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wolfSentry -- The Wolfssl Embedded Firewall/IDPS


wolfSentry is the wolfSSL embedded IDPS (Intrusion Detection and Prevention System). In simple terms, wolfSentry is an embedded firewall engine (both static and fully dynamic), with prefix-based and wildcard-capable lookup of known hosts/netblocks qualified by interface, address family, protocol, port, and other traffic parameters. Additionally, wolfSentry can be used as a dynamically configurable logic hub, arbitrarily associating user-defined events with user-defined actions, contextualized by connection attributes. The evolution of client-server relationships can thus be tracked in detail, freely passing traffic matching expected usage patterns, while efficiently rejecting abusive traffic.

wolfSentry is fully integrated with the lwIP stack, through a patchset in the lwip/ subdirectory of the source tree, and has basic integration with the wolfSSL library for application-level filtering of inbound and outbound connections.

The wolfSentry engine is dynamically configurable programmatically through an API, or from a textual input file in JSON supplied to the engine, or dynamically and incrementally with JSON fragments, or any combination of these methods. Reconfiguration is protected by transactional semantics, and advanced internal locks on threaded targets assure seamless service availability with atomic policy transition. Callbacks allow for transport-agnostic remote logging, e.g. through MQTT, syslog, or DDS message buses.

wolfSentry is designed from the ground up to function well in resource-constrained, bare-metal, and realtime environments, with algorithms to stay within designated maximum memory footprints and maintain deterministic throughput. This allows full firewall and IDPS functionality on embedded targets such as FreeRTOS, Nucleus, NUTTX, Zephyr, VxWorks, and Green Hills Integrity, and on ARM and other common embedded CPUs and MCUs. wolfSentry with dynamic firewalling can add as little as 64k to the code footprint, and 32k to the volatile state footprint, and can fully leverage the existing logic and state of applications and sibling libraries.


With doxygen installed, the HTML version of the full API reference manual can be generated from the top of the wolfSentry source tree with make doc-html. This, and the source code itself, are the recommended API references.

The PDF version of the API reference manual is pregenerated and included with source distributions in the doc/ subdirectory at doc/wolfSentry_refman.pdf. The latest version is always available on GitHub.


In its default build, wolfSentry depends on a POSIX runtime, specifically the heap allocator, clock_gettime, stdio, semaphore, pthreads, and string APIs. However, these dependencies can be avoided with various build-time options. The recipe


builds a libwolfsentry.a that depends on only a handful of basic string functions and the inet_ntop() library function (from POSIX.1-2001, and also implemented by lwIP). Allocator and time callbacks must then be set in a struct wolfsentry_host_platform_interface supplied to wolfsentry_init().

The wolfSentry Makefile depends on a modern (v4.0+) Gnu make. The library itself can be built outside make, within another project/framework, by creating a user settings macro file and passing its path to the compiler with the WOLFSENTRY_USER_SETTINGS_FILE macro.


wolfSentry was written with portability in mind, with provisions for non-POSIX and C89 targets. For example, all its dependencies can be met with the FreeRTOS/newlib-nano/lwIP runtime. If you have difficulty building wolfSentry, please don’t hesitate to seek support through our support forums or contact us directly at

The current wolfSentry release can be downloaded from the wolfSSL website as a ZIP file, and developers can browse the release history and clone the wolfSentry Git repository for the latest pre-release updates.

There are several flags that can be passed to make to control the build parameters. make will store them at build time in wolfsentry/wolfsentry_options.h in the build tree. If you are not using make, then the C macro WOLFSENTRY_USER_SETTINGS_FILE should be defined to the path to a file containing settings, both when building wolfSentry and when building the application.

