1.\" Copyright (c) 1998, Matthew Dillon. Terms and conditions are those of
2.\" the BSD Copyright as specified in the file "/usr/src/COPYRIGHT" in
3.\" the source tree.
4.\"
5.\" $FreeBSD: head/share/man/man7/security.7 130524 2004-06-15 12:48:50Z ru $
6.\"
7.Dd September 18, 1999
8.Dt SECURITY 7
9.Os
10.Sh NAME
11.Nm security
12.Nd introduction to security under
13.Fx
14.Sh DESCRIPTION
15Security is a function that begins and ends with the system administrator.
16While all
17.Bx
18multi-user systems have some inherent security, the job of building and
19maintaining additional security mechanisms to keep users
20.Dq honest
21is probably
22one of the single largest undertakings of the sysadmin.
23Machines are
24only as secure as you make them, and security concerns are ever competing
25with the human necessity for convenience.
26.Ux
27systems,
28in general, are capable of running a huge number of simultaneous processes
29and many of these processes operate as servers \(em meaning that external
30entities can connect and talk to them.
31As yesterday's mini-computers and mainframes
32become today's desktops, and as computers become networked and internetworked,
33security becomes an ever bigger issue.
34.Pp
35Security is best implemented through a layered onion approach.
36In a nutshell,
37what you want to do is to create as many layers of security as are convenient
38and then carefully monitor the system for intrusions.
39You do not want to
40overbuild your security or you will interfere with the detection side, and
41detection is one of the single most important aspects of any security
42mechanism.
43For example, it makes little sense to set the
44.Cm schg
45flags
46(see
47.Xr chflags 1 )
48on every system binary because while this may temporarily protect the
49binaries, it prevents an attacker who has broken in from making an
50easily detectable change that may result in your security mechanisms not
51detecting the attacker at all.
52.Pp
53System security also pertains to dealing with various forms of attacks,
54including attacks that attempt to crash or otherwise make a system unusable
55but do not attempt to break root.
56Security concerns can be split up into
57several categories:
58.Bl -enum -offset indent
59.It
60Denial of Service attacks (DoS)
61.It
62User account compromises
63.It
64Root compromise through accessible servers
65.It
66Root compromise via user accounts
67.It
68Backdoor creation
69.El
70.Pp
71A denial of service attack is an action that deprives the machine of needed
72resources.
73Typically, DoS attacks are brute-force mechanisms that attempt
74to crash or otherwise make a machine unusable by overwhelming its servers or
75network stack.
76Some DoS attacks try to take advantages of bugs in the
77networking stack to crash a machine with a single packet.
78The latter can
79only be fixed by applying a bug fix to the kernel.
80Attacks on servers can
81often be fixed by properly specifying options to limit the load the servers
82incur on the system under adverse conditions.
83Brute-force network attacks are harder to deal with.
84A spoofed-packet attack, for example, is
85nearly impossible to stop short of cutting your system off from the Internet.
86It may not be able to take your machine down, but it can fill up Internet
87pipe.
88.Pp
89A user account compromise is even more common than a DoS attack.
90Many
91sysadmins still run standard
92.Xr telnetd 8 ,
93.Xr rlogind 8 ,
94.Xr rshd 8 ,
95and
96.Xr ftpd 8
97servers on their machines.
98These servers, by default, do not operate over encrypted
99connections.
100The result is that if you have any moderate-sized user base,
101one or more of your users logging into your system from a remote location
102(which is the most common and convenient way to log in to a system)
103will have his or her password sniffed.
104The attentive system administrator will analyze
105his remote access logs looking for suspicious source addresses
106even for successful logins.
107.Pp
108One must always assume that once an attacker has access to a user account,
109the attacker can break root.
110However, the reality is that in a well secured
111and maintained system, access to a user account does not necessarily give the
112attacker access to root.
113The distinction is important because without access
114to root the attacker cannot generally hide his tracks and may, at best, be
115able to do nothing more than mess with the user's files or crash the machine.
116User account compromises are very common because users tend not to take the
117precautions that sysadmins take.
118.Pp
119System administrators must keep in mind that there are potentially many ways
120to break root on a machine.
121The attacker may know the root password,
122the attacker
123may find a bug in a root-run server and be able to break root over a network
124connection to that server, or the attacker may know of a bug in an SUID-root
125program that allows the attacker to break root once he has broken into a
126user's account.
127If an attacker has found a way to break root on a machine,
128the attacker may not have a need to install a backdoor.
129Many of the root holes found and closed to date involve a considerable amount
130of work by the attacker to clean up after himself, so most attackers do install
131backdoors.
132This gives you a convenient way to detect the attacker.
133Making
134it impossible for an attacker to install a backdoor may actually be detrimental
135to your security because it will not close off the hole the attacker used to
136break in in the first place.
137.Pp
138Security remedies should always be implemented with a multi-layered
139.Dq onion peel
140approach and can be categorized as follows:
141.Bl -enum -offset indent
142.It
143Securing root and staff accounts
144.It
145Securing root \(em root-run servers and SUID/SGID binaries
146.It
147Securing user accounts
148.It
149Securing the password file
150.It
151Securing the kernel core, raw devices, and file systems
152.It
153Quick detection of inappropriate changes made to the system
154.It
155Paranoia
156.El
157.Sh SECURING THE ROOT ACCOUNT AND SECURING STAFF ACCOUNTS
158Do not bother securing staff accounts if you have not secured the root
159account.
160Most systems have a password assigned to the root account.
161The
162first thing you do is assume that the password is
163.Em always
164compromised.
165This does not mean that you should remove the password.
166The
167password is almost always necessary for console access to the machine.
168What it does mean is that you should not make it possible to use the password
169outside of the console or possibly even with a
170.Xr su 1
171utility.
172For example, make sure that your PTYs are specified as being
173.Dq Li unsecure
174in the
175.Pa /etc/ttys
176file
177so that direct root logins via
178.Xr telnet 1
179or
180.Xr rlogin 1
181are disallowed.
182If using
183other login services such as
184.Xr sshd 8 ,
185make sure that direct root logins are
186disabled there as well.
187Consider every access method \(em services such as
188.Xr ftp 1
189often fall through the cracks.
190Direct root logins should only be allowed
191via the system console.
192.Pp
193Of course, as a sysadmin you have to be able to get to root, so we open up
194a few holes.
195But we make sure these holes require additional password
196verification to operate.
197One way to make root accessible is to add appropriate
198staff accounts to the
199.Dq Li wheel
200group (in
201.Pa /etc/group ) .
202The staff members placed in the
203.Li wheel
204group are allowed to
205.Xr su 1
206to root.
207You should never give staff
208members native
209.Li wheel
210access by putting them in the
211.Li wheel
212group in their password entry.
213Staff accounts should be placed in a
214.Dq Li staff
215group, and then added to the
216.Li wheel
217group via the
218.Pa /etc/group
219file.
220Only those staff members who actually need to have root access
221should be placed in the
222.Li wheel
223group.
224It is also possible, when using an
225authentication method such as Kerberos, to use Kerberos's
226.Pa .k5login
227file in the root account to allow a
228.Xr ksu 1
229to root without having to place anyone at all in the
230.Li wheel
231group.
232This
233may be the better solution since the
234.Li wheel
235mechanism still allows an
236intruder to break root if the intruder has gotten hold of your password
237file and can break into a staff account.
238While having the
239.Li wheel
240mechanism
241is better than having nothing at all, it is not necessarily the safest
242option.
