Ponder The Bits

Musings and confusings. All things DFIR.

Category: Know Your Tools

Know Your Tools: Linux (GNU) vs. Mac (BSD) Command Line Utilities

Welcome to first post in the “Know Your Tools” series!

Without further ado…

Have you ever wondered if/how *nix command line utilities may differ across distributions? Perhaps it never even occurred to you that there was even a possibility the tools were any different. I mean, they’re basic command line tools. How and why could/would they possibly differ?

Well, I’m here to say… thy basic command line utilities art not the same across different distributions. And, the differences can range from those that can cause a simple nuisance to those that can cause oversight of critical data.

Rather than going into aspects of this discussion that have already been covered such as how Linux and BSD generally differ, I would instead like to focus on a few core utilities commonly used in/for DFIR artifact analysis and some caveats that may cause you some headache or even prevent you from getting the full set of results you’d expect. In highlighting the problems, I will also help you identify some workarounds I’ve learned and developed over the years in addressing these issues, along with an overarching solution at the end to install GNU core utilities on your Mac (should you want to go that route).

Let’s get to it.

Grep

Grep is one of the most useful command-line utilities for searching within files/content, particularly for the ability to use regular expressions for searching/matching. To some, this may be the first time you’ve even heard that term or “regex” (shortened version of it). Some of you may have been using it for a while. And, nearly everyone at some point feels like…

Amirite?

Regardless of whether this is your first time hearing about regular expressions or if you use them regularly albeit with some level of discomfort, I HIGHLY suggest you take the time to learn and/or get better at using them – they will be your most powerful and best friend for grep. Though there is a definite regex learning curve (it’s really not that bad), knowing how to use regular expressions translates directly to performing effective and efficient searches for/of artifacts during an investigation.

Nonetheless, even if you feel like a near master of regular expressions, equally critical to an expression’s success is how it is implemented within a given tool. Specifically for grep, you may or may not be aware that it uses two different methods of matching that can highly impact the usefulness (and more important, validity) of results returned – Greedy vs. Lazy Matching. Let’s explore what each of these means/does.

At a very high level, greedy matching attempts to find the last (or longest) possible match, and lazy matching attempts to find the first possible match (and stops there). More specifically, greedy matching employs what is called backtracking and look-behind’s but that is a separate discussion. Suffice to say, using an incorrect, unintended, and/or unexpected matching method can completely overlook critical data or at the very least provide an inefficient or invalid set of results.

Now having established some foundational knowledge about how grep searches can work, we will drop the knowledge bomb – the exact same grep expression on Linux (using GNU grep) may produce completely different or no results on Mac (using BSD grep), especially when using these different types of matching.

…What? Why?

The first time I found this out I spent an inordinate and unnecessary amount of time banging my head against a wall typing and re-typing the same expression across systems but seeing different results. I didn’t know what I didn’t know. And, well, now I hope to let you know what I didn’t know but painfully learned.

While there is an explanation of why, it doesn’t necessarily matter for this discussion. Rather, I will get straight to the point of what you need to know and consider when using this utility across systems to perform effective searches. While GREEDY searches execute pretty much the same across systems, the main difference comes when you are attempting to perform a LAZY search with grep.

We’ll start with GREEDY searches as there is essentially little to no difference between the systems. Let’s perform a greedy search (find the last/longest possible match) for any string/line ending in “is” using grep’s Extended Regular Expressions option (“-E”).

(Linux GNU)$ echo “thisis” | grep -Eo ‘.+is'
thisis
(Mac BSD)$ echo “thisis” | grep -Eo ‘.+is'
thisis

Both systems yield the same output using a completely transferrable command. Easy peasy.

Note: When specifying Extended Regular Expressions, you can (and I often do) just use “egrep” which implies the “-E” option.

Now, let’s look at LAZY searches. First, how do we even specify a lazy search? Well, to put it simply, you append a “?” to your matching sequence. Using the same search as before, we’ll instead use lazy matching (find the first/shortest match) for the string “is” on both the Linux (GNU) and Mac (BSD) versions of grep and see what both yield.

