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4. Step by step guide

Ingredients needed:

Here follows the procedure:

4.1 Get RH

Purchase or otherwise procure the RedHat 7.0 (further referred as RH, latest version number is 7.0 at the time of updating) distribution and compatible hardware. One can get a full RH CDROM for about $3.00 including shipping and handling at 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 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. For RH 6.0 and 6.1 you might be able to add packages to Workstation setup as well, but in RH 6.2 and later (7.0) all the server services are disabled and significant amount of tweaking is required-so only Server or Custom is strongly recommended.

4.3 Some install tips

If your 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).

Here are some ideas on disk space partitioning. Read Linux Partitions HOWTO (a bit outdated) to get some general hints on functions of partitions and their sizes for different kinds of server setups.

Lest assume we are setting up a server for under one hundred users. We will need separate /tmp, /var and /home partititons (and swap, of course). If you hard drive is around 4 GB than roughly 300 MB is /tmp, 100MB swap, 1 GB /var (you want ample logging) and 1GB /home. The remaining 1.6GB will be root partition (no separate /usr). The split between /home and / might depend upon the amount of web pages you plan to host - the more pages the more space goes to /home. To enhance security it is nice to put some restricions (in /etc/fstab) to /tmp, /var and /home partitions (similar to those described in my Public Browser Station HOWTO .

If your 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 and the machine will be named ns (that gives us a fully qualified domain name (FQDN) 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 your 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 ssh1 and also all versions of ssh1 compiled with RSAREF library contain a buffer-overflow bug. Use ssh2 or the latest version of ssh1 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. RedHat 7.0 now includes OpenSSH clone that supports both ssh1 and ssh2 protocols. Its configuration is almost the same as ssh. It has some minor configuration advantages over ssh (for instance, no X11 forwarding by default) and is otherwise the same. Sshd (when run as daemon) will also refer to /etc/hosts.deny and /etc/hosts.allow for access control.

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. Also, telnet is now considered by many to be not only unnecessary, but really utterly undesirable. Use ssh, and forget telnet once and for all!

  1. Services started from /etc/inetd.conf (RedHat 7.0 introduced the more advanced /etc/xinetd.conf which uses somewhat different syntax, see below):

    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).

    Here goes the note for RedHat 7.0 users. Inetd daemon (while still present in the distribution) is now replaced with xinetd. Its configuration file format is as follows:

    # Simple configuration file for xinetd
    # Some defaults, and include /etc/xinetd.d/
            instances               = 60
            log_type                = SYSLOG authpriv
            log_on_success          = HOST PID
            log_on_failure          = HOST RECORD
    includedir /etc/xinetd.d
    where /etc/xinetd.d directory looks like (with probably more file in your case):
    -rw-r--r--    1 root     root          498 Aug 23 00:17 tftp
    -rw-r--r--    1 root     root          414 Jul 21 08:43 rsh
    -rw-r--r--    1 root     root          362 Jul 21 08:43 rexec
    -rw-r--r--    1 root     root          361 Jul 21 08:43 rlogin
    -rw-r--r--    1 root     root          347 Aug  9 05:55 wu-ftpd

    Files in the directory configure individual services like finger, telnet or ftp. There format is (this service, ftp, defaults to on on stock RedHat 7.0)

    # default: on
    # description: The wu-ftpd FTP server serves FTP connections. It uses \
    #       normal, unencrypted usernames and passwords for authentication.
    service ftp
            socket_type             = stream
            wait                    = no
            user                    = root
            server                  = /usr/sbin/in.ftpd
            server_args             = -l -a
            log_on_success          += DURATION USERID
            log_on_failure          += USERID
            nice                    = 10

    Or (this service, tftp, defaults to off on stock RedHat 7.0)

    # default: off
    # description: The tftp server serves files using the trivial file transfer \
    #       protocol.  The tftp protocol is often used to boot diskless \
    #       workstations, download configuration files to network-aware printers, \
    #       and to start the installation process for some operating systems.
    service tftp
            socket_type             = dgram
            wait                    = yes
            user                    = nobody
            log_on_success          += USERID
            log_on_failure          += USERID
            server                  = /usr/sbin/in.tftpd
            server_args             = /tftpboot
            disable                 = yes

    So, to disable services add "disable= yes" to the end of correspondent file or just remove the file.

