Re: A manager using a single socket and multiple agents?

Craig,

Would you mind sharing some more details?  You mentioned that you use an
[asyncapp] stanza in your snmp.conf that limits configuration to just your
app - I can't seem to make this work.  I've learned that if I
init_snmp("my_app") that the API will scan ~/.snmp/my_app.conf, but I can't
seem to insert "clientaddr" and "clientaddrUsesPort" tokens into this file
- a (non-fatal) diagnostic is generated when I call init_snmp(), and the
behavior reverts to "random SRC PORT for each session".

Perhaps if you could cut and paste the relevant lines from your snmp.conf I
would see something obvious I'm missing.

As mentioned, I am able to force all sessions to use the same SRC PORT by
simply appending the clientaddr and clientaddrUsesPort tokens to the end of
~/.snmp/snmp.conf, but this change affects *all* net-snmp apps.  It would
be helpful/instructive to set the SRC PORT on an app-by-app basis (e.g.
my_app, my_app_2, my_app_3 all configured to use a different SRC PORT).


On Mon, Jan 11, 2021 at 5:26 PM Ed Fair <[email protected]> wrote:

> Craig -
>
> You are exactly right - the config file allows me to control this - thank
> you so much!  I couldn't use the [application name] syntax, but simply
> adding these two lines to the end of the config file had the desired effect
> of causing all outbound SNMP from SRC PORT 30000:
>
> clientaddr :30000
> clientaddrUsesPort yes
>
> I'll work to understand how to use the [program name] approach next.  I'm
> guessing the init_snmp() call sets the program name used.
>
> ed
>
> On Sun, Jan 10, 2021 at 6:48 PM Craig Small <[email protected]> wrote:
>
>> Actually the snmplibrary already has a way of binding the ports.
>>
>> In my $HOME/.snmp/snmp.conf I just add this:
>>
>> [asyncapp]
>> #doDebugging 1
>> clientaddr 10.0.0.1:12345
>> clientaddrUsesPort yes
>>
>> asyncapp is the name of my program. I have bound outbound connections
>> with a source port of 12345 on interface 127.0.0.1 (for testing)
>> 10.0.0.1 is the local IP address of the computer that's sending the
>> request.
>>
>> Wireshark shows this:
>> User Datagram Protocol, Src Port: 12345, Dst Port: 161
>>
>>
>>
>>
>>
>>
>>
>> On Thu, 7 Jan 2021 at 15:03, Ed Fair <[email protected]> wrote:
>>
>>> Craig,
>>>
>>> The tutorials don't mention subsessions or traditional vs single session
>>> use, but the header/c files do (and that's all they do - mention them).
>>> I'm just curious what these abstractions are for since they seem, on the
>>> surface, related to my needs.
>>>
>>> As an exercise, I've tried but so far been unable to create a
>>> session/socket which uses a specific port - no errors, but no pdus/packets
>>> transmitted.  And anyway, I don't care if the port selected is random, my
>>> goal is to use *the same* port to query multiple agents.  I don't care how
>>> it's done, as long as the end result is "all outbound UDP use same SRC
>>> port".
>>>
>>>
>>> On Wed, Jan 6, 2021, 5:28 AM Craig Small <[email protected]> wrote:
>>>
>>>> On Wed, 6 Jan 2021 at 10:01, Ed Fair <[email protected]> wrote:
>>>>
>>>>> Thanks for the reply.  The Simple_Async_Application in your link uses
>>>>> one session/socket/SRC port per agent.  I've compiled and run this sample
>>>>> successfully, but I haven't been able to modify it to use a single
>>>>> session/socket/SRC port.
>>>>>
>>>> It might need to be something more low-level as reusing sockets
>>>> (therefore the ports) is generally a bad idea.
>>>>
>>>> The netsnmp_session has an attribute of local_port. If this is set to
>>>> zero (the default) then it picks it randomly. I'd try setting that and see
>>>> what happens. A quick look in the snmplib source code shows it is used for
>>>> creating the transport.
>>>>
>>>> I understand "don't hammer agents" but I don't understand your "one
>>>>> query per agent" limit - is this a limitation of the API?
>>>>>
>>>> Not at all, a lot of agents are terrible and do stupid things like have
>>>> exclusive locks on important components of the system. I've killed many
>>>> devices (the remote agents, not my code) by being too enthusiastic about
>>>> querying them.
>>>>
>>>>
>>>>> I'm new to this API, I might be missing key concepts... but I am
>>>>> confused by the "traditional vs single" distinction, and I'm curious what
>>>>> "subsessions" are.
>>>>>
>>>> Are either of those mentioned in the tutorial? They could mean multiple
>>>> things but was trying to find the context of what you are asking here.
>>>>
>>>>  - Craig
>>>>
>>>

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