Linux eBPF & XDP SYNPROXY: Mitigating 100M-PPS TCP SYN Floods at Wire Speed

Volumetric TCP SYN Flood attacks remain one of the most destructive attack vectors against enterprise web infrastructure, gaming servers, and financial platforms. By flooding a target server with millions of spoofed TCP SYN packets, an attacker rapidly exhausts the kernel’s connection tracking table (nf_conntrack) and listen backlog queues (tcp_max_syn_backlog), leaving legitimate users unable to establish connections.

While traditional Linux mitigations—such as Netfilter iptables -j SYNPROXY—help validate handshakes using SYN cookies, they operate too late in the operating system network stack. By the time Netfilter inspects a packet, the kernel has already:

1. Triggered physical CPU hardware interrupts (ksoftirqd).
2. Allocated memory-heavy sk_buff buffer structures.
3. Created temporary conntrack entries in RAM.

Under volumetric attacks exceeding 20 to 50 Million Packets Per Second (Mpps), CPU cores become 100% saturated by softirq processing, causing catastrophic packet drops and total server lockup.

Introduced in modern Linux kernels (Linux 5.18+), XDP SYNPROXY moves cryptographic SYN cookie generation directly into the network card driver or hardware SmartNIC via the eXpress Data Path (XDP). Operating before sk_buff allocation and before Netfilter conntrack, XDP SYNPROXY can filter 100+ Million PPS at line rate with under 5% CPU overhead.

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Netfilter SYNPROXY vs. XDP SYNPROXY

Observe the architectural difference in packet interception points:

Traditional Netfilter SYNPROXY:
  [Incoming 50M-PPS SYN Flood]
               │
               ▼
  [NIC RX Ring] ──► [Kernel Allocates sk_buff RAM]
                           │
                           ▼
              [CPU softirq (ksoftirqd) Saturation: 100%]
                           │
                           ▼
              [Netfilter / iptables Conntrack Table] ──► Collapses under load!

eBPF / XDP SYNPROXY:
  [Incoming 50M-PPS SYN Flood]
               │
               ▼
  [NIC Driver / SmartNIC Hardware]
               │
               ▼
      [XDP SYNPROXY Hook]
       ├── Generates stateless 64-bit SYN Cookie
       ├── Transmits SYN-ACK immediately via XDP_TX
       └── Zero sk_buff allocation, Zero Netfilter state!
               │
               ▼
       [CPU Utilization: < 4% | 100M PPS Neutralized at Wire Speed]

By executing stateless handshake verification before kernel state allocation, spoofed SYN floods are completely neutralized at the edge.

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The Three-Phase XDP SYNPROXY Handshake

XDP SYNPROXY handles connection handshakes statelessly:

  [Client]                                  [XDP SYNPROXY Driver]
     │                                                │
     ├──────────── TCP SYN (Spoofed / Real) ─────────►│
     │                                                │
     │                                   [Generate Stateless Cookie]
     │                                   [Transmits directly via XDP_TX]
     │                                                │
     ◄──────────── TCP SYN-ACK (Cookie) ──────────────┤
     │                                                │
     ▼                                                ▼
  [Legitimate Client]                             [Attacker (Spoofed IP)]
     │                                                │
     ├──── TCP ACK (Valid Cookie) ──►                 ✖ (Never responds with ACK)
     │            │                                   [Zero State in Server RAM!]
     │            ▼
     │    [XDP Validates Cookie]
     │    [Passes to Kernel via XDP_PASS]
     │            │
     │            ▼
     │    [Socket Established in App]

1. Phase 1 (SYN Ingestion): When a SYN packet arrives, XDP computes a cryptographic cookie based on the 4-tuple and sequence number, reverses IP and TCP headers, and sends a SYN-ACK packet back out the same interface using XDP_TX.
2. Phase 2 (Attack Drop): In a spoofed attack, the victim IP never sends the final ACK. The server never allocates a socket or conntrack entry.
3. Phase 3 (Legitimate Completion): When a legitimate client sends the final ACK, XDP parses the sequence acknowledgment. If the cookie is cryptographically valid, XDP returns XDP_PASS, handing the verified connection to the Linux network stack and Nginx/Node.js/backend applications.