  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 should be removed are NFS, NIS, samba etc, if they got installed by mistake. Make sure you remove NFS/NIS (if you are not using them) as bugs are often found in them. 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 your 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-2.2.17-i686.config since the machine has Pentium III 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 (to turn it on do execute the following command echo 1 > /proc/sys/net/ipv4/ip_forward or add a line into /etc/sysctl.conf). 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.


Do: ping virtual addresses as

ping 111.222.333.555
ping 111.222.333.666
ping 111.222.333.777

Should get: interfaces should be up

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, now that even bind 9 is out, but I consider bind 4.9.7 much more time-tested and stable. The arguments for upgrading to 8.x (provided and and, I guess, at many other places. Here is a message from Theo de Raadt himself (the head of OpenBSD development) where he justifies the choice of bind 4 as part of OpenBSD-the most secure UNIX OS around. He also shudders at the amount of bugs the OpenBSD auditing team saw in BIND 8 source code) 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. It is claimed that named (whatever version) should always be run in a chroot jail.

Here are the instructions, loosely following the DNS book from O'Reilly (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, 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:, and 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'Reilly 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
    ;reverse lookups
    primary        db.111.222.333
    ;localhost.localnet configs
    primary            db.127.0.0
    ;virtual Domains
    primary                     db.yoursite1
    primary                     db.yoursite2
    primary                     db.yoursite3
  4. /etc/namedb/

    ; defines our local hosts at, just one in our case, and its aliases
    @                       IN      SOA (
                                            2000012190 7200 1800 3600000 7200 )
    ;name servers and mail servers
                            IN      NS
                            IN      MX      10
                            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 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 IN    SOA (
                                            1999121501 7200 1800 3600000 7200 )
    ;name Servers
                            IN      NS
    ;addresses point to canonical name   IN      PTR
    888                             IN      PTR
    ;virtual hosts
    555                             IN      PTR
    666                             IN      PTR
    777                             IN      PTR
  6. /etc/namedb/db.127.0.0

    ;local loop config file   IN      SOA (
                                            1997072200 7200 1800 3600000 7200 )
                            IN      NS
    1                       IN      PTR     localhost.
  7. /etc/namedb/db.yoursite1

    @                       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
                            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

    @                       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
                            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

    @                       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
                            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 your real domain


Do: check address resolution


Should get: nslookup returns the correct IP addresses for all hostnames

4.8 Configure httpd

To server html pages httpd daemon is used. RH 7.0 comes with Apache 1.3.12 (latest version is currently 1.3.14 and the alpha of the upcoming 2.0 is released). 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 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>
DocumentRoot /home/httpd/yoursite1
ErrorLog yoursite1-error_log
TransferLog yoursite1-access_log

<VirtualHost 111.222.333.666>
DocumentRoot /home/httpd/yoursite2
ErrorLog yoursite2-error_log
TransferLog yoursite2-access_log

<VirtualHost 111.222.333.777>
DocumentRoot /home/httpd/yoursite3
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" will be stored in default location /home/httpd/html.

For more information, look at, 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.


Do: access the test pages via Lynx browser or telnet to port 80


Should get: Test pages will be returned (if you put them in the proper directories)

4.9 Configure sendmail


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

  1. look for the lines that starts from Dj$ and change it to point to your main ("real", not virtual) server name (, so it will looks like this Dj$
  2. locate file /etc/ and make it look like this
    # - 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 and mail getting to different mailboxes. For that look into /etc/mail/virtusertable functionality (appropriate line in /etc/ is Kvirtuser hash -o /etc/mail/virtusertable, detailed info may be added here later). Excellent documentation on that is on, 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 the 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

    Or, if using xinetd create a file sendmail in /etc/xinetd.d/ similar to

    # default: on
    service sendmail
            socket_type     = stream
            wait            = no
            user            = root
            server          = /usr/bin/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 your 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.


PROBLEM: mail that you are trying to send is denied with a message Relaying denied

SOLUTION:Look into your /etc/ Are you sure all possible variations of your hostname and of your virtual hostnames are here? Look in the message headers and see from what machine it was rejected from: does it look like another name of yours that you missed?


Do: access the SMTP port 25 via telnet

telnet 25
telnet 25
telnet 25
telnet 25

Should get: Sendmail should respond with prompt and version number! Type QUIT to get out of the prompt.

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 At the time of writing the released version is qpopper 3.0.2. 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 (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. As of April 13, Qpopper 3.0 is no longer beta, but a regular software. As of recently, the bug was discovered even in Qpopper 2.53 that allows the attacker to obtain a shell with group-id 'mail', potentially allowing read/write access to all mail.

