The purpose of this section is to briefly describe the netfilter/iptables subsystem and then delve into OpenWrt specifics.
netfilter rules require a fine level of granularity to tune packet filtering. This can cause undesirable scenarios when many rules are matching on similar packets. Be careful using the iptable application!
Netfilter is the packet filtering framework inside the Linux kernel. It allows for packet filtering, network address [and port] translation (NA[P]T) and other packet manipulations. It is far more than a simple firewall and very powerful!
Netfilter has code hooks in the kernel networking stacks (
are a set of
#define to one of the
nf_hook netfilter calls) and a set
of netfilter kernel modules (mostly starting with
iptables is the user interface to the kernel
netfilter subsystem. The iptables application uses
libiptc library to communicate between with the netfilter
libiptc, like the networking stacks, uses a BSD socket interface to
communicate between user-space and kernel-space.
There are many netfilter/iptables references so we will not repeat here, mainly because this would be almost purely a cut-and-paste effort with marginal levels of accuracy. Some good references are:
This section contains a high-level view of netfilter provided by OpenWrt.
The netfilter capabilites are exist in the kernel, either into the monolith or as loadable kernel modules. By default, OpenWrt builds the kernel with a useful set of netfilter capabilities for a robust router.
ip6tables user interfaces exist.
ebtables is no longer available in official versions due to performance implications
(https://forum.openwrt.org/viewtopic.php?pid=94379#p94379). Please employ OpenWrt Buildroot
if you need ebtables support. The kernel Kconfig settings are
arptables is not built, probably same reason as
ebtables. This is
an ipv4 function. The kernel Kconfig settings are
By default, OpenWrt uses three netfilter tables:
mangle. These are sufficient to provide the desired netfilter functionality.
Two other netfilter tables are:
raw table can be added to the kernel via
Kernel modules → Netfilter Extensions → kmod-ipt-raw . This will enable
config IP_NF_RAW tristate 'raw table support (required for NOTRACK/TRACE)' help This option adds a `raw' table to iptables. This table is the very first in the netfilter framework and hooks in at the PREROUTING and OUTPUT chains. If you want to compile it as a module, say M here and read <file:Documentation/kbuild/modules.txt>. If unsure, say `N'.
security table does not seem to be in the OpenWrt menuconfig. There is
a reference in the kernel
ipv4/netfilter/Kconfig to it but it is unclear
how to enable kernel support for it.
config IP_NF_SECURITY tristate "Security table" depends on SECURITY depends on NETFILTER_ADVANCED help This option adds a `security' table to iptables, for use with Mandatory Access Control (MAC) policy. If unsure, say N.
As was previously mentioned, there are a large number of netfilter references and examples. However, I find it helpful (to myself) to track each step of a specific fw3 rule from definition to packet processing (“Soup_to_nuts”).
This example fw3 application configuration rule allows SSH access from any station on the WAN-side of the router to any station on the LAN-side.
config rule option src 'wan' option dest 'lan' option proto 'tcp' option dest_port '22' option target 'ACCEPT' option name 'ACCEPT-SSH-WAN-LAN'
fw3 UCI parses the rule to the following iptables rule (with some others for context, implicitly created).
The rules are listed as they appear in the
fw3 print listing.
... iptables -t filter -N zone_lan_dest_ACCEPT iptables -t filter -N zone_lan_dest_REJECT ... iptables -t filter -N zone_wan_forward ... # TCP/22 from WAN jumps to zone_lan_dest_ACCEPT chain iptables -t filter -A zone_wan_forward -p tcp -m tcp --dport 22 -m comment --comment "!fw3: ACCEPT-SSH-WAN-LAN" -j zone_lan_dest_ACCEPT ... # All TCP from WAN jumps to zone_lan_dest_REJECT iptables -t filter -A zone_wan_forward -p tcp -m comment --comment "!fw3: REJECT-ALL-WAN-LAN" -j zone_lan_dest_REJECT ... # zone_lan_dest_ACCEPT jumps to final ACCEPT target iptables -t filter -A zone_lan_dest_ACCEPT -o br-lan -m comment --comment "!fw3" -j ACCEPT ... # All traffic jumps to final reject target iptables -t filter -A zone_lan_dest_REJECT -o br-lan -m comment --comment "!fw3" -j reject ... # FORWARD hook, jump to chain zone_wan_forward iptables -t filter -A FORWARD -i eth1 -m comment --comment "!fw3" -j zone_wan_forward
chains are created, primarily for convenience so they can be added and removed
The first rule added to the
zone_wan_forward chain performs a packet match
for tcp/22 (SSH) and, if it passes, jumps to the
chain. Farther down the rules, this chain matches on the output interface
br-lan (the lan-bridge) and jumps to the final
One nuiance to consider: the netfilter output interface does not provide any
routing routing information; IFF the network stack decides to route the packet
to the LAN then this rule will be invoked.
Generally each match and target is a unique kernel module. TCP/22 uses the
xt_tcpudp module, comment uses the
xt_comment module (always returns
success). The mangle TCPMSS target uses
reject target uses
are essentially no-ops:
ACCEPT allows the network stack to continue
DROP immediately discards the packet.
If the 'TCP/22' rule does not match, netfilter continues to the next rule which
matches all TCP traffic (other than SSH) and jumps to
The next rule chain jumps to the final
reject target which sends back an
ICMP packet to the originator.
zone_wan_forward chain is appended to the FORWARD hook
matching input from the
eth1, the WAN interface. This hook is statically
called in the kernel ipv4 network stack (see the
NF_HOOK call in
So that covers a single conceptual rule: TCP/SSH traffic allowed from the WAN to the LAN. NAT is a little more tricky
Beyond the standard netfilter capabilities provided in the OpenWrt release, these are useful (but not necessary.)
ipset is a netfilter mechanism to quickly manage lists of similar entities.
One powerful use is blocking spam. Typically one adds a rule to reject/drop traffic from each source. In the standard firewall, the following rules block a single source from sending a large amount of email spam to the mail server (SMTP is port 25).
Currently, the most maintainable mechanism in OpeWrt is to add rules to a
new chain in the WAN zone in
iptables -N spam_block iptables -A forwarding_rule -j spam_block iptables -t filter -A spam_block -s 126.96.36.199/22 -p tcp -m tcp --dport 25 -j DROP iptables -t filter -A spam_block -s 188.8.131.52/18 -p tcp -m tcp --dport 25 -j DROP ...
There are thousands of spam sources so the number of rules in the (custom) spam_block chain can be quite large.
In order to use ipset, it must be added to the kernel and application package.
In the OpenWrt image build directory, set it in the menu Kernel Modules → Netfilter Extensions → kmod-ipt-ipset
Once the kernel is running, add the package using
opkg install ipset.
ipset package install will fail if the kernel has not been built to
support it. DO NOT force install!!!!
There is probably a better way to add custom firewall capabilities.
This example shows how to use
ipset to block a large number of spammers!
not tested yet…
config ipset option external spam_block option match 'dest_ip dest_port' option family ipv4 option storage hash
Adding each rule to ipset will make
/etc/config/firewall unmanageable. Put
the iptable rules in
/etc/firewall.user. And there is no point to using
ipset for a small number of rules.