Add SNMP trap listener design document

docs/plans/2026-01-31-snmp-trap-listener-design.md

@@ -0,0 +1,155 @@
+# SNMP Trap Listener Design
+
+## Overview
+
+Add SNMP trap listening capability to the towerops-agent. The agent will listen on UDP port 162 for incoming SNMP traps (both v1 and v2c formats) and log them at INFO level.
+
+## Requirements
+
+- Support both SNMPv1 and SNMPv2c trap formats
+- Listen on UDP port 162 (standard SNMP trap port)
+- Log received traps at INFO level
+- Configurable log level via `LOG_LEVEL` environment variable
+- No API submission or buffering (log only for initial implementation)
+
+## Architecture
+
+### New Module: `src/snmp/trap.rs`
+
+A dedicated trap listener that:
+1. Binds a UDP socket to the configured trap port
+2. Receives incoming SNMP trap packets
+3. Parses both SNMPv1 and SNMPv2c trap PDUs using ASN.1/BER decoding
+4. Sends parsed traps through a channel for logging
+
+### Scheduler Integration
+
+The trap listener runs as a concurrent task alongside existing polling tasks in the `tokio::select!` loop. It communicates via `tokio::sync::mpsc` channel.
+
+## Trap PDU Formats
+
+### SNMPv1 Trap-PDU
+
+```
+Trap-PDU ::= [4] SEQUENCE {
+    enterprise    OBJECT IDENTIFIER,
+    agent-addr    NetworkAddress,
+    generic-trap  INTEGER (0..6),
+    specific-trap INTEGER,
+    time-stamp    TimeTicks,
+    variable-bindings VarBindList
+}
+```
+
+Generic trap types:
+- 0: coldStart
+- 1: warmStart
+- 2: linkDown
+- 3: linkUp
+- 4: authenticationFailure
+- 5: egpNeighborLoss
+- 6: enterpriseSpecific
+
+### SNMPv2c Trap-PDU
+
+```
+SNMPv2-Trap-PDU ::= [7] SEQUENCE {
+    request-id   INTEGER,
+    error-status INTEGER,
+    error-index  INTEGER,
+    variable-bindings VarBindList
+}
+```
+
+The trap OID is in the second varbind (`snmpTrapOID.0 = 1.3.6.1.6.3.1.1.4.1.0`).
+
+## Data Structures
+
+```rust
+pub enum SnmpVersion {
+    V1,
+    V2c,
+}
+
+pub struct SnmpTrap {
+    pub source_addr: SocketAddr,
+    pub version: SnmpVersion,
+    pub community: String,
+    pub trap_oid: String,           // Enterprise OID (v1) or snmpTrapOID (v2c)
+    pub generic_trap: Option<u8>,   // v1 only
+    pub specific_trap: Option<u32>, // v1 only
+    pub uptime: u32,
+    pub varbinds: Vec<(String, String)>,
+}
+
+pub struct TrapListener {
+    port: u16,
+}
+```
+
+## CLI Configuration
+
+New flag:
+```
+--trap-port <PORT>    UDP port for SNMP trap listener [default: 162] [env: TRAP_PORT]
+```
+
+## Environment Variables
+
+- `LOG_LEVEL` - Controls log verbosity: error, warn, info, debug, trace (default: info)
+- `TRAP_PORT` - UDP port for trap listener (default: 162)
+
+## Log Output Format
+
+```
+INFO SNMP trap from 192.168.1.1:161 [v1] enterprise=1.3.6.1.4.1.9.9.41 generic=6 specific=1 uptime=12345 varbinds=[1.3.6.1.2.1.2.2.1.1=2, 1.3.6.1.2.1.2.2.1.8=1]
+
+INFO SNMP trap from 192.168.1.1:161 [v2c] oid=1.3.6.1.6.3.1.1.5.4 uptime=12345 varbinds=[ifIndex.2=2, ifOperStatus.2=1]
+```
+
+## Docker Compose Configuration
+
+```yaml
+services:
+  towerops-agent:
+    image: towerops/agent:latest
+    environment:
+      - TOWEROPS_API_URL=http://host.docker.internal:4000
+      - TOWEROPS_AGENT_TOKEN=${AGENT_TOKEN}
+      - LOG_LEVEL=info
+      - TRAP_PORT=162
+    ports:
+      - "162:162/udp"
+    volumes:
+      - ./data:/data
+```
+
+## Implementation Notes
+
+### BER Parsing
+
+Implement minimal BER decoder for:
+- Reading TLV (Tag-Length-Value) structures
+- Extracting SNMP version from message wrapper
+- Parsing INTEGER, OCTET STRING, OBJECT IDENTIFIER, SEQUENCE types
+- Converting values to display strings
+
+### Error Handling
+
+- Malformed packets: log as warning and skip
+- UDP socket errors: log and continue listening
+- No crash on invalid data
+
+### Privileges
+
+Port 162 requires elevated privileges. In Docker, handled by:
+- Container running as root (default), or
+- `NET_BIND_SERVICE` capability
+
+## Files to Create/Modify
+
+1. **Create** `src/snmp/trap.rs` - Trap listener and BER parser
+2. **Modify** `src/snmp/mod.rs` - Export trap module
+3. **Modify** `src/main.rs` - Add CLI flag, configure logging with LOG_LEVEL
+4. **Modify** `src/poller/scheduler.rs` - Integrate trap listener task
+5. **Modify** `docker-compose.example.yml` - Add trap port and LOG_LEVEL