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>
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:
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:
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:
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.
Depending on the value of FirewallBackend (see firewalld.conf(5)) direct rules behave differently in some scenarios.
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:
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).
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.
Alternatively, enable only the relevant service, port, address, or otherwise in the appropriate zone.
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.
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.
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>
firewalld home page:
More documentation with examples:
Thomas Woerner <email@example.com>
Jiri Popelka <firstname.lastname@example.org>
Eric Garver <email@example.com>