xref: /freebsd/share/man/man7/security.7 (revision 814b16ba92c4194ac76eaeb0537fc7190dc9d1a0)

.\" Copyright (C) 1998 Matthew Dillon. All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY AUTHOR AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL AUTHOR OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\" $FreeBSD$
.\"
.Dd February 8, 2012
.Dt SECURITY 7
.Os
.Sh NAME
.Nm security
.Nd introduction to security under FreeBSD
.Sh DESCRIPTION
Security is a function that begins and ends with the system administrator.
While all
.Bx
multi-user systems have some inherent security, the job of building and
maintaining additional security mechanisms to keep users
.Dq 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.
.Ux
systems,
in general, are capable of running a huge number of simultaneous processes
and many of these processes operate as servers \(em meaning that external
entities can connect and talk to them.
As yesterday's mini-computers and mainframes
become today's desktops, and as computers become networked and internetworked,
security becomes an ever bigger issue.
.Pp
Security 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.
.Pp
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:
.Bl -enum -offset indent
.It
Denial of Service attacks (DoS)
.It
User account compromises
.It
Root compromise through accessible servers
.It
Root compromise via user accounts
.It
Backdoor creation
.El
.Pp
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.
.Pp
A user account compromise is even more common than a DoS attack.
Many
sysadmins still run standard
.Xr telnetd 8 ,
.Xr rlogind 8 ,
.Xr rshd 8 ,
and
.Xr 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.
.Pp
One must always assume that once an attacker has access to a user account,
the attacker can break root.
However, the reality is that in a well secured
and maintained system, access to a user account does not necessarily give the
attacker access to root.
The distinction is important because without access
to root the attacker cannot generally hide his tracks and may, at best, be
able to 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 take.
.Pp
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.
.Pp
Security remedies should always be implemented with a multi-layered
.Dq onion peel
approach and can be categorized as follows:
.Bl -enum -offset indent
.It
Securing root and staff accounts
.It
Securing root \(em root-run servers and SUID/SGID binaries
.It
Securing user accounts
.It
Securing the password file
.It
Securing the kernel core, raw devices, and file systems
.It
Quick detection of inappropriate changes made to the system
.It
Paranoia
.El
.Sh SECURING THE ROOT ACCOUNT AND SECURING STAFF ACCOUNTS
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
.Em 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
.Xr su 1
utility.
For example, make sure that your PTYs are specified as being
.Dq Li insecure
in the
.Pa /etc/ttys
file
so that direct root logins via
.Xr telnet 1
or
.Xr rlogin 1
are disallowed.
If using
other login services such as
.Xr sshd 8 ,
make sure that direct root logins are
disabled there as well.
Consider every access method \(em services such as
.Xr ftp 1
often fall through the cracks.
Direct root logins should only be allowed
via the system console.
.Pp
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
.Dq Li wheel
group (in
.Pa /etc/group ) .
The staff members placed in the
.Li wheel
group are allowed to
.Xr su 1
to root.
You should never give staff
members native
.Li wheel
access by putting them in the
.Li wheel
group in their password entry.
Staff accounts should be placed in a
.Dq Li staff
group, and then added to the
.Li wheel
group via the
.Pa /etc/group
file.
Only those staff members who actually need to have root access
should be placed in the
.Li wheel
group.
It is also possible, when using an
authentication method such as Kerberos, to use Kerberos's
.Pa .k5login
file in the root account to allow a
.Xr ksu 1
to root without having to place anyone at all in the
.Li wheel
group.
This
may be the better solution since the
.Li 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
.Li wheel
mechanism
is better than having nothing at all, it is not necessarily the safest
option.
.Pp
An indirect way to secure the root account is to secure your staff accounts
by using an alternative login access method and *'ing out the crypted password
for the staff accounts.
This way 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
.Xr kerberos 8
or
.Xr 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
.Em 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
.Xr 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.
.Pp
The more indirect security mechanisms also assume that you are logging in
from a more restrictive server to a less restrictive server.
