9.7. Ignoring Traffic¶
In some cases there are reasons to ignore certain traffic. Certain hosts may be trusted, or perhaps a backup stream should be ignored.
9.7.1. capture filters (BPF)¶
Through BPFs the capture methods pcap, af-packet, netmap and pf_ring can be told what to send to Suricata, and what not. For example a simple filter ‘tcp’ will only capture tcp packets.
If some hosts and or nets need to be ignored, use something like “not (host IP1 or IP2 or IP3 or net NET/24)”.
not host 184.108.40.206
Capture filters are specified on the commandline after all other options:
suricata -i eth0 -v not host 220.127.116.11 suricata -i eno1 -c suricata.yaml tcp or udp
Capture filters can be set per interface in the pcap, af-packet, netmap and pf_ring sections. It can also be put in a file:
echo "not host 18.104.22.168" > capture-filter.bpf suricata -i ens5f0 -F capture-filter.bpf
Using a capture filter limits what traffic Suricata processes. So the traffic not seen by Suricata will not be inspected, logged or otherwise recorded.
22.214.171.124. BPF and IPS¶
In case of IPS modes using af-packet and netmap, BPFs affect how traffic is forwarded. If a capture NIC does not capture a packet because of a BPF, it will also not be forwarded to the peering NIC.
So in the example of not host 126.96.36.199, traffic to and from the IP 188.8.131.52 is effectively dropped.
9.7.2. pass rules¶
Pass rules are Suricata rules that if matching, pass the packet and in case of TCP the rest of the flow. They look like normal rules, except that instead of alert or drop they use pass as the action.
pass ip 184.108.40.206 any <> any any (msg:"pass all traffic from/to 220.127.116.11"; sid:1;)
A big difference with capture filters is that logs such as Eve or http.log are still generated for this traffic.
Suppress rules can be used to make sure no alerts are generated for a host. This is not efficient however, as the suppression is only considered post-matching. In other words, Suricata first inspects a rule, and only then will it consider per-host suppressions.
suppress gen_id 0, sig_id 0, track by_src, ip 18.104.22.168
9.7.4. encrypted traffic¶
The TLS app layer parser has the ability to stop processing encrypted traffic after the initial handshake. By setting the app-layer.protocols.tls.encryption-handling option to bypass the rest of this flow is ignored. If flow bypass is enabled, the bypass is done in the kernel or in hardware.
9.7.5. bypassing traffic¶
Aside from using the
bypass keyword in rules, there are three other ways
to bypass traffic.
- Within suricata (local bypass). Suricata reads a packet, decodes it, checks it in the flow table. If the corresponding flow is local bypassed then it simply skips all streaming, detection and output and the packet goes directly out in IDS mode and to verdict in IPS mode.
- Within the kernel (capture bypass). When Suricata decides to bypass it calls a function provided by the capture method to declare the bypass in the capture. For NFQ this is a simple mark that will be used by the iptables/nftablesruleset. For AF_PACKET this will be a call to add an element in an eBPF hash table stored in kernel.
- Within the NIC driver. This method relies upon XDP, XDP can process the traffic prior to reaching the kernel.
Additional bypass documentation: