Setting Up a Network Firewall

#### Beyond Linux® From Scratch (systemd Edition) - Version 9.0

### Chapter 4. Security

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# Setting Up a Network Firewall

Before you read this part of the chapter, you should have already installed iptables as described in the previous section.

## Introduction to Firewall Creation

The general purpose of a firewall is to protect a computer or a network against malicious access.

In a perfect world, every daemon or service on every machine is perfectly configured and immune to flaws such as buffer overflows or other problems regarding its security. Furthermore, you trust every user accessing your services. In this world, you do not need to have a firewall.

In the real world however, daemons may be misconfigured and exploits against essential services are freely available. You may wish to choose which services are accessible by certain machines or you may wish to limit which machines or applications are allowed external access. Alternatively, you may simply not trust some of your applications or users. You are probably connected to the Internet. In this world, a firewall is essential.

Don't assume however, that having a firewall makes careful configuration redundant, or that it makes any negligent misconfiguration harmless. It doesn't prevent anyone from exploiting a service you intentionally offer but haven't recently updated or patched after an exploit went public. Despite having a firewall, you need to keep applications and daemons on your system properly configured and up to date. A firewall is not a cure all, but should be an essential part of your overall security strategy.

## Meaning of the Word "Firewall"

The word firewall can have several different meanings.

###

This is a hardware device or software program commercially sold (or offered via freeware) by companies such as Symantec which claims that it secures a home or desktop computer connected to the Internet. This type of firewall is highly relevant for users who do not know how their computers might be accessed via the Internet or how to disable that access, especially if they are always online and connected via broadband links.

###

This is a system placed between the Internet and an intranet. To minimize the risk of compromising the firewall itself, it should generally have only one role—that of protecting the intranet. Although not completely risk free, the tasks of doing the routing and IP masquerading (rewriting IP headers of the packets it routes from clients with private IP addresses onto the Internet so that they seem to come from the firewall itself) are commonly considered relatively secure.

###

This is often an old computer you may have retired and nearly forgotten, performing masquerading or routing functions, but offering non-firewall services such as a web-cache or mail. This may be used for home networks, but is not to be considered as secure as a firewall only machine because the combination of server and router/firewall on one machine raises the complexity of the setup.

### Firewall with a Demilitarized Zone [Not Further Described Here]

This box performs masquerading or routing, but grants public access to some branch of your network which, because of public IPs and a physically separated structure, is essentially a separate network with direct Internet access. The servers on this network are those which must be easily accessible from both the Internet and intranet. The firewall protects both networks. This type of firewall has a minimum of three network interfaces.

### Packetfilter

This type of firewall does routing or masquerading, but does not maintain a state table of ongoing communication streams. It is fast, but quite limited in its ability to block undesired packets without blocking desired packets.

## Now You Can Start to Build your Firewall

### Caution

This introduction on how to setup a firewall is not a complete guide to securing systems. Firewalling is a complex issue that requires careful configuration. The scripts quoted here are simply intended to give examples of how a firewall works. They are not intended to fit into any particular configuration and may not provide complete protection from an attack.

Customization of these scripts for your specific situation will be necessary for an optimal configuration, but you should make a serious study of the iptables documentation and creating firewalls in general before hacking away. Have a look at the list of links for further reading at the end of this section for more details. There you will find a list of URLs that contain quite comprehensive information about building your own firewall.

The main startup firewall is located in the file `/etc/systemd/scripts/iptables`. The sections below provide three different approaches that can be used for a system.

### Note

You should always run your firewall rules from a script. This ensures consistency and a record of what was done. It also allows retention of comments that are essential for understanding the rules long after they were written.

### Personal Firewall

A Personal Firewall is designed to let you access all the services offered on the Internet, but keep your box secure and your data private.

