To get the ball rolling, we need to download CSF and install it on our server. In this post, we’re working with a CentOS 6.0 32-bit server, so our (root) terminal commands would look like this to download and install CSF:

$ wget http://www.configserver.com/free/csf.tgz #Download CSF using wget.
$ tar zxvf csf.tgz #Unpack it.
$ yum install perl-libwww-perl #Make sure perl modules are installed ...
$ yum install perl-Time-HiRes  #Otherwise it will generate an error.
$ cd csf
$ ./install.sh #Install CSF.
 
#MAKE SURE YOU HAVE YOUR BAND-AID READY
 
$ /etc/init.d/csf start #Start CSF. (Note: You can also use '$ service csf start')

Once you start CSF, you can see a list of the default rules that load at startup. CSF defaults to a DROP policy:

$ iptables -nL | grep policy
Chain INPUT (policy DROP)
Chain FORWARD (policy DROP)
Chain OUTPUT (policy DROP)

Don’t ever run “iptables -F” unless you want to lock yourself out. In fact, you might want to add “This server is running CSF – do not run ‘iptables -F’” to your /etc/motd, just as a reminder/warning to others.

CSF loads on startup by default. This means that if you get locked out, a simple reboot probably won’t fix the problem. Runlevels 2, 3, 4, and 5 are all on:

$ chkconfig --list | grep csf
csf             0:off   1:off   2:on    3:on    4:on    5:on    6:off

Some features of CSF will not work unless you have certain iptables modules installed. I believe they are installed by default in CentOS, but if you custom-built your iptables, they might not all be installed. Run this script to see if all modules are installed:

$ /etc/csf/csftest.pl
Testing ip_tables/iptable_filter...OK
Testing ipt_LOG...OK
Testing ipt_multiport/xt_multiport...OK
Testing ipt_REJECT...OK
Testing ipt_state/xt_state...OK
Testing ipt_limit/xt_limit...OK
Testing ipt_recent...OK
Testing xt_connlimit...OK
Testing ipt_owner/xt_owner...OK
Testing iptable_nat/ipt_REDIRECT...OK
Testing iptable_nat/ipt_DNAT...OK
 
RESULT: csf should function on this server

As I mentioned, this is the default iptables installation on a minimal CentOS 6.0 image, so chances are good that these modules are already installed on your system. It never hurts to check, though.

The CSF Configuration File

The primary CSF configuration is stored in the well-documented /etc/csf/csf.conf file. CSF is extremely configurable, so there are a lot of options to read over. Let’s take a look over some of the more important features:

Testing

TESTING = "1"
TESTING_INTERVAL = "5"

This TESTING cron job runs every “5″ minutes so you don’t lock yourself out when you’re testing your rules. When you are satisfied with your rules (and confident that you won’t lock yourself out), you can set TESTING to “0″.

Globally Allowed Ports

# Allow incoming TCP ports
TCP_IN = "20,21,22,25,53,80,110,143,443,465,587,993,995"
 
# Allow outgoing TCP ports
TCP_OUT = "20,21,22,25,53,80,110,113,443"
 
# Allow incoming UDP ports
UDP_IN = "20,21,53"
 
# Allow outgoing UDP ports
# To allow outgoing traceroute add 33434:33523 to this list
UDP_OUT = "20,21,53,113,123"

Incoming Ping Requests

# Allow incoming PING
ICMP_IN = "1"

Allowing ping is usually a good option for diagnostic purposes, so I don’t recommend turning it off. Disallowing ping is an example of “security through obscurity,” and it will not typically dissuade your attackers.

Ethernet Device

ETH_DEVICE = ""
ETH6_DEVICE = ""

Here, you can configure iptables to ONLY use one Ethernet adapter. You might want to only guard your public network adapter in some situations.

IP Limit in Permanent “Deny” File

DENY_IP_LIMIT = "200"

A higher number here will obviously screen out more IP addresses in csf.deny, but higher numbers also may cause slowdowns.

IP Limit in Temporary “Deny” File

DENY_TEMP_IP_LIMIT = "100"

Similar to DENY_IP_LIMIT, the DENY_TEMP_IP_LIMIT represents the maximum number of IPs that can be stored in the temporary ban list.

SMTP Blocking

SMTP_BLOCK = "0"

When set to “1″, SMTP_BLOCK does not completely block outbound SMTP, but it does block it for most users. This will prevent malicious scripts and compromised users from making outbound connections from unauthorized mail clients on the server. SMTP_BLOCK doesn’t stop those scripts from running, but it does stop them from functioning. Mail sent through the proper channels will still be delivered normally.

