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Shell servers



Telnet was one of the first services on what is now the Internet, it allows you to login to a remote machine interactively, issue commands and see their results. It is still the primary default tools for remote administration in most environments, and has nearly universal support (even NT has a telnet daemon and client). It is also one of the most insecure protocols, susceptible to sniffing, hijacking, etc. If you have clients using telnet to come into the server you should definitely chroot their accounts if possible, as well as restricting telnet to the hosts they use with TCP_WRAPPERS. The best solution for securing telnet is to disable it and use SSL'ified telnet or ssh.

Problems with telnet include:

The best solution is to turn telnet off and use ssh. This is however not practical in all situations. If you must use telnet then I strongly suggest firewalling it, have rules to allow hosts/networks access to port 23, and then a general rule denying access to port 23, as well as using TCP_WRAPPERS (which is more efficient because the system only checks each telnet connection and not every packet against the firewall rules) however using TCP_WRAPPERS will allow people to establish the fact that you are running telnet, it allows them to connect, evaluates the connection, and then closes it if they are not listed as being allowed in. 
An example of firewalling rules:

ipfwadm -I -a accept -P tcp -S -D 23
ipfwadm -I -a accept -P tcp -S some.trusted.host -D 23
ipfwadm -I -a deny -P tcp -S -D 23

or in ipchains:

ipchains -A input -p all -j ACCEPT -s -d 23
ipchains -A input -p all -j ACCEPT -s some.trusted.host -d 23
ipchains -A input -p all -j DENY -s -d 23

An example of the same using TCP_WRAPPERS, in /etc/hosts.allow:

in.telnetd:, some.trusted.host

And in /etc/hosts.deny:

in.telnetd: ALL

There are several encrypted alternatives to telnet as mentioned before, ssh, SSLeay Telnet, and other third party utils, I personally feel that the 'best' alternative if you are going to go to the bother of ripping telnet out and replacing it with something better is to use ssh.
To secure user accounts with respect to telnet there are several things you can do. Number one would be not letting root login via telnet, this is controlled by /etc/securetty and by default in most distributions root is restricted to logging on from the console (a good thing). For a user to successfully login their shell has to be valid (this is determined by the list of shells in /etc/shells), so setting up user accounts that are allowed to login is simply a matter of setting their shell to something listed in /etc/shells, and keeping users out as simple as setting their shell to /bin/false (or something else not listed in /etc/shells. Now for some practical examples of what you can accomplish by setting the user shell to things other then shells.

For an ISP that wishes to allow customers to change their password easily, but not allow them access to the system (my ISP uses Ultrasparcs and refuses to give out user accounts for some reason, I wonder why).

in /etc/shells list:


and set the users shell to /usr/bin/passwd so you end up with something like:


and voila. The user telnets to the server, is prompted for their username and password, and is 
then prompted to change their password. If they do so successfully passwd then exits and they are disconnected. If they are unsuccessful passwd exits and they are disconnected. The following is a transcript of such a setup when a user telnets in:

Connected to localhost.
Escape character is '^]'.

Red Hat Linux release 5.2 (Apollo)
Kernel 2.2.5 on an i586
login: tester
Changing password for tester
(current) UNIX password: 
New UNIX password: 
Retype new UNIX password: 
passwd: all authentication tokens updated successfully
Connection closed by foreign host.

Telnet also displays a banner by default when someone connects. This banner typically contains systems information like the name, OS, release and sometimes other detailed information such as the kernel version. Historically this was useful if you had to work on multiple OS's, however in today's hostile Internet it is generally more harmful then useful. Telnetd displays the contents of the file /etc/issue.net (typically it is identical to /etc/issue which is displayed on terminals and so forth), this file is usually recreated at boot time in most Linux distributions, from the rc.local startup file. Simply edit the rc.local file, either modifying what it puts into /etc/issue and /etc/issue.net, or comment out the lines that create those files, then edit the files with some static information.

Typical Linux rc.local contents pertaining to /etc/issue and /etc/issue.net:

# This will overwrite /etc/issue at every boot. So, make any changes you
# want to make to /etc/issue here or you will lose them when you reboot.
echo "" > /etc/issue
echo "$R" >> /etc/issue
echo "Kernel $(uname -r) on $a $(uname -m)" >> /etc/issue
cp -f /etc/issue /etc/issue.net
echo >> /etc/issue

simply comment out the lines or remove the uname commands. If you absolutely must have telnet enabled for user logins make sure you have a disclaimer printed:

This system is for authorized users only. Trespassers will be prosecuted.

or something like the above. Legally you are in a stronger position if someone cracks into the system or otherwise abuses your telnet daemon.

