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# 30.5 The IPFILTER (IPF) Firewall

The author of IPFILTER is Darren Reed. IPFILTER is not operating system dependent: it is an open source application and has been ported to FreeBSD, NetBSD, OpenBSD, SunOS™, HP/UX, and Solaris™ operating systems. IPFILTER is actively being supported and maintained, with updated versions being released regularly.

IPFILTER is based on a kernel-side firewall and NAT mechanism that can be controlled and monitored by userland interface programs. The firewall rules can be set or deleted with the ipf(8) utility. The NAT rules can be set or deleted with the ipnat(1) utility. The ipfstat(8) utility can print run-time statistics for the kernel parts of IPFILTER. The ipmon(8) program can log IPFILTER actions to the system log files.

IPF was originally written using a rule processing logic of “the last matching rule wins” and used only stateless type of rules. Over time IPF has been enhanced to include a “quick” option and a stateful “keep state” option which drastically modernized the rules processing logic. IPF's official documentation covers only the legacy rule coding parameters and rule file processing logic. The modernized functions are only included as additional options, completely understating their benefits in producing a far superior and more secure firewall.

The instructions contained in this section are based on using rules that contain the “quick” option and the stateful “keep state” option. This is the basic framework for coding an inclusive firewall ruleset.

For detailed explanation of the legacy rules processing method see: http://www.obfuscation.org/ipf/ipf-howto.html#TOC_1 and http://coombs.anu.edu.au/~avalon/ip-filter.html.

The IPF FAQ is at http://www.phildev.net/ipf/index.html.

A searchable archive of the open-source IPFilter mailing list is available at http://marc.theaimsgroup.com/?l=ipfilter.

## 30.5.1 Enabling IPF

IPF is included in the basic FreeBSD install as a separate run time loadable module. The system will dynamically load the IPF kernel loadable module when the `rc.conf` statement `ipfilter_enable="YES"` is used. The loadable module was created with logging enabled and the `default pass all` options. There is no need to compile IPF into the FreeBSD kernel just to change the default to `block all`. This can be done just by adding a `block all` rule at the end of your ruleset.

## 30.5.2 Kernel options

It is not a mandatory requirement to enable IPF by compiling the following options into the FreeBSD kernel. It is only presented here as background information. Compiling IPF into the kernel causes the loadable module to never be used.

Sample kernel config IPF option statements are in the `/usr/src/sys/conf/NOTES` kernel source and are reproduced here:

    options IPFILTER
    options IPFILTER_LOG
    options IPFILTER_DEFAULT_BLOCK

`options IPFILTER` enables support for the “IPFILTER” firewall.

`options IPFILTER_LOG` enables the option to have IPF log traffic by writing to the `ipl` packet logging pseudo--device for every rule that has the `log` keyword.

`options IPFILTER_DEFAULT_BLOCK` changes the default behavior so any packet not matching a firewall `pass` rule gets blocked.

These settings will take effect only after installing a kernel that has been built with the above options set.

## 30.5.3 Available rc.conf Options

To activate IPF at boot time, the following statements need to be added to `/etc/rc.conf`:

    ipfilter_enable="YES"             # Start ipf firewall
    ipfilter_rules="/etc/ipf.rules"   # loads rules definition text file
    ipmon_enable="YES"                # Start IP monitor log
    ipmon_flags="-Ds"                 # D = start as daemon
                                      # s = log to syslog
                                      # v = log tcp window, ack, seq
                                      # n = map IP & port to names

If there is a LAN behind this firewall that uses the reserved private IP address ranges, the following lines will have to be added to enable NAT functionality:

    gateway_enable="YES"              # Enable as LAN gateway
    ipnat_enable="YES"                # Start ipnat function
    ipnat_rules="/etc/ipnat.rules"    # rules definition file for ipnat

## 30.5.4 IPF

The ipf(8) command is used to load your ruleset file. Your custom rules would normally be placed in a file, and the following command could then be used to replace in mass the currently running firewall rules:

    # ipf -Fa -f /etc/ipf.rules

`-Fa` means flush all internal rules tables.

`-f` means this is the file to read for the rules to load.

This gives you the ability to make changes to your custom rules file, run the above IPF command, and thus update the running firewall with a fresh copy of all the rules without having to reboot the system. This method is very convenient for testing new rules as the procedure can be executed as many times as needed.

See the ipf(8) manual page for details on the other flags available with this command.

The ipf(8) command expects the rules file to be a standard text file. It will not accept a rules file written as a script with symbolic substitution.

There is a way to build IPF rules that utilizes the power of script symbolic substitution. For more information, see Section 30.5.9.

