Tutorial For Lpi Exam 202: Part 6

Topic 212: System Security

David Mertz, Ph.D.
Professional Neophyte
February, 2006

Welcome to "System Security", the sixth of seven tutorials covering intermediate network administration on Linux. This tutorial touches on a wide array of topics related to using Linux as a security-conscious network server, of necessity each somewhat cursorily. General issues of routing, firewalls and NAT translation are discussed and the relevant tools presented. Setting security policies for FTP and SSH are addressed also. General access control with tcpd, hosts.allow and friends is reviewed. Finally, some basic scurity monitoring tools are presented, as well as where to find security resources.

Before You Start

About this series

The Linux Professional Institute (LPI) certifies Linux system administrators at junior and intermediate levels. There are two exams at each certification level. This series of seven tutorials helps you prepare for the second of the two LPI intermediate level system administrator exams--LPI exam 202. A companion series of tutorials is available for the other intermediate level exam--LPI exam 201. Both exam 201 and exam 202 are required for intermediate level certification. Intermediate level certification is also known as certification level 2.

Each exam covers several or topics and each topic has a weight. The weight indicate the relative importance of each topic. Very roughly, expect more questions on the exam for topics with higher weight. The topics and their weights for LPI exam 202 are:

Topic 205: Network Configuration (8) Topic 206: Mail and News (9) Topic 207: Domain Name System (DNS) (8) Topic 208: Web Services (6) Topic 210: Network Client Management (6) Topic 212: System Security (10) * Topic 214: Network Troubleshooting (1)

About this tutorial

Welcome to "System Security", the sixth of seven tutorials covering intermediate network administration on Linux. This tutorial touches on a wide array of topics related to using Linux as a security-conscious network server, of necessity each somewhat cursorily. General issues of routing, firewalls and NAT translation are discussed and the relevant tools presented. Setting security policies for FTP and SSH are addressed also. General access control with tcpd, hosts.allow and friends is reviewed (reiterating the discussion in LPI 201, Topic 209: File Sharing Servers). Finally, some basic scurity monitoring tools are presented, as well as where to find security resources.


To get the most from this tutorial, you should already have a basic knowledge of Linux and a working Linux system on which you can practice the commands covered in this tutorial.

Other resources

As with most Linux tools, it is always useful to examine the manpages for any utilities discussed. Versions and switches might change between utility or kernel version, or with different Linux distributions. For more in depth information, the Linux Documentation Project has a variety of useful documents, especially its HOWTOs. See http://www.tldp.org/. A variety of books on Linux networking have been published, consult your local bookseller or library for latest titles on Linux security issues.

Routing, Firewalls And Network Address Translation

About packet filtering

The Linux kernel includes "netfilter" infrastructure, which gives you the the capability of filtering network packages. Usually this capability is compiled into the base kernel, but a kernel module may be needed for the capability. However, module loading should be seamless: i.e. running iptables will load iptables_filter.o it it needs it. Packet filtering is controlled with the utility iptables in modern Linux systems; older systems used ipchains, and before that ipfwadm. While you can still use ipchains in conjunction with recent kernels if backward compatibility is needed, you will almost always prefer to use the enhanced capabilities and improved syntax in iptables. That said, most of the concepts and switches in iptables are compatible enhancements to ipchains.

Depending on the exact scenario of filtering (firewall, NAT, etc.) filtering and address translation may occur either before or after routing itself. The same ipchains tool is used in either case, but different rule sets ("chains") are used for the cases: at base, INPUT and OUTPUT. Filtering, however, can also affect the routing decision by filtering on the FORWARD chain; this may lead to dropping packets rather than routing them.


As well as filtering with iptables (or legacy ipchains), the Linux kernel performs routing of IP packets it receives. Routing is a simpler process than filtering, though the two are conceptually related. A host, during routing, simply looks at a destination IP address, and decides whether it knows how deliver to a packet directly to that address, or whether a gateway is available that knows how to deliver to that address. If a host can neither deliver a packet itself nor knows what gateway to forward it to, the packet is dropped. However, typical configurations include a "default gateway" that handles every otherwise unspecified address.

Configuration and display of routing information is peformed with the utility route. However, routing may either be static or dynamic. With static routing, delivery is determined by a routing table that is explicitly configured by invocations of the route command and its add or del commands. However, often more useful is configuring dynamic routing using the routed or gated daemons that broadcast routing information to adjacent routing daemons. The routed daemon supports the Routing Information Protocol (RIP); the gated daemon adds support for a number of other protocols (and can use multiple protocols at once): Routing Information Protocol Next Generation (RIPng), Exterior Gateway Protocol (EGP), Border Gateway Protocol (BGP) and BGP4+, Defense Communications Network Local-Network Protocol (HELLO), Open Shortest Path First (OSPF), Intermediate System to Intermediate System (IS-IS), and Internet Control Message Protocol (ICMP and ICMPv6)/Router Discovery.

Let us take a look at a fairly typical static routing table:

% /sbin/route
Kernel IP routing table
Destination   Gateway         Genmask         Flags Metric Ref  Use Iface   *        U     0      0      0 eth0    *        U     0      0      0 eth1   *        U     0      0      0 eth0   *          U     0      0      0 eth1
default       ev1s-66-98-216-         UG    0      0      0 eth0

What this means is that addresses in the 66.98.217/24 and 66.98.216/23 ranges will be directly delivered over eth0. Address ranges 10.10.12/23 and 169.254/16 will be delivered on eth1. Anything left over will be sent to the gateway ev1s-66-98-216-1.ev1servers.net (the name is cut off in the route display; you could also use route -n to see that name was IP address If you wanted to add a different gateway for some other address ranges, you might run something like:

% route add -net netmask gw dev eth0

For a machine that serves as a gateway itself, you will generally want to run dynamic routing, using the routed or gated daemons, which may supplement a smaller number of static routes. The routed daemon is configured by the contents of /etc/gateways. The gated daemon is more modern, and has more capabilities as indicated, and is configured by /etc/gated.conf. Generally if you use either of these, you will want to launch them in your startup scripts. You must not run both routed and gated on the same machine, results will be unpredictable, and almost certainly not desirable.

Filtering with iptables

The Linux kernel stores a table of filter rules for IP packets that form a sort of state-machine. Sets of rules that are processed in sequence are know as "(firewall) chains". When one chain meets a condition, one of the possible actions is to shift control to processing another chain, as in a state-machine. Before you have added any rules or states, three chains are automatically present: INPUT, OUTPUT and FORWARD. The INPUT chain is where packets addressed to the host machine passes, and potentially from there to a local application process. The FORWARD chain is where a packet addressed to a different machine passes, assuming forwarding is enabled and the routing system knows how to forward that packet. A packet generated on the local host is sent into the OUTPUT chain for filtering--if it passes the filters in the OUTPUT chain (or any linked chains), it is routed out over its network interface.

One action that a rule can take is to DROP a packet; in that case, naturally, no further rule processing or state transition is taken for that packet. But if a packet is not dropped, the next rule in a chain is examined to see if it matches the packet. In some cases, satisfaction of a rule will branch processing to a different chain, and its set of rules. Creation, deletion or modification of rules, and of chains in which rules live is performed with the tool iptables. In older Linux systems, the same function was done using ipchains instead. The concepts behind both tools, and even for the ancient ipfwadm are similar, but iptables syntax is discussed here.

A rule specifices a set of conditions that a packet might meet, and what action to take if the packet does meet that condition. As mentioned, one common action is to DROP packets. For example, suppose you wanted (for some reason) to disable ping on the loopback interace (i.e. the ICMP interface). You could enable this with:

% iptables -A INPUT -s -p icmp -j DROP

Of course, that is a silly rule, and we probably want to remove it after we test it, e.g.:

% iptables -D INPUT -s -p icmp -j DROP

Deleting a rule with the -D option requires either exactly the same options as specified when it was added, or specification by rule number (which you, therefore, must determine first), e.g.:

% iptables -D INPUT 1

A more interesting rule might look at source and destination addresses in packets. For example, suppose that a problem remote network is trying to utilize services on a particular subnet of your network. You might block this on your gateway/firewall machine with, e.g.:

% iptables -A INPUT -s 66.98.216/24 -d 64.41.64/24 -j DROP

Doing this will stop anything from the 66.98.216.* IP block from communicating with anything in the local 64.41.64.* subnet. Of course, singling out a specific IP block for blacklisting is fairly limited as protection. A more likely scenario might be to allow only a specific IP block to access a local subnet, e.g.:

% iptables -A INPUT -s ! 66.98.216/24 -d 64.41.64/24 -j DROP

In this case, only the 66.98.216.* IP block can access the specified subnet. Moreover, you can use a symbolic name for a address, and can specify a particular protocol to be filtered. You can also select a specific network interface (e.g. eth0) to filter, but that is less commonly useful. For example, to let only a specific remote network access a local web server, you might use:

% iptables -A INPUT -s ! example.com -d -p TCP -sport 80 -j DROP

There are a number of other options you can specify with iptables, including for example rate limits on the number of packets that will be allowed, or filtering on TCP flags. See the manpage for iptables for more details.

User defined chains

We have seen the basics of adding rules to the automatic chains. But much of the configurability in iptables comes with adding user defined chains, and branching to them if patterns are matched. New chains are defined with the -N option, and branching we have already seen, using the special target DROP. ACCEPT is also a special target with the obivous meaning. Also, special targets RETURN and QUEUE are available. The first means to stop processing a given chain, and return to its parent/caller. The QUEUE handler lets you pass packets to a user space process for further processing (which might be logging, modification of the packet, or more elaborate filtering than iptables supports). The simple example in Rusty Russell's "Linux 2.4 Packet Filtering HOWTO" is a good example of adding a user defined chain:

# Create chain to block new connections, except established locally
% iptables -N block
% iptables -A block -m state --state ESTABLISHED,RELATED -j ACCEPT
% iptables -A block -m state --state NEW -i ! ppp0 -j ACCEPT
% iptables -A block -j DROP  # DROP everything else not ACCEPT'd
# Jump to that chain from INPUT and FORWARD chains
% iptables -A INPUT -j block
% iptables -A FORWARD -j block

Notice that the "block" chain ACCEPTs in a limited class of cases, then the final rule DROPs everything not previously ACCEPT'd.

Once you have established some chains, whether adding rules to the automatic chains or adding user defined chains, you may use the -L option to view the current rules.

Network address translation versus firewalls

The examples we have looked at are basically in the class of firewall rules. But Network Address Translation (NAT) is also configured by iptables. Basically, NAT is a way of using connection tracking to masquerade packets coming from a local subnet address as the external WAN address before sending them out "over the wire" (on the OUTPUT chain). The gateway/router that performs NAT needs to remember which local host connected to which remote host, and reverse the address translation if packets arrive back from the remote host. From a filtering perspective though, you simply pretend that NAT did not exist. The rules you specify should simply use the "real" local addresses, regardless of how NAT might masquerade them to the outside world. Enabling masquerading, i.e. basic NAT, just uses the below iptables command. To use this you will need to make sure the kernel module iptables_nat is loaded, and also turn on IP forwarding. E.g.:

% modprobe iptables_nat    # Load the kernel module
% iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
% echo 1 > /proc/sys/net/piv4/ip_forward   # Turn on IP forwarding

This capability is what is actually called "Source NAT". That is, the address of the outgoing packet is modified. "Destination NAT" (DNAT) also exists to enable port forwarding, load sharing, and transparent proxying. In those cases, incoming packets are modified to get to the relevent local host or subnet. But most of the time when users, or even administrators, talk about NAT, they mean source NAT. If you mean to configure destination NAT, you would specify "PREROUTING" rather than "POSTROUTING". That is for DNAT, the packets are transformed before they are routed.

File Transfer Protocol

FTP servers

Many different FTP servers are available for Linux, and different distributions distribute different servers. Naturally, configuration of different servers vary, though most tend to follow similar configuration directives. A popular FTP server is vsftpd (Very Secure FTP daemon). ProFTP is also in wide use, as are wu-ftpd and ncftpd. For many purposes, FTP is not really needed at all. For example, secure transfers for users who have accounts on a server machine can often be accomplished using scp (secure copy), which relies on the underlying SSH installation, but otherwise mostly mimics the familiar cp command.

The configuration file for vsftpd is /etc/vsftpd.conf. Other FTP servers use similar files.

FTP configuration options

A few options to keep in mind in /etc/vsftpd.conf (and probably in your server if you use a different one) are listed here:

anonymous_enabled: Let anonymous users login using the usernames "anonymous" or "ftp".
anon_mkdir_write_enable: Let anonymous users create directories (within world writable parent directories).
anon_upload_enable: Let anonymous users upload files.
anon_world_readable_only: YES by default, and rarely a good idea to change. Only lets anonymous FTP access world-readable files.
chroot_list_enable: Specify a set of users (listed in /etc/vsftpd.chroot-list) in a "chroot jail" in their home directory upon login.
ssl_enable: Support SSL encrypted connections.

