"Pocket" ISP based on RedHat Linux

Anton Chuvakin, anton@chuvakin.org

v1.0.1 4 April 2000

This document outlines the setup of a single RedHat box for dialins, virtual web hosting, virtual email, POP3 and ftp servers. Why anybody might need this in one box is beyond the scope of this document. The idea is a complete ISP solution based on RedHat Linux. Any part of this setup can be implemented separately though. I will try to emphasize all the commands so one can just paste them to configure his own box. The list of documents that I borrowed from and some further reading is below (see References section). I will try to keep security in mind on all stages of the setup and to make clear all the security limitations of this setup. I should add that assets that are to be protected in this case are considered not very valuable (e.g. personal pages etc) thus efforts spent on securing the setup are allowed to be limited.

1. Introduction

2. Changes from 1.0.0 to 1.0.1

3. TODO Tue Apr 4 15:23:11 EDT 2000

4. Step by step guide

5. Conclusion

6. References

1. Introduction

The guide assumes some familiarity with Linux functionality and general Linux/UNIX setup procedure (although not very detailed). Fully functional brain is also required for some stages of the procedure. All setup would be done manually (without the use of linuxconf or other tools). Not that those are bad or that there is anything wrong with them. The reasons for that are: 1) it is comparatively hard to give step by step directions that produce predictable results as these tools pretend they are intelligent and "know better" (Windows syndrome) 2) layout of tools changes with time and is different in some distributions 3) manual setup gives better understanding of system works (not that it is always required though) 4)some tools allow only limited configuration of Linux system or do not keep up with updated features. While many improvements are possible to this setup they might be described in later editions of this document - I just outline one possible way (accidentally, the one I used). The writeup is aimed at RedHat Linux, but with trivial changes can be used on any modern Linux distribution. The resulting configuration loosely follows the setup of some particular machines built by the author..

2. Changes from 1.0.0 to 1.0.1

3. TODO Tue Apr 4 15:23:11 EDT 2000

3.1 New versions of this document

New versions of this document can be found at


3.2 Feedback

All comments, error reports, additional information (very much appreciated!!!) and criticism of all sorts should be directed to: anton@chuvakin.org


My PGP key is located at http://www.chuvakin.org/pgpkey

Please direct spelling error comments to your friendly local spell checker.

3.3 Standard disclaimer

No liability for the contents of this document can be accepted. Use the concepts, examples and other content at your own risk. Additionally, this is an early version, with many possibilities for inaccuracies and errors.

One of many possible setups will be described. In the Linux world, there is usually a number of ways in which to accomplish things.

As far as I know, only programs that under certain terms may be used or evaluated for personal purposes will be described. Most of the programs will be available complete with source under GNU-like terms.

3.4 Copyright information

This document is copyrighted (c) 2000 Anton Chuvakin and distributed under the following terms:


4. Step by step guide

Ingredients needed:

Here follows the procedure:

4.1 Get RH

Purchase or otherwise procure the RedHat 6.1 (further referred as RH, latest version number is 6.1 at the time of writing) distribution and compatible hardware. One can get a full RH CDROM for about $2.00 including shipping and handling at http://www.cheapbytes.com. This version will not contain such luxuries as secure web server and extra software. For those you should turn to RedHat website. Or probably buying the PC with Linux RH pre-installed is an option for some.

4.2 Install RH

Install the RH following the *instructions on the package* (might be added here later). CDROM install is very easy to perform. I suggest using text-mode setup, in my case their new graphical one failed miserably. When asked about the installation type (Server/Workstation/Custom) choose Server or Custom (if you know what you are doing)-you can always add software later. Some other important installation decisions are outlined further.

4.3 Some install tips

If you hardware really is compatible the installation process will detect and configure it correctly. Otherwise, refer to corresponding documentation for troubleshooting network card, modem, video card, etc problems (mostly HOWTOs and mini-HOWTOs, some are in References section below).

If you network card is detected properly you will be asked for an IP address of your machine, gateway address and network mask and the address of the DNS server (might be your own machine if you plan to set it up this way). Have all this info handy. Also you will be asked for a machine name and domain name. We will use a sample domain name you.com and the machine will be named ns (that gives us a fully qualified domain name (FQDN) ns.you.com). You should use whatever domain you registered (see Setting Up Your New Domain Mini-HOWTO, link in References section below) and intend to use as you primary domain (not a virtual). For the gateway address we will use a sample 111.222.333.111 address. Gateway is likely the router that connects your machine (or your LAN) to the outside world.

Enable shadow and MD5 passwords for greater security. First of those makes the file that contains encrypted passwords readable only to root user and the second allows longer and harder to crack passwords. As it will be a standalone machine do not enable NIS/NFS.

After installation finishes and machine reboots you will see the login prompt. Enter login and password (for the root account) and start configuring you new Linux station.

