Snort Troubleshooting

So, you’ve set up your Snort IDS exactly as described in David Gullet’s guide on and it still won’t work… Well, you’re not alone. I had the same issue. In fact, I reinstalled the operating system and followed the guide a second time – making absolutely sure I didn’t miss a single step – and it still wouldn’t work. I am not exactly sure why the guide seems to work for some and not for others. I know several other people that have set up a Snort IDS using this guide and for some it worked perfectly while others encountered a range of different issues. This blog post describes the issues that I personally encountered and how I was able to resolve them.

Wrong ELF Class

This is an easy one. Under “Download the latest Snort rules” the guide tells you to execute the following command:

sudo cp /usr/local/snort/so_rules/precompiled/Ubuntu-10-4/x86-64/* \                /usr/local/snort/lib/snort_dynamicrule

If you’re like me and you’re following the guide exactly, maybe even copying and pasting commands as you go, then you might have executed this command without realizing it didn’t fit your system. “Ubuntu-10-4” refers to the OS that is used, “x86-64” refers to your CPU architecture, and “” refers to the version of Snort that you’ve installed. Make sure all of these match to your setup. Specifically, the “wrong ELF class” error message is caused by using rules for the wrong CPU architecture, so chances are that you have to substitute “i386” for “x86-64” or vice versa.

Empty Snort.u2 files

This one in particular drove me insane for a while. After realizing that the MySQL database wasn’t being populated by Barnyard2 I started troubleshooting the issue, eventually finding out that all the snort.u2 files in the /var/log/snort directory were 0 bytes in size. Snort would create a new file every time it was run, but the files would not increase in size even when I fired up an nmap Xmas tree scan against the IDS. Several hours of using Google showed me that other people had encountered the same issue, but none of the solutions (changing file or folder permissions) worked for me.

In the end, the solution was offered to me at the first place that I really should have visited: Specifically, in their FAQ section they offer the following advice:

“This is most likely the result of a checksum offloading issue. Try adding ‘-k none’ to your Snort command line and see if it works”.

Turns out, that is exactly what did the trick for me. Although I have reviewed the documentation I still don’t fully understand that checksum offloading issue. I also don’t know if the ‘-k none’ flag has any negative side-effects while running Snort, but I do know that it is the only thing that made Snort work for me in the first place.

One more suggestion on this issue; when Snort rules are downloaded and added, many of them are commented out. This is done on purpose because Snort rules have to be customized to each and every networking environment in which Snort is deployed. Without commented out rules, Snort would provide way too many false alerts. Make sure that you’re not trying something very specific to generate an alert. If you are and the rule for that network traffic is commented out, then Snort will not issue an alert. Open up the rule files and verify that the rule for your network traffic is not commented out.

Snort Report only shows PHP code

After going through the installation process, I opened up http://localhost/snortreport-1.3.4/alerts.php only to be greeted by this:

$db->setinst($server); $db->setuser($user); $db->setpass($pass); $db->dbname($dbname); $db->persist(); $conn = $db->connect(); define(“FULL_DATETIME_FORMAT”, “Y-m-d H:i:s”); set_time_limit(1800); require_once(“info-retrieval.php”);

Snort Report error

 At first I thought that this meant that Snort could not contact the MySQL database server and therefore I was seeing the query that was supposed to be received by the database. However, I soon realized that these strings do not make up MySQL code. Additionally, I later learned that if Snort Report cannot reach the database it would simply show 0 alerts. It would still display the correct Snort Report website, just without any relevant information.

Instead, what was going on was that the srconf.php file was not recognized as PHP, resulting in part of the contents of the file being output literally. Opening up the ‘srconf.php’ file shows that the file opens with “<?”, and it closes with “?>”. These are called ‘short open tags’, and they are a shorter version of the complete “<?php” and “?>” tags.

Snort PHP tags

Your ‘php.ini’ file might not be configured to recognize these short open tags as PHP tags. First, find your php.ini file by using the command “find / -name php.ini”. My php.ini file was located at /etc/php5/apache2/php.ini. Scrolling down in this file will eventually bring you to a line that says “short_open_tag = Off”. Change this to “short_open_tag = On”.

Snort PHP.ini file

If you see the page below, that means that Snort Report is now loading correctly.

Snort Report

Hopefully it will also show alert data. If it doesn’t – like in the image – then you might have one or more of the following issues:

  • Snort Report cannot contact the MySQL database
    • Make sure that your database has been created and it has data in it. If it does, then make sure the srconf.php file for Snort Report has the correct login information.
  • Barnyard2 is not putting information in the database
    • If the srconf.php is definitely correct but the MySQL database only contains empty tables, make sure that Barnyard2 is running correctly and not erroring out. Run it in the foreground and pay attention to its output.
  • Snort is not generating alerts
    • If Barnyard2 is running without errors but still on events are put into the MySQL database then either Barnyard2 doesn’t have the correct database information (check the barnyard2.conf file) or Snort is not generating alerts. Verify that the snort.u2 files are not 0 bytes. Also run Snort in the foreground and pay attention to its output.

I hope that I was able to help you with any Snort configuration issues you might have been having. Please post a comment for any feedback.

Windows NTFS Master File Table (MFT) Analysis

In this blog post I will describe how to read file entries in the Master File Table (MFT) for an NTFS volume. I will look at specific sections of hex code for a file and discuss how they relate to the way a file is stored physically on a hard drive. Understanding this post will be easier if you follow along on your own system, but you will need to be able to access your MFT. If you have access to forensic software like EnCase then you probably already know how to access the MFT. If not, then you can download Accessdata’s FTK Imager – a free digital forensic software suite that can be downloaded here. If you know how to use The Sleuth Kit (TSK) to copy the MFT from your hard drive then that is an option too.

Analyzing a file entry

The MFT contains information on every file that is saved on the volume and as such, it usually contains tens to hundreds of thousands of entries. To make sure we can find and analyze a known file, download an image file from the Internet and give it a unique name. I downloaded a picture of the Dutch flag to my desktop and named it “Dutch_flag.jpg”. Then open up FTK Imager or another forensic software that you choose to use and add your hard drive (the one that contains the image) as an evidence file. The MFT will be located at the root of the drive, for instance at “C:\”.

FTK Imager Overview

Now we need to locate our file in the MFT, which we can do using the search function. Searching for “Dutch_flag” brings me to the following entry in the MFT:

MFT File entry overview

Every entry in the MFT is 1,024 bytes in size (1Kb) and starts with the hex value 0x46/49/4C/45/30/, which translates to “FILE0”. Before continuing, make sure that you are looking at the right MFT entry. Even though you only downloaded a single file and gave it a unique name, some operating systems will create a symbolic link for files on the system and this symbolic link will also show up in the MFT. It might have the same file name, but with “.lnk” at the end. If that is the case, keep looking for the original file because otherwise the results from the next steps will not make sense.

FTK Imager will provide you with metadata on sections of the MFT that you are looking at. For instance, if you position your cursor at the very start of the MFT entry for your file you should see a byte offset for your cursor position at the bottom of the screen. The start of my file is at offset 288,456,704 from the start of the MFT.

FTK metadata

Since we know that every entry is 1,024 bytes in size, this means that this file is entry number 281,696 in the MFT (288,456,704 / 1,024). This is not very important, but it is always good practice to verify metadata and to know what you are doing and how these numbers relate to each other.

File Allocation

The first piece of hex code that we’ll look at are bytes number 22 and 23 from the beginning. You can jump ahead a certain amount of bytes in FTK Imager by using ctrl + g. There are four values that can be found here:

0x00/00               Deleted file

0x01/00               Allocated file

0x03/00               Allocated directory

0x04/00               Deleted directory

I found the value 0x01/00, which means that “Dutch_flag.jpg” is an allocated file on the file system.

