This tutorial is designed for administrators of IBM RS/6000 systems who wish to improve the security and integrity of their servers running AIX by replacing standard insecure network services with those provided by the OpenSSH implementation of the Secure Shell protocol.

Sandor W. Sklar (ssklar@stanford.edu), Systems Administrator, Freelance Developer

Sandor W. Sklar is a Unix systems administrator at Stanford University, in beautiful Northern California. When not poking through his systems for real or imagined security holes, he enjoys spending time with his wife and two children.



01 June 2001

Before you start

About this tutorial

This tutorial is designed for administrators of IBM RS/6000 systems who wish to improve the security and integrity of their servers running AIX by replacing standard insecure network services with those provided by the OpenSSH implementation of the Secure Shell protocol.

Neither general network security, nor the use of the SSH client software is discussed in-depth in this tutorial. The primary focus of this tutorial is to detail the necessary components, the steps, and the configuration required to compile OpenSSH and its prerequisites from source to deploy across AIX systems.

About the examples in this tutorial

The example system used in this tutorial is an RS/6000 running AIX 4.3.3 Maintenance Level 08. The source software was compiled with the IBM C for AIX version 5.0.1 compiler. Note that the instructions presented apply to all currently supported versions of AIX from 4.3.x thru 5.x.

The latest stable version of each software package was used for this tutorial. Options and behavior of software may change across releases, so always refer to the documentation included with the source distribution for the most recent information.


What is OpenSSH

What is wrong with the default network services?

Like most UNIX implementations, AIX provides a large number of network services enabling remote users to log in interactively, transfer files to and from the server, and issue commands to the server in a non-interactive fashion. Unfortunately, most of the daemons (programs running on the server that fulfill requests for particular services) were designed during a time when the security of systems and network traffic was an afterthought -- if it was thought of at all.

The protocols behind such services as telnet, rsh, and ftp contain no provision for the encryption of traffic passed over the network. Most network protocols contain methods for user authentication, but the methods are extremely weak and easily forged. Protocols allowing the transmission of user IDs and passwords from the client to the server in clear-text are common-place. In addition, there is no guarantee that the data transferred through the network has not been intercepted by a third-party and possibly altered.

The Secure Shell (SSH) protocol was developed to address the aforementioned problems caused by these inherently insecure services.

The development of the Secure Shell protocol

In 1995, the original SSH protocol was developed by Tatu Ylönen, a researcher at the Helsinki University of Technology, in Finland. Along with developing the protocol, Ylönen also wrote an implementation for UNIX systems, distributing the source as free software for unlimited use. As the popularity of the SSH software grew worldwide, Ylönen formed a company, SSH Communications Security, Ltd., in order to further development of the product (now licensed commercially, but with source available) and provide support.

In time, limitations and flaws were discovered in the original definition of the protocol. These problems could not be fixed without breaking compatibility with older versions, so a new protocol was defined fixing the issues with the original SSH protocol. As the various implementations of the protocol 2-based software mature and gain features, the use of the older protocol 1-based software will fade. For now, though, implementations of both protocol 1 and protocol 2 are in widespread use around the world; to provide service to the widest audience of clients, it is important for servers to support client connections via both protocols.

What does SSH do?

The Secure Shell protocol protects against the following problems, most of which are inherent in the design of the various protocols that SSH can replace:

  • User and host authentication

    SSH uses several strong cryptographic methods to ensure that both the client and the server are who they say they are. Unless both the server and the client agree that the user and host identities are valid, the connection is denied.

  • Encryption of network traffic

    All data transmitted over the network between an SSH client and an SSH server is encrypted with algorithms of varying strength. This ensures that if the network traffic is sniffed (intercepted and read by an unauthorized party), the contents of the packets will be unreadable.

  • Integrity of data transmission

    The SSH protocol assures the integrity of all data transmitted to and from a server. If a third-party attempts to alter the data packets, SSH detects this and alerts the user.

The OpenSSH project

The creation of OpenSSH, a free implementation of both SSH protocol 1 and 2, was undertaken by the OpenBSD project in order to provide a Secure Shell implementation unencumbered by restrictive licensing. OpenSSH was first included with the release of OpenBSD 2.6. The quality and security of the code produced was excellent and it was quickly ported to other UNIX operating systems.

