Throughout this document, we will use
bold text to refer to an
application, and a monospaced
font to refer
to specific commands. Protocols will use a normal font. This
typographical distinction is useful for instances such as ssh,
since it is
a protocol as well as command.
The sections that follow will cover the methods of securing your FreeBSD system that were mentioned in the last section of this chapter.
First off, 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 the su(1) command. For example, make sure that
your ptys are specified as being insecure in the
/etc/ttys
file so that direct
root
logins
via telnet
or rlogin
are
disallowed. If using other login services such as
sshd, make sure that direct
root
logins are disabled there as well.
You can do this by editing
your /etc/ssh/sshd_config
file, and making
sure that PermitRootLogin
is set to
NO
. Consider every access method —
services such as FTP often fall through the cracks.
Direct root
logins should only be allowed
via the system console.
Of course, as a sysadmin you have to be able to get to
root
, so we open up a few holes.
But we make sure these holes require additional password
verification to operate. One way to make root
accessible is to add appropriate staff accounts to the
wheel
group (in
/etc/group
). The staff members placed in the
wheel
group are allowed to
su
to root
.
You should never give staff
members native wheel
access by putting them in the
wheel
group in their password entry. Staff
accounts should be placed in a staff
group, and
then added to the wheel
group via the
/etc/group
file. Only those staff members
who actually need to have root
access
should be placed in the
wheel
group. It is also possible, when using
an authentication method such as Kerberos, to use Kerberos'
.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 staff accounts, and ultimately
root
access is to use an alternative
login access method and
do what is known as „starring” out the encrypted
password for the staff accounts. Using the vipw(8)
command, one can replace each instance of an encrypted password
with a single „*
” character.
This command will update the /etc/master.passwd
file and user/password database to disable password-authenticated
logins.
A staff account entry such as:
foobar:R9DT/Fa1/LV9U:1000:1000::0:0:Foo Bar:/home/foobar:/usr/local/bin/tcsh
Should be changed to this:
foobar:*:1000:1000::0:0:Foo Bar:/home/foobar:/usr/local/bin/tcsh
This change will prevent normal logins from occurring,
since the encrypted password will never match
„*
”. With this done,
staff members must use
another mechanism to authenticate themselves such as
kerberos(1) or ssh(1) using a public/private key
pair. When using something like Kerberos, one generally must
secure the machines which run the Kerberos servers and your
desktop workstation. When using a public/private key pair
with ssh, one must generally secure
the machine used to login from (typically
one's workstation). An additional layer of protection can be
added to the key pair by password protecting the key pair when
creating it with ssh-keygen(1). Being able to
„star” out the passwords for staff accounts also
guarantees that staff members can only login through secure
access methods that you have set up. This forces all staff
members to use secure, encrypted connections for all of their
sessions, which closes an important hole used by many
intruders: 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 on which the staff member may have an account. If a staff member's account gets compromised, the ability to instantly change his password on all machines should not be underrated. With discrete passwords, changing a password on N machines can be a mess. You can also impose re-passwording restrictions with Kerberos: not only can a Kerberos ticket be made to timeout after a while, but the Kerberos system can require that the user choose a new password after a certain period of time (say, once a month).
The prudent sysadmin only runs the servers he needs to, no
more, no less. Be aware that third party servers are often the
most bug-prone. For example, running an old version of
imapd or
popper is like giving a universal
root
ticket out to the entire world.
Never run a server that you have not checked out carefully.
Many servers do not need to be run as root
.
For example, the ntalk,
comsat, and
finger daemons can be run in special
user sandboxes. A sandbox is not perfect,
unless you go through a large amount of trouble, but the onion
approach to security still stands: If someone is able to break
in through a server running in a sandbox, they still have to
break out of the sandbox. The more layers the attacker must
break through, the lower the likelihood of his success. Root
holes have historically been found in virtually every server
ever run as root
, including basic system servers.
If you are running a machine through which people only login via
sshd and never login via
telnetd or
rshd or
rlogind, then turn off those
services!
FreeBSD now defaults to running
ntalkd,
comsat, and
finger in a sandbox. Another program
which may be a candidate for running in a sandbox is named(8).
/etc/defaults/rc.conf
includes the arguments
necessary to run named in a sandbox in a
commented-out form. Depending on whether you are installing a new
system or upgrading an existing system, the special user accounts
used by these sandboxes may not be installed. The prudent
sysadmin would research and implement sandboxes for servers
whenever possible.
