Sentry Firewall CD(tm) Reference and Usage Guide

Stephen A. Zarkos,

v0.7.0, 2004-08-30
This document is designed to be a comprehensive reference and usage guide for the Sentry Firewall CD. This document in some cases covers some of the same material covered in the Sentry Firewall CD HOWTO and FAQ, but its primary purpose is as a reference and supplement to the other documentation available for the Sentry Firewall CD.

1. Introduction

2. Overview of Available Configuration Directives

3. Directive Reference

4. Configuration Reference

5. The sentrycd(-devel) Branches

6. The sentrycd-deb(-devel) Branches

7. The Rootdisk

8. The Sentry Firewall Configuration Scripts

1. Introduction

This document is designed to be a more comprehensive reference guide to the Sentry Firewall CD. This is not designed to be a HOWTO, but instead to be a supplement to the HOWTO and the other available documentation. The following are some of the topics covered in this document:

If you would like to add anything to this document, or if you have any questions or comments please feel free to email me,

1.1 Further Reading

1.2 Copyrights and Disclaimer

The current copyright and disclaimer can be found on the website; It applies to the Sentry Firewall CD, and all the scripts and documentation associated with it.

2. Overview of Available Configuration Directives

This section provides a general overview of the directives that are supported by all the Sentry Firewall CD branches. Since the "host" distribution varies with each branch, the configuration and initialization files used by each branch will vary. The branch-specific directives will be covered in later sections.

2.1 Replacing Files

To replace a file that is supported by the configuration scripts, you may use the following syntax:

     filename_directive = /location/of/filename

Where "filename_directive" is one of the directives supported by the configuration scripts. The file location can reference a file on a floppy disk(ie. /floppy/config/filename) or can also be in a URI format. The supported prefixed include "http://", "https://", "ftp://", "sftp://",and "scp://". For example:

     snort.conf = /floppy/snort.conf
     snort.conf = scp://user:pass@<server>/config/snort.conf

The current 'sentry.conf' file for each branch is available in the "/SENTRY/scripts/cd-config/" directory on the ISO, or on the website. The default sentry.conf file for each branch should contain all the directives currently supported.

2.2 Copying Files

To replace files not supported by the configuration scripts, use the '|=' file copy directive.

     Syntax: source_file |= dest_file, OR
             dest_file = source_file

     Example:  Copy file /floppy/daemon.conf to /etc/daemon.conf

          /floppy/daemon.conf |= /etc/daemon.conf
          /etc/daemon.conf = /floppy/daemon.conf
          /etc/daemon.conf = scp://<user>:<pass>@<server>/config/daemon.conf
          scp://<user>:<pass>@<server>/config/daemon.conf |= /etc/daemon.conf

2.3 Making Symlinks

Create a symlink

     Syntax: dest_file => source_file(where the symlink points to)

          ## Make symlink called /etc/somefile.conf that points to /etc/otherfile.conf
          /etc/somefile.conf => /etc/otherfile.conf

2.4 Make a Directory.

Note: The 'mkdir' directive is only available with the Sentry Firewall CD versions 1.5.0-rc14 or newer.

          mkdir <PATH/DIRECTORY>[:MASK]

Make a directory in the specified location with the specified permissions(MASK). MASK is optional and defaults to 0755(rwxr-xr-x). This directive can be useful if you want to copy files at boot-time to a directory that does not exist on the ramdisk by default.

2.5 'device' Directive

Set up an ethernet device to use during configuration. This device will only be used during configuration to grab files via http(s)/ftp/sftp/scp and is taken down after configuration is complete.

     device[1..10] = [device_name]:[driver_name]:[IP_Address]<|gateway>
     device[1..10] = [device_name]:[driver_name]:dhcp<|hostname>

     NOTES: 1) <hostname> and <gateway> are optional, but sometimes required.

            2) Only one <gateway> can be declared, that is, you cannot set
               up more than one default gateway.

           *3) Devices set up with the 'device{1..10}' directive are TEMPORARY
               and are taken down after the configuration process is complete.
               See rc.inet1{.conf} for more permanent network setup.

