14.2. Firewall ou Filtragem de pacotes

Um firewall é um portal de filtragem da saída de rede e é efetivo apenas em pacotes que devem passar por ele. Portanto, o firewall só será eficaz quando a única rota para estes pacotes for através dele.

CASO ESPECÍFICO Firewall Local

A firewall can be restricted to one particular machine (as opposed to a complete network), in which case its role is to filter or limit access to some services. It is, however, always better to configure services to not listen on those network interfaces, where these services are not supposed to be offered, or to disable and remove services, which are not to be offered at all, than to use a packet filter to prevent any unwanted access to these services.

Its role can also be to filter or limit connections by rogue or poorly programmed software that a user could, willingly or not, have installed. The reliability of this, however, highly depends on the goal of these actions and the integrity of the system itself. This action is not a measurement to prevent potential threats to send data.

O kernel do Linux incorpora o firewall netfilter, que pode ser controlado a partir do espaço do usuário com os comandos iptables, ip6tables, arptables e ebtables.

However, Netfilter iptables commands are being replaced by nftables, which avoids many of its problems. Its design involves less code duplication, and it can be managed with just the nft command. Since Debian Buster, the nftables framework is used by default. The commands mentioned before are provided by versions, which use the nftables kernel API, by default. If one requires the “classic“ commands, the relevant binaries can be adjusted using update-alternatives.

TOOLS fwbuilder and ufw

Although (destined to) being replaced by nftables, iptables is still widely used and suited for many use-cases. Creating a rule requires an 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, or they can be made “persistent“ using iptables-persistent.

A script usually needs to be written by hand. But there are, 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 rules are created with simple drag-and-drop actions on objects (interfaces, networks, ports, servers, etc.). A few contextual menus can change the condition (negating it, for instance). Then the action needs to be chosen and configured.

An alternative to writing and saving/loading the required rules is to use ufw from the package with the same name. This tool is a frontend to iptables as well, but it provides a command line interface only. Simple commands can enable or disable (block) network traffic to and from ports and IP addresses. The configuration files in /etc/ufw/ also allow for complex rule sets (e.g. hooking into the Docker chains), which are then saved and loaded automatically. This tool is often used for simple setups and to block

Para habilitar um firewall padrão no Debian execute:

# apt install -y nftables
Lendo listas de pacotes... Pronto
...
# systemctl enable nftables.service
Created symlink /etc/systemd/system/sysinit.target.wants/nftables.service → /lib/systemd/system/nftables.service.

14.2.1. Comportamento do nftables

As the kernel is processing a network packet, it pauses and allows us to inspect the packet and decide what to do with that packet. For example, we might want to drop or discard certain incoming packets, modify other packets in various ways, block certain outgoing packets to control against malware or redirect some packets at the earliest possible stage to bridge network interfaces or to spread the load of incoming packets between systems.

Um bom entendimento das camadas 3, 4 e 5 do modelo OSI (Open Systems Interconnection) é essencial para obter o máximo do netfilter.

CULTURA O modelo OSI

O modelo OSI é um modelo conceitual para implementar protocolos de rede sem se preocupar com as tecnologias e estruturas internas que estão por trás. O objetivo é a interoperabilidade de diversos sistemas de comunicações com protocolos de comunicações padrão.

Este modelo foi definido no padrão ISO/EIC 7498. As sete camadas seguintes são descritas:

Física: transmissão e recepção de fluxos puros de bits através de um meio físico
Link de dados: transmissão confiável de frames de dados entre dois nós conectados por uma camada física
Rede: estruturar e gerenciar uma rede multinós, incluindo endereçamento, roteamento e controle de tráfego
Transporte: transmissão confiável de segmentos de dados entre pontos em uma rede, incluindo segmentação, reconhecimento e multiplexação
Sessão: gerenciamento de comunicação de sessões, ou seja, troca contínua de informações na forma de transmissões ida e volta entre dois nós
Apresentação: tradução de dados entre um serviço de rede e uma aplicação; incluindo codificação de caracteres, compressão de dados e criptografia/descriptografia
Aplicação: APIs de alto nível, incluindo o compartilhamento de recursos, acesso a arquivos remotos.

More information can be found on Wikipedia:

→ https://en.wikipedia.org/wiki/OSI_model

O firewall é configurado com tabelas (tables), que contêm regras(rules) contidas em cadeias (chains). Ao contrário do iptables, nftables não possui nenhuma tabela padrão. O usuário decide quais e quantas tabelas criar. Cada tabela deve ter apenas uma das seguintes cinco famílias assinadas: ip, ip6, inet, arp e bridge. Usa-se ip se a família não é especificada.

