Add a util function for SNMP

On the path to instrumenting network switches, first
we'll add some framework for SNMP.  Given that we are
using eventlet and thus we need a patchable SNMP,
we employ PySNMP, despite it being a bit peculiar.
This commit tucks away the oddness and makes it
pretty easy to use for our purposes.

confluent_server/confluent/snmputil.py

@@ -0,0 +1,89 @@
+# vim: tabstop=4 shiftwidth=4 softtabstop=4
+
+# Copyright 2016 Lenovo
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# This provides a simplified wrapper around snmp implementation roughly
+# mapping to the net-snmp commands
+
+# net-snmp-python was considered as the API is cleaner, but the ability to
+# patch pysnmp to have it be eventlet friendly has caused it's selection
+# This module simplifies the complex hlapi pysnmp interface
+
+import confluent.exceptions as exc
+import eventlet
+from eventlet.support.greendns import getaddrinfo
+import socket
+snmp = eventlet.import_patched('pysnmp.hlapi')
+
+
+def _get_transport(name):
+    # Annoyingly, pysnmp does not automatically determine ipv6 v ipv4
+    res = getaddrinfo(name, 161, 0, socket.SOCK_DGRAM)
+    if res[0][0] == socket.AF_INET6:
+        return snmp.Udp6TransportTarget(res[0][4])
+    else:
+        return snmp.UdpTransportTarget(res[0][4])
+
+
+def walk(server, oid, secret, username=None, context=None):
+    """Walk over children of a given OID
+
+    This is roughly equivalent to snmpwalk.  It will automatically try to be
+    an snmpbulkwalk if possible.  If username is not given, it is assumed that
+    the secret is a community string, and v2c is used.  If a username given,
+    it'll assume SHA auth and DES privacy with the secret being the same for
+    both.
+
+    :param server: The network name/address to target
+    :param oid: The SNMP object identifier
+    :param secret: The community string or password
+    :param username: The username for SNMPv3
+    :param context: The SNMPv3 context or index for community string indexing
+    """
+    # SNMP is a complicated mess of things.  Will endeavor to shield caller
+    # from as much as possible, assuming reasonable defaults where possible.
+    # there may come a time where we add more parameters to override the
+    # automatic behavior (e.g. DES is weak, so it's a likely candidate to be
+    # overriden, but some devices only support DES)
+    tp = _get_transport(server)
+    ctx = snmp.ContextData(context)
+    if '::' in oid:
+        mib, field = oid.split('::')
+        obj = snmp.ObjectType(snmp.ObjectIdentity(mib, field))
+    else:
+        obj = snmp.ObjectType(snmp.ObjectIdentity(oid))
+    eng = snmp.SnmpEngine()
+    if username is None:
+        # SNMP v2c
+        authdata = snmp.CommunityData(secret, mpModel=1)
+    else:
+        authdata = snmp.UsmUserData(username, authKey=secret, privKey=secret)
+    walking = snmp.bulkCmd(eng, authdata, tp, ctx, 0, 10, obj,
+                           lexicographicMode=False)
+    for rsp in walking:
+        errstr, errnum, erridx, answers = rsp
+        if errstr:
+            raise exc.TargetEndpointUnreachable(str(errstr))
+        elif errnum:
+            raise exc.ConfluentException(errnum.prettyPrint())
+        for ans in answers:
+            yield ans
+
+
+if __name__ == '__main__':
+    import sys
+    for kp in walk(sys.argv[1], sys.argv[2], 'public'):
+        print(str(kp[0]))
+        print(str(kp[1]))