Add RFC 5424 syslog logging to all service templates

- decnet/logging/syslog_formatter.py: RFC 5424 formatter (local0 facility,
  decnet@55555 SD element ID, full escaping per §6.3.3)
- decnet/logging/file_handler.py: rotating file handler (10 MB / 5 backups),
  path configurable via DECNET_LOG_FILE env var
- templates/decnet_logging.py: combined syslog_line / write_syslog_file /
  forward_syslog helper distributed to all 22 service template dirs
- All templates/*/server.py: replaced ad-hoc JSON _forward/_log with RFC 5424
  syslog_line + write_syslog_file + forward_syslog
- All templates/*/Dockerfile: COPY decnet_logging.py /opt/
- DecnetConfig: added log_file field; CLI: --log-file flag;
  composer injects DECNET_LOG_FILE env var into service containers
- tests/test_syslog_formatter.py + tests/test_file_handler.py: 25 new tests

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

decnet/cli.py

@@ -185,6 +185,7 @@ def deploy(
     distro: Optional[str] = typer.Option(None, "--distro", help="Comma-separated distro slugs, e.g. debian,ubuntu22,rocky9"),
     randomize_distros: bool = typer.Option(False, "--randomize-distros", help="Assign a random distro to each decky"),
     log_target: Optional[str] = typer.Option(None, "--log-target", help="Forward logs to ip:port (e.g. 192.168.1.5:5140)"),
+    log_file: Optional[str] = typer.Option(None, "--log-file", help="Write RFC 5424 syslog to this path inside containers (e.g. /var/log/decnet/decnet.log)"),
     dry_run: bool = typer.Option(False, "--dry-run", help="Generate compose file without starting containers"),
     no_cache: bool = typer.Option(False, "--no-cache", help="Force rebuild all images, ignoring Docker layer cache"),
     config_file: Optional[str] = typer.Option(None, "--config", "-c", help="Path to INI config file"),
@@ -233,6 +234,7 @@ def deploy(
                 )
 
         effective_log_target = log_target or ini.log_target
+        effective_log_file = log_file
         decky_configs = _build_deckies_from_ini(
             ini, subnet_cidr, effective_gateway, host_ip, randomize_services
         )
@@ -282,6 +284,7 @@ def deploy(
             distros_explicit=distros_list, randomize_distros=randomize_distros,
         )
         effective_log_target = log_target
+        effective_log_file = log_file
 
     config = DecnetConfig(
         mode=mode,
@@ -290,6 +293,7 @@ def deploy(
         gateway=effective_gateway,
         deckies=decky_configs,
         log_target=effective_log_target,
+        log_file=effective_log_file,
     )
 
     if effective_log_target and not dry_run:

decnet/composer.py

@@ -58,6 +58,8 @@ def generate_compose(config: DecnetConfig) -> dict:
 
             fragment.setdefault("environment", {})
             fragment["environment"]["HOSTNAME"] = decky.hostname
+            if config.log_file:
+                fragment["environment"]["DECNET_LOG_FILE"] = config.log_file
 
             # Share the base container's network — no own IP needed
             fragment["network_mode"] = f"service:{base_key}"

decnet/config.py

@@ -43,6 +43,7 @@ class DecnetConfig(BaseModel):
     gateway: str
     deckies: list[DeckyConfig]
     log_target: str | None = None  # "ip:port" or None
+    log_file: str | None = None    # path for RFC 5424 syslog file output
 