There are two types of chains: base chains and regular chains. A base chain is an entry point for packets from the networking stack. Base chains are registered into the Netfilter hooks, e.g. they see packets flowing through the TCP/IP stack. On the other hand, a regular chain is not attached to any hook so it does not see any traffic. But it may be used as a jump target for better organization of the rules.

Regras são feitas de declarações, que incluem algumas expressões que devem ser correspondidas, e então uma declaração final, como accept, drop, queue, continue, return, jump chain e goto chain.

DE VOLTA AO BÁSICO 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 RFC 777 and RFC 792 were soon completed and extended.

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

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

Para referência, um buffer de recepção é uma pequena região de memória para armazenamento de dados entre o tempo que chega na rede e o tempo que o kernel o manipula. Se esta região está cheia, os novos dados não podem ser recebidos, e o ICMP sinaliza o problema, de modo que o emissor possa diminuir a sua taxa de transferência (que deve, idealmente, chegar a um equilíbrio após algum tempo).

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 RFC 4443.

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

14.2.2. Migrando de iptables para nftables

The iptables-translate and ip6tables-translate commands can be used to translate old iptables commands into the new nftables syntax. Whole rule sets can also be translated, in this case we migrate the rules configured in one computer which has Docker installed:

# iptables-save > iptables-ruleset.txt
# iptables-restore-translate -f iptables-ruleset.txt

# Translated by iptables-restore-translate v1.8.7 on Wed Mar 16 22:06:32 2022
add table ip filter
add chain ip filter INPUT { type filter hook input priority 0; policy accept; }
add chain ip filter FORWARD { type filter hook forward priority 0; policy drop; }
add chain ip filter OUTPUT { type filter hook output priority 0; policy accept; }
add chain ip filter DOCKER
add chain ip filter DOCKER-ISOLATION-STAGE-1
add chain ip filter DOCKER-ISOLATION-STAGE-2
add chain ip filter DOCKER-USER
add rule ip filter FORWARD counter jump DOCKER-USER
add rule ip filter FORWARD counter jump DOCKER-ISOLATION-STAGE-1
add rule ip filter FORWARD oifname "docker0" ct state related,established counter accept
add rule ip filter FORWARD oifname "docker0" counter jump DOCKER
add rule ip filter FORWARD iifname "docker0" oifname != "docker0" counter accept
add rule ip filter FORWARD iifname "docker0" oifname "docker0" counter accept
add rule ip filter DOCKER-ISOLATION-STAGE-1 iifname "docker0" oifname != "docker0" counter jump DOCKER-ISOLATION-STAGE-2
add rule ip filter DOCKER-ISOLATION-STAGE-1 counter return
add rule ip filter DOCKER-ISOLATION-STAGE-2 oifname "docker0" counter drop
add rule ip filter DOCKER-ISOLATION-STAGE-2 counter return
add rule ip filter DOCKER-USER counter return
add table ip nat
add chain ip nat PREROUTING { type nat hook prerouting priority -100; policy accept; }
add chain ip nat INPUT { type nat hook input priority 100; policy accept; }
add chain ip nat OUTPUT { type nat hook output priority -100; policy accept; }
add chain ip nat POSTROUTING { type nat hook postrouting priority 100; policy accept; }
add chain ip nat DOCKER
add rule ip nat PREROUTING fib daddr type local counter jump DOCKER
add rule ip nat OUTPUT ip daddr != 127.0.0.0/8 fib daddr type local counter jump DOCKER
add rule ip nat POSTROUTING oifname != "docker0" ip saddr 172.17.0.0/16 counter masquerade
add rule ip nat DOCKER iifname "docker0" counter return
# Completed on Wed Mar 16 22:06:32 2022
# iptables-restore-translate -f iptables-ruleset.txt > ruleset.nft
# nft -f ruleset.nft
# nft list ruleset
table inet filter {
	chain input {
		type filter hook input priority filter; policy accept;
	}

	chain forward {
		type filter hook forward priority filter; policy accept;
	}

	chain output {
		type filter hook output priority filter; policy accept;
	}
}
table ip nat {
	chain DOCKER {
		iifname "docker0" counter packets 0 bytes 0 return
		iifname "docker0" counter packets 0 bytes 0 return
	}

	chain POSTROUTING {
		type nat hook postrouting priority srcnat; policy accept;
		oifname != "docker0" ip saddr 172.17.0.0/16 counter packets 0 bytes 0 masquerade
		oifname != "docker0" ip saddr 172.17.0.0/16 counter packets 0 bytes 0 masquerade
	}

	chain PREROUTING {
		type nat hook prerouting priority dstnat; policy accept;
		fib daddr type local counter packets 1 bytes 60 jump DOCKER
		fib daddr type local counter packets 0 bytes 0 jump DOCKER
	}