The following feature control variables are recognized. True/false features (LWIP, NO_STDIO, NO_JSON, etc.) are undefined by default, and activated when defined. Macros can be supplied using the EXTRA_CFLAGS option, or by placing them in a USER_SETTINGS_FILE. More detailed documentation for macros is available in the reference manual "Startup/Configuration/Shutdown Subsystem" topic.

make Option Macro Option Description
SHELL Supplies an explicit/alternative path to bash.
AWK Supplies an explicit/alternative path to Gnu awk.
V Verbose make output
e.g. make V=1 -j test
USER_MAKE_CONF User-defined make clauses to include at the top of the main Makefile
e.g. make -j USER_MAKE_CONF=Makefile.settings
EXTRA_CFLAGS Additional arguments to be passed verbatim to the compiler
EXTRA_LDFLAGS Additional arguments to be passed verbatim to the linker
SRC_TOP The source code top level directory (default pwd -P)
BUILD_TOP Build with artifacts in an alternate location (outside or in a subdirectory of the source tree)
e.g. make BUILD_TOP=./build -j test
DEBUG Compiler debugging flag to use (default -ggdb)
OPTIM The optimizer flag to use (default -O3)
HOST The target host tuple, for cross-compilation (default unset, i.e. native targeting)
RUNTIME The target runtime ecosystem -- default unset, FreeRTOS-lwIP and Linux-lwIP are recognized
C_WARNFLAGS The warning flags to use (overriding the generally applicable defaults)
STATIC Build statically linked unit tests
STRIPPED Strip binaries of debugging symbols
FUNCTION_SECTIONS Cull any unused object code (with function granularity) to minimize total size.
BUILD_DYNAMIC Build dynamically linked library
VERY_QUIET Inhibit all non-error output during build
TAR Path to GNU tar binary for make dist, should be set to gtar for macOS
VERSION The version to package for make dist
LWIP WOLFSENTRY_LWIP True/false -- Activates appropriate build settings for lwIP
NO_STDIO_STREAMS WOLFSENTRY_NO_STDIO_STREAMS Define to omit functionality that depends on stdio stream I/O
WOLFSENTRY_NO_STDIO_H Define to inhibit inclusion of stdio.h
NO_ADDR_BITMASK_MATCHING WOLFSENTRY_NO_ADDR_BITMASK_MATCHING Define to omit support for bitmask matching of addresses, i.e. support only prefix matching.
NO_IPV6 WOLFSENTRY_NO_IPV6 Define to omit support for the IPv6 address family.
NO_JSON WOLFSENTRY_NO_JSON Define to omit JSON configuration support
CALL_TRACE WOLFSENTRY_DEBUG_CALL_TRACE Define to activate runtime call stack logging (profusely verbose)
USER_SETTINGS_FILE WOLFSENTRY_USER_SETTINGS_FILE A substitute settings file, replacing autogenerated wolfsentry_settings.h
SINGLETHREADED WOLFSENTRY_SINGLETHREADED Define to omit thread safety logic, and replace thread safety functions and macros with no-op macros.
WOLFSENTRY_NO_PROTOCOL_NAMES If defined, omit APIs for rendering error codes and source code files in human readable form. They will be rendered numerically.
WOLFSENTRY_NO_GETPROTOBY Define to disable lookup and rendering of protocols and services by name.
WOLFSENTRY_NO_ERROR_STRINGS If defined, omit APIs for rendering error codes and source code files in human readable form. They will be rendered numerically.
WOLFSENTRY_NO_MALLOC_BUILTINS If defined, omit built-in heap allocator primitives; the wolfsentry_host_platform_interface supplied to wolfSentry APIs must include implementations of all functions in struct wolfsentry_allocator.
WOLFSENTRY_HAVE_NONGNU_ATOMICS Define if gnu-style atomic intrinsics are not available. WOLFSENTRY_ATOMIC_*() macro definitions for intrinsics will need to be supplied in WOLFSENTRY_USER_SETTINGS_FILE (see wolfsentry_util.h).
WOLFSENTRY_NO_CLOCK_BUILTIN If defined, omit built-in time primitives; the wolfsentry_host_platform_interface supplied to wolfSentry APIs must include implementations of all functions in struct wolfsentry_timecbs.
WOLFSENTRY_NO_SEM_BUILTIN If defined, omit built-in semaphore primitives; the wolfsentry_host_platform_interface supplied to wolfSentry APIs must include implementations of all functions in struct wolfsentry_semcbs.
WOLFSENTRY_USE_NONPOSIX_SEMAPHORES Define if POSIX semaphore API is not available. If no non-POSIX builtin implementation is present in wolfsentry_util.c, then #WOLFSENTRY_NO_SEM_BUILTIN must be set, and the wolfsentry_host_platform_interface supplied to wolfSentry APIs must include a full semaphore implementation (shim set) in its wolfsentry_semcbs slot.
WOLFSENTRY_SEMAPHORE_INCLUDE Define to the path of a header file declaring a semaphore API.
WOLFSENTRY_THREAD_INCLUDE Define to the path of a header file declaring a threading API.
WOLFSENTRY_THREAD_ID_T Define to the appropriate type analogous to POSIX pthread_t.
WOLFSENTRY_THREAD_GET_ID_HANDLER Define to the name of a void function analogous to POSIX pthread_self, returning a value of type WOLFSENTRY_THREAD_ID_T.

