#snmp - SNMP v1 / v2c client

Import with import "snmp.j" as snmp;. An SNMP manager (client) that queries an agent over UDP: get, getNext, set, and subtree walk, with community-string authentication. The wire messages are ASN.1 BER, built and parsed with the asn1 library; the transport is net UDP. Needs the default jennifer binary (net).

import "snmp.j" as snmp;
use io;

def c as snmp.Client init snmp.client("192.0.2.10", "public");
def vbs as list of snmp.Varbind init snmp.get($c, ["1.3.6.1.2.1.1.1.0"]);
io.printf("%s = %s\n", $vbs[0].oid, $vbs[0].value);   # sysDescr

# Walk a subtree.
for (def vb in snmp.walk($c, "1.3.6.1.2.1.1")) {
    io.printf("%s [%s] %s\n", $vb.oid, $vb.type, $vb.value);
}

Runnable: examples/modules/snmp_demo.j.

#Client

export def struct Client {
    address as string, community as string,
    version as int, timeoutMs as int, retries as int
};

Constants: snmp.VERSION1 (0) and snmp.VERSION2C (1). v2c is the default; many older or embedded devices (some printers, sensors) answer v1 only - if a query times out, try clientWith(..., snmp.VERSION1, ...).

#Requests

Each call sends one PDU and returns the response bindings, throwing a snmp-kind Error if the agent reports an error status or does not answer within timeoutMs (after retries extra attempts).

#Bindings

export def struct Varbind { oid as string, type as string, value as string, number as int };

A returned Varbind carries the OID, the SNMP value type, a string value rendering, and number (the integer for a numeric type, else 0):

For a SET, build typed bindings:

#Agent (server)

The module is also an agent (server): it answers GET / GETNEXT / SET for a MIB you supply. This is a hardware simulator for testing an SNMP tool, a self-contained example (no external device), or a way to expose your own app's metrics to an SNMP-native monitoring system.

import "snmp.j" as snmp;
use net;
use task;

# A MIB: one binding per OID, any SNMP value type.
def a as snmp.Agent init snmp.agent("public", snmp.VERSION2C, [
    snmp.stringVar("1.3.6.1.2.1.1.1.0", "Jennifer SNMP agent"),
    snmp.varbind("1.3.6.1.2.1.1.3.0", "timeTicks", "", 424242),
    snmp.intVar("1.3.6.1.2.1.1.7.0", 72)
]);

# Serve on a loopback socket in a spawned goroutine, then query it.
use channel;
def sock as net.UDPSocket init net.listenUDP("127.0.0.1:0");
def stop as channel of bool init channel.make(1);
def server as task of null init spawn { snmp.serveOn($a, $sock, $stop); };
def c as snmp.Client init snmp.clientWith(net.address($sock), "public", snmp.VERSION2C, 2000, 3);
snmp.get($c, ["1.3.6.1.2.1.1.1.0"]);      # -> "Jennifer SNMP agent"
channel.send($stop, true);                # graceful shutdown ...
task.wait($server);                       # ... then join the agent

The agent answers GET (a missing OID is a v2c noSuchObject exception, or a v1 noSuchName error-status), GETNEXT (the next OID in numeric order - so walk works; past the end is endOfMibView), and SET (updates an existing binding; an unknown OID errors). A request with the wrong community, or a malformed datagram, is dropped silently, as a real agent does.

Security. Like any SNMP agent, this one is a UDP responder: a request can elicit a larger response (amplification), and the reply goes to the datagram's source address, which can be spoofed (reflection). The community string is plaintext (v1 / v2c has no real authentication). Treat the agent as a simulator / test tool or an internal-metrics endpoint - do not expose it on an untrusted network without a firewall or source allowlist in front of it.

Runnable: examples/modules/snmp_agent_demo.j runs an agent and a client in one process.

#Scope

SNMP v1 / v2c only - no SNMPv3 / USM (user-based security), which is the authentication / privacy model a later tier would add. v1 / v2c has no privacy or integrity either: the community travels in plaintext, and a response is matched only by request-id (a random 31-bit value), not by source address - so a network-level observer can read the community and a well-placed attacker can spoof a reply. Treat the community as a filter, not a secret. GETBULK is not built (a walk uses GETNEXT). number is int64-bounded, so a Counter64 (or other unsigned type) whose value is >= 2^63 is returned as a 0x-prefixed hex value string with number 0, rather than a wrong negative number. There is no MIB compiler: OIDs are dotted numeric strings, and values are returned by SNMP type, not resolved to named objects. The agent answers GET / GETNEXT / SET but does not send traps / notifications, and does not implement GETBULK (walk uses GETNEXT); its MIB is a flat list of Varbind, so lookup is linear (fine for the hundreds of OIDs a simulator holds, not a huge tree). Every OID in the MIB is writable by a client with the community - there is no read-only distinction, so filter out any OID you do not want settable when you build the agent's bindings.

#See also

The asn1 library (the BER/DER codec underneath) and net (the UDP transport); the module index.