(Linux GNU)$ echo “thisis” | grep -Eo ‘.+?is'
thisis
(Mac BSD)$ echo “thisis” | grep -Eo ‘.+?is'
this

Here the fun begins. We did the exact same command on both systems and it returned different results.

Well, for LAZY searches, Linux (GNU) grep does NOT recognize lazy searches unless you specify the “-P” option (short for PCRE, which stands for Perl Compatible Regular Expressions). So, we’ll supply that this time:

(Linux GNU)$ echo “thisis” | grep -Po ‘.+?is'
this

There we go. That’s what we expected and hoped for.

*Note: You cannot use the implied Extended expression syntax of “egrep” here as you will get a “conflicting matchers specified” error. Extended regex and PCRE are mutually exclusive in GNU grep.

Note that Mac (BSD), on the other hand, WILL do a lazy search by default with Extended grep. No changes necessary there.

While not knowing this likely won’t lead to catastrophic misses of data, it can (and in my experience will very likely) lead to massive amounts of false positives due to greedy matches that you have to unnecessarily sift through. Ever performed a grep search and got a ton of very imprecise and unnecessarily large (though technically correct) results? This implementation difference and issue could certainly have been the cause. If only you knew then what you know now…

So, now that we know how these searches differ across systems (and what we need to modify to make them do what we want), let’s see a few examples where using lazy matching can significantly help us (note: I am using my Mac for these searches, thus the successful use of Extended expressions using “egrep” to allow for both greedy and lazy matching)…

User-Agent String Matching
Let’s say I want to identify and extract the OS version from Mozilla user-agent strings from a set of logs, the format of which I know starts with “Mozilla/“ and then contains the OS version in parenthesis. The following shows some examples:

  • Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2228.0 Safari/537.36
  • Mozilla/5.0 (Windows NT 6.3; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2226.0 Safari/537.36
  • Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2227.0 Safari/537.36
  • Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2227.1 Safari/537.36

Greedy Matching (matches more than we wanted – fails)
(Mac BSD)$ echo "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2227.1 Safari/537.36" | egrep -o 'Mozilla.+\)'
Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_1) AppleWebKit/537.36 (KHTML, like Gecko)

Lazy Matching
(Mac BSD)$ echo "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2227.1 Safari/537.36" | egrep -o 'Mozilla.+?\)'
Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_1)

Searching for Malicious Eval Statements
Let’s say I want to identify and extract all of the base64 eval statements from a possibly infected web page for analysis, so that I can then pipe it into sed to extract only the base64 element and decode it for plaintext analysis.

Greedy Matching (matches more than we wanted – fails)
(Mac BSD)$ echo "date=new Date(); eval(base64_decode(\"DQplcnJvcl9yZ=\")); var ua = navigator.userAgent.toLowerCase();" | egrep -o 'eval\(base64_decode\(.+\)'
eval(base64_decode("DQplcnJvcl9yZ=")); var ua = navigator.userAgent.toLowerCase()

Lazy Matching (matches exactly what we want)
(Mac BSD)$ echo "date=new Date(); eval(base64_decode(\"DQplcnJvcl9yZ=\")); var ua = navigator.userAgent.toLowerCase();" | egrep -o 'eval\(base64_decode\(.+?\)'
eval(base64_decode("DQplcnJvcl9yZ=“))

There you have it. Hopefully you are now a bit more informed not only about the differences between Lazy and Greedy matching, but also about the difference in requirements across systems.

Strings

Strings is an important utility for use in extracting “human-readable” strings from files/binaries. It is particularly useful in extracting strings from (suspected) malicious binaries/files to attempt to acquire some insight into what may be contained within the file, its capabilities, hard-coded domains/URL’s, commands, … the list goes on.

However, not all strings are created equal. Sometimes, Unicode strings exist within a file/program/binary for various reasons, those of which are also important to identify and extract. By default, the GNU (Linux) strings utility searches for simple ASCII encoding, but also allows you to specify additional encodings for which to search, to include Unicode. Very useful.

