Command Section
SECURITY(7)        FreeBSD Miscellaneous Information Manual        SECURITY(7)

     security - introduction to security under FreeBSD

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

     Security is best implemented through a layered onion approach.  In a
     nutshell, what you want to do is to create as many layers of security as
     are convenient and then carefully monitor the system for intrusions.

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

           1.   Denial of Service attacks (DoS)

           2.   User account compromises

           3.   Root compromise through accessible servers

           4.   Root compromise via user accounts

           5.   Backdoor creation

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

     A user account compromise is even more common than a DoS attack.  Many
     sysadmins still run standard telnetd(8), rlogind(8), rshd(8), and ftpd(8)
     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 log in
     to a system) will have his or her password sniffed.  The attentive system
     administrator will analyze his remote access logs looking for suspicious
     source addresses even for successful logins.

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

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

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

           1.   Securing root and staff accounts

           2.   Securing root -- root-run servers and SUID/SGID binaries

           3.   Securing user accounts

           4.   Securing the password file

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

           6.   Quick detection of inappropriate changes made to the system

           7.   Paranoia

     Do not bother securing staff accounts if you have not secured the root
     account.  Most systems have a password assigned to the root account.  The
     first thing you do is assume that the password is always compromised.
     This does not mean that you should remove the password.  The password is
     almost always necessary for console access to the machine.  What it does
     mean is that you should not make it possible to use the password outside
     of the console or possibly even with a su(1) utility.  For example, make
     sure that your PTYs are specified as being ``insecure'' in the /etc/ttys
     file so that direct root logins via telnet(1) or rlogin(1) are
     disallowed.  If using other login services such as sshd(8), make sure
     that direct root logins are disabled there as well.  Consider every
     access method -- services such as ftp(1) often fall through the cracks.
     Direct root logins should only be allowed via the system console.

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

     An indirect way to secure the root account is to secure your staff
     accounts by using an alternative login access method and *'ing out the
     crypted password for the staff accounts.  This way an intruder may be
     able to steal the password file but will not be able to break into any
     staff accounts or root, even if root has a crypted password associated
     with it (assuming, of course, that you have limited root access to the
     console).  Staff members get into their staff accounts through a secure
     login mechanism such as kerberos(8) or ssh(1) using a private/public key
     pair.  When you use something like Kerberos you generally must secure the
     machines which run the Kerberos servers and your desktop workstation.
     When you use a public/private key pair with SSH, you must generally
     secure the machine you are logging in from (typically your workstation),
     but you can also add an additional layer of protection to the key pair by
     password protecting the keypair when you create it with ssh-keygen(1).
     Being able to *-out the passwords for staff accounts also guarantees that
     staff members can only log in through secure access methods that you have
     set up.  You can thus force all staff members to use secure, encrypted
     connections for all their sessions which closes an important hole used by
     many intruders: that of sniffing the network from an unrelated, less
     secure machine.

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

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

     The prudent sysadmin only runs the servers he needs to, no more, no less.
     Be aware that third party servers are often the most bug-prone.  For
     example, running an old version of imapd(8) or popper(8)
     (ports/mail/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
     talkd(8), comsat(8), and fingerd(8) daemons can be run in special user
     ``sandboxes''.  A sandbox is not perfect unless you go to a large amount
     of trouble, but the onion approach to security still stands: if someone
     is able to break in through a server running in a sandbox, they still
     have to break out of the sandbox.  The more layers the attacker must
     break through, the lower the likelihood of his success.  Root holes have
     historically been found in virtually every server ever run as root,
     including basic system servers.  If you are running a machine through
     which people only log in via sshd(8) and never log in via telnetd(8),
     rshd(8), or rlogind(8), then turn off those services!

     FreeBSD now defaults to running talkd(8), comsat(8), and fingerd(8) in a
     sandbox.  Depending on whether you are installing a new system or
     upgrading an existing system, the special user accounts used by these
     sandboxes may not be installed.  The prudent sysadmin would research and
     implement sandboxes for servers whenever possible.

     There are a number of other servers that typically do not run in
     sandboxes: sendmail(8), popper(8), imapd(8), ftpd(8), and others.  There
     are alternatives to some of these, but installing them may require more
     work than 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 break-ins that might occur through them.

     The other big potential root hole in a system are the SUID-root and SGID
     binaries installed on the system.  Most of these binaries, such as
     rlogin(1), reside in /bin, /sbin, /usr/bin, or /usr/sbin.  While nothing
     is 100% safe, the system-default SUID and SGID binaries can be considered
     reasonably safe.  Still, root holes are occasionally found in these
     binaries.  A root hole was found in Xlib in 1998 that made xterm(1)
     (ports/x11/xterm) (which is typically SUID) vulnerable.  It is better to
     be safe than sorry and the prudent sysadmin will restrict SUID binaries
     that only staff should run to a special group that only staff can access,
     and get rid of (``chmod 000'') any SUID binaries that nobody uses.  A
     server with no display generally does not need an xterm(1) binary.  SGID
     binaries can be almost as dangerous.  If an intruder can break an SGID-
     kmem binary the intruder might be able to read /dev/kmem and thus read
     the crypted password file, potentially compromising any passworded
     account.  Alternatively an intruder who breaks group ``kmem'' can monitor
     keystrokes sent through PTYs, including PTYs used by users who log in
     through secure methods.  An intruder that breaks the ``tty'' group can
     write to almost any user's TTY.  If a user is running a terminal program
     or emulator with a keyboard-simulation feature, the intruder can
     potentially generate a data stream that causes the user's terminal to
     echo a command, which is then run as that user.