Allowing SMTP on localhost

SMTP_ALLOWLOCAL = "1"

Custom Mail Port Designation

SMTP_PORTS = "25,465,587"

Allowing SMTP Access to Users/Groups

SMTP_ALLOWUSER = ""
SMTP_ALLOWGROUP = "mail,mailman"

SYN Flood Protection

SYNFLOOD = "0"
SYNFLOOD_RATE = "100/s"
SYNFLOOD_BURST = "150"

Per the documentation, you should only enable SYN flood protection (SYNFLOOD= “1″) if you are currently under a SYN flood attack.

Concurrent Connections Limit

CONNLIMIT = "22;5,80;20"
PORTFLOOD = "22;tcp;5;300,80;tcp;20;5

These options allow you to add customized DoS protection. CONNLIMIT handles the number of concurrent connections, and in this example, we’re limiting port 22 to 5 connections and port 80 to 20 connections.

PORTFLOOD on deals with connections per second. In this example, we’re limiting the TCP connection on port 22 to 5 connections/second with a quiet period of 300 seconds before the connection is unblocked. Additonally, we’re limiting the TCP connection on port 80 to 20 connections/second with a quiet period of 5 seconds before the connection is unblocked.

Check the readme.txt file for more information about the syntax.

Logging to Syslog

SYSLOG = "0"

When enabled, this option logs lfd (Login Failure Daemon) messages to syslog as well as to /var/log/lfd.log.

Dropping v. Rejecting Packets

DROP = "DROP"

This configuration allows you to either DROP or REJECT packets. REJECT tells the sender that the packet has been blocked by the firewall. DROP just drops the packet and does not send a response. I like DROP better for regular use, but REJECT might be more helpful if you need to diagnose a connectivity issue.

Logging Dropped Connections

DROP_LOGGING = "1"

This option logs dropped connections to syslog. I don’t see any reason to turn this off unless your hard drive is getting full.

Port Exceptions When Logging Dropped Connections

DROP_NOLOG = "67,68,111,113,135:139,445,500,513,520"

These ports are specifically blocked from being logged either to conserve hard drive space or make the log file easier to read.

“Watch Mode”

WATCH_MODE = "0"

If you are ever stuck trying to troubleshoot a large ruleset, you might consider turning this option on. You can use it to track the actions to watched IP addresses to see where they are getting blocked or accepted.

Login Failure Daemon Alert

LF_ALERT_TO = ""
LF_ALERT_FROM = ""
LF_ALERT_SMTP = ""

You can specify an email address to report errors from the Login Failure Daemon, which tracks and automatically blocks brute force login attempts.

Permanent Blocks and NetBlocks

LF_PERMBLOCK = "1"
LF_PERMBLOCK_INTERVAL = "86400"
LF_PERMBLOCK_COUNT = "4"
LF_PERMBLOCK_ALERT = "1"
LF_NETBLOCK = "0"
LF_NETBLOCK_INTERVAL = "86400"
LF_NETBLOCK_COUNT = "4"
LF_NETBLOCK_CLASS = "C"
LF_NETBLOCK_ALERT = "1"

These settings control the permanent block and netblock blocking. You probably don’t need to touch these settings, but you might want some additional security or less security depending on your company needs. If something gets permablocked, it will require your intervention to clear it, which might create downtime for your clients. Likewise, if a legitimate IP address happens to be part of a netblock which has an attacking IP address on it, it will get blocked if you have that feature turned on. A class C network encompasses 256 IP addresses. You can set this to class B or A, but that could block thousands or millions of IP addresses, respectively. Unless you find yourself under constant attack, I would advise you to leave that LF_NETBLOCK off.

Additional Protection During Updates

# Safe Chain Update. If enabled, all dynamic update chains (GALLOW*, GDENY*,
# SPAMHAUS, DSHIELD, BOGON, CC_ALLOW, CC_DENY, ALLOWDYN*) will create a new
# chain when updating, and insert it into the relevant LOCALINPUT/LOCALOUTPUT
# chain, then flush and delete the old dynamic chain and rename the new chain.
#
# This prevents a small window of opportunity opening when an update occurs and
# the dynamic chain is flushed for the new rules.
SAFECHAINUPDATE = "0"

Activating this option will increase your system resource usage and will require more rules to be running at one time, but it provides an additional layer of protection during updates. Without this option turned on, your rules will be flushed for a short amount of time, leaving your server vulnerable.

