*nix Documentation Project
·  Home
 +   man pages
·  Linux HOWTOs
·  FreeBSD Tips
·  *niX Forums

  man pages->FreeBSD man pages -> security (7)              



NAME    [Toc]    [Back]

     security -- introduction to security under FreeBSD

DESCRIPTION    [Toc]    [Back]

     Security is a function that begins and ends with the system administrator.
  While all BSD multi-user systems have some inherent security, the
     job of building and maintaining additional security mechanisms to keep
     users `honest' is probably one of the single largest undertakings of the
     sysadmin.	Machines are only as secure as you make them, and security
     concerns are ever competing with the human necessity for convenience.
     UNIX systems, in general, are capable of running a huge number of simultaneous
 processes and many of these processes operate as servers - meaning
 that external entities can connect and talk to them.  As yesterday's
     mini-computers and mainframes become today's desktops, and as computers
     become networked and internetworked, security becomes an ever bigger

     Security is best implemented through a layered onion approach.  In a nutshell,
 what you want to do is to create as many layers of security as are
     convenient and then carefully monitor the system for intrusions.  You do
     not want to overbuild your security or you will interfere with the detection
 side, and detection is one of the single most important aspects of
     any security mechanism.  For example, it makes little sense to set the
     schg flags (see chflags(1)) on every system binary because while this may
     temporarily protect the binaries, it prevents a hacker who has broken in
     from making an easily detectable change that may result in your security
     mechanisms not detecting the hacker at all.

     System security also pertains to dealing with various forms of attack,
     including attacks that attempt to crash or otherwise make a system unusable
 but do not attempt to break root.  Security concerns can be split up
     into several categories:

	   1.	Denial of service attacks

	   2.	User account compromises

	   3.	Root compromise through accessible servers

	   4.	Root compromise via user accounts

	   5.	Backdoor creation

     A denial of service attack is an action that deprives the machine of
     needed resources.	Typically, D.O.S. attacks are brute-force mechanisms
     that attempt to crash or otherwise make a machine unusable by overwhelming
 its servers or network stack.	Some D.O.S. attacks try to take advantages
 of bugs in the networking stack to crash a machine with a single
     packet.  The latter can only be fixed by applying a bug fix to the kernel.
  Attacks on servers can often be fixed by properly specifying
     options to limit the load the servers incur on the system under adverse
     conditions.  Brute-force network attacks are harder to deal with.	A
     spoofed-packet attack, for example, is nearly impossible to stop short of
     cutting your system off from the Internet.  It may not be able to take
     your machine down, but it can fill up Internet pipe.

     A user account compromise is even more common than a D.O.S. attack.  Many
     sysadmins still run standard telnetd, rlogind, rshd, and ftpd servers on
     their machines.  These servers, by default, do not operate over encrypted
     connections.  The result is that if you have any moderate-sized user
     base, one or more of your users logging into your system from a remote
     location (which is the most common and convenient way to login to a system)
 will have his or her password sniffed.  The attentive system admin
     will analyze his remote access logs looking for suspicious source
     addresses even for successful logins.

     One must always assume that once an attacker has access to a user
     account, the attacker can break root.  However, the reality is that in a
     well secured and maintained system, access to a user account does not
     necessarily give the attacker access to root.  The distinction is important
 because without access to root the attacker cannot generally hide
     his tracks and may, at best, be able to do nothing more than mess with
     the user's files or crash the machine.  User account compromises are very
     common because users tend not to take the precautions that sysadmins

     System administrators must keep in mind that there are potentially many
     ways to break root on a machine.  The attacker may know the root password,
 the attacker may find a bug in a root-run server and be able to
     break root over a network connection to that server, or the attacker may
     know of a bug in an suid-root program that allows the attacker to break
     root once he has broken into a user's account.  If an attacker has found
     a way to break root on a machine, the attacker may not have a need to
     install a backdoor.  Many of the root holes found and closed to date
     involve a considerable amount of work by the hacker to cleanup after himself,
 so most hackers do install backdoors.  This gives you a convenient
     way to detect the hacker.	Making it impossible for a hacker to install a
     backdoor may actually be detrimental to your security because it will not
     close off the hole the hacker found to break in the first place.