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

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

     Your security scripts should always check for and report changes to the
     password file (see CHECKING FILE INTEGRITY below).

     If an attacker breaks root he can do just about anything, but there are
     certain conveniences.  For example, most modern kernels have a packet
     sniffing device driver built in.  Under FreeBSD it is called the bpf(4)
     device.  An intruder will commonly attempt to run a packet sniffer on a
     compromised machine.  You do not need to give the intruder the capability
     and most systems should not have the bpf(4) device compiled in.

     But even if you turn off the bpf(4) device, you still have /dev/mem and
     /dev/kmem to worry about.  For that matter, the intruder can still write
     to raw disk devices.  Also, there is another kernel feature called the
     module loader, kldload(8).  An enterprising intruder can use a KLD module
     to install his own bpf(4) device or other sniffing device on a running
     kernel.  To avoid these problems you have to run the kernel at a higher
     security level, at least level 1.  The security level can be set with a
     sysctl(8) on the kern.securelevel variable.  Once you have set the
     security level to 1, write access to raw devices will be denied and
     special chflags(1) 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 security level is set.  This might be overdoing it, and
     upgrading the system is much more difficult when you operate at a higher
     security level.  You may compromise and run the system at a higher
     security level but not set the schg flag for every system file and
     directory under the sun.  Another possibility is to simply mount / and
     /usr read-only.  It should be noted that being too draconian in what you
     attempt to protect may prevent the all-important detection of an

     The kernel runs with five different security levels.  Any super-user
     process can raise the level, but no process can lower it.  The security
     levels are:

     -1    Permanently insecure mode - always run the system in insecure mode.
           This is the default initial value.

     0     Insecure mode - immutable and append-only flags may be turned off.
           All devices may be read or written subject to their permissions.

     1     Secure mode - the system immutable and system append-only flags may
           not be turned off; disks for mounted file systems, /dev/mem and
           /dev/kmem may not be opened for writing; /dev/io (if your platform
           has it) may not be opened at all; kernel modules (see kld(4)) may
           not be loaded or unloaded.  The kernel debugger may not be entered
           using the debug.kdb.enter sysctl.  A panic or trap cannot be forced
           using the debug.kdb.panic and other sysctl's.

     2     Highly secure mode - same as secure mode, plus disks may not be
           opened for writing (except by mount(2)) whether mounted or not.
           This level precludes tampering with file systems by unmounting
           them, but also inhibits running newfs(8) while the system is multi-

           In addition, kernel time changes are restricted to less than or
           equal to one second.  Attempts to change the time by more than this
           will log the message ``Time adjustment clamped to +1 second''.

     3     Network secure mode - same as highly secure mode, plus IP packet
           filter rules (see ipfw(8), ipfirewall(4) and pfctl(8)) cannot be
           changed and dummynet(4) or pf(4) configuration cannot be adjusted.

     The security level can be configured with variables documented in

     When it comes right down to it, you can only protect your core system
     configuration and control files so much before the convenience factor
     rears its ugly head.  For example, using chflags(1) to set the schg bit
     on most of the files in / and /usr is probably counterproductive because
     while it may protect the files, it also closes a detection window.  The
     last layer of your security onion is perhaps the most important --
     detection.  The rest of your security is pretty much useless (or, worse,
     presents you with a false sense of safety) if you cannot detect potential
     incursions.  Half the job of the onion is to slow down the attacker
     rather than stop him in order to give the detection layer a chance to
     catch him in the act.

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

     Once you give a limit-access box at least read access to the client
     systems it is supposed to monitor, you must write scripts to do the
     actual monitoring.  Given an NFS mount, you can write scripts out of
     simple system utilities such as find(1) and md5(1).  It is best to
     physically md5(1) the client-box files boxes at least once a day, and to
     test control files such as those found in /etc and /usr/local/etc even
     more often.  When mismatches are found relative to the base MD5
     information the limited-access machine knows is valid, it should scream
     at a sysadmin to go check it out.  A good security script will also check
     for inappropriate SUID binaries and for new or deleted files on system
     partitions such as / and /usr.

     When using SSH rather than NFS, writing the security script is much more
     difficult.  You essentially have to scp(1) the scripts to the client box
     in order to run them, making them visible, and for safety you also need
     to scp(1) the binaries (such as find(1)) that those scripts use.  The
     sshd(8) daemon on the client box may already be compromised.  All in all,
     using SSH may be necessary when running over unsecure links, but it is
     also a lot harder to deal with.

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

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

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

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

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

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

           1.   Limiting server forks

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

           3.   Kernel Route Cache

     A common DoS attack is against a forking server that attempts to cause
     the server to eat processes, file descriptors, and memory until the
     machine dies.  The inetd(8) server 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(8) 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(8), so
     typically a combination of options must be used.  Some standalone servers
     have self-fork-limitation parameters.

     The sendmail(8) daemon has its -OMaxDaemonChildren option which tends to
     work much better than trying to use sendmail(8)'s load limiting options
     due to the load lag.  You should specify a MaxDaemonChildren parameter
     when you start sendmail(8) high enough to handle your expected load but
     not so high that the computer cannot handle that number of sendmail's
     without falling on its face.  It is also prudent to run sendmail(8) in
     ``queued'' mode (-ODeliveryMode=queued) and to run the daemon (``sendmail
     -bd'') separate from the queue-runs (``sendmail -q15m'').  If you still
     want real-time 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(8) to prevent cascade failures.

     The syslogd(8) daemon can be attacked directly and it is strongly
     recommended that you use the -s option whenever possible, and the -a
     option otherwise.

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

     It is a very good idea to protect internal services from external access
     by firewalling them off at your border routers.  The idea here is to
     prevent saturation attacks from outside your LAN, not so much to protect
     internal services from network-based root compromise.  Always configure
     an exclusive firewall, i.e., `firewall everything except ports A, B, C,
     D, and M-Z'.  This way you can firewall off all of your low ports except
     for certain specific services such as talkd(8), sendmail(8), 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 net.inet.ip.portrange''), which can also ease the complexity of
     your firewall's configuration.  I usually use a normal first/last range
     of 4000 to 5000, and a hiport range of 49152 to 65535, then block
     everything under 4000 off in my firewall (except for certain specific
     internet-accessible ports, of course).

     Another common DoS attack is called a springboard attack -- to attack a
     server in a manner that causes the server to generate responses which
     then overload the server, the local network, or some other machine.  The
     most common attack of this nature is the ICMP PING BROADCAST attack.  The
     attacker spoofs ping packets sent to your LAN's broadcast address with
     the source IP address set to the actual machine they wish to attack.  If
     your border routers are not configured to stomp on ping's to broadcast
     addresses, your LAN winds up generating sufficient responses to the
     spoofed source address to saturate the victim, especially when the
     attacker uses the same trick on several dozen broadcast addresses over
     several dozen different networks at once.  Broadcast attacks of over a
     hundred and twenty megabits have been measured.  A second common
     springboard attack is against the ICMP error reporting system.  By
     constructing packets that generate ICMP error responses, an attacker can
     saturate a server's incoming network and cause the server to saturate its
     outgoing network with ICMP responses.  This type of attack can also crash
     the server by running it out of mbuf's, especially if the server cannot
     drain the ICMP responses it generates fast enough.  The FreeBSD kernel
     has a new kernel compile option called ICMP_BANDLIM which limits the
     effectiveness of these sorts of attacks.  The last major class of
     springboard attacks is related to certain internal inetd(8) services such
     as the UDP echo service.  An attacker simply spoofs a UDP packet with the
     source address being server A's echo port, and the destination address
     being server B's echo port, where server A and B are both on your LAN.
     The two servers then bounce this one packet back and forth between each
     other.  The attacker can overload both servers and their LANs simply by
     injecting a few packets in this manner.  Similar problems exist with the
     internal chargen port.  A competent sysadmin will turn off all of these
     inetd(8)-internal test services.

     There are a few issues with both Kerberos and SSH that need to be
     addressed if you intend to use them.  Kerberos5 is an excellent
     authentication protocol but the kerberized telnet(1) and rlogin(1) suck
     rocks.  There are bugs that make them unsuitable for dealing with binary
     streams.  Also, by default Kerberos does not encrypt a session unless you
     use the -x option.  SSH encrypts everything by default.

     SSH works quite well in every respect except when it is set up to forward
     encryption keys.  What this means is that if you have a secure
     workstation holding keys that give you access to the rest of the system,
     and you ssh(1) to an unsecure machine, your keys become exposed.  The
     actual keys themselves are not exposed, but ssh(1) installs a forwarding
     port for the duration of your login and if an attacker has broken root on
     the unsecure machine he can utilize that port to use your keys to gain
     access to any other machine that your keys unlock.

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

     chflags(1), find(1), md5(1), netstat(1), openssl(1), ssh(1), xdm(1)
     (ports/x11/xorg-clients), group(5), ttys(5), accton(8), init(8), sshd(8),
     sysctl(8), syslogd(8), vipw(8)

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

FreeBSD 11.1-RELEASE-p4        December 25, 2013       FreeBSD 11.1-RELEASE-p4
Command Section