By default, the Mac (BSD) strings utility also searches for simple ASCII encoding; however, I regret to inform you that the Mac (BSD) version of strings does NOT have the native capability to search for Unicode strings. Do not ask why. I highly encourage you to avoid the rabbit hole of lacking logic that I endured when I first found this out. Instead, we should move on and instead just be asking ourselves, “What does this mean to me?” Well, if you’ve only been using a Mac to perform string searches using the native BSD utility, you have been MISSING ALL UNICODE STRINGS. Of all the pandas, this is a very sad one.

So, what are our options?

There are several options, but I personally use one of the following (depending no the situation and my mood) when I need to extract both Unicode and ASCII strings from a file using a Mac (BSD) system:
1. Willi Ballenthin’s Python strings tool to extract both ASCII and Unicode strings from a file
2. FireEye’s FLOSS tool (though intended for binary analysis, it can also work against other types of files)
3. GNU strings*

*Wait a minute. I just went through saying how GNU strings isn’t available as a native utility on a Mac. So, how can I possibly use GNU strings on it? Well, my friends, at the end of this post I will revisit exactly how this can be achieved using a nearly irreplaceable third-party package manager.

Now, go back and re-run the above tools against various files and binaries from your previous investigations you performed from the Mac command line. You may be delighted at what new Unicode strings are now found 🙂

Sed

Sed (short for “Stream editor”) is another useful utility to perform all sorts of useful text transformations. Though there are many uses for it, I tend to use it mostly for substitutions, deletion, and permutation (switching the order of certain things), which can be incredibly useful for log files with a bunch of text.

For example, let’s say I have a messy IIS log file that somehow lost all of its newline separators and I want to extract just the HTTP status code, method, and URI from each line and output into its own separate line (restoring readability):

…2016-08-0112:31:16HTTP200GET/owa2016-08-0112:31:17HTTP200GET/owa/profile2016-08-0112:31:18HTTP404POST/owa/test…

Looking at the pattern, we’d like to insert a newline before each instance of the date, beginning with “2016-…”. Lucky for us, we’re on a Linux box with GNU sed and it can easily handle this:

(Linux GNU)$ sed 's/ \(.+\?\)2016/\1\n2016/g' logfile.txt
2016-08-0112:31:16HTTP200GET/owa
2016-08-0112:31:17HTTP200GET/owa/profile
2016-08-0112:31:18HTTP404POST/owa/test
...

You can see that it not only handles lazy matching, but also handles ANSI-C escape sequences (e.g., \n, \r, \t, …). This statement also utilizes sed variables, the understanding of which I will leave to the reader to explore.

Sweet. Let’s try that on a Mac…

(Mac BSD)$ sed 's/\(.+\?\)\(.+\)/\1\n2016/g' logfile.txt
2016-08-0112:31:16HTTP200GET/owa2016-08-0112:31:17HTTP200GET/owa/profile2016-08-0112:31:18HTTP404POST/owa/test

… Ugh. No luck.

Believe it or not, there are actually two common problems here. The first is the lack of interpretation of ANSI-C escape sequences. BSD sed simply doesn’t recognize any (except for \n, but not within the replacement portion of the statement), which means we have to find a different way of getting a properly interpreted newline into the statement.

Below are a few options that will work around this issue (and there are more clever ways to do it as well).

1. Use the literal (i.e., for a newline, literally insert a new line in the expression)
(Mac BSD)$ sed ’s//\*Press Enter*
> /g'

2. Use bash ANSI-C Quoting (I find this the easiest and least effort, but YMMV)
(Mac BSD)$ sed 's//\'$'\n/g’
3. Use Perl
(Mac BSD)$ perl -pe ‘s||\n|g'

Unfortunately, this only solves the first of two problems, the second being that BSD sed still does not allow for lazy matching (from my testing, though I am possibly just missing something). So, even if you use #1 or #2 above, it will only match the last found pattern and not all the patterns we need it to.

“So, should I bother with using BSD sed or not?”

Well, I leave that up to your judgment. Sometimes yes, sometimes no. In cases like this where you need to use both lazy matching and ANSI-C escape sequences, it may just be easier to skip the drama and use Perl (or perhaps you know of another extremely clever solution to this issue). Options are always good.

