Unofficial FreeBSD Security Checklist / Links / Resources

[vivek]

This is list of recommended security settings to protect FreeBSD server from both internal (done by your own users) and external attacks (done by crackers). Please discuss your recommendation below and I will update main post as and when required. The goal is to create a nice itemized and linked (to man files, handbook articles, etc.) article in the HOWTO section.
OpenSSH server
Unless the system needs to provide the remote login and file transfer capabilities of SSH, disable the OpenSSH server. However, it is required in 95% case.

Block ssh login attempts
Use any one of the following port:

1. security/denyhosts Works with both jaild ssh and host sshd without firewall access. Blocks ssh access.
2. security/sshguard-pf Protect hosts from brute force attacks against ssh and other services using pf
3. security/sshguard-ipfw Protect hosts from brute force attacks against ssh and other services using ipfw
4. security/sshguard-ipfilter Protect hosts from brute force attacks against ssh and other services using ipfilter
5. security/sshblock Block abusive SSH login attempts
6. security/sshit Checks for SSH/FTP bruteforce and blocks given IPs

Configure firewall such as pf to limit incomming port 22 traffic. Allow, ssh login from 192.168.1.0/24 220.1.2.3 only. A typical pf rule:

pass in on $ext_if inet proto tcp from {192.168.1.0/24, 220.1.2.3 } to 201.41.71.xxx port ssh flags S/SA synproxy state

/etc/ssh/sshd_config Settings
Set Idle Timeout Interval for User Logins (600 seconds)

ClientAliveInterval 600
ClientAliveCountMax 0

Disable .rhosts Files

IgnoreRhosts yes

Disable Host-Based Authentication

HostbasedAuthentication no

Disable Empty Passwords

PermitEmptyPasswords no

Limit Users' SSH Access, allow only foo (add foo to wheel so that he can su - root) and bar to login via ssh:

AllowUsers foo bar

Configure public keybased login for ssh - http://www.freebsd.org/doc/en/books/...k/openssh.html

Use Keychain from Gntoo project, which is a shell script which acts as a user-friendly front-end to ssh-agent(1), allowing you to have one long-running ssh-agent process per system rather than per login session. This is must for all servers, if you are configuring password less login for backup or any other purposes.

• Keychain How to http://www.gentoo.org/proj/en/keychain/
• security/keychain

Other suggestions - Disable root login, change ssh port, Bind ssh to specific IP address,Only use SSH protocol 2,Deny root user login,Setup login banner ,Disable SSHD password authentication See: http://nixcraft.com/2516-post1.html


Recommend readings:

• pf.conf
• sshd
• sshd_config
• Openssh FAQ http://www.openssh.org/faq.html
• Lots of suggestion @ slashdot thread - rundown on SSH Brute Force Attacks - http://it.slashdot.org/article.pl?sid=05/07/16/1615233

FreeBSD Jails
FreeBSD jail establish a clean cut separation between various services and users, mainly for security and ease of administration reasons. Run major services such as HTTPD, SMTPD, SQL Server and other public services in a jail. It offers the following features:

• Virtualization: Each jail is a virtual environment running on the host machine with its own files, processes, user and superuser accounts. From within a jailed process, the environment is (almost) indistinguishable from a real system.
• Security: Each jail is sealed from the others thus providing an additional level of security.
• Ease of delegation: Thanks to the limited scope of a jail, it allows administrators to painlessly delegate several tasks which require superuser access without handing out complete control over the system.
• Jail Setup Tutorial - http://www.freebsd.org/doc/en/books/handbook/jails.html
• Multiple IPs in jail - http://www.cyberciti.biz/faq/freebsd...-ipv6-address/
• Man pages - jail jls jexec

Host-based intrusion detection system
Use integrity checking software which can detect the intrusions. It monitors filesystem for unauthorized change such as find out if system binaries modified and new cracked versions installed or not.

1. security/aide - AIDE is Advanced Intrusion Detection Environment. This piece of software was written as a replacement and extension for Tripwire.
2. security/integrit - Integrit is an alternative to file integrity verification programs like tripwire and aide. It helps you determine whether an intruder has modified a computer system.
3. security/tripwire is a tool that aids system administrators and users in monitoring a designated set of files for any changes.