## 30.5.5 IPFSTAT

The default behavior of ipfstat(8) is to retrieve and display the totals of the accumulated statistics gathered as a result of applying the user coded rules against packets going in and out of the firewall since it was last started, or since the last time the accumulators were reset to zero by the `ipf -Z` command.

See the ipfstat(8) manual page for details.

The default ipfstat(8) command output will look something like this:

    input packets: blocked 99286 passed 1255609 nomatch 14686 counted 0
     output packets: blocked 4200 passed 1284345 nomatch 14687 counted 0
     input packets logged: blocked 99286 passed 0
     output packets logged: blocked 0 passed 0
     packets logged: input 0 output 0
     log failures: input 3898 output 0
     fragment state(in): kept 0 lost 0
     fragment state(out): kept 0 lost 0
     packet state(in): kept 169364 lost 0
     packet state(out): kept 431395 lost 0
     ICMP replies: 0 TCP RSTs sent: 0
     Result cache hits(in): 1215208 (out): 1098963
     IN Pullups succeeded: 2 failed: 0
     OUT Pullups succeeded: 0 failed: 0
     Fastroute successes: 0 failures: 0
     TCP cksum fails(in): 0 (out): 0
     Packet log flags set: (0)

When supplied with either `-i` for inbound or `-o` for outbound, the command will retrieve and display the appropriate list of filter rules currently installed and in use by the kernel.

`ipfstat -in` displays the inbound internal rules table with rule number.

`ipfstat -on` displays the outbound internal rules table with the rule number.

The output will look something like this:

    @1 pass out on xl0 from any to any
    @2 block out on dc0 from any to any
    @3 pass out quick on dc0 proto tcp/udp from any to any keep state

`ipfstat -ih` displays the inbound internal rules table, prefixing each rule with a count of how many times the rule was matched.

`ipfstat -oh` displays the outbound internal rules table, prefixing each rule with a count of how many times the rule was matched.

The output will look something like this:

    2451423 pass out on xl0 from any to any
    354727 block out on dc0 from any to any
    430918 pass out quick on dc0 proto tcp/udp from any to any keep state

One of the most important functions of the `ipfstat` command is the `-t` flag which displays the state table in a way similar to the way top(1) shows the FreeBSD running process table. When your firewall is under attack, this function gives you the ability to identify, drill down to, and see the attacking packets. The optional sub-flags give the ability to select the destination or source IP, port, or protocol that you want to monitor in real time. See the ipfstat(8) manual page for details.

## 30.5.6 IPMON

In order for `ipmon` to work properly, the kernel option `IPFILTER_LOG` must be turned on. This command has two different modes that it can be used in. Native mode is the default mode when the command is typed on the command line without the `-D` flag.

Daemon mode is for when a continuous system log file is desired, so that logging of past events may be reviewed. This is how FreeBSD and IPFILTER are configured to work together. FreeBSD has a built in facility to automatically rotate system logs. That is why outputting the log information to syslogd(8) is better than the default of outputting to a regular file. In the default `rc.conf` file, the `ipmon_flags` statement uses the `-Ds` flags:

    ipmon_flags="-Ds" # D = start as daemon
                      # s = log to syslog
                      # v = log tcp window, ack, seq
                      # n = map IP & port to names

The benefits of logging are obvious. It provides the ability to review, after the fact, information such as which packets had been dropped, what addresses they came from and where they were going. These can all provide a significant edge in tracking down attackers.

Even with the logging facility enabled, IPF will not generate any rule logging on its own. The firewall administrator decides what rules in the ruleset he wants to log and adds the log keyword to those rules. Normally only deny rules are logged.

It is very customary to include a default deny everything rule with the log keyword included as your last rule in the ruleset. This makes it possible to see all the packets that did not match any of the rules in the ruleset.

## 30.5.7 IPMON Logging

Syslogd uses its own special method for segregation of log data. It uses special groupings called “facility” and “level”. IPMON in `-Ds` mode uses `security` as the “facility” name. All IPMON logged data goes to `security` The following levels can be used to further segregate the logged data if desired:

    LOG_INFO - packets logged using the "log" keyword as the action rather than pass or block.
    LOG_NOTICE - packets logged which are also passed
    LOG_WARNING - packets logged which are also blocked
    LOG_ERR - packets which have been logged and which can be considered short

To setup IPFILTER to log all data to `/var/log/ipfilter.log`, the file will need to be created beforehand. The following command will do that:

    # touch /var/log/ipfilter.log

The syslogd(8) function is controlled by definition statements in the `/etc/syslog.conf` file. The `syslog.conf` file offers considerable flexibility in how syslog will deal with system messages issued by software applications like IPF.

Add the following statement to `/etc/syslog.conf`:

    security.* /var/log/ipfilter.log

The `security.*` means to write all the logged messages to the coded file location.