Below is a slightly modified version of Rusty Russell's recommendation from the Linux 2.4 Packet Filtering HOWTO. It is still applicable to the Linux 2.6 kernels.

    install -v -dm755 /etc/systemd/scripts

    cat > /etc/systemd/scripts/iptables << "EOF"
    #!/bin/sh

    # Begin /etc/systemd/scripts/iptables

    # Insert connection-tracking modules
    # (not needed if built into the kernel)
    modprobe nf_conntrack
    modprobe xt_LOG

    # Enable broadcast echo Protection
    echo 1 > /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts

    # Disable Source Routed Packets
    echo 0 > /proc/sys/net/ipv4/conf/all/accept_source_route
    echo 0 > /proc/sys/net/ipv4/conf/default/accept_source_route

    # Enable TCP SYN Cookie Protection
    echo 1 > /proc/sys/net/ipv4/tcp_syncookies

    # Disable ICMP Redirect Acceptance
    echo 0 > /proc/sys/net/ipv4/conf/default/accept_redirects

    # Do not send Redirect Messages
    echo 0 > /proc/sys/net/ipv4/conf/all/send_redirects
    echo 0 > /proc/sys/net/ipv4/conf/default/send_redirects

    # Drop Spoofed Packets coming in on an interface, where responses
    # would result in the reply going out a different interface.
    echo 1 > /proc/sys/net/ipv4/conf/all/rp_filter
    echo 1 > /proc/sys/net/ipv4/conf/default/rp_filter

    # Log packets with impossible addresses.
    echo 1 > /proc/sys/net/ipv4/conf/all/log_martians
    echo 1 > /proc/sys/net/ipv4/conf/default/log_martians

    # be verbose on dynamic ip-addresses  (not needed in case of static IP)
    echo 2 > /proc/sys/net/ipv4/ip_dynaddr

    # disable Explicit Congestion Notification
    # too many routers are still ignorant
    echo 0 > /proc/sys/net/ipv4/tcp_ecn

    # Set a known state
    iptables -P INPUT   DROP
    iptables -P FORWARD DROP
    iptables -P OUTPUT  DROP

    # These lines are here in case rules are already in place and the
    # script is ever rerun on the fly. We want to remove all rules and
    # pre-existing user defined chains before we implement new rules.
    iptables -F
    iptables -X
    iptables -Z

    iptables -t nat -F

    # Allow local-only connections
    iptables -A INPUT  -i lo -j ACCEPT

    # Free output on any interface to any ip for any service
    # (equal to -P ACCEPT)
    iptables -A OUTPUT -j ACCEPT

    # Permit answers on already established connections
    # and permit new connections related to established ones
    # (e.g. port mode ftp)
    iptables -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT

    # Log everything else. What's Windows' latest exploitable vulnerability?
    iptables -A INPUT -j LOG --log-prefix "FIREWALL:INPUT "

    # End /etc/systemd/scripts/iptables
    EOF
    chmod 700 /etc/systemd/scripts/iptables

This script is quite simple, it drops all traffic coming into your computer that wasn't initiated from your computer, but as long as you are simply surfing the Internet you are unlikely to exceed its limits.

If you frequently encounter certain delays at accessing FTP servers, take a look at BusyBox example number 4.

Even if you have daemons or services running on your system, these will be inaccessible everywhere but from your computer itself. If you want to allow access to services on your machine, such as ssh or ping, take a look at BusyBox.

### Masquerading Router

A true Firewall has two interfaces, one connected to an intranet, in this example eth0, and one connected to the Internet, here ppp0. To provide the maximum security for the firewall itself, make sure that there are no unnecessary servers running on it such as X11 et al. As a general principle, the firewall itself should not access any untrusted service (think of a remote server giving answers that makes a daemon on your system crash, or even worse, that implements a worm via a buffer-overflow).

    install -v -dm755 /etc/systemd/scripts

    cat > /etc/systemd/scripts/iptables << "EOF"
    #!/bin/sh

    # Begin /etc/systemd/scripts/iptables

    echo
    echo "You're using the example configuration for a setup of a firewall"
    echo "from Beyond Linux From Scratch."
    echo "This example is far from being complete, it is only meant"
    echo "to be a reference."
    echo "Firewall security is a complex issue, that exceeds the scope"
    echo "of the configuration rules below."

    echo "You can find additional information"
    echo "about firewalls in Chapter 4 of the BLFS book."
    echo "http://www.linuxfromscratch.org/blfs"
    echo

    # Insert iptables modules (not needed if built into the kernel).

    modprobe nf_conntrack
    modprobe nf_conntrack_ftp
    modprobe xt_conntrack
    modprobe xt_LOG
    modprobe xt_state

    # Enable broadcast echo Protection
    echo 1 > /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts

    # Disable Source Routed Packets
    echo 0 > /proc/sys/net/ipv4/conf/all/accept_source_route

    # Enable TCP SYN Cookie Protection
    echo 1 > /proc/sys/net/ipv4/tcp_syncookies

    # Disable ICMP Redirect Acceptance
    echo 0 > /proc/sys/net/ipv4/conf/all/accept_redirects

    # Don't send Redirect Messages
    echo 0 > /proc/sys/net/ipv4/conf/default/send_redirects

    # Drop Spoofed Packets coming in on an interface where responses
    # would result in the reply going out a different interface.
    echo 1 > /proc/sys/net/ipv4/conf/default/rp_filter

    # Log packets with impossible addresses.
    echo 1 > /proc/sys/net/ipv4/conf/all/log_martians

    # Be verbose on dynamic ip-addresses  (not needed in case of static IP)
    echo 2 > /proc/sys/net/ipv4/ip_dynaddr

    # Disable Explicit Congestion Notification
    # Too many routers are still ignorant
    echo 0 > /proc/sys/net/ipv4/tcp_ecn

    # Set a known state
    iptables -P INPUT   DROP
    iptables -P FORWARD DROP
    iptables -P OUTPUT  DROP

    # These lines are here in case rules are already in place and the
    # script is ever rerun on the fly. We want to remove all rules and
    # pre-existing user defined chains before we implement new rules.
    iptables -F
    iptables -X
    iptables -Z

    iptables -t nat -F

    # Allow local connections
    iptables -A INPUT  -i lo -j ACCEPT
    iptables -A OUTPUT -o lo -j ACCEPT

    # Allow forwarding if the initiated on the intranet
    iptables -A FORWARD -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
    iptables -A FORWARD ! -i ppp+ -m conntrack --ctstate NEW       -j ACCEPT

    # Do masquerading
    # (not needed if intranet is not using private ip-addresses)
    iptables -t nat -A POSTROUTING -o ppp+ -j MASQUERADE

    # Log everything for debugging
    # (last of all rules, but before policy rules)
    iptables -A INPUT   -j LOG --log-prefix "FIREWALL:INPUT "
    iptables -A FORWARD -j LOG --log-prefix "FIREWALL:FORWARD "
    iptables -A OUTPUT  -j LOG --log-prefix "FIREWALL:OUTPUT "

    # Enable IP Forwarding
    echo 1 > /proc/sys/net/ipv4/ip_forward

    # End /etc/systemd/scripts/iptables
    EOF
    chmod 700 /etc/systemd/scripts/iptables

With this script your intranet should be reasonably secure against external attacks. No one should be able to setup a new connection to any internal service and, if it's masqueraded, makes your intranet invisible to the Internet. Furthermore, your firewall should be relatively safe because there are no services running that a cracker could attack.

### Note

If the interface you're connecting to the Internet doesn't connect via PPP, you will need to change `<ppp+>` to the name of the interface (e.g., eth1) which you are using.

### BusyBox

This scenario isn't too different from the Masquerading Router, but additionally offers some services to your intranet. Examples of this can be when you want to administer your firewall from another host on your intranet or use it as a proxy or a name server.

### Note

Outlining a true concept of how to protect a server that offers services on the Internet goes far beyond the scope of this document. See the references at the end of this section for more information.

Be cautious. Every service you have enabled makes your setup more complex and your firewall less secure. You are exposed to the risks of misconfigured services or running a service with an exploitable bug. A firewall should generally not run any extra services. See the introduction to the Masquerading Router for some more details.

If you want to add services such as internal Samba or name servers that do not need to access the Internet themselves, the additional statements are quite simple and should still be acceptable from a security standpoint. Just add the following lines into the script before the logging rules.

    iptables -A INPUT  -i ! ppp+  -j ACCEPT
    iptables -A OUTPUT -o ! ppp+  -j ACCEPT

If daemons, such as squid, have to access the Internet themselves, you could open OUTPUT generally and restrict INPUT.

    iptables -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
    iptables -A OUTPUT -j ACCEPT

However, it is generally not advisable to leave OUTPUT unrestricted. You lose any control over trojans who would like to "call home", and a bit of redundancy in case you've (mis-)configured a service so that it broadcasts its existence to the world.