Monitor log files
FreeBSD installs couple of scripts and email output to root account. Read those security emails. It contains lots of useful information and warnings. You can also use the following tools:

1. sysutils/logcheck helps spot problems, anomalies and security violations in your logfiles automatically and will send the summaries to you via e-mail. Logcheck is run as a cron job.
   Logwatch is a customizable, pluggable log-monitoring system. It will go through your logs for a given period of time and make a report in the areas that you wish with the detail that you wish.
2. Use shell tools such as tail, grep, awk and friends to monitor log files. A few quick examples:

tail -f /var/log/messages
tail -f /var/log/maillog
egrep -i 'wanr|error' /var/log/messages
awk '{ print $1}'  /var/log/httpd-access.log| sort  | uniq -c  | sort -n

Root account security
You need to protect root account. A few tips:

1. Never give out root password to anyone. Delegate root level access using sudo (port security/sudo. Never give shell or vi or any command that can escape to shell access using sudo. Keep an eye on sudo log file and /var/log/auth.log.
2. Automatically log users out after a period of inactivity by setting
   

   # csh  / tcsh - 5 minutes time out
   set -r autologout 5 
   # bash / sh 300 seconds time out
   TMOUT=300
   readonly TMOUT
   export TMOUT

3. Add above to system global shell startup files.
4. Use sysutils/vlock a utility which locks a terminal so it can only be unlocked with the user's password (or the root password).
5. See Openssh and scripts for more info - http://www.cyberciti.biz/faq/linux-u...rce-time-outs/
6. See FreeBSD handbook for protecting root user - http://www.freebsd.org/doc/en/books/...g-freebsd.html

kern.securelevel

• Set kern.securelevel level 3 once *everything is configured* properly. Any super-user process can raise the level, but no process can lower it. See security man page and FreeBSD handbook security chapter - http://www.freebsd.org/doc/en/books/.../security.html
• Sample /etc/rc.conf settings:
  

  kern_securelevel_enable="YES"
  kern_securelevel="3"

Various limitations on users

• Use limitations to avoid DoS attack from both internal and external threats.
• Disk limitations via file system disk quota - See tutorial - http://www.freebsd.org/doc/en/books/...ok/quotas.html
• Configure user resource limits, accounting limits via login class. You can control cputime, memory, vm usage, umask, path, open FDs, file limits, max process and so on. See login.conf man page for details.
• ACL - Access Control Lists extend the standard UNIX® permission model in a highly compatible (POSIX®.1e) way. This feature permits an administrator to make use of and take advantage of a more sophisticated security model.

NOTE: I'm still working on a list...

[vivek]

Continued from post # 1...

Apache Security
Run minimal built-in modules. To see all compiled modules:

httpd -l

Only following 4 compiled in modules should be listed to reduce the risk:

1. core.c
2. prefork.c
3. http_core.c
4. mod_so.c

Rebuild Apache if compiled-in modules is significantly larger than the above list.

Hide Apache version (/usr/local/etc/apache22/extra/httpd-default.conf

ServerTokens Prod
ServerSignature Off

Remove and minimize loadable modules. Open httpd.conf and start removing unwanted modules. After each module run the following to make sure module is not breaking something:

/usr/local/etc/rc.d/apache22 configtest

Turn off directory browsing (default index).

Turn off server side includes and server side scripting such as php, python and so on. Only grant access to certian diretories to run perl, python, php. Use mod_fastcgi or mod_fcgi

Disable symbolic links.

Secure /tmp and /var/tmp directories and mount it with noexec, nosuid, nosymfollow.

Disable .htaccess if not required.

Make sure you use mod_ssl to encrypt content.

Make sure you install and use mod security, which provides an application level firewall for Apache. A sample pf.conf(5) rule:

http_servers = "{ 202.54.1.1, 202.54.1.3, 202.54.1.5}"
https_servers = "{ 202.54.1.2, 202.54.1.3}"
# ....
pass in on $ext_if inet proto tcp from any to $http_servers port http flags S/SA synproxy state
pass in on $ext_if inet proto tcp from any to $https_servers port https flags S/SA synproxy state

Make sure you use DoS service protection modules such as mod_throttle mod_bwshare www/mod_limitipconn mod_dosevasive.

Configure php securely use suexec or other wrappers. If possible use www/mod_fastcgi (see FreeBSD apache FastCGI PHP tutorial) to start php externally with other UID.

Restrict file and directory access, use chmod and chgrp command to set permission on documentroot.

Configure firewall to allow access to the web server.

Run Apache in a chroot jail if possible. Use chroot(8), FreeBSD jails, www/mod_chroot, www/mod_security (mod_security tutorial and project website) SecChrootDir directive. Some basic examples of mod_security:

# Prevent directory traversal
SecFilter "\.\./"

# Filter on specific system specific paths
SecFilter /etc/passwd
SecFilter /bin/

# Prevent cross-site scripting
SecFilter "<[[:space:]]* script"

# Prevent SQL injection
SecFilter "delete[[:space:]]+from"
SecFilter "insert[[:space:]]+into"
SecFilter "select.+from"

Run updated apache version:

portsnap fetch update
pkg_version -vl '<'

Use portmaster(8) or other tool to update Apache version.

