Making the Distribution, Part 2

Nftables

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What is nftables?

nftables is the successor to iptables. It replaces the existing iptables, ip6tables, arptables and ebtables framework. It uses the Linux kernel and a new userspace utility called nft. nftables provides a compatibility layer for the ip(6)tables and framework.

Introduction

As with the iptables framework, nftables is build upon rules which specify the actions. These rules are attached to chains. A chain can contain a collection of rules and is registered into the netfilter hooks. Chains are stored inside tables. A table is specific for one of the layer 3 protocols. One of the main differences with iptables is that there are no predefined tables and chains anymore.

Tables

A table is nothing more than a container for your chains. With nftables there are no predefined tables (filter, raw, mangle...) anymore. You are free to recreate the iptables-like structure, but anything might do. Currently there are 5 different families of tables:

• ip: Used for IPv4 related chains;
• ip6: Used for IPv6 related chains;
• arp: Used for ARP related chains;
• bridge: Used for bridging related chains;
• inet: Mixed ipv4/ipv6 chains (kernel 3.14 and up).

It is not hard to recognize the old tables framework in these tables. The only new one is the inet table which is used for both IPv4 and IPv6 traffic. It should make firewalling for dual-stack hosts easier by combining the rules for IPv4 and IPv6.

Chains

Chains are used to group together rules. As with the tables, nftables does not have any predefined chains. Chains are grouped in base and non-base types. Base chains are registered in one of the netfilter hooks. A base chain has a hook its registered with, a type and a priority. Non-base chains are not attached to a hook and they don't see any traffic by default. They can be used to arrange a rule-set in a tree of chains. There are currently three types of chains:

• filter: for filtering packets
• route: for rerouting packets
• nat: for performing Network Address Translation. Only the first packet of a flow hits this chain, making it impossible to use it for filtering.

The hooks that can be used are:

• prerouting: This is before the routing decision, all packets entering the machine hits this chain
• input: All packets for the local system hits this hook
• forward: Packets not for the local system, those that need to be forwarded hits this hook
• output: Packets that originate from the local system pass this hook
• postrouting: This hook is after the routing decision, all packets leaving the machine hits this chain

   Note

The ARP address family only supports the input and output hook

   Note

The bridge address family only seems to supports the input, forward and output hook

Priorities

   Note

Priorities do not currently appear to have any effect on which chain sees packets first.

   Note

Since the priority seems to be an unsigned integer, negative priorities will be converted into very high priorities.

Rules

Rules specify which action has to be taken for which packets. Rules are attached to chains. Each rule can has an expression to match packets with and one or multiple actions when matching. Main differences with iptables is that it is possible to specify multiple actions and that by default counters are off. It must be specified explicitly in rules if you want packet- and byte-counters for a rule. Each rule has a unique handle number by which it can be distinguished. The following matches are available:

• ip: IP protocol
• ip6: IPv6 protocol
• tcp: TCP protocol
• udp: UDP protocol
• udplite: UDP-lite protocol
• sctp: SCTP protocol
• dccp: DCCP protocol
• ah: Authentication headers
• esp: Encrypted security payload headers
• ipcomp: IPcomp headers
• icmp: icmp protocol
• icmpv6: icmpv6 protocol
• ct: Connection tracking
• meta: meta properties such as interfaces