Read the manpages for your FTP server for more complete options. Generally, running an FTP server is as simple as tweaking a configuration file and running the server within your intilization scripts.

Secure Shell

Client and server

Most every Linux machine, and most other operating systems, should have a secure shell (SSH) client. Often, the OpenSSH version is used, but a variety of compatible SSH clients are sometimes used. While an SSH client is essential to connect to a host, the larger security issues arise in properly configuring an SSH server. Since a client initiates a connection to a server, the client is actively choosing to trust the server. Just having an SSH client does not allow any kind of access into a machine, and hence does not expose vulnerabilities. Configuring a server is also not particularly complex, and the server daemon is designed to enable and enforce good security practices. But clearly it is a server that is sharing resources with clients, based on requests from the clients the server decides to honor.

The SSH protocol has two versions, version 1 and version 2. In modern systems, using protocol version 2 is always preferred, but generally both clients and servers maintain backward compatibility with version 1 (unless this capability is disabled with configuration options); this lets you connect to increasingly uncommon version 1-only systems. Somewhat different configuration files are used between version 1 and version 2 protocols. For protocol version 1, a client first creates an RSA key pair using ssh-keygen, and stores the private key in $HOME/.ssh/identity and the public key in $HOME/.ssh/identity.pub. This same identity.pub should be appended to the remote $HOME/.ssh/authorized_keys files. Obviously, there is a chicken-and-egg problem here: how can you copy a file to a remote system before you have access? Fortunately, SSH also supports a fallback authentication method of sending encrypted-on-the-wire passwords that are evaluated through the usual remote-system login tests (i.e. the user account must exist, and the right password must be provided).

Protocol 2 supports both RSA and DSA keys, but RSA authentication is somewhat enhanced rather than identical to that in protocol 1. For protocol 2, private keys are stored in $HOME/.ssh/id_rsa and $HOME/.ssh/id_dsa. Protocol 2 also supports a number of extra confidentiality and integrity algorithms: AES, 3DES, Blowfish, CAST128, HMAC-MD5, HMAC-SHA1, and so on. The server can be configured as to preferred algorithms, and order of fallbacks.

For general configuration options, rather than key information, the client stores its in /etc/ssh/ssh_config or if available /$HOME/.ssh/config. Client options can also be configured with the -o switch; a particularly common switch is the -X or -x to enable or disable X11 forwarding. If enabled, the X11 port is tunnelled through SSH to enable encrypted X11 connections. Tools like scp also use similar port forwarding over SSH. For example, to on the local machine I am working on I can launch onto the local display an X11 application that only exists remotely (on my local subnet in this case), e.g.:

$ which gedit  # not on local system
$ ssh -X dqm@
Linux averatec 2.6.10-5-386 #1 Mon Oct 10 11:15:41 UTC 2005 i686 GNU/Linux
No mail.
Last login: Thu Feb 23 03:51:15 2006 from
dqm@averatec:~$ gedit &

Configuring the server

The sshd daemon, specifically the OpenSSH version available from <http://www.openssh.org/>, enables secure encrypted communications between two untrusted hosts over an insecure network. The base sshd server is normally started during initialization, and listens for cleitn connections forking a new daemon for each client connection. The forked daemons handle key exchange, encryption, authentication, command execution, and data exchange.

As with the client tool, the sshd server accepts a variety of options on the command-line, but is normally configured by the file /etc/ssh/sshd_config. A number of other configuration files are used also. For example, the access controls /etc/hosts.allow and /etc/hosts.deny are honored. Keys are stored in a similar fashion to the client side, in /etc/ssh/ssh_host_key (protocol 1), /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key, and public keys in /etc/ssh/ssh_host_dsa_key.pub and friends. Also as with the client, you will use ssh-keygen to generate keys in the first place. See the manpage for sshd and ssh-keygen for details on configuration files and copying generated keys to appropriate files.

Theere are a large number of configuration options in /etc/ssh/sshd_config, and the default values are generally sensible (and sensibly secure). A few options are particularly notable though. AllowTcpForwarding enables or disables port forwarding (tunneling), and is "yes" by default. Ciphers controls the list and order of encryption algorithms to be utilized. AllowUsers and AllowGroups accept wildcard patterns and allow you to control which users may even attempt further authentication. DenyGroups and DenyUsers act symmetrically, as you would expect. PermitRootLogin lets the "root" user SSH into a machine. Protocol lets you specify whether both protocol versions are accepted (and if not, which one is). TCPKeepAlive is good to look at if you are loosing SSH connections. A "keepalive" message is sent to check connections if this is enabled, but this can cause disconnection if transient errors occur in the route.

