wolfguard: Modern VPN Architecture Design

Document Version: 1.0 Date: 2025-10-29 Based on: Industry research and draft planning documents Target: wolfguard v2.0.0 (C23, ISO/IEC 9899:2024)

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Executive Summary

This document describes the modern architecture design for wolfguard, based on comprehensive research of industry-leading VPN implementations including ExpressVPN Lightway, CloudFlare BoringTun, WireGuard, Tailscale, and OpenVPN 3.x. The architecture emphasizes event-driven patterns, pure C implementation, and maximum performance through modern kernel interfaces.

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Architecture Philosophy

Core Principles

1. Event-Driven Architecture: Single-threaded event loop with worker pool (not thread-per-connection)
2. Pure C Implementation: No C++ dependencies, modern C23 standards
3. Zero-Copy Networking: Minimize memory copies, leverage kernel features
4. NUMA-Aware: Optimize for multi-socket systems
5. Callback-Based Crypto: Asynchronous TLS/DTLS operations
6. Protocol Agnostic: Clean abstraction layers for future protocols

Industry Research Insights

ExpressVPN Lightway (Analyzed):

• Callback-based wolfSSL integration
• Event-driven architecture
• DTLS 1.3 for UDP transport
• Rust optimization layer (2x improvement over OpenVPN)
• Minimal attack surface (2000 LOC core)

CloudFlare BoringTun:

• Rust WireGuard implementation
• 200 Gbps on single CPU core
• UDP GSO/GRO optimizations
• MASQUE/HTTP3/QUIC integration

WireGuard:

• Minimalist design (4000 LOC)
• Kernel integration
• 1011 Mbps throughput (vs OpenVPN 258 Mbps - 4x improvement)
• Modern cryptography (ChaCha20-Poly1305, Curve25519)

Tailscale:

• wireguard-go with UDP GSO/GRO
• 10+ Gbps achieved on modern hardware
• NAT traversal and coordination layer

OpenVPN 3.x:

• C++20 rewrite
• Event-driven ASIO (Boost.Asio)
• Clean abstraction layers
• Modern cipher suites

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System Architecture

High-Level Overview

┌─────────────────────────────────────────────────────────────────┐
│                    wolfguard Process                         │
│                                                                   │
│  ┌─────────────┐  ┌─────────────┐  ┌─────────────┐             │
│  │   Main      │  │   Worker    │  │   Worker    │             │
│  │   Process   │  │   Pool #0   │  │   Pool #1   │  ...        │
│  │  (libuv)    │  │  (libuv)    │  │  (libuv)    │             │
│  └──────┬──────┘  └──────┬──────┘  └──────┬──────┘             │
│         │                │                │                      │
│         ▼                ▼                ▼                      │
│  ┌──────────────────────────────────────────────────┐           │
│  │         TLS Abstraction Layer (wolfSSL)          │           │
│  └──────────────────────────────────────────────────┘           │
│         │                                                        │
│         ▼                                                        │
│  ┌──────────────────────────────────────────────────┐           │
│  │       Network I/O (io_uring / epoll)             │           │
│  └──────────────────────────────────────────────────┘           │
│         │                                                        │
└─────────┼────────────────────────────────────────────────────────┘
          │
          ▼
┌─────────────────────────────────────────────────────────────────┐
│                       Linux Kernel                               │
│                                                                   │
│  ┌──────────────┐  ┌──────────────┐  ┌──────────────┐          │
│  │   TCP/IP     │  │   UDP/DTLS   │  │   TUN/TAP    │          │
│  │   Stack      │  │   Stack      │  │   Device     │          │
│  └──────────────┘  └──────────────┘  └──────────────┘          │
│                                                                   │
│  ┌──────────────────────────────────────────────────┐           │
│  │       eBPF/XDP (Optional Performance Layer)      │           │
│  └──────────────────────────────────────────────────┘           │
└─────────────────────────────────────────────────────────────────┘

Component Details

1. Main Process (Control Plane)

Responsibilities:

• Configuration management
• Worker process spawning and monitoring
• Client authentication (aggregate auth framework)
• Session management and tracking
• Health monitoring and metrics
• IPC coordination (protobuf-c)

Event Loop: libuv Threading: Single-threaded with async I/O

Key Libraries:

• libuv 1.51.0+ (event loop)
• wolfSSL 5.8.2+ (TLS/DTLS for control channel)
• llhttp 9.2+ (HTTP/HTTPS control protocol)
• cJSON 1.7.19+ (configuration, JSON API)
• protobuf-c (IPC with workers)
• mimalloc 3.1.5+ (memory allocation)

2. Worker Pool (Data Plane)

Responsibilities:

• VPN tunnel handling (TLS/DTLS)
• Packet forwarding (TUN ↔ Network)
• Compression/decompression (optional)
• Per-connection state management
• Performance-critical operations

Architecture:

• One worker per CPU core (NUMA-aware)
• Event-driven (libuv)
• Zero-copy where possible
• Lock-free data structures

Key Libraries:

• libuv 1.51.0+ (async I/O)
• wolfSSL 5.8.2+ (TLS/DTLS data plane)
• mimalloc 3.1.5+ (per-worker heaps)
• LZ4 (optional compression)

3. TLS Abstraction Layer

Design:

• Callback-based API (inspired by Lightway)
• Dual-build support (GnuTLS + wolfSSL)
• Asynchronous operations
• Session caching and resumption
• DTLS 1.3 ready

Implementation: See /opt/projects/repositories/wolfguard/src/crypto/tls_abstract.h

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Event-Driven Architecture

libuv Integration

Why libuv?

• Cross-platform (Linux, BSD, macOS, Windows)
• Production-proven (Node.js, neovim, Julia)
• Excellent performance (epoll/kqueue/io_uring)
• Active development and community
• Clean C API

Event Loop Pattern:

// Main event loop (C23)
#include <uv.h>
#include <stdatomic.h>

typedef struct ocserv_context {
    uv_loop_t *loop;
    _Atomic int running;
    struct {
        uv_tcp_t *listeners[16];
        size_t count;
    } tcp;
    struct {
        uv_udp_t *listeners[16];
        size_t count;
    } udp;
} ocserv_context_t;

[[nodiscard]]
int ocserv_run(ocserv_context_t *ctx) {
    atomic_store(&ctx->running, 1);

    while (atomic_load(&ctx->running)) {
        uv_run(ctx->loop, UV_RUN_ONCE);
    }

    return 0;
}

Callback-Based wolfSSL

Pattern (inspired by Lightway):

// TLS I/O callbacks for non-blocking operation
static int tls_recv_callback(WOLFSSL *ssl, char *buf, int sz, void *ctx) {
    connection_t *conn = (connection_t *)ctx;

    // Non-blocking read from libuv stream
    ssize_t ret = uv_try_read(&conn->stream, buf, sz);

    if (ret == UV_EAGAIN) {
        return WOLFSSL_CBIO_ERR_WANT_READ;
    } else if (ret < 0) {
        return WOLFSSL_CBIO_ERR_GENERAL;
    }

    return (int)ret;
}

static int tls_send_callback(WOLFSSL *ssl, char *buf, int sz, void *ctx) {
    connection_t *conn = (connection_t *)ctx;

    // Non-blocking write via libuv
    uv_write_t *req = malloc(sizeof(*req));
    uv_buf_t wbuf = uv_buf_init(buf, sz);

    int ret = uv_write(req, &conn->stream, &wbuf, 1, on_write_complete);

    if (ret < 0) {
        free(req);
        return WOLFSSL_CBIO_ERR_GENERAL;
    }

    return sz;
}

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Performance Optimization Strategies

1. Zero-Copy Networking

io_uring Integration (Linux 5.19+):

// Zero-copy send using io_uring
#include <liburing.h>

[[nodiscard]]
int zero_copy_send(struct io_uring *ring, int fd,
                   const void *buf, size_t len) {
    struct io_uring_sqe *sqe = io_uring_get_sqe(ring);

    io_uring_prep_send(sqe, fd, buf, len, MSG_ZEROCOPY);
    io_uring_sqe_set_data(sqe, (void *)buf);

    return io_uring_submit(ring);
}

UDP GSO/GRO (Generic Segmentation/Receive Offload):

// Enable UDP GSO for batching
#include <linux/udp.h>

static int enable_udp_gso(int sockfd) {
    int val = 1;
    return setsockopt(sockfd, SOL_UDP, UDP_SEGMENT, &val, sizeof(val));
}

2. NUMA-Aware Allocation

mimalloc with NUMA Support:

#include <mimalloc.h>

// Per-worker heap allocation
typedef struct worker_context {
    mi_heap_t *heap;
    int numa_node;
    int cpu_id;
} worker_context_t;

[[nodiscard]]
void* worker_alloc(worker_context_t *ctx, size_t size) {
    return mi_heap_malloc(ctx->heap, size);
}

3. Multi-Queue TUN Interface

Modern TUN/TAP Setup:

#include <linux/if_tun.h>

// Create multi-queue TUN device
[[nodiscard]]
int create_multiqueue_tun(const char *dev_name, int num_queues) {
    int fd = open("/dev/net/tun", O_RDWR);

    struct ifreq ifr = {0};
    ifr.ifr_flags = IFF_TUN | IFF_NO_PI | IFF_MULTI_QUEUE;
    strncpy(ifr.ifr_name, dev_name, IFNAMSIZ - 1);

    if (ioctl(fd, TUNSETIFF, &ifr) < 0) {
        close(fd);
        return -1;
    }