  1. wget

    Retrieve the archive from Eudora site.

  2. tar zxvf qpopper3.0.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. /bin/cp popper/popper /usr/local/bin

    Copies the binary to /usr/local/bin

  7. Now set the mode to
    -rwx------   1 root     root       297008 Feb 16 15:41 /usr/local/bin/popper
    by using the command:
    chmod 700 /usr/local/bin/popper
  8. 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
    That will allow people from domains and 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/ shadow 
    account    required     /lib/security/
    password   required     /lib/security/
    password   required     /lib/security/ nullok use_authtok md5 shadow
    session    required     /lib/security/

  9. 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.


PROBLEM: you are connecting to your POP server with valid password and username and they are rejected with a message Password incorrect.

SOLUTION: PAM doesn't like your setup. This message is common for qpopper 2.53, use 3.0 and it should disappear. Otherwise, look into /etc/pam.d/pop3 that you created. Is it OK?


Do: access the POP3 port 110 via telnet

telnet 110

Should get: Qpopper should respond with prompt and version number! Type QUIT to get out of the prompt.

4.11 Configure FTP server

Anonymous FTP setup

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. But you definitely should restrict access via tcp wrappers and watch for "warez puppies" (people who will try to exchange stolen software via your ftp site if you allow unlimited downloads!).

I suggest using the stock RH wu-ftpd (version 2.6.1 at the time of writing). While it is rumored that there are "more secure" ftp daemons (Pro-ftpd), wu-ftp appears to be one most commonly used. Recently a series of bugs was again discovered in wu-ftp (even in 2.6.x versions) and its reputation as the most popular ftp daemon seem to be dwindling. CERT has issued an advisory concerning WU-FTPD and all ftp daemons derived from BSD's final release.

RH installs the wu-ftpd (package wu-ftpd-2.6.1-1) 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 structure 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
-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
lrwxrwxrwx   1 root     root           11 Feb 15 06:22 ->
-rwxr-xr-x   1 root     root      1031004 Mar 21  1999
lrwxrwxrwx   1 root     root           13 Feb 15 06:22 ->
-rwxr-xr-x   1 root     root        77196 Mar 21  1999
lrwxrwxrwx   1 root     root           15 Feb 15 06:22 ->
-rwxr-xr-x   1 root     root        33596 Mar 21  1999
lrwxrwxrwx   1 root     root           21 Feb 15 06:22 -> 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="" name=""> 
#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 your 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. Make sure you update the permissions on files that you changed after you upgrade the RPM packages next time.

Guest FTP setup

Guest FTP users are those that have valid usernames and passwords (unlike anonymous), but do not have access to the whole directory structure (unlike real ones). So they are chrooted after authentication. Guest users can do uploads in this configuration.

Easy 21-step directions for that are provided below ;-)

Software used: wu-ftpd-2.6.1

Sample username will be created: ftpguy, user ID=505.

Her group will be: lusers, group ID=701.

If you want more users of the same sort, they should be the members of the same group. For that it might be good to change the directory structure somewhat so that all of them use the same passwd file and the same static ls. But, for better separation you can give each of them their own files.

