Using DNSBLs to Monitor Network Security

By Luis E. Munoz, October 25, 2006

Many email administrators are turning to DNSBLs -- DNS Block Lists -- as useful weapons in the arsenal against spam. There are DNSBLs covering many aspects of the security spectrum related to spam. A brief sample of the overall focus of the most common lists include: Open HTTP proxies Open SMTP proxies Zombies or trojaned machines Miscellaneous open proxies Hosts that send spam to spamtrap addresses

SysAdminMag.com

Using DNSBLs to Monitor Network Security

Luis E. Muñoz

Open HTTP proxies
Open SMTP proxies
Zombies or trojaned machines
Miscellaneous open proxies
Hosts that send spam to spamtrap addresses

These lists continue to grow despite the efforts of the community to educate the general public and, more importantly, the administrators responsible for the operation or security of the network. No matter how many security measures we implement in our network, the reality is that a lot of computers in the public network and in our datacenters, are compromised each day.

This article will introduce another useful application for the DNSBLs. I'll show how to use this valuable information source to diagnose and monitor the overall security level of a given network. I'll do so by generating a sort of "reputation" or index, based in the information collected from the lists themselves.

The code I will use for this, although simply an example, is available from the Sys Admin Web site:

http://www.sysadminmag.com

The Lists One of the first things to do is research the existing DNSBLs. To save you some time, I will be using the following lists in this article. Be aware that you must thoroughly understand how each list work and what is really represented by a listing there:

l2.spews.org (SPEWS level 2 list): IP ranges listed here usually have long-standing problems that have not been properly addressed by their operators. Alternatively, the IP ranges were associated with spamming operations. Level 1 and Level 2 are "severity levels" associated with a listing. Quoting from the SPEWS FAQ, "... A common practice is to bounce based on the SPEWS Level 1 list, and tag based on the SPEWS Level 2 list."
A listing in SPEWS probably is something you need to address immediately as those listings have been known to grow progressively to sizeable chunks of IP space. In this article I'll use the level 2, which tends to include more IP space. In practice, this can work as an early warning of an upcoming level 1 listing.
psbl.surriel.com (Passive Spam Block List): This list includes hosts that have sent spam to certain spamtraps with the rationale that anyone can quickly and easily delist them.
In my experience, many spammers tend to hit psbl's spamtraps, so this may be a good early warning of problems. However, your mileage will vary.
list.dsbl.org (Distributed Sender Blackhole List): This list specializes in detecting SMTP relay hosts. An open SMTP relay will likely be listed in dsbl.org pretty quickly when one of the testers receives spam coming from it and subsequently tests the relay.
This particular version of the list contains hosts that have been included by trusted users of dsbl.org. My experience shows that this is the most common variant of the list used to block email. For statistical purposes, you may want to include unconfirmed.dsbl.org and multihop.dsbl.org, which can also glean useful information about what is leaking out of your network.
Note that it is actually hard for an ISP not to have its customer-serving mail servers listed in multihop, so this list is less common in mail filtering.
spam.dnsbl.sorbs.net (SORBS' Spam Database): This is the "Database of hosts sending to SORBS' spamtraps". The SORBS' FAQs provide a thorough description of what is listable in this DNSBL. A quick summary is that any host sending spam, in the vicinity of a spammer or providing spam support can be listed.
smtp.dnsbl.sorbs.net (Open SMTP Relay Servers): A list of SMTP servers that can be used to relay spam, commonly known as open relays.
http.dnsbl.sorbs.net (Open HTTP Proxies): A list of HTTP proxy servers that can be used by anyone without (or with trivial) authentication.
socks.dnsbl.sorbs.net (Open SOCKS Proxies): A list of SOCKS proxy servers that can be used by anyone without (or with trivial) authentication.
web.dnsbl.sorbs.net (Web Servers with Spammer-Abusable Vulnerabilities): It is a well-known fact that notorious bugs in common Web applications can be exploited to send bulk email, making it seem to come from the Web server hosting those faulty applications. This list is designed to catch those hosts and prevent spam sent in this way from spreading.
misc.dnsbl.sorbs.net (Non-HTTP and Non-SOCKS Open Proxies): Any other type of open proxy will be listed here. This deals mostly with worm infections that cause a host to become a "zombie". Zombies are commonly used to send huge amounts of spam for the benefit of those who control them. Those zombies are also used to orchestrate DDoS attacks.
sbl.spamhaus.org (Spamhaus Block List): Straight from the Spamhaus site, "The SBL is a realtime database of IP addresses of verified spam sources ... maintained by the Spamhaus Project team and supplied as a free service to help email administrators better manage incoming email streams."
A listing in the SBL, especially if it is associated with a spammer in the Register of Known Spamming Operations (ROKSO), can be a very serious matter warranting prompt attention.
xbl.spamhaus.org (Exploits Block List): Again, straight from the Spamhaus site, this list is "a realtime database of IP addresses of illegal third party exploits, including open proxies..., worms/viruses with built-in spam engines, and other types of trojan-horse exploits."
The data in the XBL is composed from two public lists: the CBL and the NJABL Open Proxy list. Listings in the XBL, just as listings in other similar DNSBLs, are reliable indicators of host security compromise.