For example,
if your main box is running all sorts of servers, your workstation 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.
.Pp
Using something like Kerberos also gives you the ability to disable or
change the password for a staff account in one place and have it immediately
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).
.Sh SECURING ROOT \(em ROOT-RUN SERVERS AND SUID/SGID BINARIES
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
.Xr imapd 8
or
.Xr popper 8 Pq Pa 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
.Xr talkd 8 ,
.Xr comsat 8 ,
and
.Xr fingerd 8
daemons can be run in special user
.Dq 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
.Xr sshd 8
and never log in via
.Xr telnetd 8 ,
.Xr rshd 8 ,
or
.Xr rlogind 8 ,
then turn off those services!
.Pp
.Fx
now defaults to running
.Xr talkd 8 ,
.Xr comsat 8 ,
and
.Xr fingerd 8
in a sandbox.
Another program which may be a candidate for running in a sandbox is
.Xr named 8 .
The default
.Pa rc.conf
includes the arguments necessary to run
.Xr named 8
in a sandbox in a commented-out form.
Depending on whether you
are installing a new system or upgrading an existing system, the special
user accounts used by these sandboxes may not be installed.
The prudent
sysadmin would research and implement sandboxes for servers whenever possible.
.Pp
There are a number of other servers that typically do not run in sandboxes:
.Xr sendmail 8 ,
.Xr popper 8 ,
.Xr imapd 8 ,
.Xr 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.
.Pp
The other big potential root hole in a system are the SUID-root and SGID
binaries installed on the system.
Most of these binaries, such as
.Xr rlogin 1 ,
reside in
.Pa /bin , /sbin , /usr/bin ,
or
.Pa /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
.Xr xterm 1 Pq Pa 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
.Pq Dq Li "chmod 000"
any SUID binaries that nobody uses.
A server with no display generally does not need an
.Xr 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
.Pa /dev/kmem
and thus read the crypted password
file, potentially compromising any passworded account.
Alternatively an
intruder who breaks group
.Dq Li kmem
can monitor keystrokes sent through PTYs,
including PTYs used by users who log in through secure methods.
An intruder
that breaks the
.Dq Li 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.
.Sh SECURING USER ACCOUNTS
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.
.Sh SECURING THE 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
.Pq Pa /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.
.Pp
Your security scripts should always check for and report changes to
the password file
(see
.Sx CHECKING FILE INTEGRITY
below).
.Sh SECURING THE KERNEL CORE, RAW DEVICES, AND FILE SYSTEMS
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
.Fx
it is called
the
.Xr 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
.Xr bpf 4
device compiled in.
.Pp
But even if you turn off the
.Xr bpf 4
device, you still have
.Pa /dev/mem
and
.Pa /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,
.Xr kldload 8 .
An enterprising intruder can use a KLD module to install
his own
.Xr 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
.Xr sysctl 8
on the
.Va kern.securelevel
variable.
Once you have
set the security level to 1, write access to raw devices will be denied and
special
.Xr chflags 1
flags, such as
.Cm schg ,
will be enforced.
You must also ensure
that the
.Cm schg
flag is set on critical startup binaries, directories, and
script files \(em 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
.Cm schg
flag for every
system file and directory under the sun.
Another possibility is to simply
mount
.Pa /
and
.Pa /usr
read-only.
It should be noted that being too draconian in
what you attempt to protect may prevent the all-important detection of an
intrusion.
.Pp
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:
.Bl -tag -width flag
.It Ic -1
Permanently insecure mode \- always run the system in insecure mode.
This is the default initial value.
.It Ic 0
Insecure mode \- immutable and append-only flags may be turned off.
All devices may be read or written subject to their permissions.
.It Ic 1
Secure mode \- the system immutable and system append-only flags may not
be turned off;
disks for mounted file systems,
.Pa /dev/mem
and
.Pa /dev/kmem
may not be opened for writing;
.Pa /dev/io
(if your platform has it) may not be opened at all;
kernel modules (see
.Xr kld 4 )
may not be loaded or unloaded.