Bytes 44-47 of the entry contain the record number. For my file, these bytes had the value 0x60/4C/04/00, in little-Endian encoding. Disregarding the trailing zeros, this makes 0x04/4C/60 in big-Endian encoding. Converting this value to decimal results in the number 281,696, which is exactly the record number that we calculated previously.

Standard Information Attribute

The next hex value we’ll look at are bytes 56-59 of the MFT entry. These four bytes indicate the Standard Information Attribute marker. An entry in the MFT typically has three of these markers; the Standard Information Attribute, the File Name Attribute, and the Data Attribute. The bytes for the Standard Information Attribute (SIA) should be 0x10/00/00/00.

SIA overview

The next four bytes, offsets 4-7 from the SIA marker, record the attribute’s length. In most cases, only the first of these bytes will be anything but zeros. For my file entry, the value of this byte is 0x60, which translates to 96 in decimal. This means the entire SIA is 96 bytes long. Bytes 16-19 of the SIA indicate the size of the content in the SIA, and bytes 20-23 indicate when the content starts. Again, chances are only bytes 16 and 20 are anything but zeros. In my file entry, these values were 0x48 and 0x18, which converts to 72 and 24 respectively. This means that the SIA content starts at byte 24 from the SIA marker, and the content is 72 bytes in size.

If you jump 24 bytes from the start of the SIA, there should be four 8-byte sequences of data. These sequences are timestamps for the following: (1) Time of file creation, (2) Time when file was last modified, (3) Time when the MFT entry was last modified, and (4) Time when the file was last accessed. There are programs that will decode these values to normal timestamps for you. One such program is Dcode, which can be downloaded for free here.  These values are updated constantly.

Filename Attribute

These 32 bytes of data are really all the information of interest in the 72 bytes of content contained in the SIA. Immediately after the SIA is the Filename Attribute (FNA) marker, indicated by the value 0x/30/00/00/00. It is not uncommon to find more than one FNA in a file entry. For instance, immediately after the FNA in my file entry there is another 4-byte sequence with a value of 0x30/00/00/00.

FNA overview

Again, the length of the attribute can be found at bytes 4-7 from the start of the FNA marker. For my file entry, byte 4 contained the value 0x78, or 120 in decimal, meaning the attribute was 120 bytes in size. Bytes 16 and 20 show that the content in the FNA was 0x5A, or 90 bytes in size, and the content started at offset 0x18, or 24. The first 8 bytes of content contain the $MFT record number of the parent (bytes 0-5) and the sequence value of the parent (bytes 6-7).

The next four 8-byte chunks, starting at offset 32 bytes, show the same four 8-byte streams (another 32 bytes) that represent the time and dates relevant to the file. These date-time stamps are redundant to the data in the Standard Information Attribute (SIA) from above. Unlike the timestamps in the SIA, these ones are not updated constantly and are generally not reliable.

The eight bytes after the last 8-byte timestamp entry (offset 40-47) represent the size of the file on disk, in little-Endian encoding. Disregarding the four bytes of training zeros for my file, the value was 0x00/40/03/00, which makes 0x00/03/40/00 in big-Endian encoding. Converted to decimal, this value represents the number 212,992, which is the size on disk in bytes for the file. The size on disk includes file slack – it represents the amount of sectors needed to store the file. Dividing 212,992 by 512 (the size of a sector) shows that 416 sectors are needed to store this file.

File size in Windows explorer

The actual size of the file without file slack can be found in the next eight bytes. At offset 48-55). Again disregarding the four bytes of trailing zeros, the value for my file was 0x79/35/03/00, or 0x00/03/35/79 in big-Endian. Converting this value to decimal results in 210,297, which is the actual size of the file.

Offset 64 stores the length of the filename and the actual filename is stored beginning at offset 66. The value of byte 64 for my file entry was 0x0C, which is 12. “Dutch_flag.jpg” has 14 characters, but the file name stored in this particular FNA was actually “Dutch_~1.jpg”, which has 12 characters. Looking at the second FNA – the 120 byte sequence following the first FNA – we can see that the majority of the byte values are the same but this time the value indicating the length of the filename is 0x0E, which is 14, and the filename is stored as “Dutch_flag.jpg”.

