14.2. Firewall or Packet Filtering

A firewall is a filtering network gateway and is only effective on packets that must go through it. Therefore, it can only be effective when going through the firewall is the only route for these packets.

The lack of a standard configuration (and the “process, not product” motto) explains the lack of a turn-key solution. There are, however, tools that make it simpler to configure the netfilter firewall, with a graphical representation of the filtering rules. fwbuilder is undoubtedly among the best of them.

The Linux kernel embeds the netfilter firewall. It can be controlled from user space with the iptables and ip6tables commands. The difference between these two commands is that the former acts on the IPv4 network, whereas the latter acts on IPv6. Since both network protocol stacks will probably be around for many years, both tools will need to be used in parallel.

14.2.1. Netfilter Behavior

netfilter uses four distinct tables which store rules regulating three kinds of operations on packets:

filter concerns filtering rules (accepting, refusing or ignoring a packet);
nat concerns translation of source or destination addresses and ports of packages;
mangle concerns other changes to the IP packets (including the ToS — Type of Service — field and options);
raw allows other manual modifications on packets before they reach the connection tracking system.

Each table contains lists of rules called chains. The firewall uses standard chains to handle packets based on predefined circumstances. The administrator can create other chains, which will only be used when referred to by one of the standard chains (either directly or indirectly).

The filter table has three standard chains:

INPUT: concerns packets whose destination is the firewall itself;
OUTPUT: concerns packets emitted by the firewall;
FORWARD: concerns packets transiting through the firewall (which is neither their source nor their destination).

The nat table also has three standard chains:

PREROUTING: to modify packets as soon as they arrive;
POSTROUTING: to modify packets when they are ready to go on their way;
OUTPUT: to modify packets generated by the firewall itself.

圖形 14.1. How netfilter chains are called

Each chain is a list of rules; each rule is a set of conditions and an action to execute when the conditions are met. When processing a packet, the firewall scans the appropriate chain, one rule after another; when the conditions for one rule are met, it “jumps” (hence the -j option in the commands) to the specified action to continue processing. The most common behaviors are standardized, and dedicated actions exist for them. Taking one of these standard actions interrupts the processing of the chain, since the packet's fate is already sealed (barring an exception mentioned below):

BACK TO BASICS ICMP

ICMP (Internet Control Message Protocol) is the protocol used to transmit complementary information on communications. It allows testing network connectivity with the ping command (which sends an ICMP echo request message, which the recipient is meant to answer with an ICMP echo reply message). It signals a firewall rejecting a packet, indicates an overflow in a receive buffer, proposes a better route for the next packets in the connection, and so on. This protocol is defined by several RFC documents; the initial RFC777 and RFC792 were soon completed and extended.

→ http://www.faqs.org/rfcs/rfc777.html

→ http://www.faqs.org/rfcs/rfc792.html

For reference, a receive buffer is a small memory zone storing data between the time it arrives from the network and the time the kernel handles it. If this zone is full, new data cannot be received, and ICMP signals the problem, so that the emitter can slow down its transfer rate (which should ideally reach an equilibrium after some time).

Note that although an IPv4 network can work without ICMP, ICMPv6 is strictly required for an IPv6 network, since it combines several functions that were, in the IPv4 world, spread across ICMPv4, IGMP (Internet Group Membership Protocol) and ARP (Address Resolution Protocol). ICMPv6 is defined in RFC4443.

→ http://www.faqs.org/rfcs/rfc4443.html

ACCEPT: allow the packet to go on its way;
REJECT: reject the packet with an ICMP error packet (the --reject-with type option to iptables allows selecting the type of error);
DROP: delete (ignore) the packet;
LOG: log (via syslogd) a message with a description of the packet; note that this action does not interrupt processing, and the execution of the chain continues at the next rule, which is why logging refused packets requires both a LOG and a REJECT/DROP rule;
ULOG: log a message via ulogd, which can be better adapted and more efficient than syslogd for handling large numbers of messages; note that this action, like LOG, also returns processing to the next rule in the calling chain;
chain_name: jump to the given chain and evaluate its rules;
RETURN: interrupt processing of the current chain, and return to the calling chain; in case the current chain is a standard one, there's no calling chain, so the default action (defined with the -P option to iptables) is executed instead;
SNAT (only in the nat table): apply Source NAT (extra options describe the exact changes to apply);
DNAT (only in the nat table): apply Destination NAT (extra options describe the exact changes to apply);
MASQUERADE (only in the nat table): apply masquerading (a special case of Source NAT);
REDIRECT (only in the nat table): redirect a packet to a given port of the firewall itself; this can be used to set up a transparent web proxy that works with no configuration on the client side, since the client thinks it connects to the recipient whereas the communications actually go through the proxy.

Other actions, particularly those concerning the mangle table, are outside the scope of this text. The iptables(8) and ip6tables(8) have a comprehensive list.

14.2.2. Syntax of `iptables` and `ip6tables`

The iptables and ip6tables commands allow manipulating tables, chains and rules. Their -t table option indicates which table to operate on (by default, filter).

14.2.2.1. Commands

The -N chain option creates a new chain. The -X chain deletes an empty and unused chain. The -A chain rule adds a rule at the end of the given chain. The -I chain rule_num rule option inserts a rule before the rule number rule_num. The -D chain rule_num (or -D chain rule) option deletes a rule in a chain; the first syntax identifies the rule to be deleted by its number, while the latter identifies it by its contents. The -F chain option flushes a chain (deletes all its rules); if no chain is mentioned, all the rules in the table are deleted. The -L chain option lists the rules in the chain. Finally, the -P chain action option defines the default action, or “policy”, for a given chain; note that only standard chains can have such a policy.

14.2.2.2. Rules

Each rule is expressed as conditions -j action action_options. If several conditions are described in the same rule, then the criterion is the conjunction (logical and) of the conditions, which is at least as restrictive as each individual condition.

The -p protocol condition matches the protocol field of the IP packet. The most common values are tcp, udp, icmp, and icmpv6. Prefixing the condition with an exclamation mark negates the condition, which then becomes a match for “any packets with a different protocol than the specified one”. This negation mechanism is not specific to the -p option and it can be applied to all other conditions too.

The -s address or -s network/mask condition matches the source address of the packet. Correspondingly, -d address or -d network/mask matches the destination address.

The -i interface condition selects packets coming from the given network interface. -o interface selects packets going out on a specific interface.

There are more specific conditions, depending on the generic conditions described above. For instance, the -p tcp condition can be complemented with conditions on the TCP ports, with clauses such as --source-port port and --destination-port port.

The --state state condition matches the state of a packet in a connection (this requires the ipt_conntrack kernel module, for connection tracking). The NEW state describes a packet starting a new connection; ESTABLISHED matches packets belonging to an already existing connection, and RELATED matches packets initiating a new connection related to an existing one (which is useful for the ftp-data connections in the “active” mode of the FTP protocol).

The previous section lists available actions, but not their respective options. The LOG action, for instance, has the following options:

--log-level, with default value warning, indicates the syslog severity level;
--log-prefix allows specifying a text prefix to differentiate between logged messages;
--log-tcp-sequence, --log-tcp-options and --log-ip-options indicate extra data to be integrated into the message: respectively, the TCP sequence number, TCP options, and IP options.

The DNAT action provides the --to-destination address:port option to indicate the new destination IP address and/or port. Similarly, SNAT provides --to-source address:port to indicate the new source IP address and/or port.

The REDIRECT action (only available if NAT is available) provides the --to-ports port(s) option to indicate the port, or port range, where the packets should be redirected.

14.2.3. Creating Rules

Each rule creation requires one invocation of iptables/ip6tables. Typing these commands manually can be tedious, so the calls are usually stored in a script so that the same configuration is set up automatically every time the machine boots. This script can be written by hand, but it can also be interesting to prepare it with a high-level tool such as fwbuilder.

# apt install fwbuilder

The principle is simple. In the first step, one needs to describe all the elements that will be involved in the actual rules:

the firewall itself, with its network interfaces;
the networks, with their corresponding IP ranges;
the servers;
the ports belonging to the services hosted on the servers.

The rules are then created with simple drag-and-drop actions on the objects. A few contextual menus can change the condition (negating it, for instance). Then the action needs to be chosen and configured.

As far as IPv6 is concerned, one can either create two distinct rulesets for IPv4 and IPv6, or create only one and let fwbuilder translate the rules according to the addresses assigned to the objects.

圖形 14.2. Fwbuilder's main window

fwbuilder can then generate a script configuring the firewall according to the rules that have been defined. Its modular architecture gives it the ability to generate scripts targeting different systems (iptables for Linux, ipf for FreeBSD and pf for OpenBSD).

14.2.4. Installing the Rules at Each Boot

In other cases, the recommended way is to register the configuration script in an up directive of the /etc/network/interfaces file. In the following example, the script is stored under /usr/local/etc/arrakis.fw.

範例 14.1. interfaces file calling firewall script

auto eth0
iface eth0 inet static
    address 192.168.0.1
    network 192.168.0.0
    netmask 255.255.255.0
    broadcast 192.168.0.255
    up /usr/local/etc/arrakis.fw

This obviously assumes that you are using ifupdown to configure the network interfaces. If you are using something else (like NetworkManager or systemd-networkd), then refer to their respective documentation to find out ways to execute a script after the interface has been brought up.