Resolving MIBs

This page covers the two common operations: resolving a name to an OID, and translating a received OID back to a name. It ends with how to point pysmi at a local directory instead of, or ahead of, the published tree.

Resolving a name

IF-MIB is not among the modules pysnmp ships, so naming an object in it requires pysmi and a source to read from:

from pysnmp.smi import builder, compiler, rfc1902, view

mibBuilder = builder.MibBuilder()
compiler.addMibCompiler(
    mibBuilder, sources=["https://pysnmp.github.io/mibs/asn1/@mib@"]
)
mibView = view.MibViewController(mibBuilder)

identity = rfc1902.ObjectIdentity("IF-MIB", "ifInOctets", 1)
identity.resolveWithMib(mibView)

print(identity.getOid())

1.3.6.1.2.1.2.2.1.10.1

@mib@ is a placeholder that pysmi substitutes with the module name it needs. It is not a URL you open in a browser.

The first resolution compiles IF-MIB. Later ones read the compiled copy. Pass the same ObjectIdentity to getCmd or nextCmd to poll by name.

Translating a trap

A notification arrives as bare numbers. Translating it uses the same machinery in reverse, with one requirement.

An OID carries no indication of which module defines it. Load the modules the receiver expects before the lookup. Resolving against an empty builder raises SmiError: ... (MIB not loaded?).

from pysnmp.smi import builder, compiler, rfc1902, view

mibBuilder = builder.MibBuilder()
compiler.addMibCompiler(
    mibBuilder, sources=["https://pysnmp.github.io/mibs/asn1/@mib@"]
)
# Load what this receiver expects to see. Without this the reverse lookup
# has nothing to search.
mibBuilder.loadModules("SNMPv2-MIB", "IF-MIB")
mibView = view.MibViewController(mibBuilder)

# What a linkDown trap actually carries on the wire.
varBinds = [
    ("1.3.6.1.6.3.1.1.4.1.0", "1.3.6.1.6.3.1.1.5.3"),
    ("1.3.6.1.2.1.2.2.1.1.1", 1),
    ("1.3.6.1.2.1.2.2.1.8.1", 2),
]

for oid, value in varBinds:
    varBind = rfc1902.ObjectType(rfc1902.ObjectIdentity(oid), value)
    varBind.resolveWithMib(mibView)
    print(varBind.prettyPrint())

SNMPv2-MIB::snmpTrapOID.0 = IF-MIB::linkDown
IF-MIB::ifIndex.1 = 1
IF-MIB::ifOperStatus.1 = down

In the last line, 2 is rendered as down by the textual convention in IF-MIB. The distribution carries the enumeration labels, not only the object names.

Where a receiver cannot know in advance which modules a trap will name, core.db resolves in the other direction: one indexed query per arc returns the module, the name, the syntax and the access level, with no compile on the trap path.

Pointing pysmi somewhere else

The source list is ordered and the first hit wins. Put a local directory ahead of the published tree to override a module or to compile without network access:

compiler.addMibCompiler(
    mibBuilder,
    sources=[
        "/usr/share/snmp/mibs",
        "https://pysnmp.github.io/mibs/asn1/@mib@",
    ],
)

mibdump takes the same list on the command line. Most vendor MIBs are not published, so a local directory ahead of the tree is the usual arrangement.

An unpacked mibs-asn1.zip or a mounted corpus-compact image is a local directory like any other. See getting the MIBs.

Warning

A MIB module compiled by pysmi becomes Python that pysnmp executes. Treat an ASN.1 MIB source as code you are about to run, and compile only from a source you trust.