            4) Please see file: /SENTRY/scripts/cd-config/ for
               list of supported devices.  Most 10/100BaseT ethernet devices
               should be supported.

          device1 = eth0:tulip:|
          device2 = eth1:via-rhine:dhcp

2.6 'nameserver' Directive

Set up a nameserver to use during configuration.

     Syntax: nameserver = <DNS_IP>

2.7 Proxy Support Directives

Set up a proxy for pulling files via http(s), or ftp.

          http_proxy = http://<hostname>/
          ftp_proxy = http://<hostname>/
          proxy-user = <PROXY_USER>
          proxy-passwd = <PROXY_PASSWORD>

2.8 Passive FTP Support

Use passive ftp instead of active ftp to retrieve files via ftp.

          passive-ftp = <on|off>  ## Default == off

2.9 'include' Directive

Retrieve and parse another 'sentry.conf' file.

          include = </location/of/sentry.conf>

          Or, with network support -


2.10 The 'path<#>' Directive.

Note: The 'path<#>' directive is only available with the Sentry Firewall CD versions 1.5.0-rc13 or newer.

Path statements tell the configuration scripts where to look for files. These can specify a path on a local or remote system. The variables "path1" to "path10" are allowed.

          path<#> = <PATH>
          path<#> = <URI>

NOTE: <URI> should point to a directory on a remote system, NOT just a file.

          path1 = /floppy/node1/config/
          path2 = scp://user:pass@someserver/node123/config/
          path3 = http://user:pass@someserver/node123-backup/config/
          etc etc...

You may then use the following syntax when declaring a file within your sentry.conf:

          squid.conf = squid.conf
          /etc/someconf.conf = someconf.conf

The configuration scripts will first look for "squid.conf" or "someconf.conf" in $m_point, which is usually /floppy. If it isn't found, then the system will try path1..path10 in order until "squid.conf" or "someconf.conf" is found. This not only makes for less typing when creating your sentry.conf, but it also allows you to add some redundancy to the configuration process.

2.11 'cdrom' Directive

Defines which device the CDROM is. If not declared the configuration scripts will still try to probe for and mount the CD. But declaring this is much easier/faster/safer.

          cdrom = <DEVICE>

          cdrom = /dev/hdc

2.12 'cron' Directive

Replace a user's crontab file.


2.13 'hostname' Directive

Defines the hostname of the local machine. This directive can be used to either point to a file containing the hostname of the local machine, or to simply define the hostname itself.

          hostname = </path/to/file>
          hostname = MYHOSTNAME

2.14 The 'add_swap' directive

Note: The 'add_swap' directive is only available with the Sentry Firewall CD versions 1.5.0-rc11 or newer.

The 'add_swap' directive tells the configuration scripts to add a swap partition at configuration time. If the ":format" option is appended to the variable, then the configuration scripts will also format the partition before activating it.

Warning: An improper setting of this variable could cause serious damage to data.

     add_swap = /dev/hda1
     add_swap = /dev/hda1:format

2.15 The 'root_size' directive

Note: The 'root_size' directive is only available with the Sentry Firewall CD versions 1.5.0-rc11 or newer.

The 'root_size' directive allows one to change size of root(/) at configuration time(before any other files are copied). By default the root filesystem is around 18MB in size. This option allows you to change the size of the root filesystem if you need more/less space. Also - since root is mounted on a tmpfs filesystem - this area can be swapped out as needed. The suffix g, m, or k is accepted for binary kilo, mega and giga. If no suffix is added, a size in megabytes is presumed.

     root_size = "18M"

The size of the root file system can also be changed after configuration by simply remounting it, ie "mount -oremount,size=24M /"

3. Directive Reference

The following is a list of file directives currently supported by the various branches of the Sentry Firewall CD. Please note that this information is highly subject to change. Currently supported configuration directives can be found in the sample sentry.conf file available on the ISO image in the "SENTRY/scripts/cd-config" directory, or online at

3.1 SENTRYCD(-DEVEL) Branches

The following is a list of file directives currently supported in the SENTRYCD and SENTRYCD-DEVEL branches(slackware-based).

