[lwip-users] Discrepancy in SNMP GET responses

[Jesus Alvarez]

I am using the lwIP 1.3.0 SNMP agent on an embedded controller using the
Luminary ARM-Cortex microcontrollers. The implementation works but appears
to be sensitive to the version of the SNMP tool used to query it.

Using net-snmp 5.4.2.1 under Windows a simple query to sysLocation.0 works.
Yet the same query fails from net-snmp 5.4.1 under Debian Linux. The UDP
query and response is included below for both cases. A router used as a
reference responds to queries properly with both versions.

If I use debug mode (-D ALL) on the query that fails I get

  trace: _sess_process_packet(): snmp_api.c, 5388:
  sess_process_packet: unhandled PDU

It appears that the lwIP SNMP agent is processing one of the queries
incorrectly but I can't identify what portion of the ASN-1 encoded response
packet is causing the rejection. I also don't understand why the content of
the query packets is different on the two versions. Any suggestions would be
appreciated.

Thanks,
Jesus Alvarez
----------
C:\>snmpget -V
NET-SNMP version: 5.4.2.1

C:\>snmpget -v 1 -c public -d -r 0 192.168.1.222 sysLocation.0

Sending 41 bytes to UDP: [0.0.0.0]->[192.168.1.222]:161
0000: 30 27 02 01  00 04 06 70  75 62 6C 69  63 A0 1A 02    0'.....publicá..
0016: 02 32 0B 02  01 00 02 01  00 30 0E 30  0C 06 08 2B    .2.......0.0...+
0032: 06 01 02 01  01 06 00 05  00                          .........


Received 54 bytes from UDP: [0.0.0.0]->[192.168.1.222]:161
0000: 30 34 02 01  00 04 06 70  75 62 6C 69  63 A2 27 02    04.....publicó'.
0016: 02 32 0B 02  01 00 02 01  00 30 1B 30  19 06 08 2B    .2.......0.0...+
0032: 06 01 02 01  01 06 00 04  0D 4C 61 6B  65 20 4D 61    .........Lake Ma
0048: 72 79 2C 20  46 4C                                    ry, FL

SNMPv2-MIB::sysLocation.0 = STRING: Lake Mary, FL
----------
$ snmpget -V
NET-SNMP version: 5.4.1
$ snmpget -v 1 -c public -d -r 0 192.168.1.222 sysLocation.0

Sending 43 bytes to UDP: [192.168.1.222]:161
0000: 30 29 02 01  00 04 06 70  75 62 6C 69  63 A0 1C 02    0).....public...
0016: 04 1C 7B EC  CB 02 01 00  02 01 00 30  0E 30 0C 06    ..{........0.0..
0032: 08 2B 06 01  02 01 01 06  00 05 00                    .+.........


Received 55 bytes from UDP: [192.168.1.222]:161
0000: 30 35 02 01  00 04 06 70  75 62 6C 69  63 A2 28 02    05.....public.(.
0016: 03 00 EC CB  02 01 00 02  01 00 30 1B  30 19 06 08    ..........0.0...
0032: 2B 06 01 02  01 01 06 00  04 0D 4C 61  6B 65 20 4D    +.........Lake M
0048: 61 72 79 2C  20 46 4C                                 ary, FL

Timeout: No Response from 192.168.1.222.