There are a number of other servers that typically do not run
in sandboxes: sendmail,
popper,
imapd, ftpd,
and others. There are alternatives to some of these, but
installing them may require more work than you are willing to
perform (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
holes in a
system are the
suid-root and sgid binaries installed on the system. Most of
these binaries, such as rlogin, reside
in /bin
, /sbin
,
/usr/bin
, or /usr/sbin
.
While nothing is 100% safe, the system-default suid and sgid
binaries can be considered reasonably safe. Still,
root
holes are occasionally found in these
binaries. A root
hole was found in
Xlib
in 1998 that made
xterm (which is typically suid)
vulnerable. It is better to be safe than sorry and the prudent
sysadmin will restrict suid binaries, that only staff should run,
to a special group that only staff can access, and get rid of
(chmod 000
) any suid binaries that nobody uses.
A server with no display generally does not need an
xterm binary. Sgid binaries can be
almost as dangerous. If an intruder can break an sgid-kmem binary,
the intruder might be able to read /dev/kmem
and thus read the encrypted 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 login through secure
methods. An intruder that breaks the tty
group can write to
almost any user's tty. If a user is running a terminal program or
emulator with a keyboard-simulation feature, the intruder can
potentially generate a data stream that causes the user's terminal
to echo a command, which is then run as that user.
User accounts are usually the most difficult to secure. While you can impose Draconian access restrictions on your staff and „star” 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 encrypted
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 the Checking file integrity section below).
If an attacker breaks root
he can do
just about anything, but
there are certain conveniences. For example, most modern kernels
have a packet sniffing device driver built in. Under FreeBSD it
is called the bpf
device. An intruder
will commonly attempt to run a packet sniffer on a compromised
machine. You do not need to give the intruder the capability and
most systems do not have the need for the
bpf
device compiled in.
But even if you turn off the bpf
device, you still have
/dev/mem
and
/dev/kmem
to worry about. For that matter, the intruder can still write to
raw disk devices. Also, there is another kernel feature called
the module loader, kldload(8). An enterprising intruder can
use a KLD module to install his own bpf
device, or other sniffing
device, on a running kernel. To avoid these problems you have to
run the kernel at a higher secure level, at least securelevel 1.
The securelevel can be set with a sysctl
on
the kern.securelevel
variable. Once you have
set the securelevel to 1, write access to raw devices will be
denied and special chflags
flags,
such as schg
,
will be enforced. You must also ensure that the
schg
flag is set on critical startup binaries,
directories, and script files — everything that gets run up
to the point where the securelevel is set. This might be overdoing
it, and upgrading the system is much more difficult when you
operate at a higher secure level. You may compromise and run the
system at a higher secure level but not set the
schg
flag for every system file and directory
under the sun. Another possibility is to simply mount
/
and /usr
read-only.
It should be noted that being too Draconian in what you attempt to
protect may prevent the all-important detection of an
intrusion.
When it comes right down to it, you can only protect your core
system configuration and control files so much before the
convenience factor rears its ugly head. For example, using
chflags
to set the schg
bit
on most of the files in /
and
/usr
is probably counterproductive, because
while it may protect the files, it also closes a detection window.
The last layer of your security onion is perhaps the most
important — detection. The rest of your security is pretty
much useless (or, worse, presents you with a false sense of
safety) if you cannot detect potential incursions. Half the job
of the onion is to slow down the attacker, rather than stop him, in
order to give the detection side of the equation a chance to catch
him in the act.
The best way to detect an incursion is to look for modified, missing, or unexpected files. The best way to look for modified files is from another (often centralized) limited-access system. Writing your security scripts on the extra-secure limited-access system makes them mostly invisible to potential 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 key-pairs to allow the limited-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 limited-access box, at least read access to the
client systems it is supposed to monitor, you must write scripts
to do the actual monitoring. Given an NFS mount, you can write
scripts out of simple system utilities such as find(1) and
md5(1). It is best to physically md5 the client-box files
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
the scripts to the client
box in order to
run them, making them visible, and for safety you also need to
scp
the binaries (such as find) that those
scripts use. The ssh client on the
client box may already be compromised. All in all, using
ssh may be necessary when running over
insecure 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 and devices on those
partitions is a good idea. The nodev
and
nosuid
options (see mount(8)) are what you
want to look into. You should probably 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 might help 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 document verbatim, you give away your methodologies to the prospective attacker who also has access to this document.
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.
Limiting server forks.
Limiting springboard attacks (ICMP response attacks, ping broadcast, etc.).
Kernel Route Cache.