Currently, the development of OpenSSH is divided into two teams. One team does strictly OpenBSD-based development, aiming to produce code that is as clean, simple, and secure as possible. The other team takes the clean version and makes it portable so it builds and runs on many different operating systems, including AIX. The portable releases can be identified by the "p" in the version number (e.g., OpenSSH 3.0.1p1); source distributions without the "p" compile only on OpenBSD.


Gathering the pieces

Recipe Ingredients

OpenSSH, like many other open source software projects, builds on the work and components of other applications to perform its tasks. This allows the developers of OpenSSH to focus on creating the stable and secure code that is at the core of the application, while relying on the expertise and ability of the developers of other applications to ensure that those components perform as designed.

Unfortunately, this model can make the deployment of OpenSSH a bit like a recipe: numerous components need to be downloaded and compiled separately, and the various applications often use different systems for configuration, compilation, and installation of their code.

Obtain the prerequisites

The following open source software projects are required to compile and deploy OpenSSH. The version numbers for each package listed is the most current stable version at the time of this tutorial. Check the project's Web site to find out about updated releases.

Build and install gzip

GNU Zip (gzip) is an open source data compression program similar to the standard UNIX compress/uncompress applications, but unencumbered by patents that might affect its status as free software.

Although gzip is not a prerequisite for building OpenSSH, its use is required in decompressing the source bundles used later in this tutorial. The gzip format is used most often for the distribution of free software on the Internet, and so its presence on an AIX system is almost a requirement.

Fortunately, the source for gzip is available in an uncompressed tape archive (tar) format. After downloading the tarball and saving it into /usr/local/src, execute the following commands:

tar xvf gzip-1.2.4a.tar
cd gzip-1.2.4a
./configure && make check

When the auto-configuration and compilation is complete, the following lines are output:

gzip test OK
rm -f _gztest*

Now, run the make install command (as root); the following files will be installed in the appropriate subdirectories of /usr/local:

/usr/local/man/man1/gzip.1
/usr/local/man/man1/gzexe.1
/usr/local/man/man1/zdiff.1
/usr/local/man/man1/zgrep.1
/usr/local/man/man1/zmore.1
/usr/local/man/man1/znew.1
/usr/local/man/man1/zforce.1
/usr/local/man/man1/zcat.1
/usr/local/man/man1/zcmp.1
/usr/local/man/man1/gunzip.1
/usr/local/bin/gzip
/usr/local/bin/zdiff
/usr/local/bin/zgrep
/usr/local/bin/zmore
/usr/local/bin/znew
/usr/local/bin/zforce
/usr/local/bin/gzexe
/usr/local/bin/zcmp
/usr/local/bin/gunzip
/usr/local/bin/zcat
/usr/local/info/gzip.info

Build and install zlib

Zlib is a lossless, general-purpose compression library used by many open source software projects. The library uses the same compression algorithms used by the gzip program, which are more efficient than those used by UNIX compress.

After downloading the source for the latest version of zlib, place it in /usr/local/src, and run the following commands:

gunzip -c zlib-1.1.3.tar.gz | tar xvf -
cd zlib-1.1.3
vi Makefile

Edit Makefile, adding -qmaxmem=-1 to the end of the CFLAGS line:

CFLAGS=-O -qmaxmem=-1

Run the make test command to compile and test the library. When that process is complete, the last line displayed on the screen will be:

*** zlib test OK ***

As root run make install to install the following header files and library to their correct location:

/usr/local/lib/libz.a
/usr/local/include/zlib.h
/usr/local/include/zconf.h

Note: -qmaxmem=-1 is a option specific to IBM's C for AIX compiler; it tells the compiler to use as much memory as necessary during the compilation in order to obtain the best optimization of the binary.

Build and install prngd

The Pseudo Random Number Generator Daemon, prngd, provides a source of entropy on platforms that do not include a /dev/random file for that purpose. Entropy, or randomness, is an integral part of any encryption process. Generating a quantity of true random data is critical in securing password, secret phrases, and other encrypted data. Many UNIX platforms provide a kernel-level source of random data, via /dev/random.