243.Pp
244An indirect way to secure the root account is to secure your staff accounts
245by using an alternative login access method and *'ing out the crypted password
246for the staff accounts.
247This way an intruder may be able to steal the password
248file but will not be able to break into any staff accounts or root, even if
249root has a crypted password associated with it (assuming, of course, that
250you have limited root access to the console).
251Staff members
252get into their staff accounts through a secure login mechanism such as
253.Xr kerberos 1
254or
255.Xr ssh 1
256using a private/public
257key pair.
258When you use something like Kerberos you generally must secure
259the machines which run the Kerberos servers and your desktop workstation.
260When you use a public/private key pair with SSH, you must generally secure
261the machine you are logging in
262.Em from
263(typically your workstation),
264but you can
265also add an additional layer of protection to the key pair by password
266protecting the keypair when you create it with
267.Xr ssh-keygen 1 .
268Being able
269to *-out the passwords for staff accounts also guarantees that staff members
270can only log in through secure access methods that you have set up.
271You can
272thus force all staff members to use secure, encrypted connections for
273all their sessions which closes an important hole used by many intruders: that
274of sniffing the network from an unrelated, less secure machine.
275.Pp
276The more indirect security mechanisms also assume that you are logging in
277from a more restrictive server to a less restrictive server.
278For example,
279if your main box is running all sorts of servers, your workstation should not
280be running any.
281In order for your workstation to be reasonably secure
282you should run as few servers as possible, up to and including no servers
283at all, and you should run a password-protected screen blanker.
284Of course, given physical access to
285a workstation, an attacker can break any sort of security you put on it.
286This is definitely a problem that you should consider but you should also
287consider the fact that the vast majority of break-ins occur remotely, over
288a network, from people who do not have physical access to your workstation or
289servers.
290.Pp
291Using something like Kerberos also gives you the ability to disable or
292change the password for a staff account in one place and have it immediately
293affect all the machines the staff member may have an account on.
294If a staff
295member's account gets compromised, the ability to instantly change his
296password on all machines should not be underrated.
297With discrete passwords, changing a password on N machines can be a mess.
298You can also impose
299re-passwording restrictions with Kerberos: not only can a Kerberos ticket
300be made to timeout after a while, but the Kerberos system can require that
301the user choose a new password after a certain period of time
302(say, once a month).
303.Sh SECURING ROOT \(em ROOT-RUN SERVERS AND SUID/SGID BINARIES
304The prudent sysadmin only runs the servers he needs to, no more, no less.
305Be aware that third party servers are often the most bug-prone.
306For example,
307running an old version of
308.Xr imapd 8
309or
310.Xr popper 8
311is like giving a universal root
312ticket out to the entire world.
313Never run a server that you have not checked
314out carefully.
315Many servers do not need to be run as root.
316For example,
317the
318.Xr talkd 8 ,
319.Xr comsat 8 ,
320and
321.Xr fingerd 8
322daemons can be run in special user
323.Dq sandboxes .
324A sandbox is not perfect unless you go to a large amount of trouble, but the
325onion approach to security still stands: if someone is able to break in
326through a server running in a sandbox, they still have to break out of the
327sandbox.
328The more layers the attacker must break through, the lower the
329likelihood of his success.
330Root holes have historically been found in
331virtually every server ever run as root, including basic system servers.
332If you are running a machine through which people only log in via
333.Xr sshd 8
334and never log in via
335.Xr telnetd 8 ,
336.Xr rshd 8 ,
337or
338.Xr rlogind 8 ,
339then turn off those services!
340.Pp
341.Fx
342now defaults to running
343.Xr talkd 8 ,
344.Xr comsat 8 ,
345and
346.Xr fingerd 8
347in a sandbox.
348Another program which may be a candidate for running in a sandbox is
349.Xr named 8 .
350The default
351.Pa rc.conf
352includes the arguments necessary to run
353.Xr named 8
354in a sandbox in a commented-out form.
355Depending on whether you
356are installing a new system or upgrading an existing system, the special
357user accounts used by these sandboxes may not be installed.
358The prudent
359sysadmin would research and implement sandboxes for servers whenever possible.
360.Pp
361There are a number of other servers that typically do not run in sandboxes:
362.Xr sendmail 8 ,
363.Xr popper 8 ,
364.Xr imapd 8 ,
365.Xr ftpd 8 ,
366and others.
367There are alternatives to
368some of these, but installing them may require more work then you are willing
369to put
370(the convenience factor strikes again).
371You may have to run these
372servers as root and rely on other mechanisms to detect break-ins that might
373occur through them.
374.Pp
375The other big potential root hole in a system are the SUID-root and SGID
376binaries installed on the system.
377Most of these binaries, such as
378.Xr rlogin 1 ,
379reside in
380.Pa /bin , /sbin , /usr/bin ,
381or
382.Pa /usr/sbin .
383While nothing is 100% safe,
384the system-default SUID and SGID binaries can be considered reasonably safe.
385Still, root holes are occasionally found in these binaries.
386A root hole
387was found in Xlib in 1998 that made
388.Xr xterm 1
389(which is typically SUID)
390vulnerable.
391It is better to be safe than sorry and the prudent sysadmin will restrict SUID
392binaries that only staff should run to a special group that only staff can
393access, and get rid of
394.Pq Dq Li "chmod 000"
395any SUID binaries that nobody uses.
396A server with no display generally does not need an
397.Xr xterm 1
398binary.
399SGID binaries can be almost as dangerous.
400If an intruder can break an SGID-kmem binary the
401intruder might be able to read
402.Pa /dev/kmem
403and thus read the crypted password
404file, potentially compromising any passworded account.
405Alternatively an
406intruder who breaks group
407.Dq Li kmem
408can monitor keystrokes sent through PTYs,
409including PTYs used by users who log in through secure methods.
410An intruder
411that breaks the
412.Dq Li tty
413group can write to almost any user's TTY.
414If a user
415is running a terminal
416program or emulator with a keyboard-simulation feature, the intruder can
417potentially
418generate a data stream that causes the user's terminal to echo a command, which
419is then run as that user.
420.Sh SECURING USER ACCOUNTS
421User accounts are usually the most difficult to secure.
422While you can impose
423draconian access restrictions on your staff and *-out their passwords, you
424may not be able to do so with any general user accounts you might have.
425If
426you do have sufficient control then you may win out and be able to secure the
427user accounts properly.
428If not, you simply have to be more vigilant in your
429monitoring of those accounts.
430Use of SSH and Kerberos for user accounts is
431more problematic due to the extra administration and technical support
432required, but still a very good solution compared to a crypted password
433file.
434.Sh SECURING THE PASSWORD FILE
435The only sure fire way is to *-out as many passwords as you can and
436use SSH or Kerberos for access to those accounts.
437Even though the
438crypted password file
439.Pq Pa /etc/spwd.db
440can only be read by root, it may
441be possible for an intruder to obtain read access to that file even if the
442attacker cannot obtain root-write access.
443.Pp
444Your security scripts should always check for and report changes to
445the password file
446(see
447.Sx CHECKING FILE INTEGRITY
448below).
449.Sh SECURING THE KERNEL CORE, RAW DEVICES, AND FILE SYSTEMS
450If an attacker breaks root he can do just about anything, but there
451are certain conveniences.
452For example, most modern kernels have a packet sniffing device driver built in.
453Under
454.Fx
455it is called
456the
457.Xr bpf 4
458device.
459An intruder will commonly attempt to run a packet sniffer
460on a compromised machine.
461You do not need to give the intruder the
462capability and most systems should not have the
463.Xr bpf 4
464device compiled in.
465.Pp
466But even if you turn off the
467.Xr bpf 4
468device, you still have
469.Pa /dev/mem
470and
471.Pa /dev/kmem
472to worry about.
473For that matter,
474the intruder can still write to raw disk devices.
475Also, there is another kernel feature called the module loader,
476.Xr kldload 8 .
477An enterprising intruder can use a KLD module to install
478his own
479.Xr bpf 4
480device or other sniffing device on a running kernel.
481To avoid these problems you have to run
482the kernel at a higher secure level, at least securelevel 1.
483The securelevel can be set with a
484.Xr sysctl 8
485on the
486.Va kern.securelevel
487variable.
488Once you have
489set the securelevel to 1, write access to raw devices will be denied and
490special
491.Xr chflags 1
492flags, such as
493.Cm schg ,
494will be enforced.
495You must also ensure
496that the
497.Cm schg
498flag is set on critical startup binaries, directories, and
499script files \(em everything that gets run up to the point where the securelevel
500is set.
501This might be overdoing it, and upgrading the system is much more
502difficult when you operate at a higher secure level.
503You may compromise and
504run the system at a higher secure level but not set the
505.Cm schg
506flag for every
507system file and directory under the sun.
508Another possibility is to simply
509mount
510.Pa /
511and
512.Pa /usr
513read-only.
514It should be noted that being too draconian in
515what you attempt to protect may prevent the all-important detection of an
516intrusion.
517.Sh CHECKING FILE INTEGRITY: BINARIES, CONFIG FILES, ETC
518When it comes right down to it, you can only protect your core system
519configuration and control files so much before the convenience factor
520rears its ugly head.
521For example, using
522.Xr chflags 1
523to set the
524.Cm schg
525bit on most of the files in
526.Pa /
527and
528.Pa /usr
529is probably counterproductive because
530while it may protect the files, it also closes a detection window.
531The
532last layer of your security onion is perhaps the most important \(em detection.
533The rest of your security is pretty much useless (or, worse, presents you with
534a false sense of safety) if you cannot detect potential incursions.
535Half
536the job of the onion is to slow down the attacker rather than stop him
537in order to give the detection layer a chance to catch him in
538the act.
539.Pp
540The best way to detect an incursion is to look for modified, missing, or
541unexpected files.
542The best
543way to look for modified files is from another (often centralized)
544limited-access system.
545Writing your security scripts on the extra-secure limited-access system
546makes them mostly invisible to potential attackers, and this is important.
547In order to take maximum advantage you generally have to give the
548limited-access box significant access to the other machines in the business,
549usually either by doing a read-only NFS export of the other machines to the
550limited-access box, or by setting up SSH keypairs to allow the limit-access
551box to SSH to the other machines.
552Except for its network traffic, NFS is
553the least visible method \(em allowing you to monitor the file systems on each
554client box virtually undetected.
555If your
556limited-access server is connected to the client boxes through a switch,
557the NFS method is often the better choice.
558If your limited-access server
559is connected to the client boxes through a hub or through several layers
560of routing, the NFS method may be too insecure (network-wise) and using SSH
561may be the better choice even with the audit-trail tracks that SSH lays.
562.Pp
563Once you give a limit-access box at least read access to the client systems
564it is supposed to monitor, you must write scripts to do the actual
565monitoring.
566Given an NFS mount, you can write scripts out of simple system
567utilities such as
568.Xr find 1
569and
570.Xr md5 1 .
571It is best to physically
572.Xr md5 1
573the client-box files boxes at least once a
574day, and to test control files such as those found in
575.Pa /etc
576and
577.Pa /usr/local/etc
578even more often.
579When mismatches are found relative to the base MD5
580information the limited-access machine knows is valid, it should scream at
581a sysadmin to go check it out.
582A good security script will also check for
583inappropriate SUID binaries and for new or deleted files on system partitions
584such as
585.Pa /
586and
587.Pa /usr .
588.Pp
589When using SSH rather than NFS, writing the security script is much more
590difficult.
591You essentially have to
592.Xr scp 1
593the scripts to the client box in order to run them, making them visible, and
594for safety you also need to
595.Xr scp 1
596the binaries (such as
597.Xr find 1 )
598that those scripts use.
599The
600.Xr sshd 8
601daemon on the client box may already be compromised.
602All in all,
603using SSH may be necessary when running over unsecure links, but it is also a
604lot harder to deal with.
605.Pp
606A good security script will also check for changes to user and staff members
607access configuration files:
608.Pa .rhosts , .shosts , .ssh/authorized_keys
609and so forth... files that might fall outside the purview of the MD5 check.
610.Pp
611If you have a huge amount of user disk space it may take too long to run
612through every file on those partitions.
613In this case, setting mount
614flags to disallow SUID binaries and devices on those partitions is a good
615idea.
616The
617.Cm nodev
618and
619.Cm nosuid
620options
621(see
622.Xr mount 8 )
623are what you want to look into.
624I would scan them anyway at least once a
625week, since the object of this layer is to detect a break-in whether or
626not the break-in is effective.
627.Pp
628Process accounting
629(see
630.Xr accton 8 )
631is a relatively low-overhead feature of
632the operating system which I recommend using as a post-break-in evaluation
633mechanism.
634It is especially useful in tracking down how an intruder has
635actually broken into a system, assuming the file is still intact after
636the break-in occurs.
637.Pp
638Finally, security scripts should process the log files and the logs themselves
639should be generated in as secure a manner as possible \(em remote syslog can be
640very useful.
641An intruder tries to cover his tracks, and log files are critical
642to the sysadmin trying to track down the time and method of the initial
643break-in.
644One way to keep a permanent record of the log files is to run
645the system console to a serial port and collect the information on a
646continuing basis through a secure machine monitoring the consoles.
647.Sh PARANOIA
648A little paranoia never hurts.
649As a rule, a sysadmin can add any number
650of security features as long as they do not affect convenience, and
651can add security features that do affect convenience with some added
652thought.
653Even more importantly, a security administrator should mix it up
654a bit \(em if you use recommendations such as those given by this manual
655page verbatim, you give away your methodologies to the prospective
656attacker who also has access to this manual page.
657.Sh SPECIAL SECTION ON DoS ATTACKS
658This section covers Denial of Service attacks.
659A DoS attack is typically a packet attack.
660While there is not much you can do about modern spoofed
661packet attacks that saturate your network, you can generally limit the damage
662by ensuring that the attacks cannot take down your servers.
663.Bl -enum -offset indent
664.It
665Limiting server forks
666.It
667Limiting springboard attacks (ICMP response attacks, ping broadcast, etc.)
668.It
669Kernel Route Cache
670.El
671.Pp
672A common DoS attack is against a forking server that attempts to cause the
673server to eat processes, file descriptors, and memory until the machine
674dies.
675The
676.Xr inetd 8
677server
678has several options to limit this sort of attack.
679It should be noted that while it is possible to prevent a machine from going
680down it is not generally possible to prevent a service from being disrupted
681by the attack.
682Read the
683.Xr inetd 8
684manual page carefully and pay specific attention
685to the
686.Fl c , C ,
687and
688.Fl R
689options.
690Note that spoofed-IP attacks will circumvent
691the
692.Fl C
693option to
694.Xr inetd 8 ,
695so typically a combination of options must be used.
696Some standalone servers have self-fork-limitation parameters.
697.Pp
698The
699.Xr sendmail 8
700daemon has its
701.Fl OMaxDaemonChildren
702option which tends to work much
703better than trying to use
704.Xr sendmail 8 Ns 's
705load limiting options due to the
706load lag.
