Matching packet headers: Difference between revisions
(→Matching Ethernet header fields: missing source tag) |
(→Matching ICMP traffic: Use nft describe to list available nft type keywords) |
||
Line 83: | Line 83: | ||
= Matching ICMP traffic = | = Matching ICMP traffic = | ||
You can drop all ICMP echo requests (popularly known as ''pings'') via: | You can drop all [https://en.wikipedia.org/wiki/Internet_Control_Message_Protocol ICMP] echo requests (popularly known as ''pings'') via: | ||
<source lang="bash"> | <source lang="bash"> | ||
Line 89: | Line 89: | ||
</source> | </source> | ||
You can use ''nft describe'' to find ''nft'''s available ''icmp type'' keywords: | |||
<source lang="bash"> | |||
% nft describe icmp type | |||
payload expression, datatype icmp_type (ICMP type) (basetype integer), 8 bits | |||
pre-defined symbolic constants (in decimal): | |||
echo-reply 0 | |||
destination-unreachable 3 | |||
source-quench 4 | |||
redirect 5 | |||
echo-request 8 | |||
router-advertisement 9 | |||
router-solicitation 10 | |||
time-exceeded 11 | |||
parameter-problem 12 | |||
timestamp-request 13 | |||
timestamp-reply 14 | |||
info-request 15 | |||
info-reply 16 | |||
address-mask-request 17 | |||
address-mask-reply 18 | |||
</source> | |||
= Matching UDP/TCP headers in the same rule = | = Matching UDP/TCP headers in the same rule = |
Revision as of 22:41, 14 February 2021
The nft command line utility supports the following layer 4 protocols: AH, ESP, UDP, UDPlite, TCP, DCCP, SCTP and IPComp.
Matching transport protocol
The following rule shows how to match any kind of TCP traffic:
% nft add rule filter output ip protocol tcp
Matching Ethernet header fields
If you want to match ethernet traffic whose destination address is ff:ff:ff:ff:ff:ff, you can type the following command:
% nft add rule filter input ether daddr ff:ff:ff:ff:ff:ff counter
Do not forget that the layer 2 header information is only available in the input path.
Matching IPv4 header fields
You can also match traffic based on the IPv4 source and destination, the following example shows how to account all traffic that comes from 192.168.1.100 and that is addressed to 192.168.1.1:
% nft add rule filter input ip saddr 192.168.1.100 ip daddr 192.168.1.1 counter
Note that, since the rule is attached to the input chain, your local machine needs to use the 192.168.1.1 address, otherwise you won't see any matching ;-).
To filter on a layer 4 protocol like TCP, you can use the protocol keyword:
% nft add rule filter input protocol tcp counter
Matching IPv6 header fields
If you want to account IPv6 traffic that is addressed to abcd::100, you can type the following command:
% nft add rule filter output ip6 daddr abcd::100 counter
To filter on a layer 4 protocol like TCP, you can use the nexthdr keyword:
% nft add rule filter input ip6 nexthdr tcp counter
Do not forget to create an ip6 table and register the corresponding chains to run the examples.
NOTE: the syntax mixing IPv6/IPv4 notation is not supported yet: '::ffff:192.168.1.0'
Matching TCP/UDP/UDPlite traffic
The following examples show how to drop all tcp traffic for low TCP ports (1-1024):
% nft add rule filter input tcp dport 1-1024 counter drop
Note that this rule is using an interval (from 1 to 1024).
To match on TCP flags, you need to use a binary operation. For example, to count packets that are not SYN ones:
% nft add rule filter input tcp flags != syn counter
More complex filters can be used. For example, to count and log TCP packets with flags SYN and ACK set:
% nft -i
nft> add rule filter output tcp flags & (syn | ack) == syn | ack counter log
This example drops TCP SYN packets which a MSS lower than 500:
% nft add rule inet filter input tcp flags syn tcp option maxseg size 1-500 drop
Matching ICMP traffic
You can drop all ICMP echo requests (popularly known as pings) via:
% nft add rule filter input icmp type echo-request counter drop
You can use nft describe to find nft's available icmp type keywords:
% nft describe icmp type
payload expression, datatype icmp_type (ICMP type) (basetype integer), 8 bits
pre-defined symbolic constants (in decimal):
echo-reply 0
destination-unreachable 3
source-quench 4
redirect 5
echo-request 8
router-advertisement 9
router-solicitation 10
time-exceeded 11
parameter-problem 12
timestamp-request 13
timestamp-reply 14
info-request 15
info-reply 16
address-mask-request 17
address-mask-reply 18
Matching UDP/TCP headers in the same rule
Matching several transport protocols in a single rule is a new feature of nftables that wasn't present in iptables.
The following example matches the tcp/udp port 53 in the transport header:
% nft add rule filter input meta l4proto { tcp, udp } @th,16,16 53 counter packets 0 bytes 0 accept comment \"accept DNS\"