SSH tunneling

OpenSSH lets you create a tunnel to encapsulate another protocol within an encrypted SSH channel. This capability is enabled on the sshd server by default, but could have been disabled with command-line or configuration file options. Assuming the capability is enabled, a client can easily emulate whatever port/protocol they wish to use for a connection. For example, to create a tunnel for telnet:

% ssh -2 -N -f -L 5023:localhost:23 user@foo.example.com
% telnet localhost 5023

This example, of course, if fairly pointless since a SSH command shell does the same thing as a telnet shell. But you could create a POP3, HTTP, SMTP, FTP, X11, or other protocol connection in the analogous manner. The basic concept is that a particular localhost port acts as if it were the remote service, with actual communication packets travelling over the SSH connection in encrypted form.

The options we used in the example are: -2 (use protocol 2); -N (no command/tunnel only), -f (SSH in background); -L (describe tunnel as "localport:remotehost:remoteport". The server (with username) are also specified.

Tcp Wrappers

What is tcp_wrappers?

The first thing to know about TCP_wrappers is that you should not use it, and it is not actively maintained. However, you might find the tcpd daemon from TCP_wrappers still running on a legacy system. In its time, this was a good application, but its functionality has been superceded by iptables and other tools. The general purpose of TCP_wrappers is to monitor and filter incoming requests for the SYSTAT, FINGER, FTP, TELNET, RLOGIN, RSH, EXEC, TFTP, TALK, and other network services.

TCP_wrappers can be configured in a couple manners. One is to substitute tcpd for other servers, providing arguments to pass control on to the particular server once tcpd has done its logging and filtering. Another method leave the network daemons alone and modifies the inetd configuration file. For example, an entry such as:

tftp  dgram  udp  wait  root  /usr/etc/tcpd  in.tftpd -s /tftpboot

Causes an incoming tftp request to run through the wrapper program (tcpd) with a process name in.tftpd.

Security Tasks


A few tools and sites are worth keeping in mind for a Linux administrator minding security. Websites worth monitoring for security issues and patches include:

* Security Focus news <http://www.securityfocus.com/>. The Security Focus website is one of the best sites for reporting and discussion of security issues and specific vulnerabilities. The site includes a number of newsletters and alertes you can subscribe to, as well as general columns and searchable bug reports.
* The Bugtraq mailing list <http://www.securityfocus.com/archive/1> is a full disclosure moderated mailing list for the detailed discussion and announcement of computer security vulnerabilities: what they are, how to exploit them, and how to fix them.
* CERT Coordination Center <http://www.cert.org/>. Hosted by Carnegie Mellon University, CERT has a similar range of advisories as Security Focus site, with a bit more emphasis on tutorials and guidelines. Keeping track of multiple such sites is a good way to make sure you are current on all the security incident affecting your OS, distribution, and specific tools or servers.
* Computer Incident Adisory Capability <http://www.ciac.org/ciac/index.html>. CIAC Information Bulletins are distributed to the Department of Energy community to notify sites of computer security vulnerabilities and recommended actions. Similarly, CIAC Advisory Notices serve to alert sites to severe, time-critical vulnerabilities and solutions to be applied as soon as is possible. CIAC Technical Bulletins cover technical security issues and analyses of a less time sensitive nature.
* Information on securing open mail relays <http://www.ordb.org/faq/>. A common vulnerability on systems with mail servers is failure to properly secure systems against malicious use by spammer and fraudulent mailers. The Open Relay Database provides both tutorials on security particular mail tools, open relay testing online tools, and a database of known problem servers that can be used to configure blacklists if site administrators so desire.

Tools to monitor security you might consider running are:

* Open Source Tripwire <http://sourceforge.net/projects/tripwire/>: A security and data integrity tool for monitoring and alerting on specific file changes.
* scanlogd <http://www.openwall.com/scanlogd/>: A TCP port scan detection tool.
* Snort <http://www.snort.org/>: Network intrusion prevention and detection, utilizing a rule-driven language. Uses signature, protocol and anomaly based inspection methods.