FNA number 2


Data Attribute

Immediately after the end of the last FNA is the value 0x80/00/00/00, which is the marker for the Data Attribute. Byte four again indicates the length of the attribute. For my file entry this value is 0x48, or 72.

DA overview

Eight bytes after the Data Attribute marker there will be a one byte value of either 0x00 or 0x01. If the value is 0x01 that means that the file for this MFT entry is ‘non-resident’ – the data resides somewhere on the hard drive. If the value is 0x00 that means the file is a resident file and the data is actually contained within the MFT entry itself. For certain small files, there is actually enough room in the 1,024 byte MFT entry to store the file data. The value for any image file should be 0x01 because such a file would typically be larger than 1,024 bytes. If the value for your file is 0x00 then you are likely looking at the wrong MFT entry. Also, the results for the next steps will be different if you’re looking at a resident file entry.

Jumping 32 bytes from the start of the Data Attribute marker brings us to a sequence of two bytes that indicate where the ‘data run’ is located. The data run is the sequence of bytes that indicate where the file is actually stored on disk. For my file, the value of these two bytes was 0x40/00 or 0x/00/40 in big-Endian. Converting this to decimal results in 64, meaning the data run is located 64 bytes from the start of the Data Attribute Marker.

Reference to data run

Jumping to this location reveals the 8-byte sequence 0x31/34/61/9C/0A/00/FF/FF. The first byte of this sequence is the data run header, which is viewed as two nibbles (3 and 1). Adding these two nibbles together reveals the number of bytes following the header that are used in the data run. For my file that means that four bytes after the header are used for the data run, so we’re actually only looking at the value 0x31/34/61/9C/0A. Additionally, the second nibble in the header (1) is used to indicate the number of bytes after the header that are used to indicate how many contiguous clusters are used to store the file data. The first nibble in the header (3) is used to indicate the number of bytes after the header that are used to indicate the starting sector of the file data. If 0x00 follows the run list, then there are no more data runs for this file. If something else than 0x00 follows, then the file is fragmented and spread out over multiple locations on the hard disk. In that case, another data run will immediately follow the first one, indicating the starting point and length of the cluster where the data is continued. This can go on for as many fragments of space are needed to store the file.

For my file, there is only one data run since it is followed by 0x00. After going through this file, I will also provide an example for a file that has multiple data runs. The value 0x34 indicates how many clusters are needed to store the file and the value 0x61/9C/0A indicates the starting sector of the file. Converting these values to big-Endian and then to decimal results in 52 and 695,393. FTK Imager has the ability to ‘go to sector/cluster’ (ctrl + s), but it only seems to be able to go to a specific sector, not a cluster. Therefore, you want to find out what sector the file is on.

How many sectors go into a cluster depends on what operating system you are using, and what settings you chose when the operating system was installed or when the hard drive was formatted. I believe the Windows 7 default is 8 sectors per cluster. Knowing this, look again at the value that we found previously under the Filename Attribute Marker for the amount of sectors needed to store the file. This value was 416. Divide 416 by 8 and you have 52. Again – it’s always good to understand how these numbers fit together.

I multiplied the number 695,393 by 8, which results in 5,563,144. In FTK Imager, I navigated out of the MFT and up to the actual partition so that the contents of the entire partition is displayed and then I jumped to sector 5,563,144. This brought me right to the start of the “Dutch_flag.jpg” file.

Start of the file on disk

Jumping 210,297 bytes ahead positioned my cursor right behind the value 0xFF/D9 – the footer for the JPEG file signature which indicates the end of a JPEG file. Looking at the metadata at the bottom of the screen revealed that this data was located on sector 5,563,554 – 410 sectors after the starting sector. It makes sense that this number is not 416 because not all of the last cluster was used to store the file. This tells us that the file slack in the last cluster is approximately 6 sectors in size.