Multi-Server Deployment Options

LF_GLOBAL = "0"
GLOBAL_ALLOW = ""
GLOBAL_DENY = ""
GLOBAL_IGNORE = ""

Like APF, you can configure global lists for multiple server deployments. You’ll need to specify a URL of the text file with the IP addresses for the global lists.

SPAMHAUSE Blocklist

LF_SPAMHAUS = "0"

This option enables the SPAMHAUS blocklist. Specify the number of seconds between refreshes. Recommended setting is 86400 (1 day).

Blocking TOR Exit IP Addresses

LF_TOR = "0"

Enabling this option will block TOR exit IP addresses. If you are not familiar with TOR, it is a completely anonymous proxy network. This could block some legitimate users who are trying to protect their anonymity, so I would recommend only turning this on if you are already under attack from a TOR exit address.

Blocking Bogon Addresses

LF_BOGON = "0"
LF_BOGON_URL = "http://www.cymru.com/Documents/bogon-bn-agg.txt"
LF_BOGON_SKIP = ""

Blocking bogon addresses (addresses that should not be possible) is usually a good decision. To enable, set the number of seconds between refreshes. I recommend enabling this option and setting the refresh at 86400 (1 day). If you do so, be sure to add your private network adapters to the skip list.

Country-Specific Access to Your Server

CC_DENY = ""
CC_ALLOW = ""

With these options, you can block or allow entire countries from accessing your server. To do so, enter the country codes in a comma separated list. Even though this generates a lot of additional rules, it’s valuable to some sysadmins.

CC_ALLOW_FILTER = ""

Alternatively, you can set your server to exclusively accept traffic from a list of country codes. All other countries not listed will have their traffic dropped. There are many other settings related to these options that I don’t have time to cover in this blog.

Blocking Login Failures

LF_TRIGGER = "0"

This enables blocking of login failures (per service). There are a lot of great customization options in this section.

Scanning Directories for Malicious Files

LF_DIRWATCH = "300"

This feature scans /tmp and /dev/shm for potentially malicious files and alerts you to their presence based on the interval you designate. You can also have CSF automatically quarantine malicious files with this option:

LF_DIRWATCH_DISABLE = "0"

Distributed Attack Protection

LF_DISTATTACK = "0"

By enabling this option, you activate additional protection against distributed attacks.

Blocking Based on Abusive Email Usage

LT_POP3D = "0"
LT_IMAPD = "0"

If a user checks email too many times per hour (more than the non-zero value specified), the user’s IP address is blocked.

Email Alert Following Block

LT_EMAIL_ALERT = "1"

This will send you email when something is blocked. I’d recommend leaving it on.

Blocking IP Addresses Based on Number of Connections

CT_LIMIT = "0"

This feature tracks connections and blocks the IP if the number of connections is too high. Use caution because if you enable this option and set this value too low, it will block legitimate traffic.

Application-Level Protection

PT_LIMIT = "60"

This feature provides application level protection against malicious scripts that take a long time to execute.

Blocking Port Scanners

PS_INTERVAL = "300"
PS_LIMIT = "10"

Enabling HTML User Interface for CSF

UI = "0"

CSF has a built-in HTML user interface. You can enable this by setting UI = “1″. There are a list of prerequisites for this option in the readme.txt.

Notifying Blocked IP Addresses

MESSENGER = "0"

This option will notify blocked IP addresses when they have been blocked by the firewall.

Port Knocking

PORTKNOCKING = ""

CSF supports port knocking, which is a technique that provides an additional layer of security. See http://www.portknocking.org/ for details.

Allow and Deny Lists

As we walked through the CSF configuration file, you saw that I referenced the csf.deny file, so it should come as no surprise that CSF also includes csf.allow to customize “allow” rules as well. If you are familiar with APF, these files have a very similar syntax … Each entry is made up of the same four components: protocol|flow|port|IP. The only real difference being that APF uses the colon as a delimiter while CSF uses the pipe:

#APF Version
tcp:in:d=48000_48020:s=10.0.0.0/8
 
#CSF Version
tcp|in|d=48000_48020|s=10.0.0.0/8

Fortunately, replacing your colon with a pipe is a minimally invasive procedure that can be automated with a tool like vi.

