Junk in SNMP responses

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Junk in SNMP responses

Gisle Vanem-3
While testing the lwIP-SNMP client or agent (correct term?), using
Net-SNMP's snmpwalk, I see some responses with "junk" like
this (in the lwIP host's ARP table):

   snmpwalk.exe -M mibs -m ALL -v2c -c public 10.0.0.23 RFC1213-MIB::atPhysAddress
   RFC1213-MIB::atPhysAddress.1.10.0.0.1 = STRING: "╪δù0ëí"
   RFC1213-MIB::atPhysAddress.1.10.0.0.10 = Hex-STRING: E0 3F 49 81 2E EA
   RFC1213-MIB::atPhysAddress.1.10.0.0.21 = Hex-STRING: 94 A1 A2 07 04 BB

and:
   snmpwalk.exe -M mibs -m ALL -v2c -c public 10.0.0.23 RFC1213-MIB::ipNetToMediaPhysAddress
   RFC1213-MIB::ipNetToMediaPhysAddress.1.10.0.0.1 = STRING: "╪δù0ëí"
   RFC1213-MIB::ipNetToMediaPhysAddress.1.10.0.0.10 = Hex-STRING: E0 3F 49 81 2E EA
   RFC1213-MIB::ipNetToMediaPhysAddress.1.10.0.0.21 = Hex-STRING: 94 A1 A2 07 04 BB

(the lwIP host is at 10.0.0.23).

I do not claim to understand SNMP. Why are there seemingly 2 types
of responses here? Maybe Net-SNMP is to blame here?

The 'E0-3F-49-81-2E-EA' and '94 A1 A2 07 04 BB' are the correct MAC-addresses
of machines connecting to the lwIP web-server. 10.0.0.1 is my default GW.


--
--gv

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Re: Junk in SNMP responses

Dirk Ziegelmeier-2
Place a breakpoint in
ip_NetToMediaTable_get_cell_value_core()
and check whether the correct MAC is returned for you GW. If it is correct, please check wireshark if the response is comparable to the other entries. I guess it is only an snmpwalk problem that tries to figure out if the returned value is a "printable"/"visible" string or not.

Dirk



On Mon, Oct 22, 2018 at 1:57 PM Gisle Vanem <[hidden email]> wrote:
While testing the lwIP-SNMP client or agent (correct term?), using
Net-SNMP's snmpwalk, I see some responses with "junk" like
this (in the lwIP host's ARP table):

   snmpwalk.exe -M mibs -m ALL -v2c -c public 10.0.0.23 RFC1213-MIB::atPhysAddress
   RFC1213-MIB::atPhysAddress.1.10.0.0.1 = STRING: "╪δù0ëí"
   RFC1213-MIB::atPhysAddress.1.10.0.0.10 = Hex-STRING: E0 3F 49 81 2E EA
   RFC1213-MIB::atPhysAddress.1.10.0.0.21 = Hex-STRING: 94 A1 A2 07 04 BB

and:
   snmpwalk.exe -M mibs -m ALL -v2c -c public 10.0.0.23 RFC1213-MIB::ipNetToMediaPhysAddress
   RFC1213-MIB::ipNetToMediaPhysAddress.1.10.0.0.1 = STRING: "╪δù0ëí"
   RFC1213-MIB::ipNetToMediaPhysAddress.1.10.0.0.10 = Hex-STRING: E0 3F 49 81 2E EA
   RFC1213-MIB::ipNetToMediaPhysAddress.1.10.0.0.21 = Hex-STRING: 94 A1 A2 07 04 BB

(the lwIP host is at 10.0.0.23).

I do not claim to understand SNMP. Why are there seemingly 2 types
of responses here? Maybe Net-SNMP is to blame here?

The 'E0-3F-49-81-2E-EA' and '94 A1 A2 07 04 BB' are the correct MAC-addresses
of machines connecting to the lwIP web-server. 10.0.0.1 is my default GW.


--
--gv

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Re: Junk in SNMP responses

Gisle Vanem-3
Dirk Ziegelmeier wrote:

> Place a breakpoint in
> ip_NetToMediaTable_get_cell_value_core()
> and check whether the correct MAC is returned for you GW. If it is correct, please check wireshark if the response is
> comparable to the other entries. I guess it is only an snmpwalk problem that tries to figure out if the returned value
> is a "printable"/"visible" string or not.

I did not manage to set a break-point. But adding some trace-code, shows
lwIP returns the correct MAC-addresses.

And in Wireshark, the 2 responses are the same;
So a problem with snmpwalk I guess.

--
--gv

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