     @field_validator("log_target")
     @classmethod

decnet/logging/file_handler.py

@@ -0,0 +1,57 @@
+"""
+Rotating file handler for DECNET syslog output.
+
+Writes RFC 5424 syslog lines to a local file.
+Path is controlled by the DECNET_LOG_FILE environment variable
+(default: /var/log/decnet/decnet.log).
+"""
+
+import logging
+import logging.handlers
+import os
+from pathlib import Path
+
+_LOG_FILE_ENV = "DECNET_LOG_FILE"
+_DEFAULT_LOG_FILE = "/var/log/decnet/decnet.log"
+_MAX_BYTES = 10 * 1024 * 1024  # 10 MB
+_BACKUP_COUNT = 5
+
+_handler: logging.handlers.RotatingFileHandler | None = None
+_logger: logging.Logger | None = None
+
+
+def _get_logger() -> logging.Logger:
+    global _handler, _logger
+    if _logger is not None:
+        return _logger
+
+    log_path = Path(os.environ.get(_LOG_FILE_ENV, _DEFAULT_LOG_FILE))
+    log_path.parent.mkdir(parents=True, exist_ok=True)
+
+    _handler = logging.handlers.RotatingFileHandler(
+        log_path,
+        maxBytes=_MAX_BYTES,
+        backupCount=_BACKUP_COUNT,
+        encoding="utf-8",
+    )
+    _handler.setFormatter(logging.Formatter("%(message)s"))
+
+    _logger = logging.getLogger("decnet.syslog")
+    _logger.setLevel(logging.DEBUG)
+    _logger.propagate = False
+    _logger.addHandler(_handler)
+
+    return _logger
+
+
+def write_syslog(line: str) -> None:
+    """Write a single RFC 5424 syslog line to the rotating log file."""
+    try:
+        _get_logger().info(line)
+    except Exception:
+        pass
+
+
+def get_log_path() -> Path:
+    """Return the configured log file path (for tests/inspection)."""
+    return Path(os.environ.get(_LOG_FILE_ENV, _DEFAULT_LOG_FILE))

decnet/logging/syslog_formatter.py

@@ -0,0 +1,84 @@
+"""
+RFC 5424 syslog formatter for DECNET.
+
+Produces fully-compliant syslog messages:
+  <PRI>1 TIMESTAMP HOSTNAME APP-NAME PROCID MSGID [SD-ELEMENT] MSG
+
+Facility: local0 (16)
+PEN for structured data: decnet@55555
+"""
+
+from datetime import datetime, timezone
+from typing import Any
+
+
+FACILITY_LOCAL0 = 16
+NILVALUE = "-"
+_SD_ID = "decnet@55555"
+
+SEVERITY_INFO    = 6
+SEVERITY_WARNING = 4
+SEVERITY_ERROR   = 3
+
+# RFC 5424 field length limits
+_MAX_HOSTNAME = 255
+_MAX_APPNAME  = 48
+_MAX_MSGID    = 32
+
+
+def _pri(severity: int) -> str:
+    return f"<{FACILITY_LOCAL0 * 8 + severity}>"
+
+
+def _truncate(value: str, maxlen: int) -> str:
+    return value[:maxlen] if len(value) > maxlen else value
+
+
+def _sd_escape(value: str) -> str:
+    """Escape SD-PARAM-VALUE per RFC 5424 §6.3.3."""
+    return value.replace("\\", "\\\\").replace('"', '\\"').replace("]", "\\]")
+
+
+def _sd_element(fields: dict[str, Any]) -> str:
+    if not fields:
+        return NILVALUE
+    params = " ".join(
+        f'{k}="{_sd_escape(str(v))}"'
+        for k, v in fields.items()
+    )
+    return f"[{_SD_ID} {params}]"
+
+
+def format_rfc5424(
+    service: str,
+    hostname: str,
+    event_type: str,
+    severity: int = SEVERITY_INFO,
+    timestamp: datetime | None = None,
+    msg: str | None = None,
+    **fields: Any,
+) -> str:
+    """
+    Return a single RFC 5424-compliant syslog line (no trailing newline).
+
+    Args:
+        service:    APP-NAME field (e.g. "http", "mysql")
+        hostname:   HOSTNAME field (the decky node name)
+        event_type: MSGID field (e.g. "request", "login_attempt")
+        severity:   Syslog severity integer (default: INFO=6)
+        timestamp:  Datetime to use; defaults to utcnow
+        msg:        Optional free-text MSG suffix
+        **fields:   Arbitrary key=value pairs encoded in structured data
+    """
+    ts = (timestamp or datetime.now(timezone.utc)).isoformat()
+
+    pri       = _pri(severity)
+    version   = "1"
+    host      = _truncate(hostname or NILVALUE, _MAX_HOSTNAME)
+    appname   = _truncate(service or NILVALUE, _MAX_APPNAME)
+    procid    = NILVALUE
+    msgid     = _truncate(event_type or NILVALUE, _MAX_MSGID)
+    sd        = _sd_element(fields)
+    message   = f" {msg}" if msg else ""
+
+    return f"{pri}{version} {ts} {host} {appname} {procid} {msgid} {sd}{message}"