CSF Command Line Tool

The command line tool for CSF is much more robust than the one for APF:

$ csf --help
csf: v5.79 (cPanel)
 
ConfigServer Security & Firewall
(c)2006-2013, Way to the Web Limited (http://www.configserver.com)
 
Usage: /usr/sbin/csf [option] [value]
 
Option              Meaning
-h, --help          Show this message
-l, --status        List/Show iptables configuration
-l6, --status6      List/Show ip6tables configuration
-s, --start         Start firewall rules
-f, --stop          Flush/Stop firewall rules (Note: lfd may restart csf)
-r, --restart       Restart firewall rules
-q, --startq        Quick restart (csf restarted by lfd)
-sf, --startf       Force CLI restart regardless of LF_QUICKSTART setting
-a, --add ip        Allow an IP and add to /etc/csf.allow
-ar, --addrm ip     Remove an IP from /etc/csf.allow and delete rule
-d, --deny ip       Deny an IP and add to /etc/csf.deny
-dr, --denyrm ip    Unblock an IP and remove from /etc/csf.deny
-df, --denyf        Remove and unblock all entries in /etc/csf.deny
-g, --grep ip       Search the iptables rules for an IP match (incl. CIDR)
-t, --temp          Displays the current list of temp IP entries and their TTL
-tr, --temprm ip    Remove an IPs from the temp IP ban and allow list
-td, --tempdeny ip ttl [-p port] [-d direction]
                    Add an IP to the temp IP ban list. ttl is how long to
                    blocks for (default:seconds, can use one suffix of h/m/d).
                    Optional port. Optional direction of block can be one of:
                    in, out or inout (default:in)
-ta, --tempallow ip ttl [-p port] [-d direction]
                    Add an IP to the temp IP allow list (default:inout)
-tf, --tempf        Flush all IPs from the temp IP entries
-cp, --cping        PING all members in an lfd Cluster
-cd, --cdeny ip     Deny an IP in a Cluster and add to /etc/csf.deny
-ca, --callow ip    Allow an IP in a Cluster and add to /etc/csf.allow
-cr, --crm ip       Unblock an IP in a Cluster and remove from /etc/csf.deny
-cc, --cconfig [name] [value]
                    Change configuration option [name] to [value] in a Cluster
-cf, --cfile [file] Send [file] in a Cluster to /etc/csf/
-crs, --crestart    Cluster restart csf and lfd
-w, --watch ip      Log SYN packets for an IP across iptables chains
-m, --mail [addr]   Display Server Check in HTML or email to [addr] if present
-lr, --logrun       Initiate Log Scanner report via lfd
-c, --check         Check for updates to csf but do not upgrade
-u, --update        Check for updates to csf and upgrade if available
-uf                 Force an update of csf
-x, --disable       Disable csf and lfd
-e, --enable        Enable csf and lfd if previously disabled
-v, --version       Show csf version

The command line tool will also tell you if the testing mode is enabled (which is a very useful feature). If TESTING were enabled, we’d see this line at the bottom of the output:

*WARNING* TESTING mode is enabled - do not forget to disable it in the configuration

Did you make it all the way through?! Great! I know it’s a lot to take in, but it’s not terribly complicated when we break it down and understand how each piece works. Next time, I’ll be back with some tips on integrating CSF into cPanel.

-Mark

APF is an iptables wrapper that works alongside iptables and extends its functionality. I personally don’t use iptables wrappers, but I have a lot of experience with them, and I’ve seen that they do offer some additional features that streamline policy management. For example, by employing APF, you’ll get several simple on/off toggles (set via configuration files) that make some complex iptables configurations available without extensive coding requirements. The flip-side of a wrapper’s simplicity is that you aren’t directly in control of the iptables commands, so if something breaks it might take longer to diagnose and repair. Before you add a wrapper like APF, be sure that you know what you are getting into. Here are a few points to consider:

• Make sure that what you’re looking to use adds a feature you need but cannot easily incorporate with iptables on its own.
• You need to know how to effectively enable and disable the iptables wrapper (the correct way … read the manual!), and you should always have a trusted failsafe iptables ruleset handy in the unfortunate event that something goes horribly wrong and you need to disable the wrapper.
• Learn about the basic configurations and rule changes you can apply via the command line. You’ll need to understand the way your wrapper takes rules because it may differ from the way iptables handles rules.
• You can’t manually configure your iptables rules once you have your wrapper in place (or at least you shouldn’t).
• Be sure to know how to access your server via the IPMI management console so that if you completely lock yourself out beyond repair, you can get back in. You might even go so far as to have a script or set of instructions ready for tech support to run, in the event that you can’t get in via the management console.