     Security remedies should always be implemented with a multi-layered
     `onion peel' approach and can be categorized as follows:

	   1.	Securing root and staff accounts

	   2.	Securing root - root-run servers and suid/sgid binaries

	   3.	Securing user accounts

	   4.	Securing the password file

	   5.	Securing the kernel core, raw devices, and file systems

	   6.	Quick detection of inappropriate changes made to the system

	   7.	Paranoia


     Don't bother securing staff accounts if you haven't secured the root
     account.  Most systems have a password assigned to the root account.  The
     first thing you do is assume that the password is `always' compromised.
     This does not mean that you should remove the password.  The password is
     almost always necessary for console access to the machine.  What it does
     mean is that you should not make it possible to use the password outside
     of the console or possibly even with a su(1) command.  For example, make
     sure that your pty's are specified as being unsecure in the `/etc/ttys'
     file so that direct root logins via telnet or rlogin are disallowed.  If
     using other login services such as sshd, make sure that direct root
     logins are disabled there as well.  Consider every access method - services
 such as ftp often fall through the cracks.  Direct root logins
     should only be allowed via the system console.

     Of course, as a sysadmin you have to be able to get to root, so we open
     up a few holes.  But we make sure these holes require additional password
     verification to operate.  One way to make root accessible is to add
     appropriate staff accounts to the wheel group (in /etc/group).  The staff
     members placed in the wheel group are allowed to `su' to root.  You
     should never give staff members native wheel access by putting them in
     the wheel group in their password entry.  Staff accounts should be placed
     in a `staff' group, and then added to the wheel group via the
     `/etc/group' file.  Only those staff members who actually need to have
     root access should be placed in the wheel group.  It is also possible,
     when using an authentication method such as kerberos, to use kerberos's
     `.k5login' file in the root account to allow a ksu(1) to root without
     having to place anyone at all in the wheel group.	This may be the better
     solution since the wheel mechanism still allows an intruder to break root
     if the intruder has gotten hold of your password file and can break into
     a staff account.  While having the wheel mechanism is better than having
     nothing at all, it isn't necessarily the safest option.

     An indirect way to secure the root account is to secure your staff
     accounts by using an alternative login access method and *'ing out the
     crypted password for the staff accounts.  This way an intruder may be
     able to steal the password file but will not be able to break into any
     staff accounts (or, indirectly, root, even if root has a crypted password
     associated with it).  Staff members get into their staff accounts through
     a secure login mechanism such as kerberos(1) or ssh(1) using a private/public
 key pair.  When you use something like kerberos you generally
     must secure the machines which run the kerberos servers and your desktop
     workstation.  When you use a public/private key pair with ssh, you must
     generally secure the machine you are logging in FROM (typically your
     workstation), but you can also add an additional layer of protection to
     the key pair by password protecting the keypair when you create it with
     ssh-keygen(1).  Being able to *-out the passwords for staff accounts also
     guarantees that staff members can only login through secure access methods
 that you have setup.  You can thus force all staff members to use
     secure, encrypted connections for all their sessions which closes an
     important hole used by many intruders:  That of sniffing the network from
     an unrelated, less secure machine.

     The more indirect security mechanisms also assume that you are logging in
     from a more restrictive server to a less restrictive server.  For example,
 if your main box is running all sorts of servers, your workstation
     shouldn't be running any.	In order for your workstation to be reasonably
     secure you should run as few servers as possible, up to and including no
     servers at all, and you should run a password-protected screen blanker.
     Of course, given physical access to a workstation an attacker can break
     any sort of security you put on it.  This is definitely a problem that
     you should consider but you should also consider the fact that the vast
     majority of break-ins occur remotely, over a network, from people who do
     not have physical access to your workstation or servers.

     Using something like kerberos also gives you the ability to disable or
     change the password for a staff account in one place and have it immediately
 effect all the machine the staff member may have an account on.  If
     a staff member's account gets compromised, the ability to instantly
     change his password on all machines should not be underrated.  With discrete
 passwords, changing a password on N machines can be a mess.	You
     can also impose re-passwording restrictions with kerberos:  not only can
     a kerberos ticket be made to timeout after a while, but the kerberos system
 can require that the user choose a new password after a certain
     period of time (say, once a month).

