Re: Discrepancy in queries to lwIP
Dave Shield <[email protected]>
| Newsgroups | gmane.network.net-snmp.user |
|---|---|
| Message-ID | <[email protected]> |
2009/2/18 Jesus Alvarez <[email protected]>: > 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. > 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. The first (successful) query is using a 16-bit request ID. This is handled correctly by the remote agent, which returns a response with the same ID: > 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... > 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... Note that the last four octets are the same in both cases: 04 02 32 0B The second (failing) query is using a 32-bit request ID. This is perfectly valid, but the remote agent is mangling it, and only returning the lower 16-bits in the response: > 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.... > 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.... Note: 02 04 1C 7B EC EB becomes 02 03 00 EC CB The client notes that the request ID in the response doesn't match the original one, so discards the message. This is a bug in the remote agent. Complain to the lwIP people. As a workaround: $ man snmp.conf 16bitIDs yes restricts requestIDs, etc to 16-bit values. The SNMP specifications define these ID fields as 32-bit quantities, and the Net-SNMP library typically ini- tialises them to random values for security. However certain (broken) agents cannot handle ID values greater than 2^16 - this option allows interoperability with such agents. [Note: this goes in snmp.conf, *not* snmpd.conf] Dave ------------------------------------------------------------------------------ Open Source Business Conference (OSBC), March 24-25, 2009, San Francisco, CA -OSBC tackles the biggest issue in open source: Open Sourcing the Enterprise -Strategies to boost innovation and cut costs with open source participation -Receive a $600 discount off the registration fee with the source code: SFAD http://p.sf.net/sfu/XcvMzF8H _______________________________________________ Net-snmp-users mailing list [email protected] Please see the following page to unsubscribe or change other options: https://lists.sourceforge.net/lists/listinfo/net-snmp-users