Ubuntu Manpage: firewalld.direct - firewalld direct configuration file

NAME

       firewalld.direct - firewalld direct configuration file

SYNOPSIS

       /etc/firewalld/direct.xml

DEPRECATED

       The direct interface has been deprecated. It will be removed in a future release. It is superseded by
       policies, see firewalld.policies(5).

DESCRIPTION

       Direct configuration gives a more direct access to the firewall. It requires user to know basic
       ip(6)tables/ebtables concepts, i.e.  table (filter/mangle/nat/...), chain (INPUT/OUTPUT/FORWARD/...),
       commands (-A/-D/-I/...), parameters (-p/-s/-d/-j/...) and targets (ACCEPT/DROP/REJECT/...). Direct
       configuration should be used only as a last resort when it's not possible to use firewalld.zone(5). See
       also Direct Options in firewall-cmd(1).

       A firewalld direct configuration file contains information about permanent direct chains, rules and
       passthrough ...

       This is the structure of a direct configuration file:

           <?xml version="1.0" encoding="utf-8"?>
           <direct>
             [ <chain ipv="ipv4|ipv6|eb" table="table" chain="chain"/> ]
             [ <rule ipv="ipv4|ipv6|eb" table="table" chain="chain" priority="priority"> args </rule> ]
             [ <passthrough ipv="ipv4|ipv6|eb"> args </passthrough> ]
           </direct>

   direct
       The mandatory direct start and end tag defines the direct. This tag can only be used once in a direct
       configuration file. There are no attributes for direct.

   chain
       Is an optional empty-element tag and can be used several times. It can be used to define names for
       additional chains. A chain entry has exactly three attributes:

       ipv="ipv4|ipv6|eb"
           The IP family where the chain will be created. This can be either ipv4, ipv6 or eb.

       table="table"
           The table name where the chain will be created. This can be one of the tables that can be used for
           iptables, ip6tables or ebtables. For the possible values, see TABLES section in the iptables,
           ip6tables or ebtables man pages.

       chain="chain"
           The name of the chain, that will be created. Please make sure that there is no other chain with this
           name already.

       Please remember to add a rule or passthrough rule with an --jump or --goto option to connect the chain to
       another one.

   rule
       Is an optional element tag and can be used several times. It can be used to add rules to a built-in or
       added chain. A rule entry has exactly four attributes:

       ipv="ipv4|ipv6|eb"
           The IP family where the rule will be added. This can be either ipv4, ipv6 or eb.

       table="table"
           The table name where the rule will be added. This can be one of the tables that can be used for
           iptables, ip6tables or ebtables. For the possible values, see TABLES section in the iptables,
           ip6tables or ebtables man pages.

       chain="chain"
           The name of the chain where the rule will be added. This can be either a built-in chain or a chain
           that has been created with the chain tag. If the chain name is a built-in chain, then the rule will
           be added to chain_direct, else the supplied chain name is used.  chain_direct is created internally
           for all built-in chains to make sure that the added rules do not conflict with the rules created by
           firewalld.

       priority="priority"
           The priority is used to order rules. Priority 0 means add rule on top of the chain, with a higher
           priority the rule will be added further down. Rules with the same priority are on the same level and
           the order of these rules is not fixed and may change. If you want to make sure that a rule will be
           added after another one, use a low priority for the first and a higher for the following.

       The args can be any arguments of iptables or ip6tables, that do not conflict with the table or chain
       attributes.

   passthrough
       Is an optional element tag and can be used several times. It can be used to add rules to a built-in or
       added chain. A rule entry has exactly one attribute:

       ipv="ipv4|ipv6|eb"
           The IP family where the passthrough rule will be added. This can be either ipv4, ipv6 or eb.

       The args can be any arguments of iptables or ip6tables.

       The passthrough rule will be added to the chain directly. There is no mechanism like for the direct rule
       above. The user of the passthrough rule has to make sure that there will be no conflict with the rules
       created by firewalld.

CAVEATS

Depending on the value of FirewallBackend (see firewalld.conf(5)) direct rules behave differently in some
scenarios.

Packet accept/drop precedence
Due to implementation details of netfilter inside the kernel, if FirewallBackend=nftables is used direct
rules that ACCEPT packets don't actually cause the packets to be immediately accepted by the system.
Those packets are still be subject to firewalld's nftables ruleset. This basically means there are two
independent firewalls and packets must be accepted by both (iptables and nftables). As an aside, this
scenario also occurs inside of nftables (again due to netfilter) if there are multiple chains attached to
the same hook - it's not as simple as iptables vs nftables.

There are a handful of options to workaround the ACCEPT issue:

1. Rich Rules

If a rich rule can be used, then they should always be preferred over direct rules. Rich Rules will
be converted to the enabled FirewallBackend. See firewalld.richlanguage(5).

2. Blanket Accept

Users can add an explicit accept to the nftables ruleset. This can be done by adding the interface or
source to the trusted zone.

This strategy is often employed by things that perform their own filtering such as: libvirt, podman,
docker.

Warning: This means firewalld will do no filtering on these packets. It must all be done via direct
rules or out-of-band iptables rules.

3. Selective Accept

Alternatively, enable only the relevant service, port, address, or otherwise in the appropriate zone.

4. Revert to the iptables backend

A last resort is to revert to the iptables backend by setting FirewallBackend=iptables. Users should
be aware that firewalld development focuses on the nftables backend.

For direct rules that DROP packets the packets are immediately dropped regardless of the value of
FirewallBackend. As such, there is no special consideration needed.

Firewalld guarantees the above ACCEPT/DROP behavior by registering nftables hooks with a lower precedence
than iptables hooks.

Direct interface precedence
With FirewallBackend=iptables firewalld's top-level internal rules apply before direct rules are
executed. This includes rules to accept existing connections. In the past this has surprised users. As an
example, if a user adds a direct rule to drop traffic on destination port 22 existing SSH sessions would
continue to function, but new connections would be denied.

With FirewallBackend=nftables direct rules were deliberately given a higher precedence than all other
firewalld rules. This includes rules to accept existing connections.

EXAMPLE

       Denylisting of the networks 192.168.1.0/24 and 192.168.5.0/24 with logging and dropping early in the raw
       table:

           <?xml version="1.0" encoding="utf-8"?>
           <direct>
             <chain ipv="ipv4" table="raw" chain="denylist"/>
             <rule ipv="ipv4" table="raw" chain="PREROUTING" priority="0">-s 192.168.1.0/24 -j denylist</rule>
             <rule ipv="ipv4" table="raw" chain="PREROUTING" priority="1">-s 192.168.5.0/24 -j denylist</rule>
             <rule ipv="ipv4" table="raw" chain="denylist" priority="0">-m limit --limit 1/min -j LOG --log-prefix "denylisted: "</rule>
             <rule ipv="ipv4" table="raw" chain="denylist" priority="1">-j DROP</rule>
           </direct>

NOTES

       firewalld home page:
           http://firewalld.org

AUTHORS

       Thomas Woerner <twoerner@redhat.com>
           Developer

       Jiri Popelka <jpopelka@redhat.com>
           Developer

       Eric Garver <eric@garver.life>
           Developer