End of the file on disk

Fragmented Files

So what happens if a file is fragmented and has multiple data runs? As mentioned earlier, the second data run will immediately follow the first one and this will continue for as many data runs as necessary to store the file on disk. Here is an example of a file with 13 data runs.

fragmented file

The first data run indicates that four bytes after the header are used for information on where data is stored and over how many contiguous clusters. Following immediately after the first data run is another data run with the header 0x21, and the one after that has the header 0x31. This continuous until the last data run is encountered which is 0x31/19/09/23/FB. This data run can be identified as the last one because it is immediately followed by the bytes 0x00, indicating the data runs have ended.

So with this file being fragmented across the hard drive, how can the clusters that contain information for this file be found? The process here is a little different from when there is only a single data run. Not for the first data run – that one works exactly the same. However, for the second data run and every one after that, the starting location for the data is actually calculated from the start of the previous data run.

For this example, the first data run (0x31/0F/AA/B3/03) indicates that there are 15 (0x0F) clusters of information stored for this file starting at cluster 242,602 (0x03/B3/AA). Data run two, which is 0x21/1F/74/25, indicates that there are 31 (0x1F) clusters of information starting 9,588 (0x25/74) bytes from the start of the first data run. This would be at cluster 252,190. The third data run indicates that there are 58 clusters of information stored 34,870 clusters from the start of the second data run, which would be cluster 287,060. This continues for all 13 data runs for this file.

Negative Data Runs

However, something strange occurs at data run 7, which is 0x31/11/0D/22/FB. Applying the same process we’ve been following suggests that there are 17 (0x11) clusters of information stored 16,458,253 clusters from the start of the sixth data run. You can’t tell from the picture but the hard drive that was used for this example only contained a total of 1,024,134 clusters, meaning that the designated cluster falls well outside the range of total available clusters.

What happened here is that the 7th data run actually points back to an earlier cluster on the hard drive – one that comes before the 6th data run rather than after it. The key indicator is the last byte of the data run; if the byte falls between the values 0x00 and 0x7F then the amount of clusters to the next data run is positive. If the value of the last byte falls between 0x80 and 0xFF then the amount of clusters to the next data run is negative. For data run 7 the last byte is 0xFB, meaning the amount of clusters to the next data run is negative.

With a data run that points backwards, the process to follow is as follows:

  1. Convert the hexadecimal value to Big Endian.
  2. Convert the value to binary.
  3. Apply a XOR calculation to each bit in the binary value. This means you make each bit the opposite bit, so a 0 becomes a 1 and a 1 becomes a 0.
  4. Add 1 to the result.
  5. Convert the result to decimal.
  6. Subtract the value from the start of the previous data run.

While at first glance this may seem like a crazy amount of work, it’s really not so bad. For data run 7, these steps look like this:

  1. 0x0D/22/FB = 0xFB/22/0D.
  2. 0xFB/22/0D = 1111 1011 0010 0010 0000 1101
  3. 1111 1011 0010 0010 0000 1101 becomes 0000 0100 1101 1101 1111 0010
  4. 0000 0100 1101 1101 1111 0010 becomes 0000 0100 1101 1101 1111 0011
  5. 0000 0100 1101 1101 1111 0011 = 318,963
  6. The start of the 6th data run was 328,508, so 328,508 – 318,963 = 9,545.

The start of the 7th data run is at cluster 9,545.

Now that we know how to find out where the clusters with data for this fragmented file are located we can carve (extract, or retrieve) the data. Doing so simply requires you to copy all the data indicated by each data run. So data run 1 indicates that 61,440 bytes (15 clusters; 15 * 4,096 bytes) need to be copied starting at cluster 242,602. Data run 2 indicates that 126,976 bytes need to be copied starting at cluster 252,190 and these bytes need to be appended (added to the end) of the data copied from data run 1. Next, the data from data run 3 is added and this continues until the data for all 13 data runs is added together. If done correctly, the result will be a complete file.

Hopefully this walkthrough made sense. If you have any feedback or if the walkthrough did not match up with what you found on your system, please let me know by leaving me a comment.