NOTE: CentOS Enterprise Linux 5 is built from the Red Hat Enterprise Linux source code. Other than logo and name changes CentOS Enterprise Linux 5 is compatible with the equivalent Red Hat version. This document applies equally to both Red Hat and CentOS Enterprise Linux 5.
Information security is commonly thought of as a process and not a product. However, standard security implementations usually employ some form of dedicated mechanism to control access privileges and restrict network resources to users who are authorized, identifiable, and traceable. Red Hat Enterprise Linux includes several tools to assist administrators and security engineers with network-level access control issues.
Firewalls are one of the core components of a network security implementation. Several vendors market firewall solutions catering to all levels of the marketplace: from home users protecting one PC to data center solutions safeguarding vital enterprise information. Firewalls can be stand-alone hardware solutions, such as firewall appliances by Cisco, Nokia, and Sonicwall. Vendors such as Checkpoint, McAfee, and Symantec have also developed proprietary software firewall solutions for home and business markets.
Apart from the differences between hardware and software firewalls, there are also differences in the way firewalls function that separate one solution from another. Table 42.2, “Firewall Types” details three common types of firewalls and how they
function:
Method
Description
Advantages
Disadvantages
NAT
Network Address Translation (NAT) places private IP subnetworks behind one or a small pool of public IP addresses, masquerading all requests to one source rather than several. The Linux kernel has built-in NAT functionality through the Netfilter kernel subsystem.
· Can be configured transparently to machines on a LAN
· Protection of many machines and services behind one or more external IP addresses simplifies administration duties
· Restriction of user access to and from the LAN can be configured by opening and closing ports on the NAT firewall/gateway
· Cannot prevent malicious activity once users connect to a service outside of the firewall
Packet Filter
A packet filtering firewall reads each data packet that passes through a LAN. It can read and process packets by header information and filters the packet based on sets of programmable rules implemented by the firewall administrator. The Linux kernel has built-in packet filtering functionality through the Netfilter kernel subsystem.
· Customizable through the iptables front-end utility
· Does not require any customization on the client side, as all network activity is filtered at the router level rather than the application level
· Since packets are not transmitted through a proxy, network performance is faster due to direct connection from client to remote host
· Cannot filter packets for content like proxy firewalls
· Processes packets at the protocol layer, but cannot filter packets at an application layer
· Complex network architectures can make establishing packet filtering rules difficult, especially if coupled with IP masquerading or local subnets and DMZ networks
Proxy
Proxy firewalls filter all requests of a certain protocol or type from LAN clients to a proxy machine, which then makes those requests to the Internet on behalf of the local client. A proxy machine acts as a buffer between malicious remote users and the internal network client machines.
· Gives administrators control over what applications and protocols function outside of the LAN
· Some proxy servers can cache frequently-accessed data locally rather than having to use the Internet connection to request it. This helps to reduce bandwidth consumption
· Proxy services can be logged and monitored closely, allowing tighter control over resource utilization on the network
· Proxies are often application-specific (HTTP, Telnet, etc.), or protocol-restricted (most proxies work with TCP-connected services only)
· Application services cannot run behind a proxy, so your application servers must use a separate form of network security
· Proxies can become a network bottleneck, as all requests and transmissions are passed through one source rather than directly from a client to a remote service
Table 42.2. Firewall Types
42.8.1. Netfilter and IPTables
The Linux kernel features a powerful networking subsystem called Netfilter. The Netfilter subsystem provides stateful or stateless packet filtering as well as NAT and IP masquerading services. Netfilter also has the ability to mangle IP header information for advanced routing and connection state management. Netfilter is controlled using the iptables tool.
42.8.1.1. IPTables Overview
The power and flexibility of Netfilter is implemented using the iptables administration tool, a command line tool similar in syntax to its predecessor, ipchains.
A similar syntax does not mean similar implementation, however. ipchains requires intricate rule sets for: filtering source paths; filtering destination paths; and filtering both source and destination connection ports.
By contrast, iptables uses the Netfilter subsystem to enhance network connection, inspection, and processing. iptables features advanced logging, pre- and post-routing actions, network address translation, and port forwarding, all in one command line interface.
This section provides an overview of iptables. For more detailed information, refer to Section 42.9, “IPTables”.
42.8.2. Basic Firewall Configuration
Just as a firewall in a building attempts to prevent a fire from spreading, a computer firewall attempts to prevent malicious software from spreading to your computer. It also helps to prevent unauthorized users from accessing your computer.