To activate the changes to `/etc/syslog.conf` you can reboot or bump the syslogd(8) daemon into re-reading `/etc/syslog.conf` by running `/etc/rc.d/syslogd reload`

Do not forget to change `/etc/newsyslog.conf` to rotate the new log created above.

## 30.5.8 The Format of Logged Messages

Messages generated by `ipmon` consist of data fields separated by white space. Fields common to all messages are:

1. The date of packet receipt.

2. The time of packet receipt. This is in the form HH:MM:SS.F, for hours, minutes, seconds, and fractions of a second (which can be several digits long).

3. The name of the interface the packet was processed on, e.g. `dc0`.

4. The group and rule number of the rule, e.g. `@0:17`.

These can be viewed with `ipfstat -in`.

1. The action: p for passed, b for blocked, S for a short packet, n did not match any rules, L for a log rule. The order of precedence in showing flags is: S, p, b, n, L. A capital P or B means that the packet has been logged due to a global logging setting, not a particular rule.

2. The addresses. This is actually three fields: the source address and port (separated by a comma), the -> symbol, and the destination address and port, e.g.: `209.53.17.22,80 -> 198.73.220.17,1722`.

3. `PR` followed by the protocol name or number, e.g.: `PR tcp`.

4. `len` followed by the header length and total length of the packet, e.g.: `len 20 40`.

If the packet is a TCP packet, there will be an additional field starting with a hyphen followed by letters corresponding to any flags that were set. See the ipmon(8) manual page for a list of letters and their flags.

If the packet is an ICMP packet, there will be two fields at the end, the first always being “ICMP”, and the next being the ICMP message and sub-message type, separated by a slash, e.g. ICMP 3/3 for a port unreachable message.

## 30.5.9 Building the Rule Script with Symbolic Substitution

Some experienced IPF users create a file containing the rules and code them in a manner compatible with running them as a script with symbolic substitution. The major benefit of doing this is that only the value associated with the symbolic name needs to be changed, and when the script is run all the rules containing the symbolic name will have the value substituted in the rules. Being a script, symbolic substitution can be used to code frequently used values and substitute them in multiple rules. This can be seen in the following example.

The script syntax used here is compatible with the sh(1), csh(1), and tcsh(1) shells.

Symbolic substitution fields are prefixed with a dollar sign: `$`.

Symbolic fields do not have the $ prefix.

The value to populate the symbolic field must be enclosed with double quotes (`"`).

Start your rule file with something like this:

    ############# Start of IPF rules script ########################

    oif="dc0"            # name of the outbound interface
    odns="192.0.2.11"    # ISP's DNS server IP address
    myip="192.0.2.7"     # my static IP address from ISP
    ks="keep state"
    fks="flags S keep state"

    # You can choose between building /etc/ipf.rules file
    # from this script or running this script "as is".
    #
    # Uncomment only one line and comment out another.
    #
    # 1) This can be used for building /etc/ipf.rules:
    #cat > /etc/ipf.rules << EOF
    #
    # 2) This can be used to run script "as is":
    /sbin/ipf -Fa -f - << EOF

    # Allow out access to my ISP's Domain name server.
    pass out quick on $oif proto tcp from any to $odns port = 53 $fks
    pass out quick on $oif proto udp from any to $odns port = 53 $ks

    # Allow out non-secure standard www function
    pass out quick on $oif proto tcp from $myip to any port = 80 $fks

    # Allow out secure www function https over TLS SSL
    pass out quick on $oif proto tcp from $myip to any port = 443 $fks
    EOF
    ################## End of IPF rules script ########################

That is all there is to it. The rules are not important in this example; how the symbolic substitution fields are populated and used are. If the above example was in a file named `/etc/ipf.rules.script`, these rules could be reloaded by entering the following command:

    # sh /etc/ipf.rules.script

There is one problem with using a rules file with embedded symbolics: IPF does not understand symbolic substitution, and cannot read such scripts directly.

This script can be used in one of two ways:

- Uncomment the line that begins with `cat`, and comment out the line that begins with `/sbin/ipf`. Place `ipfilter_enable="YES"` into `/etc/rc.conf` as usual, and run script once after each modification to create or update `/etc/ipf.rules`.

- Disable IPFILTER in system startup scripts by adding `ipfilter_enable="NO"` (this is default value) into `/etc/rc.conf` file.

  Add a script like the following to your `/usr/local/etc/rc.d/` startup directory. The script should have an obvious name like `ipf.loadrules.sh`. The `.sh` extension is mandatory.

      #!/bin/sh
      sh /etc/ipf.rules.script

  The permissions on this script file must be read, write, execute for owner `root`.

      # chmod 700 /usr/local/etc/rc.d/ipf.loadrules.sh

Now, when your system boots, your IPF rules will be loaded.

