Databases, Systems & Networks

As someone says, security is a not a product, but a process. While SSH protocol itself is cryptographically secure by design, someone can wreak havoc on your SSH service if it is not administered properly, be it weak passwords, compromised keys or outdated SSH client.

As far as SSH authentication is concerned, public key authentication is in general considered more secure than password authentication. However, key authentication is actually not desirable or even less secure if you are logging in from a public or shared computer, where things like stealth keylogger or memory scraper can always a possibility. If you cannot trust the local computer, it is better to use something else. This is when « one-time passwords » come in handy. As the name implies, each one-time password is for single-use only. Such disposable passwords can be safely used in untrusted environments as they cannot be re-used even when they are stolen.

One way to generate disposable passwords is via Google Authenticator. In this tutorial, I am going to demonstrate another way to create one-time passwords for SSH login: OTPW, a one-time password login package. Unlike Google Authenticator, you do not rely on any third party for one-time password generation and verification.

What is OTPW?

OTPW consists of one-time password generator and PAM-integrated verification routines. In OTPW, one-time passwords are generated apriori with the generator, and carried by a user securely (e.g., printed in a paper sheet). Cryptographic hash of the generated passwords are then stored in the SSH server host. When a user logs in with a one-time password, OTPW’s PAM module verifies the password, and invalidates it to prevent re-use.

Step One: Install and Configure OTPW on Linux

Debian, Ubuntu or Linux Mint:

Install OTPW packages with apt–get.

$ sudo apt-get install libpam-otpw otpw-bin

Open a PAM configuration file for SSH (/etc/pam.d/sshd) with a text editor, and comment out the following line (to disable password authentication).

#@include common-auth

and add the following two lines (to enable one-time password authentication):

auth       required     pam_otpw.so
session    optional     pam_otpw.so

Fedora or CentOS/RHEL:

OTPW is not available as a prebuilt package on Red Hat based systems. So let’s install OTPW by building it from the source.

First, install prerequites:

$ sudo yum git gcc pam-devel
$ git clone https://www.cl.cam.ac.uk/~mgk25/git/otpw
$ cd otpw

Open Makefile with a text editor, and edit a line that starts with « PAMLIB= » as follows.

On 64-bit system:

PAMLIB=/usr/lib64/security

On 32-bit system:

PAMLIB=/usr/lib/security

Compile and install it. Note that installation will automatically restart an SSH server. So be ready to be disconnected if you are on an SSH connection.

$ make
$ sudo make install

Now you need to update SELinux policy since /usr/sbin/sshd tries to write to user’s home directory, which is not allowed by default SELinux policy. The following commands will do. If you are not using SELinux, skip this step.

$ sudo grep sshd /var/log/audit/audit.log | audit2allow -M mypol
$ sudo semodule -i mypol.pp

Next, open a PAM configuration file for SSH (/etc/pam.d/sshd) with a text editor, and comment out the following line (to disable password authentication).

#auth       substack     password-auth

and add the following two lines (to enable one-time password authentication):

auth       required     pam_otpw.so
session    optional     pam_otpw.so

Lire la suite…

If you have customized iptables rules, and would like to load the customized iptables rules persistently across reboots on Debian, you can leverage if-up.d scripts that are located in /etc/network/if-up.d. On Debian, any script that is marked as executable and placed in /etc/network/if-up.d gets executed when a network interface is brought up.

In order to run iptables automatically after reboot on Debian, do the following.

First, customize iptables as you wish, and then save the current iptables rule-set using iptables-save command.

$ sudo iptables-save > /etc/firewall.conf

The above command will dump the current iptables rule set into /etc/firewall.conf file which iptables-restore command can later use to restore the same rule set.

Now create the following if-up.d script called iptables that restores the saved iptables rule set.

#!/bin/sh
iptables-restore < /etc/firewall.conf
$ sudo chmod  x /etc/network/if-up.d/iptables

Alternatively, you can add « iptables-restore < /etc/firewall.conf » command to /etc/rc.local, which gets executed at the end of system boot-up.

Source: Xmodulo

Sécuriser Linux

Ce guide va vous apprendre à sécuriser un serveur et donc vous initier aux thématiques de la sécurité informatique. En quoi est-ce important ? Par définition, un serveur est ouvert sur le monde, un minimum de sécurité est donc intéressant afin de se prémunir des attaques les plus simplistes.

La marche à suivre sera donc la suivante :

• présentation des failles ;
• présentation des outils pour y pallier.

Bien entendu, je ne fais pas un cours complet de sécurité informatique, ce tutoriel est une initiation. Pour faire simple, c’est un peu comme fermer les volets la nuit.

Point de vue matériel, voilà ce qu’il vous faudra :

• un serveur embarquant une distribution xBuntu ou Debian (pour les autres distribs, le principe sera le même, mais les commandes risquent d’être différentes) ;
• un accès root (en ssh par exemple) et une console.

Tout le tutoriel se passe en ligne de commande afin de pouvoir être accessible à tous. Comme il faudra éditer des fichiers, veuillez vous assurer d’avoir un éditeur de fichier en ligne de commande (j’utiliserai personnellement nano qui est très simple).

C’est parti !

apt-get install iptables

nano /etc/init.d/firewall

iptables -t filter -F
iptables -t filter -X

3. On bloque par défaut tout le trafic (si vous êtes en ssh, bien entendu, n’exécutez pas encore le script !) :

iptables -t filter -P INPUT DROP
iptables -t filter -P FORWARD DROP
iptables -t filter -P OUTPUT DROP

4. On ne ferme pas les connexions déjà établies :

iptables -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -m state --state RELATED,ESTABLISHED -j ACCEPT

Nous indiquons avec les paramètres -m et --state de ne pas fermer les connexions qui sont déjà établies.

5. On autorise le loopback (on ne va pas se bloquer nous-mêmes !)

iptables -t filter -A INPUT -i lo -j ACCEPT
iptables -t filter -A OUTPUT -o lo -j ACCEPT

Note : lo signifie localhost (le serveur lui-même).

Tout est bloqué, il ne nous reste plus qu’à ouvrir les ports utilisés.

Lire la suite…

System administrators (sysadmins) are responsible for day-to-day operations of production systems and services. One of the critical roles of sysadmins is to ensure that operational services are available round the clock. For that, they have to carefully plan backup policies, disaster management strategies, scheduled maintenance, security audits, etc. Like every other discipline, sysadmins have their tools of trade. Utilizing proper tools in the right case at the right time can help maintain the health of operating systems with minimal service interruptions and maximum uptime.

This article will present some of the most popular and useful CLI tools recommended for sysadmins in their day to day activities. If you would like to recommend any useful tool which is not listed here, don’t forget to share it in the comment section.

Network Tools

1. ping: Check end-to-end connectivity (RTT delay, jitter, packet loss) of a remote host with ICMP echo/reply. Useful to check system status and reachability.

2. hping: Network scanning and testing tool that can generate ICMP/TCP/UDP ping packets. Often used for advanced port scanning, firewall testing, manual path MTU discovery and fragmentation testing.

3. traceroute: Discover a layer-3 forwarding path from a local host to a remote destination host with TTL-limited ICMP/UDP/TCP probe packets. Useful to troubleshoot network reachability and routing problems.

4. mtr: A variation of traceroute which characterizes per-hop packet loss/jitter with running statistics. Useful to characterize routing path delays.

5. netcat/socat: A swiss army knife of TCP/IP networking, allowing to read/write byte streams over TCP/UDP. Useful to troubleshoot firewall policies and service availability.

6. dig: DNS troubleshooting tool that can generate forward queries, reverse queries, find authoritative name servers, check CNAME, MX and other DNS records. Can be instructed to query a specific DNS server of your choosing.

7. nslookup: Another DNS checking/troubleshooting tool. Works with all DNS queries and records. Can query a particular DNS server.

8. dnsyo: A DNS testing tool which checks DNS propagation by performing DNS lookup from over a number of open resolvers located across 1,500 different networks around the world.

9. lsof: Show information about files (e.g., regular files, pipes or sockets) which are opened by processes. Useful to monitor processes or users in terms of their open network connections or opened files.

10. iftop: A ncurses-based TUI utility that can be used to monitor in real time bandwidth utilization and network connections for individual network interfaces. Useful to keep track of bandwidth hogging applications, users, destinations and ports.

11. netstat: A network statistics utility that can show status information and statistics about open network connections (TCP/UDP ports, IP addresses), routing tables, TX/RX traffic and protocols. Useful for network related diagnosis and performance tuning.

12. tcpdump: A popular packet sniffer tool based on libpcap packet capture library. Can define packet capturing filters in Berkeley Packet Filters format.

13. tshark: Another CLI packet sniffer software with full compatibility with its GUI counterpart, Wireshark. Supports 1,000 protocols and the list is growing. Useful to troubleshoot, analyze and store information on live packets.

14. ip: A versatile CLI networking tool which is part of iproute2 package. Used to check and modifying routing tables, network device state, and IP tunneling settings. Useful to view routing tables, add/remove static routes, configure network interfaces, and otherwise troubleshoot routing issues.

15. ifup/ifdown: Used to bring up or shut down a particular network interface. Often a preferred alternative to restarting the entire network service.

16. autossh: A program which creates an SSH session and automatically restarts the session should it disconnect. Often useful to create a persistent reverse SSH tunnel across restrictive corporate networks.

17. iperf: A network testing tool which measures maximum bi-directional throughput between a pair of hosts by injecting customizable TCP/UDP data streams in between.

18. elinks/lynx: text-based web browsers for CLI-based server environment.

Lire la suite…

In an operating system, logs are all about keeping track of events, be it critical system errors, resource usage warnings, transaction history, application status, or user activities. These logs, which are stored as (text or binary) files in the system, are useful for system auditing, debugging and maintenance. However, with so many different system entities generating log files, and even at growing rate, the challenge as a system admin is to how to « consume » these log files effectively.

That’s when log monitoring tools come into the picture, which streamline the often laborious process of collecting, parsing and analyzing log files, as well as alerting system admins for any interesting events. These tools are designed from ground up focused on log monitoring, so they offer many attractive features, such as scalable log aggregation and filtering, human-readable display, event correlation, visual or email notification, flexible log retention policy, and so on.

In this post, I am going to introduce a list of popular open-source log monitoring software for Linux, ranging from simple log file viewers to full-blown log monitoring frameworks.

Log Aggregation and Filtering

Log monitoring would not be possible without efficient and scalable mechanisms to collect and pre-process log files. Tools in this category focus on shipping, collecting, filtering, indexing and storing log files, so that they can be further analyzed and visualized in subsequent monitoring pipelines.

1. rsyslog: an open-source log collector server which can filter and consolidate log data (based on syslog protocol) from different hosts and devices in the network. rsyslog can be configured as a server or a client, where the former plays the role of a log collector and the latter runs as a log sender.

2. syslog-ng: another open-source implementation of the syslog protocol with more advanced and user-friendly features such as content-based filtering, easier-to-understand config format, and real-time event correlation.

3. systemd journal: systemd journal can be configured for remote journal logging, where locally logged events are forwarded to a remote server over HTTP. In this setup, systemd-journal-upload on a client host serializes and forwards journal messages to systemd-journal-remote running on a remote collector server.

4. logstash: an open-source tool that collects, parses, and stores log files for offline search and analysis. logstash can run in various pipelines due to many plugins supporting different input/output interfaces, decoding/encoding, and filtering rules. Input plugins allows logstash to gather log files from different sources and protocols (e.g., files, S3, RabbitMQ, syslog, collectd, TCP/UDP sockets). Filter/codec plugins are used to parse, convert, modify and add metadata to log files. Output plugins pass processed log files to various target storages (e.g., file, Google cloud storage, Nagios, S3, Zabbix).

5. collectd: a daemon service which gathers various system-level statistics, and stores them for historical analysis or real-time graphing. Similar to logstash, collectd is an extensible architecture, where you can enable various input/output plugins to change its collection behaviors. For log collection, collectd can leverage LogFile and Network plugins to aggregate remote log files.

6. Logster: an open-source utility for parsing log files for any interesting data, and aggregating extracted data into metrics for subsequent reporting and graphing pipelines.

7. Fluentd: a unified log aggregation layer which allows in-stream processing for a variety of streaming data and log files. It comes with a huge plugin ecosystem with more than 300 plugins to support various input sources and output interfaces.

8. Nxlog: a unified log collector and forwarder which supports a variety of log sources, formats and protocols. Advanced features include multi-threaded log collection and processing, message buffering and prioritization, built-in log rotation, and TLS/SSL transport.

9. Scribe:: a scalable log collector server developed by Facebook. Scribe can aggregate log data which is streamed in real time from a large number of clients. It uses Apache Thrift for protocol encoding, so its interface is compatible with pretty much any languages. While a proven solution, Scribe is not something you can deploy quickly as a turnkey. Also, note that Scribe is no longer updated and maintained.

10. Flume: a highly scalable and reliable service to transport and collect large volumes of streaming log data from any clients, and store them in backend storage such as Apache Hadoops’ HDFS.

Lire la suite…