I am attempting to understand SNMP (in general, and v3). The goal is to include an snmp agent in an embedded device running an RTOS.
I've already been through over a dozen RFCs with at least another dozen more to go. Each one creates more questions than it answers. (1052, 1065, 1067, 1155, 1156, 1157, 1212, 1213, 1592, 1905, 2578, 2579, 2580, 3410, 3411, 3412, 3413, 3414, 3415, 3416, 3417, 3418, 3584... )
I implemented mDNS-SD and 802.1X EAPOL with just a couple RFCs and it wasn't this confusing.
Many of the reviews of books I considered all complain of the same inconsistent and vagueness of the material. I bought a couple books that had better reviews.
Searching online isn't getting anywhere largely because the keywords aren't finding things I want answers to. So I must not even know the best keywords to search with.
Eventually, I decided to just try to reverse engineer what's going on, I installed WireShark on a Linux PC, and the snmpd and snmp tools, so I could sniff it. Here is what I have, and can't align what I see with what I read.
This is a v3 sniff, It's a reply to the first request from a manager. This question is just zeroing in on one of the things that I want to understand. I can't decode and examine a plaintext PDU, because I can't get a request in v2 or v1.
Wireshark shows this reply to a manager. It's apparently the first step in whatever authentication it to be used.
The book I have shows this as the protocol on the wire. And I am trying to parse out the variable bindings.
Here are the variable bindings from Wireshark
A "sequence" that is 15 bytes long (x30 x0f)
This, from the RFC, says that the list is a SEQUENCE of VarBinds, where each VarBind is the object name, and the value in ObjectSyntax. So it's looking okay so far.
Here is the next segment inside the SEQUENCE (Wireshark highlighted all 14 bytes)
An object ID that is 10 bytes long (x06, x0a)
Here is the actual object:
The objectName is the object ID, and it is x2b x6 x1 x6 x3 xf x1 x1 xx4 x0 or (1.3).6.1.6.3.15.1.1.4.0
Given that this is ISO, ORG, DOD, INTERNET, 6?... I have to assume "6" is an object under internet branch I've not yet come across. Likely something to do with the v3 security.
Next, is the value.
This is a type x41 (65), with a length of 1, and a value of 7.
Well, in "ObjectSyntax" what is x41? I can't find it defined anywhere.
For that matter, all these RFCs use words for identifiers, and I can find only a fraction of what their actual numeric values are.
Wireshark knew what it was... It's saying "Counter32"... is that what x41 is supposed to be? If so, it's nowhere near 32 bits. It's only one byte. Again, I'd like to find it's definition.
Also, somewhere, (I can't even recall which RFC) it said the reply to an OID request is to append the value to the requested object, not replace the zero (example: request: 1.3.6.1.4.300.1 -> reply 1.3.6.1.4.300.1.15 so it is a value of 15 ). This OID has a trailing zero, nad I'm not sure why.
Can anyone point me to some useful, concise, condensed information explaining this material? Every RFC requires that I go back and read some previous (and sometimes obsoleted) RFC, and I've now got over 25 of them already. I don't think it should take this many RFCs to be able to write an "simple" snmp agent. A month of researching, and most of what I have to show for it is how to read MIB files. Although that take some mental gymnastics too.
"Simple" is rather deceptive (as more than one book reviewer has stated).
RFC 1157 specifies that SNMP messages are encoded with "a subset of the basic encoding rules of ASN.1". I don't think the official basic encoding rules (BER) specification is available for free, but it's not hard to find explainers online (here's one I found with a simple search). To your question about the 0x41 byte, this is a BER identifier. The 2 most-significant bits (01) tell you the "class" (i.e. something like a namespace) is "application". The "form" bit (0) tells you that it's a primitive type (i.e. not a sequence). Finally the "tag" is 1. Consulting the SNMPv2-SMI MIB (RFC 2578) you can find this definition:
Counter32 ::=
[APPLICATION 1]
IMPLICIT INTEGER (0..4294967295)
You also asked about why a 32-bit integer is encoded with a single byte. This requires you to distinguish between the scope of the SNMP standard versus the ASN.1 standard. ASN.1 only has a single INTEGER type, which 1) has an unlimited range, 2) is always signed (two's complement), and 3) should be encoded in the least number of octets possible. This actually means that a Counter32 (or any other 32-bit unsigned integer type) might use up to 5 bytes for its encoding (see this answer I gave to a question about that).