Telnet - SSL

A drop in replacement for telnet, SSLtelnet and MZtelnet provide a much higher level of security then plain old telnet, although SSLtelnet and MZtelnet are not as flexible as SSH, they are perfectly free (i.e., GNU licensed) which SSH is not (although OpenSSH is *BSD licensed). The server and client packages are available as tarballs at: ftp://ftp.uni-mainz.de/pub/internet/security/ssl/, and as RPM packages at ftp://ftp.replay.com/pub/replay/linux/redhat/.


Slush is based on OpenSSL and supports X.509 certificates currently, which for a large organization is a much better (and saner) bet then trying to remember several dozen passwords on various servers. Slush is GPL, but not finished yet (it implements most of the required functionality to be useful, but has limits). On the other hand it is based completely in open source software making the possibilities of backdoors/etc remote. Ultimately it could replace SSH with something much nicer. You can get it from: http://violet.ibs.com.au/slush/.


NSH is a commercial product with all the bells and whistles (and I do mean all). It's got built in support for encryption, so it's relatively safe to use (I cannot verify this completely however, as it isn't open source). Ease of use is high, you cd //computername and that 'logs' you into that computer, you can then easily copy/modify/etc. files, run ps and get the process listing for that computer, etc. NSH also has a Perl module available, making scripting of commands pretty simple, and is ideal for administering many like systems (such as workstations). In addition to this NSH is available on multiple platforms (Linux, BSD, Irix, etc.) with RPM's available for Red Hat systems. NSH is available from: http://www.networkshell.com/, and 30 day evaluation versions are easily downloaded. 


SSH is a secure protocol and set of tools to replace some common (insecure) ones. It was designed from the beginning to offer a maximum of security and allows remote access to servers in a secure manner. SSH can be used to secure any network based traffic, by setting it up as a 'pipe' (i.e. binding it to a certain port at both ends). This is quite kludgy but good for such things as using X across the Internet. In addition to this the server components runs on most UNIX systems, and NT, and the client components runs on pretty much anything. Unfortunately SSH is no longer free; however, there is a project to create a free implementation of the SSH protocol.

There aren't any problems with SSH per se like there are with telnet, all session traffic is encrypted and the key exchange is done relatively securely (alternatively you can preload keys at either end to prevent them from being transmitted and becoming vulnerable to man in the middle attacks). SSH typically runs as a daemon, and can easily be locked down by using the sshd_config file. You can also run sshd out of inetd, and thus use TCP_WRAPPERS, and by default the ssh rpm's from ftp://ftp.replay.com/ have TCP_WRAPPERS check option compiled into them. Thus using TCP_WRAPPERS you can easily restrict access to ssh. Please note earlier versions of ssh do contain bugs, and several sites have been hacked (typically with man in the middle attacks or problems with buffer overflows in the ssh code), but later version of ssh address these problems. The main issue with ssh is it's license, it is only free for non-commercial use, however you can download source code from a variety of sites. If you want to easily install ssh there is a script called "install-ssh" that will download, compile and install ssh painlessly, it is available from: ftp://ftp.yellowdoglinux.com/pub/yellowdog/install-ssh/.

The firewalling rules for ssh are pretty much identical to telnet. There is of course TCP_WRAPPERS, the problem with TCP_WRAPPERS being that an attacker connects to the port, but doesn't get a daemon, HOWEVER they know that there is something on that port, whereas with firewalling they don't even get a connection to the port. The following is an example of allowing people to ssh from internal machines, and a certain C class on the internet (say the C class your ISP uses for it's dial-up pool of modems). 

ipfwadm -I -a accept -P tcp -S -D 22
ipfwadm -I -a accept -P tcp -S isp.dial.up.pool/24 -D 22
ipfwadm -I -a deny -P tcp -S -D 22


ipchains -A input -p tcp -j ACCEPT -s -d 22
ipchains -A input -p tcp -j ACCEPT -s isp.dial.up.pool/24 -d 22
ipchains -A input -p tcp -j DENY -s -d 22

Or via TCP_WRAPPERS, hosts.allow:

sshd:, isp.dial.up.pool/



In addition to this, ssh has a wonderful configuration file, /etc/sshd/sshd_config by default in most installations. You can easily restrict who is allowed to login, which hosts, and what type of authentication they are allowed to use. The default configuration file is relatively safe but following is a more secure one with explanations. Please note all this info can be obtained by a "man sshd" which is one of the few well written man pages out there. The following is a typical sshd_config file:

