First, a picture: 10.1.1.0/24 --------------------- | | eth0 manager ---- router1 router2 | | eth1 --------------------- 10.2.2.0/24
The manager is singly-homed, with the address 10.3.3.3. It sends a request to router2's 10.1.1.2 address, so the request comes in eth0. router2's routing table points 10.3.3.0/24 to eth1, however -- the next hop is 10.2.2.1, router1's address. IP normally works fine in this situation, of course, and Net-SNMP 5.5 works as well -- manager can talk to router2 just fine. It stopped working in Net-SNMP 5.6, due to the use of the interface index in IP_PKTINFO. Specifying the interface index causes Linux to try to send the packet out eth0, so it just ARPs for it -- but since the destination is not on the local network, the ARPs don't get replied to. This means that router2 cannot reply to requests from manager that are sent to its eth0. This use of the interface index was introduced in r17855, a little over a year ago: Modified Tue Dec 1 07:34:43 2009 UTC (12 months, 1 week ago) by jsafranek CHANGES: snmplib: Fix responding to broadcast requests. UDP responses are sent with source IP address which was destination of appropriate requests (implemented in SVN rev. 15215). But if the destination of a request is broadcast IP address, the request was sent with the broadcast address as source. sendmsg() on Linux does not support this and returns error -> response is not sent. In order to send responses from the same interface, interface index of the appropriate interface must be used. I'm not sure what to suggest as a solution. For those who use net-snmp as the agent on a router, where asymmetric routing like this is very common, this black hole for the responses is a real problem. I think a better solution for responding to broadcast requests is to not use the if_index, and just don't specify the source address when the request was to a broadcast address. (In fact, the current solution won't work for directed broadcasts, e.g., in the above picture, if router1 allows manager to send a directed broadcast to the top network -- neither router will be able to reply.) Unfortunately, this means having to figure out if a given address is a broadcast, you have to get a list of interfaces on the system. Obviously, the ipAddrTable has this info, but that's not available to snmplib (especially if, like us, you have your own IP-MIB implementation). Any other ideas? Thanks, Bill ------------------------------------------------------------------------------ Oracle to DB2 Conversion Guide: Learn learn about native support for PL/SQL, new data types, scalar functions, improved concurrency, built-in packages, OCI, SQL*Plus, data movement tools, best practices and more. http://p.sf.net/sfu/oracle-sfdev2dev _______________________________________________ Net-snmp-coders mailing list Net-snmp-coders@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/net-snmp-coders
