14.2. Firewall atau Penyaringan Paket

Firewall adalah gateway jaringan yang menyaring dan hanya efektif pada paket yang harus melaluinya. Oleh karenanya, itu hanya dapat efektif ketika melalui firewall adalah satu-satunya rute untuk paket-paket ini.

KASUS SPESIFIK Firewall Lokal

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.

Kernel Linux menyertakan firewall netfilter. Ini dapat dikontrol dari ruang pengguna dengan perintah iptables, ip6tables, arptables, dan 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 incoming traffic on ports which cannot be assigned to a limited set of interfaces or closed appropriately.

Untuk mengaktifkan firewall default di Debian, jalankan:

# apt install -y nftables
Reading package lists... Done
...
# systemctl enable nftables.service
Created symlink /etc/systemd/system/sysinit.target.wants/nftables.service → /lib/systemd/system/nftables.service.

14.2.1. Perilaku 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.

Pemahaman yang baik dari layer 3, 4, dan 5 dari model OSI (Open System Interconnection) sangat penting untuk mendapatkan yang terbaik dari netfilter.

KULTUR Model OSI

Model OSI adalah model konseptual untuk mengimplementasikan protokol jaringan tanpa memperhatikan struktur internal dan teknologi yang mendasari. Tujuannya adalah interoperabilitas dari sistem komunikasi yang beragam dengan protokol komunikasi standar.

Model ini didefinisikan dalam standar ISO/EIC 7498. Berikut ini adalah tujuh lapisan yang diuraikan:

Fisik: transmisi dan penerimaan aliran bit mentah melalui media fisik
Data Link: transmisi frame data yang handal antara dua simpul yang terhubung oleh lapisan fisik
Jaringan: menata dan mengelola jaringan multi-simpul, termasuk pengalamatan, routing, dan kontrol lalu lintas
Transport: transmisi segmen data yang handal antar titik pada jaringan, termasuk segmentasi, acknowledgement, dan multiplexing
Session: mengelola sesi komunikasi, yaitu pertukaran informasi terus-menerus dalam bentuk beberapa transmisi bolak-balik antara dua node
Presentasi: penerjemahan data antara layanan jaringan dan aplikasi; termasuk pengkodean karakter, kompresi data dan enkripsi/dekripsi
Aplikasi: API tingkat tinggi, termasuk berbagi sumber daya, akses berkas jarak jauh.

More information can be found on Wikipedia:

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

Firewall dikonfigurasi dengan tabel, yang memegang aturan yang terkandung dalam rantai. Tidak seperti iptables, nftables tidak memiliki tabel default. Pengguna memutuskan mana dan berapa banyak tabel yang dibuat. Setiap tabel harus ditugaskan hanya ke satu dari lima keluarga berikut: ip, ip6, inet, arp dan bridge. ip digunakan jika keluarga tidak dinyatakan.

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.

Aturan yang terbuat dari pernyataan, yang mencakup beberapa ungkapan yang harus dicocokkan dan kemudian pernyataan putusan, seperti accept, drop, queue, continue, return, jump chain, dan goto chain.

KEMBALI KE DASAR 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

Untuk referensi, suatu penyangga penerima adalah sebuah zona memori kecil yang menyimpan data antara saat itu datang dari jaringan dan saat kernel menanganinya. Jika zona ini penuh, data baru tidak dapat diterima, dan ICMP mengirim sinyal adanya masalah, sehingga pengirim dapat memperlambat laju transfer (yang idealnya harus mencapai suatu keseimbangan setelah beberapa waktu).

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. Berpindah dari iptables ke 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;
	}
}

Alat iptables-nft, ip6tables-nft, arptables-nft, ebtables-nft adalah versi iptables yang menggunakan API nftables, sehingga pengguna dapat tetap menggunakan sintaks iptables lama dengan mereka, tapi itu tidak dianjurkan; alat ini hanya boleh digunakan untuk kesesuaian ke belakang.

14.2.3. Sintaks `nft`

Perintah nft memungkinkan memanipulasi tabel, rantai, dan aturan. Opsi table mendukung beberapa operasi: add, create, delete, list, dan flush. nft add table ip6 mangle menambahkan tabel baru dari keluarga ip6.

Untuk memasukkan rantai dasar baru ke tabel filter, Anda dapat mengeksekusi perintah berikut (perhatikan bahwa titik koma di-escape dengan garis miring terbalik saat menggunakan Bash):

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

Aturan biasanya ditambahkan dengan sintaks berikut: nft add rule [family] table chain handle handle pernyataan .

insert mirip dengan perintah add, namun aturan yang diberikan ditambahkan ke awal rantai atau sebelum aturan dengan handle yang diberikan bukan pada akhir atau setelah aturan itu. Sebagai contoh, perintah berikut ini memasukkan aturan sebelum aturan dengan handler nomor 8:

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

Perintah nft yang dieksekusi tidak membuat perubahan permanen untuk konfigurasi, sehingga mereka akan hilang jika mereka tidak disimpan. Aturan firewall terletak di /etc/nftables.conf. Sebuah cara sederhana untuk menyimpan konfigurasi firewall saat ini secara permanen adalah dengan mengeksekusi nft list ruleset > /etc/nftables.conf sebagai root.

nft memungkinkan lebih banyak operasi, lihat halaman manual nft(8) untuk informasi lebih lanjut.

14.2.4. Menginstal Aturan Pada Setiap Boot

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.

Dalam kasus lain, cara yang disarankan adalah mendaftarkan skrip konfigurasi dalam direktif up dari berkas /etc/network/interfaces. Pada contoh berikut, skrip disimpan di bawah /usr/local/etc/arrakis.fw.

Contoh 14.1. berkas interface yang memanggil skrip 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

Ini jelas mengasumsikan bahwa Anda menggunakan ifupdown untuk mengkonfigurasi antarmuka jaringan. Jika Anda menggunakan sesuatu yang lain (seperti NetworkManager atau systemd-networkd), maka rujuklah ke dokumentasi mereka masing-masing untuk mengetahui cara menjalankan skrip setelah antarmuka telah dihidupkan.