OpenWrt uses the firewall3 (
fw3) netfilter/iptable rule builder application.
It runs in user-space to parse a configuration file into a set of
iptables rules, sending each to the kernel netfilter modules.
The netfilter rule set can be very complex for a typical router. This is by
necessity; each rule is tailored to a discrete capability provided by the router
to protect its supported networks, provide
NAT to conserve scarce IPv4
mangle the packets during routing. A typical router has
over 100 rules designed to support packet routing.
The fw3 application is used by OpenWRT to “safely” construct a rule set while hiding much of the details.
On inspecting the netfilter rule set using
fw3 print, you will see
a number of netfilter/iptables rules either not explicitly defined in the fw3 configuration files,
or more difficult to understand (thank goodness for the
The netfilter rules include:
_rule) for each special target and zone
option syn_flood 1or
option mtu_fix 1each translate to complex iptable rules
option masq 1translates to the '-j MASQUERADE' target for NAT
manglerules that match bits in the packets TCP header and then modify the packet
The fw3 configuration is fairly straight forward and automatically provides the router with a base rule set of rules and an understandable configuration file for additional rules.
The rules consumed by netfilter are, at best, difficult to comprehend due to the exacting nature of netfilter. However, every rule provides desired capability or blocks malicious capability, and therefore necessary.
fw3 is a user-space application similar in nature to the iptables application.
Both use the
libiptc library to communication with the netfilter kernel
modules, and follow the same basic pattern:
iptc_initto establish a socket and, using a
getsockoptcall, read the netfilter table into the application. This is on a per-table (
filterby default) basis.
iptc_committo replace the table in the kernel.
The fw3 application uses the procd subsystem to for management. The conventional
/etc/init.d/firewall, a shell script calling
fw3 with a small
set of arguments:
boot: this is invoked during system init (bootup)
start: parse configuration files and write to the netfilter kernel modules
stop: flush configuration rules from the kernel modules (they will not be unloaded)
reload: read the netfilter rules from the kernel, replace using the
configuration files, and write back to the netfilter kernel modules.
stop, only the rules in the configuration files will be flushed.
Those rules automatically generated by
fw3 will be retained. If ALL the rules are flushed,
and the default policy is set to
DROP then all communication to
and through the router would be dropped. Conversely if the default policy is
ACCEPT then the
router would provide no security.