Recently I have been trying to save list of hostnames from captured DHCP packets. I have found out, every DHCP hostname (option 12) should have form defined in RFC 1035. So if I understand it correctly, hostname should be encoded in 7-bit ASCII and have other restrictions like:
- name should not start with digit and should omit some forbidden characters.
Almost every device I have encountered in packets fulfill this constraint, but not Windows devices (Vendor ID MSFT 5.0). IMHO Windows DHCP client takes computer (mobile) name and fill it in hostname option.
Problem occurs, when computer name is set for example to "Lukáš-PC". Wireshark display this hostname as Luk\240\347-PC. (240 and 347 are numbers in octal). To see for myself I have printed values in packets with printf("%hhu", c) (C language).
á = 160
š = 231
IMHO I think this is simple char variable overflow. I tried deduce original value from overflow value, but I haven't found any relation between character and known encodings. So my questions are:
Is there any way to convert these values back to original?
If yes, what was original character encoding, when overflow happened?
Thanks.
Default char is usually signed, and extends to int when passed to a variadic function. To ensure that it is printed unsigned, use printf("%hhu", c) or printf("%d", (unsigned char)c);.
The correct encoding is impossible to know because it depends on each system's settings.
Note that any compliant systems MUST encode names according to RFC 3490, but Windows seems to enjoy violating standards.
The characters á and š that you are seing are encoded using code page 852 (Latin-2 - Central European languages).
Unfortunately there is no simple way how you can figure out the encoding used only by looking at DHCP requests. In principle the DHCP client can use any code page it wants. If you are working in a private/controlled network, then it is probably safe to assume the all clients are using the same code page and explicitly encode the strings using that particular code page.
Related
With a friend we are currently working on a library to create and read SNMPv3 packet.
The idea is "only" to create the content of the packet and it will be sent independently.
I know that many libraries exist for that but not in the language that we need. Our major problem now is to specify the content of the different packets. Which part is mandatory? Which part comes in which type of request?
With some examples available on Wireshark's website and the RFCs we can have a beginning of an idea but as it is a protocol, we need to be very clear and sure of what is required in each type of request (get-request, set-request, get-bulk, trap, etc.).
Is there a way to know exactly how each type of packet is created or the only information sources are the RFCs?
First, I want to offer some clarification about the terminology. A UDP packet encodes an SNMP "message". The format of the message varies with the SNMP version, but in all cases, it contains a single PDU. I think when you say "packet", you really mean "PDU".
As for your question, there's no better source than the RFCs, and they are actually easier to read than you think, as long as you know which parts to read (that's the tricky part).
RFC 3416 specifies everything to do with PDUs, including the format (p. 8), a comprehensive list of PDU types (pp. 7-8), and an explanation of how each PDU is used (under section 4.2, starting on p. 10).
The format of all PDUs is the same (though the BulkPDU replaces error-status and error-index with two integer fields of different meanings):
PDU ::= SEQUENCE {
request-id INTEGER (-214783648..214783647),
error-status -- sometimes ignored
INTEGER {
noError(0),
tooBig(1),
noSuchName(2), -- for proxy compatibility
badValue(3), -- for proxy compatibility
readOnly(4), -- for proxy compatibility
genErr(5),
noAccess(6),
wrongType(7),
wrongLength(8),
wrongEncoding(9),
wrongValue(10),
noCreation(11),
inconsistentValue(12),
resourceUnavailable(13),
commitFailed(14),
undoFailed(15),
authorizationError(16),
notWritable(17),
inconsistentName(18)
},
error-index -- sometimes ignored
INTEGER (0..max-bindings),
variable-bindings -- values are sometimes ignored
VarBindList
}
I need to send a UNICODE string message to A16 COMS (Mainframe) via TCP/IP. What algorithm do I need , what transformation of a string. String can contain one or more UNICODE Characters.
While sending ASCII only based string I convert(map) it to EBCDIC and send via TCP/IP connection. I know that EBCDIC doesn't handle UNICODE Character. Besides, I can send via TCP IP only byte array, where in case of ASCII string one character maps to one array cell. In the case of UNICODE character - it can occupy from 1 to 4 byte array cells.
The question is how do I send the UNICODE containing string to A16 Mainframe.
Further clarification:
When I run the code, the TCP client cannot receive any response. It passes timeout and gives an error. Increasing timeout does not help. C# can convert an UNC string to UTF-8 either using System.Text.Encoding or even with an algorithm - almost manually. Those are not a problem. Problem is that A16 COMS expects “one character = one byte”, (mapped to EBCDIC). And with UTF-8 one character may occupy 2, 3 or 4 cells of an array. Now EBCDIC mapping itself does not help, because EBCDIC is designed to work with non-unicode (ASCII based) strings.
I hope that someone whoever did this at some point in his career might read my post because not much can be done by figuring out. Can it be done with TCP Client and its NetworkStream? Send method has only array of bytes in its signature, but with utf-8 array of bytes can be so much longer than the limit.
It is a question asking to share experience, not knowledge.
I am debugging some snmp code for an integer overflow problem. Basically we use an integer to store disk/raid capacity in KB. However when a disk/raid of more than 2TB is used, it'll overflow.
I read from some internet forums that snmp v2c support integer64 or unsigned64. In my test it'll still just send the lower 32 bits even though I have set the type to integer64 or unsigned64.
