Nftables

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net-firewall/nftables


Source Repository:Gentoo Portage Tree

Summary: Linux kernel (3.13+) firewall, NAT and packet mangling tools


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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 Under Network 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:

# emerge net-firewall/nftables


OpenRC configuration

Don't forget to add nftables service to startup:

# 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:

# rmmod iptable_nat

Start nftables:

# /etc/init.d/nftables start


Using nftables

All nftable commands are done with the nft ultility from Nftables.

Tables

Creating tables

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

# nft add table ip filter

Likewise a table for arp can be created with

# 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

# nft list tables ip
table filter

The contents of the table filter can be listed with:

# 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:

# 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:

# 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.

# 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

# 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:

# 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:

# 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:

# nft delete rule ip filter input handle 2

Management

Backup

You can also backup your rules:

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

Restoration

And load it atomically:

# nft -f backup.nft

OpenRC configuration

Don't forget to add nftables service to startup:

# rc-update add nftables default

Init script (firewall nftables like a 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

Personal Rules

And configure your personal rules:

# A simple firewall
#

table ip filter {
        chain input {
                type filter hook input priority 0;
                ct state established,related accept

                # invalid connections
                ct state invalid drop

                # loopback interface
                iifname "lo" accept

                # icmp
                ip protocol icmp accept

                # open ports
                #tcp dport {ssh, http} accept
                #udp dport {5060} accept
                #udp dport {4000-20000} accept

                # Bind DNS Server
                udp sport domain accept
                tcp sport domain accept


                # LAN
                #ip saddr 192.168.100.0/24 ct state new counter accept

                # everything else
                drop
        }
        chain forward {
                #ip daddr 192.168.100.0/24 accept
        }
}


table ip6 filter {
        chain input {
                type filter hook input priority 0;

                # established/related connections
                ct state established,related accept

                # invalid connections
                ct state invalid drop

                # loopback interface
                iifname lo accept

                # icmp
                ip6 nexthdr icmpv6 accept

                # open tcp ports: sshd (22), httpd (80)
                #tcp dport {ssh, http} accept
                #udp dport {5060} accept
                #udp dport {4000-20000} accept

                # everything else
                drop
        }
}


table ip nat {
        chain prerouting {
                #tcp dport 8081 ip protocol tcp counter dnat 192.168.100.24
        }
        chain postrouting {
                #masquerade random,persistent
                #ip saddr 192.168.100.0/24 oif eth0 snat 192.168.25.3
        }

}