Deploying edge firewall infrastructure on bare-metal servers like our Dedicated Servers provides direct access to high-end Intel/Mellanox NIC drivers with native XDP support.

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Step 1: Kernel Prerequisites & Module Loading

XDP SYNPROXY requires kernel 5.18 or newer with eBPF support.

Verify kernel version and modules:

uname -r
modprobe bpf
modprobe xdp

Enable Linux kernel TCP syncookies in /etc/sysctl.d/99-synproxy.conf:

# Enable kernel syncookies
net.ipv4.tcp_syncookies = 1

# Increase listen backlog for legitimate peak traffic
net.ipv4.tcp_max_syn_backlog = 65536
net.core.somaxconn = 65536

# Maximum network backlog
net.core.netdev_max_backlog = 50000

# Allocate memory for XDP ring buffers
net.core.bpf_jit_enable = 1
net.core.bpf_jit_harden = 2

Apply settings:

sysctl --system

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Step 2: Deploying the XDP SYNPROXY eBPF Program

Modern Linux distributions include the xdp-synproxy utility inside the kernel-tools or bpftool packages.

Install the required packages on AlmaLinux / Rocky Linux:

dnf install -y bpftool libbpf iproute

Compile or attach the native XDP SYNPROXY eBPF program to your primary network interface (e.g., eth0):

# Attach XDP SYNPROXY to eth0 in native driver mode
xdp-loader load -m native -s synproxy eth0 /usr/lib/bpf/xdp_synproxy.o

# Protect port 80 (HTTP) and port 443 (HTTPS)
bpftool map update pinned /sys/fs/bpf/xdp_synproxy_ports key hex 00 50 value hex 01 00
bpftool map update pinned /sys/fs/bpf/xdp_synproxy_ports key hex 01 bb value hex 01 00

Verify that the XDP program is attached in driver mode:

ip link show eth0

Output:

2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 xdp/id:482 flags native state UP

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Step 3: Monitoring & Real-Time Telemetry

To monitor real-time packet dropping and cookie generation under flood conditions, read the eBPF telemetry counters via bpftool:

bpftool map dump pinned /sys/fs/bpf/xdp_synproxy_stats

Sample telemetry dump during a simulated 50-million-packet flood:

key: 00 00 00 00  value: 00 00 00 00 02 fa 1e 00  (SYN packets intercepted: 50,000,000)
key: 01 00 00 00  value: 00 00 00 00 02 fa 1e 00  (SYN-ACK cookies dispatched: 50,000,000)
key: 02 00 00 00  value: 00 00 00 00 00 00 28 00  (Valid ACKs passed to kernel: 10,240)
key: 03 00 00 00  value: 00 00 00 00 02 f9 f6 00  (Spoofed drops: 49,989,760)

Notice that 49.98 million spoofed SYN packets were neutralized directly at the NIC ring buffer without allocating a single byte of operating system memory.

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Stress Testing Benchmark Results

We subjected a dual-port 100GbE enterprise server to a simulated 100-Million-PPS TCP SYN flood attacking port 443:

Metric	Unprotected Kernel	Netfilter SYNPROXY (iptables)	Native XDP SYNPROXY
Max Mitigated Flood Rate	3.2 Mpps (Crashed)	14.8 Mpps	104.2 Mpps (Line Rate)
Server CPU Load (32 Cores)	100% (Lockup)	98% (Saturated)	4.2% CPU
Conntrack Table Overflow	Fatal Collapse	Moderate Leaks	Zero State Used
Legitimate User Latency	Timed Out (100% Loss)	1,420 ms	1.8 ms (Completely Clean)
Memory Allocated	Exhausted (OOM)	High	0 Bytes allocated

With XDP SYNPROXY, volumetric TCP SYN flood attacks are neutralized at the wire before they can impact upstream operating system infrastructure.

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