	chain OUTPUT {
		type nat hook output priority -100; policy accept;
		ip daddr != 127.0.0.0/8 fib daddr type local counter packets 0 bytes 0 jump DOCKER
		ip daddr != 127.0.0.0/8 fib daddr type local counter packets 0 bytes 0 jump DOCKER
	}

	chain INPUT {
		type nat hook input priority 100; policy accept;
	}
}
table ip filter {
	chain DOCKER {
	}

	chain DOCKER-ISOLATION-STAGE-1 {
		iifname "docker0" oifname != "docker0" counter packets 0 bytes 0 jump DOCKER-ISOLATION-STAGE-2
		counter packets 0 bytes 0 return
		iifname "docker0" oifname != "docker0" counter packets 0 bytes 0 jump DOCKER-ISOLATION-STAGE-2
		counter packets 0 bytes 0 return
	}

	chain DOCKER-ISOLATION-STAGE-2 {
		oifname "docker0" counter packets 0 bytes 0 drop
		counter packets 0 bytes 0 return
		oifname "docker0" counter packets 0 bytes 0 drop
		counter packets 0 bytes 0 return
	}

	chain FORWARD {
		type filter hook forward priority filter; policy drop;
		counter packets 0 bytes 0 jump DOCKER-USER
		counter packets 0 bytes 0 jump DOCKER-ISOLATION-STAGE-1
		oifname "docker0" ct state related,established counter packets 0 bytes 0 accept
		oifname "docker0" counter packets 0 bytes 0 jump DOCKER
		iifname "docker0" oifname != "docker0" counter packets 0 bytes 0 accept
		iifname "docker0" oifname "docker0" counter packets 0 bytes 0 accept
		counter packets 0 bytes 0 jump DOCKER-USER
		counter packets 0 bytes 0 jump DOCKER-ISOLATION-STAGE-1
		oifname "docker0" ct state established,related counter packets 0 bytes 0 accept
		oifname "docker0" counter packets 0 bytes 0 jump DOCKER
		iifname "docker0" oifname != "docker0" counter packets 0 bytes 0 accept
		iifname "docker0" oifname "docker0" counter packets 0 bytes 0 accept
	}

	chain DOCKER-USER {
		counter packets 0 bytes 0 return
		counter packets 0 bytes 0 return
	}

	chain INPUT {
		type filter hook input priority filter; policy accept;
	}

	chain OUTPUT {
		type filter hook output priority filter; policy accept;
	}
}

As ferramentas iptables-nft, ip6tables-nft, arptables-nft, ebtables-nft são versões de iptables que usam a API nftables, de modo que os(as) usuários(as) podem continuar usando a antiga sintaxe iptables com elas, mas isto não é recomendado; essas ferramentas somente devem ser usadas para retrocompatibilidade.

14.2.3. Sintaxe do `ntf`

Os comandos nft permitem manipular tabelas, cadeias e regras. A opção table suporta múltiplas operações: add, create, delete, list e flush. O comando nft add table ip6 mangle adiciona uma nova tabela da família ip6.

Para inserir uma nova cadeia de base na tabela de filtro, você pode executar o seguinte comando (observe que o ponto e virgula é protegido por uma barra invertida quando se usa o Bash):

# nft add chain filter input { type filter hook input priority 0 \; }

Regras são geralmente adicionadas com a seguinte sintaxe: nft add rule [família] tabela cadeia handle handle statement.

insert é similar ao comando add, mas a regra oferecida é prefixada à cadeia ou antes da regra com o dadohandle em vez do fim ou após aquela regra. Por exemplo, o comando seguinte insere uma regra antes da regra com handler número 8:

# nft insert rule filter output position 8 ip daddr 127.0.0.8 drop

Os comandos nft executados não fazem modificações permanentes na configuração, por isso serão perdidas se não forem salvas. As regras de firewall estão localizadas em /etc/nftables.conf. Uma maneira simples de gravar a configuração de firewall permanentemente é executar nft list ruleset > /etc/nftables.conf como root.

nft permite muitas outras operações, consulte sua página de manual nft(8) para mais informações.

14.2.4. Instalando as Regras em Cada Inicialização

To enable a default firewall in Debian, you need to store the rules in /etc/nftables.conf and execute systemctl enable nftables as root. You can stop the firewall by executing nft flush ruleset as root.

Em outros casos, a maneira recomendada é registrar o script de configuração em diretivas up do /etc/network/interfaces. No exemplo a seguir, o script é armazenado em /usr/local/etc/arrakis.fw.

Exemplo 14.1. arquivo interfaces chamando script firewall

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

Isso obviamente assume que você está usando o ifupdown para configurar as interfaces de rede. Se você está usando algo diferente (como o NetworkManager ou systemd-networkd), então consulte suas respectivas documentações para encontrar maneiras de executar um script após a interface ter sido levantada.