svcadm(8)을 검색하려면 섹션에서 8 을 선택하고, 맨 페이지 이름에 svcadm을 입력하고 검색을 누른다.
security(7)
Security is a function that begins and ends with the system ad‐
ministrator. While all multi-user systems have some inherent se‐
curity, the job of building and maintaining additional security
mechanisms to keep users 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 hu‐
man necessity for convenience. systems, in general, are capable
of running a huge number of simultaneous processes and many of
these processes operate as servers — meaning that external enti‐
ties 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 is‐
sue. Security is best implemented through a layered onion ap‐
proach. 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 at‐
tempt to crash or otherwise make a system unusable but do not at‐
tempt to break root. Security concerns can be split up into sev‐
eral categories: Denial of Service attacks (DoS) User account
compromises Root compromise through accessible servers Root com‐
promise via user accounts Backdoor creation A denial of service
attack is an action that deprives the machine of needed re‐
sources. Typically, DoS attacks are brute-force mechanisms that
attempt to crash or otherwise make a machine unusable by over‐
whelming its servers or network stack. Some DoS attacks try to
take advantages of bugs in the networking stack to crash a ma‐
chine with a single packet. The latter can only be fixed by ap‐
plying 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 cut‐
ting 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 and servers on their machines.
These servers, by default, do not operate over encrypted connec‐
tions. 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 re‐
ality 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. System administrators must keep in mind that there are po‐
tentially 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 in‐
volve 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 im‐
possible 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 approach and
can be categorized as follows: Securing root and staff accounts
Securing root — root-run servers and SUID/SGID binaries Securing
user accounts Securing the password file Securing the kernel
core, raw devices, and file systems Quick detection of inappro‐
priate changes made to the system 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 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 util‐
ity. For example, make sure that your PTYs are specified as be‐
ing in the file so that direct root logins via or are disallowed.
If using other login services such as make sure that direct root
logins are disabled there as well. Consider every access method
— services such as 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 pass‐
word verification to operate. One way to make root accessible is
to add appropriate staff accounts to the group (in The staff mem‐
bers placed in the group are allowed to to root. You should
never give staff members native access by putting them in the
group in their password entry. Staff accounts should be placed
in a group, and then added to the group via the file. Only those
staff members who actually need to have root access should be
placed in the group. It is also possible, when using an authen‐
tication method such as Kerberos, to use Kerberos's file in the
root account to allow a to root without having to place anyone at
all in the group. This may be the better solution since the
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 mechanism is better than having noth‐
ing 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 or
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 (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 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 in‐
direct 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 work‐
station 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 physi‐
cal 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 abil‐
ity 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 Ker‐
beros system can require that the user choose a new password af‐
ter 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 or 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 and daemons can be run in special
user 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 sand‐
box, they still have to break out of the sandbox. The more lay‐
ers the attacker must break through, the lower the likelihood of
his success. Root holes have historically been found in virtu‐
ally every server ever run as root, including basic system
servers. If you are running a machine through which people only
log in via and never log in via or then turn off those services!