## 30.5.10 IPF Rulesets

A ruleset is a group of IPF rules coded to pass or block packets based on the values contained in the packet. The bi-directional exchange of packets between hosts comprises a session conversation. The firewall ruleset processes both the packets arriving from the public Internet, as well as the packets produced by the system as a response to them. Each TCP/IP service (i.e.: telnet, www, mail, etc.) is predefined by its protocol and privileged (listening) port. Packets destined for a specific service, originate from the source address using an unprivileged (high order) port and target the specific service port on the destination address. All the above parameters (i.e.: ports and addresses) can be used as selection criteria to create rules which will pass or block services.

IPF was originally written using a rules processing logic of “the last matching rule wins” and used only stateless rules. Over time IPF has been enhanced to include a “quick” option and a stateful “keep state” option which drastically modernized the rule processing logic.

The instructions contained in this section are based on using rules that contain the “quick” option and the stateful “keep state” option. This is the basic framework for coding an inclusive firewall rule set.

> Warning: When working with the firewall rules, be very careful. Some configurations will lock you out of the server. To be on the safe side, you may wish to consider performing the initial firewall configuration from the local console rather than doing it remotely e.g. via ssh.

## 30.5.11 Rule Syntax

The rule syntax presented here has been simplified to only address the modern stateful rule context and “first matching rule wins” logic. For the complete legacy rule syntax description see the ipf(8) manual page.

A `#` character is used to mark the start of a comment and may appear at the end of a rule line or on its own line. Blank lines are ignored.

Rules contain keywords. These keywords have to be coded in a specific order from left to right on the line. Keywords are identified in bold type. Some keywords have sub-options which may be keywords themselves and also include more sub-options. Each of the headings in the below syntax has a bold section header which expands on the content.

`ACTION IN-OUT OPTIONS SELECTION STATEFUL PROTO SRC_ADDR,DST_ADDR OBJECT PORT_NUM TCP_FLAG STATEFUL`

`ACTION` = block | pass

`IN-OUT` = in | out

`OPTIONS` = log | quick | on interface-name

`SELECTION` = proto value | source/destination IP | port = number | flags flag-value

`PROTO` = tcp/udp | udp | tcp | icmp

`SRC_ADD,DST_ADDR` = all | from object to object

`OBJECT` = IP address | any

`PORT_NUM` = port number

`TCP_FLAG` = S

`STATEFUL` = keep state

### 30.5.11.1 ACTION

The action indicates what to do with the packet if it matches the rest of the filter rule. Each rule must have an action. The following actions are recognized:

`block` indicates that the packet should be dropped if the selection parameters match the packet.

`pass` indicates that the packet should exit the firewall if the selection parameters match the packet.

### 30.5.11.2 IN-OUT

A mandatory requirement is that each filter rule explicitly state which side of the I/O it is to be used on. The next keyword must be either `in` or `out` and one or the other has to be coded or the rule will not pass syntax checks.

`in` means this rule is being applied against an inbound packet which has just been received on the interface facing the public Internet.

`out` means this rule is being applied against an outbound packet destined for the interface facing the public Internet.

### 30.5.11.3 OPTIONS

> Note: These options must be used in the order shown here.

`log` indicates that the packet header will be written to the `ipl` log (as described in the LOGGING section below) if the selection parameters match the packet.

`quick` indicates that if the selection parameters match the packet, this rule will be the last rule checked, allowing a “short-circuit” path to avoid processing any following rules for this packet. This option is a mandatory requirement for the modernized rules processing logic.

`on` indicates the interface name to be incorporated into the selection parameters. Interface names are as displayed by ifconfig(8). Using this option, the rule will only match if the packet is going through that interface in the specified direction (in/out). This option is a mandatory requirement for the modernized rules processing logic.

When a packet is logged, the headers of the packet are written to the IPL packet logging pseudo-device. Immediately following the `log` keyword, the following qualifiers may be used (in this order):

`body` indicates that the first 128 bytes of the packet contents will be logged after the headers.

`first` If the `log` keyword is being used in conjunction with a `keep state` option, it is recommended that this option is also applied so that only the triggering packet is logged and not every packet which thereafter matches the “keep state” information.

### 30.5.11.4 SELECTION

The keywords described in this section are used to describe attributes of the packet to be checked when determining whether rules match or not. There is a keyword subject, and it has sub-option keywords, one of which has to be selected. The following general-purpose attributes are provided for matching, and must be used in this order:

### 30.5.11.5 PROTO

`proto` is the subject keyword and must be coded along with one of its corresponding keyword sub-option values. The value allows a specific protocol to be matched against. This option is a mandatory requirement for the modernized rules processing logic.