Unfortunately, AIX 4.3 or 5.1 does not include this source of randomness. On AIX and other systems lacking /dev/random, the prngd application can provide the entropy required by OpenSSH and other cryptographic software.

After downloading the source for the latest version of prngd into /usr/local/src, run the following commands:

gunzip -c prngd-0.9.23.tar.gz | tar xvf -
cd prngd-0.9.23.tar.gz
vi Makefile

Find the AIX 4.3 w/cc section in Makefile; uncomment and add the-qmaxmem=-1 flag to the CFLAGS line so that it appears like the following:

# AIX 4.3 w/cc ("Joerg Petersen <j.petersen@msh.de>)
# Please also check out contrib/AIX-4.3/00README.aix-src
CFLAGS=-O -DAIX43 -qmaxmem=-1
# SYSLIBS=

The source can then be compiled by issuing the make command. The prngd Makefile does not include a rule for installing the daemon; it must be installed manually by running the following command:

mkdir /usr/local/sbin ; cp prngd /usr/local/sbin/
cp contrib/prngd.conf.aix43 /etc/prngd.conf

The longer the prngd daemon process is running, the better the quality of randomness it can provide to other applications that use entropy. Thus, this daemon should be run at startup and should never exit. There are numerous methods of running daemons at startup; this tutorial will present one using the AIX System Resource Controller (SRC). By using SRC, a consistent interface for starting, stopping, and querying the status of the subsystem will be made available.

To create a subsystem for controlling the prngd daemon, issue the followingcommand:

/usr/bin/mkssys -s prngd -p /usr/local/sbin/prngd -a '-f -c 
/etc/prngd.conf -s /var/tmp/egd-seed /dev/egd-pool' -u 0 -S -n 15 
-f 9 -R -G local

The prngd subsystem can now be started by executing the startsrc -s prngd command. To have the prngd subsystem start at system boot, enter the following command, which adds an entry to /etc/inittab:

/usr/sbin/mkitab "prngd:2:wait:startsrc -s prngd > 
/dev/console 2>&1"

Build and install OpenSSL

OpenSSL is an open source implementation of the Secure Sockets Layer (SSL) and Transport Layer Security (TLS) protocols. The general-purpose cryptology libraries provided with OpenSSL are used by a number of encryption-related applications, including OpenSSH.

After downloading the latest source release of OpenSSL into /usr/local/src, run the following commands:

gunzip -c openssl-0.9.6b.tar.gz | tar xvf 
-cd openssl-0.9.6b
./config && make && make test

Note: OpenSSL is a large and complicated package. The compilation and testing can take a very long time, especially on slower systems. When the test suite has completed, text similar to the following will be printed to the screen:

OpenSSL 0.9.6b 9 Jul 2001
built on: Sat Nov 17 17:41:15 PST 2001
platform: aix43-cc
options: bn(64,32) md2(int) rc4(ptr,char) des(idx,cisc,4,long) idea(int) blowfish(idx)    
compiler: cc -DDSO_DLFCN -DHAVE_DLFCN_H -O -DAIX -DB_ENDIAN -qmaxmem=16384
Target "test" is up to date.

Now as root run make install to install the requisite program files:

/usr/local/ssl/man/man1/CA.pl.1
/usr/local/ssl/man/man1/asn1parse.1
/usr/local/ssl/man/man1/ca.1
/usr/local/ssl/man/man1/ciphers.1
/usr/local/ssl/man/man1/crl.1
/usr/local/ssl/man/man1/crl2pkcs7.1
/usr/local/ssl/man/man1/dgst.1
/usr/local/ssl/man/man1/dhparam.1

---[snip]---
many, many similar lines...
/usr/local/ssl/include/openssl/ssl3.h
/usr/local/ssl/include/openssl/ssl23.h
/usr/local/ssl/include/openssl/tls1.h
/usr/local/ssl/misc/CA.sh
/usr/local/ssl/misc/CA.pl
/usr/local/ssl/misc/der_chop
/usr/local/ssl/misc/c_hash
/usr/local/ssl/misc/c_info
/usr/local/ssl/misc/c_issuer
/usr/local/ssl/misc/c_name
/usr/local/ssl/openssl.cnf

Build and install TCP Wrappers (optional)

TCP Wrappers provides a simple application, tcpd, that can be used to limit access to various network services based on the IP address of the client. It is often used, and in fact, was designed for "wrapping" services spawned by inetd. The package also provides a library, libwrap.a, that applications, including OpenSSH, can link to and gain the access controls that TCP Wrappers provides. While it is not necessary for deploying OpenSSH, TCP Wrappers adds another level of access control and logging capability that an administrator might appreciate.