Matches

Match	Arguments	Description/Example
ip	version	Ip Header version
	hdrlength	IP header length
	tos	Type of Service
	length	Total packet length
	id	IP ID
	frag-off	Fragmentation offset
	ttl	Time to live
	protocol	Upper layer protocol
	checksum	IP header checksum
	saddr	Source address
	daddr	Destination address
ip6	version	IP header version
	priority
	flowlabel	Flow label
	length	Payload length
	nexthdr	Next header type (Upper layer protocol number)
	hoplimit	Hop limit
	saddr	Source Address
	daddr	Destination Address
tcp	sport	Source port
	dport	Destination port
	sequence	Sequence number
	ackseq	Acknowledgement number
	doff	Data offset
	flags	TCP flags
	window	Window
	checksum	Checksum
	urgptr	Urgent pointer
udp	sport	Source port
	dport	destination port
	length	Total packet length
	checksum	Checksum
udplite	sport	Source port
	dport	destination port
	cscov	Checksum coverage
	checksum	Checksum
sctp	sport	Source port
	dport	destination port
	vtag	Verification tag
	checksum	Checksum
dccp	sport	Source port
	dport	destination port
ah	nexthdr	Next header protocol (Upper layer protocol)
	hdrlength	AH header length
	spi	Security Parameter Index
	sequence	Sequence Number
esp	spi	Security Parameter Index
	sequence	Sequence Number
ipcomp	nexthdr	Next header protocol (Upper layer protocol)
	flags	Flags
	cfi	Compression Parameter Index
icmp	type	icmp packet type
icmpv6	type	icmpv6 packet type
ct	state	State of the connection
	direction	Direction of the packet relative to the connection
	status	Status of the connection
	mark	Connection mark
	expiration	Connection expiration time
	helper	Helper associated with the connection
	l3proto	Layer 3 protocol of the connection
	saddr	Source address of the connection for the given direction
	daddr	Destination address of the connection for the given direction
	protocol	Layer 4 protocol of the connection for the given direction
	proto-src	Layer 4 protocol source for the given direction
	proto-dst	Layer 4 protocol destination for the given direction
meta	length	Length of the packet in bytes: meta length > 1000
	protocol	ethertype protocol: meta protocol vlan
	priority	TC packet priority
	mark	Packet mark
	iif	Input interface index
	iifname	Input interface name
	iiftype	Input interface type
	oif	Output interface index
	oifname	Output interface name
	oiftype	Output interface hardware type
	skuid	UID associated with originating socket
	skgid	GID associated with originating socket
	rtclassid	Routing realm

Statements

Statements represent the action to be performed when the rule matches. They exist in two kinds: Terminal statements, unconditionally terminate the evaluation of the current rules and non-terminal statements that either conditionally or never terminate the current rules. There can be an arbitrary amount of non-terminal statements, but there must be only a single terminal statement. The terminal statements can be:

• accept: Accept the packet and stop the ruleset evaluation.
• drop: Drop the packet and stop the ruleset evaluation.
• reject: Reject the packet with an icmp message
• queue: Queue the packet to userspace and stop the ruleset evaluation.
• continue:
• return: Return from the current chain and continue at the next rule of the last chain. In a base chain it is equivalent to accept
• jump <chain>: Continue at the first rule of <chain>. It will continue at the next rule after a return statement is issued
• goto <chain>: Similar to jump, but after the new chain the evaluation will continue at the last chain instead of the one containing the goto statement

Installing nftables

Kernel

These kernel options must be set:

[*] Networking support  --->
   Networking options  --->
       [*] Network packet filtering framework (Netfilter)  --->
           Core Netfilter Configuration  --->
               <M> Netfilter nf_tables support
               <M>   Netfilter nf_tables IPv6 exthdr module
               <M>   Netfilter nf_tables meta module
               <M>   Netfilter nf_tables conntrack module
               <M>   Netfilter nf_tables rbtree set module
               <M>   Netfilter nf_tables hash set module
               <M>   Netfilter nf_tables counter module
               <M>   Netfilter nf_tables log module
               <M>   Netfilter nf_tables limit module
               <M>   Netfilter nf_tables nat module
               <M>   Netfilter x_tables over nf_tables module
           IP: Netfilter Configuration  --->
               <M> IPv4 nf_tables support
               <M>   nf_tables IPv4 reject support
               <M>   IPv4 nf_tables route chain support
               <M>   IPv4 nf_tables nat chain support
           IPv6: Netfilter Configuration  --->
               <M> IPv6 nf_tables support
               <M>   IPv6 nf_tables route chain support
               <M>   IPv6 nf_tables nat chain support
           <M>   Ethernet Bridge nf_tables support

Emerging

To install nftables, run the following command:

root # emerge net-firewall/nftables

OpenRC configuration

Don't forget to add nftables service to startup:

root # rc-update add nftables default

You cannot use iptables and nft to perform NAT at the same time. So make sure that the iptable_nat module is unloaded. Remove iptables_nat module:

root # rmmod iptable_nat

Start nftables:

root # /etc/init.d/nftables start

Using nftables

All nftable commands are done with the nft ultility from net-firewall/nftables.