     The prudent sysadmin only runs the servers he needs to, no more, no less.
     Be aware that third party servers are often the most bug-prone.  For
     example, running an old version of imapd or popper is like giving a universal
 root ticket out to the entire world.  Never run a server that you
     have not checked out carefully.  Many servers do not need to be run as
     root.  For example, the ntalk, comsat, and finger daemons can be run in
     special user `sandboxes'.	A sandbox isn't perfect unless you go to a
     large amount of trouble, but the onion approach to security still stands:
     If someone is able to break in through a server running in a sandbox,
     they still have to break out of the sandbox.  The more layers the
     attacker must break through, the lower the likelihood of his success.
     Root holes have historically been found in virtually every server ever
     run as root, including basic system servers.  If you are running a
     machine through which people only login via sshd and never login via telnetd
 or rshd or rlogind, then turn off those services!

     FreeBSD now defaults to running ntalkd, comsat, and finger in a sandbox.
     Another program which may be a candidate for running in a sandbox is
     named(8).	The default rc.conf includes the arguments necessary to run
     named in a sandbox in a commented-out form.  Depending on whether you are
     installing a new system or upgrading an existing system, the special user
     accounts used by these sandboxes may not be installed.  The prudent
     sysadmin would research and implement sandboxes for servers whenever possible.

     There are a number of other servers that typically do not run in sandboxes:
 sendmail, popper, imapd, ftpd, and others.	There are alternatives
     to some of these, but installing them may require more work then you are
     willing to put (the convenience factor strikes again).  You may have to
     run these servers as root and rely on other mechanisms to detect breakins
 that might occur through them.

     The other big potential root hole in a system are the suid-root and sgid
     binaries installed on the system.	Most of these binaries, such as
     rlogin, reside in /bin, /sbin, /usr/bin, or /usr/sbin.  While nothing is
     100% safe, the system-default suid and sgid binaries can be considered
     reasonably safe.  Still, root holes are occasionally found in these binaries.
  A root hole was found in Xlib in 1998 that made xterm (which is
     typically suid) vulnerable.  It is better to be safe than sorry and the
     prudent sysadmin will restrict suid binaries that only staff should run
     to a special group that only staff can access, and get rid of (chmod 000)
     any suid binaries that nobody uses.  A server with no display generally
     does not need an xterm binary.  Sgid binaries can be almost as dangerous.
     If an intruder can break an sgid-kmem binary the intruder might be able
     to read /dev/kmem and thus read the crypted password file, potentially
     compromising any passworded account.  Alternatively an intruder who
     breaks group kmem can monitor keystrokes sent through pty's, including
     pty's used by users who login through secure methods.  An intruder that
     breaks the tty group can write to almost any user's tty.  If a user is
     running a terminal program or emulator with a keyboard-simulation feature,
 the intruder can potentially generate a data stream that causes the
     user's terminal to echo a command, which is then run as that user.


     User accounts are usually the most difficult to secure.  While you can
     impose Draconian access restrictions on your staff and *-out their passwords,
 you may not be able to do so with any general user accounts you
     might have.  If you do have sufficient control then you may win out and
     be able to secure the user accounts properly.  If not, you simply have to
     be more vigilant in your monitoring of those accounts.  Use of ssh and
     kerberos for user accounts is more problematic due to the extra administration
 and technical support required, but still a very good solution
     compared to a crypted password file.


     The only sure fire way is to *-out as many passwords as you can and use
     ssh or kerberos for access to those accounts.  Even though the crypted
     password file (/etc/spwd.db) can only be read by root, it may be possible
     for an intruder to obtain read access to that file even if the attacker
     cannot obtain root-write access.

     Your security scripts should always check for and report changes to the
     password file (see `Checking file integrity' below).

     If an attacker breaks root he can do just about anything, but there are
     certain conveniences.  For example, most modern kernels have a packet
     sniffing device driver built in.  Under FreeBSD it is called the `bpf'
     device.  An intruder will commonly attempt to run a packet sniffer on a
     compromised machine.  You do not need to give the intruder the capability
     and most systems should not have the bpf device compiled in.

     But even if you turn off the bpf device, you still have /dev/mem and
     /dev/kmem to worry about.	For that matter, the intruder can still write
     to raw disk devices.  Also, there is another kernel feature called the
     module loader, kldload(8).  An enterprising intruder can use a KLD module
     to install his own bpf device or other sniffing device on a running kernel.
  To avoid these problems you have to run the kernel at a higher
     secure level, at least securelevel 1.  The securelevel can be set with a
     sysctl on the kern.securelevel variable.  Once you have set the
     securelevel to 1, write access to raw devices will be denied and special
     chflags flags, such as `schg', will be enforced.  You must also ensure
     that the `schg' flag is set on critical startup binaries, directories,
     and script files - everything that gets run up to the point where the
     securelevel is set.  This might be overdoing it, and upgrading the system
     is much more difficult when you operate at a higher secure level.	You
     may compromise and run the system at a higher secure level but not set
     the schg flag for every system file and directory under the sun.  Another
     possibility is to simply mount / and /usr read-only.  It should be noted
     that being too draconian in what you attempt to protect may prevent the
     all-important detection of an intrusion.