The kernel debugger may not be entered using the
.Va debug.kdb.enter
sysctl.
A panic or trap cannot be forced using the
.Va debug.kdb.panic
and other sysctl's.
.It Ic 2
Highly secure mode \- same as secure mode, plus disks may not be
opened for writing (except by
.Xr mount 2 )
whether mounted or not.
This level precludes tampering with file systems by unmounting them,
but also inhibits running
.Xr newfs 8
while the system is multi-user.
.Pp
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
.Dq Time adjustment clamped to +1 second .
.It Ic 3
Network secure mode \- same as highly secure mode, plus
IP packet filter rules (see
.Xr ipfw 8 ,
.Xr ipfirewall 4
and
.Xr pfctl 8 )
cannot be changed and
.Xr dummynet 4
or
.Xr pf 4
configuration cannot be adjusted.
.El
.Pp
The security level can be configured with variables documented in
.Xr rc.conf 5 .
.Sh CHECKING FILE INTEGRITY: BINARIES, CONFIG FILES, ETC
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
.Xr chflags 1
to set the
.Cm schg
bit on most of the files in
.Pa /
and
.Pa /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 \(em 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.
.Pp
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 \(em 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.
.Pp
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
.Xr find 1
and
.Xr md5 1 .
It is best to physically
.Xr md5 1
the client-box files boxes at least once a
day, and to test control files such as those found in
.Pa /etc
and
.Pa /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
.Pa /
and
.Pa /usr .
.Pp
When using SSH rather than NFS, writing the security script is much more
difficult.
You essentially have to
.Xr scp 1
the scripts to the client box in order to run them, making them visible, and
for safety you also need to
.Xr scp 1
the binaries (such as
.Xr find 1 )
that those scripts use.
The
.Xr 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.
.Pp
A good security script will also check for changes to user and staff members
access configuration files:
.Pa .rhosts , .shosts , .ssh/authorized_keys
and so forth, files that might fall outside the purview of the MD5 check.
.Pp
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
.Cm nosuid
option
(see
.Xr 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.
.Pp
Process accounting
(see
.Xr 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.
.Pp
Finally, security scripts should process the log files and the logs themselves
should be generated in as secure a manner as possible \(em 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.
.Sh PARANOIA
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 \(em 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.
.Sh SPECIAL SECTION ON DoS ATTACKS
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.
.Bl -enum -offset indent
.It
Limiting server forks
.It
Limiting springboard attacks (ICMP response attacks, ping broadcast, etc.)
.It
Kernel Route Cache
.El
.Pp
A common DoS attack is against a forking server that attempts to cause the
server to eat processes, file descriptors, and memory until the machine
dies.
The
.Xr 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
.Xr inetd 8
manual page carefully and pay specific attention
to the
.Fl c , C ,
and
.Fl R
options.
Note that spoofed-IP attacks will circumvent
the
.Fl C
option to
.Xr inetd 8 ,
so typically a combination of options must be used.
Some standalone servers have self-fork-limitation parameters.
.Pp
The
.Xr sendmail 8
daemon has its
.Fl OMaxDaemonChildren
option which tends to work much
better than trying to use
.Xr sendmail 8 Ns 's
load limiting options due to the
load lag.
You should specify a
.Va MaxDaemonChildren
parameter when you start
.Xr sendmail 8
high enough to handle your expected load but not so high that the
computer cannot handle that number of
.Nm sendmail Ns 's
without falling on its face.
It is also prudent to run
.Xr sendmail 8
in
.Dq queued
mode
.Pq Fl ODeliveryMode=queued
and to run the daemon
.Pq Dq Nm sendmail Fl bd
separate from the queue-runs
.Pq Dq Nm sendmail Fl q15m .
If you still want real-time delivery you can run the queue
at a much lower interval, such as
.Fl q1m ,
but be sure to specify a reasonable
.Va MaxDaemonChildren
option for that
.Xr sendmail 8
to prevent cascade failures.