Finally, you asked about the way the replies are modifying the requested OID. I was confused about this for a long time, but when I figured it out, I realized it's actually pretty simple. I think the best place to start is with this excerpt from RFC 1157:
Each instance of any object type defined in the MIB is identified in
SNMP operations by a unique name called its "variable name." In
general, the name of an SNMP variable is an OBJECT IDENTIFIER of the
form x.y, where x is the name of a non-aggregate object type defined
in the MIB and y is an OBJECT IDENTIFIER fragment that, in a way
specific to the named object type, identifies the desired instance.
This naming strategy admits the fullest exploitation of the semantics
of the GetNextRequest-PDU (see Section 4), because it assigns names
for related variables so as to be contiguous in the lexicographical
ordering of all variable names known in the MIB.
The type-specific naming of object instances is defined below for a
number of classes of object types. Instances of an object type to
which none of the following naming conventions are applicable are
named by OBJECT IDENTIFIERs of the form x.0, where x is the name of
said object type in the MIB definition.
For example, suppose one wanted to identify an instance of the
variable sysDescr The object class for sysDescr is:
iso org dod internet mgmt mib system sysDescr
1 3 6 1 2 1 1 1
Hence, the object type, x, would be 1.3.6.1.2.1.1.1 to which is
appended an instance sub-identifier of 0. That is, 1.3.6.1.2.1.1.1.0
identifies the one and only instance of sysDescr.
So, to summarize, the OID that comes from the MIB doesn't refer to a concrete object, but to the "object type". Each concrete object (i.e. "instance") is identified by a suffix of one or more sub-identifiers (i.e. the y in this explanation). For singleton objects, this suffix is always 0. However, I think most SNMP objects are found in tables, not in singleton objects. I don't actually know of a good explanation of this in the standards, so I'll give it my best shot.
Like any table, SNMP tables are made up of rows and columns. In SNMP, however, the rows are called "entries", and each entry defines a custom type to describe the columns. Here's a simple example from the IF-MIB:
ifTable OBJECT-TYPE
SYNTAX SEQUENCE OF IfEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"A list of interface entries. The number of entries is
given by the value of ifNumber."
::= { interfaces 2 }
ifEntry OBJECT-TYPE
SYNTAX IfEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"An entry containing management information applicable to a
particular interface."
INDEX { ifIndex }
::= { ifTable 1 }
IfEntry ::=
SEQUENCE {
ifIndex InterfaceIndex,
ifDescr DisplayString,
ifType IANAifType,
ifMtu Integer32,
ifSpeed Gauge32,
ifPhysAddress PhysAddress,
ifAdminStatus INTEGER,
ifOperStatus INTEGER,
ifLastChange TimeTicks,
ifInOctets Counter32,
ifInUcastPkts Counter32,
ifInNUcastPkts Counter32, -- deprecated
ifInDiscards Counter32,
ifInErrors Counter32,
ifInUnknownProtos Counter32,
ifOutOctets Counter32,
ifOutUcastPkts Counter32,
ifOutNUcastPkts Counter32, -- deprecated
ifOutDiscards Counter32,
ifOutErrors Counter32,
ifOutQLen Gauge32, -- deprecated
ifSpecific OBJECT IDENTIFIER -- deprecated
}
So, ifTable has an OID of 1.3.6.1.2.1.2.2, and ifEntry has an OID of 1.3.6.1.2.1.2.2.1. Each item in IfEntry also has its own definition, which includes the OID relative to ifEntry. Generally they match up with the entry's data type, so, for example, ifIndex, as the first column in IfEntry, has an OID of ifEntry.1. Confusingly, when you do a simple Get-Next walk, you will traverse in column-major order, meaning you will get all the ifIndexes, followed by all the ifDescrs, and so on.
So, with all that explained, I'm now prepared to explain the instance identifiers for these tables. Notice above that ifEntry defines
INDEX { ifIndex }
This means, first, that each row is guaranteed to have a unique ifIndex, and, more importantly, that the ifIndex is used as the instance identifier for the entire entry. For example, you can pick any column in the IfEntry data type, let's say ifOperStatus (1.3.6.1.2.1.2.2.1.8), and use Get-Next to find the first instance of that column. Let's say its OID is 1.3.6.1.2.1.2.2.1.8.1, and it's value is 1 (up). The last sub-identifier tells you that it belongs to the row whose ifIndex is 1. To find the name of that interface, you can then query ifDescr.1, and to find its speed setting, you can query ifSpeed.1, and so forth. In this case, it is possible to query ifIndex.1, which will just return 1, but in many tables, the INDEX columns are not-accessible, meaning you can only find out what instances there are by walking some other column. Some tables also use multiple indices, or use OCTET STRING or even OBJECT IDENTIFIER rather than INTEGER typed indices. The rules for encoding and decoding those are in RFC 2578 section 7.7.
