How can I compute the checksum of an ICMP echo request or reply when the checksum should include the data portion, the data portion can be variable sized, and there's no way to anticipate the data size?
Here's documentation on how to compute the checksum of an ICMP header.
ICMP Header Checksum. 16 bits.
The 16-bit one's complement of the
one's complement sum of the ICMP message, starting with the ICMP Type
field. When the checksum is computed, the checksum field should first
be cleared to 0. When the data packet is transmitted, the checksum is
computed and inserted into this field. When the data packet is
received, the checksum is again computed and verified against the
checksum field. If the two checksums do not match then an error has
occurred.
When the sender is calculating the checksum, the value is inserted into the zero'd field. The receiver then does the reverse, it pulls out the checksum, zeros the field and computes the checksum with this field set to zeros. It compares the value it calculated to the one it extracted.
Both sides of the transmission calculate the checksum with the field zero'd out.
Update
An example of how to perform this calculation exists on this Scribd presentation, starting on slide 44. I'm also including the relevant example slide below.
Figure 9.19 shows an example of checksum calculation for a simple
echo-request message (see Figure 9.14). We randomly chose the
identifier to be 1 and the sequence number to be 9. The message is
divided into 16-bit (2-byte) words. The words are added together and
the sum is complemented. Now the sender can put this value in the
checksum field.
You split the ICMP header and data into 16 bit words (using 0x0000 for the checksum field), get the sum of these words and then the ones complement of the sum. This is then inserted into the checksum field.
You can calculate the ICMP message length by subtracting the size of the IP header from the "Total length" field in the IP header.
Bear in mind that in IPv6, a pseudo header of the IPv6 header, is also included in the checksum calculation. In IPv4 this is not done, because the header already checksums itself.
Related
I try to parse WMV (ASF) files without any SDK, just by decoding raw bytes. Now I have problem with ASF_Data_Object, where I can't find length of data packet. More precise, Single payload data packet.
See image:
Here I have 9 packets, but unable to find size of individual packet. How I can determine border between packets?
I think, my problem at byte 0x411, where field "Length type flags". As you can see, here 0 value, so all flags are zero. Even Packet Length Type.
Yes, 0 value here allowed here. But how to read this type of content?
This is now compressed payload, as replication data is 8, not 1. So, this is single payload without additional fields of size.
Sample of WMV file: https://files.catbox.moe/b51l2j.wmv
You seem to be having fixed size packets with no explicit payload length included, meaning that payload data size is derived from top level data object structure.
Spec quote commented:
That is, the ASF data object carries 9 packets, 3200 bytes each, then internally the packets contain payload 3174 bytes of payload per packet except the last one which has less data and some padding.
I'm currently working on reverse engineering a device I have serial protocol.
I'm mostly there however I can't figure out one part of the string.
For each string the machine returns it always has !XXXX where the XXXX varies in a hex value. From what I can find this may be CRC16?
However I can't figure out how to calculate the CRC myself to confirm it is correct.
Here's an example of 3 Responses.
U;0;!1F1B
U;1;!0E92
U;2;!3C09
The number can be replaced with a range of ascii characters. For example here's what I'll be using most often.
U;RYAN W;!FF0A
How do I calculate how the checksum is generated?
You need more examples with different lengths.
With reveng, you will want to reverse the CRC byte, e.g. 1b1f, not 1f1b. It appears that the CRC is calculated over what is between the semicolons. With reveng I get that the polynomial is 0x1021, which is a very common 16-bit polynomial, and that the CRC is reflected.
% reveng -w 16 -s 301b1f 31920e 32093c 5259414e20570aff
width=16 poly=0x1021 init=0x1554 refin=true refout=true xorout=0x07f0 check=0xfa7e name=(none)
width=16 poly=0x1021 init=0xe54b refin=true refout=true xorout=0xffff check=0xfa7e name=(none)
With more examples, you will be able to determine the initial value of the CRC register and what the result is exclusive-or'ed with.
There is a tool available to reverse-engineer CRC calculations: CRC RevEng http://reveng.sourceforge.net/
You can give it hex strings of the input and checksum and ask it what CRC algorithm matches the input. Here is the input for the first three strings (assuming the messages are U;0;, U;1; and U;2;):
$ reveng -w 16 -s 553b303b1f1b 553b313b0e92 553b323b3c09
width=16 poly=0xa097 init=0x63bc refin=false refout=false xorout=0x0000 check=0x6327 residue=0x0000 name=(none)
The checksum follows the input messages. Unfortunately this doesn't work if I try the RYAN W message. You'll probably want to try editing the input messages to see which part of the string is being input into the CRC.
I'm programming a server that accepts an incoming connection from a client and then reads from it (via net.Conn.Read()). I'm going to be reading the message into a []byte slice obviously and then processing it in an unrelated way, but the question is - how do I find out the length of this message first to create a slice of according length?