now defaults to running and 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 in‐
stalled. The prudent sysadmin would research and implement sand‐
boxes for servers whenever possible. There are a number of other
servers that typically do not run in sandboxes: 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 reside in or While nothing is 100% safe,
the system-default SUID and SGID binaries can be considered rea‐
sonably safe. Still, root holes are occasionally found in these
binaries. A root hole was found in Xlib in 1998 that made (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 ac‐
cess, and get rid of any SUID binaries that nobody uses. A
server with no display generally does not need an binary. SGID
binaries can be almost as dangerous. If an intruder can break an
SGID-kmem binary the intruder might be able to read and thus read
the crypted password file, potentially compromising any pass‐
worded account. Alternatively an intruder who breaks group can
monitor keystrokes sent through PTYs, including PTYs used by
users who log in through secure methods. An intruder that breaks
the group can write to almost any user's TTY. If a user is run‐
ning 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 vigi‐
lant in your monitoring of those accounts. Use of SSH and Ker‐
beros for user accounts is more problematic due to the extra ad‐
ministration 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 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 be‐
low). 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 it
is called the 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
device compiled in. But even if you turn off the device, you
still have and 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, An enterprising intruder can
use a KLD module to install his own device or other sniffing de‐
vice 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 on the variable. Once you have
set the security level to 1, write access to raw devices will be
denied and special flags, such as will be enforced. You must
also ensure that the 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 overdo‐
ing 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 flag for
every system file and directory under the sun. Another possibil‐
ity is to simply mount and 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. The kernel runs
with five different security levels. Any super-user process can
raise the level, but no process can lower it. The security lev‐
els are: Permanently insecure mode - always run the system in in‐
secure mode. This is the default initial value. Insecure mode -
immutable and append-only flags may be turned off. All devices
may be read or written subject to their permissions. Secure mode
- the system immutable and system append-only flags may not be
turned off; disks for mounted file systems, and may not be opened
for writing; (if your platform has it) may not be opened at all;
kernel modules (see may not be loaded or unloaded. The kernel
debugger may not be entered using the sysctl. A panic or trap
cannot be forced using the and other sysctl's. Highly secure
mode - same as secure mode, plus disks may not be opened for
writing (except by whether mounted or not. This level precludes
tampering with file systems by unmounting them, but also inhibits
running while the system is multi-user. In addition, kernel time
changes are restricted to less than or equal to one second. At‐
tempts to change the time by more than this will log the message
Network secure mode - same as highly secure mode, plus IP packet
filter rules (see and cannot be changed and or configuration can‐
not be adjusted. The security level can be configured with vari‐
ables 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 to set the bit on most of the files in and 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 se‐
curity 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) lim‐
ited-access system. Writing your security scripts on the extra-
secure limited-access system makes them mostly invisible to po‐
tential attackers, and this is important. In order to take maxi‐
mum 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 un‐
detected. 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 and It is best to
physically the client-box files boxes at least once a day, and to
test control files such as those found in and 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 When using SSH rather than
NFS, writing the security script is much more difficult. You es‐
sentially have to the scripts to the client box in order to run
them, making them visible, and for safety you also need to the
binaries (such as that those scripts use. The 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:
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 option (see 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 is a rela‐
tively low-overhead feature of the operating system which I rec‐
ommend using as a post-break-in evaluation mechanism. It is es‐
pecially 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 se‐
cure 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 ini‐
tial 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 at‐
tacker 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 gener‐
ally 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. The server has several op‐
tions 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 manual page carefully and pay specific
attention to the and options. Note that spoofed-IP attacks will
circumvent the option to so typically a combination of options
must be used. Some standalone servers have self-fork-limitation
parameters. The daemon has its option which tends to work much
better than trying to use load limiting options due to the load
lag. You should specify a parameter when you start high enough
to handle your expected load but not so high that the computer
cannot handle that number of without falling on its face. It is
also prudent to run in mode and to run the daemon separate from
the queue-runs If you still want real-time delivery you can run
the queue at a much lower interval, such as but be sure to spec‐
ify a reasonable option for that to prevent cascade failures.
The daemon can be attacked directly and it is strongly recom‐
mended that you use the option whenever possible, and the option
otherwise. You should also be fairly careful with connect-back
services such as tcpwrapper's reverse-identd, which can be at‐
tacked 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 fire‐
walling 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
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 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 a couple of ser‐
vices 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 allows
you to control the range of port numbers used for dynamic binding
via the various sysctl's 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 cer‐
tain 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 ac‐
tual 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 especially if the server cannot drain the ICMP responses it
generates fast enough. The kernel has a new kernel compile op‐
tion called which limits the effectiveness of these sorts of at‐
tacks. The last major class of springboard attacks is related to
certain internal services such as the UDP echo service. An at‐
tacker 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 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 authentica‐
tion protocol but the kerberized and 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
option. SSH encrypts everything by default. SSH works quite
well in every respect except when it is set up to forward encryp‐
tion keys. What this means is that if you have a secure worksta‐
tion holding keys that give you access to the rest of the system,
and you to an unsecure machine, your keys become exposed. The
actual keys themselves are not exposed, but 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 when‐
ever possible for staff logins. SSH can be compiled with Ker‐
beros support. This reduces your reliance on potentially expos‐
able 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 option that SSH allows
in its file to make the key only usable to entities logging in
from specific machines. The manual page was originally written
by and first appeared in December 1998.