In a default Red Hat Enterprise Linux installation, a firewall exists between your computer or network and any untrusted networks, for example the Internet. It determines which services on your computer remote users can access. A properly configured firewall can greatly increase the security of your system. It is recommended that you configure a firewall for any Red Hat Enterprise Linux system with an Internet connection.
42.8.2.1. Security Level Configuration Tool
During the Firewall Configuration screen of the Red Hat Enterprise Linux installation, you were given the option to enable a basic firewall as well as to allow specific devices, incoming services, and ports.
After installation, you can change this preference by using the Security Level Configuration Tool.
To start this application, use the following command:
The Security Level Configuration Tool only configures a basic firewall. If the system needs more complex rules, refer to Section 42.9, “IPTables” for details on
configuring specific iptables rules.
42.8.2.2. Enabling and Disabling the Firewall
Select one of the following options for the firewall:
Disabled — Disabling the firewall provides complete access to your system and does no security checking. This should only be selected if you are running on a trusted network (not the Internet) or need to configure a custom firewall using the iptables command line tool.
Warning
Firewall configurations and any customized firewall rules are stored in the /etc/sysconfig/iptables file. If you choose Disabled and click OK, these configurations and firewall rules will be lost.
Enabled — This option configures the system to reject incoming connections that are not in response to outbound requests, such as DNS replies or DHCP requests. If access to services running on this machine is needed, you can choose to allow specific services through the firewall.
If you are connecting your system to the Internet, but do not plan to run a server, this is the safest choice.
42.8.2.3. Trusted Services
Enabling options in the Trusted services list allows the specified service to pass through the firewall.
WWW (HTTP)
The HTTP protocol is used by Apache (and by other Web servers) to serve web pages. If you plan on making your Web server publicly available, select this check box. This option is not required for viewing pages locally or for developing web pages. This service requires that the httpd package be installed.
Enabling WWW (HTTP) will not open a port for HTTPS, the SSL version of HTTP. If this service is required, select the Secure WWW (HTTPS) check box.
FTP
The FTP protocol is used to transfer files between machines on a network. If you plan on making your FTP server publicly available, select this check box. This service requires that the vsftpd package be installed.
SSH
Secure Shell (SSH) is a suite of tools for logging into and executing commands on a remote machine. To allow remote access to the machine via ssh, select this check box. This service requires that the openssh-server package be installed.
Telnet
Telnet is a protocol for logging into remote machines. Telnet communications are unencrypted and provide no security from network snooping. Allowing incoming Telnet access is not recommended. To allow remote access to the machine via telnet, select this check box. This service requires that the
telnet-server package be installed.
Mail (SMTP)
SMTP is a protocol that allows remote hosts to connect directly to your machine to deliver mail. You do not need to enable this service if you collect your mail from your ISP's server using POP3 or IMAP, or if you use a tool such as fetchmail. To allow delivery of mail to your machine, select this check box. Note that an improperly configured SMTP server can allow remote machines to use your server to send spam.
NFS4
The Network File System (NFS) is a file sharing protocol commonly used on *NIX systems. Version 4 of this protocol is more secure than its predecessors. If you want to share files or directories on your system with other network users, select this check box.
Samba
Samba is an implementation of Microsoft's proprietary SMB networking protocol. If you need to share files, directories, or locally-connected printers with Microsoft Windows machines, select this check box.
42.8.2.4. Other Ports
The Security Level Configuration Tool includes an Other ports section for specifying custom IP ports as being trusted by iptables. For example, to allow IRC and Internet printing protocol (IPP) to pass through the firewall, add the following to the Other ports section:
194:tcp,631:tcp
42.8.2.5. Saving the Settings
Click OK to save the changes and enable or disable the firewall. If Enable firewall was selected, the options selected are translated to iptables commands and written to the /etc/sysconfig/iptables file. The iptables service is also started so that the firewall is activated immediately after saving the selected options. If Disable firewall was selected, the /etc/sysconfig/iptables file is removed and the iptables service is stopped immediately.
The selected options are also written to the /etc/sysconfig/system-config-selinux file so that the settings can be restored the next time the application is started. Do not edit this file by hand.
Even though the firewall is activated immediately, the iptables service is not configured to start automatically at boot time. Refer to Section 42.8.2.6, “Activating the IPTables Service” for more information.
