182 lines
6.2 KiB
Plaintext
182 lines
6.2 KiB
Plaintext
SNMPv1 agent for lwIP
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Author: Christiaan Simons
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This is a brief introduction how to use and configure the SNMP agent.
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Note the agent uses the raw-API UDP interface so you may also want to
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read rawapi.txt to gain a better understanding of the SNMP message handling.
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0 Agent Capabilities
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====================
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SNMPv1 per RFC1157
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This is an old(er) standard but is still widely supported.
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For SNMPv2c and v3 have a greater complexity and need many
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more lines of code. IMHO this breaks the idea of "lightweight IP".
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Note the S in SNMP stands for "Simple". Note that "Simple" is
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relative. SNMP is simple compared to the complex ISO network
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management protocols CMIP (Common Management Information Protocol)
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and CMOT (CMip Over Tcp).
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MIB II per RFC1213
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The standard lwIP stack management information base.
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This is a required MIB, so this is always enabled.
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When builing lwIP without TCP, the mib-2.tcp group is omitted.
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The groups EGP, CMOT and transmission are disabled by default.
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Most mib-2 objects are not writable except:
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sysName, sysLocation, sysContact, snmpEnableAuthenTraps.
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Writing to or changing the ARP and IP address and route
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tables is not possible.
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Note lwIP has a very limited notion of IP routing. It currently
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doen't have a route table and doesn't have a notion of the U,G,H flags.
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Instead lwIP uses the interface list with only one default interface
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acting as a single gateway interface (G) for the default route.
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The agent returns a "virtual table" with the default route 0.0.0.0
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for the default interface and network routes (no H) for each
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network interface in the netif_list.
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All routes are considered to be up (U).
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Loading additional MIBs
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MIBs can only be added in compile-time, not in run-time.
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There is no MIB compiler thus additional MIBs must be hand coded.
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Large SNMP message support
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The packet decoding and encoding routines are designed
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to use pbuf-chains. Larger payloads then the minimum
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SNMP requirement of 484 octets are supported if the
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PBUF_POOL_SIZE and IP_REASS_BUFSIZE are set to match your
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local requirement.
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1 Building the Agent
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====================
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First of all you'll need to add the following define
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to your local lwipopts.h:
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#define LWIP_SNMP 1
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and add the source files in lwip/src/core/snmp
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and some snmp headers in lwip/src/include/lwip to your makefile.
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Note you'll might need to adapt you network driver to update
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the mib2 variables for your interface.
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2 Running the Agent
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===================
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The following function calls must be made in your program to
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actually get the SNMP agent running.
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Before starting the agent you should supply pointers
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to non-volatile memory for sysContact, sysLocation,
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and snmpEnableAuthenTraps. You can do this by calling
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snmp_set_syscontact()
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snmp_set_syslocation()
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snmp_set_snmpenableauthentraps()
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Additionally you may want to set
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snmp_set_sysdescr()
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snmp_set_sysobjid() (if you have a private MIB)
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snmp_set_sysname()
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Also before starting the agent you need to setup
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one or more trap destinations using these calls:
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snmp_trap_dst_enable();
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snmp_trap_dst_ip_set();
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In the lwIP initialisation sequence call snmp_init() just after
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the call to udp_init().
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Exactly every 10 msec the SNMP uptime timestamp must be updated with
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snmp_inc_sysuptime(). You should call this from a timer interrupt
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or a timer signal handler depending on your runtime environment.
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An alternative way to update the SNMP uptime timestamp is to do a call like
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snmp_add_sysuptime(100) each 1000ms (which is bigger "step", but call to
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a lower frequency). Another one is to not call snmp_inc_sysuptime() or
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snmp_add_sysuptime(), and to define the SNMP_GET_SYSUPTIME(sysuptime) macro.
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This one is undefined by default in mib2.c. SNMP_GET_SYSUPTIME is called inside
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snmp_get_sysuptime(u32_t *value), and enable to change "sysuptime" value only
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when it's queried (any function which need "sysuptime" have to call
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snmp_get_sysuptime).
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3 Private MIBs
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==============
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If want to extend the agent with your own private MIB you'll need to
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add the following define to your local lwipopts.h:
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#define SNMP_PRIVATE_MIB 1
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You must provide the private_mib.h and associated files yourself.
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Note we don't have a "MIB compiler" that generates C source from a MIB,
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so you're required to do some serious coding if you enable this!
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Note the lwIP enterprise ID (26381) is assigned to the lwIP project,
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ALL OBJECT IDENTIFIERS LIVING UNDER THIS ID ARE ASSIGNED BY THE lwIP
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MAINTAINERS!
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If you need to create your own private MIB you'll need
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to apply for your own enterprise ID with IANA: http://www.iana.org/numbers.html
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You can set it by passing a struct snmp_obj_id to the agent
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using snmp_set_sysobjid(&my_object_id), just before snmp_init().
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Note the object identifiers for thes MIB-2 and your private MIB
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tree must be kept in sorted ascending (lexicographical) order.
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This to ensure correct getnext operation.
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An example for a private MIB is part of the "minimal Unix" project:
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contrib/ports/unix/proj/minimal/lwip_prvmib.c
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The next chapter gives a more detailed description of the
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MIB-2 tree and the optional private MIB.
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4 The Gory Details
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==================
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4.0 Object identifiers and the MIB tree.
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We have three distinct parts for all object identifiers:
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The prefix
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.iso.org.dod.internet
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the middle part
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.mgmt.mib-2.ip.ipNetToMediaTable.ipNetToMediaEntry.ipNetToMediaPhysAddress
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and the index part
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.1.192.168.0.1
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Objects located above the .internet hierarchy aren't supported.
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Currently only the .mgmt sub-tree is available and
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when the SNMP_PRIVATE_MIB is enabled the .private tree
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becomes available too.
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Object identifiers from incoming requests are checked
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for a matching prefix, middle part and index part
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or are expanded(*) for GetNext requests with short
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or inexisting names in the request.
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(* we call this "expansion" but this also
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resembles the "auto-completion" operation)
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The middle part is usually located in ROM (const)
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to preserve precious RAM on small microcontrollers.
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However RAM location is possible for an dynamically
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changing private tree.
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The index part is handled by functions which in
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turn use dynamically allocated index trees from RAM.
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These trees are updated by e.g. the etharp code
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when new entries are made or removed form the ARP cache.
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/** @todo more gory details */
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