To build TCP Wrappers, issue the following commands after downloading the source distribution into /usr/local/src:

gunzip -c tcp_wrappers_7.6.tar.gz | tar xvf -
cd tcp_wrappers_7.6
vi Makefile

Before compiling the source, several changes need to be made to the Makefile:

  • Uncomment the REAL_DAEMON_DIR line for AIX, so that it appears:
    # SysV.4 Solaris 2.x OSF AIX
    REAL_DAEMON_DIR=/usr/sbin
  • Uncomment the following line:
    #STYLE = -DPROCESS_OPTIONS # Enable language extensions.
  • Change the line:
    FACILITY= LOG_MAIL # LOG_MAIL is what most sendmail daemons use

    to

    FACILITY= LOG_LOCAL7 # tcpd messages will be logged to facility local7
  • Change the line:
     TABLES = -DHOSTS_DENY=\"/etc/hosts.deny\"   
     -DHOSTS_ALLOW=\"/etc/hosts.allow\"

    to

     TABLES = -DHOSTS_DENY=\"/etc/tcpd.conf\"   
     -DHOSTS_ALLOW=\"/etc/tcpd.conf\"
  • Add -qmaxmem=-1 to the CFLAGS block:
    CFLAGS  = -O -DFACILITY=$(FACILITY) $(ACCESS) $(PARANOID) $(NETGROUP) \
            $(BUGS) $(SYSTYPE) $(AUTH) $(UMASK) \
            -DREAL_DAEMON_DIR=\"$(REAL_DAEMON_DIR)\" $(STYLE) $(KILL_OPT) \
            -DSEVERITY=$(SEVERITY) -DRFC931_TIMEOUT=$(RFC931_TIMEOUT) \
            $(UCHAR) $(TABLES) $(STRINGS) $(TLI) $(EXTRA_CFLAGS) $(DOT) \
            $(VSYSLOG) $(HOSTNAME) -qmaxmem=-1

After saving the above changes to the Makefile, run the make aix command to compile the source.

The Makefile for TCP Wrappers does not include an install target. To place the files in the proper locations, enter the following root commands:

cp tcpdchk safe_finger try-from tcpdmatch tcpd /usr/local/sbin/
cp libwrap.a /usr/local/lib/
cp hosts_access.3 /usr/local/man/man3/
cp hosts_access.5 hosts_options.5 /usr/local/man/man5/
cp tcpd.8 tcpdchk.8 tcpdmatch.8 /usr/local/man/man8/
mkdir -p /usr/local/share/tcpd/
cp Banners.Makefile /usr/local/share/tcpd/
mkdir /usr/local/include/
cp tcpd.h /usr/local/include/
touch /etc/tcpd.conf

Configuration of TCP Wrappers will not be detailed in this tutorial. See the included README and man pages for instructions on usage and configuration settings.


Building OpenSSH

Build configuration options

Now that all of the prerequisites are in place, the OpenSSH source can be compiled. After downloading the latest version the OpenSSH source into /usr/local/src, extract the contents with the following commands:

gunzip -c openssh-3.0.1p1.tar.gz | tar xvf 
-cd openssh-3.0.1p1

There are a number of options that must be defined at compile-time and numerous other options that can have their default values set during compilation. For a list and description of all of the compile-time configuration options, type the ./configure --help command in the source directory. For the purposes of this tutorial, the following options are specified:

./configure --sysconfdir=/etc/ssh --with-cflags="-qmaxmem=-1" --with-tcp-wrappers
 --with-xauth=/usr/bin/X11/xauth --with-prngd-socket=/dev/egd-pool --with-ipv4-default
 --with-pid-dir=/var/tmp