Tables

Creating tables

The following command adds a table called filter for the ip(v4) layer

root # nft add table ip filter

Likewise a table for arp can be created with

root # nft add table arp filter

   Note

The name "filter" used here is completly arbitrary. It could have any name

Listing tables

The following command lists all tables for the ip(v4) layer

root # nft list tables ip

table filter

The contents of the table filter can be listed with:

root # nft list table ip filter

table ip filter {
        chain input {
                 type filter hook input priority 0;
                 ct state established,related accept
                 iifname "lo" accept
                 ip protocol icmp accept
                 drop
        }
}

using -a with the nft command, it shows the handle of each rule. Handles are used for various operations on specific rules:

root # nft -a list table ip filter

table ip filter {
        chain input {
                 type filter hook input priority 0;
                 ct state established,related accept # handle 2
                 iifname "lo" accept # handle 3
                 ip protocol icmp accept # handle 4
                 drop # handle 5
        }
}

Deleting tables

The following command deletes the table called filter for the ip(v4) layer:

root # nft delete table ip filter

chains

Adding chains

The following command adds a chain called input to the ip filter table and registered to the input hook with priority 0. It is of the type filter.

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

   Note

If You're running this command from Bash you need to escape the semicolon

A non-base chain can be added by not specifying the chain configurations between the curly braces.

Removing chains

The following command deletes the chain called input

root # nft delete chain ip filter input

   Note

Chains can only be deleted if there are no rules in them.

rules

Adding rules

The following command adds a rule to the chain called input, on the ip filter table, dropping all traffic to port 80:

root # nft add rule ip filter input tcp dport 80 drop

Deleting Rules

To delete a rule, you first need to get the handle number of the rule. This can be done by using the -a flag on nft:

root # nft  rule ip filter input tcp dport 80 drop

table ip filter {
        chain input {
                 type filter hook input priority 0;
                 tcp dport http drop # handle 2
        }
}

It is then possible to delete the rule with:

root # nft delete rule ip filter input handle 2

Management

Backup

You can also backup your rules:

root # echo "nft flush ruleset" > backup.nft

root # nft list ruleset >> backup.nft

Restoration

And load it atomically:

root # nft -f backup.nft

OpenRC configuration

Don't forget to add nftables service to startup:

root # rc-update add nftables default

Init script - firewall nftables like a firewall iptables

#!/sbin/runscript
#      Raphael Bastos aka coffnix        #
#      Init Script for Funtoo Linux      #
##########################################

depend() {
        need net
        need nftables
        }

start(){
##################### PARTE 1 #####################
ebegin "Starting Firewall NFTables"

#######################################################################
### Incompatibilities ###
# You cannot use iptables and nft to perform NAT at the same time.
# So make sure that the iptable_nat module is unloaded
rmmod iptable_nat

#######################################################################

echo 1 > /proc/sys/net/ipv4/ip_forward
echo 1 > /proc/sys/net/ipv4/ip_dynaddr
echo 1 > /proc/sys/net/ipv4/conf/all/rp_filter
for f in /proc/sys/net/ipv4/conf/*/rp_filter ; do echo 1 > $f ; done

#######################################################################

iptables -t nat -F

#######################################################################

# ipv4
nft -f /etc/nftables/ipv4-filter

# ipv4 nat
nft -f /etc/nftables/ipv4-nat

# ipv6
nft -f /etc/nftables/ipv6-filter

# Rules firewall NTFtables
nft -f /etc/nftables/firewall.rules

#######################################################################

}

stop(){
ebegin "Stoping Firewall NFTables"

#######################################################################

#iptables -t nat -F
NFT=nft
FAMILIES="ip ip6 arp bridge"

for FAMILY in $FAMILIES; do
  TABLES=$($NFT list tables $FAMILY | grep "^table\s" | cut -d' ' -f2)

  for TABLE in $TABLES; do
    CHAINS=$($NFT list table $FAMILY $TABLE | grep "^\schain\s" | cut -d' ' -f2)

    for CHAIN in $CHAINS; do
      echo "Flushing chain: $FAMILY->$TABLE->$CHAIN"
      $NFT flush chain $FAMILY $TABLE $CHAIN
      $NFT delete chain $FAMILY $TABLE $CHAIN
    done

    echo "Flushing table: $FAMILY->$TABLE"
    $NFT flush table $FAMILY $TABLE
    $NFT delete table $FAMILY $TABLE
  done
done
}

status(){
nft list ruleset
}

# End