Note: There are also other issues with BSD sed like line numbers and using the “-i” parameter. Should you be interested beyond the scope of this post, this StackExchange thread actually has some useful information on the differences between GNU and BSD sed. Though, I’ve found that YMMV on posts like this where the theory and “facts” may not necessarily match up to what you find in testing. So, when in doubt, always test for yourself.

Find

Of all commands, you might wonder how something so basic as find could differ across *nix operating systems. I mean, what could possibly differ? It’s just find, the path, the type, the name… how or why could that even be complicated? Well, for the most part they are the same, except in one rather important use case – using find with regular expressions (regex).

Let’s take for example a regex to find all current (non-archived/rotated) log files.

On a GNU Linux system this is somewhat straight forward:

(Linux GNU)$ find /var/log -type f -regextype posix-extended -regex "/var/log/[a-zA-Z\.]+(/[a-zA-Z\.]+)*"

You can see here that rather than using the standard “-name” parameter, we instead used the “-regextype” flag to enable extended expressions (remember egrep from earlier?) and then used the “-regex” flag to denote our expression to utilize. And, that’s it. Bless you, GNU!

Obviously, Mac BSD is not this straight forward, otherwise I wouldn’t be writing about it. It’s not exactly SUPER complicated, but it’s different enough to cause substantial frustration as your Google searches will show that the internet is very confused about how to do this properly. I know. Shocking. Nonetheless, there is value in traveling down the path of frustration here so that you don’t have to when it really matters. So, let’s just transfer the command verbatim over to a Mac and see what happens.

(Mac BSD)$ find /var/log -type f -regextype posix-extended -regex "/var/log/[a-zA-Z\.]+(/[a-zA-Z\.]+)*"
find: -regextype: unknown primary or operator

Great, because why would BSD find use the same operators, right? That would be too easy. By doing a “man find” (on the terminal, not in Google, as that will produce very different results from what we are looking for here) you will see that BSD find does not use that operator. Though, it still does use the “-regex” operator. Easy enough, we’ll just remove that bad boy:

(Mac BSD)$ find /var/log -type f -regex "/var/log/[a-zA-Z\.]+(/[a-zA-Z\.]+)*
(Mac BSD)$

No results. Ok. Let’s look at the manual again… ah ha, to enable extended regular expressions (brackets, parenthesis, etc.), we need to use the “-E” option. Easy enough:

(Mac BSD)$ find /var/log -E -type f -regex "/var/log/[a-zA-Z\.]+(/[a-zA-Z\.]+)*"
find: -E: unknown primary or operator

Huh? The manual says the “-E” parameter is needed, yet we get the same error message we got earlier about the parameter being an unknown option. I’ll spare you a bit of frustration and tell you that it is VERY picky about where this flag is put – it must be BEFORE the path, like so:

(Mac BSD) $> find -E /var/log -type f -regex "/var/log/[a-zA-Z\.]+(/[a-zA-Z\.]+)*"
/var/log/alf.log
/var/log/appfirewall.log
/var/log/asl/StoreData
/var/log/CDIS.custom
/var/log/corecaptured.log
/var/log/daily.out
/var/log/DiagnosticMessages/StoreData
/var/log/displaypolicyd.log
/var/log/displaypolicyd.stdout.log
/var/log/emond/StoreData
/var/log/install.log
/var/log/monthly.out
/var/log/opendirectoryd.log
/var/log/powermanagement/StoreData
/var/log/ppp.log
/var/log/SleepWakeStacks.bin
/var/log/system.log
/var/log/Tunnelblick/tunnelblickd.log
/var/log/vnetlib
/var/log/weekly.out
/var/log/wifi.log

Success. And, that’s that. Nothing earth shattering here, but different and unnecessarily difficult enough to be aware of in your switching amongst systems.

So, now what?

Are you now feeling a bit like you know too much about these little idiosyncrasies? Well, there’s no going back now. If for no other reason, maybe you can use them to sound super smart or win bets or something.

