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14.2. ファイアウォールとパケットフィルタリング

ファイアウォールはネットワークゲートウェイをフィルタするもので、ゲートウェイを通過しなければいけないパケットだけに有効です。それ故、フィルタしたいパケットをファイアウォール以外の経路で宛先に配送することが可能な場合、ファイアウォールは無意味です。
The Linux kernel embeds the netfilter firewall, which can be controlled from user space with the iptables, ip6tables, arptables and ebtables commands.
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. Debian Buster uses the nftables framework by default.
To enable a default firewall in Debian execute:
# 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. nftables Behavior

As the kernel is processing a network packet it pauses and allows us to inspect the packet and decide what to do with that package. For example, we might want to drop or discard certain incoming packages, modify other packages 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.
A good understanding of the layers 3, 4 and 5 of the OSI (Open Systems Interconnection) model is essential to get the most from netfilter.
The firewall is configured with tables, which hold rules contained in chains. Unlike iptables, nftables does not have any default table. The user decides which and how many tables to create. Every table must have only one of the following five families assigned: ip, ip6, inet, arp and bridge. ip is used if the family is not specified.
There are two types of chains: base chains and regular chains. A base chain is an entry point for packets from the networking stack, they are registered into the Netfilter hooks, ie. these chains see packets flowing through the TCP/IP stack. On the other hand, and a regular chain is not attached to any hook, so they do not see any traffic, but it may be used as a jump target for better organization.
Rules are made of statements, which includes some expressions to be matched and then a verdict statement, like accept, drop, queue, continue, return, jump chain and goto chain.

14.2.2. Moving from iptables to nftables

The iptables-translate and ip6tables-translate commands can be used to translate old iptables commands into the new nftables syntax. Whole rulesets 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 v1.8.2 on Thu Jul 18 10:39:33 2019
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 POSTROUTING { type nat hook postrouting priority 100; policy accept; }
add chain ip nat OUTPUT { type nat hook output 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 POSTROUTING oifname != "docker0" ip saddr 172.17.0.0/16 counter masquerade
add rule ip nat OUTPUT ip daddr != 127.0.0.0/8 fib daddr type local counter jump DOCKER
add rule ip nat DOCKER iifname "docker0" counter return
# Completed on Thu Jul 18 10:39:33 2019
# iptables-restore-translate -f iptables-ruleset.txt > ruleset.nft
# nft -f ruleset.nft
# nft list ruleset
table ip filter {
	chain INPUT {
		type filter hook input priority 0; policy accept;
	}

	chain FORWARD {
		type filter hook forward priority 0; 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
	}

	chain OUTPUT {
		type filter hook output priority 0; policy accept;
	}

	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
	}

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

	chain DOCKER-USER {
		counter packets 0 bytes 0 return
	}
}
table ip nat {
	chain PREROUTING {
		type nat hook prerouting priority -100; policy accept;
		fib daddr type local counter packets 0 bytes 0 jump DOCKER
	}

	chain INPUT {
		type nat hook input priority 100; policy accept;
	}

	chain POSTROUTING {
		type nat hook postrouting priority 100; policy accept;
		oifname != "docker0" ip saddr 172.17.0.0/16 counter packets 0 bytes 0 masquerade
	}

	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
	}

	chain DOCKER {
		iifname "docker0" counter packets 0 bytes 0 return
	}
}
table ip mangle {
	chain PREROUTING {
		type filter hook prerouting priority -150; policy accept;
	}

	chain INPUT {
		type filter hook input priority -150; policy accept;
	}

	chain FORWARD {
		type filter hook forward priority -150; policy accept;
	}

	chain OUTPUT {
		type route hook output priority -150; policy accept;
	}

	chain POSTROUTING {
		type filter hook postrouting priority -150; policy accept;
	}
}
The tools iptables-nft, ip6tables-nft, arptables-nft, ebtables-nft are versions of iptables that use the nftables API, so users can keep using the old iptables syntax with them, but that is not recommended; these tools should only be used for backwards compatibility.

14.2.3. Syntax of nft

The nft commands allow manipulating tables, chains and rules. The table option supports multiple operations: add, create, delete, list and flush. nft add table ip6 mangle adds a new table from the family ip6.
To insert a new base chain to the filter table, you can execute the following command (note that the semicolon is escaped with a backslash when using Bash):
# nft add chain filter input { type filter hook input priority 0 \; }
Rules are usually added with the following syntax: nft add rule [family] table chain handle handle statement.
insert is similar to the add command, but the given rule is prepended to the beginning of the chain or before the rule with the given handle instead of at the end or after that rule. For example, the following command inserts a rule before the rule with handler number 8:
# nft insert rule filter output position 8 ip daddr 127.0.0.8 drop
The executed nft commands do not make permanent changes to the configuration, so they are lost if they are not saved. The firewall rules are located in /etc/nftables.conf. A simple way to save the current firewall configuration permanently is to execute nft list ruleset > /etc/nftables.conf as root.
nft allows many more operations, refer to its manual page nft(8) for more information.

14.2.4. 起動時にルールを適用する

To enable a default firewall in Debian, you need to store the rules in /etc/nftables.conf and execute systemctl enable nftables.service as root. You can stop the firewall executing nft flush ruleset as root.
In other cases, the recommended way is to register the configuration script in up directive of the /etc/network/interfaces file. In the following example, the script is stored under /usr/local/etc/arrakis.fw.

例 14.1 ファイアウォールスクリプトを呼び出す interfaces ファイル

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
見ての通りこの例ではネットワークインターフェースを設定するために ifupdown を使っています。他の方法 (たとえば NetworkManagersystemd-networkd など) を使ってネットワークインターフェースを設定している場合、それぞれの文書を参照して、インターフェースを起動した後にスクリプトを実行する方法を見つけてください。