A common DoS attack is against a forking server that attempts
to cause the server to eat processes, file descriptors, and memory,
until the machine dies. inetd
(see inetd(8)) has several
options to limit this sort of attack. It should be noted that
while it is possible to prevent a machine from going down, it is
not generally possible to prevent a service from being disrupted
by the attack. Read the inetd manual
page carefully and pay
specific attention to the -c
, -C
,
and -R
options. Note that spoofed-IP attacks
will circumvent the -C
option to
inetd, so
typically a combination of options must be used. Some standalone
servers have self-fork-limitation parameters.
Sendmail has its
-OMaxDaemonChildren
option, which tends to work
much better than trying to use sendmail's load limiting options
due to the load lag. You should specify a
MaxDaemonChildren
parameter, when you start
sendmail, high enough to handle your
expected load, but not so high that the computer cannot handle that
number of sendmails without falling on
its face. It is also prudent to run sendmail in queued mode
(-ODeliveryMode=queued
) and to run the daemon
(sendmail -bd
) separate from the queue-runs
(sendmail -q15m
). If you still want 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 to prevent cascade failures.
Syslogd can be attacked directly
and it is strongly recommended that you use the -s
option whenever possible, and the -a
option
otherwise.
You should also be fairly careful with connect-back services such as TCP Wrapper's reverse-identd, which can be attacked directly. You generally do not want to use the reverse-ident feature of TCP Wrapper for this reason.
It is a very good idea to protect internal services from
external access by firewalling them off at your border routers.
The idea here is to prevent saturation attacks from outside your
LAN, not so much to protect internal services from network-based
root
compromise.
Always configure an exclusive firewall, i.e.,
„firewall everything except ports A, B,
C, D, and M-Z”. This way you can firewall off all of your
low ports except for certain specific services such as
named (if you are primary for a zone),
ntalkd,
sendmail, and other Internet-accessible
services. If you try to configure the firewall the other way
— as an inclusive or permissive firewall, there is a good
chance that you will forget to „close” a couple of
services, or that you will add a new internal service and forget
to update the firewall. You can still open up the high-numbered
port range on the firewall, to allow permissive-like operation,
without compromising your low ports. Also take note that FreeBSD
allows you to control the range of port numbers used for dynamic
binding, via the various net.inet.ip.portrange
sysctl
's (sysctl -a | fgrep
portrange
), which can also ease the complexity of your
firewall's configuration. For example, you might use a normal
first/last range of 4000 to 5000, and a hiport range of 49152 to
65535, then block off everything under 4000 in your 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 overloads 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.
Use the sysctl
variable net.inet.icmp.icmplim
to limit these attacks.
The last major class of springboard
attacks is related to certain internal
inetd services such as the
udp echo service. An attacker simply spoofs a UDP packet with the
source address being server A's echo port, and the destination
address being server B's echo port, where server A and B are both
on your LAN. The two servers then bounce this one packet back and
forth between each other. The attacker can overload both servers
and their LANs simply by injecting a few packets in this manner.
Similar problems exist with the internal
chargen port. A
competent sysadmin will turn off all of these inetd-internal test
services.
Spoofed packet attacks may also be used to overload the kernel
route cache. Refer to the net.inet.ip.rtexpire
,
rtminexpire
, and rtmaxcache
sysctl
parameters. A spoofed packet attack
that uses a random source IP will cause the kernel to generate a
temporary cached route in the route table, viewable with
netstat -rna | fgrep W3
. These routes
typically timeout in 1600 seconds or so. If the kernel detects
that the cached route table has gotten too big it will dynamically
reduce the rtexpire
but will never decrease it
to less than rtminexpire
. There are two
problems:
The kernel does not react quickly enough when a lightly loaded server is suddenly attacked.
The rtminexpire
is not low enough for
the kernel to survive a sustained attack.
If your servers are connected to the Internet via a T3 or
better, it may be prudent to manually override both
rtexpire
and rtminexpire
via sysctl(8). Never set either parameter to zero (unless
you want to crash the machine). Setting both
parameters to 2 seconds should be sufficient to protect the route
table from attack.
There are a few issues with both Kerberos and
ssh that need to be addressed if
you intend to use them. Kerberos V is an excellent
authentication protocol, but there are bugs in the kerberized
telnet and
rlogin applications 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 that it forwards encryption keys by default. What
this means is that if you have a secure workstation holding keys
that give you access to the rest of the system, and you
ssh to an insecure machine, your keys
are usable. The actual keys themselves are not exposed, but
ssh installs a forwarding port for the
duration of your login, and if an attacker has broken
root
on the
insecure 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 exposed
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 do). 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.
All FreeBSD documents are available for download at http://ftp.FreeBSD.org/pub/FreeBSD/doc/
Questions that are not answered by the
documentation may be
sent to <freebsd-questions@FreeBSD.org>.
Send questions about this document to <freebsd-doc@FreeBSD.org>.