  1. adduser ftpguy

    creates an entry in /etc/passwd

  2. passwd ftpguy change password to whatever
  3. Edit file /etc/passwd, last line (that contains our new user) should look like this
    yes, that is "slash"-"dot"-"slash" after his home directory.
  4. Edit file /etc/shells, add line, below
    This file has to exist in some newer Linux distributions (contrary to what is claimed at Guest FTP HOWTO). Sometimes one can put /bin/true in its place.
  5. Edit file /etc/group, add line, below
  6. cd /home
  7. chown ftpguy.lusers ftpguy
    this directory is created by adduser command
  8. cd ftpguy; mkdir etc bin ; chown root.daemon etc bin
    this creates a directory tree for chroot
  9. chmod 111 etc bin
    this sets very conservative permissions on directories within the chrooted tree
  10. cp ~/static_ls /home/ftpguy/bin/ls
    obtaining static (not calling any libraries) version of /bin/ls: this directory ( contains static version of many RH 6.x/7.x-compatible utilities, including ls (local copy is here, gunzip ls.gz to run)
  11. cd bin ; chown root.bin ls
  12. chmod 111 ls
    this sets very conservative permissions on binaries within chroot
  13. cd ../etc
  14. Create file /home/ftpguy/etc/passwd as follows
  15. Create file /home/ftpguy/etc/group, contents follow
  16. chown root.daemon passwd group
    this sets proper ownership of these files
  17. chmod 444 passwd group
    this sets minimum necessary permission on that file
  18. cd ~ftpguy; touch .forward
    this creates .forward file
  19. chown root.root .forward ; chmod 400 .forward
    and locks it for security reasons
  20. cd /etc
  21. Add the facilities for handling guest users into /etc/ftpaccess
    class anonftp guest,anonymous *                                                 
    delete     no   anonymous,guest               # delete permission?              
    overwrite  no   anonymous,guest               # overwrite permission?           
    rename     no   anonymous,guest               # rename permission?              
    chmod      no   anonymous,guest               # chmod permission?               
    umask      no   anonymous,guest               # umask permission?               
    guestgroup lusers                                                               
    limit   remote  10  Any                 /toomany.msg                            
    upload /home/ftp * no                                                           
    readme  README*    login                                                        
    readme  README*    cwd=*                                                        
    message /welcome.msg            login                                           
    message .message                cwd=*                                           
    alias   inc:    /incoming                                                       
    cdpath  /incoming                                                               
    cdpath  /pub                                                                    
    cdpath  /                                                                       
    path-filter  anonymous  /etc/pathmsg  ^[-A-Za-z0-9_\.]*$  ^\.  ^-               
    noretrieve .notar                                                               
    upload    /home/ftp    /incoming   yes root wheel 0400 nodirs                   
    noretrieve /home/ftp/incoming

Lets test this beast:

localhost[anton]#1008: ftp localhost
Connected to anton.
220 anton FTP server (Version wu-2.6.1(1) Mon Feb 28 10:30:36 EST 2000) ready.
Name (localhost:anton): ftpguy
331 Password required for ftpguy.
230 User ftpguy logged in.  Access restrictions apply.
Remote system type is UNIX.
Using binary mode to transfer files.
ftp> ls -la
200 PORT command successful.
150 Opening ASCII mode data connection for /bin/ls.
total 4
drwx------   4 505      701          1024 Apr  8 02:16 .
drwx------   4 505      701          1024 Apr  8 02:16 ..
-r--------   1 0        0               0 Apr  8 02:16 .forward
d--x--x--x   2 0        2            1024 Apr  8 02:09 bin
d--x--x--x   2 0        2            1024 Apr  8 02:15 etc
226 Transfer complete.
ftp> mkdir TEST
257 "/TEST" new directory created.
ftp> ls -l
200 PORT command successful.
150 Opening ASCII mode data connection for /bin/ls.
total 3
-r--------   1 0        0               0 Apr  8 02:16 .forward
drwxr-xr-x   2 505      701          1024 Apr  8 02:32 TEST
d--x--x--x   2 0        2            1024 Apr  8 02:09 bin
d--x--x--x   2 0        2            1024 Apr  8 02:15 etc
226 Transfer complete.

and so on.


Do: access the ftp server using ftp client


Should get: ftp daemon should respond with prompt and version number!

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)

    Another note is appropriate here. Some people reported that they had more success with +chap -pap in authenticating both Windows and Linux dial-up clients. If you are having problems, try changing /etc/ppp/options to have +chap -pap. In this case the new file /etc/ppp/chap-secrets should be created (same contents as recommended /etc/ppp/pap-secrets).

    Some other people reported that having default line from /etc/mgetty+sendfax/login.config works fine. I am very happy to hear that, and I never claimed that my way to set things up is the only true way.

  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.

As was noted by one of the readers some steps are to be taken to prevent users from sharing their dialin password with others. A simple perl/shell script will do the job by killing and logging connections that use the same username.

Also, if it is desirable to prevent users from using dialing in their usernames should not be put into /etc/ppp/pap-secrets .

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 ;-) )


Do: try to dial in using terminal program (UNIX:minicom /Windows:terminal or other)

Should get: Mgetty should respond with prompt and you Linux distribution version!

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). In case of RH 7 the xinetd will check the same access control files itself without any need to wrap services with /usr/sbin/tcpd. These facilities are 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.
#we rely on anti-relaying features of sendmail 8.9+ to fight spam
#and also restrict some sites that we don't want to see email from
sendmail: ALL EXCEPT .kr .cn
popper: .com .edu .gov .mil
#these people can upload/download stuff, make it restrictive to avoid warez!

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