42.8.2.6. Activating the IPTables Service
The firewall rules are only active if the iptables service is running. To manually start the service, use the following command:
[root@myServer ~] # service iptables restart
To ensure that iptables starts when the system is booted, use the following command:
[root@myServer ~] # chkconfig --level 345 iptables on
The ipchains service is not included in Red Hat Enterprise Linux. However, if ipchains is installed (for example, an upgrade was performed and the system had ipchains previously installed), the ipchains and iptables services should not be activated simultaneously. To make sure the ipchains service is disabled and configured not to start at boot time, use the following two commands:
[root@myServer ~] # service ipchains stop
[root@myServer ~] # chkconfig --level 345 ipchains off
42.8.3. Using IPTables
The first step in using iptables is to start the iptables service. Use the following command to start the iptables service:
[root@myServer ~] # service iptables start
Note
The ip6tables service can be turned off if you intend to use the iptables service only. If you deactivate the ip6tables service, remember to deactivate the IPv6 network also. Never leave a network device active without the matching firewall.
To force iptables to start by default when the system is booted, use the following command:
[root@myServer ~] # chkconfig --level 345 iptables on
This forces iptables to start whenever the system is booted into runlevel 3, 4, or 5.
42.8.3.1. IPTables Command Syntax
The following sample iptables command illustrates the basic command syntax:
[root@myServer ~ ] # iptables -A <chain> -j <target>
The -A option specifies that the rule be appended to <chain>. Each chain is comprised of one or more rules, and is therefore also known as a ruleset.
The three built-in chains are INPUT, OUTPUT, and FORWARD. These chains are permanent and cannot be deleted. The chain specifies the point at which a packet is manipulated.
The -j <target> option specifies the target of the rule; i.e., what to do if the packet matches the rule. Examples of built-in targets are ACCEPT, DROP, and REJECT.
Refer to the iptables man page for more information on the available chains, options, and targets.
42.8.3.2. Basic Firewall Policies
Establishing basic firewall policies creates a foundation for building more detailed, user-defined rules.
Each iptables chain is comprised of a default policy, and zero or more rules which work in concert with the default policy to define the overall ruleset for the firewall.
The default policy for a chain can be either DROP or ACCEPT. Security-minded administrators typically implement a default policy of DROP, and only allow specific packets on a case-by-case basis. For example, the following policies block all incoming and outgoing packets on a network gateway:
[root@myServer ~ ] # iptables -P INPUT DROP
[root@myServer ~ ] # iptables -P OUTPUT DROP
It is also recommended that any forwarded packets — network traffic that is to be routed from the firewall to its destination node — be denied as well, to restrict internal clients from inadvertent exposure
to the Internet. To do this, use the following rule:
[root@myServer ~ ] # iptables -P FORWARD DROP
When you have established the default policies for each chain, you can create and save further rules for your particular network and security requirements.
The following sections describe how to save iptables rules and outline some of the rules you might implement in the course of building your iptables firewall.
42.8.3.3. Saving and Restoring IPTables Rules
Changes to iptables are transitory; if the system is rebooted or if the iptables service is restarted, the rules are automatically flushed and reset. To save the rules so that they are loaded when the iptables service is started, use the following command:
[root@myServer ~ ] # service iptables save
The rules are stored in the file /etc/sysconfig/iptables and are applied whenever the service is started or the machine is rebooted.
42.8.4. Common IPTables Filtering
Preventing remote attackers from accessing a LAN is one of the most important aspects of network security. The integrity of a LAN should be protected from malicious remote users through the use of stringent firewall rules.
However, with a default policy set to block all incoming, outgoing, and forwarded packets, it is impossible for the firewall/gateway and internal LAN users to communicate with each other or with external resources.
To allow users to perform network-related functions and to use networking applications, administrators must open certain ports for communication.
For example, to allow access to port 80 on the firewall, append the following rule:
This allows users to browse websites that communicate using the standard port 80. To allow access to secure websites (for example, https://www.example.com/), you also need to provide access to port 443, as follows:
When creating an iptables ruleset, order is important.
If a rule specifies that any packets from the 192.168.100.0/24 subnet be dropped, and this is followed by a rule that allows packets from 192.168.100.13 (which is within the dropped subnet), then the second rule is ignored.
The rule to allow packets from 192.168.100.13 must precede the rule that drops the remainder of the subnet.
To insert a rule in a specific location in an existing chain, use the -I option. For example:
[root@myServer ~ ] # iptables -I INPUT 1 -i lo -p all -j ACCEPT
This rule is inserted as the first rule in the INPUT chain to allow local loopback device traffic.