Run lightweight web server. If you just run a wordpress blog or static html site, switch to www/lighttpd or www/nginx web server.

1. Official Apache 2.2 Security Tips
   
2. FreeBSD Apache HTTP Server tutorial
   
3. 20 ways to Secure your Apache Configuration
   
4. Recommended book - Apache Security book.

BIND DNS Server
BIND9 is in base system.

Isolate DNS from Other Services. FreeBSD support chrooting bind server out of box. Add following to /etc/rc.conf

named_enable="YES"
named_chrootdir="/var/named"

Configure firewalls to protect the DNS server. Sample pf rules:

dns_servers = "{ 202.1.2.3 }"
# ...
pass in on $ext_if inet proto udp from any to $dns_servers  port domain
pass in on $ext_if inet proto tcp from any to $dns_servers  port domain flags S/SA synproxy state

Run separate DNS servers for External and Internal Queries (use jails).

Use Views to partition External and Internal information.


Authenticate Zone Transfers using TSIG. On primary ns:

cd /tmp
dnssec-keygen -a HMAC-MD5 -b 128 -n HOST ns1.freebsd.org

Note downl base64-key-string:

cat Kns1.freebsd.org.+NNN+MMMMM.key

Edit /var/named/etc/namedb/named.conf on the primary nameserver. Add the following:

key zone-transfer-key {
       algorithm hmac-md5;
       secret "base64-key-string";
     };
     zone "example.com " IN {
       type master;
       allow-transfer { key zone-transfer-key; };
       ...
     }

Edit /var/named/etc/namedb/named.conf on the secondary nameserver. Add the directives:

     key zone-transfer-key {
       algorithm hmac-md5;
       secret "base64-key-string ";
     };

Disable dynamic updates

zone "freebsd.org " IN {
  allow-update { none; };
  ...
}

Configure the logging options for security and monitoring purpose:

logging {
    channel security_channel {
        # Send log messages to the specified file
        file            "/var/log/security.log" versions 3 size 50m;
        # Log all messages
        severity        debug;
        # Log the date and time of the message
        print-time      yes;
        # Log the category of the message
        print-category  yes;
        # Log the severity level of the message
        print-severity  yes;
    };

    channel default {
        # Send logs to the syslog 'local0' facility
        syslog          local0;
        # Log messages of severity 'info' or higher
        severity        info;
        print-category  yes;
        print-severity  yes;
    };

    # Logs about approval and denial of requests
    category security {
        security_channel;
        default;
    };

    # Ignore logs about misconfigured remote servers
    category lame-servers { null; };

    # Default logging options
    category default { default; };

   channel "querylog" { file "/var/log/query.log" versions 3 size 50m; print-time yes; };
   category queries { querylog; };
};

Note /var/log/query.log will be created in /var/named/ jail i.e. actual location will be /var/named/var/log/query.log.

• BIND/named manual pages: rndc(8), named(8), and named.conf(5)
• FreeBSD BIND Setup Tutorial
• BIND TSIG tutorial for more information.
• ISC BIND Software and Documentations
• BIND and DNS Book

FreeBSD Hardening System Via /etc/sysctl.conf

# Security networking
# Limit ICMP
net.inet.icmp.icmplim=50
net.inet.icmp.maskrepl=0
net.inet.icmp.drop_redirect=1
net.inet.icmp.bmcastecho=0
net.inet.tcp.icmp_may_rst=0
# Drop synfin packets
net.inet.tcp.drop_synfin=1
# a single pass through the firewall
# net.inet.ip.fw.one_pass=1
# adds more queue buckets for ipfw dummynet
# net.inet.ip.dummynet.hash_size=2048
# increase the size of network mbufs to allocate
# kern.ipc.nmbclusters=65536
# If above used add the following to /boot/loader.conf - reboot needed
# kern.ipc.nmbclusers="65536"
# This is for dos protection 
# net.inet.tcp.msl=7500
# Turn off stealth IP networking 
net.inet.ip.stealth=0
# Try to protect against scans
net.inet.tcp.blackhole=2
net.inet.udp.blackhole=1
# Try to stop some syn flood attacks, and route cache degregation
net.inet.ip.rtexpire=2
net.inet.ip.rtminexpire=2
net.inet.ip.rtmaxcache=256
# Drop evil sourcerouted packets
net.inet.ip.accept_sourceroute=0
net.inet.ip.sourceroute=0
# Turn it on when you have two interfaces on same switch
# net.link.ether.inet.log_arp_wrong_iface=0
# IPCS - memory optimization 
kern.ipc.shmmax=134217728
kern.ipc.shmall=32768
kern.ipc.semmap=256

# Hide UID and GID from other users
security.bsd.see_other_gids=0
security.bsd.see_other_uids=0
security.bsd.unprivileged_read_msgbuf=0
# Max open file?
kern.maxfiles=65536

See sysctl.conf(5) and sysctl(8) for more info,

[vivek]

Continued from post # 2...
General security tips

1. Encrypt transmitted data whenever possible – Do not use rservices or insecure protocol such as telnet / ftp etc. Use scp, ssh and other secure alternative.
   