`tcp/udp | udp | tcp | icmp` or any protocol names found in `/etc/protocols` are recognized and may be used. The special protocol keyword `tcp/udp` may be used to match either a TCP or a UDP packet, and has been added as a convenience to save duplication of otherwise identical rules.

### 30.5.11.6 SRC_ADDR/DST_ADDR

The `all` keyword is essentially a synonym for “from any to any” with no other match parameters.

`from src to dst`: the `from` and `to` keywords are used to match against IP addresses. Rules must specify both source and destination parameters. `any` is a special keyword that matches any IP address. Examples of use: `from any to any` or `from 0.0.0.0/0 to any` or `from any to 0.0.0.0/0` or `from 0.0.0.0 to any` or `from any to 0.0.0.0`.

There is no way to match ranges of IP addresses which do not express themselves easily using the dotted numeric form / mask-length notation. The `net-mgmt/ipcalc` port may be used to ease up the calculations. Additional information is available in the utility's web page: http://jodies.de/ipcalc.

### 30.5.11.7 PORT

If a port match is included, for either or both of source and destination, then it is only applied to TCP and UDP packets. When composing port comparisons, either the service name from `/etc/services` or an integer port number may be used. When the port appears as part of the `from` object, it matches the source port number; when it appears as part of the `to` object, it matches the destination port number. The use of the port option with the `to` object is a mandatory requirement for the modernized rules processing logic. Example of use: `from any to any port = 80`

Single port comparisons may be done in a number of ways, using a number of different comparison operators. Port ranges may also be specified.

port "=" | "!=" | "<" | ">" | "<=" | ">=" | "eq" | "ne" | "lt" | "gt" | "le" | "ge".

To specify port ranges, port "<>" | "><"

> Warning: Following the source and destination matching parameters, the following two parameters are mandatory requirements for the modernized rules processing logic.

### 30.5.11.8 TCP_FLAG

Flags are only effective for TCP filtering. The letters represent one of the possible flags that can be matched against the TCP packet header.

The modernized rules processing logic uses the `flags S` parameter to identify the tcp session start request.

### 30.5.11.9 STATEFUL

`keep state` indicates that on a pass rule, any packets that match the rules selection parameters should activate the stateful filtering facility.

> Note: This option is a mandatory requirement for the modernized rules processing logic.

## 30.5.12 Stateful Filtering

Stateful filtering treats traffic as a bi-directional exchange of packets comprising a session conversation. When activated, keep-state dynamically generates internal rules for each anticipated packet being exchanged during the bi-directional session conversation. It has sufficient matching capabilities to determine if the session conversation between the originating sender and the destination are following the valid procedure of bi-directional packet exchange. Any packets that do not properly fit the session conversation template are automatically rejected as impostors.

Keep state will also allow ICMP packets related to a TCP or UDP session through. So if you get ICMP type 3 code 4 in response to some web surfing allowed out by a keep state rule, they will be automatically allowed in. Any packet that IPF can be certain is part of an active session, even if it is a different protocol, will be let in.

What happens is:

Packets destined to go out through the interface connected to the public Internet are first checked against the dynamic state table. If the packet matches the next expected packet comprising an active session conversation, then it exits the firewall and the state of the session conversation flow is updated in the dynamic state table. Packets that do not belong to an already active session, are simply checked against the outbound ruleset.

Packets coming in from the interface connected to the public Internet are first checked against the dynamic state table. If the packet matches the next expected packet comprising an active session conversation, then it exits the firewall and the state of the session conversation flow is updated in the dynamic state table. Packets that do not belong to an already active session, are simply checked against the inbound ruleset.

When the conversation completes it is removed from the dynamic state table.

Stateful filtering allows you to focus on blocking/passing new sessions. If the new session is passed, all its subsequent packets will be allowed through automatically and any impostors automatically rejected. If a new session is blocked, none of its subsequent packets will be allowed through. Stateful filtering has technically advanced matching abilities capable of defending against the flood of different attack methods currently employed by attackers.

## 30.5.13 Inclusive Ruleset Example

The following ruleset is an example of how to code a very secure inclusive type of firewall. An inclusive firewall only allows services matching `pass` rules through, and blocks all others by default. Firewalls intended to protect other machines, also called “network firewalls”, should have at least two interfaces, which are generally configured to trust one side (the LAN) and not the other (the public Internet). Alternatively, a firewall might be configured to protect only the system it is running on--this is called a “host based firewall”, and is particularly appropriate for servers on an untrusted network.

All UNIX® flavored systems including FreeBSD are designed to use interface `lo0` and IP address `127.0.0.1` for internal communication within the operating system. The firewall rules must contain rules to allow free unmolested movement of these special internally used packets.

The interface which faces the public Internet is the one to place the rules that authorize and control access of the outbound and inbound connections. This can be your user PPP `tun0` interface or your NIC that is connected to your DSL or cable modem.