Initialization scripts:

   rc.M                         Multiuser init script(runlevel 3).
   rc.6                         Halt or reboot
   rc.dhcpd                     Startup script for ISC DHCP daemon.
   rc.netdevice                 Load modules for network devices(before rc.inet1 is run).
   rc.inet1                     Set up ethernet interfaces.
   rc.inet1.conf                Configuration file for rc.inet1.
   rc.inet2                     Start network daemons.
   rc.inet2.conf                Configuration file for rc.inet2.
   rc.keymap                    Load keyboard map.
   rc.local                     Local system init script.
   rc.modules                   Load any needed modules.
   rc.firewall                  Firewall script.
   rc.firewall.nat              Nat-specific firewall script(not always used).             Use if firewall script was generated using iptables-save.
   rc.ntpd                      ntpd initialization script.
   rc.sendmail                  Sendmail initialization script.
   rc.snort                     Snort initialization script.

Important system configuration files:

   fstab                        fstab(5)
   ftpusers                     ftpusers(5)
   group                        group(5)
   hosts                        hosts(5)
   hosts.equiv                  hosts.equiv(5)
   hosts.allow                  hosts_access(5)
   hosts.deny                   hosts_access(5)
   inittab                      inittab
   modules.conf                 modules.conf(5)
   openssl.cnf                  OpenSSL configuration file.
   passwd                       passwd(5)
   profile                      bash(1)
   resolv.conf                  resolv.conf(5)
   shadow                       shadow(5)
   shells                       shells(5)

Daemon configuration files:

   bgpd.conf                    Configuration file for bgpd(
   dhcpd.conf                   Configuration file for dhcpd.
   dnsmasq.conf                 Configuration file for dnsmasq.
   httpd.conf                   Configuration file for Apache HTTP Daemon(
   inetd.conf                   Configuration file for inetd(8).
   ipsec.conf                   Configuration file for ipsec(
   ipsec.secrets                IPSec secrets file for IKE/IPsec authentication.
   named.conf                   Configuration file for named(8).
   l2tpd.conf                   Configuration file for l2tpd, Layer 2 Tunnelling Protocol Daemon(
   newsyslog.conf               Configuration file for newsyslog.
   ospfd.conf                   Configuration file for ospfd(
   portsentry.conf              Configuration file for portsentry.
   pppoe.conf                   Configuration file for RP-PPPOE(
   pptpd.conf                   Configuration file for pptpd(
   proftpd.conf                 Configuration file for proftpd(
   ntp.conf                     Configuration file for ntpd.
   rinetd.conf                  Configuration file for rinetd.
   ripd.conf                    Configuration file for ripd(
   rndc.conf                    Configuration file for named control utility, rndc(8).                  Configuration file for sendmail(
   smb.conf                     Configuration file for Samba(
   snort.conf                   Configuration file for snort(
   squid.conf                   Configuration file for squid(
   ss5.conf                     Configuration file for Socks Server 5(
   stunnel.conf                 Configuration file for stunnel(
   stunnel.pem                  Certificate chain PEM file for stunnel.
   syslog.conf                  syslogd(8) configuration file.
   syslog-ng.conf               Configuration file for syslog-ng(
   vsftpd.conf                  Configuration file for vsftp daemon(
   wlan.conf                    Configuration for prism based wireless cards(
   gated.conf                   Configuration for GateD.
   ulogd.conf                   Configuration file for ulogd.
   zebra.conf                   Configuration file for zebra(

OpenSSH configuration files:

   ssh_config                   Configuration file for ssh(1).
   sshd_config                  Configuration file for sshd(8).
   shosts.equiv                 Like hosts.equiv(5) but for ssh.
   ssh_host_key                 Private rsa1 host key file.             Private rsa1 host key file.
   ssh_host_dsa_key             Private rsa1 host key file.         Private rsa1 host key file.
   ssh_host_rsa_key             Private rsa1 host key file.         Private rsa1 host key file.
   ssh_known_hosts              Public host keys of known ssh servers.
   ssh_known_hosts2             Public host keys of known ssh servers.

Configuration directives for Webmin(

   start_webmin                 enable|disable Webmin.  Default is "disable".
   webmin_config                Main Webmin configuration file(/etc/webmin/config).
   miniserv.conf                Config file for Webmin http(s) daemon.
   miniserv.pem                 SSL cert for Webmin http(s) daemon.
                                An SSL cert will be created by rc.webmin if
                                one is not specified.
   miniserv.users               Password file used for Webmin.
                                Default user:pass is sentry:SENTRY.
                                NOTE: If this file is not replaced webmin
                                will NOT start.