These are just a few examples relevant to the commands and utilities often used in performing DFIR. There are still plenty of other utilities that differ as well that can make life a pain. So, now that we know this, what can we do about it? Are we doomed to live in constant translation of GNU <—> BSD and live without certain GNU utility capabilities on our Macs? Fret not, there is a light at the end of the tunnel…

If you would like to not have to deal with many of these cross-platform issues on your Mac, you may be happy to know that the GNU core utilities can be rather easily installed on OS X. There are a few options to do this, but I will go with my personal favorite method (for a variety of reasons) called Homebrew.

Homebrew (or brew) has been termed “The missing package manager for OS X”, and rightfully so. It allows simple command-line installation of a huge set of incredibly useful utilities (using Formulas) that aren’t installed by default and/or easily installed via other means. And, the GNU core utilities are no exception.

As a resource, Hong’s Technology Blog provides a great walk-through of installation and considerations.

You may already be thinking, “Great! But wait… how will the system know which utility I want to run if both the BSD and GNU version are installed?” Great question! By default, homebrew installs the binaries to /usr/local/bin. So, you have a couple options, depending on which utility in particular you are using. Some GNU utilities (such as sed) are prepended with a “g” and can be run without conflict (e.g., “gsed” will launch GNU sed). Others may not have the “g” prepended. In those cases, you will need to make sure that /usr/local/bin is in your path (or has been added to it) AND that it precedes those of the standard BSD utilities’ locations of /usr/bin, /bin, etc. So, your path should look something like this:

$ echo $PATH
/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin

With that done, it will by default now launch the GNU version installed in /usr/local/bin instead of the standard system one located in /usr/bin. And, to use the native system utilities when there is a GNU version installed with the same name, you will just need to provide their full path (i.e., “/usr/bin/<utility>”).

Please feel free to sound off in the comments with any clever/ingenious solutions not covered here or stories of epic failure in switching between Linux and Mac systems 😃

/JP

Knowing Your Tools

We all have our favorite tools – whether it’s a simple python script to parse/search a specific type of log or a mainstream commercial forensics tool to ingest a bunch of evidence and search in parallel across a multitude of artifacts. We typically use tools to help us by performing some set of actions that:

  • We are not able to perform due to lack of knowledge (e.g., parsing proprietary format objects, identifying all available information within an object, scanning for anomalies in an obscure technology/file, etc.)
  • We could perform on our own, but are better (more efficient/effectively) performed by a tool (e.g., parallelized searching, parsing of artifacts for specific information, etc.)

Regardless of the tool, one of the main benefits of using them is to save time. Time saved in performing our job as Incident Responders and Forensic Experts means less time between compromise and mitigation/recovery/return to safe operations. This is one of the core responsibilities of our line of work and why we have these jobs – to utilize our expertise and tools to reduce the impact to the business.

With this responsibility to find answers and find them fast comes both the responsibility and propensity to find/use a tool whenever possible. And, given the awesome increasingly collaborative DFIR field of ours, this is quite easily done a large majority of the time. However, with this responsibility also comes an an incredibly important onus on each and every one of us to not just be able to use a tool to get quick answers but to also ensure that the tool(s) being utilized are both well understood and performing as expected/intended. After all, time saved getting a wrong answer is not time saved at all. In fact, it often creates an additional cost of not just time but also both non-monetary and monetary costs/losses.

Do you actually know what each of your tools does?

This is a VERY important question in our line of work. Frankly, it’s an important question in most any line of work in which people rely on an expert to provide them with assistance that is accurate, effective, and efficient during times of which the wrong decision or information can yield substantial consequences. The onus is on you/us, as experts in the field, to not just provide answers spit out from a tool, but to be able to know how that information was processed and how it arrived as output from each tool you used. The effects of not understanding this can range from simply looking unintelligent when queried about what the tool does and why it does it to a catastrophic result of providing incorrect information and losing key insight into a compromise.