In cases where one or more NICs are cabled to private network segments, those interfaces may require rules to allow packets originating from those LAN interfaces transit to each other and/or to the outside (Internet).

The rules should be organized into three major sections: first trusted interfaces, then the public interface outbound, and last the public untrusted interface inbound.

The rules in each of the public interface sections should have the most frequently matched rules placed before less commonly matched rules, with the last rule in the section blocking and logging all packets on that interface and direction.

The Outbound section in the following ruleset only contains `pass` rules which contain selection values that uniquely identify the service that is authorized for public Internet access. All the rules have the `quick`, `on`, `proto`, `port`, and `keep state` options set. The `proto tcp` rules have the `flag` option included to identify the session start request as the triggering packet to activate the stateful facility.

The Inbound section has all the blocking of undesirable packets first, for two different reasons. The first is that malicious packets may be partial matches for legitimate traffic. These packets have to be discarded rather than allowed in, based on their partial matches against `allow` rules. The second reason is that known and uninteresting rejects may be blocked silently, rather than being caught and logged by the last rules in the section. The final rule in each section, blocks and logs all packets and can be used to create the legal evidence needed to prosecute the people who are attacking your system.

Another thing that should be taken care of, is to ensure there is no response returned for any of the undesirable traffic. Invalid packets should just get dropped and vanish. This way the attacker has no knowledge if his packets have reached your system. The less the attackers can learn about your system, the more time they must invest before actually doing something bad. Rules that include a `log first` option, will only log the event the first time they are triggered. This option is included in the sample `nmap OS fingerprint` rule. The `security/nmap` utility is commonly used by attackers who attempt to identify the operating system of your server.

Any time there are logged messages on a rule with the `log first` option, an `ipfstat -hio` command should be executed to evaluate how many times the rule has actually matched. Large number of matches usually indicate that the system is being flooded (i.e.: under attack).

The `/etc/services` file may be used to lookup unknown port numbers. Alternatively, visit http://www.securitystats.com/tools/portsearch.php and do a port number lookup to find the purpose of a particular port number.

Check out this link for port numbers used by Trojans http://www.simovits.com/trojans/trojans.html.

The following ruleset creates a complete and very secure `inclusive` type of firewall ruleset that has been tested on production systems. It can be easily modified for your own system. Just comment out any `pass` rules for services that should not be authorized.

To avoid logging unwanted messages, just add a `block` rule in the inbound section.

The `dc0` interface name has to be changed in every rule to the real interface name of the NIC card that connects your system to the public Internet. For user PPP it would be `tun0`.

Add the following statements to `/etc/ipf.rules`:

    #################################################################
    # No restrictions on Inside LAN Interface for private network
    # Not needed unless you have LAN
    #################################################################

    #pass out quick on xl0 all
    #pass in quick on xl0 all

    #################################################################
    # No restrictions on Loopback Interface
    #################################################################
    pass in quick on lo0 all
    pass out quick on lo0 all

    #################################################################
    # Interface facing Public Internet (Outbound Section)
    # Match session start requests originating from behind the
    # firewall on the private network
    # or from this gateway server destined for the public Internet.
    #################################################################

    # Allow out access to my ISP's Domain name server.
    # xxx must be the IP address of your ISP's DNS.
    # Dup these lines if your ISP has more than one DNS server
    # Get the IP addresses from /etc/resolv.conf file
    pass out quick on dc0 proto tcp from any to xxx port = 53 flags S keep state
    pass out quick on dc0 proto udp from any to xxx port = 53 keep state

    # Allow out access to my ISP's DHCP server for cable or DSL networks.
    # This rule is not needed for 'user ppp' type connection to the
    # public Internet, so you can delete this whole group.
    # Use the following rule and check log for IP address.
    # Then put IP address in commented out rule & delete first rule
    pass out log quick on dc0 proto udp from any to any port = 67 keep state
    #pass out quick on dc0 proto udp from any to z.z.z.z port = 67 keep state

    # Allow out non-secure standard www function
    pass out quick on dc0 proto tcp from any to any port = 80 flags S keep state

    # Allow out secure www function https over TLS SSL
    pass out quick on dc0 proto tcp from any to any port = 443 flags S keep state

    # Allow out send & get email function
    pass out quick on dc0 proto tcp from any to any port = 110 flags S keep state
    pass out quick on dc0 proto tcp from any to any port = 25 flags S keep state

    # Allow out Time
    pass out quick on dc0 proto tcp from any to any port = 37 flags S keep state

    # Allow out nntp news
    pass out quick on dc0 proto tcp from any to any port = 119 flags S keep state

    # Allow out gateway & LAN users' non-secure FTP ( both passive & active modes)
    # This function uses the IPNAT built in FTP proxy function coded in
    # the nat rules file to make this single rule function correctly.
    # If you want to use the pkg_add command to install application packages
    # on your gateway system you need this rule.
    pass out quick on dc0 proto tcp from any to any port = 21 flags S keep state