Port 22
# runs on port 22, the standard
# listens to all interfaces, you might only want to bind a firewall
# internally, etc
HostKey /etc/ssh/ssh_host_key
# where the host key is
RandomSeed /etc/ssh/ssh_random_seed
# where the random seed is
ServerKeyBits 768
# how long the server key is
LoginGraceTime 300
# how long they get to punch their credentials in
KeyRegenerationInterval 3600
# how often the server key gets regenerated 
PermitRootLogin no
# permit root to login? no
IgnoreRhosts yes
# ignore .rhosts files in users dir? yes
StrictModes yes
# ensures users don't do silly things
QuietMode no
# if yes it doesn't log anything. yikes. we want to log logins/etc.
X11Forwarding no
# forward X11? shouldn't have to on a server
FascistLogging no
# maybe we don't want to log too much.
PrintMotd yes
# print the message of the day? always nice
KeepAlive yes
# ensures sessions will be properly disconnected
SyslogFacility DAEMON
# who's doing the logging?
RhostsAuthentication no
# allow rhosts to be used for authentication? the default is no
# but nice to say it anyways
RhostsRSAAuthentication no
# is authentication using rhosts or /etc/hosts.equiv sufficient
# not in my mind. the default is yes so lets turn it off. 
RSAAuthentication yes
# allow pure RSA authentication? this one is pretty safe
PasswordAuthentication yes
# allow users to use their normal login/passwd? why not.
PermitEmptyPasswords no
# permit accounts with empty password to log in? no

Other useful sshd_config directives include:

AllowGroups - explicitly allow groups (/etc/group) to login using ssh
DenyGroups - explicitly disallows groups (/etc/groups) from logging in
AllowUsers - explicitly allow users to login in using ssh
DenyUsers - explicitly blocks users from logging in
AllowHosts - allow certain hosts, the rest will be denied
DenyHosts - blocks certain hosts, the rest will be allowed
IdleTimeout time - time in minutes/hours/days/etc, forces a logout by SIGHUP'ing the process.

SSH software: 


Fresh Free FiSSH

Most of us still have to sit in front of windows workstations, and ssh clients for windows are a pain to find. Fresh Free FiSSH is a free ssh client for Windows 95/NT 4.0. Although not yet completed, I would recommend keeping your eye on it if you are like me and have many Windows workstations. The URL is: http://www.massconfusion.com/ssh/.

Tera Term

Tera Term is a free Telnet client for Windows, and has an add-on DLL to enable ssh support. Tera Term is available from: http://hp.vector.co.jp/authors/VA002416/teraterm.html. The add-on DLL for SSH support is available from: http://www.zip.com.au/~roca/ttssh.html


putty is a Windows SSH client, pretty good, and completely free, and also small (184k currently). You can download it from: http://www.chiark.greenend.org.uk/~sgtatham/putty.html.


mindterm is a free java ssh client, you can get it at: http://www.mindbright.se/mindterm/.


LSH is a free implementation of the SSH protocol (both client and server), LSH is GNU licensed and is starting to look like the alternative (commercially speaking) to SSH (which is not free anymore). You can download it from: http://www.net.lut.ac.uk/psst/, please note it is under development.

Secure CRT

A commercial Telnet / SSH client from Vandyke software. You can download / purchase it at: http://www.vandyke.com/.


Fsh is stands for "Fast remote command execution" and is similar in concept to rsh/rcp. It avoids the expense of constantly creating encrypted sessions by bring up an encrypted tunnel using SSH or LSH, and running all the commands over it. You can get it from: http://www.lysator.liu.se/fsh/.


LSH is a free implementation of the SSH protocol, LSH is GNU licensed and is starting to look like the alternative (commercially speaking) to SSH (which is not free anymore). You can download it from: http://www.net.lut.ac.uk/psst/, please note it is under development.

SSH Win32 ports

Ports of SSH to Win32 available at: http://guardian.htu.tuwien.ac.at/therapy/ssh/.

R services

R services such as rsh, rcp, rexec and so forth are very insecure. There is simply no other way to state it. Their basis of security is based on the hostname/IP address of the machine connecting, which can easily be spoofed or, using techniques such as DNS poisoning, otherwise compromised. By default they are not all disabled, please do so immediately. Edit /etc/inetd.conf and look for rexec, rsh and so on, and comment them out, followed by a "killall -1 inetd" to restart inetd.

If you absolutely must run these services use TCP_WRAPPERS to restrict access, it's not much but it will help. Also make sure you firewall them as TCP_WRAPPERS will allow an attacker to see that they are running, which might result in a spoofed attack, something TCP_WRAPPERS cannot defend against if done properly. Access to the various R services is controlled via rhosts files, usually each user has their own rhosts file, unfortunately this is susceptible to packet spoofing. The problem with r services is also that once there is a minor security breach that can be used to modify files, editing a users (like root's) .rhost file makes it very easy to crack a system wide open.

If you need remote administration tools that are easy to use and similar to rsh/etc I would recommend NSH (Network SHell) or SSH, they both support encryption, and a much higher level of security. Alternatively using VPN software will reduce some of the risk as you can deny packet spoofers the chance to compromise your system(s) (part of IPSec is authentication of sender and source, which is almost more important then encrypting the data in some cases).


Security Portal

Written by Kurt Seifried