I am trying to send a trap message from my agent to net-snmp but in my agent it looks like it's fine, however net-snmp doesn't show the message.
I want to know if net-snmp knows how to handle a trap message?
Normally, you would use net-snmp as an Agent.
Agents generally send traps, and don't receive them. They need to be able to handle get-requests, set-requests and get-next-requests, but not traps.
The system which receives SNMP traps is known as a Manager. Net-snmp is not an SNMP manager, but it contains all the (C) programming libraries needed for you to create your own SNMP manager, should you want to. There is also snmptrapd, which can log incoming traps into a file, or pass them to a perl script for processing.
Perhaps you're indeed asking about snmptrapd?
If traps are not logged on the receiver, normal network troubleshooting procedures apply. Snoop the traffic using Wireshark, first on the receiving system, then if needed, on the sending side. This can verify whether the traffic is dropped by the network (firewalls etc).
If you want to study the basics of the protocol, this RFC is a good starting point: "Protocol Operations for Version 2 of the Simple Network Management Protocol" http://www.ietf.org/rfc/rfc1905.txt
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As my knowledge, in normal behavior, client will make random port to connect the SNMP service port, now the client use SNMP trap port (162) to communicate with server.
My questions:
As the client setting, I do not have configure any SNMP setting for the client, why WireShark able to capture SNMP traffic came from the client?
Why the client use SNMP trap port (162) to communicate with server, rather than using random port?
SNMP is usually transmitted over UDP, so there is actually no "connection", and speaking technically the source port doesn't matter. You can just send out datagrams (e.g. traps) without binding to a port.
However, even when run over UDP, SNMP does involve some two-way communication. If you are expecting a response (which a client does if it's sending an SNMP Get or Set request), the only place the other end knows to send it is back where the request came from, i.e. the original IP/port combination. There's no information in a SNMP packet that provides any alternative "return address" information.
So, in order to get a response on a predictable port, you'll send the request datagram from a bound socket. Typically the client will run its own listening "server" on port 162, send requests from there, and then it can receive responses there too. Otherwise you wouldn't see the responses. This also allows us to set up simple firewall rules (though you can often get away without firewall rules for the return path, due to hole punching*).
This is also true for the server, which emits traps and informs on a known, standard, predictable port not only so that you can configure your trap receiver and firewalls in a reliable way, but so that inform responses can be sent back to a known, standard, predictable port that you're listening on.
tl;dr: You can send your requests from an arbitrary port if you like, but it's not very useful.
* My SNMP implementation seemed buggy when the client/receiver only saw traps emitted during the ~15 minutes after it had last poked out some kind of request packet. Subsequent traps seemed to be completely missing. After much debugging on the server end, it turned out that we'd forgotten to open the correct port on the inbound firewall for the client, and were accidentally relying on hole punching, which has a time limit. :D
As for why Wireshark is seeing traffic from an unconfigured SNMP client, well, either your SNMP client actually is configured to send requests, or you're misinterpreting the results. Wireshark doesn't invent traffic. Without a more complete picture of your network setup, software setup, and those packets you're seeing, we could only speculate as to the exact cause of your confusion.
I'm trying to send and receive an SNMP trap using an Ubuntu VM. Works well if I send and receive it in the same VM. But is it possible to send it to another Ubuntu VM in a different network or sending it through the internet?
Used a ton of tutorial like this but with no luck.
Any tutorials or guides for this? Been searching for days. Any help would be great. Thanks!
Yes, of course, but you will need to open up your firewalls and/or set up port forwarding so that the packets go where they need to go when they cross any network boundaries. The specific details depend on your network layout and cannot be further explained in the abstract.
I am new to SNMP agent implementation. A confusion regarding the usage of SNMP4J API is, which SNMP4J Api should be used to create the agent? Should it be SNMP4J or SNMP4J-Agent?
An agent is some kind of server: it mainly replies to requests (it may occasionally also send traps to managers).
A manager is a some kind of client: it mainly send requests to agents (it may occasionally also receive traps from managers).
SNMP4J is the library for managers to send requests to remote agents.
SNMP4J-Agent is the library for agents to reply to requests.
So, to write a SNMP agent, you need to use SNMP4J-Agent jar file.
I have an application in PHP(running on ubuntu) which executes some commands depending on some received traps. I have a windows test client to send traps and it works well for snmp v2. I check everything from the snmptrapd log. Unfortunately the client doesn't support v3 and I must use a real device. When I use a NoBreak which sends only v3 traps snmptrapd does nothing. In fact I can see these traps on my windows client, therefore some setup in linux appears to be missing. I've been reading net-snmp page and tutorials on the internet, but I'm not really sure about what to do, because I didn't configure any aspect of security in windows and it simply works.
If you go to net-snmp docs, it's said that we need to create a user and set a password, however the nobreak seems to send a kind of broadcast. Also I didn't receive instructions for setup(I'm not saying that I don't need to do so).
The first question is:
How can I listen to a port in order to check trap reception? Maybe it's arriving but not being processed.
The second question:
How can I configure snmptrapd to receive all v3 traps? Is that possible?
Net-SNMP's snmptrapd very much does handly SNMPv3 traps. But, it does so only after you've configured it. In fact, the it won't receive any traps over any version of the protocol unless you configure it. This is for security, since it is capable of running applications when a trap is received, etc. Imagine getting a fake trap that told you the machine across town was having an issue? You'd drive all the way over there just to find out someone had sent you a bugs trap because you were accepting and believing any message that came it's way.
To set up SNMPv3 for snmptrapd, you might start with this tutorial in particular: SNMPv3 Traps
I want to build an application using snmp for searching my LAN and exploring the devices which have shared some resources. I need some assistance to start with it, I have trying to do the same using net-snmp, but still no success.
SNMP is on UDP, so you can simply do a broadcast.
It is quite normal that net-snmp times out in such cases, as no SNMP agent would like to respond to you or no SNMP agent is there in the same LAN.