4.4 Some preliminary security configuration

First (and fast), add a line: ALL:ALL to your /etc/hosts.deny file. That would (to some known extent) prevent other people from accessing your machine while you are doing the configuration. That will also prevent you from doing the same. For further configuration efforts (that can be done remotely, by the way) secure shell is recommended. Download the RPM package for RH from one of the many sites and install it (as root) using: rpm -U ssh*rpm or similar command (depends upon the version). You will have to get both client and server packages (if you want to ssh from this machines as well as to this machine). Upon installation all necessary post-installation commands (like server key generation) are run automatically by the RPM package. You will have to start server manually using command /etc/rc.d/init.d/sshd start.Some early versions of ssh-1 and also all versions of ssh-1 compiled with RSAREF library contain a buffer-overflow bug. Use ssh-2 or the latest version of ssh-1 without RSAREF. If you do this you will have to allow access using ssh from some trusted machine (described later) in /etc/hosts.allow file.

If you want to be really rigorous in you configuration pursuits go to single use mode by giving the command init 1, in this case all work is to be done locally and you would not be able to test you network-related configuration as network is not available in this mode.

To further enhance your security ipchains software (that is usually part of your Linux distribution) can be used (for that refer to IPCHAINS HOWTO, link in References). It takes quite a bit more efforts to configure it than TCP wrappers, although some automated tools are available for that too.

4.5 Remove unnecessary services

Now lets deal with unnecessary services. Please note that my idea of "unnecessary" might not be 100% same as yours.

  1. Services started from /etc/inetd.conf:

    comment out all the lines, but those

    ftp     stream  tcp     nowait  root    /usr/sbin/tcpd  in.ftpd -L -l -i -a
    telnet  stream  tcp     nowait  root    /usr/sbin/tcpd  in.telnetd
    Check this by using the command: grep -v '\#' /etc/inetd.conf

    If you will be using the secure shell (ssh), telnet is also not necessary and can be removed. Secure shell can either be started as a daemon on system startup or as a service from /etc/inetd.conf. Default configuration (used by the RPM package) is to start is as a daemon. Sshd can be compiled to refer to /etc/hosts.allow file for access control. In this case, while you will not have it in your /etc/inetd.conf, it will still use the settings from /etc/hosts.allow and /etc/hosts.deny. The advantages of this method is faster connection as the sshd will not have to regenerate server key every time somebody connects. On the other hand, if you start it from /etc/inetd.conf it will be more isolated from the outside world. More lines will be added to /etc/inetd.conf as necessary (POP3 is one of those).

  2. Services started on system startup from /etc/rc.d directory:

    Check what services are running by using: ps ax. You will get something similar to the sample output below:

        1 ?        S      0:04 init
        2 ?        SW     0:30 [kflushd]
        3 ?        SW     0:32 [kupdate]
        4 ?        SW     0:00 [kpiod]
        5 ?        SW     0:03 [kswapd]
        6 ?        SW<    0:00 [mdrecoveryd]
      296 ?        SW     0:00 [apmd]
      349 ?        S      0:00 syslogd -m 0
      360 ?        S      0:00 klogd
      376 ?        S      0:00 /usr/sbin/atd
      392 ?        S      0:00 crond
      412 ?        S      0:00 inetd
      454 ttyS0    S      0:00 gpm -t ms
      533 tty2     SW     0:00 [mingetty]
      534 tty3     SW     0:00 [mingetty]
      535 tty4     SW     0:00 [mingetty]
      536 tty5     SW     0:00 [mingetty]
      537 tty6     SW     0:00 [mingetty]
      667 tty1     SW     0:00 [mingetty]
     4540 ?        S      0:00 httpd
     5176 ?        S      0:00 httpd
     5177 ?        S      0:00 httpd
     5178 ?        S      0:00 httpd
     5179 ?        S      0:00 httpd
     5180 ?        S      0:00 httpd
     5181 ?        S      0:00 httpd
     5182 ?        S      0:00 httpd
     5183 ?        S      0:00 httpd
     7321 ?        S      0:00 /usr/sbin/sshd                 <<<<< only after you installed sshd to run on startup
     7323 pts/0    S      0:00 -bash
     7336 pts/0    R      0:00 ps ax

    Lets concentrate on processes that listen to network, such as lpd. Since we do not plan to use our server for printing (we sure might, I just don't describe it here), I suggest we remove the printer daemon by: rpm -e lpd . If rpm complains about any dependencies (like, in my case, printfilter and rhprinttool), add them to your rpm -e command and repeat it. Other services that might be removed are NFS, NIS, samba etc, if they got installed by mistake. Again, these are useful things, I am just following the *golden rule* "remove the software you don't currently use". And, with RH RPM it is really easy to add it any time in the future.

Some more basic security settings can be obtained from Armoring Linux paper. As suggested there, lets make a wheel group with trusted users (in our case, only user youwill be able to do /bin/su and to run cron jobs (together with root).

I suggest you do not install X Windows as it will bring new concern that you might not be prepared to deal with.