     When it comes right down to it, you can only protect your core system
     configuration and control files so much before the convenience factor
     rears its ugly head.  For example, using chflags to set the schg bit on
     most of the files in / and /usr is probably counterproductive because
     while it may protect the files, it also closes a detection window.  The
     last layer of your security onion is perhaps the most important - detection.
  The rest of your security is pretty much useless (or, worse,
     presents you with a false sense of safety) if you cannot detect potential
     incursions.  Half the job of the onion is to slow down the attacker
     rather than stop him in order to give the detection side of the equation
     a chance to catch him in the act.

     The best way to detect an incursion is to look for modified, missing, or
     unexpected files.	The best way to look for modified files is from
     another (often centralized) limited-access system.  Writing your security
     scripts on the extra-secure limited-access system makes them mostly
     invisible to potential hackers, and this is important.  In order to take
     maximum advantage you generally have to give the limited-access box significant
 access to the other machines in the business, usually either by
     doing a read-only NFS export of the other machines to the limited-access
     box, or by setting up ssh keypairs to allow the limit-access box to ssh
     to the other machines.  Except for its network traffic, NFS is the least
     visible method - allowing you to monitor the file systems on each client
     box virtually undetected.	If your limited-access server is connected to
     the client boxes through a switch, the NFS method is often the better
     choice.  If your limited-access server is connected to the client boxes
     through a hub or through several layers of routing, the NFS method may be
     too insecure (network-wise) and using ssh may be the better choice even
     with the audit-trail tracks that ssh lays.

     Once you give a limit-access box at least read access to the client systems
 it is supposed to monitor, you must write scripts to do the actual
     monitoring.  Given an NFS mount, you can write scripts out of simple system
 utilities such as find(1) and md5(1) It is best to physically md5 the
     client-box files boxes at least once a day, and to test control files
     such as those found in /etc and /usr/local/etc even more often.  When
     mismatches are found relative to the base md5 information the limitedaccess
 machine knows is valid, it should scream at a sysadmin to go check
     it out.  A good security script will also check for inappropriate suid
     binaries and for new or deleted files on system partitions such as / and

     When using ssh rather than NFS, writing the security script is much more
     difficult.   You essentially have to scp the scripts to the client box in
     order to run them, making them visible, and for safety you also need to
     scp the binaries (such as find) that those scripts use.  The ssh daemon
     on the client box may already be compromised.  All in all, using ssh may
     be necessary when running over unsecure links, but it's also a lot harder
     to deal with.

     A good security script will also check for changes to user and staff members
 access configuration files: .rhosts, .shosts, .ssh/authorized_keys
     and so forth... files that might fall outside the purview of the MD5

     If you have a huge amount of user disk space it may take too long to run
     through every file on those partitions.  In this case, setting mount
     flags to disallow suid binaries and devices on those partitions is a good
     idea.  The `nodev' and `nosuid' options (see mount(8)) are what you want
     to look into.  I would scan them anyway at least once a week, since the
     object of this layer is to detect a break-in whether or not the breakin
     is effective.

     Process accounting (see accton(8)) is a relatively low-overhead feature
     of the operating system which I recommend using as a post-break-in evaluation
 mechanism.  It is especially useful in tracking down how an
     intruder has actually broken into a system, assuming the file is still
     intact after the break-in occurs.

     Finally, security scripts should process the log files and the logs themselves
 should be generated in as secure a manner as possible - remote
     syslog can be very useful.  An intruder tries to cover his tracks, and
     log files are critical to the sysadmin trying to track down the time and
     method of the initial break-in.  One way to keep a permanent record of
     the log files is to run the system console to a serial port and collect
     the information on a continuing basis through a secure machine monitoring
     the consoles.