    # Allow out ssh/sftp/scp (telnet/rlogin/FTP replacements)
    # This function is using SSH (secure shell)
    pass out quick on dc0 proto tcp from any to any port = 22 flags S keep state

    # Allow out insecure Telnet
    pass out quick on dc0 proto tcp from any to any port = 23 flags S keep state

    # Allow out FreeBSD CVSup
    pass out quick on dc0 proto tcp from any to any port = 5999 flags S keep state

    # Allow out ping to public Internet
    pass out quick on dc0 proto icmp from any to any icmp-type 8 keep state

    # Allow out whois from LAN to public Internet
    pass out quick on dc0 proto tcp from any to any port = 43 flags S keep state

    # Block and log only the first occurrence of everything
    # else that's trying to get out.
    # This rule implements the default block
    block out log first quick on dc0 all

    #################################################################
    # Interface facing Public Internet (Inbound Section)
    # Match packets originating from the public Internet
    # destined for this gateway server or the private network.
    #################################################################

    # Block all inbound traffic from non-routable or reserved address spaces
    block in quick on dc0 from 192.168.0.0/16 to any    #RFC 1918 private IP
    block in quick on dc0 from 172.16.0.0/12 to any     #RFC 1918 private IP
    block in quick on dc0 from 10.0.0.0/8 to any        #RFC 1918 private IP
    block in quick on dc0 from 127.0.0.0/8 to any       #loopback
    block in quick on dc0 from 0.0.0.0/8 to any         #loopback
    block in quick on dc0 from 169.254.0.0/16 to any    #DHCP auto-config
    block in quick on dc0 from 192.0.2.0/24 to any      #reserved for docs
    block in quick on dc0 from 204.152.64.0/23 to any   #Sun cluster interconnect
    block in quick on dc0 from 224.0.0.0/3 to any       #Class D & E multicast

    ##### Block a bunch of different nasty things. ############
    # That I do not want to see in the log

    # Block frags
    block in quick on dc0 all with frags

    # Block short tcp packets
    block in quick on dc0 proto tcp all with short

    # block source routed packets
    block in quick on dc0 all with opt lsrr
    block in quick on dc0 all with opt ssrr

    # Block nmap OS fingerprint attempts
    # Log first occurrence of these so I can get their IP address
    block in log first quick on dc0 proto tcp from any to any flags FUP

    # Block anything with special options
    block in quick on dc0 all with ipopts

    # Block public pings
    block in quick on dc0 proto icmp all icmp-type 8

    # Block ident
    block in quick on dc0 proto tcp from any to any port = 113

    # Block all Netbios service. 137=name, 138=datagram, 139=session
    # Netbios is MS/Windows sharing services.
    # Block MS/Windows hosts2 name server requests 81
    block in log first quick on dc0 proto tcp/udp from any to any port = 137
    block in log first quick on dc0 proto tcp/udp from any to any port = 138
    block in log first quick on dc0 proto tcp/udp from any to any port = 139
    block in log first quick on dc0 proto tcp/udp from any to any port = 81

    # Allow traffic in from ISP's DHCP server. This rule must contain
    # the IP address of your ISP's DHCP server as it's the only
    # authorized source to send this packet type. Only necessary for
    # cable or DSL configurations. This rule is not needed for
    # 'user ppp' type connection to the public Internet.
    # This is the same IP address you captured and
    # used in the outbound section.
    pass in quick on dc0 proto udp from z.z.z.z to any port = 68 keep state

    # Allow in standard www function because I have apache server
    pass in quick on dc0 proto tcp from any to any port = 80 flags S keep state

    # Allow in non-secure Telnet session from public Internet
    # labeled non-secure because ID/PW passed over public Internet as clear text.
    # Delete this sample group if you do not have telnet server enabled.
    #pass in quick on dc0 proto tcp from any to any port = 23 flags S keep state

    # Allow in secure FTP, Telnet, and SCP from public Internet
    # This function is using SSH (secure shell)
    pass in quick on dc0 proto tcp from any to any port = 22 flags S keep state

    # Block and log only first occurrence of all remaining traffic
    # coming into the firewall. The logging of only the first
    # occurrence avoids filling up disk with Denial of Service logs.
    # This rule implements the default block.
    block in log first quick on dc0 all
    ################### End of rules file #####################################

## 30.5.14 NAT

NAT stands for Network Address Translation. To those familiar with Linux®, this concept is called IP Masquerading; NAT and IP Masquerading are the same thing. One of the many things the IPF NAT function enables is the ability to have a private Local Area Network (LAN) behind the firewall sharing a single ISP assigned IP address on the public Internet.

