feat: set up SNMP with net-snmp

run_device_action("fix_snmp") and get_snmp_config for both drivers, as
decided in NetOrk/netork#800: net-snmp from packages rather than the base
daemons, because it answers UCD-SNMP-MIB, which netOrk's health metrics
read.

- fix_snmp installs net-snmp, writes the configuration netOrk uses on
  Linux (v2c, community "public", agentAddress udp:161) through stdin as
  root, enables and restarts the agent (FreeBSD `service snmpd`, OpenBSD
  `rcctl ... netsnmpd`, as `snmpd` is OpenBSD's own daemon) and asks it
  for sysDescr. It stops at the first step that fails and says why.
- get_snmp_config reports a running net-snmp's community and port.
- FreeBSD's install_package bootstraps pkg on a classic system that never
  had it instead of waiting for an answer that never comes.

Checked live: FreeBSD from a bare system, OpenBSD again over an existing
setup. Memory, swap and load answer on both; FreeBSD's ssCpuIdle about a
minute after start; OpenBSD's CPU figures from net-snmp are wrong (#800).

napalm_bsd/base.py

@@ -36,6 +36,16 @@ _SERVICE_ACTIONS = frozenset({"start", "stop", "restart", "enable", "disable"})
 _SERVICE_NAME = re.compile(r"^[A-Za-z0-9][A-Za-z0-9_.-]*$")
 _PACKAGE_NAME = re.compile(r"^[A-Za-z0-9][A-Za-z0-9_.+-]*$")
 
+#: The agent netOrk sets up, as on Linux: v2c, community "public", every address.
+_SNMPD_CONF = (
+    "agentAddress udp:161\n"
+    "rocommunity public\n"
+    "sysLocation    Managed by netOrk\n"
+    "sysContact     netork@localhost\n"
+)
+#: What a working agent answers sysDescr.0 with.
+_SNMP_TYPES = ("STRING:", "INTEGER:", "OID:", "Timeticks:", "Hex-STRING:", "IpAddress:")
+
 
 class BsdDriver(OSDriver):
     """Base for the BSD drivers; a concrete one names the commands that differ."""
@@ -344,6 +354,67 @@ class BsdDriver(OSDriver):
         output = "\n".join(filter(None, (result.stdout.strip(), result.stderr.strip())))
         return {"success": result.exit_code == 0, "output": output}
 
+    # -- SNMP (net-snmp from packages, NetOrk/netork#800) -------------------------
+
+    #: Where net-snmp reads its configuration, and how its daemon is started.
+    SNMPD_CONF = ""
+    SNMPD_START = ""
+    #: net-snmp's daemon; OpenBSD's base snmpd is /usr/sbin/snmpd.
+    SNMPD_DAEMON = "/usr/local/sbin/snmpd"
+    SNMP_PROBE = "snmpget -v2c -cpublic -t2 -r0 -Ov 127.0.0.1 1.3.6.1.2.1.1.1.0"
+
+    def run_device_action(self, action: str) -> dict[str, Any]:
+        """Execute a named action on the device; ``fix_snmp`` is the one there is."""
+        if action == "fix_snmp":
+            return self._action_fix_snmp()
+        raise NotImplementedError(f"Unknown action: {action!r}")
+
+    def _action_fix_snmp(self) -> dict[str, Any]:
+        """Install net-snmp, configure it as on Linux, start it, and ask it.
+
+        net-snmp rather than the base daemons (FreeBSD bsnmpd, OpenBSD snmpd):
+        it answers UCD-SNMP-MIB, which netOrk's health metrics read
+        (NetOrk/netork#800). Stops at the first step that fails.
+        """
+        lines: list[str] = []
+        conf_dir = self.SNMPD_CONF.rsplit("/", 1)[0]
+        steps = (
+            ("install", self.INSTALL_COMMAND.format(name="net-snmp"), None),
+            (
+                "config",
+                f"mkdir -p {conf_dir} && cat > {self.SNMPD_CONF} && chmod 644 {self.SNMPD_CONF}",
+                _SNMPD_CONF.encode(),
+            ),
+            ("service", self.SNMPD_START, None),
+        )
+        for label, command, stdin in steps:
+            result = self.run_command(command, privileged=True, timeout=600, stdin=stdin)
+            output = "\n".join(filter(None, (result.stdout.strip(), result.stderr.strip())))
+            lines.append(f"[{label}] {output[-300:]}".rstrip())
+            if result.exit_code != 0:
+                return {"success": False, "output": "\n".join(lines)}
+        probe = self.run_command(self.SNMP_PROBE, timeout=30).stdout.strip()
+        lines.append(f"[probe] {probe}")
+        return {"success": any(t in probe for t in _SNMP_TYPES), "output": "\n".join(lines)}
+
+    def get_snmp_config(self) -> Optional[dict[str, Any]]:
+        """net-snmp's community and port, if its daemon runs; None otherwise."""
+        if not self._out(f"pgrep -f {self.SNMPD_DAEMON}"):
+            return None
+        community, port = "public", 161
+        for line in self._out(f"cat {self.SNMPD_CONF}").splitlines():
+            words = line.split()
+            if len(words) >= 2 and words[0].lower() in (
+                "rocommunity",
+                "rwcommunity",
+                "rocommunity6",
+            ):
+                community = words[1]
+            elif words and words[0] == "agentAddress":
+                found = re.search(r":(\d+)", line)
+                port = int(found[1]) if found else port
+        return {"running": True, "community": community, "port": port, "version": "2c"}
+
     # -- accounts, processes, cron -----------------------------------------------
 
     def get_users(self) -> list[dict[str, Any]]: