152 lines
5.3 KiB
Markdown
152 lines
5.3 KiB
Markdown
# MIB Resolution with net-snmp C Library
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## Implementation Summary
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Successfully integrated the net-snmp C library for ultra-fast MIB name resolution, achieving LibreNMS-style performance.
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## Performance Comparison
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| Method | First Resolution | Cached Resolution |
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|--------|------------------|-------------------|
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| **Subprocess (old)** | ~300-400ms | <1µs (ETS cache) |
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| **net-snmp C library (new)** | ~1-20µs | ~1-20µs (in-memory) |
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## Benchmark Results
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```
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sysDescr avg: 1.6µs min: 1µs max: 6µs
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IF-MIB::ifDescr avg: 17.4µs min: 9µs max: 23µs
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SNMPv2-MIB::sysUpTime avg: 12.3µs min: 7µs max: 19µs
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IF-MIB::ifInOctets avg: 3.5µs min: 2µs max: 6µs
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IP-MIB::ipAdEntAddr avg: 4.8µs min: 1µs max: 14µs
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```
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**Result**: 17-300x faster than subprocess approach!
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## Architecture
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### Stack
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```
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Elixir (MibCache GenServer)
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↓
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Rustler NIF (towerops_native)
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↓
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Rust FFI bindings (netsnmp_ffi.rs)
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↓
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net-snmp C library (libnetsnmp)
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```
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### Key Components
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1. **netsnmp_ffi.rs** - Rust FFI bindings to net-snmp C functions
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- `initialize_snmp_library()` - One-time MIB loading (~1-2s startup)
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- `resolve_mib_name()` - Fast OID resolution (<20µs)
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2. **mib.rs** - Rust NIF implementation
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- `load_mib_directory_impl()` - Configure MIB search paths
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- `init_mib_library_impl()` - Initialize net-snmp (DirtyCpu scheduler)
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- `resolve_oid_impl()` - Resolve MIB names with Rust HashMap cache
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3. **lib.rs** - Rustler NIF exports
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- Exports functions to Elixir as `ToweropsNative` module
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4. **lib/towerops_native.ex** - Elixir NIF wrapper
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- `load_mib_directory/1` - Set MIB search paths
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- `init_mib_library/0` - Initialize net-snmp library
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- `resolve_oid/1` - Resolve MIB name to numeric OID
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5. **lib/towerops/profiles/mib_cache.ex** - GenServer cache layer
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- Async pre-resolution of common MIBs at startup
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- ETS cache for ultra-fast lookups
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- Runtime fallback for cache misses
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### Initialization Flow
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1. Application starts → `load_mib_directory/1` called with MIB paths
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2. First `resolve_oid/1` call triggers lazy initialization:
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- Sets `MIBDIRS` and `MIBS=ALL` environment variables
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- Calls `init_snmp()` and `netsnmp_init_mib()`
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- Calls `read_all_mibs()` to load all MIBs into memory (~1-2s)
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3. Subsequent calls use in-memory MIB tree (fast!)
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### Resolution Flow
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1. Elixir calls `ToweropsNative.resolve_oid("sysDescr")`
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2. Rust checks Rust HashMap cache (miss on first call)
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3. Calls C `read_objid()` to resolve MIB name → OID array
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4. Calls C `snprint_objid()` to format OID as string
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5. Caches result in Rust HashMap
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6. Returns to Elixir
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7. MibCache GenServer caches in ETS
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## Files Created/Modified
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### New Files
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- `native/towerops_native/src/netsnmp_ffi.rs` - C library FFI bindings
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- `native/towerops_native/build.rs` - Cargo build script for linking
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### Modified Files
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- `native/towerops_native/Cargo.toml` - Added `libc` and `cc` dependencies
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- `native/towerops_native/src/lib.rs` - NIF function exports
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- `native/towerops_native/src/mib.rs` - Refactored for C library integration
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- `lib/towerops_native.ex` - Added `init_mib_library/0` function
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### Existing Files (unchanged)
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- `lib/towerops/profiles/mib_cache.ex` - Still provides caching layer
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- `lib/towerops/profiles/yaml_profiles.ex` - Uses MibCache as before
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- `lib/towerops/snmp/client.ex` - Uses MibCache as before
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## Build Requirements
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### Development (macOS)
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- Homebrew net-snmp: `brew install net-snmp`
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- Headers: `/usr/include/net-snmp/*` or `/opt/homebrew/include/net-snmp/*`
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- Library: `/usr/lib/libnetsnmp.dylib` or `/opt/homebrew/lib/libnetsnmp.dylib`
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### Production (Docker/Linux)
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- Package: `net-snmp` and `net-snmp-devel` (CentOS) or `libsnmp-dev` (Debian/Ubuntu)
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- Headers: `/usr/include/net-snmp/*`
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- Library: `/usr/lib/x86_64-linux-gnu/libnetsnmp.so`
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### Build Process
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1. `build.rs` uses `net-snmp-config --libs` to get library paths
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2. Links against `libnetsnmp` dynamically
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3. Works in both dev (macOS) and prod (Linux Docker)
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## Advantages
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1. **Performance**: 17-300x faster than subprocess calls
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2. **LibreNMS-aligned**: Same C library as LibreNMS PHP extension
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3. **Memory-efficient**: MIBs loaded once, shared across all resolutions
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4. **Thread-safe**: C library handles concurrent access
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5. **Works in Docker**: Dynamic linking finds system libnetsnmp
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6. **No MIB pre-compilation**: MIBs loaded as text files (easy updates)
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## Trade-offs
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1. **Startup time**: ~1-2 seconds to load all MIBs (one-time cost)
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- Happens lazily on first resolution call
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- Runs on DirtyCpu scheduler (doesn't block main BEAM schedulers)
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2. **C dependency**: Requires net-snmp library installed
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- Already required for `snmptranslate` command
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- Standard package on all Linux distros
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3. **Complexity**: Added Rust FFI layer
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- Well-documented and type-safe
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- Similar to PHP's SNMP extension internals
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## Testing
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All existing tests pass:
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```bash
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mix test test/towerops/profiles/yaml_profiles_test.exs # 12 tests, 0 failures
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mix test test/towerops/snmp/discovery_test.exs # 13 tests, 0 failures
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```
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## Future Enhancements
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1. **Lazy MIB loading**: Only load MIBs for matched vendors (faster startup)
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2. **MIB dependency tracking**: Load only required MIB dependencies
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3. **Shared library caching**: Share MIB tree across multiple NIF instances
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4. **Telemetry**: Track resolution times and cache hit rates
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