4.6 Enable multiple IP addresses

Now we are ready to enable our machine to handle multiple IP addresses for virtual hosting. At that point, the IP Aliasing HOWTO might come handy (see link in References). For several reasons, IP-based virtual hosting is better (if you have enough IP addresses, that is). For instance, reverse lookups would succeed, if done from the browser side. It might also be needed for hosting cryptographically enabled websites (commonly known as "secure websites"). Older browsers (not supporting HTTP 1.1) will get unhappy too.

The changes would be concentrated in /etc/rc.d/ directory. To enable multiple IP addresses you kernel should support this. On a freshly installed RH Linux it does. To verify it one should look into the config file that was used to compile the kernel. In my case, it was /usr/src/linux/configs/kernel-2.2.12-i686.config since the machine has Pentium II processor. This file exists, if the kernel-source RPM package was installed. If line CONFIG_IP_ALIAS=y is present in the file than you are OK. While we are here, we can also confirm the ability to forward IP packets (needed for dialup users PPP). This ability is present, but not turned on by default. Also needed is the support for PPP protocol (line CONFIG_PPP=m, this means PPP support is compiled as a kernel loadable module, CONFIG_PPP=y is also OK)

The examples will use the ridiculous IP addresses 111.222.333.444-111.222.333.777 from C block 111.222.333.0. 111.222.333.444 is a real host IP (that is configured during RH installation), 111.222.333.555-777 are virtual addresses and 111.222.333.888 is a dialin user address (can be more of those).

Lets assume we want to configure 3 virtual hosts.

Two sets of commands will be used:

  1. /sbin/ifconfig eth0:0 111.222.333.555
    /sbin/ifconfig eth0:1 111.222.333.666
    /sbin/ifconfig eth0:2 111.222.333.777

    These will bind the IP addresses to (virtual) interfaces eth0:0-eth0:2.

  2. /sbin/route add -host  111.222.333.555 dev eth0
    /sbin/route add -host  111.222.333.666 dev eth0
    /sbin/route add -host  111.222.333.777 dev eth0

    These commands will add routes for those addresses and connect those to real interface eth0 (ethernet card).

After doing them the ifconfig command output (ifconfig) will look like this:
eth0      Link encap:Ethernet  HWaddr 02:60:8C:4D:24:CE  
          inet addr:111.222.333.444  Bcast:  Mask:
          RX packets:901597 errors:33 dropped:0 overruns:0 frame:823
          TX packets:433589 errors:0 dropped:0 overruns:0 carrier:0
          collisions:128327 txqueuelen:100 
          Interrupt:5 Base address:0x280 

eth0:0    Link encap:Ethernet  HWaddr 02:60:8C:4D:24:CE  
          inet addr:111.222.333.555  Bcast:111.222.333.255  Mask:
          Interrupt:5 Base address:0x280 

eth0:1    Link encap:Ethernet  HWaddr 02:60:8C:4D:24:CE  
          inet addr:111.222.333.666  Bcast:111.222.333.255  Mask:
          Interrupt:5 Base address:0x280

eth0:2    Link encap:Ethernet  HWaddr 02:60:8C:4D:24:CE  
          inet addr:111.222.333.777  Bcast:111.222.333.255  Mask:
          Interrupt:5 Base address:0x280

lo        Link encap:Local Loopback  
          inet addr:  Mask:
          UP LOOPBACK RUNNING  MTU:3924  Metric:1
          RX packets:26232 errors:0 dropped:0 overruns:0 frame:0
          TX packets:26232 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
All commands can be added to the bottom of /etc/rc.d/rc.local so that the changes are saved after reboot. Strictly speaking, rebooting machine is not required for adding new IP addresses. Please, do document all changes you do to your machines. Many a good sysadmin (or, should I say not-so-good?) were burned on that at some point in their careers.

4.7 Configure DNS

Now we are ready to configure DNS. The easiest way would be to add the hostnames (real and all the virtual) that we want to be seen by the world to the configuration of some machine that already has bind (DNS daemon) running. But, since we are setting up ISP-in-a-box we might not be able to avoid "DNS fun".

Now, let me also try to defend the (well, questionable) choice of "outdated" version of bind 4.9.7 (last of the pre-8 series). I know that my arguments can be beaten, but I consider bind 4.9.7 much more time-tested and stable. The arguments for upgrading to 8.x (provided http://www.acmebw.com/askmrdns/00444.htm and http://www.dns.net/dnsrd/servers.html and, I guess, at many other places) still didn't seem to convince many people. And, lets not forget the "exploit of 1999" - ADMROCKS, that gives remote root access to almost any Linux machine running bind prior to 8.1.2 patch 3. Judging by the INCIDENTS mailing list, this is still a very popular way to attack RH versions 5.0-6.1 if no recommended upgrades are installed.

Here are the instructions, loosely following the DNS book from O'Reily (a good one, highly recommended to all, but very casual DNS user).