To accomplish this, you should restrict INPUT and OUTPUT on all ports except those that it's absolutely necessary to have open. Which ports you have to open depends on your needs: mostly you will find them by looking for failed accesses in your log files.

Have a Look at the Following Examples:

- Squid is caching the web:

      iptables -A OUTPUT -p tcp --dport 80 -j ACCEPT
      iptables -A INPUT  -p tcp --sport 80 -m conntrack --ctstate ESTABLISHED \
        -j ACCEPT

- Your caching name server (e.g., named) does its lookups via UDP:

      iptables -A OUTPUT -p udp --dport 53 -j ACCEPT

- You want to be able to ping your computer to ensure it's still alive:

      iptables -A INPUT  -p icmp -m icmp --icmp-type echo-request -j ACCEPT
      iptables -A OUTPUT -p icmp -m icmp --icmp-type echo-reply   -j ACCEPT

- If you are frequently accessing FTP servers or enjoy chatting, you might notice certain delays because some implementations of these daemons have the feature of querying an identd on your system to obtain usernames. Although there's really little harm in this, having an identd running is not recommended because many security experts feel the service gives out too much additional information.

  To avoid these delays you could reject the requests with a 'tcp-reset':

      iptables -A INPUT  -p tcp --dport 113 -j REJECT --reject-with tcp-reset

- To log and drop invalid packets (packets that came in after netfilter's timeout or some types of network scans) insert these rules at the top of the chain:

      iptables -I INPUT 0 -p tcp -m conntrack --ctstate INVALID \
        -j LOG --log-prefix "FIREWALL:INVALID "
      iptables -I INPUT 1 -p tcp -m conntrack --ctstate INVALID -j DROP

- Anything coming from the outside should not have a private address, this is a common attack called IP-spoofing:

      iptables -A INPUT -i ppp+ -s 10.0.0.0/8     -j DROP
      iptables -A INPUT -i ppp+ -s 172.16.0.0/12  -j DROP
      iptables -A INPUT -i ppp+ -s 192.168.0.0/16 -j DROP

  There are other addresses that you may also want to drop: 0.0.0.0/8, 127.0.0.0/8, 224.0.0.0/3 (multicast and experimental), 169.254.0.0/16 (Link Local Networks), and 192.0.2.0/24 (IANA defined test network).

- If your firewall is a DHCP client, you need to allow those packets:

      iptables -A INPUT  -i ppp0 -p udp -s 0.0.0.0 --sport 67 \
         -d 255.255.255.255 --dport 68 -j ACCEPT

- To simplify debugging and be fair to anyone who'd like to access a service you have disabled, purposely or by mistake, you could REJECT those packets that are dropped.

  Obviously this must be done directly after logging as the very last lines before the packets are dropped by policy:

      iptables -A INPUT -j REJECT

These are only examples to show you some of the capabilities of the firewall code in Linux. Have a look at the man page of iptables. There you will find much more information. The port numbers needed for this can be found in `/etc/services`, in case you didn't find them by trial and error in your log file.

## Conclusion

Finally, there is one fact you must not forget: The effort spent attacking a system corresponds to the value the cracker expects to gain from it. If you are responsible for valuable information, you need to spend the time to protect it properly.

## Extra Information

### Where to Start with Further Reading on Firewalls

> www.netfilter.org - Homepage of the netfilter/iptables project
> Netfilter related FAQ
> Netfilter related HOWTO's
> en.tldp.org/LDP/nag2/x-087-2-firewall.html
> en.tldp.org/HOWTO/Security-HOWTO.html
> en.tldp.org/HOWTO/Firewall-HOWTO.html
> www.linuxsecurity.com/docs/
> www.little-idiot.de/firewall (German & outdated, but very comprehensive)
> linux.oreillynet.com/pub/a/linux/2000/03/10/netadmin/ddos.html
> staff.washington.edu/dittrich/misc/ddos
> www.e-infomax.com/ipmasq
> www.circlemud.org/~jelson/writings/security/index.htm
> www.securityfocus.com
> www.cert.org - tech_tips
> security.ittoolbox.com
> www.insecure.org/reading.html

Last updated on 2016-06-04 22:57:10 -0700

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