    // Enable per-queue checksum offload
    int offload = TUN_F_CSUM | TUN_F_TSO4 | TUN_F_TSO6 | TUN_F_UFO;
    ioctl(fd, TUNSETOFFLOAD, offload);

    return fd;
}

4. eBPF/XDP Integration (Optional)

Packet Filtering at NIC Level:

// XDP program for early packet filtering (BPF bytecode)
// This would be compiled from restricted C to BPF

SEC("xdp")
int xdp_vpn_filter(struct xdp_md *ctx) {
    void *data_end = (void *)(long)ctx->data_end;
    void *data = (void *)(long)ctx->data;

    struct ethhdr *eth = data;
    if ((void *)(eth + 1) > data_end)
        return XDP_PASS;

    // Filter VPN traffic (UDP port 443 for DTLS)
    if (eth->h_proto == htons(ETH_P_IP)) {
        struct iphdr *ip = (void *)(eth + 1);
        if ((void *)(ip + 1) > data_end)
            return XDP_PASS;

        if (ip->protocol == IPPROTO_UDP) {
            struct udphdr *udp = (void *)(ip + 1);
            if ((void *)(udp + 1) > data_end)
                return XDP_PASS;

            if (ntohs(udp->dest) == 443) {
                // Fast-path to user-space VPN handler
                return XDP_PASS;
            }
        }
    }

    return XDP_DROP;  // Not VPN traffic
}

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Pure C Library Stack

Rationale: No C++ Dependencies

Problems with C++ Libraries:

1. Binary size bloat (exception handling, RTTI)
2. Complex ABI compatibility issues
3. Slower compile times
4. Runtime overhead
5. Mixing C and C++ memory management

Solution: Pure C ecosystem

Category	C++ Library (Rejected)	Pure C Library (Approved)
Logging	spdlog	zlog 1.2.18
Metrics	prometheus-cpp	libprom 0.1.3
Config	tomlplusplus	tomlc99 1.0
YAML	yaml-cpp	libyaml
JSON	nlohmann/json	cJSON 1.7.19

Selected Libraries

zlog (Logging):

#include <zlog.h>

zlog_category_t *zc;

int init_logging(void) {
    if (zlog_init("/etc/ocserv/zlog.conf") != 0) {
        return -1;
    }

    zc = zlog_get_category("vpn");
    return 0;
}

void log_connection(const char *client_ip) {
    zlog_info(zc, "Client connected: %s", client_ip);
}

libprom (Metrics):

#include <prom.h>

prom_counter_t *connections_total;
prom_histogram_t *handshake_duration;

void init_metrics(void) {
    connections_total = prom_counter_new("vpn_connections_total",
                                         "Total VPN connections", 0, NULL);

    handshake_duration = prom_histogram_new("vpn_handshake_duration_seconds",
                                            "TLS handshake duration",
                                            prom_histogram_buckets_linear(0.001, 0.05, 10),
                                            0, NULL);
}

tomlc99 (Configuration):

#include <toml.h>

typedef struct vpn_config {
    char *server_cert;
    char *server_key;
    int max_clients;
    bool dtls_enabled;
} vpn_config_t;

[[nodiscard]]
int load_config(const char *path, vpn_config_t *cfg) {
    FILE *fp = fopen(path, "r");
    if (!fp) return -1;

    char errbuf[200];
    toml_table_t *conf = toml_parse_file(fp, errbuf, sizeof(errbuf));
    fclose(fp);

    if (!conf) {
        fprintf(stderr, "TOML parse error: %s\n", errbuf);
        return -1;
    }

    toml_datum_t server_cert = toml_string_in(conf, "server_cert");
    if (server_cert.ok) {
        cfg->server_cert = server_cert.u.s;
    }

    toml_datum_t max_clients = toml_int_in(conf, "max_clients");
    if (max_clients.ok) {
        cfg->max_clients = (int)max_clients.u.i;
    }

    toml_free(conf);
    return 0;
}

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C23 Modern Features

Adopted C23 Features

1. [[nodiscard]] Attribute: Error checking enforcement
2. nullptr Constant: Type-safe null pointer
3. constexpr Functions: Compile-time evaluation
4. _Static_assert: Compile-time validation
5. _Atomic Types: Lock-free concurrency
6. _BitInt(N): Arbitrary-width integers (crypto operations)
7. typeof Operator: Generic macros
8. Designated Initializers: Structure clarity
9. Compound Literals: Inline initialization

Example: Modern Error Handling

// C23 error handling pattern
#include <stddef.h>
#include <stdint.h>

typedef enum [[nodiscard]] result {
    RESULT_OK = 0,
    RESULT_ERR_NOMEM = -1,
    RESULT_ERR_INVALID = -2,
    RESULT_ERR_TIMEOUT = -3
} result_t;

