Category Archives: Networking

How to Restrict a Login Shell Using Linux Namespaces

Firejail is a SUID sandbox program that reduces the risk of security breaches by restricting the running environment of untrusted applications using Linux namespaces. It allows a process and all its descendants to have their own private view of the globally shared kernel resources, such as the network stack, process table, mount table.

Started as a simple sandbox for Mozilla Firefox, Firejail was expanded to work on any type of executable, such as servers, graphic programs, and even as login shell.

The program is written in C and only needs libc and POSIX threads (libpthreads), available by default on any Linux platform. The download page provides source code (./configure && make && sudo make install), deb (dpkg -i firejail.deb) and rpm (rpm -i firejail.rpm) packages. Once installed, you can start a program in sandbox as:

$ firejail [options] program and arguments
Example:
$ firejail --debug firefox

Default sandbox

To login into a Firejail sandbox, you need to set /usr/bin/firejail as user shell in /etc/passwd. You can change the shell for an existing user with chsh command (chsh –shell /usr/bin/firejail), or you can use adduser command to define the shell when the user account is created (adduser –shell /usr/bin/firejail username). Below is a ssh login session into a sandboxed account:

SSH login into a default Firejail sandbox

SSH login into a default Firejail sandbox

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Debian Virtualization: Back to the Basics, part 3

The traditional Linux security model starts with file permissions. The model lets the kernel decide whether or not a process may access a resource based on permissions set as part of the filesystem. The coarse-grained granularity of this model often causes Linux processes to have too many rights. If more granularity is needed, one has to resort to adding security related code into the program source.

This series of articles is about Linux namespaces, a lightweight virtualization technology implemented in Linux kernel. In part 1 I’ve talked about building chroot jails using mount namespace, and in part 2 I’ve looked into isolating processes using PID namespace. The next step is to isolate the TCP/IP networking stack using network namespaces.

Security at this level is always reactive. Assuming the bad guy breaks into your server, he will realize he doesn’t have root privileges (classic Unix privilege separation implemented in server software), he runs on top of a fake filesystem (chroot), and he cannot get outside on the network. The later is usually done by placing the computer in a Demilitarized Zone (DMZ) behind a firewall.

The same effect can be achieved on the cheap using Linux namespaces. For this, I place the server in a container (vm1) running its own network segment (10.10.20.0/24). The container is connected to the host through a Linux bridge interface (br0). On the host I configure iptables firewall, isolating the server and effectively limiting the potential damage that could be inflicted on the larger network. The final setup looks like this:

Network setup

Network setup

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RCPlive: Inter-VLAN Routing

Ethernet networks can be partitioned into multiple distinct broadcast domains using VLANs. VLAN domains are mutually isolated. Whenever a hosts in one VLAN domain needs to communicate with a hosts in another VLAN domain, the traffic must be routed between the two domains. This is known as inter-VLAN routing.

This document provides a VLAN configuration example for a small network split into two separate VLAN domains: SALES and ENGINEERING. The backbone consists of two VLAN bridges connected by a VLAN trunk. I will use a Linux-based router, RCPlive, connected to the trunk to provide routing between the two VLAN domains and the outside world. On the router I will also enable a number of services such as DHCP and stateful firewall.

VLAN network

VLAN network

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Debian Virtualization: LXC debootstrap filesystem

Virtualization refers to the creation of virtual machines that acts like real computers with an operating system. Software executed on these virtual machines is separated from the underlying hardware resources.

This article discusses LXC, a lightweight virtualization technology built into Linux kernel. The user space LXC tool is distributed with a number of templates that allow the creation of different Linux distro filesystems, usually one template for each major Linux distribution. The problem with these templates is they never work, or they stop working with every new release of LXC tool or of the particular Linux distribution. This is the case with all Linux distributions, and Debian is no exception. Currently, the Debian template is borken under “wheezy”. The relevant Debian bug is here, and history shows that as soon such a bug gets fixed, lxc user space driver changes again and breaks it. It could be worse, in Fedora LXC was broken in Fedora 15 and it was never fixed.

The simple way to handle the problem is to forget all about the template mechanism and roll your own containers. In Debian you can build the container filesystem using the standard debootstrap, or mount read-only the host filesystem, and then use lxc-execute to start a simple bash session inside the container. In this session you can than start all the programs you need to run in the container. It is an application container, very similar to the containers created using the official ssh template distributed with LXC.

The virtual machine I will describe in this article uses a root filesystem build using debootstrap (apt-get install debootstrap). The procedure is simple and it should work on any Debian machine. It will probably work also on any other distro based on Debian, such as Ubuntu, Mint etc.

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Linux SNMP MIB Browser

An SNMP MIB browser is an indispensable tool for engineers and system administrators to manage SNMP enabled network devices such as routers, switches, servers and workstations. The information provided by SNMP includes uptime, interface traffic data, routing information, TCP and UDP connection information, installed software, and much more.

