Building a DGA Classifier: Part 3, Model Selection

October 6, 2014

(This article was first published on Data Driven Security, and kindly contributed to R-bloggers)

This is part two of a three-part blog series on building a DGA
classifier and it is split into the three phases of building a
classifier: 1) Data

2) Feature

and 3) Model selection (this post)

Back in part
, we
prepared the data and we are starting with a nice clean list of domains
labeled as either legitimate (“legit”) or generated by an algorithm
(“dga”). Then in part 2, we calculated various features which
included length, entropy, several combinations of n-grams and finally a
dictionary matching feature that calculated the percentage of characters
that can be explained by dictionary words. Now we want to select a model
and generate an algorithm to classify new domains. While we are doing
that we will also double check how well each of the features we
generated in part 2 perform. You should fully expect to remove several
of our features during this step. If you’d like to follow along, you can
grab the sampledga CSV
or from R run the following code.

# this code will load up the compressed CSV from the dds website:
dataurl <- ""
# create a gzip connection
con <- gzcon(url(dataurl))
# read in the (uncompressed) data
txt <- readLines(con)
# read in the text as a CSV 
sampledga <- read.csv(textConnection(txt))

As we create an algorithm to classify, we have to answer a very, very
important question… “How do we know this algorithm will perform well
with data we haven’t seen yet?”
All we have to work with is the data we
have, so how can we get feedback about the data we haven’t seen? It
would be a whole lot of work to go out and get a second labeled data set
to test this against. The solution is a whole lot simpler: rather than
use all of the data to generate the algorithm, you can set aside some
samples to test how well the algorithm performs on data it hasn’t seen.
These “test” samples will not be used to generate or “train” the
algorithm so they will give you a fairly good sense of how well you are doing.

How we generate the “test” data and the “training” data can get a little
tricky on real data. The sample data in this example is fairly well
balanced with half being “legit” and the other half is “dga”. But if you
separate on the subclass you can see 4,948 samples are from alexa and
only 52 are from the opendns list of domains. If you were to randomly
separate the training from test data, you may get all the opendns
samples in one set and none in the other. To make sure we good
representation from each subclass we will do stratified random sampling,
meaning we will randomly assign each subclass to either the training or
test data to ensure even distribution. In order to do that quickly (and
most everything else in this step), we will leverage the caret

An Brief Introduction to Caret

Directly from the package website,
“The caret package (short for Classification And REgression Training) is
a set of functions that attempt to streamline the process for creating
predictive models.” It not only has an overwhelming list of models
(over 150), it
also has several supporting functions that will make this part of the
process a whole lot easier. But it’s not enough to just install the
caret package because it’s mostly just a wrapper. You’ll also have to
install the package to support the model you are using. The huge benefit
is that the way we will call different model is standardized and the
results will be directly comparable. This is a huge advantage when
selecting models.

Okay, let’s do a stratified sample based on the subclass using the caret
package. I am going to split based on the subclass to ensure each source
is represented evenly and I’m going to use 75% of the data to train the
algorithm and hold out 25% of the data for testing later. As I create
the training data, I will also remove fields we don’t need, for example
the full hostname, domain and tld fields will not be used directly in
the classification.

# make this repeatable
# if we pass in a factor, it will do the stratified sampling on it.
# this will return the row numbers to include in the training data
trainindex <- createDataPartition(sampledga$subclass, p=0.75, list=F)
# only train with these fields:
fields <- c("class", "length", "entropy", "dict", "gram345", "onegram",
            "twogram", "threegram", "fourgram", "fivegram")
# Now you can create a training and test data set 
traindga <- sampledga[trainindex, fields]
# going to leave all the fields in the test data
testdga <- sampledga[-trainindex, ]

Just to verify, we can look the before and after samples of the
subclass. You should expect 75% of each subclass to be in the training data.


##        alexa cryptolocker          goz       newgoz      opendns 
##         4948         1667         1667         1666           52


##        alexa cryptolocker          goz       newgoz      opendns 
##         3711         1251         1251         1250           39

And now it gets a little complicated

Many of the models you can try have tuning parameters which are
attributes of the model that don’t have a direct method of computing
their value. For example, the Random Forest algorithm can be tuned for
how many trees it grows in the forest (and no, I am not making up these
terms). So the challenge with the tuning parameters is that we have to
derive the best value for each parameter. Once again, the caret package
will take care of most of that for you. For each model, you can tell the
caret package to further split up the training data and do a process
called cross-validation, which will hold out a portion of the training
data to do internal checking and derive it’s best guess for the tuning
parameters. To be sure you don’t allow a single bad split to influence
the outcome, you can tell it to repeat it 5 times. There is a lot of
stuff going into this code and I apologize for not explaining all of it.
If you are really interested, you could read an introductory
on the caret package.

I’m also going to run three different models here and compare them to
see which performs better. I’m going to compare a random
(rf), a support
vector machine

(svmRadial) and the C4.5
(it’s super fast).
Note that while the random forest and c4.5 models don’t care, the
support vector machine gets thrown off if the data is not preprocesed by
being centered and scaled. The support vector model also has an
additional tuneLength parameter that I needed to set.

# set up the training control attributes:
ctrl <- trainControl(method="repeatedcv", 
rfFit <- train(class ~ .,
               data = traindga,
               method = "rf",
               trControl = ctrl)
svmFit <- train(class ~ .,
                data = traindga,
                method = "svmRadial",
                preProc = c("center", "scale"),
                tuneLength = 10,
                trControl = ctrl)
c45Fit <- train(class ~ .,
                data = traindga,
                method = "J48",
                trControl = ctrl)

If you tried running these at home, you may have noticed those commands
were not exactly speedy (about 20 minutes for me). If you are working on
a multi-core system you could load up the doMC (multicore) package and
tell it how many cores to use with the registerDoMC() function. But be
warned, each core requires a lot of memory, and I run out of memory long
before I’m’ able to leverage all the cores.

But now you’ve got two models, in order to compare them, you can compare
the resampling results it stored during the repeated cross-validation in
the training.

resamp <- resamples(list(rf=rfFit, svm=svmFit, c45=c45Fit))

## Call:
## summary.resamples(object = resamp)
## Models: rf, svm, c45 
## Number of resamples: 50 
## ROC 
##       Min. 1st Qu. Median   Mean 3rd Qu. Max. NA's
## rf  0.9982  0.9997 0.9999 0.9997  1.0000    1    0
## svm 0.9983  0.9993 0.9999 0.9996  1.0000    1    0
## c45 0.9901  0.9943 0.9972 0.9963  0.9986    1    0
## Sens 
##       Min. 1st Qu. Median   Mean 3rd Qu. Max. NA's
## rf  0.9893  0.9947 0.9973 0.9959  0.9993    1    0
## svm 0.9787  0.9920 0.9947 0.9948  0.9973    1    0
## c45 0.9840  0.9947 0.9973 0.9951  0.9973    1    0
## Spec 
##       Min. 1st Qu. Median   Mean 3rd Qu. Max. NA's
## rf  0.9813  0.9920 0.9960 0.9953  0.9973    1    0
## svm 0.9867  0.9947 0.9973 0.9955  0.9973    1    0
## c45 0.9867  0.9893 0.9947 0.9938  0.9973    1    0

Just looking at these numbers the SVM and random forest are close, the
C4.5 isn’t so close. It’s possible to test the differences with the
diff command and we see that the there is no significant difference
between the svm and rf model, but the c4.5 is significantly different
from both the rf and svm model (with a p-value of 4.629e-10 and
5.993e-10 respectively). With the decision between the random forest and
support vector machine, I am going with the random forest here. It’s a
little simpler to comprehend and explain and it’s less complicated
overall, either one would be fine though.

Now with just the random forest, let’s look at how the variables
contributed by using the varImp function (variable importance).

importance <- varImp(rfFit, scale=F)

rfFit Variable Importance

It’s no surprise that the dict feature is at the top here. We could
see the huge difference in the plot back in part 2. It’s interesting to
see the 2 and 5 n-gram at the bottom. Now we’ve got an iterative
process. We could drop a few variables and re-run, maybe add some back
in, and so on. All the while keeping an eye on this plot. After doing
this, I found I could (surprisingly) drop the entropy and all the
single values n-grams and just go with the gram345 feature (as Click
Security used) along with the length and dict features.

# we still have the trainindex value from before, so just trim
# the fields and re-run the random forest
fields <- c("class", "length", "dict", "gram345")
traindga <- sampledga[trainindex, fields]
rfFit2 <- train(class ~ .,
                data = traindga,
                method = "rf",
                trControl = ctrl)

plot(varImp(rfFit2, scale=F))

rfFit2 Variable Importance


resamp <- resamples(list(rf1=rfFit, rf2=rfFit2))
diffs <- diff(resamp)

##  One Sample t-test
## data:  x
## t = 2.243, df = 49, p-value = 0.02948
## alternative hypothesis: true mean is not equal to 0
## 95 percent confidence interval:
##  3.419e-05 6.241e-04
## sample estimates:
## mean of x 
## 0.0003291

Even though the model with all of the initial features is slightly
better, the difference may be considered negligible (p-value of 0.02 on
testing if they are different). Plus by cutting all of those features we
are saving the time and effort to generate the features and classify
them based on all those features. Depending on the environment the
classifier is going to run, that improvement in speed may be worth the
decrease in overall accuracy.

The confusion matrix

One other thing I do quite a bit of is look at the confusion matrix.
Remember we pulled out that test data so we could see how the model does
on “new” data? Well we can do that by looking at what’s called the
confusion matrix (see Dan Geer’s October “For Good Measure”
for a nice
discussion of the concepts). With the test data you held out, you can
run the predict function on and then see how well the classification
performed by printing the confusion matrix:

pred <- predict(rfFit2, testdga)
print(confusionMatrix(pred, testdga$class))

## Confusion Matrix and Statistics
##           Reference
## Prediction  dga legit
##      dga   1242     5
##      legit    6  1245
##                Accuracy : 0.996         
##                  95% CI : (0.992, 0.998)
##     No Information Rate : 0.5           
##     P-Value [Acc > NIR] : <2e-16        
##                   Kappa : 0.991         
##  Mcnemar's Test P-Value : 1             
##             Sensitivity : 0.995         
##             Specificity : 0.996         
##          Pos Pred Value : 0.996         
##          Neg Pred Value : 0.995         
##              Prevalence : 0.500         
##          Detection Rate : 0.497         
##    Detection Prevalence : 0.499         
##       Balanced Accuracy : 0.996         
##        'Positive' Class : dga           

That’s pretty good, out of the 1,247 domains generated by a DGA, only 5
were mis-classified (looking at the first table in the output above),
with a simliar ratio on the legitimate side. If you
were curious, you could pull out the misclassified from the test data
and see which domains failed to be correct. But all in all, this is a
fairly accurate classifier!

This isn’t quite done though, now that you know what features to include
and what algorithm to train, you should go back to your original data
set and generate the final model on all the data. In order to use this
on data you haven’t seen yet, you would need to generate the features
you used in the final model (length, dict and gram345) and then run the
predict function with the algorithm generated with all of your labeled data.


As a final word and note of caution, it’s important to keep in mind the
data used here and be very aware that any classifier is only as good as
the training data. The training data used here included domains from the
Cryptolocker and GOZ botnets. We should have a fairly high degree of
confidence that this will do well classifying those two botnets from
legitimate traffic, but we shouldn’t have the same confidence that this will apply to all
domains generated from any current or future DGA. Instead, as new domains are observed and new
botnets emerge, this process should be repeated with new training data.

Unfortunately I had to gloss over many, many points here. And if you’ve
read through this post, you realize that feature engineering isn’t
isolated from model selection and often exist in an iterative process.
Overall though, hopefully this gave you a glimpse of the process for
generating a classifier or least what went into building a DGA classifier.

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