Senior Software Engineer — Embedded C / Firewall Client

SonicWall is a cybersecurity forerunner with more than 30 years of expertise and is recognized as a leading partner-first company, ensuring our partners and their customers are never alone in the fight against cybercrime. With the ability to build, scale and manage security across the cloud, hybrid and traditional environments in real-time, SonicWall provides relentless security against the most evasive cyberattacks across endless exposure points for increasingly remote, mobile and cloud-enabled users. With its own threat research center, SonicWall can quickly and economically provide purpose-built security solutions to enable any organization—enterprise, government agencies and SMBs—around the world. For more information, visit www.sonicwall.com or follow us on Twitter, LinkedIn, Facebook and Instagram.
About the Role
SonicWall is hiring a Senior Software Engineer to develop and maintain the Content Filtering Service client embedded in SonicOS — the firmware running on SonicWall firewall appliances worldwide. This module intercepts live traffic, queries the CFS rating servers in real time, and enforces URL-based content filtering policies at the packet level. This is security-critical code: a policy bug is a traffic bypass on a deployed security appliance.
The codebase is in C with a split Control Plane / Data Plane architecture, multi-blade cache synchronization, and multiple firmware version upgrade paths.
Responsibilities
Maintain, harden, and extend the CFS client module across both the control plane and data plane.
Own the binary UDP protocol implementation between the firewall and the CFS rating servers, ensuring correctness and compatibility across protocol versions.
Develop and maintain complete IPv4 and IPv6 support across all code paths.
Own the firmware upgrade and migration paths, ensuring they are atomic, failure-safe, and correctly handle all supported version transitions.
Improve reliability of inter-blade cache synchronization on multi-blade appliances.
Collaborate with the server-side on coordinated protocol changes.
Document the concurrency model, upgrade invariants, and dataplane integration for long-term maintainability.
Collaborate asynchronously with a US-based team and the India-based server team.
Requirements
Expert-level C (not C++): disciplined manual memory management, pointer arithmetic, and systems design.
Multi-core concurrency in C: reader-writer locks, spinlocks, atomic operations, and memory ordering.
Kernel-level networking: deep understanding of how a packet moves through a firewall dataplane — TCP/UDP processing, flow cache, connection tracking. Userspace socket programming alone is not sufficient.
Strong IPv4 and IPv6 across all code paths.
Comfortable navigating and owning a large C codebase with multiple subsystems.
Experience with network testing and debugging tools: packet capture (Wireshark/tcpdump), traffic generators, GDB.
Good written English for remote, asynchronous collaboration.
Nice to have:
Experience with multi-blade or multi-ASIC hardware platforms (Octeon/CVMX or similar).
Understanding of symmetric encryption (AES-CBC, DES) at the C implementation level.
Prior experience on a security appliance, network device, or UTM firmware codebase.
Familiarity with TLV binary protocol design and versioning.
Experience with C fuzz testing or sanitizers (AFL, AddressSanitizer, ThreadSanitizer).
Education & Experience
At least 8 years of related experience with a Bachelor's degree; or 6 years with a Master's degree; or a PhD with 3 years' experience; or equivalent experience preferred. Degree in Computer Science, Engineering, or related field preferred.
#C
#Firewall
#Networking
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At SonicWall, we pride ourselves on recruiting a diverse mix of talented people and providing active security solutions in 100+ countries.
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Work arrangement
Hybrid