When configuration completes, a summary of the options will be printed to the screen, similar to:

OpenSSH has been configured with the following options:
                 User binaries: /usr/local/bin
               System binaries: /usr/local/sbin
           Configuration files: /etc/ssh
               Askpass program: /usr/local/libexec/ssh-askpass
                  Manual pages: /usr/local/man/manX
                      PID file: /var/tmp
        sshd default user PATH: /usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin
      Random number collection: PRNGD/EGD (socket /dev/egd-pool)
                Manpage format: man
                   PAM support: no
            KerberosIV support: no
             Smartcard support: no
                   AFS support: no
                 S/KEY support: no
          TCP Wrappers support: yes
          MD5 password support: no
   IP address in $DISPLAY hack: no
      Use IPv4 by default hack: yes
       Translate v4 in v6 hack: no
              Host: powerpc-ibm-aix4.3.3.0
          Compiler: cc
    Compiler flags: -g -qmaxmem=-1
Preprocessor flags: -I/usr/local/ssl/include  -I/usr/local/include
      Linker flags: -L/usr/local/ssl/lib  -L/usr/local/lib 
        -blibpath:/usr/lib:/lib:/usr/local/lib
         Libraries: -lwrap -lz  -lcrypto

Compile and install the components

Issue the make command to compile the source. When the build is complete, as root run jmake install to install the various files in their proper places.

The following files are installed into /usr/local:

/usr/local/man/man1/ssh.1
/usr/local/man/man1/scp.1
/usr/local/man/man1/ssh-add.1
/usr/local/man/man1/ssh-agent.1
/usr/local/man/man1/ssh-keygen.1
/usr/local/man/man1/ssh-keyscan.1
/usr/local/man/man1/sftp.1
/usr/local/man/man8/sshd.8
/usr/local/man/man8/sftp-server.8
/usr/local/bin/ssh
/usr/local/bin/scp
/usr/local/bin/ssh-add
/usr/local/bin/ssh-agent
/usr/local/bin/ssh-keygen
/usr/local/bin/ssh-keyscan
/usr/local/bin/sftp
/usr/local/sbin/sshd
/usr/local/share/Ssh.bin
/usr/local/libexec/sftp-server

The configuration directory /etc/ssh is also created. It contains following files:

/etc/ssh/ssh_config
/etc/ssh/sshd_config
/etc/ssh/ssh_prng_cmds
/etc/ssh/moduli
/etc/ssh/ssh_host_key
/etc/ssh/ssh_host_key.pub
/etc/ssh/ssh_host_dsa_key
/etc/ssh/ssh_host_dsa_key.pub
/etc/ssh/ssh_host_rsa_key
/etc/ssh/ssh_host_rsa_key.pub

Configuration and usage

The host keys

As part of the install process, host keys are generated and placed in /etc/ssh. There are three types of host keys, explained below. Each host key is comprised of two files (a key pair): a secret portion, whose contents should not be accessable by any user other then root, and a public portion, whose contents are transferred to the client system each time a connection is initiated.

At the start of each new connection to a server, the client compares the public portion of the server's host key to one from a previous connection, saved in a file in the user's home directory. If the current version and previous versions are not identical, the client issues a warning to the user that the server connection may have been spoofed or compromised and should not be trusted. For this reason, it is important to back up the server keys and ensure that those files are not replaced during an upgrade of the OpenSSH software.

The three types of key pairs are:

  • ssh_host_key and ssh_host_key.pub

    This pair of files contains the host key that will be used when clients connect to the server using version 1 of the Secure Shell protocol. This key pair uses the Rivest-Shamir-Adleman (RSA) algorithm for encryption.

  • ssh_host_dsa_key and ssh_host_dsa_key.pub

    This key pair contains the host key used for clients connecting via version 2 of the Secure Shell protocol. It uses the Digital Signature Algorithm (DSA) public-key algorithm for encryption.

  • ssh_host_rsa_key and ssh_host_rsa_key.pub

    This key pair is used with clients connecting with version 2 of the Secure Shell protocol. It also uses the RSA algorithm for encryption, and is most commonly used with key pairs that have been converted from the protocol 1 version for use with protocol 2.

