Filtering by Packet Size tcpdump allows capturing packets of a specified size using two options: less — captures packets smaller than the specified number of bytes. greater — captures packets larger than the specified number of bytes. Here are some examples: Capture traffic with packets that are no more than 43 bytes in size: tcpdump less 43 Capture traffic with packets larger than 43 bytes: tcpdump greater 43 Note that the packet size includes header size: an Ethernet header without CRC occupies 14 bytes, an IPv4 header occupies 20 bytes, and an ICMP header occupies 8 bytes. Filtering by MAC Address To filter by MAC address, use the ether host option. For example, to capture any traffic sent to or from a specified MAC address (e.g., 00:0c:29:c7:00:3f), use: tcpdump ether host 00:0c:29:c7:00:3f Filtering by Source or Destination Address You can filter traffic using the IP address or hostname of the source or destination. To capture traffic originating from a specific host, use the src option: tcpdump -nn src 192.168.36.132 To capture traffic directed to a specific host, use the dst option: tcpdump -nn dst 192.168.36.132 Using Logical Operators in tcpdump tcpdump supports various logical operators, allowing you to combine options. The following operators are supported: and or && — logical "AND." Combines multiple conditions and shows results matching all conditions. or or || — logical "OR." Combines multiple conditions and shows results matching at least one condition. not or ! — logical "NOT." Excludes specified conditions, showing results that do not match the given condition. Here are examples of using logical operators: Capture packets sent from the host 192.168.36.132 and only those listening on port 22: tcpdump -nn src 192.168.36.132 and port 22 Capture packets on all available interfaces that are listening on either port 22 or port 80: tcpdump -nn port 22 or port 80 Capture all packets except ICMP packets: tcpdump -nn not icmp Saving Output to a File As previously mentioned, tcpdump does not save its output to a file by default. To save captured data to a file, use the -w option, specifying the filename with a .pcap extension: tcpdump -nn src 192.168.36.132 -w results.pcap While saving to a file, results will not display in the terminal. To stop capturing packets, press CTRL + C. To read the data saved in the file, use the -r option, followed by the filename where the tcpdump results were saved: tcpdump -r results.pcap Conclusion tcpdump is a powerful command-line tool for analyzing networks and identifying issues. The utility supports a wide array of options, enabling users to filter for specific packet information.