  1. Find and install bind 4.9.7 either from RPM package (RH 4.2, if I am not mistaken - for that you can use RPMFIND.net, personally I didn't try this and so I am somewhat skeptical about installing RH 4.2 package on RH 6.1 system, but it might work) or from source ( bind 4.9.7, compiling it is a bit troublesome, but reading all the README files in the archive will definitely help)
  2. Create files and directories needed for bind: This will be used for 3 virtual domains: yoursite1.com,yoursite2.com and yoursite3.com. One more important comment refers to secondary DNS issue. As all your domains and all their services will be hosted on the same machine, DNS backup in the form of secondary server doesn't make much sense: if your primary DNS is down everything else (mail, www, ftp, pop, etc) is down as well. But you do have to have a secondary DNS to register a domain. Try to convince somebody to put you in as a secondary or use a free DNS service (link is in Setting Up Your New Domain Mini-HOWTO).
  3. That is how they look like (if you are unfamiliar with bind 4.x configuration file format, please, do read either the O'Reily DNS book or any of the HOWTOs or documents at bind pages, or, better, all of the above. You also have an option of using them without understanding, but this is a bad idea in general):


    This is the main config file for bind 4.9.x.

    directory /etc/namedb
    ;cache-obtained from internic, usually
    cache   .                               db.cache
    ;main config files
    primary you.com                         db.you
    ;reverse lookups
    primary 333.222.111.in-addr.arpa        db.111.222.333
    ;localhost.localnet configs
    primary 0.0.127.in-addr.arpa            db.127.0.0
    ;virtual Domains
    primary yoursite1.net                     db.yoursite1
    primary yoursite2.net                     db.yoursite2
    primary yoursite3.net                     db.yoursite3
  4. /etc/namedb/db.you

    ; defines our local hosts at you.com, just one in our case, and its aliases
    @                       IN      SOA     ns.you.com. root.ns.you.com. (
                                            2000012190 7200 1800 3600000 7200 )
    ;name servers and mail servers
                            IN      NS      ns.you.com.
                            IN      MX      10 ns.you.com.
                            IN      A       111.222.333.444
    ns                      IN      A       111.222.333.444
    ;address of the canonical names
    localhost               IN      A
    gateway                 IN      A       111.222.333.111
    ;aliases (to use in ftp: ftp ftp.you.com etc, for clarity)
    www                     CNAME   ns
    mail                    CNAME   ns
    ftp                     CNAME   ns
    pop3                    CNAME   ns
  5. /etc/namedb/db.111.222.333

    ;reverse mapping of our IP addresses
    ;origin is 333.222.111.in-addr.arpa
    333.222.111.in-addr.arpa. IN    SOA     ns.you.com. root.ns.you.com. (
                                            1999121501 7200 1800 3600000 7200 )
    ;name Servers
                            IN      NS      ns.you.com.
    ;addresses point to canonical name
    444.333.222.111.in-addr.arpa.   IN      PTR     ns.you.com.
    888                             IN      PTR     dialup.you.com.
    ;virtual hosts
    555                             IN      PTR    yoursite1.com.
    666                             IN      PTR    yoursite2.com.
    777                             IN      PTR    yoursite3.com.
  6. /etc/namedb/db.127.0.0

    ;local loop config file
    0.0.127.in-addr.arpa.   IN      SOA     ns.you.com. root.ns.you.com. (
                                            1997072200 7200 1800 3600000 7200 )
                            IN      NS      ns.you.com.
    1                       IN      PTR     localhost.
  7. /etc/namedb/db.yoursite1

    ; yoursite1.com
    @                       IN      SOA     virtual root.virtual (
                                            1999092201      ; Serial: update each time the file is changed
                                            7200            ; refresh, sec
                                            1800            ; retry, sec
                                            3600000         ; expire, sec
                                            7200 )          ; minimum TTL
    ;name Servers
                            IN      NS      ns.you.com.
                            IN      MX      10 virtual
                            IN      A       111.222.333.555 
    ;address of the canonical names
    localhost               IN      A
    gateway                 IN      A       111.222.333.111
    virtual                 IN      A       111.222.333.555
                            IN      MX      10 virtual
    www                     CNAME   virtual
    mail                    CNAME   virtual
    ftp                     CNAME   virtual
    pop3                    CNAME   virtual
  8. /etc/namedb/db.yoursite2

    ; yoursite2.com
    @                       IN      SOA     virtual root.virtual (
                                            1999092201      ; Serial: update each time the file is changed
                                            7200            ; refresh, sec
                                            1800            ; retry, sec
                                            3600000         ; expire, sec
                                            7200 )          ; minimum TTL
    ;name Servers
                            IN      NS      ns.you.com.
                            IN      MX      10 virtual
                            IN      A       111.222.333.666 
    ;address of the canonical names
    localhost               IN      A
    gateway                 IN      A       111.222.333.111
    virtual                 IN      A       111.222.333.666
                            IN      MX      10 virtual
    www                     CNAME   virtual
    mail                    CNAME   virtual
    ftp                     CNAME   virtual
    pop3                    CNAME   virtual
  9. /etc/namedb/db.yoursite3