If the file containing the public half of a key pair is damaged, it can easily be regenerated from the secret half with the ssh-keygen utility. If the private half is damaged or compromised by a security breach, the entire key pair is useless and must be regenerated. If this occurs, all users of the system must be told that there is a new host key, and they will have to delete the public portion of the old server key from their ~/.ssh/known_hosts or known_hosts2 file. If they don't, they will receive a warning message each time they connect to the server, or, depending on their local configuration, they will not be able to connect at all.

Setting configuration options

Secure Shell server (sshd) options are defined in the /etc/ssh/sshd_config file. A default file is placed into that location during the install process. For a list and description of all the possible options, see the sshd(8) man page. A few of the more common options, and their recommended settings are:

  • DenyGroups

    This keyword can be followed by a number of group names, separated by spaces. Users whose primary group or supplementary group list matches one of the patterns aren't allowed to log in. "*" and "?" can be used as wildcards in the patterns. Only group names are valid; a numerical group ID is not recognized. By default, login is allowed regardless of the group list.

  • DenyUsers

    This keyword can be followed by a number of user names, separated by spaces. Login is disallowed for user names that match one of the patterns. "*" and "?" can be used as wildcards in the patterns. Only user names are valid; a numerical user ID is not recognized. By default login is allowed regardless of the user name.

  • HostbasedAuthentication

    Specifies whether rhosts or /etc/hosts.equiv authentication together with successful public key client host authentication is allowed (hostbased authentication). This option is similar to RhostsRSAAuthentication and applies to protocol version 2 only. The default is "no".

  • IgnoreRhosts

    Specifies that .rhosts and .shosts files will not be used in RhostsAuthentication, RhostsRSAAuthentication or HostbasedAuthentication. /etc/hosts.equiv and /etc/shosts.equiv are still used. The default is "yes".

  • LogLevel

    Gives the verbosity level that is used when logging messages from sshd. The possible values are: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG. The default is INFO. Logging with level DEBUG violates the privacy of users and is not recommended.

  • PermitRootLogin

    Specifies whether root can login using ssh(1). The argument must be "yes", "without-password", "forced-commands-only" or "no". The default is "yes". If this option is set to "without-password" password authentication is disabled for root. If this option is set to "forced-commands-only", root login with public key authentication will be allowed, but only if the command option has been specified (which may be useful for taking remote backups even if root login is normally not allowed). All other authentication methods are disabled for root. If this option is set to "no" root is not allowed to login.

  • Protocol

    Specifies the protocol versions sshd should support. The possible values are "1" and "2". Multiple versions must be comma-separated. The default is "2,1".

  • RhostsAuthentication

    Specifies whether authentication using rhosts or /etc/hosts.equiv files is sufficient. Normally, this method should not be permitted because it is insecure. RhostsRSAAuthentication should be used instead, because it performs RSA-based host authentication in addition to normal rhosts or /etc/hosts.equiv authentication. The default is "no". This option applies to protocol version 1 only.

  • RhostsRSAAuthentication

    Specifies whether rhosts or /etc/hosts.equiv authentication together with successful RSA host authentication is allowed. The default is "no". This option applies to protocol version 1 only.

  • StrictModes

    Specifies whether sshd should check file modes and ownership of the user's files and home directory before accepting login. This is normally desirable because novices sometimes accidentally leave their directory or files world-writable. The default is "yes".

  • Subsystem

    Configures an external subsystem (e.g., file transfer daemon). Arguments should be a subsystem name and a command to execute upon subsystem request. The sftp-server(8) command implements the "sftp" file transfer subsystem. By default no subsystems are defined. Note that this option applies to protocol version 2 only.

  • SyslogFacility

    Gives the facility code that is used when logging messages from sshd. The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The default is AUTH.

  • X11Forwarding

    Specifies whether X11 forwarding is permitted. The default is "no". Note that disabling X11 forwarding does not improve security in any way, as users can always install their own forwarders. X11 forwarding is automatically disabled if UseLogin is enabled.