2. Minimize software to minimize vulnerability - Only install required ports and applications. The simplest way to avoid vulnerabilities in software is to avoid installing that software. Run the following command to see open ports
   

   sockstat -4
   sockstat -6
   sockstat -4 | grep something

3. Run different network services on separate systems - If possible, a server should be dedicated to serving exactly one network service. This limits the number of other services that can be compromised in the event that an attacker is able to successfully exploit a software flaw in one network service. Use FreeBSD jails to save hardware costs.
   
4. Use and configure security tools to improve system robustness - Use firewall for host based firewalling and kernel protection, MAC etc for protection against vulnerable services. Configure log auditing for detecting problems. There are plenty of security tools for various purposes provides by FreeBSD security ports (located at/usr/ports/security)
5. Updating Software - You need to update both base system + kernel (via buildworld or binary tool called freebsd-update) and applications (ports) via various tools. It is recommended that you use the following tools to keep systems up to date with the latest security patches.
   1. Updating and Upgrading FreeBSD
   2. FreeBSD Update Software and Apply Security Patches
   3. Install ports-mgmt/portaudit to monitor 3rd party apps and ports for security issues.
   4. Subscribe to freebsd-security-notifications mailing list or RSS feed.
6. Avoid weak and default passwords - Do not leave network ports open. Always follow close all, open required port policy using firewall. Do not expose internal hosts such as sql servers, backup servers to the Internet. Use nating / proxy to hide internal server IPs.
7. Do not run insecure and badly configured programs - For e.g. do not run apache, dns or mail server as a root user. Do not grant full system access to php or perl script. Restrict them to directories.
8. Delete all unwanted account - For e.g. laid-off employee may seek revenge
9. You need both host and firewall security.
10. Never ever assumed that you are not target - you can be targeted by both humans and automated worms and virus. All you can do is set tight permissions and make sure you are always prepared for attacks.
11. Always make a backup. Keep offsite backups on tape or dvd. RAID is not backup solution. Second hard disk on the same system is not a backup solution. Mirroring (to other server or disk) is not a backup. Backups are physically removed from the machine and stored where they can't be altered until they're needed for a restore. Always, check backup media and run dummy restore procedure. Use tools such as dump(8), restore(8), tar(1) etc. You can also use net/rsync, sysutils/rsnapshot and other 3rd party apps.
12. Always read /usr/ports/UPDATING before updating ports.
13. Always read /usr/src/UPDATING before starting buildworld procedure.

Tools for monitoring systems

1. nmap - scan your server for open ports.
2. top(1) - display and update information about the top cpu processes.
3. vmstat(8) - report virtual memory statistics.
4. fstat(1) - identify active files
5. lsof - list open files, network connections and much more.
6. systat(1) - display system statistics
7. iostat(8) - report I/O statistics
8. pstat(8) and swapinfo(8) - display system data structures
9. netstat(1) - show network status
10. sockstat(1) - list open sockets
11. sysctl(8) - get or set kernel state. Many security settings and system information can be displayed using this tool. Use /etc/sysctl.conf to store configuration.
12. ps(1) - list process status.
13. w(1) / who(1) - display who is logged in and what they are doing
14. uptime(1) - show how long system has been running
15. last(1) - - indicate last logins of users and ttys
16. lastcomm(1) - - show last commands executed
17. ac(8) - connect time accounting
18. sa(8) - print system accounting statistics

Resources
This is a just tiny list. Try the following resources / books:

• Official FreeBSD handbook
• The Six Dumbest Ideas in Computer Security
• Mastering FreeBSD and OpenBSD Security
• The Book of PF - A No-Nonsense Guide to the OpenBSD Firewall
• BSD Hacks 100 Industrial Tip & Tools
• Essential System Administration - This book covers many fundamental tasks in system administration.
• Learn shell scripting to automate the tasks. Checkout TLDP.org shell scripting guide.

I've tried to keep this small but useful list. Please add other information and comments below. Good luck!

[Beastie]

Originally Posted by vivek

OpenSSH server

[...]
Disable .rhosts Files

IgnoreRhosts yes

Disable Host-Based Authentication

HostbasedAuthentication no

Disable Empty Passwords

PermitEmptyPasswords no

I know it's not terribly important, but aren't these the default?