.Pp
The
.Xr syslogd 8
daemon can be attacked directly and it is strongly recommended that you use
the
.Fl s
option whenever possible, and the
.Fl a
option otherwise.
.Pp
You should also be fairly careful
with connect-back services such as tcpwrapper's reverse-identd, which can
be attacked directly.
You generally do not want to use the reverse-ident
feature of tcpwrappers for this reason.
.Pp
It is a very good idea to protect internal services from external access
by firewalling them off at your border routers.
The idea here is to prevent
saturation attacks from outside your LAN, not so much to protect internal
services from network-based root compromise.
Always configure an exclusive
firewall, i.e.,
.So
firewall everything
.Em except
ports A, B, C, D, and M-Z
.Sc .
This
way you can firewall off all of your low ports except for certain specific
services such as
.Xr named 8
(if you are primary for a zone),
.Xr talkd 8 ,
.Xr sendmail 8 ,
and other internet-accessible services.
If you try to configure the firewall the other
way \(em as an inclusive or permissive firewall, there is a good chance that you
will forget to
.Dq 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
.Fx
allows you to
control the range of port numbers used for dynamic binding via the various
.Va net.inet.ip.portrange
sysctl's
.Pq Dq Li "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).
.Pp
Another common DoS attack is called a springboard attack \(em 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
.Vt mbuf Ns 's ,
especially if the server cannot drain the
ICMP responses it generates fast enough.
The
.Fx
kernel has a new kernel
compile option called
.Dv ICMP_BANDLIM
which limits the effectiveness of these
sorts of attacks.
The last major class of springboard attacks is related to
certain internal
.Xr 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
.Xr inetd 8 Ns -internal
test services.
.Pp
Spoofed packet attacks may also be used to overload the kernel route cache.
Refer to the
.Va net.inet.ip.rtexpire , net.inet.ip.rtminexpire ,
and
.Va net.inet.ip.rtmaxcache
.Xr sysctl 8
variables.
A spoofed packet attack that uses a random source IP will cause
the kernel to generate a temporary cached route in the route table, viewable
with
.Dq Li "netstat -rna | fgrep W3" .
These routes typically timeout in 1600
seconds or so.
If the kernel detects that the cached route table has gotten
too big it will dynamically reduce the
.Va rtexpire
but will never decrease it to
less than
.Va rtminexpire .
There are two problems: (1) The kernel does not react
quickly enough when a lightly loaded server is suddenly attacked, and (2) The
.Va rtminexpire
is not low enough for the kernel to survive a sustained attack.
If your servers are connected to the internet via a T3 or better it may be
prudent to manually override both
.Va rtexpire
and
.Va rtminexpire
via
.Xr sysctl 8 .
Never set either parameter to zero
(unless you want to crash the machine :-)).
Setting both parameters to 2 seconds should be sufficient to protect the route
table from attack.
.Sh ACCESS ISSUES WITH KERBEROS AND SSH
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
.Xr telnet 1
and
.Xr 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
.Fl x
option.
SSH encrypts everything by default.
.Pp
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
.Xr ssh 1
to an
unsecure machine, your keys become exposed.
The actual keys themselves are
not exposed, but
.Xr 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.
.Pp
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
.Va from Ns = Ns Ar IP/DOMAIN
option that SSH allows in its
.Pa authorized_keys
file to make the key only usable to entities logging in from specific
machines.
.Sh SEE ALSO
.Xr chflags 1 ,
.Xr find 1 ,
.Xr md5 1 ,
.Xr netstat 1 ,
.Xr openssl 1 ,
.Xr ssh 1 ,
.Xr xdm 1 Pq Pa ports/x11/xorg-clients ,
.Xr group 5 ,
.Xr ttys 5 ,
.Xr accton 8 ,
.Xr init 8 ,
.Xr sshd 8 ,
.Xr sysctl 8 ,
.Xr syslogd 8 ,
.Xr vipw 8
.Sh HISTORY
The
.Nm
manual page was originally written by
.An Matthew Dillon
and first appeared
in
.Fx 3.1 ,
December 1998.