Maintaining configuration defaults

It is useful to set options even if the setting is to the default value, as defaults can change between version releases. Using the option settings discussed on the previous panel, the /etc/ssh/sshd_config configuration file look as follows:

#############################################################
#
# /etc/ssh/sshd_config
#
# configuration file for the OpenSSH ssh daemon
#
############################################################

# deny connections from members of these groups:

DenyGroups uucp, mail, nobody, nogroup

# deny connections from these users:

DenyUsers daemon, bin, sys, adm, uucp, guest, nobody, lpd

# allow host-based authentication (.rhosts and /etc/hosts.equiv)
# if the host key exchange was successful.  I realize that this
# is reducing the security of my server.  [  protocol 2 only]

HostbasedAuthentication yes

# permit the use of user .rhosts and .shosts files.  Again, I 
# release that I am reducing the security of my server in favor
# of functionality for clients:

IgnoreRhosts no

# messages logged to syslog from sshd will be at priority INFO:

LogLevel INFO

# root will not be permitted to log in interactively, but can
# run commands remotely ...

PermitRootLogin forced-commands-only

# accept protocol 2 connections first, then fall back to protocol 1

Protocol 2,1

# straight .rhosts authentication will not be permitted, as this is
# exactly the same as "rsh/rcp" ...

RhostsAuthentication no

# however, .rhosts authentication with successful RSA host 
# authentication will be permitted [protocol 1 only]:

RhostsRSAAuthentication yes

# ensure that the permissions on a user's ssh-related files are
# set properly; deny connections if they are not:

StrictModes yes

# define the subsystem "sftp" to enable the secure replacement for
# the ftp protocol:

Subsystem   sftp   /usr/local/libexec/sftp-server

# have syslogd dispatch sshd messages to the AUTH facility ...

SyslogFacility AUTH

# permit the forwarding of X11 connections.  This doesn't decrease
# security at all ...

X11Forwarding yes
############################################################

Running the SSH daemon at system boot

AIX provides a number of ways a program or service can be automatically started on system reboot. This tutorial presents one possibility using the System Resource Controller (SRC) feature of AIX. Using the SRC provides a method of controlling the daemon consistent with other subsystems present on AIX systems.

To create a subsystem that will control the sshd daemon, issue the following command as root:

/usr/bin/mkssys -s sshd -p 
/usr/local/sbin/sshd -a '-D' -u 0 -S -n 15 -f 9 -R -G local

This creates a new subsystem named "sshd". The program started by this subsystem is /usr/local/sbin/sshd with the "-D" argument. The program will be run as root, and will use signals for communication with the SRC. When requested to stop, the daemon is sent the TERM signal, and if that fails, the KILL signal. The subsystem will be restarted if it stops abnormally, and it will be included in the SRC group named "local".

To have the subsystem started at system boot, run the following command to add an entry to /etc/inittab, after the prngd entry:

/usr/sbin/mkitab -i prngd "sshd:2:wait:startsrc -s sshd > 
/dev/console 2>&1"

Replacing insecure network services

Replacing the telnet service

Replacing the use of the insecure telnet protocol with the much more secure ssh protocol is simple. Users will have to substitute the use of an ssh client for their telnet client. When connecting via telnet, the user enters $ telnet earth.

They are then prompted to enter their user name and password:

Trying 123.456.789.012 ...
Connected to earth.galaxy.com
Escape character is '^]

AIX Version 4
(C) Copyrights by IBM and by others 1982, 1996.
login: user
user's Password: ******

If the user account and password are correct, the user is authenticated and provided access to the system. To perform the same action using ssh, the user types:

$ ssh earth
user@earth's password: ******

As with telnet, the user will then be logged in if the user account and password specified are valid. The difference, though, is that all network traffic between the client and the server, including the user name and password, is encrypted, making them immune from packet sniffing attacks. SSH clients usually use the name of the user that is logged in on the client system when connecting to the remote system. If the end user wishes to use a different user account, they will need to add that account name before the host name, joined with an "@" sign. For example:

$ ssh user@earth

The telnet service should be disabled on the server by either deleting or commenting out the telnet entry in /etc/inetd.conf.