Obtaining List Feeds There are many more DNSBLs that work under different guidelines, so do your own research before selecting one or more as part of an indicator for your network. I've chosen these lists for my own use, because they measure variables consistent with the indicators I need to track in my job. The next step is to obtain regular copies or "feeds" of the lists. Those feeds are used when you want to set up a mirror for the list for your own use and contain all the data in a file you're supposed to pass to your DNS server. In our case, we'll be concerned with using the information for our statistics. While this process is specific to each list, most include the use of either wget, curl, or rsync. These are examples of the crontab entries I use to fetch some of these lists:

16 9 * * * cd $DNSBL_WORK_DIR && \ 
   rsync -q psbl.surriel.com::psbl/psbl.txt ./psbl.surriel.com && \ 
   cp ./psbl.surriel.com ../lists 


21 7,20 * * * cd $DNSBL_WORK_DIR && \ 
   curl -s -o ./l2.spews.org \ 
   http://www.spews.org/spews_list_level2.txt && \ 
   cut -f1 '-d ' l2.spews.org | egrep '^[0-9]' | \ 
   sort --temporary-directory=/var/tmp| uniq > ../lists/l2.spews.org 


3 10 * * * cd $DNSBL_WORK_DIR && \ 
  for i in http misc smtp socks spam web; do \ 
  rsync -q rsync://rsync.us.sorbs.net/rbldnszones/ \
    $i.dnsbl.sorbs.net .;\ 
  cut -f1 '-d ' $i | egrep '^[0-9]' | \ 
  sort --temporary-directory=/var/tmp | uniq > ../lists/$i; done

Notice the seemingly odd times I use for the entries. This is done to help randomize the hits on the sync servers operated by the lists. Also, please make sure you obtain permission from the list operators before mirroring their data. Each list usually has information about how to request and obtain copies of its data that you should review before trying to obtain their source. As you can see, these crontab entries leave the list data in a mnemonically named file in the $DNSBL_WORK_DIR/lists/ directory, which I will be using throughout this article as the repository for list information. One of the most important metrics that can be extracted from this list data is the proportion of our IP space that has been listed on each DNSBL. This is in fact what this article is about. Tracking this variable over time can give you useful insights about what is really going on in the network and what are its most visible symptoms. More importantly, it can show you whether what you're doing to secure your network is working and to what degree. Additionally, this kind of analysis can provide a robust metric for security incidents, which can help justify the business case for special tools or resources for your area. I've been able to use information like this a few times. We will calculate this variable once a day and store this sample in a database. Later, we will produce a simple graph that shows the progress of this variable over time. Let's begin with the tools we'll need:

sqlite3 (Database)
rrdtool (Graph)
Perl
NetAddr::IP (Perl module to handle IP addresses -- use the latest version)
Class::DBI (an OO interface to databases supported by Perl's DBI)

I'll assume you can install these packages if you don't already have them. As for the database, the examples in this article are quite portable and simple, so you can use whatever you have around. The first step is finding out which IP addresses are included in which lists. A little script called "count-ips" can take care of that. Let's see its interesting parts:

    3   use strict; 
    4   use warnings; 
    6   use IO::File; 
    7   use File::Find; 
   10   use NetAddr::IP 4.00; 
   13   use Getopt::Std; 
   14   use vars qw/$opt_i $opt_s $opt_h/; 
   15   my $opts = 'hi:s:'; 
   16   getopts($opts);

Lines 3-16 load the modules we will be using and parse the command-line options:

   18   die <<HELP 
   19   Usage: count-ips [-h] [-i isp-ip-space] [-s source] 
   ... 
   34   HELP 
   35       if $opt_h;