Misc. configuration directives.

   cron:<user>                       Replace <user>'s crontab with specified file.
   add_swap                     Initialize a swap partition at configuration time.
   root_size                    Change the size of root(/) at configuration time.

3.2 SENTRYCD-DEB(-DEVEL) Branch Directives

The following is a list of file directives currently supported in the SENTRYCD-DEB and SENTRYCD-DEB-DEVEL branches(debian-based).


4. Configuration Reference

This section contains general information about how to configure the Sentry Firewall CD, including information on which files to edit and how to setup specific daemons and services on a running system.

More information about configuring specific daemons or services can be found in the HOWTO.

4.1 Init script configuration

This section covers specifically the configuration of the system via the various init scripts kept in /etc/rc.d/(Slackware) or /etc/init.d/(Debian).

rc.inet1 and rc.inet1.conf

NOTE: This section applies to the SENTRYCD and SENTRYCD-DEVEL(Slackware-based) branches.

Versions of the Sentry Firewall CD before 1.5.0-rc7 utilized a perl-based rc.inet1 script to add and configure interfaces. The syntax of this file was similiar to the syntax used in the sentry.conf file for network configuration support -

   $interface(1..10) = "<IF>:<IP ADDRESS[/NETMASK]>|<DHCP>"

## Examples:
   $interface1 = "eth0:";        ## Set up eth0 with ip and
                                               ## netmask
   $interface2 = "eth0:";        ## Bind second IP to eth0.
   $interface3 = "eth1:dhcp";                  ## Use DHCP to set up eth1.

As you can see, the syntax is fairly simple. You can configure an interface or add an IP address by adding a "$interface#" variable.

As of version 1.5.0-rc7, the Sentry Firewall CD utilizes a modified version of the rc.inet1 and rc.inet1.conf init files that appeared in Slackware 9.1. Once again, you alter the network setup by altering a bunch of variables, except this time you edit /etc/rc.d/rc.inet1.conf instead of rc.inet1 itself. By default, this file supports the setup of up to four ethernet devices. A basic entry to set up eth0 looks something like the following:

   ETH0_ALIAS[0]=""        ## For multiple IPs on interface.
   ETH0_ALIAS[1]=""        ## Ditto.


To use DHCP instead of static IP addresses, set the "USE_DHCP[0]" directive to "yes". You may also add any number of additional "alias" IP addresses to each interface with the "ETHx_ALIAS[#]" variable.

NOTE: Keep in mind that if neither of these rc files suit your needs you may simply replace the rc.inet1 file at boot time with your own script by using the 'rc.inet1' directive in your sentry.conf file.

4.2 Further Reading

If there is something you wish to add to this list or to the documentation in general, please feel free to email

5. The sentrycd(-devel) Branches

This section contains information about features or directives that are unique to the SENTRYCD and SENTRYCD-DEVEL(slackware-based)branches.

This section is still under construction.

6. The sentrycd-deb(-devel) Branches

This section contains information about features or directives that are unique to the SENTRYCD and SENTRYCD-DEVEL(slackware-based)branches.

This section is still under construction.

6.1 Start/Stop a Service or Daemon

This directive gives you the ability to start or stop a service at bootup. The syntax looks like the following:

     service:[start|stop] = <path/to/service_init_file>

For example:

     httpd:start = /floppy/config/httpd

In the above example, we are telling the Sentry Firewall CD to either start or stop the http daemon at bootup. The optional argument "<path/to/service_init_file>" is usually not necessary, but is used to actually replace the startup script located in /etc/rc.d/init.d/, in case you ever wanted to do so.

7. The Rootdisk

7.1 Overview

A rootdisk is a gzip-compressed RAMdisk image that is mounted as root(/) during the boot process. Currently, the rootdisk for the Sentry Firewall CD is around 16-18 MB in size, uncompressed.

It is, of course, possible to increase the size of the rootdisk to accommodate your needs. When you increase the size of this image you also need to increase the "ramdisk_size" parameter passed to the kernel at boot time by syslinux. This parameter can be adjusted either manually at the initial boot prompt or in the "isolinux.cfg" file kept in the isolinux directory on the CD. If the uncompressed ramdisk size is larger than this parameter, the boot process will not continue passed the loading of the kernel. That is, root(/) will never be mounted and you will get a kernel panic.