    ; yoursite3.com
    @                       IN      SOA     virtual root.virtual (
                                            1999092201      ; Serial: update each time the file is changed
                                            7200            ; refresh, sec
                                            1800            ; retry, sec
                                            3600000         ; expire, sec
                                            7200 )          ; minimum TTL
    ;name Servers
                            IN      NS      ns.you.com.
                            IN      MX      10 virtual
                            IN      A       111.222.333.777 
    ;address of the canonical names
    localhost               IN      A
    gateway                 IN      A       111.222.333.111
    virtual                 IN      A       111.222.333.777
                            IN      MX      10 virtual
    www                     CNAME   virtual
    mail                    CNAME   virtual
    ftp                     CNAME   virtual
    pop3                    CNAME   virtual
These configuration files will allow you to host these three virtual domains and you real domain you.com.

4.8 Configure httpd

To server html pages httpd daemon is used. RH 6.1 comes with Apache 1.3.9 (latest version is currently 1.3.12). At that point it is wise to check RH site or its mirrors ( RH Mirrors) for updates.

Most changes that we are about to make concentrate in /etc/httpd/httpd.conf (RH standard location for Apache configuration). Default location for html pages (shown when you go to www.you.com) is /home/httpd/html. You can allocate directories for virtual hosts within the same /home/httpd, shown below are the following locations for them: /home/httpd/yoursite1, /home/httpd/yoursite2 and /home/httpd/yoursite3.

Below I provide the minimum necessary changes for your /etc/httpd/httpd.conf file:

<VirtualHost 111.222.333.555>
ServerAdmin webmaster@you.com
DocumentRoot /home/httpd/yoursite1
ServerName www.yoursite1.com
ErrorLog yoursite1-error_log
TransferLog yoursite1-access_log

<VirtualHost 111.222.333.666>
ServerAdmin webmaster@you.com
DocumentRoot /home/httpd/yoursite2
ServerName www.yoursite2.com
ErrorLog yoursite2-error_log
TransferLog yoursite2-access_log

<VirtualHost 111.222.333.777>
ServerAdmin webmaster@you.com
DocumentRoot /home/httpd/yoursite3
ServerName www.yoursite3.com
ErrorLog yoursite3-error_log
TransferLog yoursite3-access_log

That configuration will cause all logs to be stored in one directory (whatever is specified as such) for all sites. If that is not desired the ErrorLog and TransferLog directives can be changed to point to the proper location separately for each virtual host. The pages for the "real" www.you.com will be stored in default location /home/httpd/html.

For more information, look at http://www.apache.org, Apache http server homepage. They have a lot of support pages, including those for virtual hosting setup (both IP-based and name-based [uses just 1 IP address]). Also useful is Linux WWW HOWTO (link in References section), section on virtual hosting.

4.9 Configure sendmail

Now we will deal with sendmail. Again, proposed are the minimum necessary changes to the stock RH /etc/sendmail.cf and /etc/sendmail.cw.

  1. look for the lines that starts from Dj$w.foo.com and change it to point to your main ("real", not virtual) server name (you.com, so it will looks like this Dj$w.you.com).
  2. locate file /etc/sendmail.cw and make it look like this
    # sendmail.cw - include all aliases for your machine here.
    These are necessary so that sendmail accepts mail for these domains.

This does not address the issue of user@yoursite1.com and user@yoursite2.com mail getting to different mailboxes. For that look into /etc/mail/virtusertable functionality (appropriate line in /etc/sendmail.cw is Kvirtuser hash -o /etc/mail/virtusertable, detailed info may be added here later). Excellent documentation on that is on http://www.sendmail.org/virtual, sendmail reference on virtual hosting.

It is worthwhile to add that linuxconf proposes a somewhat different scheme for virtual email with separate spool directories for all domains (that cleanly solves the above "name-conflict" issue"), but that requires a special virtual-aware POP/IMAP server (included with RH) and is somewhat more complicated. It is recommended for bigger email volume sites with many users within each domain.

A few words about sendmail, it is a good idea (good from a security standpoint) to have sendmail run from inetd.conf and not as a standalone daemon. For that we need to add it to /etc/inetd.conf, remove it from /etc/rc.d/init.d, add the sendmail queue processing to cron. Here is what you have to do:

  1. Add the following line to /etc/inetd.conf:
    smtp  stream  tcp     nowait  root    /usr/sbin/tcpd  /usr/sbin/sendmail -bs
  2. Edit /etc/rc.d/init.d/sendmail to have exit 0 somewhere in the very beginning (might not be the best way, be sure to document the changes you do to these files) so that this file does nothing instead of starting sendmail
  3. By editing your (root's) crontab (to edit do crontab -e) add a line like this
    */20 * * * * /usr/sbin/sendmail -q
    That would process sendmail queue every 20 min (if it exists). The described steps will simplify sendmail access control and will let you regulate who can talk to you 25 port, not just who can send email through you. The lines in /etc/hosts.allow that let all machines from .com and .org domains send you email are as follows
    sendmail: .com .org
    Please, note, that the daemon name, not protocol name is used here (sendmail, NOT smtp).

That would allow your system to handle email for all those domains.

4.10 Configure POP3

POP3 configuration is easy (no "virtualization" is required for this setup). RH comes equipped with imapd IMAP server. If you do not want to use IMAP functionality or do not like this particular implementation (buffer overflow bugs were discovered in it at some point) the good idea is to use qpopper, free POP3 daemon from Eudora http://www.eudora.com/freeware/qpop.html. At the time of writing they release the "stable" version (qpopper 2.53) and "public beta" ( qpopper 3.0, release 34, now called "final beta"). It is important to note that versions earlier than 2.5 contain a buffer overflow error that allows remote root exploit to be executed. Same problem plagues "public betas" up to 3.0 release 21. Use either 2.53 or the latest 3.0 beta (the former is better audited and the latter is better suited for RH - seamlessly works with PAM authentication). I suggest using 3.0, so the instructions below apply to that case.

  1. wget ftp://ftp.qualcomm.com/eudora/servers/unix/popper/qpopper3.0b34.tar.Z

    Retrieve the archive from Eudora site.

  2. tar zxvf qpopper3.0b34.tar.Z

    Uncompress and untar the contents.

  3. cd qpopper

    If you need explanation for this step, please, discontinue reading the document.

  4. ./configure --enable-specialauth --with-pam --enable-log-login --enable-shy

    The options here are:

    --enable-specialauth : allows MD5 and shadow passwords

    --with-pam: allows the use of RH Pluggable Authentication Modules (PAM) technology

    --enable-log-login: log successful logins, not only failures (not really that useful as it will use tcpd wrappers logging anyway)

    --enable-shy: conceal version number (yeah, a little pesky manifestation of "security through obscurity")

  5. make

    That compiles the popper

  6. /bon/cp popper/popper /usr/local/bin

    Now copy it to /usr/local/bin and set the mode to

    -rwx------   1 root     root       297008 Feb 16 15:41 /usr/local/lib/popper
    by using the command:
    chmod 700 /usr/local/bin/popper
  7. Add a line to /etc/inetd.conf
    pop3  stream tcp        nowait root /usr/sbin/tcpd /usr/local/bin/popper -s
    That would cause the tcpd wrapper to control access to popper. The lines to add in /etc/hosts.allow are
    popper: .good.com .nice.org
    That will allow people from domains good.com and nice.org to read email via POP3 client from your machine.

    To cause qpopper to use PAM authentication one must create a file for POP3 service in /etc/pam.d/ directory. File should be named "pop3" (same as line in /etc/services and qpopper compile-time option). The file looks like this:

    auth       required     /lib/security/pam_pwdb.so shadow 
    account    required     /lib/security/pam_pwdb.so
    password   required     /lib/security/pam_cracklib.so
    password   required     /lib/security/pam_pwdb.so nullok use_authtok md5 shadow
    session    required     /lib/security/pam_pwdb.so

  8. For whatever reason stock RH lists line in /etc/services file for POP3 protocol as "pop-3". And since qpopper prefers to see "pop3", it should be edited to be:
    pop3            110/tcp         # pop3 service
That would allow all user to get their email via any reasonable mail client.

4.11 Configure FTP server

We will use only anonymous ftp and will not allow any non-anonymous user any access. Here we describe the anonymous ftp server setup that allows anonymous uploads. Any self-respecting guide on the subject will tell you that "this is a bad thing". But how is it worse than allowing users to ftp from untrusted location and transfer their passwords in clear text? Not everybody (especially, using Windows) can easily setup an ftp tunnel via ssh.

I suggest using the stock RH wu-ftpd (version 2.6.0 at the time of writing). While it is rumored that there are "more secure" ftp daemons (Pro-FTP?), wu-ftp appears to be one most commonly used.

RH installs the wu-ftpd (package wu-ftpd-2.6.0-1) by default in server configuration. You are encouraged to check for updates as running ftp is an important security concern. There is also a separate rpm package that creates a separate directory for anonymous ftp home (anonftp-2.8-1). As anonymous ftp always does a chroot() system call (puts the user in the restricted file system) all necessary binaries and libraries are required. The typical directory looks like this (output of ls -lRa in /home/ftp):

total 20
d--x--x--x   2 root     root         4096 Feb 15 06:22 bin
d--x--x--x   2 root     root         4096 Feb 15 06:22 etc
drwxrws-wt   2 root     wheel        4096 Feb 18 19:51 incoming
drwxr-xr-x   2 root     root         4096 Feb 15 06:22 lib
drwxr-sr-x   3 root     ftp          4096 Feb 15 23:34 pub