Replacing the 'r' services

The r services are those that use only the /etc/hosts.equiv file and the ~/.rhosts files within users home directories to perform authentication. These services include rsh, rlogin, and rcp. All of these services can be replaced by OpenSSH, exponentially increasing the overall security of the server. Several "layers" of security may be imposed on the use of these services, depending on the options set in the /etc/ssh/sshd_config file by the server's administrator.

The OpenSSH distribution includes both the client and server programs necessary to replace the insecure r commands. For the examples presented in the following table, it is assumed that:

  • The fully qualified domain name of the client system is listed in either the /etc/hosts.equiv file on the server named earth, or in the .rhosts file within the user's home directory
  • If the client is a UNIX system, the ssh program is set-UID root, and has the UsePriveligedPort yes option in the /etc/ssh_config configuration file
  • The /etc/sshd_config file on the server earth contains the options: HostbasedAuthentication yes, IgnoreRhosts no, and RhostsRSAAuthentication yes
  • The public key for the client system is in either the server's global known hosts file, or the user's known hosts file.
Insecure commandSecure equivalentAction performedSecurity advantage
rsh earthrlogin earthssh earthslogin earthprovides the user with an interactive login session on the server named earth, without having to enter a password. The host key of the client system is checked against the server's known hosts file. If they do not match, the connection is refused. All communications between the server and the client are encrypted.
rsh earth uptimessh earth uptimeexecutes the uptime command on the server named earth, without having to enter a password.
rcp earth:/etc/passwd /tmp/earth-passwdscp earth:/etc/passwd /tmp/earth-passwdcopies the /etc/passwd file from the server named earth and saves it on the local system as /tmp/earth-passwd, without the user needing to enter a password.

The rexec command, though similarly named, uses a different but also insecure method of authorizing a remote user to run a command on a server without entering her password. Instead of the .rhosts file, a .netrc file in the user's home directory on the client system contains the user name and password. This data, and all other data transferred over the network is sent in clear-text. By using the ssh client's ability to execute commands, use of the rexec service can be avoided, and the daemon that provides this service can be disabled on the server.

In order to take advantage of the increased security provided by the OpenSSH replacements, the login, shell, and exec services should be commented out or deleted from the server's /etc/inetd.conf.

Replacing the ftp service

For those systems that are required to provide an interactive file transfer service, the sftp-server is included with the OpenSSH distribution. Using the same authentication and encryption methods as ssh, users can use the sftp client program to connect to and transfer files to and from remote servers. Operation of the sftp program is similar to standard FTP clients, though the sftp-server program lacks some of the "bells and whistles" of the ftpd daemon.

If the features provided by the sftpd-server program meet the requirements for your FTP service, the standard ftpd daemon should be disabled by commenting out or deleting the ftp entry in /etc/inetd.conf.


Summary

Summary

The Secure Shell protocol is a flexible and powerful tool; this tutorial has only scratched the surface of its capabilities. SSH can be used in many different ways, including but not limited to: securing remote X11 sessions, providing encryption for services not designed with such protection, the use of public keys to provide seamless login and the secure execution of specific commands, and so on.

OpenSSH can be extended to include Kerberos authentication, AFS token passing, Smart Card support, and a number of other related technologies. For more information about using these and other features with OpenSSH, refer to the items listed in the references and resources section on the next panel.

Resources

Learn

  • The web site for the OpenSSH project, http://www.openssh.com/, is the primary source for information about new releases of OpenSSH. It also contains a Frequently Asked Questions (FAQ) page, a form for reporting bugs, and an archive of the various OpenSSH related mailing lists.
  • SSH, The Secure Shell: The Definitive Guide, by Daniel J. Barrett and Richard E. Silverman, published by O'Reilly & Associates, is, as the title states, the definitive guide to the Secure Shell protocol, providing in-depth explanations about almost everything SSH-related. The book also has a Web site, http://www.snailbook.com/, containing news, an FAQ, and discussion forums dedicated to SSH.
  • A number of Internet Drafts (working documents of the Internet Engineering Task Force, IETF) related to Secure Shell have been published: they can be found at the web site http://www.ietf.org/html.charters/secsh-charter.html.

Discuss

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