Lines 18-35 use the HEREDOC syntax for specifying multi-line strings in Perl in order to produce some help text when count-ips is invoked with the -h option:

   41   my @input = ();      # The IP space itself 
   43   my $fh = IO::File->new($opt_i, "r") 
   44       or die "Unable to open input file $opt_i: $!\n";

Lines 41-44 prepare @input, the variable where the IP space we want to match against the DNSBLs will be stored. This IP space will be read from a file that will be available through the $fh filehandle:

   46   while (my $l = $fh->getline) 
   47   { 
   48       $l =~ s!(?:\#.*$|\s+)!!g; 
   49       chomp $l; 
   50       next unless $l =~ /\S/; 
   51       my $ip = new NetAddr::IP $l; 
   52       unless ($ip) 
   53       { 
   54           warn "$opt_i:$.: Invalid IP address $l\n"; 
   55           next; 
   56       } 
   58       push @input, $ip; 
   59   }

This loop at lines 46-59 reads one line at a time from the given file, stripping whitespace and Perl comments. This allows for some familiar formatting and documentation to be included in the file. Blank lines are skipped. The rest -- hopefully a CIDR block or lone IP address -- is passed to NetAddr::IP, which can understand most formats for IPv4 network specification (CIDR, range notation, etc.). If NetAddr::IP fails to recognize an IP address or subnet, its ->new() method returns undef, which then causes a warn() to output a message referencing the bogus entry in the input file. The $. variable holds the line number of the last filehandle read, which allows the script to generate a more useful message:

   61   @input = NetAddr::IP::Compact @input; 
   62   my @ire =  map { qr/$_/ } map { $_->re } @input;

After this is done, NetAddr::IP::Compact() is used to merge contiguous ranges of IP space. That is, two contiguous /25s will become a single /24. IP blocks will also be nicely sorted as a side effect. This happens at line 61. Line 62 uses some map magic to convert each subnet to a Perl regular expression that will match any IP address within it. This is a handy feature for this application, and here is why. In many cases, the DNSBLs contain lots of /32s. The overhead of parsing the IP address out of its textual representation to use NetAddr::IP->contains() method to determine whether that IP address is listed is usually too high. Alternatively, the /32s could be simply matched against the regular expression, which is normally much faster. Note that two maps are used at line 62. This is so because in the future, ->re() might return an array of regular expressions. Whatever the case, this leaves a list of regular expressions that will match any IP address within our network in @ire. Next comes the slightly complex matching function, responsible for reading in each DNSBL source file and checking it against the @input and @ire representations of our IP space. Let's analyze it piece by piece:

   66   sub slurp_n_check 
   67   { 
   68       my $file = $File::Find::name; 
   69       if ($file =~ m/[\[\]\(\)\{\}\$]/) 
   70       { 
   71           warn "Possibly dangerous filename: $file - Skipping\n"; 
   72           return; 
   73       }

For paranoia's sake, lines 68-73 verify that the file name supplied by File::Find is not dangerous. (We'll use this module later to scan the directory where the DNSBLs sources are.) Basically, the code looks for potential variable interpolations or metacharacters in the file name and refuses to work with them:

   75       my $fh = IO::File->new($file, "r"); 
   77       unless ($fh) 
   78       { 
   79           warn "Failed to open $file: $!\n"; 
   80           return; 
   81       }

If the filename is considered safe enough, it is opened for reading with IO::File. If the open fails, a suitable warning is returned and this DNSBL is skipped. This happens at lines 75-81. The loop at lines 83-140 reads and processes each line on the DNSBL source file. This seemingly simple task is implemented with comparatively much code, as a few tradeoffs are done for speed:

   87           $l =~ s!(?:\s+|/32$)!!g;

Line 87 removes the (for us) useless /32 mask from some entries:

   89           if ($l =~ m!/(\d{1,2})$!) 
   90           { 
   91               my $m_len = $1;     # Cheap mask len 
   92               my $ip = new NetAddr::IP $l; 
   93               unless ($ip) 
   94               { 
   95                   warn "$file:$.: Invalid IP spec <$l>\n"; 
   96                   next IP; 
   97               } 
   98 
   99               for my $n (@input) 
  100               { 
  101                   if ($n->masklen < $m_len) 
  102                   { 
  103                       if ($n->contains($ip)) 
  104                       { 
  105                           print "$file $ip ", 
  106                           $ip->broadcast->numeric - \         
                                  $ip->network->numeric + 1, 
  107                           "\n"; 
  108                           next IP; 
  109                       } 
  110                   } 
  111                   else 
  112                   { 
  113                       if ($ip->contains($n)) 
  114                       { 
  115                           print "$file $n", 
  116                           $n->broadcast->numeric - \      
                                  $n->network->numeric + 1, 
  117                           "\n"; 
  118                           next IP; 
  119                       } 
  120                   } 
  121               } 
  122                   
  123           }

Lines 89-123 take care of the case where the address has a netmask. In this case, we parse it using NetAddr::IP and use its ->contains() method to find out how to report the listing. Lines 93-97 take care of the potentially corrupt entries that cannot be parsed by NetAddr::IP.