As of version 1.5.0-rc11 the Sentry Firewall CD utilizes a tmpfs file system for its root partition. The tmpfs file system, also known as "virtual memory file system" or "shm fs", provides two major advantages:

With this new development it is now possible to resize root(/) at configuration time(or any time afterward) without rebuilding the rootdisk or ISO. Please see the "root_size" and "add_swap" configuration directives for more details.

7.2 Creating the rootdisk

I use a script called ''. This is a bash shell script that formats/mounts the disk image, and then creates or copies the files to the disk image as needed.

If you would like to attempt to use the script please be sure to read through it first, as it tends to be a bit hacky at times. It runs perfectly on my development system, but may not run well at all on yours. The output from the script should look something like the following:

   Sentry Firewall CD-ROM:
   Copyright (C) Stephen A. Zarkos,
   Ok, let's get to it...

   [+] Creating /root/rootdisk/root... Done.
   [+] Ok, starting to copy stuff to the rootdisk...
   [+] Making directories: root dev proc etc sbin bin lib mnt mnt1 mnt2 mnt3 mnt4 opt cdrom floppy tmp
   tmp/drivers var initrd... Done.
   [+] Copying /dev files... Done.
   [+] Working in /var... Done.
   [+] Working in /home... Done.
   [+] Working in /bin... Done.
   [+] Working in /sbin... Done.
   [+] Working in /lib... Done.
   [+] Working in /etc... Done.
   [+] Building drivers-2.4.tar.gz(network config support).
       [+] Using /cdrom/lib/modules/2.4.25GENERIC.
   [+] Tar/Gzipping /root/rootdisk/root... Done.
   [+] Zeroing out file: /root/rootdisk/initrd.img... Done.
   [+] Creating ext2 file system on /root/rootdisk/initrd.img... Done.
   [+] Mounting initrd.img on /root/rootdisk/mnt... Done.
   [+] Copying files to rootdisk... Done.

   [+] /root/rootdisk/initrd.img is still mounted, do you want me
       to unmount it? (y/n) y
       [+] Unmounting /root/rootdisk/mnt... Done.
       [+] Gzipping /root/rootdisk/initrd.img... Done.

   Location of new rootdisk --> /root/rootdisk/initrd.img

The finished ramdisk image is then copied to the isolinux directory before creating the actual ISO image.

7.3 Editing the rootdisk

To look at and modify the initrd.img image, do something like the following:

blah@wherever:~$ cp /cdrom/isolinux/initrd.img /tmp/initrd.img.gz
blah@wherever:~$ gzip -d /tmp/initrd.img.gz
blah@wherever:~$ mount -o loop /tmp/initrd.img /MOUNT_POINT

You may then cd to /MOUNT_POINT and edit the files on the rootdisk. Once you are finished you can then unmount and gzip the initrd.img file and place it back in the isolinux directory.

7.4 Details of the rootdisk layout

The following are some notes about the layout and design of the rootdisk.

8. The Sentry Firewall Configuration Scripts

This section is designed as a technical outline of the configuration scripts built for the Sentry Firewall CD. These configuration are responsible for finding and parsing the sentry.conf file and the directives contained therein.

As of version 1.5.0-rc14 there are six configuration scripts overall on the system. These are kept in the "/etc/rc.d/SENTRY/" or the "/etc/init.d/SENTRY/" directory on the rootdisk. They are also available on the CD in the "<CDROM>/SENTRY/scripts/cd-config/" directory or online. The configuration scripts are called `', `', `', `', `', and `'. The details of these files are outlined below.


This is the first perl script to run. It is usually called from the rc.S or rcS file, depending on the branch. This file contains the following:


This file contains the following:


This file contains the following:


In general, this is the only configuration file you would need to modify if you would like to create a Sentry Firewall CD for any Linux distribution. This file contains the following:


This script contains a number of general functions that are used throughout the other configuration scripts. Most of the functions are related to working with files and directories. Some of these were contained in in earlier versions of the CD.


This file contains the following: