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14.3. Supervisi: Pencegahan, Deteksi, Deteren

Monitoring is an integral part of any security policy for several reasons. Among them, that the goal of security is usually not restricted to guaranteeing data confidentiality, but it also includes ensuring availability of the services. It is therefore imperative to check that everything works as expected, and to detect in a timely manner any deviant behavior or change in quality of the service(s) rendered. Monitoring activity can help detecting intrusion attempts and enable a swift reaction before they cause grave consequences. This section reviews some tools that can be used to monitor several aspects of a Debian system. As such, it completes Bagian 12.4, “Pemantauan”.

14.3.1. Memantau Log dengan logcheck

Program logcheck memonitor berkas log setiap jam secara default. Ia akan mengirimkan pesan-pesan log yang tidak biasa melalui email ke administrator untuk analisa lebih lanjut.
The list of monitored files is stored in /etc/logcheck/logcheck.logfiles; the default values work fine if the /etc/rsyslog.conf file has not been completely overhauled.
logcheck can work in one of three more or less detailed modes: paranoid, server and workstation. The first one is very verbose, and should probably be restricted to specific servers such as firewalls. The second (and default) mode is recommended for most servers. The last one is designed for workstations, and is even terser (it filters out more messages).
In all three cases, logcheck should probably be customized to exclude some extra messages (depending on installed services), unless the admin really wishes to receive hourly batches of long uninteresting emails. Since the message selection mechanism is rather complex, /usr/share/doc/logcheck-database/README.logcheck-database.gz is a required — if challenging — read.
Aturan yang diterapkan dapat dipecah menjadi beberapa jenis:
  • those that qualify a message as a cracking attempt (stored in a file in the /etc/logcheck/cracking.d/ directory);
  • those canceling such a qualification (/etc/logcheck/cracking.ignore.d/);
  • those classifying a message as a security alert (/etc/logcheck/violations.d/);
  • those canceling this classification (/etc/logcheck/violations.ignore.d/);
  • finally, those applying to the remaining messages (considered as system events).
A system event is always signaled unless a rule in one of the /etc/logcheck/ignore.d.{paranoid,server,workstation}/ directories states the event should be ignored. Of course, the only directories taken into account are those corresponding to verbosity levels equal or greater than the selected operation mode.

14.3.2. Memantau Aktivitas Secara Real Time

top is an interactive tool that displays a list of currently running processes. The default sorting is based on the current amount of processor use and can be obtained with the P key. Other sort orders include a sort by occupied memory (M key), by total processor time (T key) and by process identifier (N key). The k key allows killing a process by entering its process identifier. The r key allows renicing a process, i.e. changing its priority.
When the system seems to be overloaded, top is a great tool to see which processes are competing for processor time or consume too much memory. In particular, it is often interesting to check if the processes consuming resources match the real services that the machine is known to host. An unknown process running as the www-data user should really stand out and be investigated, since it's probably an instance of software installed and executed on the system through a vulnerability in a web application.
top is a very flexible tool and its manual page gives details on how to customize its display and adapt it to one's personal needs and habits.
The gnome-system-monitor graphical tool is similar to top and it provides roughly the same features. Riwayat

Beban prosesor, lalu lintas jaringan, dan ruang bebas disk adalah informasi yang terus-menerus berubah. Mencatat riwayat evolusi mereka sering berguna dalam menentukan persis bagaimana komputer digunakan.
Ada banyak alat khusus untuk tugas ini. Sebagian dapat mengambil data melalui SNMP (Simple Network Management Protocol) untuk memusatkan informasi ini. Manfaat tambahan adalah bahwa hal ini memungkinkan pengambilan data dari elemen-elemen jaringan yang mungkin bukan komputer untuk keperluan umum, seperti router jaringan atau switch.
This book deals with Munin in some detail (see Bagian 12.4.1, “Setting Up Munin”) as part of Bab 12: “Administrasi Tingkat Lanjut. Debian also provides a similar tool, cacti. Its deployment is slightly more complex, since it is based solely on SNMP. Despite having a web interface, grasping the concepts involved in configuration still requires some effort. Reading the HTML documentation (/usr/share/doc/cacti/html/index.html) should be considered a prerequisite.

14.3.3. Mendeteksi Perubahan

Once the system is installed and configured, and barring security upgrades, there's usually no reason for most of the files and directories to evolve, data excepted. It is therefore interesting to make sure that files actually do not change: any unexpected change would therefore be worth investigating. This section presents a few tools able to monitor files and to warn the administrator when an unexpected change occurs (or simply to list such changes). Mengaudit Paket-paket dengan dpkg --verify

dpkg --verify (or dpkg -V) is an interesting tool since it allows finding what installed files have been modified (potentially by an attacker), but this should be taken with a grain of salt. To do its job it relies on checksums stored in dpkg's own database which is stored on the hard disk (they can be found in /var/lib/dpkg/info/package.md5sums); a thorough attacker will therefore update these files so they contain the new checksums for the subverted files.
Running dpkg -V will verify all installed packages and will print out a line for each file with a failing test. The output format is the same as the one of rpm -V where each character denotes a test on some specific meta-data. Unfortunately dpkg does not store the meta-data needed for most tests and will thus output question marks for them. Currently only the checksum test can yield a "5" on the third character (when it fails).
# dpkg -V
??5??????   /lib/systemd/system/ssh.service
??5?????? c /etc/libvirt/qemu/networks/default.xml
??5?????? c /etc/lvm/lvm.conf
??5?????? c /etc/salt/roster
In the sample above, dpkg reports a change to SSH's service file that the administrator made to the packaged file instead of using an appropriate /etc/systemd/system/ssh.service override (which would be stored below /etc like any configuration change should be). It also lists multiple configuration files (identified by the "c" letter on the second field) that had been legitimately modified. Mengaudit Paket: debsums dan keterbatasannya

debsums is the ancestor of dpkg -V and is thus mostly obsolete. It suffers from the same limitations than dpkg. Fortunately, some of the limitations can be worked-around (whereas dpkg does not offer similar work-arounds).
Since the data on the disk cannot be trusted, debsums offers to do its checks based on .deb files instead of relying on dpkg's database. To download trusted .deb files of all the packages installed, we can rely on APT's authenticated downloads. This operation can be slow and tedious, and should therefore not be considered a proactive technique to be used on a regular basis.
# apt-get --reinstall -d install `grep-status -e 'Status: install ok installed' -n -s Package`
[ ... ]
# debsums -p /var/cache/apt/archives --generate=all
Note that this example uses the grep-status command from the dctrl-tools package, which is not installed by default. Memantau Berkas: AIDE

The AIDE tool (Advanced Intrusion Detection Environment) allows checking file integrity, and detecting any change against a previously recorded image of the valid system. This image is stored as a database (/var/lib/aide/aide.db) containing the relevant information on all files of the system (fingerprints, permissions, timestamps and so on). This database is first initialized with aideinit; it is then used daily (by the /etc/cron.daily/aide script) to check that nothing relevant changed. When changes are detected, AIDE records them in log files (/var/log/aide/*.log) and sends its findings to the administrator by email.
Many options in /etc/default/aide can be used to tweak the behavior of the aide package. The AIDE configuration proper is stored in /etc/aide/aide.conf and /etc/aide/aide.conf.d/ (actually, these files are only used by update-aide.conf to generate /var/lib/aide/aide.conf.autogenerated). Configuration indicates which properties of which files need to be checked. For instance, the contents of log files changes routinely, and such changes can be ignored as long as the permissions of these files stay the same, but both contents and permissions of executable programs must be constant. Although not very complex, the configuration syntax is not fully intuitive, and reading the aide.conf(5) manual page is therefore recommended.
A new version of the database is generated daily in /var/lib/aide/; if all recorded changes were legitimate, it can be used to replace the reference database.

14.3.4. Mendeteksi Intrusi (IDS/NIDS)

suricata (in the Debian package of the same name) is a NIDS — a Network Intrusion Detection System. Its function is to listen to the network and try to detect infiltration attempts and/or hostile acts (including denial of service attacks). All these events are logged in multiple files in /var/log/suricata. There are third party tools (Kibana/logstash) to better browse all the data collected.
Configuring suricata involves reviewing and editing /etc/suricata/suricata-debian.yaml, which is very long because each parameter is abundantly commented. A minimal configuration requires describing the range of addresses that the local network covers (HOME_NET parameter). In practice, this means the set of all potential attack targets. But getting the most of it requires reading it in full and adapting it to the local situation.
On top of this, you should also edit /etc/default/suricata to define the network interface to monitor and to enable the init script (by setting RUN=yes). You might also want to set LISTENMODE=pcap because the default LISTENMODE=nfqueue requires further configuration to work properly (the netfilter firewall must be configured to pass packets to some user-space queue handled by suricata via the NFQUEUE target).
To detect bad behaviour, suricata needs a set of monitoring rules: you can find such rules in the snort-rules-default package. snort is the historical reference in the IDS ecosystem and suricata is able to reuse rules written for it. Unfortunately that package is missing from Debian Jessie and should be retrieved from another Debian release like Testing or Unstable.
Alternatively, oinkmaster (in the package of the same name) can be used to download Snort rulesets from external sources.