SNMP Support for Crystal Lang

NOTE:: I consider the project ready to use. Usage won't change significantly between now and v1.0.0

Usage

This library can be used to build either an SNMP Agent or Client application. The examples below indicate how to use it as a client.

SNMP v2c

# Connect to server
socket = UDPSocket.new
socket.connect("demo.snmplabs.com", 161)
socket.sync = false

# Make request
session = SNMP::Session.new
socket.write_bytes session.get("1.3.6.1.2.1.1.4.0")
socket.flush

# Process response
response = session.parse(socket.read_bytes(ASN1::BER))
response.value.get_string # "SNMP Laboratories, info@snmplabs.com"

SNMP v3

# Connect to server
socket = UDPSocket.new
socket.connect("demo.snmplabs.com", 161)
socket.sync = false

# Setup session
session = SNMP::V3::Session.new("usr-md5-aes", "authkey1", "privkey1", priv_protocol: SNMP::V3::Security::PrivacyProtocol::AES)

# This is required to get the engine ID, boot and tick times
# You can read about it here: https://www.snmpsharpnet.com/?page_id=28
if session.must_revalidate?
  socket.write_bytes session.engine_validation_probe
  socket.flush
  session.validate socket.read_bytes(ASN1::BER)
end

# Make the request
# NOTE:: with SNMPv3 you need to prepare the message for transmission
unencrypted_message = session.get("1.3.6.1.2.1.1.4.0")
socket.write_bytes session.prepare(unencrypted_message)
socket.flush

# Process response
response = session.parse(socket.read_bytes(ASN1::BER))
response.value.get_string # "SNMP Laboratories, info@snmplabs.com"

Setting values

NOTE:: set currently supports:

• Strings
• Integers
• Boolean
• Nil

More crystal classes will be added over time (such as Float and Socket::IPAddress etc)

# Setting a string
session.set("1.3.6.1.2.1.1.3.0", "some string value")

# Setting an integer
session.set("1.3.6.1.2.1.1.3.0", 34)

For more complex or currently unsupported types you can build a custom ASN1.BER.

ber = ASN1::BER.new
ber.tag_class = ASN1::BER::TagClass::Application
ber.tag_number = 12
ber.payload = Bytes[1,2,3,4,5]

session.set("1.3.6.1.2.1.1.3.0", ber)

Extracting response values

The response value is always an ASN1::BER

response = session.parse(socket.read_bytes(ASN1::BER))
response.value

You can extract common data types using helper methods:

• .get_string
• .get_object_id for SNMP OIDs such as 1.3.6.1.2.1
• .get_hexstring for a hex representation of the payload bytes
• .get_bytes for the raw byte data
• .get_boolean
• .get_integer returning an Int64

Notes on IO

Writing to Sockets

When writing SNMP messages to the socket, be aware that you should be buffering the write.

session = SNMP::V3::Session.new
message = session.engine_validation_probe

# Ensure sync is false so the message is buffered
socket.sync = false
socket.write_bytes message

# This requires you to call `flush`
socket.flush

This is because the call to to_io on message involves multiple writes to the IO as the message is progressively constructed. However you don't want each write to be sending packets as this will result in a lot of overhead and most SNMP servers will not accept fragmented messages.

Reading from sockets

Whilst you'll probably be OK reading data like socket.read_bytes(ASN1::BER) you should probably be buffering requests based on SNMP PDU Max Size (defaulting to 65507 bytes) and throwing away any buffered data that can't be read after buffering or a short timeout.