It is entirely dependent on the design of the protocol you are attempting to read from the connection.
If you are designing your own protocol you will need to design some way for your reader to determine when to stop reading or predeclare the length of the message.
For binary protocols, you will often find some sort of fixed size header that will contain a length value (for example, a big-endian int64) at some known/discoverable header offset. You can then parse the value at the length offset and use that value to read the correct amount of data once you reach the offset beginning the variable length data. Some examples of binary protocols include DNS and HTTP/2.
For text protocols, when to stop reading will be encoded in the parsing rules. Some examples of text protocols include HTTP/1.x and SMTP. An HTTP/1.1 request for example, declares the protocol to look something like:
METHOD /path HTTP/1.1\r\n
Header-1: value\r\n
Header-2: value\r\n
Content-Length: 20\r\n
\r\n
This is the content.
The first line (where line is denoted as ending with \r\n) must include the HTTP method, followed by the path (could be absolute or relative), followed by the version.
Subsequent lines are defined to be headers, made up of a key and a value.
The key includes any text from the beginning of the line up to, but not including, the colon. After the colon comes a variable number of insignificant space characters followed by the value.
One of these headers is special and denotes the length of the upcoming body: Content-Length. The value for this header contains the number of bytes to read as the body. For our simple case (ignoring trailers, chunked encoding, etc.), we will assume that the end of the body denotes the end of the request and another request may immediately follow.
After the last header comes a blank line denoting the end of the header block and the beginning of the body (\r\n\r\n).
Once you are finished reading all the headers you would then take the value from the Content-Length header you parsed and read the next number of bytes corresponding to its value.
For more information check out:
https://nerdland.net/2009/12/designing-painless-protocols/
http://www.catb.org/esr/writings/taoup/html/ch05s03.html
https://www.ietf.org/rfc/rfc3117.txt
In the end what I did was create a 1024 byte slice, then read the message from the connection, then shorten the slice to the number of integers read.
The solution selected as correct is not good. What happens if the messages is more than 1024 Bytes? You need to have a protocol of the form TLV (Type Length Value) or just LV if you don't have different types of messages. For example, Type can be 2 Bytes and Length can be 2 Bytes. Then you first always read 4 Bytes, then based on the length indicated in Bytes 2 and 3, you will know how many Bytes come later and then you read the rest. There is something else you need to take into account: TCP is stream oriented so in order to read the complete TCP message you might need to read many times. Read this (it is for Java but useful for any language): How to read all of Inputstream in Server Socket JAVA
I'm parsing an Jpeg/JFIF file and I noticed that after the SOI (0xFF D8) I parse the different "streams" starting with 0xFFXX (where XX is a hexadecimal number) until I find the EOI (0XFFD9). Now the structure of the diffrent chunks is:
APP0 marker 2 Bytes
Length 2 Bytes
Now when I parse the a chunk I parse until i reach the length written in the 2 Bytes of the length field. After that I thought I would immediately find another Marker, followed by a length for the next chunk. According to my parser that is not always true, there might be data between the chunks. I couldn't find out what that data is, and if it is relevant to the image. Do you have any hints what this could be and how to interpret those bytes?
I'm lost and would be happy if somebody could point me in the correct direction. Thanks in advance
I've recently noticed this too. In my case it's an APP2 chunk which is the ICC profile which doesn't contain the length of the chunk.
In fact so far as I can see the length of the chunk needn't be the first 2 bytes (though it usually is).
In JFIF all 0xFF bytes are replaced with 0xFF 0x00 in the data section, so it should just be a matter of calculating the length from that. I just read until I hit another header, however I've noticed that sometimes (again in the ICC profile) there are byte sequences which don't make sense such as 0xFF 0x6D, so I may still be missing something.
I am currently looking at checksums but am having trouble fully understanding how they work.
FYI, I have been looking at UDP checksums and Internet checksums. I have learned that UDP at the sender side performs 1s complement, however I am unclear as to what 1s complement is.
I have a rough idea of 1s complement being something to do with 'reversing' the values of all 1s and 0s, so that a 1 becomes a 0 and a 0 becomes a 1, but I do not know why this is done in the first place.
Could somebody kindly provide some information about checksums in general?
Thank you.
Checksum is mostly the hash (a one way encryption function) of some value to make sure that the data is consistent when it gets to the other end. The checksum is taken before the data is sent, then when the data is received at the other, the checksum of the same value is taken again, and matched with the checksum from the sender, if they are the same, then the data is in good state, else we know something is wrong.
Fairly simplified explanation.
A checksum is just an integer which is calculated by these rules:
Sum everything in the packet except the checksum (I call it sum). Save in the checksum: -sum.
When the packet is arriving, sum everything that is in the packet. If the sum gives 0, then the packet is valid.