707You should specify a
708.Va MaxDaemonChildren
709parameter when you start
710.Xr sendmail 8
711high enough to handle your expected load but not so high that the
712computer cannot handle that number of
713.Nm sendmail Ns 's
714without falling on its face.
715It is also prudent to run
716.Xr sendmail 8
717in
718.Dq queued
719mode
720.Pq Fl ODeliveryMode=queued
721and to run the daemon
722.Pq Dq Nm sendmail Fl bd
723separate from the queue-runs
724.Pq Dq Nm sendmail Fl q15m .
725If you still want real-time delivery you can run the queue
726at a much lower interval, such as
727.Fl q1m ,
728but be sure to specify a reasonable
729.Va MaxDaemonChildren
730option for that
731.Xr sendmail 8
732to prevent cascade failures.
733.Pp
734The
735.Xr syslogd 8
736daemon can be attacked directly and it is strongly recommended that you use
737the
738.Fl s
739option whenever possible, and the
740.Fl a
741option otherwise.
742.Pp
743You should also be fairly careful
744with connect-back services such as tcpwrapper's reverse-identd, which can
745be attacked directly.
746You generally do not want to use the reverse-ident
747feature of tcpwrappers for this reason.
748.Pp
749It is a very good idea to protect internal services from external access
750by firewalling them off at your border routers.
751The idea here is to prevent
752saturation attacks from outside your LAN, not so much to protect internal
753services from network-based root compromise.
754Always configure an exclusive
755firewall, i.e.,
756.So
757firewall everything
758.Em except
759ports A, B, C, D, and M-Z
760.Sc .
761This
762way you can firewall off all of your low ports except for certain specific
763services such as
764.Xr named 8
765(if you are primary for a zone),
766.Xr talkd 8 ,
767.Xr sendmail 8 ,
768and other internet-accessible services.
769If you try to configure the firewall the other
770way \(em as an inclusive or permissive firewall, there is a good chance that you
771will forget to
772.Dq close
773a couple of services or that you will add a new internal
774service and forget to update the firewall.
775You can still open up the
776high-numbered port range on the firewall to allow permissive-like operation
777without compromising your low ports.
778Also take note that
779.Fx
780allows you to
781control the range of port numbers used for dynamic binding via the various
782.Va net.inet.ip.portrange
783sysctl's
784.Pq Dq Li "sysctl net.inet.ip.portrange" ,
785which can also
786ease the complexity of your firewall's configuration.
787I usually use a normal
788first/last range of 4000 to 5000, and a hiport range of 49152 to 65535, then
789block everything under 4000 off in my firewall
790(except for certain specific
791internet-accessible ports, of course).
792.Pp
793Another common DoS attack is called a springboard attack \(em to attack a server
794in a manner that causes the server to generate responses which then overload
795the server, the local network, or some other machine.
796The most common attack
797of this nature is the ICMP PING BROADCAST attack.
798The attacker spoofs ping
799packets sent to your LAN's broadcast address with the source IP address set
800to the actual machine they wish to attack.
801If your border routers are not
802configured to stomp on ping's to broadcast addresses, your LAN winds up
803generating sufficient responses to the spoofed source address to saturate the
804victim, especially when the attacker uses the same trick on several dozen
805broadcast addresses over several dozen different networks at once.
806Broadcast attacks of over a hundred and twenty megabits have been measured.
807A second common springboard attack is against the ICMP error reporting system.
808By
809constructing packets that generate ICMP error responses, an attacker can
810saturate a server's incoming network and cause the server to saturate its
811outgoing network with ICMP responses.
812This type of attack can also crash the
813server by running it out of
814.Vt mbuf Ns 's ,
815especially if the server cannot drain the
816ICMP responses it generates fast enough.
817The
818.Fx
819kernel has a new kernel
820compile option called
821.Dv ICMP_BANDLIM
822which limits the effectiveness of these
823sorts of attacks.
824The last major class of springboard attacks is related to
825certain internal
826.Xr inetd 8
827services such as the UDP echo service.
828An attacker
829simply spoofs a UDP packet with the source address being server A's echo port,
830and the destination address being server B's echo port, where server A and B
831are both on your LAN.
832The two servers then bounce this one packet back and
833forth between each other.
834The attacker can overload both servers and their
835LANs simply by injecting a few packets in this manner.
836Similar problems
837exist with the internal chargen port.
838A competent sysadmin will turn off all
839of these
840.Xr inetd 8 Ns -internal
841test services.
842.Pp
843Spoofed packet attacks may also be used to overload the kernel route cache.
844Refer to the
845.Va net.inet.ip.rtexpire , net.inet.ip.rtminexpire ,
846and
847.Va net.inet.ip.rtmaxcache
848.Xr sysctl 8
849variables.
850A spoofed packet attack that uses a random source IP will cause
851the kernel to generate a temporary cached route in the route table, viewable
852with
853.Dq Li "netstat -rna | fgrep W3" .
854These routes typically timeout in 1600
855seconds or so.
856If the kernel detects that the cached route table has gotten
857too big it will dynamically reduce the
858.Va rtexpire
859but will never decrease it to
860less than
861.Va rtminexpire .
862There are two problems: (1) The kernel does not react
863quickly enough when a lightly loaded server is suddenly attacked, and (2) The
864.Va rtminexpire
865is not low enough for the kernel to survive a sustained attack.
866If your servers are connected to the internet via a T3 or better it may be
867prudent to manually override both
868.Va rtexpire
869and
870.Va rtminexpire
871via
872.Xr sysctl 8 .
873Never set either parameter to zero
874(unless you want to crash the machine :-)).
875Setting both parameters to 2 seconds should be sufficient to protect the route
876table from attack.
877.Sh ACCESS ISSUES WITH KERBEROS AND SSH
878There are a few issues with both Kerberos and SSH that need to be addressed
879if you intend to use them.
880Kerberos5 is an excellent authentication
881protocol but the kerberized
882.Xr telnet 1
883and
884.Xr rlogin 1
885suck rocks.
886There are bugs that make them unsuitable for dealing with binary streams.
887Also, by default
888Kerberos does not encrypt a session unless you use the
889.Fl x
890option.
891SSH encrypts everything by default.
892.Pp
893SSH works quite well in every respect except when it is set up to
894forward encryption keys.
895What this means is that if you have a secure workstation holding
896keys that give you access to the rest of the system, and you
897.Xr ssh 1
898to an
899unsecure machine, your keys become exposed.
900The actual keys themselves are
901not exposed, but
902.Xr ssh 1
903installs a forwarding port for the duration of your
904login and if an attacker has broken root on the unsecure machine he can utilize
905that port to use your keys to gain access to any other machine that your
906keys unlock.
907.Pp
908We recommend that you use SSH in combination with Kerberos whenever possible
909for staff logins.
910SSH can be compiled with Kerberos support.
911This reduces
912your reliance on potentially exposable SSH keys while at the same time
913protecting passwords via Kerberos.
914SSH keys
915should only be used for automated tasks from secure machines (something
916that Kerberos is unsuited to).
917We also recommend that you either turn off
918key-forwarding in the SSH configuration, or that you make use of the
919.Va from Ns = Ns Ar IP/DOMAIN
920option that SSH allows in its
921.Pa authorized_keys
922file to make the key only usable to entities logging in from specific
923machines.