Related
I understood the process to shorten the URL with base 62 at How do I create a URL shortener?.
Steps given are
Think of an alphabet we want to use. In your case, that's [a-zA-Z0-9]. It contains 62 letters.
Take an auto-generated, unique numerical key (the auto-incremented id of a MySQL table for example).
For this example, I will use 12510 (125 with a base of 10).
Now you have to convert 12510 to X62 (base 62)
My question is why not just create unique numerical key and return it ? What is the advantage of concerting numerical key > Base 62 > then Finally some alphanumeric number ?
Is it because final alphanumeric number will be much smaller than unique numerical key ?
Yes. The idea is to make it short and usable in a URL. A number in base 62 will use fewer characters than the same number in base 10. Notice also that URL shorteners use short hosts, such as g.co.
I can see you understand that, yes, a number written in base 62 takes less characters than a number in base 10 just like a number in base 10 takes less characters than a number in base 2 (e.g. 0101 is 3 characters longer than just '5').
So, I'll answer specifically "Why".
Sometimes a link is shortened to be more visually pleasing. A company worried about their public perception likely doesn't want their links to look like an error code due to how long they are so they resort to shortening. That's why some url shortening services allow you to add your own "vanity url" which customizes the domain name, so that a link can be shortened and branded.
Other times a link is shortened to minimize character count when working with constraints, like Twitter. For example, at my company we shortened the links in our automated Twilio messages because SMS messages that contain more than 160 characters are technically 2 concatenated messages so it is more expensive to send.
And finally if the link is being shared through a medium that cannot be directly clicked on (e.g. verbally, on paper), making it shorter makes it much easier to type into an address bar manually. (Imagine trying to type the url to this SO question when someone is reading it to you.) I assume this is also at least partially why the base used for these links usually stop at around 62. If you start including other arbitrary characters to higher the base and consequentially make the link marginally shorter, it'll become harder to communicate, read and type. ("domain.name/5omeC0d3" vs "domian.name/🈲}♠"
When creating a PKCS#7 signed message with Win32 low level functions like CryptMsgOpenToEncode and CryptMsgUpdate, the resulting message is a message with OID 1.2.840.113549.1.7.2 signedData (PKCS #7), which contains a sequence with OID 1.2.840.113549.1.7.1 data (PKCS #7).
Can I use the low level message functions to change this latter OID? For example, Authenticode uses OID 1.3.6.1.4.1.311.2.1.4 spcIndirectDataContext (Microsoft code signing).
I saw CryptMsgOpenToEncode CMSG_BARE_CONTENT_FLAG flag, but I'm not sure if this is what I want or how to use it.
The (inner) content type of the message is the 5th parameter to CryptMsgOpenToEncode (pszInnerContentObjID).
It should accept any ASCII dotted decimal OID value as input, including the predefined value for the OID you mentioned (SPC_INDIRECT_DATA_OBJID / "1.3.6.1.4.1.311.2.1.4").
Can you please tell me the maximum length allowed for SNMPv2c read and write community .I didn't find any relevant doc which can provide description about the same .
Thanks
-Ravi
The community based model also refers to the entries in the USM tables. Following the SNMP USM MIBs defined in RFC3414 the definition of usmUserName and usmSecurityName is as below which limits the user name to 32 characters. The textual convention SnmpAdminString itself is 255 Octet long
usmUserName OBJECT-TYPE
SYNTAX SnmpAdminString (SIZE(1..32))
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION "A human readable string representing the name of
the user.
This is the (User-based Security) Model dependent
security ID.
"
::= { usmUserEntry 2 }
usmUserSecurityName OBJECT-TYPE
SYNTAX SnmpAdminString
MAX-ACCESS read-only
STATUS current
DESCRIPTION "A human readable string representing the user in
Security Model independent format.
The default transformation of the User-based Security
Model dependent security ID to the securityName and
vice versa is the identity function so that the
securityName is the same as the userName.