As an example, what if you used a tool that misrepresented an executable’s create/modify time on disk? You may have still found it via other methods, or you may have missed that insight completely. This may not necessarily be of substantial consequence to, say, a single-source ransomware case. But, what if an entire decision of possible data exfiltration for a multi-billion dollar company hinged on when, specifically, the executable was created on the system and you incorrectly reported a date/time that was MONTHS OFF because you didn’t know (or your tool misstated) that the dates/times were extracted from the NTFS $SI attribute* (easily timestomped) versus the $FN attribute? I think (hope) you get the point here. THESE ARE THINGS YOU NEED TO KNOW!

*Note: It’s not just small tools susceptible to this poor practice of displaying an NTFS $SI attribute for date/time information during forensic analysis. I know of one very large commercial forensic tool that does this by default, which is not only poor practice for such a broadly utilized and expensive tool but something of which I’ve learned many people are not aware! Gah. But, I digress…

Feeling like I’ve appropriately berated and re-stated the “why” of needing to know how your tools work, let’s move on to formulating an approach for “how” to understand them. As someone who researches, peruses, tests, utilizes, and submits bugs/improvements for a wide variety of both FOSS and commercial tools (mostly the former, I’m a true FOSS junky), my quest for understanding each tool to the fullest extent possible comes down to asking three questions in a variety of different ways:

  • Why does it do or not do <X>?
  • What does it mean when <Y>?
  • How would I know if <Z>?

The basis of these questions can be broken down into many more granular questions of which the sky is the limit. However, as a baseline of where to start, the below is a set of questions I typically ask myself when using a tool that someone else has built (and I encourage you all to add/modify as appropriate):

  • How does it acquire the specific information it is parsing?
  • How, specifically, is it parsing the data?
  • What does it mean if something is not present that I would expect?
  • What does it mean when something is present that I would not expect? 
  • Do I fully understand the intent of the tool?
  • Am I using the tool for its intended usage?
  • How would I know if it was not successfully achieving its intent/goal(s)?
  • Would I know if the tool produced an error?
  • Upon error, how do I find out whether the error may be in the source data and/or the tool’s processing of the data?

… and I could go on, but you get the point. These are all questions you should be asking yourself when you use each of your tools (yes, this includes BOTH commercial and FOSS tools). From these questions comes scenarios in which you should test the tool for expected results with all types of data you expect to encounter. To that end, I would also suggest you test it with data you do not expect to encounter in the event that it can yield better understanding of the tool.

So, we’ve covered “why” you need to understand your tools, “how” you might go about doing that, but what if you do encounter an error with the tool? Should you just give up and try other tools? I would encourage you not to give up and move on as that helps no one (not the author, not you, and not others in the community who are likely experiencing the same issue or issues you are). If there is an issue, try to find an answer! This is a community. Someone has taken the time to create a tool to save you time and effort, the least you can do is return the favor and invest your time to understand the tool and help make it better. Too often I hear about issues with tools and come to find out that the issue was never investigated, posed, or posted to gather feedback from others. And, even if the issue was posed/posted somewhere, they never attempted to contact the author(s).

ASK THE AUTHOR.

They are often the primary source of information for the tool, not to mention doing so reinforces the fact that people appreciate the time they took to create a tool and are invested in it enough to contact them about how to make it better. Just a simple question about how/why something works can be invaluable to an author. I continue to be amazed at how responsive the authors of various tools are within our community for bug reports and/or improvement requests.

As you can see, I’m very passionate about, and a huge proponent of, investing time into understanding each and every one of your tools (not to mention building your own and contributing back to the community if you are so inclined). It not only makes you better, it also makes the community better. Pointing and clicking without understanding what is going on behind the scenes is a disservice not only to yourself but to the community. Don’t be someone that can be replaced by another tool. Add value to your company/clients by going above and beyond and taking the time to understand the tools for your profession from the ground up. If you do, the investment will come back to you and to the community multiple times over.

If this topic interests you and/or you are simply interested in getting to know your Linux/Mac command line and/or FOSS tools (better), stay tuned for the future “Know Your Tools” series/posts where I will be covering various tools used in my DFIR investigations, to include basic tool usage, lesser known (and useful) capabilities, as well as tips/tricks I’ve collected over the years in refining various tools and processes.

Happy 2017!

/JP

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