924.Sh SEE ALSO
925.Xr chflags 1 ,
926.Xr find 1 ,
927.Xr md5 1 ,
928.Xr netstat 1 ,
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2.\" the BSD Copyright as specified in the file "/usr/src/COPYRIGHT" in
3.\" the source tree.
4.\"
5.\" $FreeBSD: head/share/man/man7/security.7 130524 2004-06-15 12:48:50Z ru $
6.\"
7.Dd September 18, 1999
8.Dt SECURITY 7
9.Os
10.Sh NAME
11.Nm security
12.Nd introduction to security under
13.Fx
14.Sh DESCRIPTION
15Security is a function that begins and ends with the system administrator.
16While all
17.Bx
18multi-user systems have some inherent security, the job of building and
19maintaining additional security mechanisms to keep users
20.Dq honest
21is probably
22one of the single largest undertakings of the sysadmin.
23Machines are
24only as secure as you make them, and security concerns are ever competing
25with the human necessity for convenience.
26.Ux
27systems,
28in general, are capable of running a huge number of simultaneous processes
29and many of these processes operate as servers \(em meaning that external
30entities can connect and talk to them.
31As yesterday's mini-computers and mainframes
32become today's desktops, and as computers become networked and internetworked,
33security becomes an ever bigger issue.
34.Pp
35Security is best implemented through a layered onion approach.
36In a nutshell,
37what you want to do is to create as many layers of security as are convenient
38and then carefully monitor the system for intrusions.
39You do not want to
40overbuild your security or you will interfere with the detection side, and
41detection is one of the single most important aspects of any security
42mechanism.
43For example, it makes little sense to set the
44.Cm schg
45flags
46(see
47.Xr chflags 1 )
48on every system binary because while this may temporarily protect the
49binaries, it prevents an attacker who has broken in from making an
50easily detectable change that may result in your security mechanisms not
51detecting the attacker at all.
52.Pp
53System security also pertains to dealing with various forms of attacks,
54including attacks that attempt to crash or otherwise make a system unusable
55but do not attempt to break root.
56Security concerns can be split up into
57several categories:
58.Bl -enum -offset indent
59.It
60Denial of Service attacks (DoS)
61.It
62User account compromises
63.It
64Root compromise through accessible servers
65.It
66Root compromise via user accounts
67.It
68Backdoor creation
69.El
70.Pp
71A denial of service attack is an action that deprives the machine of needed
72resources.
73Typically, DoS attacks are brute-force mechanisms that attempt
74to crash or otherwise make a machine unusable by overwhelming its servers or
75network stack.
76Some DoS attacks try to take advantages of bugs in the
77networking stack to crash a machine with a single packet.
78The latter can
79only be fixed by applying a bug fix to the kernel.
80Attacks on servers can
81often be fixed by properly specifying options to limit the load the servers
82incur on the system under adverse conditions.
83Brute-force network attacks are harder to deal with.
84A spoofed-packet attack, for example, is
85nearly impossible to stop short of cutting your system off from the Internet.
86It may not be able to take your machine down, but it can fill up Internet
87pipe.
88.Pp
89A user account compromise is even more common than a DoS attack.
90Many
91sysadmins still run standard
92.Xr telnetd 8 ,
93.Xr rlogind 8 ,
94.Xr rshd 8 ,
95and
96.Xr ftpd 8
97servers on their machines.
98These servers, by default, do not operate over encrypted
99connections.
100The result is that if you have any moderate-sized user base,
101one or more of your users logging into your system from a remote location
102(which is the most common and convenient way to log in to a system)
103will have his or her password sniffed.
104The attentive system administrator will analyze
105his remote access logs looking for suspicious source addresses
106even for successful logins.
107.Pp
108One must always assume that once an attacker has access to a user account,
109the attacker can break root.
110However, the reality is that in a well secured
111and maintained system, access to a user account does not necessarily give the
112attacker access to root.
113The distinction is important because without access
114to root the attacker cannot generally hide his tracks and may, at best, be
115able to do nothing more than mess with the user's files or crash the machine.
116User account compromises are very common because users tend not to take the
117precautions that sysadmins take.
118.Pp
119System administrators must keep in mind that there are potentially many ways
120to break root on a machine.
121The attacker may know the root password,
122the attacker
123may find a bug in a root-run server and be able to break root over a network
124connection to that server, or the attacker may know of a bug in an SUID-root
125program that allows the attacker to break root once he has broken into a
126user's account.
127If an attacker has found a way to break root on a machine,
128the attacker may not have a need to install a backdoor.
129Many of the root holes found and closed to date involve a considerable amount
130of work by the attacker to clean up after himself, so most attackers do install
131backdoors.
132This gives you a convenient way to detect the attacker.
133Making
134it impossible for an attacker to install a backdoor may actually be detrimental
135to your security because it will not close off the hole the attacker used to
136break in in the first place.
137.Pp
138Security remedies should always be implemented with a multi-layered
139.Dq onion peel
140approach and can be categorized as follows:
141.Bl -enum -offset indent
142.It
143Securing root and staff accounts
144.It
145Securing root \(em root-run servers and SUID/SGID binaries
146.It
147Securing user accounts
148.It
149Securing the password file
150.It
151Securing the kernel core, raw devices, and file systems
152.It
153Quick detection of inappropriate changes made to the system
154.It
155Paranoia
156.El
157.Sh SECURING THE ROOT ACCOUNT AND SECURING STAFF ACCOUNTS
158Do not bother securing staff accounts if you have not secured the root
159account.
160Most systems have a password assigned to the root account.
161The
162first thing you do is assume that the password is
163.Em always
164compromised.
165This does not mean that you should remove the password.
166The
167password is almost always necessary for console access to the machine.
168What it does mean is that you should not make it possible to use the password
169outside of the console or possibly even with a
170.Xr su 1
171utility.
172For example, make sure that your PTYs are specified as being
173.Dq Li unsecure
174in the
175.Pa /etc/ttys
176file
177so that direct root logins via
178.Xr telnet 1
179or
180.Xr rlogin 1
181are disallowed.
182If using
183other login services such as
184.Xr sshd 8 ,
185make sure that direct root logins are
186disabled there as well.
187Consider every access method \(em services such as
188.Xr ftp 1
189often fall through the cracks.
190Direct root logins should only be allowed
191via the system console.
192.Pp
193Of course, as a sysadmin you have to be able to get to root, so we open up
194a few holes.
195But we make sure these holes require additional password
196verification to operate.
197One way to make root accessible is to add appropriate
198staff accounts to the
199.Dq Li wheel
200group (in
201.Pa /etc/group ) .
202The staff members placed in the
203.Li wheel
204group are allowed to
205.Xr su 1
206to root.
207You should never give staff
208members native
209.Li wheel
210access by putting them in the
211.Li wheel
212group in their password entry.
213Staff accounts should be placed in a
214.Dq Li staff
215group, and then added to the
216.Li wheel
217group via the
218.Pa /etc/group
219file.
220Only those staff members who actually need to have root access
221should be placed in the
222.Li wheel
223group.
224It is also possible, when using an
225authentication method such as Kerberos, to use Kerberos's
226.Pa .k5login
227file in the root account to allow a
228.Xr ksu 1
229to root without having to place anyone at all in the
230.Li wheel
231group.
232This
233may be the better solution since the
234.Li wheel
235mechanism still allows an
236intruder to break root if the intruder has gotten hold of your password
237file and can break into a staff account.
238While having the
239.Li wheel
240mechanism
241is better than having nothing at all, it is not necessarily the safest
242option.
243.Pp
244An indirect way to secure the root account is to secure your staff accounts
245by using an alternative login access method and *'ing out the crypted password
246for the staff accounts.
247This way an intruder may be able to steal the password
248file but will not be able to break into any staff accounts or root, even if
249root has a crypted password associated with it (assuming, of course, that
250you have limited root access to the console).
251Staff members
252get into their staff accounts through a secure login mechanism such as
253.Xr kerberos 1
254or
255.Xr ssh 1
256using a private/public
257key pair.
258When you use something like Kerberos you generally must secure
259the machines which run the Kerberos servers and your desktop workstation.
260When you use a public/private key pair with SSH, you must generally secure
261the machine you are logging in
262.Em from
263(typically your workstation),
264but you can
265also add an additional layer of protection to the key pair by password
266protecting the keypair when you create it with
267.Xr ssh-keygen 1 .
268Being able
269to *-out the passwords for staff accounts also guarantees that staff members
270can only log in through secure access methods that you have set up.
271You can
272thus force all staff members to use secure, encrypted connections for
273all their sessions which closes an important hole used by many intruders: that
274of sniffing the network from an unrelated, less secure machine.
275.Pp
276The more indirect security mechanisms also assume that you are logging in
277from a more restrictive server to a less restrictive server.
278For example,
279if your main box is running all sorts of servers, your workstation should not
280be running any.
281In order for your workstation to be reasonably secure
282you should run as few servers as possible, up to and including no servers
283at all, and you should run a password-protected screen blanker.
284Of course, given physical access to
285a workstation, an attacker can break any sort of security you put on it.
286This is definitely a problem that you should consider but you should also
287consider the fact that the vast majority of break-ins occur remotely, over
288a network, from people who do not have physical access to your workstation or
289servers.
290.Pp
291Using something like Kerberos also gives you the ability to disable or
292change the password for a staff account in one place and have it immediately
293affect all the machines the staff member may have an account on.
294If a staff
295member's account gets compromised, the ability to instantly change his
296password on all machines should not be underrated.
297With discrete passwords, changing a password on N machines can be a mess.
298You can also impose
299re-passwording restrictions with Kerberos: not only can a Kerberos ticket
300be made to timeout after a while, but the Kerberos system can require that
301the user choose a new password after a certain period of time
302(say, once a month).
303.Sh SECURING ROOT \(em ROOT-RUN SERVERS AND SUID/SGID BINARIES
304The prudent sysadmin only runs the servers he needs to, no more, no less.
305Be aware that third party servers are often the most bug-prone.
306For example,
307running an old version of
308.Xr imapd 8
309or
310.Xr popper 8
311is like giving a universal root
312ticket out to the entire world.
313Never run a server that you have not checked
314out carefully.
315Many servers do not need to be run as root.
316For example,
317the
318.Xr talkd 8 ,
319.Xr comsat 8 ,
320and
321.Xr fingerd 8
322daemons can be run in special user
323.Dq sandboxes .
324A sandbox is not perfect unless you go to a large amount of trouble, but the
325onion approach to security still stands: if someone is able to break in
326through a server running in a sandbox, they still have to break out of the
327sandbox.
328The more layers the attacker must break through, the lower the
329likelihood of his success.
330Root holes have historically been found in
331virtually every server ever run as root, including basic system servers.
332If you are running a machine through which people only log in via
333.Xr sshd 8
334and never log in via
335.Xr telnetd 8 ,
336.Xr rshd 8 ,
337or
338.Xr rlogind 8 ,
339then turn off those services!
340.Pp
341.Fx
342now defaults to running
343.Xr talkd 8 ,
344.Xr comsat 8 ,
345and
346.Xr fingerd 8
347in a sandbox.
348Another program which may be a candidate for running in a sandbox is
349.Xr named 8 .
350The default
351.Pa rc.conf
352includes the arguments necessary to run
353.Xr named 8
354in a sandbox in a commented-out form.
355Depending on whether you
356are installing a new system or upgrading an existing system, the special
357user accounts used by these sandboxes may not be installed.
358The prudent
359sysadmin would research and implement sandboxes for servers whenever possible.
360.Pp
361There are a number of other servers that typically do not run in sandboxes:
362.Xr sendmail 8 ,
363.Xr popper 8 ,
364.Xr imapd 8 ,
365.Xr ftpd 8 ,
366and others.
367There are alternatives to
368some of these, but installing them may require more work then you are willing
369to put
370(the convenience factor strikes again).
371You may have to run these
372servers as root and rely on other mechanisms to detect break-ins that might
373occur through them.
374.Pp
375The other big potential root hole in a system are the SUID-root and SGID
376binaries installed on the system.
377Most of these binaries, such as
378.Xr rlogin 1 ,
379reside in
380.Pa /bin , /sbin , /usr/bin ,
381or
382.Pa /usr/sbin .
383While nothing is 100% safe,
384the system-default SUID and SGID binaries can be considered reasonably safe.
385Still, root holes are occasionally found in these binaries.
386A root hole
387was found in Xlib in 1998 that made
388.Xr xterm 1
389(which is typically SUID)
390vulnerable.
391It is better to be safe than sorry and the prudent sysadmin will restrict SUID
392binaries that only staff should run to a special group that only staff can
393access, and get rid of
394.Pq Dq Li "chmod 000"
395any SUID binaries that nobody uses.
396A server with no display generally does not need an
397.Xr xterm 1
398binary.
399SGID binaries can be almost as dangerous.
400If an intruder can break an SGID-kmem binary the
401intruder might be able to read
402.Pa /dev/kmem
403and thus read the crypted password
404file, potentially compromising any passworded account.
405Alternatively an
406intruder who breaks group
407.Dq Li kmem
408can monitor keystrokes sent through PTYs,
409including PTYs used by users who log in through secure methods.
410An intruder
411that breaks the
412.Dq Li tty
413group can write to almost any user's TTY.
414If a user
415is running a terminal
416program or emulator with a keyboard-simulation feature, the intruder can
417potentially
418generate a data stream that causes the user's terminal to echo a command, which
419is then run as that user.
420.Sh SECURING USER ACCOUNTS
421User accounts are usually the most difficult to secure.
422While you can impose
423draconian access restrictions on your staff and *-out their passwords, you
424may not be able to do so with any general user accounts you might have.
425If
426you do have sufficient control then you may win out and be able to secure the
427user accounts properly.
428If not, you simply have to be more vigilant in your
429monitoring of those accounts.
430Use of SSH and Kerberos for user accounts is
431more problematic due to the extra administration and technical support
432required, but still a very good solution compared to a crypted password
433file.
434.Sh SECURING THE PASSWORD FILE
435The only sure fire way is to *-out as many passwords as you can and
436use SSH or Kerberos for access to those accounts.
437Even though the
438crypted password file
439.Pq Pa /etc/spwd.db
440can only be read by root, it may
441be possible for an intruder to obtain read access to that file even if the
442attacker cannot obtain root-write access.
443.Pp
444Your security scripts should always check for and report changes to
445the password file
446(see
447.Sx CHECKING FILE INTEGRITY
448below).
449.Sh SECURING THE KERNEL CORE, RAW DEVICES, AND FILE SYSTEMS
450If an attacker breaks root he can do just about anything, but there
451are certain conveniences.
452For example, most modern kernels have a packet sniffing device driver built in.
453Under
454.Fx
455it is called
456the
457.Xr bpf 4
458device.
459An intruder will commonly attempt to run a packet sniffer
460on a compromised machine.
461You do not need to give the intruder the
462capability and most systems should not have the
463.Xr bpf 4
464device compiled in.
465.Pp
466But even if you turn off the
467.Xr bpf 4
468device, you still have
469.Pa /dev/mem
470and
471.Pa /dev/kmem
472to worry about.
473For that matter,
474the intruder can still write to raw disk devices.
475Also, there is another kernel feature called the module loader,
476.Xr kldload 8 .
477An enterprising intruder can use a KLD module to install
478his own
479.Xr bpf 4
480device or other sniffing device on a running kernel.
481To avoid these problems you have to run
482the kernel at a higher secure level, at least securelevel 1.
483The securelevel can be set with a
484.Xr sysctl 8
485on the
486.Va kern.securelevel
487variable.
488Once you have
489set the securelevel to 1, write access to raw devices will be denied and
490special
491.Xr chflags 1
492flags, such as
493.Cm schg ,
494will be enforced.
495You must also ensure
496that the
497.Cm schg
498flag is set on critical startup binaries, directories, and
499script files \(em everything that gets run up to the point where the securelevel
500is set.
501This might be overdoing it, and upgrading the system is much more
502difficult when you operate at a higher secure level.
503You may compromise and
504run the system at a higher secure level but not set the
505.Cm schg
506flag for every
507system file and directory under the sun.
508Another possibility is to simply
509mount
510.Pa /
511and
512.Pa /usr
513read-only.
514It should be noted that being too draconian in
515what you attempt to protect may prevent the all-important detection of an
516intrusion.
517.Sh CHECKING FILE INTEGRITY: BINARIES, CONFIG FILES, ETC
518When it comes right down to it, you can only protect your core system
519configuration and control files so much before the convenience factor
520rears its ugly head.
521For example, using
522.Xr chflags 1
523to set the
524.Cm schg
525bit on most of the files in
526.Pa /
527and
528.Pa /usr
529is probably counterproductive because
530while it may protect the files, it also closes a detection window.
531The
532last layer of your security onion is perhaps the most important \(em detection.
533The rest of your security is pretty much useless (or, worse, presents you with
534a false sense of safety) if you cannot detect potential incursions.
535Half
536the job of the onion is to slow down the attacker rather than stop him
537in order to give the detection layer a chance to catch him in
538the act.
539.Pp
540The best way to detect an incursion is to look for modified, missing, or
541unexpected files.
542The best
543way to look for modified files is from another (often centralized)
544limited-access system.
545Writing your security scripts on the extra-secure limited-access system
546makes them mostly invisible to potential attackers, and this is important.
547In order to take maximum advantage you generally have to give the
548limited-access box significant access to the other machines in the business,
549usually either by doing a read-only NFS export of the other machines to the
550limited-access box, or by setting up SSH keypairs to allow the limit-access
551box to SSH to the other machines.
552Except for its network traffic, NFS is
553the least visible method \(em allowing you to monitor the file systems on each
554client box virtually undetected.
555If your
556limited-access server is connected to the client boxes through a switch,
557the NFS method is often the better choice.
558If your limited-access server
559is connected to the client boxes through a hub or through several layers
560of routing, the NFS method may be too insecure (network-wise) and using SSH
561may be the better choice even with the audit-trail tracks that SSH lays.
562.Pp
563Once you give a limit-access box at least read access to the client systems
564it is supposed to monitor, you must write scripts to do the actual
565monitoring.
566Given an NFS mount, you can write scripts out of simple system
567utilities such as
568.Xr find 1
569and
570.Xr md5 1 .
571It is best to physically
572.Xr md5 1
573the client-box files boxes at least once a
574day, and to test control files such as those found in
575.Pa /etc
576and
577.Pa /usr/local/etc
578even more often.
579When mismatches are found relative to the base MD5
580information the limited-access machine knows is valid, it should scream at
581a sysadmin to go check it out.
582A good security script will also check for
583inappropriate SUID binaries and for new or deleted files on system partitions
584such as
585.Pa /
586and
587.Pa /usr .
588.Pp
589When using SSH rather than NFS, writing the security script is much more
590difficult.
591You essentially have to
592.Xr scp 1
593the scripts to the client box in order to run them, making them visible, and
594for safety you also need to
595.Xr scp 1
596the binaries (such as
597.Xr find 1 )
598that those scripts use.
599The
600.Xr sshd 8
601daemon on the client box may already be compromised.
602All in all,
603using SSH may be necessary when running over unsecure links, but it is also a
604lot harder to deal with.
605.Pp
606A good security script will also check for changes to user and staff members
607access configuration files:
608.Pa .rhosts , .shosts , .ssh/authorized_keys
609and so forth... files that might fall outside the purview of the MD5 check.
610.Pp
611If you have a huge amount of user disk space it may take too long to run
612through every file on those partitions.
613In this case, setting mount
614flags to disallow SUID binaries and devices on those partitions is a good
615idea.
616The
617.Cm nodev
618and
619.Cm nosuid
620options
621(see
622.Xr mount 8 )
623are what you want to look into.
624I would scan them anyway at least once a
625week, since the object of this layer is to detect a break-in whether or
626not the break-in is effective.
627.Pp
628Process accounting
629(see
630.Xr accton 8 )
631is a relatively low-overhead feature of
632the operating system which I recommend using as a post-break-in evaluation
633mechanism.
634It is especially useful in tracking down how an intruder has
635actually broken into a system, assuming the file is still intact after
636the break-in occurs.
637.Pp
638Finally, security scripts should process the log files and the logs themselves
639should be generated in as secure a manner as possible \(em remote syslog can be
640very useful.
641An intruder tries to cover his tracks, and log files are critical
642to the sysadmin trying to track down the time and method of the initial
643break-in.
644One way to keep a permanent record of the log files is to run
645the system console to a serial port and collect the information on a
646continuing basis through a secure machine monitoring the consoles.
647.Sh PARANOIA
648A little paranoia never hurts.
649As a rule, a sysadmin can add any number
650of security features as long as they do not affect convenience, and
651can add security features that do affect convenience with some added
652thought.
653Even more importantly, a security administrator should mix it up
654a bit \(em if you use recommendations such as those given by this manual
655page verbatim, you give away your methodologies to the prospective
656attacker who also has access to this manual page.
657.Sh SPECIAL SECTION ON DoS ATTACKS
658This section covers Denial of Service attacks.
659A DoS attack is typically a packet attack.
660While there is not much you can do about modern spoofed
661packet attacks that saturate your network, you can generally limit the damage
662by ensuring that the attacks cannot take down your servers.
663.Bl -enum -offset indent
664.It
665Limiting server forks
666.It
667Limiting springboard attacks (ICMP response attacks, ping broadcast, etc.)
668.It
669Kernel Route Cache
670.El
671.Pp
672A common DoS attack is against a forking server that attempts to cause the
673server to eat processes, file descriptors, and memory until the machine
674dies.
675The
676.Xr inetd 8
677server
678has several options to limit this sort of attack.
679It should be noted that while it is possible to prevent a machine from going
680down it is not generally possible to prevent a service from being disrupted
681by the attack.
682Read the
683.Xr inetd 8
684manual page carefully and pay specific attention
685to the
686.Fl c , C ,
687and
688.Fl R
689options.
690Note that spoofed-IP attacks will circumvent
691the
692.Fl C
693option to
694.Xr inetd 8 ,
695so typically a combination of options must be used.
696Some standalone servers have self-fork-limitation parameters.
697.Pp
698The
699.Xr sendmail 8
700daemon has its
701.Fl OMaxDaemonChildren
702option which tends to work much
703better than trying to use
704.Xr sendmail 8 Ns 's
705load limiting options due to the
706load lag.
707You should specify a
708.Va MaxDaemonChildren
709parameter when you start
710.Xr sendmail 8
711high enough to handle your expected load but not so high that the
712computer cannot handle that number of
713.Nm sendmail Ns 's
714without falling on its face.
715It is also prudent to run
716.Xr sendmail 8
717in
718.Dq queued
719mode
720.Pq Fl ODeliveryMode=queued
721and to run the daemon
722.Pq Dq Nm sendmail Fl bd
723separate from the queue-runs
724.Pq Dq Nm sendmail Fl q15m .
725If you still want real-time delivery you can run the queue
726at a much lower interval, such as
727.Fl q1m ,
728but be sure to specify a reasonable
729.Va MaxDaemonChildren
730option for that
731.Xr sendmail 8
732to prevent cascade failures.
733.Pp
734The
735.Xr syslogd 8
736daemon can be attacked directly and it is strongly recommended that you use
737the
738.Fl s
739option whenever possible, and the
740.Fl a
741option otherwise.
742.Pp
743You should also be fairly careful
744with connect-back services such as tcpwrapper's reverse-identd, which can
745be attacked directly.
746You generally do not want to use the reverse-ident
747feature of tcpwrappers for this reason.
748.Pp
749It is a very good idea to protect internal services from external access
750by firewalling them off at your border routers.
751The idea here is to prevent
752saturation attacks from outside your LAN, not so much to protect internal
753services from network-based root compromise.
754Always configure an exclusive
755firewall, i.e.,
756.So
757firewall everything
758.Em except
759ports A, B, C, D, and M-Z
760.Sc .
761This
762way you can firewall off all of your low ports except for certain specific
763services such as
764.Xr named 8
765(if you are primary for a zone),
766.Xr talkd 8 ,
767.Xr sendmail 8 ,
768and other internet-accessible services.
769If you try to configure the firewall the other
770way \(em as an inclusive or permissive firewall, there is a good chance that you
771will forget to
772.Dq close
773a couple of services or that you will add a new internal
774service and forget to update the firewall.
775You can still open up the
776high-numbered port range on the firewall to allow permissive-like operation
777without compromising your low ports.
778Also take note that
779.Fx
780allows you to
781control the range of port numbers used for dynamic binding via the various
782.Va net.inet.ip.portrange
783sysctl's
784.Pq Dq Li "sysctl net.inet.ip.portrange" ,
785which can also
786ease the complexity of your firewall's configuration.
787I usually use a normal
788first/last range of 4000 to 5000, and a hiport range of 49152 to 65535, then
789block everything under 4000 off in my firewall
790(except for certain specific
791internet-accessible ports, of course).
792.Pp
793Another common DoS attack is called a springboard attack \(em to attack a server
794in a manner that causes the server to generate responses which then overload
795the server, the local network, or some other machine.
796The most common attack
797of this nature is the ICMP PING BROADCAST attack.
798The attacker spoofs ping
799packets sent to your LAN's broadcast address with the source IP address set
800to the actual machine they wish to attack.
801If your border routers are not
802configured to stomp on ping's to broadcast addresses, your LAN winds up
803generating sufficient responses to the spoofed source address to saturate the
804victim, especially when the attacker uses the same trick on several dozen
805broadcast addresses over several dozen different networks at once.
806Broadcast attacks of over a hundred and twenty megabits have been measured.
807A second common springboard attack is against the ICMP error reporting system.
808By
809constructing packets that generate ICMP error responses, an attacker can
810saturate a server's incoming network and cause the server to saturate its
811outgoing network with ICMP responses.
812This type of attack can also crash the
813server by running it out of
814.Vt mbuf Ns 's ,
815especially if the server cannot drain the
816ICMP responses it generates fast enough.
817The
818.Fx
819kernel has a new kernel
820compile option called
821.Dv ICMP_BANDLIM
822which limits the effectiveness of these
823sorts of attacks.
824The last major class of springboard attacks is related to
825certain internal
826.Xr inetd 8
827services such as the UDP echo service.
828An attacker
829simply spoofs a UDP packet with the source address being server A's echo port,
830and the destination address being server B's echo port, where server A and B
831are both on your LAN.
832The two servers then bounce this one packet back and
833forth between each other.
834The attacker can overload both servers and their
835LANs simply by injecting a few packets in this manner.
836Similar problems
837exist with the internal chargen port.
838A competent sysadmin will turn off all
839of these
840.Xr inetd 8 Ns -internal
841test services.
842.Pp
843Spoofed packet attacks may also be used to overload the kernel route cache.
844Refer to the
845.Va net.inet.ip.rtexpire , net.inet.ip.rtminexpire ,
846and
847.Va net.inet.ip.rtmaxcache
848.Xr sysctl 8
849variables.
850A spoofed packet attack that uses a random source IP will cause
851the kernel to generate a temporary cached route in the route table, viewable
852with
853.Dq Li "netstat -rna | fgrep W3" .
854These routes typically timeout in 1600
855seconds or so.
856If the kernel detects that the cached route table has gotten
857too big it will dynamically reduce the
858.Va rtexpire
859but will never decrease it to
860less than
861.Va rtminexpire .
862There are two problems: (1) The kernel does not react
863quickly enough when a lightly loaded server is suddenly attacked, and (2) The
864.Va rtminexpire
865is not low enough for the kernel to survive a sustained attack.
866If your servers are connected to the internet via a T3 or better it may be
867prudent to manually override both
868.Va rtexpire
869and
870.Va rtminexpire
871via
872.Xr sysctl 8 .
873Never set either parameter to zero
874(unless you want to crash the machine :-)).
875Setting both parameters to 2 seconds should be sufficient to protect the route
876table from attack.
877.Sh ACCESS ISSUES WITH KERBEROS AND SSH
878There are a few issues with both Kerberos and SSH that need to be addressed
879if you intend to use them.
880Kerberos5 is an excellent authentication
881protocol but the kerberized
882.Xr telnet 1
883and
884.Xr rlogin 1
885suck rocks.
886There are bugs that make them unsuitable for dealing with binary streams.
887Also, by default
888Kerberos does not encrypt a session unless you use the
889.Fl x
890option.
891SSH encrypts everything by default.
892.Pp
893SSH works quite well in every respect except when it is set up to
894forward encryption keys.
895What this means is that if you have a secure workstation holding
896keys that give you access to the rest of the system, and you
897.Xr ssh 1
898to an
899unsecure machine, your keys become exposed.
900The actual keys themselves are
901not exposed, but
902.Xr ssh 1
903installs a forwarding port for the duration of your
904login and if an attacker has broken root on the unsecure machine he can utilize
905that port to use your keys to gain access to any other machine that your
906keys unlock.
907.Pp
908We recommend that you use SSH in combination with Kerberos whenever possible
909for staff logins.
910SSH can be compiled with Kerberos support.
911This reduces
912your reliance on potentially exposable SSH keys while at the same time
913protecting passwords via Kerberos.
914SSH keys
915should only be used for automated tasks from secure machines (something
916that Kerberos is unsuited to).
917We also recommend that you either turn off
918key-forwarding in the SSH configuration, or that you make use of the
919.Va from Ns = Ns Ar IP/DOMAIN
920option that SSH allows in its
921.Pa authorized_keys
922file to make the key only usable to entities logging in from specific
923machines.
924.Sh SEE ALSO
925.Xr chflags 1 ,
926.Xr find 1 ,
927.Xr md5 1 ,
928.Xr netstat 1 ,
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