Network Packet Filtering
Overview
The Network Packet Filtering facility provides the infrastructure to construct custom rules for accepting and/or denying packet transmission and reception. This can be used to create a basic firewall, control network traffic, etc.
The CONFIG_NET_PKT_FILTER must be set in order to enable the
relevant APIs.
Both the transmission and reception paths may have a list of filter rules. Each rule is made of a set of conditions and a packet outcome. Every packet is subjected to the conditions attached to a rule. When all the conditions for a given rule are true then the packet outcome is immediately determined as specified by the current rule and no more rules are considered. If one condition is false then the next rule in the list is considered.
Packet outcome is either NET_OK to accept the packet or NET_DROP to
drop it.
A rule is represented by a npf_rule object. It can be inserted to,
appended to or removed from a rule list contained in a
npf_rule_list object using npf_insert_rule(),
npf_append_rule(), and npf_remove_rule().
Currently, two such rule lists exist: npf_send_rules for outgoing packets,
and npf_recv_rules for incoming packets.
If a filter rule list is empty then NET_OK is assumed. If a non-empty
rule list runs to the end then NET_DROP is assumed. However it is
recommended to always terminate a non-empty rule list with an explicit
default termination rule, either npf_default_ok or npf_default_drop.
Rule conditions are represented by a npf_test. This structure
can be embedded into a larger structure when a specific condition requires
extra test data. It is up to the test function for such conditions to
retrieve the outer structure from the provided npf_test structure pointer.
Convenience macros are provided in include/zephyr/net/net_pkt_filter.h
to statically define condition instances for various conditions, and
NPF_RULE() to create a rule instance to tie them.
Examples
Here’s an example usage:
static NPF_SIZE_MAX(maxsize_200, 200);
static NPF_ETH_TYPE_MATCH(ip_packet, NET_ETH_PTYPE_IP);
static NPF_RULE(small_ip_pkt, NET_OK, ip_packet, maxsize_200);
void install_my_filter(void)
{
npf_insert_recv_rule(&npf_default_drop);
npf_insert_recv_rule(&small_ip_pkt);
}
The above would accept IP packets that are 200 bytes or smaller, and drop all other packets.
Another (less efficient) way to achieve the same result could be:
static NPF_SIZE_MIN(minsize_201, 201);
static NPF_ETH_TYPE_UNMATCH(not_ip_packet, NET_ETH_PTYPE_IP);
static NPF_RULE(reject_big_pkts, NET_DROP, minsize_201);
static NPF_RULE(reject_non_ip, NET_DROP, not_ip_packet);
void install_my_filter(void) {
npf_append_recv_rule(&reject_big_pkts);
npf_append_recv_rule(&reject_non_ip);
npf_append_recv_rule(&npf_default_ok);
}