total 344
---x--x--x   1 root     root        15204 Mar 21  1999 compress
---x--x--x   1 root     root        52388 Mar 21  1999 cpio
---x--x--x   1 root     root        50384 Mar 21  1999 gzip
---x--x--x   1 root     root        29308 Mar 21  1999 ls
----------   1 root     root        62660 Mar 21  1999 sh
---x--x--x   1 root     root       110668 Mar 21  1999 tar
lrwxrwxrwx   1 root     root            4 Feb 15 06:22 zcat -> gzip

total 40
-r--r--r--   1 root     root           53 Mar 21  1999 group
-rw-r--r--   1 root     root        31940 Mar 21  1999 ld.so.cache
-r--r--r--   1 root     root           79 Mar 21  1999 passwd

total 0

total 1212
-rwxr-xr-x   1 root     root        77968 Mar 21  1999 ld-2.1.1.so
lrwxrwxrwx   1 root     root           11 Feb 15 06:22 ld-linux.so.2 -> ld-2.1.1.so
-rwxr-xr-x   1 root     root      1031004 Mar 21  1999 libc-2.1.1.so
lrwxrwxrwx   1 root     root           13 Feb 15 06:22 libc.so.6 -> libc-2.1.1.so
-rwxr-xr-x   1 root     root        77196 Mar 21  1999 libnsl-2.1.1.so
lrwxrwxrwx   1 root     root           15 Feb 15 06:22 libnsl.so.1 -> libnsl-2.1.1.so
-rwxr-xr-x   1 root     root        33596 Mar 21  1999 libnss_files-2.1.1.so
lrwxrwxrwx   1 root     root           21 Feb 15 06:22 libnss_files.so.2 -> libnss_fi

total 0

Notice though, that for whatever reason, RH puts a copy of /bin/sh in /home/ftp/bin. I do not feel good about having it there, so it is chmoded to 0 by chmod 0 sh (can also be removed completely, but RPM might be slightly unhappy if you attempt to remove the package afterwards).

Permissions on /home/ftp directories and files should be carefully considered. In the above example, all of the system files are owned by root and are only readable (executable where necessary) by all. Files in bin are only executable (as is the directory itself to prevent listing of its contents).

The interesting part is permissions on pub and incoming.

Below follows the configuration file for ftp daemon (/etc/ftpaccess). It is well commented to the degree of being self-explanatory:

#ideas from <htmlurl url="ftp://ftp.wu-ftpd.org/pub/wu-ftpd/upload.configuration.HOWTO" name="ftp://ftp.wu-ftpd.org/pub/wu-ftpd/upload.configuration.HOWTO"> 
#only allow anonymous users-no other classes defined
class anonftp anonymous *

#number of users restriction with message shown when too many
limit   remote  10  Any                 /toomany.msg

#prevent uploads everywhere (for now)
upload /home/ftp * no

#display the contents of some files upon login/cd
readme  README*    login
readme  README*    cwd=*
message /welcome.msg            login
message .message                cwd=*

#log all file transfers DISABLED
#log transfers anonymous

#prevent these file operations for anon users
delete          no      anonymous
overwrite       no      anonymous

#fast cd and aliasing for the same reason (not really necessary, but convenient)
alias   inc:    /incoming
cdpath  /incoming
cdpath  /pub
cdpath  /

#what is allowed in paths
path-filter  anonymous  /etc/pathmsg  ^[-A-Za-z0-9_\.]*$  ^\.  ^-

#prevent the retrieval of some file
noretrieve .notar

#allow upload with NO subdirectory creation by anon users
upload    /home/ftp    /incoming   yes root wheel 0400 nodirs

#allow upload with subdirectory creation by anon users DISABLED
#upload    /home/ftp    /incoming   yes root wheel 0400 dirs

#prevent anon users to GET files from incoming (you might not like it, but it
#is a good idea-to prevent some people from using you ftp server to store
#their own stuff, pics, warez etc)
noretrieve /home/ftp/incoming
That would allow only anonymous users to do downloads and uploads in somewhat (!) controlled manner.

4.12 Configure dialin

Now the fun part starts. We want the machine to allow dial-in access via attached (inserted?) modem or modems. It will provide either regular shell or restricted shell (that only executes pppd daemon). Windows 95/98 users should be able to effortlessly dial in using all default settings of their computers.

Linux setup

To handle login via serial line some version of getty program is needed. This program monitors the serial line (/dev/ttyS1 will be used throughout the document, see serial HOWTO for details) and upon connection shown the login prompt or starts a program.

I suggest using the mgetty program (as it has more features and is easier to setup than some of the competitors).

RH comes with mgetty-1.1.21-2, that also has extensions to receive faxes and voice mail (if the modem supports this). Check whether mgetty is installed by doing: rpm -qa | grep mgetty.

After installing mgetty some reconfiguration is necessary. The files that should be changed and the details follow:

  1. /etc/inittab

    That enables mgetty to start when system is booted and be respawned accordingly. These lines should be added in the end.

