Copyright (c) 1991, 1993
The Regents of the University of California. 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.
3. All advertising materials mentioning features or use of this software
must display the following acknowledgement:
This product includes software developed by the University of
California, Berkeley and its contributors.
4. Neither the name of the University nor the names of its contributors
may be used to endorse or promote products derived from this software
without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE REGENTS 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 THE REGENTS 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.

@(#)security.1 8.2 (Berkeley) 12/30/93
$Id: security.1,v 1.3 1998/12/20 20:05:44 dillon Exp $

.Dd December 30, 1993 .Dt SECURITY 7 .Os .Sh NAME .Nm security .Nd introduction to security under FreeBSD .Sh DESCRIPTION

p Security is a function that begins and ends with the system administrator. While all x systems are inherently multi-user capable, the job of building and maintaining security mechanisms to keep those users 'honest' is probably one of the single largest undertakings of the sysad. 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 simultanious 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.

p Security concerns can be split up into several categories: l -enum -offset indent t Denial of service attacks t User account compromises t Root Hacks through accessible servers t Root Hacks via user accounts .El

p A denial of service attack is an action that deprives the machine of needed resources. Typically, D.O.S. attacks are brute-force mechanisms that attempt to crash or otherwise make a machine unusable by overwhelming its servers or network stack. Some D.O.S. 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 servers to limit the load they 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.

p A user account compromise is even more common then a D.O.S. attack. Many sysops still run standard telnetd, rlogind, rshd, and ftpd 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 login to a system) will have his or her password sniffed. The attentive system admin will analyze his remote access logs occassionally looking for suspicious source addresses even for successful logins.

p 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 remove that user's files and crash the machine, but not touch anyone else's files.

p System administrators must keep in mind that there are several 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.

p Security remedies are always implemented in a multi-layered 'onion peel' approach and can be categorized as follows: l -enum -offset indent t Securing root and staff accounts t Securing root - root-run servers and suid/sgid binaries t Securing user accounts t Securing the password file t Securing the kernel core, raw devices, and filesystems t Checking file integrity: binaries, config files, and so forth t Paranoia .El .Sh SECURING THE ROOT ACCOUNT AND SECURING STAFF ACCOUNTS

p Don't bother securing staff accounts if you haven't 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. To secure the root account you make sure that it is not possible to login to the root account using the root password from a random user account or over the network. If you haven't already, configure telnetd, rlogind, and all other servers that handle login operations to refuse root logins, period, whether the right password is given or not. Allow direct root logins only via the system console. The '/etc/ttys' file comes in handy here and is secure by default on most systems, but a good sysad always checks to make sure.

p Of course, as a sysad 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 via their entry in the password file... put staff in a 'staff' group or something and only add those that really need root to the wheel group. Unfortunately the wheel mechanism still allows a hacker to break root if the hacker has gotten hold of your password file - he need only break the root password and the password of one of the staff accounts that happens to be in the wheel group. So while the wheel mechanism is useable, it isn't much safer then not having a wheel group at all.

p 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 a hacker may be able to steal the password file but will not be able to break into any staff accounts (or, indirectly, root, even if root has a crypted password associated with it). Staff members get into their staff accounts through a secure login mechanism such as kerberos(1) or ssh(1) (see /usr/ports/security/ssh) using a private/public keypair. 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 keypair 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 keypair by password protecting the keypair when you create it with ssh-keygen(1). Being able to *-out the passwords for staff accounts also guarentees that staff members can only login through secure access methods that you have setup. You can thus force all staff members to use secure, encrypted connections for all their sessions which closes an important hole used by many hackers: That of sniffing the network from an unrelated, less secure machine.

p 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 shouldn't 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 breakins occur remotely, over a network, from peopl who do not have physical access to your workstation or servers.

p 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 effect all the machine 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 - ROOT-RUN SERVERS AND SUID/SGID BINARIES

p The prudent sysop 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 isn't perfect unless you go to a hellofalot 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!

p 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). The default 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 sysop would research and implement sandboxes for servers whenever possible.

p 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 then 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 breakins that might occur through them.

p 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, 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 occassionaly 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 then sorry and the prudent sysad 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 a hacker can break an sgid-kmem binary the hacker might be able to read /dev/kmem and thus read the crypted password file, potentially compromising any passworded account. A hacker that breaks the tty group can write to almost user's tty. If a user is running a terminal program or emulator with a talk-back feature, the hacker 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

p 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, but still a very good solution compared to a crypted password. .Sh SECURING THE PASSWORD FILE

p 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 a hacker to obtain read access to that file even if the attacker cannot obtain root-write access.

p Your security scripts should always check for and report changes to the password file (see 'Checking file integrity' below). .Sh SECURING THE KERNEL CORE, RAW DEVICES, AND FILESYSTEMS

p 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. A hacker will commonly attempt to run a packet sniffer on a compromised machine. You do not need to give the hacker the capability and most systems should not have the bpf device compiled in. Unfortunately, there is another kernel feature called the Loadable Kernel Module interface. An enterprising hacker can use an LKM to install his own bpf device or other sniffing device on a running kernel. If you do not need to use the module loader, turn it off in the kernel config with the NO_LKM option.

p But even if you turn off the bpf device, and turn off the module loader, you still have /dev/mem and /dev/kmem to worry about. For that matter, the hacker can still write raw devices. To avoid this 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. .Sh CHECKING FILE INTEGRITY: BINARIES, CONFIG FILES, ETC

p 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. The last layer of your security onion is perhaps the most important - detection.

p The only correct way to check a system's file integrity is via another, more secure system. It is fairly easy to setup a 'secure' system: you simply do not run any services on it. With a secure system in place you can then give it access to other system's root spaces via ssh. This may seem like a security breech, but you have to put your trust somewhere and as long as you don't do something stupid like run random servers it really is possible to build a secure machine. When I say 'secure' here, I assuming physical access security as well, of course. Given a secure machine with root access on all your other machines, you can then write security scripts ON the secure machine to check the other machines on the system. The most common way of checking is to have the security script scp(1) over a find and md5 binary and then ssh a shell command to the remote machine to md5 all the files in the system (or, at least, the /, /var, and /usr partitions!). The security machine copies the results to a file and diff's them against results from a previous run (or compares the results against its own binaries), then emails each staff member a daily report of differences.

p Another way to do this sort of check is to NFS export the major filesystems from every other machine to the security machine. This is somewhat more network intensive but also virtually impossible for a hacker to detect or spoof.

p 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 pervue of the MD5 check.

p A good security script will check for suid and sgid binaries on all filesystems and report their absolute existance as well as a diff against the previous report or some baseline (say, make a baseline once a week). While you can turn off the ability to run suid and sgid binaries on certain filesystems through the 'nosuid' option in fstab/mount, you cannot turn this off on root and anyone who breaks root can just install their binary their. If you have a huge amount of user disk space, though, it may be useful to disallow suid binaries and devices ('nodev' option) on the user partitions so you do not have to scan them for such. I would scan them anyway, though, at least once a week, since the object of this onion layer is detection of a breakin.

p Process accounting (see accton(1)) is a relatively low-overhead feature of the operating system which I recommend using as a post-breakin evaluation mechanism. It is especially useful in tracking down how a hacker has actually broken root on a system, assuming the file is still intact after the breakin occurs.

p Finally, security scripts should process the log files and the logs themselves should be generated in as secured a manner as possible - remote syslog can be very useful. A hacker tries to cover his tracks, and log files are critical to the sysop trying to track down the time and method of the initial breakin. .Sh PARANOIA

p A little paranoia never hurts. As a rule, a sysop can add any number of security features as long as they do not effect convenience, and can add security features that do effect convenience with some added thought. .Sh SPECIAL SECTION ON D.O.S. ATTACKS

p This section covers Dential of Service attacks. A DOS attack is typically a packet attack. While there isn't 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. l -enum -offset indent t Limiting server forks t Limiting springboard attacks (ICMP response attacks, ping broadcast, etc...) t Kernel Route Cache .El

p A common DOS attack is against a forking server that attempts to cause the server to eat processes, file descirptors, 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.

p Sendmail has its -OMaxDaemonChildren option which tends to work much better then 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 no 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 realtime 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.

p Syslogd can be attacked directly and it is strongly recommended that you use the -s option whenever possible, and the -a option otherwise.

p 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.

p 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 root network-based root hacks. 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. 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 ).

p 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 spoofed 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 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 sysad will turn off all of these inetd-internal test services.

p 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 then rtminexpire. There are two problems: (1) The kernel does not react quickly enough when a lightly loaded server is suddenly attacked, and (2) 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. .Sh SEE ALSO

p .Xr ssh 1 , .Xr sshd 1 , .Xr kerberos 1 , .Xr accton 1 , .Xr xdm 1 , .Xr syslogd 1 , .Xr chflags 1 , .Xr find 1 , .Xr md5 1 , .Xr sysctl 8 .Sh HISTORY The .Nm manual page was originally written by Matthew Dillon and first appeared in FreeBSD-3.0.1, December 1998.