There may be times when you require remote access to the LAN. Secure services, for example SSH, can be used for encrypted remote connection to LAN services.
Administrators with PPP-based resources (such as modem banks or bulk ISP accounts), dial-up access can be used to securely circumvent firewall barriers. Because they are direct connections, modem connections are typically behind a firewall/gateway.
For remote users with broadband connections, however, special cases can be made. You can configure iptables to accept connections from remote SSH clients. For example, the following rules allow remote SSH access:
These rules allow incoming and outbound access for an individual system, such as a single PC directly connected to the Internet or a firewall/gateway. However, they do not allow nodes behind the firewall/gateway to access these services. To allow LAN access to these services, you can use Network Address Translation (NAT) with iptables filtering rules.
42.8.5. FORWARD and NAT Rules
Most ISPs provide only a limited number of publicly routable IP addresses to the organizations they serve.
Administrators must, therefore, find alternative ways to share access to Internet services without giving public IP addresses to every node on the LAN. Using private IP addresses is the most common way of allowing all nodes on a LAN to properly access internal and external network services.
Edge routers (such as firewalls) can receive incoming transmissions from the Internet and route the packets to the intended LAN node. At the same time, firewalls/gateways can also route outgoing requests from a LAN node to the remote Internet service.
This forwarding of network traffic can become dangerous at times, especially with the availability of modern cracking tools that can spoof internal IP addresses and make the remote attacker's machine act as a node on your LAN.
To prevent this, iptables provides routing and forwarding policies that can be implemented to prevent abnormal usage of network resources.
The FORWARD chain allows an administrator to control where packets can be routed within a LAN. For example, to allow forwarding for the entire LAN (assuming the firewall/gateway is assigned an internal IP address on eth1), use the following rules:
This rule gives systems behind the firewall/gateway access to the internal network. The gateway routes packets from one LAN node to its intended destination node, passing all packets through its eth1 device.
Note
By default, the IPv4 policy in Red Hat Enterprise Linux kernels disables support for IP forwarding. This prevents machines that run Red Hat Enterprise Linux from functioning as dedicated edge routers. To enable IP forwarding, use the following command:
This configuration change is only valid for the current session; it does not persist beyond a reboot or network service restart. To permanently set IP forwarding, edit the /etc/sysctl.conf file as follows:
Locate the following line:
net.ipv4.ip_forward = 0
Edit it to read as follows:
net.ipv4.ip_forward = 1
Use the following command to enable the change to the sysctl.conf file:
[root@myServer ~ ] # sysctl -p /etc/sysctl.conf
42.8.5.1. Postrouting and IP Masquerading
Accepting forwarded packets via the firewall's internal IP device allows LAN nodes to communicate with each other; however they still cannot communicate externally to the Internet.
To allow LAN nodes with private IP addresses to communicate with external public networks, configure the firewall for IP masquerading, which masks requests from LAN nodes with the IP address of the firewall's external device (in this case, eth0):
This rule uses the NAT packet matching table (-t nat) and specifies the built-in POSTROUTING chain for NAT (-A POSTROUTING) on the firewall's external networking device (-o eth0).
POSTROUTING allows packets to be altered as they are leaving the firewall's external device.
The -j MASQUERADE target is specified to mask the private IP address of a node with the external IP address of the firewall/gateway.
42.8.5.2. Prerouting
If you have a server on your internal network that you want make available externally, you can use the -j DNAT target of the PREROUTING chain in NAT to specify a destination IP address and port where incoming packets requesting a connection to your internal service can be forwarded.
For example, if you want to forward incoming HTTP requests to your dedicated Apache HTTP Server at 172.31.0.23, use the following command:
This rule specifies that the nat table use the built-in PREROUTING chain to forward incoming HTTP requests exclusively to the listed destination IP address of 172.31.0.23.
Note
If you have a default policy of DROP in your FORWARD chain, you must append a rule to forward all incoming HTTP requests so that destination NAT routing is possible. To do this, use the following command:
This rule forwards all incoming HTTP requests from the firewall to the intended destination; the Apache HTTP Server behind the firewall.
42.8.5.3. DMZs and IPTables
You can create iptables rules to route traffic to certain machines, such as a dedicated HTTP or FTP server, in a demilitarized zone (DMZ). A DMZ is a special local subnetwork dedicated to providing services on a public carrier, such as the Internet.
