Moving security page to section 7

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2024-11-16 07:11:05 +01:00 · 1998-12-20 20:11:25 +00:00 · 1998-12-20 20:11:25 +00:00 · 1641c009a5 · 2020-12-20 02:59:44 +00:00
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 #	@(#)Makefile	8.1 (Berkeley) 6/5/93
-#	$Id: Makefile,v 1.5 1998/12/19 09:33:03 dillon Exp $
+#	$Id: Makefile,v 1.6 1998/12/20 06:27:00 bde Exp $
-MAN1=	cd.1 intro.1 security.1 wait.1
+MAN1=	cd.1 intro.1 wait.1
 MLINKS=	intro.1 introduction.1
 .include <bsd.prog.mk>
--- a/share/man/man1/security.1
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@ -1,474 +0,0 @@
 .\" Copyright (c) 1991, 1993
 .\"	The Regents of the University of California.  All rights reserved.
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 .\"	This product includes software developed by the University of
 .\"	California, Berkeley and its contributors.
 .\" 4. Neither the name of the University nor the names of its contributors
 .\"    may be used to endorse or promote products derived from this software
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 .\"
 .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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 .\"     @(#)security.1	8.2 (Berkeley) 12/30/93
 .\"	$Id: security.1,v 1.2 1998/12/20 19:49:43 dillon Exp $
 .\"
 .Dd December 30, 1993
 .Dt SECURITY 1
 .Os
 .Sh NAME
 .Nm security
 .Nd introduction to security under FreeBSD
 .Sh DESCRIPTION
 .Pp
 Security is a function that begins and ends with the system administrator.
 While all
 .Bx
 systems are inherently multi-user capable, the job of building and
 maintaining security mechanisms to keep those users 'honest' is probably
 one of the single largest undertakings of the sysad.  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 simultanious 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 issue.
 .Pp
 Security concerns can be split up into several categories:
 .Bl -enum -offset indent
 .It
 Denial of service attacks
 .It
 User account compromises
 .It
 Root Hacks through accessible servers
 .It
 Root Hacks via user accounts
 .El
 .Pp
 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 servers to limit the load
 they 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.
 .Pp
 A user account compromise is even more common then a D.O.S. attack.  Many
 sysops 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 occassionally looking for suspicious source addresses
 even for successful logins.
 .Pp
 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 remove that user's files and crash the machine, but not touch anyone
 else's files.
 .Pp
 System administrators must keep in mind that there are several 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.
 .Pp
 Security remedies are always implemented in a multi-layered 'onion peel' 
 approach and can be categorized as follows:
 .Bl -enum -offset indent
 .It
 Securing root and staff accounts
 .It
 Securing root - root-run servers and suid/sgid binaries
 .It
 Securing user accounts
 .It
 Securing the password file 
 .It
 Securing the kernel core, raw devices, and filesystems
 .It
 Checking file integrity: binaries, config files, and so forth
 .It
 Paranoia
 .El
 .Sh SECURING THE ROOT ACCOUNT AND SECURING STAFF ACCOUNTS
 .Pp
 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.
 To secure the root account you make sure that it is not possible to login
 to the root account using the root password from a random user account or 
 over the network.  If you haven't already, configure telnetd, rlogind, and
 all other servers that handle login operations to refuse root logins, period,
 whether the right password is given or not.  Allow direct root logins only
 via the system console.  The '/etc/ttys' file comes in handy here and is
 secure by default on most systems, but a good sysad always checks to make sure.
 .Pp
 Of course, as a sysad 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 via their entry in the password file... put staff
 in a 'staff' group or something and only add those that really need root to
 the wheel group.  Unfortunately the wheel mechanism still allows a hacker to
 break root if the hacker has gotten hold of your password file - he need only
 break the root password and the password of one of the staff accounts that
 happens to be in the wheel group.  So while the wheel mechanism is useable,
 it isn't much safer then not having a wheel group at all.
 .Pp
 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 a hacker 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) (see /usr/ports/security/ssh) using a private/public
 keypair.  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 keypair 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 keypair by password 
 protecting the keypair when you create it with ssh-keygen(1).  Being able
 to *-out the passwords for staff accounts also guarentees 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 hackers:  That
 of sniffing the network from an unrelated, less secure machine.
 .Pp
 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 breakins occur remotely, over
 a network, from peopl who do not have physical access to your workstation or
 servers.
 .Pp
 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). 
 .Sh SECURING ROOT - ROOT-RUN SERVERS AND SUID/SGID BINARIES
 .Pp
 The prudent sysop 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 hellofalot 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!
 .Pp
 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
 sysop would research and implement sandboxes for servers whenever possible.
 .Pp
 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.
 .Pp
 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 occassionaly 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 then sorry and the prudent sysad 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 a hacker can break an sgid-kmem binary the
 hacker might be able to read /dev/kmem and thus read the crypted password
 file, potentially compromising any passworded account.  A hacker that breaks
 the tty group can write to almost user's tty.  If a user is running a terminal
 program or emulator with a talk-back feature, the hacker can potentially 
 generate a data stream that causes the user's terminal to echo a command, which
 is then run as that user.
 .Sh SECURING USER ACCOUNTS
 .Pp
 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, but still a very good solution compared to a crypted
 password. 
 .Sh SECURING THE PASSWORD FILE
 .Pp
 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 a hacker to obtain read access to that file even if the 
 attacker cannot obtain root-write access.
 .Pp
 Your security scripts should always check for and report changes to 
 the password file (see 'Checking file integrity' below).
 .Sh SECURING THE KERNEL CORE, RAW DEVICES, AND FILESYSTEMS
 .Pp
 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.  A hacker will commonly attempt to run a packet sniffer
 on a compromised machine.  You do not need to give the hacker the 
 capability and most systems should not have the bpf device compiled in.
 Unfortunately, there is another kernel feature called the Loadable Kernel
 Module interface.  An enterprising hacker can use an LKM to install 
 his own bpf device or other sniffing device on a running kernel.  If you
 do not need to use the module loader, turn it off in the kernel config
 with the NO_LKM option.
 .Pp
 But even if you turn off the bpf device, and turn off the module loader,
 you still have /dev/mem and /dev/kmem to worry about.  For that matter,
 the hacker can still write raw devices.  To avoid this 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.
 .Sh CHECKING FILE INTEGRITY: BINARIES, CONFIG FILES, ETC
 .Pp
 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.  The last layer of your security onion is perhaps
 the most important - detection.
 .Pp
 The only correct way to check a system's file integrity is via another,
 more secure system.  It is fairly easy to setup a 'secure' system: you
 simply do not run any services on it.  With a secure system in place you
 can then give it access to other system's root spaces via ssh.  This may
 seem like a security breech, but you have to put your trust somewhere and
 as long as you don't do something stupid like run random servers it really
 is possible to build a secure machine.  When I say 'secure' here, I assuming
 physical access security as well, of course.  Given a secure machine with
 root access on all your other machines, you can then write security scripts
 ON the secure machine to check the other machines on the system.  The most
 common way of checking is to have the security script scp(1) over a find
 and md5 binary and then ssh a shell command to the remote machine to md5
 all the files in the system (or, at least, the /, /var, and /usr partitions!).
 The security machine copies the results to a file and diff's them against
 results from a previous run (or compares the results against its own 
 binaries), then emails each staff member a daily report of differences.
 .Pp
 Another way to do this sort of check is to NFS export the major filesystems
 from every other machine to the security machine.  This is somewhat more
 network intensive but also virtually impossible for a hacker to detect
 or spoof.
 .Pp
 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 pervue of the MD5 check.
 .Pp
 A good security script will check for suid and sgid binaries on all 
 filesystems and report their absolute existance as well as a diff against
 the previous report or some baseline (say, make a baseline once a week).
 While you can turn off the ability to run suid and sgid binaries on certain
 filesystems through the 'nosuid' option in fstab/mount, you cannot turn this
 off on root and anyone who breaks root can just install their binary their.
 If you have a huge amount of user disk space, though, it may be useful to
 disallow suid binaries and devices ('nodev' option) on the user partitions
 so you do not have to scan them for such.  I would scan them anyway, though,
 at least once a week, since the object of this onion layer is detection of
 a breakin.
 .Pp
 Process accounting (see accton(1)) is a relatively low-overhead feature of
 the operating system which I recommend using as a post-breakin evaluation
 mechanism.  It is especially useful in tracking down how a hacker has 
 actually broken root on a system, assuming the file is still intact after
 the breakin occurs.
 .Pp
 Finally, security scripts should process the log files and the logs themselves
 should be generated in as secured a manner as possible - remote syslog can be
 very useful.  A hacker tries to cover his tracks, and log files are critical
 to the sysop trying to track down the time and method of the initial breakin.
 .Sh PARANOIA
 .Pp
 A little paranoia never hurts.  As a rule, a sysop 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.
 .Sh SPECIAL SECTION ON D.O.S. ATTACKS
 .Pp
 This section covers Dential 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.  
 .Bl -enum -offset indent
 .It
 Limiting server forks
 .It
 Limiting springboard attacks (ICMP response attacks, ping broadcast, etc...)
 .It
 Kernel Route Cache
 .El
 .Pp
 A common DOS attack is against a forking server that attempts to cause the
 server to eat processes, file descirptors, 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.  
 .Pp
 Sendmail has its -OMaxDaemonChildren option which tends to work much
 better then 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.
 .Pp
 Syslogd can be attacked directly and it is strongly recommended that you use
 the -s option whenever possible, and the -a option otherwise.
 .Pp
 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.
 .Pp
 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 root network-based root hacks.  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 ).
 .Pp
 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 spoofed 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 sysad will turn off all
 of these inetd-internal test services.
 .Pp
 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 then 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.
 .Sh SEE ALSO
 .Pp
 .Xr ssh 1 ,
 .Xr sshd 1 ,
 .Xr kerberos 1 ,
 .Xr accton 1 ,
 .Xr xdm 1 ,
 .Xr syslogd 1 ,
 .Xr chflags 1 ,
 .Xr find 1 ,
 .Xr md5 1 ,
 .Xr sysctl 8
 .Sh HISTORY
 The
 .Nm
 manual page was originally written by Matthew Dillon and first appeared 
 in FreeBSD-3.0.1, December 1998.