TL;DR: Have a Band-Aid ready!

APF Configuration

Now that you have been sufficiently advised about the potential challenges of using a wrapper (and you’ve got your Band-Aid ready), we can check out some of the useful APF rules that make iptables administration a lot easier. Most of the configuration for APF is in conf.apf. This file handles the default behavior, but not necessarily the specific blocking rules, and when we make any changes to the configuration, we’ll need to restart the APF service for the changes to take effect.

Let’s jump into conf.apf and break down what we see. The first code snippit is fairly self-explanatory. It’s another way to make sure you don’t lock yourself out of your server as you are making configuration changes and testing them:

# !!! Do not leave set to (1) !!!
# When set to enabled; 5 minute cronjob is set to stop the firewall. Set
# this off (0) when firewall is determined to be operating as desired.
DEVEL_MODE="1"

The next configuration options we’ll look at are where you can make quick high-level changes if you find that legitimate traffic is being blocked and you want to make APF a little more lenient:

# This controls the amount of violation hits an address must have before it
# is blocked. It is a good idea to keep this very low to prevent evasive
# measures. The default is 0 or 1, meaning instant block on first violation.
RAB_HITCOUNT="1"
 
# This is the amount of time (in seconds) that an address gets blocked for if
# a violation is triggered, the default is 300s (5 minutes).
RAB_TIMER="300"
# This allows RAB to 'trip' the block timer back to 0 seconds if an address
# attempts ANY subsiquent communication while still on the inital block period.
RAB_TRIP="1"
 
# This controls if the firewall should log all violation hits from an address.
# The use of LOG_DROP variable set to 1 will override this to force logging.
RAB_LOG_HIT="1"
 
# This controls if the firewall should log all subsiqent traffic from an address
# that is already blocked for a violation hit, this can generate allot of logs.
# The use of LOG_DROP variable set to 1 will override this to force logging.
RAB_LOG_TRIP="0"

Next, we have an option to adjust ICMP flood protection. This protection should be useful against some forms of DoS attacks, and the associated rules show up in your INPUT chain:

# Set a reasonable packet/time ratio for ICMP packets, exceeding this flow
# will result in dropped ICMP packets. Supported values are in the form of:
# pkt/s (packets/seconds), pkt/m (packets/minutes)
# Set value to 0 for unlimited, anything above is enabled.
ICMP_LIM="30/s"

If you wanted to add more ports to block for p2p traffic (which will show up in the P2P chain), you’ll update this code:

# A common set of known Peer-To-Peer (p2p) protocol ports that are often
# considered undesirable traffic on public Internet servers. These ports
# are also often abused on web hosting servers where clients upload p2p
# client agents for the purpose of distributing or downloading pirated media.
# Format is comma separated for single ports and an underscore separator for
# ranges (4660_4678).
BLK_P2P_PORTS="1214,2323,4660_4678,6257,6699,6346,6347,6881_6889,6346,7778"

The next few lines let you designate the ports that you want to have closed at all times. They will be blocked for INPUT and OUTPUT chains:

# These are common Internet service ports that are understood in the wild
# services you would not want logged under normal circumstances. All ports
# that are defined here will be implicitly dropped with no logging for
# TCP/UDP traffic inbound or outbound. Format is comma separated for single
# ports and an underscore separator for ranges (135_139).
BLK_PORTS="135_139,111,513,520,445,1433,1434,1234,1524,3127"

The next important section to look at deals with conntrack. If you get “conntrack full” errors, this is where you’d increase the allowed connections. It’s not uncommon to need more connections than the default, so if you need to adjust that value, you’d do it here:

# This is the maximum number of "sessions" (connection tracking entries) that
# can be handled simultaneously by the firewall in kernel memory. Increasing
# this value too high will simply waste memory - setting it too low may result
# in some or all connections being refused, in particular during denial of
# service attacks.
SYSCTL_CONNTRACK="65536"

We’ve talked about the ports we want closed at all times, so it only makes sense that we’d specify which ports we want open for all interfaces:

# Common inbound (ingress) TCP ports
IG_TCP_CPORTS="22"
# Common inbound (ingress) UDP ports
IG_UDP_CPORTS=""
# Common outbound (egress) TCP ports
EG_TCP_CPORTS="21,25,80,443,43"
# Common outbound (egress) UDP ports
EG_UDP_CPORTS="20,21,53"

And when we want a special port allowance for specific users, we can declare it easily. For example, if we want port 22 open for user ID 0, we’d use this code:

# Allow outbound access to destination port 22 for uid 0
EG_TCP_UID="0:22"

The next few sections on Remote Rule Imports and Global Trust are a little more specialized, and I encourage you to read a little more about them (since there’s so much to them and not enough space to cover them here on the blog). An important feature of APF is that it imports block lists from outside sources to keep you safe from some attackers, so the Remote Rule Imports can prove to be very useful. The Global Trust section is incredibly useful for multi-server deployments of APF. Here, you can set up your global allow/block lists and have them all pull from a central location so that you can make a single update to the source and have the update propogated to all servers in your configuration. These changes are synced to the glob_allow/deny.rules files, and they will be downloaded (and overwritten) on a regular basis from your specified source, so don’t make any manual edits in glob_allow/deny.rules.

As you can see, apf.conf is no joke. It has a lot of stuff going on, but it’s very straightforward and documented well. Once we’ve set up apf.conf with the configurations we need, it’s time to look at the more focused allow_hosts.rules and deny_hosts.rules files. These .rules files are where where you put your typical firewall rules in place. If there’s one piece of advice I can give you about these configurations, it would be to check if your traffic is already allowed or blocked. Having multiple rules that do the same thing (possibly in different places) is confusing and potentially dangerous.

The deny_hosts.rules configuration will look just like allow_hosts.rules, but it’s performing the opposite function. Let’s check out an allow_hosts.rules configuration that will allow the Nimsoft service to function:

tcp:in:d=48000_48020:s=10.0.0.0/8
tcp:out:d=48000_48020:d=10.0.0.0/8

The format is somewhat simplistic, but the file gives a little more context in the comments:

# The trust rules can be made in advanced format with 4 options
# (proto:flow:port:ip);
# 1) protocol: [packet protocol tcp/udp]
# 2) flow in/out: [packet direction, inbound or outbound]
# 3) s/d=port: [packet source or destination port]
# 4) s/d=ip(/xx) [packet source or destination address, masking supported]
# Syntax:
# proto:flow:[s/d]=port:[s/d]=ip(/mask)

APF also uses ds_hosts.rules to load the DShield.org blocklist, and I assume the ecnshame_hosts.rules does something similar (can’t find much information about it), so you won’t need to edit these files manually. Additionally, you probably don’t need to make any changes to log.rules, unless you want to make changes to what exactly you log. As it stands, it logs certain dropped connections, which should be enough. Also, it might be worth noting that this file is a script, not a configuration file.

The last two configuration files are the preroute.rules and postroute.rules that (unsurprisingly) are used to make routing changes. If you have been following my articles, this corresponds to the iptables chains for PREROUTING and POSTROUTING where you would do things like port forwarding and other advanced configuration that you probably don’t want to do in most cases.

APF Command Line Management

As I mentioned in the “points to consider” at the top of this post, it’s important to learn the changes you can perform from the command line, and APF has some very useful command line tools:

[root@server]# apf --help
APF version 9.7 <apf@r-fx.org>
Copyright (C) 2002-2011, R-fx Networks <proj@r-fx.org>
Copyright (C) 2011, Ryan MacDonald <ryan@r-fx.org>
This program may be freely redistributed under the terms of the GNU GPL
 
usage /usr/local/sbin/apf [OPTION]
-s|--start ......................... load all firewall rules
-r|--restart ....................... stop (flush) & reload firewall rules
-f|--stop........ .................. stop (flush) all firewall rules
-l|--list .......................... list all firewall rules
-t|--status ........................ output firewall status log
-e|--refresh ....................... refresh & resolve dns names in trust rules
-a HOST CMT|--allow HOST COMMENT ... add host (IP/FQDN) to allow_hosts.rules and
                                     immediately load new rule into firewall
-d HOST CMT|--deny HOST COMMENT .... add host (IP/FQDN) to deny_hosts.rules and
                                     immediately load new rule into firewall
-u|--remove HOST ................... remove host from [glob]*_hosts.rules
                                     and immediately remove rule from firewall
-o|--ovars ......................... output all configuration options

You can use these command line tools to turn your firewall on and off, add allowed or blocked hosts and display troubleshooting information. These commands are very easy to use, but if you want more fine-tuned control, you’ll need to edit the configuration files directly (as we looked at above).

I know it seems like a lot of information, but to a large extent, that’s all you need to know to get started with APF. Take each section slowly and understand what each configuration file is doing, and you’ll master APF in no time at all.

-Mark