"
::= { usmUserEntry 3 }
The Textual Convention usmUserSecurityName is defined in RFC3411
SnmpAdminString ::= TEXTUAL-CONVENTION
DISPLAY-HINT "255t"
STATUS current
DESCRIPTION "An octet string containing administrative
information, preferably in human-readable form.
..
Note that when this TC is used for an object that
is used or envisioned to be used as an index, then
a SIZE restriction MUST be specified so that the
number of sub-identifiers for any object instance
does not exceed the limit of 128, as defined by
[RFC3416].
Note that the size of an SnmpAdminString object is
measured in octets, not characters.
"
SYNTAX OCTET STRING (SIZE (0..255))
On a Cisco switch/ router that also appears to be enforced when you are setting this via CLI.
There is no explicit limit on the length according to RFC 3584.
The limits are going to be practical (message size, etc).
SNMP version 2c maximum community length on Cisco routers is 128 characters.
Let me preface this with I know next to nothing about SNMP but I am learning. I am trying to get the device name from a printer.
I get the '1.3.6.1.2.1.1.5.0' OID. But it has a lot of additional information in it and I think it's some type of wrapper, but I don't know how to unwrap it.
Here are the results of my get
varBinds=[ObjectType(ObjectIdentity(ObjectName('1.3.6.1.2.1.1.5.0')), DisplayString(b'OFHP1', subtypeSpec=ConstraintsIntersection(ConstraintsIntersection(ConstraintsIntersection(ConstraintsIntersection(), ValueSizeConstraint(0, 65535)), ValueSizeConstraint(0, 255)), ValueSizeConstraint(0, 255))))]
the printer name is OFHP1. That's all I need. Is there a command to unwrap this, or do I need to just parse it by brute force?
When it comes to SNMP, you typically deal with so called variable-binding or OID-value pairs. That is conceptually similar to key-value pairs that you may encounter in other applications.
So your varBinds is a list of objects, each object represent a ket-value pair. To get the value you need to traverse that down to the component you need:
varBind = varBinds[0] # first var-bind
oid, value = varBind # unpack var-bind into OID and value
Now, values in SNMP are typed and constrained (they are actually ASN.1 types). That is why they are not just base Python types, but specialized objects. But you can strip extra information they carry and get a pure Python string (or int) from any SNMP scalar:
py_value = str(value) # turn SNMP value object into Python str
py_value = value.prettyPrint() # turn SNMP value object into a MIB-guided, human friendly representation
I am writing an extension to my companies existing SNMP MIB. I have a whole list of objects, with the same properties on each. I want to be able to get and set these through SNMP.
So for example, consider my object has name, desc, arg0, arg1. What I want is to be able to refer to these as:
fullpath.objects.ObjectA.name
fullpath.objects.ObjectA.desc
fullpath.objects.ObjectA.arg0
fullpath.objects.ObjectB.name
fullpath.objects.ObjectB.desc
fullpath.objects.ObjectB.arg0
However the leaf nodes appear to have to have unique names, so I am unable to define this.
I can use a SNMP table to produce:
fullpath.objects.table.name.1
fullpath.objects.table.desc.1
fullpath.objects.table.arg0.1
fullpath.objects.table.name.2
fullpath.objects.table.desc.2
fullpath.objects.table.arg0.2
But there is nowhere to look up that 2 means ObjectB. This leaves it open to user error looking up the wrong value and setting the wrong thing.
At the moment the best solution I can see is:
fullpath.objects.ObjectAName
fullpath.objects.ObjectADesc
fullpath.objects.ObjectAArg0
fullpath.objects.ObjectBName
fullpath.objects.ObjectBDesc
fullpath.objects.ObjectBArg0
which involves defining name for every object (there are 20 or so of them). The set of objects is fixed, so this is ok...just not very tidy.
Is there some way to define names for index in the table?
Is there some way of defining a container type?
Is there some way of allowing leaf nodes to be non-unique?
Any other ideas?
You should definitely use SNMP tables to accomplish what is required. This is the only way.
MIB Object names must be unique within entire MIB file.
You can easily use object of OCTET STRING type as Table index. So each byte/symbol/char of OCTET STRING value will be translated to corresponding numeric ASCII code in OID.
I ended up just using a naming convention and adding each of the settings directly into the MIB.
Not really the answer I wanted, but it means that all of the settings show up in the MIB, and that reduces the chance of users setting the wrong setting.