    #for dialins use mgetty
    #note this S1 in the beginning of the line and ttyS1 in the end
    S1:2345:respawn:/sbin/mgetty ttyS1

  2. /etc/ppp/options

    This file controls the pppd daemon whenever it is started. Some of the options here are optional (hey, that why they are called options, right?).

    auth -chap +pap login modem crtscts debug proxyarp lock
    ms-dns 111.222.333.444

    Here is their brief meaning:

    Look at pppd man page for all the juicy details (parts of the above info is adapted from there)

  3. /etc/ppp/options.ttyS1

    This file serves purpose similar to the previous one, but only applies to particular modem line. It specifies the IP address given to the remote machine (dynamic, in some sense, if you have more than one line) and the local IP as well.


  4. /etc/mgetty+sendfax/login.config

    This file is the main mgetty control file. Mgetty is Windows-PPP-aware, so it has provisions to start pppd automatically upon receiving connect from the Windows machine.

    These lines should be present:

    /AutoPPP/ -     -       /usr/sbin/pppd

    Before adding them, check that some other version of similar command is absent there (commented out by default).

  5. /etc/ppp/pap-secrets

    This is similar to /etc/password file, but only used for dialins and contains plain text passwords (apparently, only visible to root). All users that you want to be able to dialin must have their usernames and password listed in this file. They should enter the same username and password into Windows Dial Up Networking configuration.

    # Secrets for authentication using PAP
    # these two users below can use dialin
    # client        server  secret pword   remote IP addresses
    dialinuser1     *       b1ab1a!?         111.222.333.888
    dialinuser2     *       p8sSw0rD         111.222.333.888

Check that mgetty is running by looking for similar line in the output of ps ax command.

 4625 ?        S      0:00 /sbin/mgetty ttyS1

Now this machine will allow modem calls from any Windows 95/98 box.

Windows setup

This is really straightforward.

  1. Click on My Computer
  2. Click on Dial Up networking
  3. Click on Make New Connection
  4. Proceed according to directions, enter the phone number etc
  5. After a new connection is created click on it and enter the username and password (same as mentioned in /etc/passwd and /etc/ppp/pap-secrets)
  6. Click Connect and it should work (it did in my case ;-) )

4.13 Open access

Now, after testing all the services, we are ready to open the access to this machine. The main access control facility in our case is TCP wrappers (tcpd). Their behavior is controlled by 2 files /etc/hosts.allow and /etc/hosts.deny, as was mentioned in the sections devoted to various network services. TCP wrappers configuration can be done in 2 distinct ways and we will employ the simplest.

Let our /etc/hosts.deny contain ALL:ALL clause, thus denying the access to all services (started from /etc/inetd.conf ) for all hosts and all users on them. Now we can allow what we need explicitly in /etc/hosts.allow, thus following the philosophy "what is not expressly allowed is denied".

Lets assume we want to allow people to read and send email, we want some trusted hosts to update contents of the web pages and we want admin workstation to have full access. So we arrive at the following /etc/hosts.allow:

# hosts.allow   This file describes the names of the hosts which are
#               allowed to use the local INET services, as decided
#               by the '/usr/sbin/tcpd' server.
ALL: adminbox.some.net
#we rely on anti-relaying features of sendmail 8.9.x to fight spam
#and also restrict some sites that we don't want to see email from
sendmail: ALL EXCEPT .kr
popper: .com .edu .gov .mil
#these people can upload/download stuff
in.ftpd: .this.net .that.net

5. Conclusion

There must be the conclusion, right?

6. References

Useful LDP HOWTOs (well, actually, all others are useful too)

  1. Setting Up Your New Domain Mini-HOWTO., really good guide of DNS setup and general network setup (recommended reading)
  2. Linux WWW HOWTO, provides more details on Apache setup, including virtual hosting
  3. Red Hat Linux 6.X as an Internet Gateway for a Home Network, some hints on network setup
  4. IP Aliasing On A Linux Machine, used for multiple IP on the same interface
  5. Ethernet HOWTO, look here in case of network card trouble
  6. IPCHAINS HOWTO, turn to this if more security is desired
  7. Serial HOWTO, serial ports, lines, modems and related stuff
  8. PPP HOWTO,some notes on PPP server setup

Software (used or mentioned) websites

  1. Eudora POP3 server
  2. WU-FPTD ftp server
  3. Sendmail MTA
  4. Mgetty pages
  5. Apache httpd server

Other documents

  1. Armoring Linux
  2. Setting Up POP/PPP server
  3. Mgetty and Windows dialin info
  4. Using RedHat 5.1 to Start an ISP, the short article on how to start an ISP if all you have is a Linux RH ;-)

Resources, not related to the topic of the document ;-)

  1. I also maintain a list of computer/network security related books with (where available) reviews and online availability. It is posted at http://www.chuvakin.org/books. If you have a book that I don't list please use the form on the page and I will add it to the list and maybe review it later.