  126           else 
  127           { 
  128               for my $re (@ire) 
  129               { 
  130                   if ($l =~ m/$re/) 
  131                   { 
  132                       print "$file $l 1\n"; 
  133                       next IP; 
  134                   } 
  135               } 
  136           }

Lines 126-136 take care of the simpler case of a lone /32, now without a mask. This can simply be matched against the regular expressions in @ire, reporting in the same way as for the previous case:

  139       close $fh;

Line 139 explicitly closes the $fh pointing to the DNSBL data file:

  143   File::Find::find 
  144       ( 
  145        { 
  146            no_chdir       => 'yes', 
  147            wanted         => \&slurp_n_check, 
  148        }, 
  149        $opt_s 
  150        );

Finally, lines 143-150 summon File::Find to scan the directory provided with the -s option to our script and process it with the function discussed before. Now I will create the file isp-networks, with content like this:

  # This is a list of all my IP space 

  200.11.128.0/17 
  ...

A simple run of count-ips would produce output as follows:

  $ mkdir results 
  $ count-ips -i isp-networks -s lists > results/hits-'date '+%Y%m%d''
  $ head results/hits-20060910 
  lists/http.dnsbl.sorbs.net 200.11.177.138 1 
  lists/http.dnsbl.sorbs.net 200.11.178.204 1 
  lists/http.dnsbl.sorbs.net 200.11.181.28 1 
  lists/http.dnsbl.sorbs.net 200.11.182.113 1 
  lists/http.dnsbl.sorbs.net 200.11.182.114 1 
  lists/http.dnsbl.sorbs.net 200.11.182.115 1 
  lists/http.dnsbl.sorbs.net 200.11.182.187 1 
  lists/http.dnsbl.sorbs.net 200.11.182.58 1 
  lists/http.dnsbl.sorbs.net 200.11.182.59 1 
  lists/http.dnsbl.sorbs.net 200.11.182.62 1

In fact, I could now put this line in my crontab, as follows:

7 1 * * * cd $DNSBL_WORK_DIR && \ 
  ./count-ips -i ./isp-networks -s ./lists > \ 
  ./results/hits-'date '+%Y%m%d''; \ 
  find ./results/ -type f -mtime +10 | xargs rm

This would produce a hits-<date> file with each day's result. It probably isn't useful to generate more than one datapoint per day. Note that many DNSBLs place a limit on the frequency for downloading their sources, so this may also limit the number of samples per day you can generate. In practice, I've been working with a single sample per day. This provides good results as the trends are usually a few weeks long. Note the find | rm added at the end of the command. This should allow you to keep the last 10 days of results, in case you need them. You can adjust this value based on the amount of listings you have (and disk space). These hits files are very useful. As an example, here is a script -- warn-count -- that can alert you whenever a critical part of your network appears in one of your lists. The beginning of the script, up to line 44 is very similar to count-ips, so I won't discuss it. The first relevant code is found below:

   46   my @C = ();                     # The critical IP space 
   48   my $fh = IO::File->new($opt_c, "r") 
   49       or die "Unable to open critical file $opt_c: $!\n"; 
   50 
   51   while (my $l = $fh->getline) 
   52   { 
   53       $l =~ s!\#.*$!!g; 
   54       chomp $l; 
   55       next unless $l =~ /\S/; 
   56       my ($n, $d) = split(m/\s+/, $l, 2); 
   57       my $ip = new NetAddr::IP $n; 
   58       unless ($ip) 
   59       { 
   60           warn "$opt_c:$.: Invalid IP address $n\n"; 
   61           next; 
   62       } 
   63 
   64       push @C, { ip => $ip, desc => $d }; 
   65   }