For example, to set a rule for routing incoming HTTP requests to a dedicated HTTP server at 10.0.4.2 (outside of the 192.168.1.0/24 range of the LAN), NAT uses the PREROUTING table to forward the packets to the appropriate destination:
With this command, all HTTP connections to port 80 from outside of the LAN are routed to the HTTP server on a network separate from the rest of the internal network. This form of network segmentation can prove safer than allowing HTTP connections to a machine on the network.
If the HTTP server is configured to accept secure connections, then port 443 must be forwarded as well.
42.8.6. Malicious Software and Spoofed IP Addresses
More elaborate rules can be created that control access to specific subnets, or even specific nodes, within a LAN. You can also restrict certain dubious applications or programs such as trojans, worms, and other client/server viruses from contacting their server.
For example, some trojans scan networks for services on ports from 31337 to 31340 (called the elite ports in cracking terminology).
Since there are no legitimate services that communicate via these non-standard ports, blocking them can effectively diminish the chances that potentially infected nodes on your network independently communicate with their remote master servers.
The following rules drop all TCP traffic that attempts to use port 31337:
[root@myServer ~ ] # iptables -A OUTPUT -o eth0 -p tcp --dport 31337 --sport 31337 -j DROP
[root@myServer ~ ] # iptables -A FORWARD -o eth0 -p tcp --dport 31337 --sport 31337 -j DROP
You can also block outside connections that attempt to spoof private IP address ranges to infiltrate your LAN.
For example, if your LAN uses the 192.168.1.0/24 range, you can design a rule that instructs the Internet-facing network device (for example, eth0) to drop any packets to that device with an address in your LAN IP range.
Because it is recommended to reject forwarded packets as a default policy, any other spoofed IP address to the external-facing device (eth0) is rejected automatically.
[root@myServer ~ ] # iptables -A FORWARD -s 192.168.1.0/24 -i eth0 -j DROP
Note
There is a distinction between the DROP and REJECT targets when dealing with appended rules.
The REJECT target denies access and returns a connection refused error to users who attempt to connect to the service. The DROP target, as the name implies, drops the packet without any warning.
Administrators can use their own discretion when using these targets. However, to avoid user confusion and attempts to continue connecting, the REJECT target is recommended.
42.8.7. IPTables and Connection Tracking
You can inspect and restrict connections to services based on their connection state. A module within iptables uses a method called connection tracking to store information about incoming connections. You can allow or deny access based on the following connection states:
NEW — A packet requesting a new connection, such as an HTTP request.
ESTABLISHED — A packet that is part of an existing connection.
RELATED — A packet that is requesting a new connection but is part of an existing connection. For example, FTP uses port 21 to establish a connection, but data is transferred on a different port (typically port 20).
INVALID — A packet that is not part of any connections in the connection tracking table.
You can use the stateful functionality of iptables connection tracking with any network protocol, even if the protocol itself is stateless (such as UDP). The following example shows a rule that uses connection tracking to forward only the packets that are associated with an established connection:
[root@myServer ~ ] # iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT
42.8.8. IPv6
The introduction of the next-generation Internet Protocol, called IPv6, expands beyond the 32-bit address limit of IPv4 (or IP). IPv6 supports 128-bit addresses, and carrier networks that are IPv6 aware are therefore able to address a larger number of routable addresses than IPv4.
Red Hat Enterprise Linux supports IPv6 firewall rules using the Netfilter 6 subsystem and the ip6tables command. In Red Hat Enterprise Linux 5, both IPv4 and IPv6 services are enabled by default.
The ip6tables command syntax is identical to iptables in every aspect except that it supports 128-bit addresses. For example, use the following command to enable SSH connections on an IPv6-aware network server:
For more information about IPv6 networking, refer to the IPv6 Information Page at https://www.ipv6.org/.
42.8.9. Additional Resources
There are several aspects to firewalls and the Linux Netfilter subsystem that could not be covered in this chapter. For more information, refer to the following resources.
42.8.9.1. Installed Documentation
Refer to Section 42.9, “IPTables” for more detailed information on the iptables command, including definitions for many command options.
The iptables man page contains a brief summary of the various options.
Red Hat Linux Firewalls, by Bill McCarty; Red Hat Press — a comprehensive reference to building network and server firewalls using open source packet filtering technology such as Netfilter and iptables. It includes topics that cover analyzing firewall logs, developing firewall rules, and customizing your firewall using various graphical tools.
Linux Firewalls, by Robert Ziegler; New Riders Press — contains a wealth of information on building firewalls using both 2.2 kernel ipchains as well as Netfilter and iptables. Additional security topics such as remote access issues and intrusion detection systems are also covered.
