DDoS attacks are rising at an alarming rate, with one recent report showing a 40% increase YOY. Part of the challenge in preventing these attacks is that there are many different types, using different tactics and techniques. While some protections work against all types of DDoS attacks, like robust monitoring and alerting and DDoS scrubbing, each type also requires a different protection strategy, and sometimes different tools.
Protocol attacks represent an especially insidious type of DDoS, aimed at the L3/L4 network and transport layers. The category includes SYN floods, TCP connection floods, fragmented packet attacks, and Smurf (ICMP) attacks. Because L3 and L4 are upstream in the enterprise infrastructure, such attacks can cause wider damage than L6 or L7 attacks.
If a protocol attack succeeds, it can impact many services, including entire hosts, networks, links, apps, edge devices, and security devices that protect against other types of attacks.
It can be harder to ward off protocol attacks because they don't always look like the typical large-bandwidth DDoS surge. They can appear as legitimate traffic, but they target state and device resources by forcing them to track a lot of fake connections. That's why enterprises need to deploy DDoS protection services and methodologies that are tuned specifically to combat protocol attacks, in order to keep their systems safe.
1. Dropping Malformed and Fragmented Packets
Protocol DDoS attacks often employ abusive packet behavior that forces systems to do extra work parsing, reassembling, tracking, or validating packets.
Dropping packets that are malformed, fragmented, break protocol rules, or look suspicious stops you from wasting device resources and CPU work, and eases the strain on middleboxes.
This usually involves abandoning malformed packets and unwanted fragments entirely at the edge, blocking weird fragmentation patterns, and using firewall or IPS rules that detect malformed and fragment-based evasion patterns.
2. Turning on Protocol-Rate Limiting
Protocol DDoS attacks often win by abusing the fact that networks and devices will process packets quickly and automatically, even when the packets don't represent real users. The good news is that, unlike app-layer attacks, it's relatively easy to spot and rate-limit damaging spam requests.
Protocol-rate limiting sets caps on how much traffic of a certain type you'll accept. It includes ICMP rate limits to block ping flood-style traffic, UDP rate limits for raw UDP floods, and DNS rate limits that protect resolvers or authoritative DNS.
It's best to deploy it at the edge router or firewall, before deeper systems see it, and to build in burst allowances so that normal spikes don't break real usage needs. Implementing protocol-rate limiting per source IP or subnet prevents one host from dominating.
3. Adopting RTBH and FlowSpec Tools
Protocol attacks don't alwaysuse volume as a weapon, but sometimes they do. Attackers use high packet rates like SYN floods, UDP floods, ICMP floods to overwhelm upstream links and edge devices before traffic even reaches your app stack. This makes it crucial to stop bad packets as far upstream and as fast as possible.
A Remotely Triggered Black Hole (RTBH) is an excellent but blunt tool. It quickly reroutes all the traffic for a particular IP, dropping it into a "black hole" upstream. The downside is that it takes the entire IP offline. You have to sacrifice availability for one target to keep the rest of your infrastructure alive.
There are even tools that push specific attack traffic patterns upstream using granular filtering rules, so that the rest of your IP remains up. Either way, you'll cut off L3/L4 attack traffic fast, before it melts your infrastructure.
4. Implementing SYN Cookies and TCP Backlog Tuning
Protocol DDoS attacks frequently focus on exhausting connection and state resources, rather than (only) bandwidth. One tactic they use is to fill up the TCP connection state table with TCP SYNs (connection-start packets). Each half-open connection uses up state and memory, so even low-level traffic can create a backlog and cause timeouts for legit users.
A good way to prevent this is to use SYN cookies together with TCP backlog tuning. SYN cookies let the server avoid storing connection state when it receives a SYN, instead waiting for the final ACK from the client before allocating resources. Tuning the TCP backlog can increase backlog size so spikes don't instantly overwhelm it, and reduce timeouts and retry behavior so bad entries clear faster.
Together, they keep your systems open to accept new TCP handshakes.
5. Utilizing Connection Limits and State-Table Protections
Another strategy to prevent state exhaustion involves adjusting connection limits and state-table protections.
This broadens defenses beyond the TCP handshake queue to include entities like firewalls, load balancers, VPN gateways, and NAT routers, thereby enforcing sane limits and keeping enough capacity for real users.
Limiting connections puts hard caps on how much state any one source or network block can force your resources to track, like the maximum number of new connections per second per IP, or maximum NAT translations or sessions per source. Common state protections include aggressive timeouts for half-open or idle sessions to clear junk faster, and prioritizing established connections over new suspicious ones.
The Right Tactics Can Limit the Dangers of Protocol Attacks
Protocol DDoS attacks can be highly damaging, but there are tools and strategies that hold them up and neutralize their threat. Reducing the potential impact of protocol attacks will keep your networks available for legitimate users, cut your costs, and ensure availability for your services.
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