In this tutorial, I introduce qtmib, an easy-to-use SNMP browser available for Linux and published under GPLv2 license. The program is build as a front-end for net-snmp tools using QT4 library.

qtmib browser window

qtmib browser window

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Debian Virtualization: LXC Desktop Virtualization

Virtualization allows the creation of multiple virtual machines (VM) on top of an existing computer, each VM configured in a very specific way. All virtual machines run in parallel alongside the regular host applications, without affecting the host system. The type of virtualization I am currently using is Linux containers (LXC), a lightweight virtualization technology built into Linux kernel.

This is my third Debian virtualization article. In the first article, I’ve described the steps to create and run a basic virtual machine using LXC. In the second article I’ve isolated the VM on its own network segment, with its own TCP/IP networking stack. Both articles were dealing with server VMs, I’ve used Lighttpd as an example throughout my articles.

I will describe now how to run desktop applications such as Mozilla Firefox and LibreOffice in a virtual machine. I will use virtenv to build and run the VM. virtenv is a QT4 application released under GPLv2 license. It is basically a configuration wizard that allows you to configure and start the LXC-based virtual machines. Once the VM is started, you get a regular controlling terminal (xterm) and a desktop window running the lightweight Openbox window manager. In this window you can run your GUI applications, very similar to VMware Workstation or Oracle’s VirtualBox.

virtenv desktop and controlling terminal

virtenv desktop and controlling terminal

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Debian Virtualization: LXC Network Isolation

Linux containers (LXC) is a lightweight virtualization technology built into Linux kernel. In my previous article, Debian Virtualization: LXC Application Containers, I have detailed the steps to configure and run a simple application container using LXC. LXC application containers are very lean and consume strictly the resources the application requires. This is in sharp contrast with other virtualization technologies which are running a full Linux distribution in VM.

The container uses its own file system, built by mounting read-only the relevant directories from the host file system. The host is an older computer running Debian 7 “wheezy”. The virtual machine is controlled through GNU screen if the VM was started automatically at boot time, or through a regular xterm.

One thing I left out was the networking stack. In my Lighttpd web server example, the VM uses the same networking stack as the host. This could become a problem if someone manages to compromise the web server: the intruder could then probe the networks connected to our host, in search for the next victim.

In this article I’ll modify the VM to run on a separate networking stack. I will place the VM on its own network segment, connected to the host through a Linux bridge interface. I will then go and set up the host firewall using iptables. This effectively isolates the VM and limits the potential damage that could be inflicted on the larger network. The final setup looks like this:

Network setup

Network setup

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How to Install and Configure RCP100 Routing Suite on Debian 7

Software-based routers have always played a role in the Internet, and are becoming increasingly important in data centers due to the convergence of video, mobile, and cloud services. Data traffic no longer moves simply from the subscriber into the network and then out again. Instead, most of the traffic is located inside the data center between various application servers within the network.

All this traffic can be routed easily using software-based routers running on commodity PC hardware. Such a router looks like just another server in the data center, and most of the time it is implemented using open-source software. The availability of the source code and the right to modify the software enables the unlimited tuning and optimization of the network traffic.

This article describes how to set up RCP100 routing suite on a Debian 7 computer. RCP100 is a full OSPF/RIP router for Linux. It works on 64bit computers, it is licensed under GPL, and it is actively developed.

The computer I am setting up has two Ethernet interfaces, eth0 (192.168.20.20) and eth1 (10.1.10.1), and it is meant to connect a small private network segment (10.1.10.0/24) to the larger public network. To isolate the private network, I configure Network Address Translation on the router and enable the firewall. Computers on the private network are assigned IP addresses using DHCP. The router also provides NTP and DNS proxy services.

Network setup

Network setup

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Installing net-snmp MIBs on Ubuntu and Debian

Simple Network Management Protocol (SNMP) is an Internet-standard protocol for managing devices on IP networks. net-snmp is the main SNMP implementation for Linux and BSD platforms. On Ubuntu or Debian net-snmp tools are installed as follows:

$ sudo apt-get install snmp

You can also install snmpd package. This package contains the SNMP agent.

Installing MIBs

For licensing reasons, net-snmp package installs only a small number of MIBs in /usr/share/mibs directory. A large number of standard MIBs can be installed using snmp-mibs-downloader package:

$ sudo apt-get install snmp-mibs-downloader
$ sudo download-mibs

To have the new MIBs recognized by net-snmp, edit /etc/snmp/snmp.conf file as follows:

$ cat /etc/snmp/snmp.conf
mibs +ALL
$

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Network and Filesystem Isolation with LXC and virtenv

For my memory comparison of light Linux desktops I needed a tool that would allow me to install on my computer about 20 window managers/desktop environments. After looking at several common virtualization packages, I ended up using Linux containers and virtenv for the job.

LXC and virtenv

Probably the best way to describe virtenv is as a graphic interface for Linux containers utilities developed and distributed by LXC project. Linux containers is the virtualization technology build into Linux kernel, available in any kernel after 2.6.32.

The virtual machines (VM) are driven without any overhead by the kernel already running on the computer. You don’t need to run a different kernel in the virtual machine, run only the processes you need, without even going trough the regular SysV or Linux init. This all means that memory is used very conservatively. For example, on a 1GB RAM computer you can run easily 10 SSH/DHCP servers, or 10 different xorg/X11 servers with LXDE window managers on top.

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