Ubuntu Manpage: firewalld.direct - firewalld direct configuration file

NAME

       firewalld.direct - firewalld direct configuration file

SYNOPSIS

       /etc/firewalld/direct.xml

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 informations 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

       Blacklisting 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="blacklist"/>
             <rule ipv="ipv4" table="raw" chain="PREROUTING" priority="0">-s 192.168.1.0/24 -j blacklist</rule>
             <rule ipv="ipv4" table="raw" chain="PREROUTING" priority="1">-s 192.168.5.0/24 -j blacklist</rule>
             <rule ipv="ipv4" table="raw" chain="blacklist" priority="0">-m limit --limit 1/min -j LOG --log-prefix "blacklisted: "</rule>
             <rule ipv="ipv4" table="raw" chain="blacklist" priority="1">-j DROP</rule>
           </direct>

NOTES

       firewalld home page:
           http://firewalld.org

       More documentation with examples:
           http://fedoraproject.org/wiki/FirewallD

AUTHORS

       Thomas Woerner <twoerner@redhat.com>
           Developer

       Jiri Popelka <jpopelka@redhat.com>
           Developer

       Eric Garver <eric@garver.life>
           Developer