Lines 46-65 are responsible for reading in the "critical" file. This is a file in a format similar to the input file of count-ips, which specifies network ranges that are critical. For instance, your mail servers should be there. After the IP subnet specification, that should be the first whitespace-separated column, a legend must be added. This legend should be mnemonic, so that the warnings make more sense. The critical space is stored in a list of hashrefs at @C:

   70   @C = sort { $b->{ip}->masklen <=> $a->{ip}->masklen } @C;

Line 70 sorts @C so that the network specifications are arranged from most specific to least specific. In some cases, you may want to have an entry for a whole datacenter and then more specific entries for groups of servers:

   75   while (<>) 
   76   { 
   77       print $_ if $opt_f; 
   78       chomp; 
   79       my ($bl, $n, $num) = split(/\s+/, $_, 3); 
   80       my $ip = new NetAddr::IP $n; 
   81       unless ($ip) 
   82       { 
   83           warn "Unrecognized IP spec $n (line $.) - Ignoring\n"; 
   84           next; 
   85       } 
   86 
   87       # Iterate through the critical networks 
   88       for my $c (@C) 
   89       { 
   90           if ($c->{ip}->contains($ip)) 
   91           { 
   92               warn "Listing $bl ($ip) [$num hosts] matches $c->{desc}\n"; 
   93               last; 
   94           } 
   95           elsif ($ip->contains($c->{ip})) 
   96           { 
   97               warn "Listing $bl ($ip) [$num hosts] contains $c->{desc}\n"; 
  100           } 
  101       } 
  102   }

Lines 75-102 iterate through the hits being read and match the critical networks. The sorting done at line 70 allows the last in line 93 to break the loop early, avoiding unnecessary work. Now, I'll create a "critical" file with the following contents:

  # Our datacenter space 
  200.11.128.0/20    IP space for internal use 
  200.11.128.0/24    Firewalls 
  200.11.130.0/24    Misc public servers 
  200.11.132.0/24    Access servers 
  200.11.134.0/24    Misc private servers 
  200.11.130.0/25    Public mail servers 
  ...

Warn-count could be run as in this example:

  $ cd $DNSBL_WORK_DIR 
  $ ./warn-count -c critical ./results/hits-20060910 
  Listing lists/spam.dnsbl.sorbs.net (200.11.130.10/32) [1 hosts] 
    matches Public mail servers 
  Listing lists/spam.dnsbl.sorbs.net (200.11.130.10/32) [1 hosts] 
    matches Misc public servers 
  Listing lists/spam.dnsbl.sorbs.net (200.11.130.10/32) [1 hosts] 
    matches IP space for internal use

The report contains the input file, the IP block that was found and the number of IP addresses it contains. If placed in the proper crontab file, it would cause this information to be sent via email to the person in charge of dealing with listings of critical infrastructure. The example above would tell you that one of your public mail servers was listed in spam.dnsbl.sorbs.net. At this point, we know how to generate daily samples of our IP space listings in the DNSBLs we're tracking. Now it's time to store that information in a database, so that we can easily build reports out of it. Let's start with defining a schema like the one shown in Figure 1 . The "dnsbl" table will hold the name of each of the DNSBLs we will consider. The "sample" table will hold each of the individual samples that will describe our variable over time. The file count.sql ( Listing 1 ) contains the SQL description of this schema in SQLite syntax. To create the database in the file $DNSBL_WORK_DIR/count.db, the following command can be used:

  $ cd $DNSBL_WORK_DIR 
  $ sqlite3 count.db < count.sql

That's all there is to it, so now let's move on to filling the database with samples. To make that job even easier, let's use Class::DBI. This provides for a very nice OO wrap around the database, making the code considerably cleaner. Our Class::DBI-derived class will be in $DNSBL_WORK_DIR/lib/Net/Count.pm and the module will be called "Net::Count" for lack of a better name. Within this file, I will define three classes: Net::Count (the base class inheriting from Class::DBI), Net::Count::Dnsbl, and Net::Count::Sample. Let's see some code:

    9   package Net::Count; 
   10   use base 'Class::DBI';

Lines 9 and 10 are pretty much it for Net::Count. However, this allows for a handy place to put common code, if the need arises. Most of my Class::DBI hierarchies have a common base like this, just in case:

   12   package Net::Count::Dnsbl; 
   13   use base 'Net::Count'; 
   15   __PACKAGE__->table('dnsbl'); 
   16   __PACKAGE__->columns(All => qw/id name/); 
   17   __PACKAGE__->has_many(samples => 'Net::Count::Sample');

Lines 12-17 define Net::Count::Dnsbl base properties: The name of the table, the columns that will be managed by Class::DBI, and the relationship with the Net::Count::Sample class, expressed in this case with a call to ->has_many():

   19   sub normalize_column_values 
   20   { 
   21       my $self    = shift;     # Object or string - Careful! 
   22       my $r       = shift; 
   23       $r->{name} =~ s/\W/_/g; 
   24   }

Since we want to be sure that the names of the lists do not contain dangerous characters, I provide a normalize_column_values() function. This is invoked by Class::DBI automatically every time a row is to be inserted or updated in the table, and provides for an opportunity to alter the data:

   26   __PACKAGE__->set_sql(names => qq{ 
   27       SELECT DISTINCT name 
   28       FROM __TABLE__ 
   29   }); 
   30 
   31   sub unique_names 
   32   { 
   33       my $sth = Net::Count::Dnsbl->sql_names(); 
   34       $sth->execute;     
   35       map { $_->[0] } @{$sth->fetchall_arrayref(['name'])}; 
   36   }

Finally, lines 26-36 use the extended ->set_sql() provided by Class::DBI to provide a custom ->unique_names() method returning a list of unique DNSBL names in the database. This will be useful later, when we try to generate the graphs from our data:

   38   package Net::Count::Sample; 
   39   use base 'Net::Count'; 
   40 
   41   __PACKAGE__->table('sample'); 
   42   __PACKAGE__->columns(Primary    => qw/dnsbl_id sample_utime source/); 
   43   __PACKAGE__->columns(Other      => qw/count/); 
   44   __PACKAGE__->has_a(dnsbl_id     => 'Net::Count::Dnsbl');

Lines 38-44 define the basic details of the Net::Count::Sample. The relationship with the dnsbl table, through the Net::Count::Dnsbl class, is expressed by Class::DBI->has_a(). Note the calls to ->columns(), which allow the specification of multi-column primary keys as well as regular columns:

   53   __PACKAGE__->set_sql(sources    => q{ 
   54   SELECT DISTINCT source 
   55   FROM __TABLE__ 
   56   }); 
   58   __PACKAGE__->set_sql(historic_data => q{ 
   59   SELECT __ESSENTIAL__ 
   60   FROM dnsbl, __TABLE__ 
   61   WHERE 
   62     dnsbl.name = ? 
   63     AND sample.source = ? 
   64     AND sample.sample_utime >= ? 
   65     AND dnsbl.id = sample.dnsbl_id 
   66   ORDER BY sample.sample_utime 
   67   }); 
   69   sub unique_sources 
   70   { 
   71       my $sth = Net::Count::Sample->sql_sources(); 
   72       $sth->execute; 
   73       map { $_->[0] } @{$sth->fetchall_arrayref(['source'])}; 
   74   }

Lines 52-74 use ->set_sql() again to produce the list of unique sources and also to query the database looking for the latest samples corresponding to a given DNSBL and source. These 73 lines of code, completely abstract the interaction with the database and allow our data loading script, dbi-count, to be much simpler. Let's see its relevant parts:

   47   while (<>) 
   48   { 
   49       chomp; 
   50       my ($bl, $num) = (split(/\s+/, $_, 3))[0, 2]; 
   51       $T{$bl} += $num; 
   52   }

The first step is to load and summarize all the hits in each DNSBL. This is done by a accumulating the total in a simple hash, at lines 47-52:

   55   Net::Count->connection($opt_d, undef, undef);

Line 55 initializes the Class::DBI machinery using either the argument to the -d command-line flag or a default DSN specified earlier in the code:

   58   for my $bl (sort keys %T) 
   59   { 
   60       my $db_bl = Net::Count::Dnsbl->find_or_create(name => $bl); 
   61       my $db_sample = Net::Count::Sample->insert 
   62           ( 
   63            { 
   64                dnsbl_id           => $db_bl, 
   65                sample_utime       => $time, 
   66                source             => $opt_s, 
   67                count              => $T{$bl}, 
   68            } 
   69            ); 
   70       # Some fancy output 
   71       print "$opt_s\t$bl\t$T{$bl}\n"; 
   72   }

Lines 58-72 trivially insert each sample in the database. Line 60 uses Class::DBI->find_or_create() to either search for or create a new DNSBL entry corresponding with the total we intend to store. This allows our database to automatically adapt as we incorporate new DNSBLs to our monitoring, sparing us from having to remember to add columns or modify scripts. Lines 61-69 insert the sample corresponding to this list, with this source, at the time the program is running. Thanks to having used Class::DBI, our script is extremely portable. It will support any database already supported by the Perl DBI. Running the script could not be easier:

  $ cd $DNSBL_WORK_DIR 
  $ ./dbi-count ./results/hits-20060910

Alternatively, you could modify the line in your crontab file where you invoke count-ips like this:

7 1 * * * cd $DNSBL_WORK_DIR && \ 
  ./count-ips -i ./isp-networks -s ./lists > \ 
  ./results/hits-'date '+%Y%m%d''; \ 
  ./dbi-count -d $DSN ./results/hits-'date '+%Y%m%d''; \ 
  find ./results/ -type f -mtime +10 | xargs rm

This causes the results to be loaded to the database as soon as the scanning of the DNSBL data is finished. All that's left now is producing the eye candy by converting the data stored in the database to a nice graph we can show the boss. We will use rrdtool for this task. This process has two parts: populating the RRD data and generating the graph. To accomplish the first task, let's take a look at count-rrd, a simple script that takes the results stored in the database, creates the missing RRD files, and updates them. This makes the process of maintaining the RRDs automatic, thus reducing our workload:

   48   my @dnsbls      = map { s!\W+!_!g; $_ } \             
                          Net::Count::Dnsbl->unique_names; 
   49   my @sources     = map { s!\W+!_!g; $_ } \               
                          Net::Count::Sample->unique_sources;

Lines 48-49 use the ->unique_* methods defined in Net::Count::* to obtain the list of DNSBLs and sample sources in the database. Note the use of map { ... } as an additional measure against dangerous names coming from the database:

   52   for my $d (@dnsbls) 
   53   { 
   54       for my $s (@sources) 
   55       { 
   56           my $name = "$path/$d-$s.rrd"; 
   57           if (-f $name) 
   58           { 
   59               update_rrd($name, $d, $s, 86400); 
   60           } 
   61           else 
   62           { 
   63               init_rrd($name, $d, $s); 
   64           } 
   65       } 
   66   }

Lines 52-66 iterate through all possible combinations of DNSBL and sample sources. If the RRD corresponding to that combination is missing, the function init_rrd() is called. Otherwise, the update_rrd method is invoked:

   72   sub init_rrd 
   73   { 
   74       my $file    = shift; 
   75       my $dnsbl   = shift; 
   76       my $source  = shift; 
   77 
   78       # Which data sources and aggregator functions to 
   79       # include in each RRD 
   80 
   81       my @dss = (qw/listings/); 
   82       my @agg = (qw/AVERAGE MIN MAX/); 
   83 
   84       # Create the RRDs 
   85       RRDs::create 
   86           ( 
   87            $file, 
   88            '--start'      => '-1 year', 
   89            '--step'       => 86400, 
   90            (map { "DS:$_:GAUGE:172800:0:U" } @dss), 
   91            (map { "RRA:$_:0.5:1:365" } ('LAST', @agg)), 
   92            (map { "RRA:$_:0.5:5:365" } ('LAST', @agg)), 
   93            (map { "RRA:$_:0.5:30:365" } ('LAST', @agg)), 
   94            ); 
   ... 
   98       # Verify creation errors 
   99       my $error = RRDs::error; 
  100       die "Failed to create RRD $file: $error\n"
  101           if $error; 
  102 
  103       # Perform the update 
  104       update_rrd($file, $dnsbl, $source, 31536000); 
  105   }

Lines 72-105 define the ->init_rrd() method. This is a rather thin wrapper around RRDs::create() that supplies the parameters to create the RRD as required. This is precisely what allows count-rrd to automatically adapt to new DNSBLs or sample sources. RRDs for new combinations can be created on the fly. In fact, you can erase the RRDs and rerun count-rrd. This will re-create the erased files:

  107   sub update_rrd 
  108   { 
  ... 
  115       my @samples = Net::Count::Sample->search_historic_data($dnsbl, 
  116                                                    $source, 
  117                                                    $time); 
  118       # Now update the RRD file 
  119       for my $sample (@samples) 
  120       { 
  121           # The actual update command 
  122           RRDs::update( 
  123                        $file, 
  124                        '-t'       => 'listings', 
  125                        $sample->sample_utime . ":" . \            
                               $sample->count 
  126                        ); 
  127 
  128             # Verify update errors 
  129             my $error = RRDs::error; 
  130             warn "Failed to update RRD $file: $error\n"
  131                 if $error; 
  132         } 
  133   }

Lines 107-133 obtain data for a given period of time using the Net::Count::Sample->search_historic_data() method, the data is then passed to the RRDs::update() method for updating of the corresponding RRD file. I can now update all my RRD files by invoking the following command:

  $ cd $DNSBL_WORK_DIR 
  $ mkdir rrd 
  $ ./count-rrd rrd

This will create all the missing RRDs and update their information. Of course, I could also add this step to the crontab file. The new line would look a lot like this:

7 1 * * * cd $DNSBL_WORK_DIR && \ 
  ./count-ips -i ./isp-networks -s ./lists > \ 
  ./results/hits-'date '+%Y%m%d''; \ 
  ./dbi-count -d $DSN ./results/hits-'date '+%Y%m%d''; \ 
  ./count-rrd -d $DSN ./rrd; \ 
  find ./results/ -type f -mtime +10 | xargs rm

All that's left is converting the RRDs into graphs that can be put on Web pages -- probably with rrdcgi -- or added to reports. For demonstration purposes, I prepared graph.sh, which creates a very simple area graph with the lists I've discussed in this article:

#!/bin/bash 

rrdtool graph 'rrd/summary.svg' --imgformat SVG \ 
    --start '-6 months' --end '-1 second' --step 86400 \ 
    --width 600 --height 240 --lower-limit 0 \ 
    --slope-mode \ 
    --title 'Listing summary' --watermark 'SysAdmin Magazine' \ 
    DEF:var0=rrd/lists_http_dnsbl_sorbs_net-default.rrd:listings:LAST \ 
    DEF:var1=rrd/lists_smtp_dnsbl_sorbs_net-default.rrd:listings:LAST \ 
    DEF:var2=rrd/lists_socks_dnsbl_sorbs_net-default.rrd:listings:LAST \ 
    DEF:var3=rrd/lists_spam_dnsbl_sorbs_net-default.rrd:listings:LAST \ 
    DEF:var4=rrd/lists_web_dnsbl_sorbs_net-default.rrd:listings:LAST \ 
    DEF:var5=rrd/lists_misc_dnsbl_sorbs_net-default.rrd:listings:LAST \ 
    DEF:var6=rrd/lists_list_dsbl_org-default.rrd:listings:LAST \ 
    DEF:var7=rrd/lists_psbl_surriel_com-default.rrd:listings:LAST \ 
    DEF:var8=rrd/lists_sbl_spamhaus_org-default.rrd:listings:LAST \ 
    DEF:var9=rrd/lists_xbl_spamhaus_org-default.rrd:listings:LAST \ 
    AREA:var0#FF0000:'rrd/lists_http_dnsbl_sorbs_net-default.rrd' \ 
    AREA:var1#FFFF00:'rrd/lists_smtp_dnsbl_sorbs_net-default.rrd':STACK \ 
    AREA:var2#FF00FF:'rrd/lists_socks_dnsbl_sorbs_net-default.rrd':STACK \ 
    AREA:var3#CCDD22:'rrd/lists_spam_dnsbl_sorbs_net-default.rrd':STACK \ 
    AREA:var4#0000FF:'rrd/lists_web_dnsbl_sorbs_net-default.rrd':STACK \ 
    AREA:var5#0044EE:'rrd/lists_misc_dnsbl_sorbs_net-default.rrd':STACK \ 
    AREA:var6#CCAAFF:'rrd/lists_list_dsbl_org-default.rrd':STACK \ 
    AREA:var7#3322DD:'rrd/lists_psbl_surriel_com-default.rrd':STACK \ 
    AREA:var8#888800:'rrd/lists_sbl_spamhaus_org-default.rrd':STACK \ 
    AREA:var9#882288:'rrd/lists_xbl_spamhaus_org-default.rrd':STACK

Please see the documentation for rrdtool, as this excellent package includes lots of options for customizing your graphs. As shown, this script produced a graph similar to Figure 2 when fed with data for a few months.

Another example of the look you can achieve can be seen in Figure 3 . This graph, in Spanish, shows the results of applying this technique in a real network.

And this eye candy provides an excellent excuse to finish this article, so you can go play with the rrdtool options for changing how your graphics look. I hope this information proves as useful to you as it has been for me.

Luis has been working in various areas of computer science since the late 1980s. Some people blame him for conspiring to bring the Internet into his home country, where currently he spends most of his time teaching others about Perl and taking care of network security at the largest ISP there as its CISO. He also believes that being a sys admin is supposed to be fun.

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