iptables-1.8.3

Introduction to iptables

iptables is a userspace command line program used to configure Linux 2.4 and later kernel packet filtering ruleset.

This package is known to build and work properly using an LFS-9.0 platform.

Package Information

iptables Dependencies

Optional

nftables-0.9.2

User Notes: http://wiki.linuxfromscratch.org/blfs/wiki/iptables

Kernel Configuration

A firewall in Linux is accomplished through the netfilter interface. To use iptables to configure netfilter, the following kernel configuration parameters are required:

[*] Networking support  --->                                    [CONFIG_NET]
      Networking Options  --->
        [*] Network packet filtering framework (Netfilter) ---> [CONFIG_NETFILTER]
          Core Netfilter Configuration --->

Include any connection tracking protocols that will be used, as well as any protocols that you wish to use for match suppport under the "Core Netfilter Configuration" section.

Installation of iptables

[Note]

Note

The installation below does not include building some specialized extension libraries which require the raw headers in the Linux source code. If you wish to build the additional extensions (if you aren't sure, then you probably don't), you can look at the INSTALL file to see an example of how to change the KERNEL_DIR= parameter to point at the Linux source code. Note that if you upgrade the kernel version, you may also need to recompile iptables and that the BLFS team has not tested using the raw kernel headers.

Install iptables by running the following commands:

./configure --prefix=/usr      \
            --sbindir=/sbin    \
            --disable-nftables \
            --enable-libipq    \
            --with-xtlibdir=/lib/xtables &&
make

This package does not come with a test suite.

Now, as the root user:

make install &&
ln -sfv ../../sbin/xtables-legacy-multi /usr/bin/iptables-xml &&

for file in ip4tc ip6tc ipq iptc xtables
do
  mv -v /usr/lib/lib${file}.so.* /lib &&
  ln -sfv ../../lib/$(readlink /usr/lib/lib${file}.so) /usr/lib/lib${file}.so
done

Command Explanations

--disable-nftables: This switch disables building nftables compat. Omit this switch if you have installed nftables-0.9.2.

--enable-libipq: This switch enables building of libipq.so which can be used by some packages outside of BLFS.

--with-xtlibdir=/lib/xtables: Ensure all iptables modules are installed in the /lib/xtables directory.

--enable-nfsynproxy: This switch enables installation of nfsynproxy SYNPROXY configuration tool.

ln -sfv ../../sbin/xtables-legacy-multi /usr/bin/iptables-xml: Ensure the symbolic link for iptables-xml is relative.

Configuring iptables

[Note]

Note

If you intend to use firewalld-0.7.2 to configure your firewall rules, you should not use the example configurations provided here, nor should you enable the systemd unit.

[Note]

Note

In the following example configurations, LAN1 is used for the internal LAN interface, and WAN1 is used for the external interace connected to the Internet. You will need to replace these values with appropriate interface names for your system.

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 3.x 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 with iptable 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 Creating a BusyBox With iptables.

Masquerading Router

A network Firewall has two interfaces, one connected to an intranet, in this example LAN1, and one connected to the Internet, here WAN1. 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 WAN1 -m conntrack --ctstate NEW       -j ACCEPT

# Do masquerading
# (not needed if intranet is not using private ip-addresses)
iptables -t nat -A POSTROUTING -o WAN1 -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

# The following sections allow inbound packets for specific examples
# Uncomment the example lines and adjust as necessary

# Allow ping on the external interface
#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

# Reject ident packets with TCP reset to avoid delays with FTP or IRC
#iptables -A INPUT  -p tcp --dport 113 -j REJECT --reject-with tcp-reset

# Allow HTTP and HTTPS to 192.168.0.2
#iptables -A PREROUTING -t nat -i WAN1 -p tcp --dport 80 -j DNAT --to 192.168.0.2
#iptables -A PREROUTING -t nat -i WAN1 -p tcp --dport 443 -j DNAT --to 192.168.0.2
#iptables -A FORWARD -p tcp -d 192.168.0.2 --dport 80 -j ACCEPT
#iptables -A FORWARD -p tcp -d 192.168.0.2 --dport 443 -j ACCEPT

# 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.

BusyBox

This scenario isn't too different from the Creating a Masquerading Router With iptables, 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]

Note

Outlining specifically how to protect a server that offers services on the Internet goes far beyond the scope of this document. See the references in the section called “Extra Information” 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 Creating a Masquerading Router With iptables 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 ! WAN1  -j ACCEPT
iptables -A OUTPUT -o ! WAN1  -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 delays because some implementations of these daemons query an identd daemon 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' response:

    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 WAN1 -s 10.0.0.0/8     -j DROP
    iptables -A INPUT -i WAN1 -s 172.16.0.0/12  -j DROP
    iptables -A INPUT -i WAN1 -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 WAN1 -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.

Systemd Unit

To set up the iptables firewall at boot, install the iptables.service unit included in the blfs-systemd-units-20191026 package.

make install-iptables

Contents

Installed Programs: ip6tables, ip6tables-restore, ip6tables-save, iptables, iptables-restore, iptables-save, iptables-xml, nfsynproxy (optional) and xtables-multi
Installed Libraries: libip4tc.so, libip6tc.so, libipq.so, libiptc.so, and libxtables.so
Installed Directories: /lib/xtables and /usr/include/libiptc

Short Descriptions

iptables

is used to set up, maintain, and inspect the tables of IP packet filter rules in the Linux kernel.

iptables-restore

is used to restore IP Tables from data specified on STDIN. Use I/O redirection provided by your shell to read from a file.

iptables-save

is used to dump the contents of an IP Table in easily parseable format to STDOUT. Use I/O-redirection provided by your shell to write to a file.

iptables-xml

is used to convert the output of iptables-save to an XML format. Using the iptables.xslt stylesheet converts the XML back to the format of iptables-restore.

ip6tables*

are a set of commands for IPV6 that parallel the iptables commands above.

nfsynproxy

(optional) configuration tool. SYNPROXY target makes handling of large SYN floods possible without the large performance penalties imposed by the connection tracking in such cases.

xtables-multi

is a binary that behaves according to the name it is called by.

Last updated on 2019-10-26 12:41:52 -0500