[[nodiscard]]
static result_t allocate_session(session_t **out) {
    _Static_assert(sizeof(session_t) <= 4096,
                   "Session structure too large");

    session_t *sess = mi_calloc(1, sizeof(session_t));
    if (sess == nullptr) {
        return RESULT_ERR_NOMEM;
    }

    *out = sess;
    return RESULT_OK;
}

Example: Atomic Operations

#include <stdatomic.h>

typedef struct connection_pool {
    _Atomic size_t active_count;
    _Atomic size_t total_count;
    size_t max_connections;
} connection_pool_t;

[[nodiscard]]
static bool acquire_connection_slot(connection_pool_t *pool) {
    size_t current = atomic_load(&pool->active_count);

    do {
        if (current >= pool->max_connections) {
            return false;  // Pool full
        }
    } while (!atomic_compare_exchange_weak(&pool->active_count,
                                           &current, current + 1));

    atomic_fetch_add(&pool->total_count, 1);
    return true;
}

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Security Architecture

Defense in Depth

1. Privilege Separation: Main process (root) → Workers (unprivileged)
2. Capability Dropping: Use libcap to drop unnecessary privileges
3. Seccomp Filters: Restrict syscalls per process type
4. Address Space Layout Randomization (ASLR): Enabled by default
5. Stack Canaries: -fstack-protector-strong
6. FORTIFY_SOURCE: -D_FORTIFY_SOURCE=3 (gcc 12+)
7. Position Independent Executables (PIE): -fPIE -pie

Secure Coding Practices

Memory Safety:

// Bounds-checked string operations
#include <string.h>

static void safe_copy(char *dst, size_t dst_size, const char *src) {
    size_t src_len = strnlen(src, dst_size);
    if (src_len >= dst_size) {
        // Truncation occurred, log warning
        src_len = dst_size - 1;
    }
    memcpy(dst, src, src_len);
    dst[src_len] = '\0';
}

Timing-Safe Operations:

// Constant-time comparison (crypto)
[[nodiscard]]
static bool timing_safe_compare(const uint8_t *a, const uint8_t *b, size_t len) {
    volatile uint8_t diff = 0;

    for (size_t i = 0; i < len; i++) {
        diff |= a[i] ^ b[i];
    }

    return diff == 0;
}

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Performance Targets

Benchmarks (Target vs Baseline)

Metric	GnuTLS Baseline	wolfSSL Target	Actual (PoC)
TLS Handshakes/sec	800 hs/s	≥1000 hs/s	1200 hs/s ✅
Throughput	500 Mbps	≥550 Mbps	TBD
CPU Usage	60% @ 1000 conn	≤55%	TBD
Memory/Connection	120 KB	≤130 KB	TBD
Latency (p99)	15 ms	≤12 ms	TBD

Achieved in PoC: 50% handshake improvement (validated 2025-10-29)

Optimization Roadmap

Phase 1 (Sprint 2-5): Core Implementation

• wolfSSL integration
• Event-driven architecture
• Basic performance tuning

Phase 2 (Sprint 6-10): Advanced Optimizations

• io_uring integration (Linux 5.19+)
• UDP GSO/GRO
• Multi-queue TUN

Phase 3 (Sprint 11+): Extreme Performance

• eBPF/XDP packet filtering
• NUMA awareness
• Custom memory allocators
• SIMD optimizations (crypto operations)

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References

Industry Research

1. ExpressVPN Lightway: https://github.com/expressvpn/lightway-core
2. CloudFlare BoringTun: https://blog.cloudflare.com/boringtun-userspace-wireguard-rust/
3. WireGuard: https://www.wireguard.com/papers/wireguard.pdf
4. Tailscale Performance: https://tailscale.com/blog/throughput-improvements/
5. OpenVPN 3.x: https://github.com/OpenVPN/openvpn3

Technical Documentation

6. libuv Design Overview: http://docs.libuv.org/en/v1.x/design.html
7. io_uring: https://kernel.dk/io_uring.pdf
8. UDP GSO: https://lwn.net/Articles/768317/
9. eBPF/XDP: https://www.kernel.org/doc/html/latest/bpf/

Draft Research Documents (Analyzed)

10. /opt/projects/repositories/wolfguard/docs/draft/wolfguard-architecture-research.md
11. /opt/projects/repositories/wolfguard/docs/draft/wolfguard-c-libraries.md
12. /opt/projects/repositories/wolfguard/docs/draft/wolfguard-technical-implementation.md
13. /opt/projects/repositories/wolfguard/docs/draft/ocserv-refactoring-plan-networking.md

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Document Status: Architecture Reference Maintainer: wolfguard architecture team Review Schedule: Quarterly Next Review: 2026-01-29

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Co-Authored-By: Claude <[email protected]>