Here is how I did it:
a standalone program will obtain the capacity and write the data to a file. example lines for raid capacity
my-sub-oid
Counter64
7813857280
/etc/snmp/snmpd.conf has a clause to pass thru the oids:
pass_persist mymiboid /path/to/snmpagent
in the mysnmpagent source, read the oidmap into oid/type/value structure from the file, and print to stdout.
printf("%s\n", it->first.c_str());
printf("%s\n", it->second.type.c_str());
printf("%s\n", it->second.value.c_str());
fflush(stdout);
use snmpget to get the sub-oid, and it returns:
mysuboid = Counter32: 3518889984
I use tcpdump and the last segment of the value portion is:
41 0500 d1be 0000
41 should be the tag, 05 should be the length, and the value is only carrying the lower 32-bit of the capacity. (note 7813857280 is 0x1.d1.be.00.00)
I do find that using string type would send correct value (in octetstring format). But I want to know if there is a way to use 64-bit integer in snmp v2c.
I am running NET-SNMP 5.4.2.1 though.
thanks a lot.
Update:
Found the following from snmpd.conf regarding pass (and probably also pass_persist) in net-snmp doc page. I guess it's forcing the Counter64 to Counter32.
Note:
The SMIv2 type counter64 and SNMPv2 noSuchObject exception are not supported.
You are supposed to use two Unsigned32 for lower and upper bytes of your large number.
Counter64 is not meant to be used for large numbers this way.
For reference : 17 Common MIB Design Errors (last one)
SNMP SMIv2 defines a new type Counter64,
https://www.rfc-editor.org/rfc/rfc2578#page-24
which is in fact unsigned 64 bit integer. So if your data fall into the range, using Counter64 is proper.
"In my test it'll still just send the lower 32 bits even though I have set the type to integer64 or unsigned64" sounds like a problem, but unless you show more details (like showing some code) on how you tested it out and received the result, nobody might help further.
Is 74DC::02BA a valid IPv6 address?
I am trying to break it down, but the various shortcuts are confusing me.
Valid address, yes. See this question. Also, this validator breaks it down nicely.
Correct address, probably not. See RFC 4291, section 2.4, where this address is defined as a Global Unicast address. (the first bits are 0111 0100, which falls under "everything else" in the table) Then see the IPv6 address assignments. You'll notice this address range has not been assigned for use.
Normally you wouldn't see an address written like this, since it contains extra information. (the leading 0 in the second group of digits) So you would probably see it written like 74dc::2ba. (The IETF makes recommendations about how to print IPv6 addresses in RFC 5952.)
If you want to know the rules for IPv6 address shortening, they are specified in RFC 4291, section 2.2.
Here's what I believe to be the best online IPv6 validator (and not just because I wrote it). It will show you the various address forms and show you how the different representations relate to each other (try hovering over each address group).
The "::" means there's all 0s in between the colons. The address expands to 74dc:0000:0000:0000:0000:0000:0000:02ba
IPv6 Address Validator
I'm using the WML function "providelocalinfo" to put location information into Short Messages send via a WIB menu on a GSM handset.
I'm using the WIG WML v.4 Spec from SmartTrust. The relevant section is "9.4 providelocalinfo Element"
I use the code as in the example, and then transmit the variable via SMS, and use Kannel to retrieve the message from the SMSC.
Here's the code that I'm using, with the exception of [myservicecentre] being my actual service centre:
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE wml PUBLIC "-//SmartTrust//DTD WIG-WML 4.0//EN"
"http://www.smarttrust.com/DTD/WIG-WML4.0.dtd">
<wml wibletenc="UCS2">
<card id="s">
<p>
<providelocalinfo cmdqualifier="location" destvar="LOC"/>
<setvar name="X" value="loc=" class="binary"/>
<sendsm>
<destaddress value="367"/>
<userdata docudenc="hex-binary" dcs="245">
$(X)$(LOC)
</userdata>
<servicecentreaddress value="[myservicecentre]"/>
</sendsm>
</p>
</card>
</wml>
What I see in my received messages is "loc=" followed by 7 bytes (octets) or binary data. I have tried to find documentation explaining how to decode this data, but found nothing the explains this clearly.
Of the decoded 7 octets,
the first 3 octets are always the same,
The next 2 octets tend to vary between three unique values,
the last 2 octets appear to be the cellid.
So I have coded the receiver to pull the last two octets and construct a 16-bit GSM cellid. Most of the time it matches known cellids from the network. But quite often, the value does not match.
So I'm trying to find information on the following:
How to properly transmit the location information in a safe manner (encodings, casts, etc)
How to decode the information properly
How to configure Kannel to honor binary location data
I've examined the following documents in my vain searching, but not found the relevant data:
GSM 03.38, GSM 04.07, GSM 04.08, GSM 11.15, as well as the WIG WML Spec V .4
Any insight into what I might be doing wrong would be appreciated!
To decode the location info, you need to look in GSM 11.14 page 48
1.19 LOCATION INFORMATION
Byte(s) Description Length
1 Location Information tag 1
2 Length (X) of bytes following 1
3-5 Mobile Country & Network Codes (MCC & MNC) 3
6-7 Location Area Code (LAC) 2
8-9 Cell Identity Value (Cell ID) 2
The mobile country code (MCC), the mobile network code (MNC), the location area code (LAC) and the
cell ID are coded as in TS GSM 04.08 [8].
From personal experience, the first octet mentioned here is usually left off, so your first three unchanging bytes are the length and the country. The next 2 are the network operator code.
Not too many bites on this question! I wanted to summarize my findings in case others can find them useful:
Need to send messages with a dcs setting not equal to 0. dcs="0" sends data packed (honoring the lower 7-bits of each octet; this allows 160 character SMS messages when the max message size is actually 140 octets)
Need to parse the data in a binary safe manner: regex expressions that stop searching when 0x0A is encountered will fail when the binary data itself can be that value.
I found no need to change Kannel's default configuration.
Cheers
Disclaimer: Safe transmission of 16-bit GSM Cell-Ids requires dealing with a few settings that I understand only because they weren't configured by default. There are probably other defaults that I've depended on but am unaware that they can vary.