PARANOIA    [Toc]    [Back]

     A little paranoia never hurts.  As a rule, a sysadmin can add any number
     of security features as long as they do not effect convenience, and can
     add security features that do effect convenience with some added thought.
     Even more importantly, a security administrator should mix it up a bit -
     if you use recommendations such as those given by this manual page verbatim,
 you give away your methodologies to the prospective hacker who also
     has access to this manual page.


     This section covers Denial of Service attacks.  A DOS attack is typically
     a packet attack.  While there isn't much you can do about modern spoofed
     packet attacks that saturate your network, you can generally limit the
     damage by ensuring that the attacks cannot take down your servers.

	   1.	Limiting server forks

	   2.	Limiting springboard attacks (ICMP response attacks, ping
		broadcast, etc...)

	   3.	Kernel Route Cache

     A common DOS attack is against a forking server that attempts to cause
     the server to eat processes, file descriptors, and memory until the
     machine dies.  Inetd (see inetd(8)) has several options to limit this
     sort of attack.  It should be noted that while it is possible to prevent
     a machine from going down it is not generally possible to prevent a service
 from being disrupted by the attack.  Read the inetd manual page
     carefully and pay specific attention to the -c, -C, and -R options.  Note
     that spoofed-IP attacks will circumvent the -C option to inetd, so typically
 a combination of options must be used.  Some standalone servers
     have self-fork-limitation parameters.

     Sendmail has its -OMaxDaemonChildren option which tends to work much better
 than trying to use sendmail's load limiting options due to the load
     lag.  You should specify a MaxDaemonChildren parameter when you start
     sendmail high enough to handle your expected load but no so high that the
     computer cannot handle that number of sendmails without falling on its
     face.  It is also prudent to run sendmail in queued mode
     (-ODeliveryMode=queued) and to run the daemon (sendmail -bd) separate
     from the queue-runs (sendmail -q15m).  If you still want realtime delivery
 you can run the queue at a much lower interval, such as -q1m, but be
     sure to specify a reasonable MaxDaemonChildren option for that sendmail
     to prevent cascade failures.

     Syslogd can be attacked directly and it is strongly recommended that you
     use the -s option whenever possible, and the -a option otherwise.

     You should also be fairly careful with connect-back services such as tcpwrapper's
 reverse-identd, which can be attacked directly.	You generally
     do not want to use the reverse-ident feature of tcpwrappers for this reason.

     It is a very good idea to protect internal services from external access
     by firewalling them off at your border routers.  The idea here is to prevent
 saturation attacks from outside your LAN, not so much to protect
     internal services from network-based root compromise.  Always configure
     an exclusive firewall, i.e. `firewall everything *except* ports A, B, C,
     D, and M-Z'.  This way you can firewall off all of your low ports except
     for certain specific services such as named (if you are primary for a
     zone), ntalkd, sendmail, and other internet-accessible services.  If you
     try to configure the firewall the other way - as an inclusive or permissive
 firewall, there is a good chance that you will forget to `close' a
     couple of services or that you will add a new internal service and forget
     to update the firewall.  You can still open up the high-numbered port
     range on the firewall to allow permissive-like operation without compromising
 your low ports.  Also take note that FreeBSD allows you to control
     the range of port numbers used for dynamic binding via the various
     net.inet.ip.portrange sysctl's (sysctl -a | fgrep portrange), which can
     also ease the complexity of your firewall's configuration.  I usually use
     a normal first/last range of 4000 to 5000, and a hiport range of 49152 to
     65535, then block everything under 4000 off in my firewall (except for
     certain specific internet-accessible ports, of course).

     Another common DOS attack is called a springboard attack - to attack a
     server in a manner that causes the server to generate responses which
     then overload the server, the local network, or some other machine.  The
     most common attack of this nature is the ICMP PING BROADCAST attack.  The
     attacker spoofs ping packets sent to your LAN's broadcast address with
     the source IP address set to the actual machine they wish to attack.  If
     your border routers are not configured to stomp on ping's to broadcast
     addresses, your LAN winds up generating sufficient responses to the
     spoofed source address to saturate the victim, especially when the
     attacker uses the same trick on several dozen broadcast addresses over
     several dozen different networks at once.	Broadcast attacks of over a
     hundred and twenty megabits have been measured.  A second common springboard
 attack is against the ICMP error reporting system.  By constructing
     packets that generate ICMP error responses, an attacker can saturate a
     server's incoming network and cause the server to saturate its outgoing
     network with ICMP responses.  This type of attack can also crash the
     server by running it out of mbuf's, especially if the server cannot drain
     the ICMP responses it generates fast enough.  The FreeBSD kernel has a
     new kernel compile option called ICMP_BANDLIM which limits the effectiveness
 of these sorts of attacks.  The last major class of springboard
     attacks is related to certain internal inetd services such as the udp
     echo service.  An attacker simply spoofs a UDP packet with the source
     address being server A's echo port, and the destination address being
     server B's echo port, where server A and B are both on your LAN.  The two
     servers then bounce this one packet back and forth between each other.
     The attacker can overload both servers and their LANs simply by injecting
     a few packets in this manner.  Similar problems exist with the internal
     chargen port.  A competent sysadmin will turn off all of these inetdinternal
 test services.

     Spoofed packet attacks may also be used to overload the kernel route
     cache.  Refer to the net.inet.ip.rtexpire, rtminexpire, and rtmaxcache
     sysctl parameters.  A spoofed packet attack that uses a random source IP
     will cause the kernel to generate a temporary cached route in the route
     table, viewable with `netstat -rna | fgrep W3'.  These routes typically
     timeout in 1600 seconds or so.  If the kernel detects that the cached
     route table has gotten too big it will dynamically reduce the rtexpire
     but will never decrease it to less than rtminexpire.  There are two problems:
  (1) The kernel does not react quickly enough when a lightly loaded
     server is suddenly attacked, and (2) The rtminexpire is not low enough
     for the kernel to survive a sustained attack.  If your servers are connected
 to the internet via a T3 or better it may be prudent to manually
     override both rtexpire and rtminexpire via sysctl(8).  Never set either
     parameter to zero (unless you want to crash the machine :-)).  Setting
     both parameters to 2 seconds should be sufficient to protect the route
     table from attack.


     There are a few issues with both kerberos and ssh that need to be
     addressed if you intend to use them.  Kerberos V is an excellent authentication
 protocol but the kerberized telnet and rlogin suck rocks.  There
     are bugs that make them unsuitable for dealing with binary streams.
     Also, by default kerberos does not encrypt a session unless you use the
     -x option.  Ssh encrypts everything by default.

     Ssh works quite well in every respect except when it is set up to forward
     encryption keys.  What this means is that if you have a secure workstation
 holding keys that give you access to the rest of the system, and you
     ssh to an unsecure machine, your keys become exposed.  The actual keys
     themselves are not exposed, but ssh installs a forwarding port for the
     duration of your login and if a hacker has broken root on the unsecure
     machine he can utilize that port to use your keys to gain access to any
     other machine that your keys unlock.

     We recommend that you use ssh in combination with kerberos whenever possible
 for staff logins.  Ssh can be compiled with kerberos support.  This
     reduces your reliance on potentially exposable ssh keys while at the same
     time protecting passwords via kerberos.  Ssh keys should only be used for
     automated tasks from secure machines (something that kerberos is unsuited
     to).  We also recommend that you either turn off key-forwarding in the
     ssh configuration, or that you make use of the from=IP/DOMAIN option that
     ssh allows in its authorized_keys file to make the key only usable to
     entities logging in from specific machines.

SEE ALSO    [Toc]    [Back]

     chflags(1), find(1), md5(1), netstat(1), openssl(1), ssh(1), xdm(1),
     group(5), ttys(5), accton(8), init(8), kerberos(8), sshd(8), sysctl(8),
     syslogd(8), vipw(8)

HISTORY    [Toc]    [Back]

     The security manual page was originally written by Matthew Dillon and
     first appeared in FreeBSD 3.1, December 1998.

FreeBSD 5.2.1		      September 18, 1999		 FreeBSD 5.2.1
[ Back ]
 Similar pages
Name OS Title
development FreeBSD introduction to development with the FreeBSD codebase
mac FreeBSD introduction to the MAC security API
acl FreeBSD introduction to the POSIX.1e ACL security API
posix1e FreeBSD introduction to the POSIX.1e security API
posix1e OpenBSD introduction to the POSIX.1e security API
sec_intro HP-UX Introduction to the DCE Security administrative commands
sec_intro HP-UX Introduction to the DCE Security administrative files
mtio FreeBSD FreeBSD magtape interface
firewall FreeBSD simple firewalls under FreeBSD
crash FreeBSD FreeBSD system failures
Copyright © 2004-2005 DeniX Solutions SRL
newsletter delivery service