Build and Self-Test Examples

Building and testing libwolfsentry.a on Linux:

make -j test

Build verbosely:

make V=1 -j test

Build with artifacts in an alternate location (outside or in a subdirectory of the source tree):

make BUILD_TOP=./build -j test

Install from an alternate build location to a non-standard destination:

make BUILD_TOP=./build INSTALL_DIR=/usr INSTALL_LIBDIR=/usr/lib64 install

Build libwolfsentry.a and test it in various configurations:

make -j check

Build and test libwolfsentry.a without support for multithreading:

make -j SINGLETHREADED=1 test

Other available make flags are STATIC=1, STRIPPED=1, NO_JSON=1, and NO_JSON_DOM=1, and the defaults values for DEBUG, OPTIM, and C_WARNFLAGS can also be usefully overridden.

Build with a user-supplied makefile preamble to override defaults:

make -j USER_MAKE_CONF=Makefile.settings

(Makefile.settings can contain simple settings like OPTIM := -Os, or elaborate makefile code including additional rules and dependency mechanisms.)

Build the smallest simplest possible library:


Build and test with user settings:

make -j USER_SETTINGS_FILE=user_settings.h test

Build for FreeRTOS on ARM32, assuming FreeRTOS and lwIP source trees are located as shown:

make -j HOST=arm-none-eabi RUNTIME=FreeRTOS-lwIP FREERTOS_TOP=../third/FreeRTOSv202212.00 LWIP_TOP=../third/lwip EXTRA_CFLAGS=-mcpu=cortex-m7

Project Examples

In the wolfsentry/examples/ subdirectory are a set of example ports and applications, including a demo pop-up notification system implementing a toy TLS-enabled embedded web server, integrating with the Linux D-Bus facility.

More comprehensive examples of API usage are in tests/unittests.c, particularly test_static_routes(), test_dynamic_rules(), and test_json(), and the JSON configuration files at tests/test-config*.json.

In the wolfSSL repository, see code in wolfssl/test.h gated on WOLFSSL_WOLFSENTRY_HOOKS, including wolfsentry_store_endpoints(), wolfSentry_NetworkFilterCallback(), wolfsentry_setup(), and tcp_connect_with_wolfSentry(). See also code in examples/server/server.c and examples/client/client.c gated on WOLFSSL_WOLFSENTRY_HOOKS. Configure wolfssl with --enable-wolfsentry to build with wolfSentry integration, and use --with-wolfsentry=/the/install/path if wolfSentry is installed in a nonstandard location. The wolfSSL test client/server can be loaded with user-supplied wolfSentry JSON configurations from the command line, using --wolfsentry-config <file>.