You may ask why would someone want to do this. ISPs normally assign a dynamic IP address to their non-commercial users. Dynamic means that the IP address can be different each time you dial in and log on to your ISP, or for cable and DSL modem users, when the modem is power cycled. This dynamic IP address is used to identify your system to the public Internet.

Now lets say you have five PCs at home and each one needs Internet access. You would have to pay your ISP for an individual Internet account for each PC and have five phone lines.

With NAT only a single account is needed with your ISP. The other four PCs may then be cabled to a switch and the switch to the NIC in your FreeBSD system which is going to service your LAN as a gateway. NAT will automatically translate the private LAN IP address for each separate PC on the LAN to the single public IP address as it exits the firewall bound for the public Internet. It also does the reverse translation for returning packets.

There is a special range of IP addresses reserved for NATed private LANs. According to RFC 1918, the following IP ranges may be used for private nets which will never be routed directly to the public Internet:

Start IP `10.0.0.0` | - | Ending IP `10.255.255.255`
Start IP `172.16.0.0` | - | Ending IP `172.31.255.255`
Start IP `192.168.0.0` | - | Ending IP `192.168.255.255`

## 30.5.15 IPNAT

NAT rules are loaded by using the `ipnat` command. Typically the NAT rules are stored in `/etc/ipnat.rules`. See ipnat(1) for details.

When changing the NAT rules after NAT has been started, make your changes to the file containing the NAT rules, then run the `ipnat` command with the `-CF` flags to delete the internal in use NAT rules and flush the contents of the translation table of all active entries.

To reload the NAT rules issue a command like this:

    # ipnat -CF -f /etc/ipnat.rules

To display some statistics about your NAT, use this command:

    # ipnat -s

To list the NAT table's current mappings, use this command:

    # ipnat -l

To turn verbose mode on, and display information relating to rule processing and active rules/table entries:

    # ipnat -v

## 30.5.16 IPNAT Rules

NAT rules are very flexible and can accomplish many different things to fit the needs of commercial and home users.

The rule syntax presented here has been simplified to what is most commonly used in a non-commercial environment. For a complete rule syntax description see the ipnat(5) manual page.

The syntax for a NAT rule looks something like this:

    map IF LAN_IP_RANGE -> PUBLIC_ADDRESS

The keyword `map` starts the rule.

Replace `IF` with the external interface.

The `LAN_IP_RANGE` is what your internal clients use for IP Addressing, usually this is something like `192.168.1.0/24`.

The `PUBLIC_ADDRESS` can either be the external IP address or the special keyword `0/32`, which means to use the IP address assigned to `IF`.

## 30.5.17 How NAT works

A packet arrives at the firewall from the LAN with a public destination. It passes through the outbound filter rules, NAT gets its turn at the packet and applies its rules top down, first matching rule wins. NAT tests each of its rules against the packet's interface name and source IP address. When a packet's interface name matches a NAT rule then the source IP address (i.e.: private LAN IP address) of the packet is checked to see if it falls within the IP address range specified to the left of the arrow symbol on the NAT rule. On a match the packet has its source IP address rewritten with the public IP address obtained by the `0/32` keyword. NAT posts an entry in its internal NAT table so when the packet returns from the public Internet it can be mapped back to its original private IP address and then passed to the filter rules for processing.

## 30.5.18 Enabling IPNAT

To enable IPNAT add these statements to `/etc/rc.conf`.

To enable your machine to route traffic between interfaces:

    gateway_enable="YES"

To start IPNAT automatically each time:

    ipnat_enable="YES"

To specify where to load the IPNAT rules from:

    ipnat_rules="/etc/ipnat.rules"

## 30.5.19 NAT for a very large LAN

For networks that have large numbers of PC's on the LAN or networks with more than a single LAN, the process of funneling all those private IP addresses into a single public IP address becomes a resource problem that may cause problems with the same port numbers being used many times across many NATed LAN PC's, causing collisions. There are two ways to relieve this resource problem.

### 30.5.19.1 Assigning Ports to Use

A normal NAT rule would look like:

    map dc0 192.168.1.0/24 -> 0/32

In the above rule the packet's source port is unchanged as the packet passes through IPNAT. By adding the `portmap` keyword, IPNAT can be directed to only use source ports in the specified range. For example the following rule will tell IPNAT to modify the source port to be within the range shown:

    map dc0 192.168.1.0/24 -> 0/32 portmap tcp/udp 20000:60000

Additionally we can make things even easier by using the `auto` keyword to tell IPNAT to determine by itself which ports are available to use:

    map dc0 192.168.1.0/24 -> 0/32 portmap tcp/udp auto

### 30.5.19.2 Using a Pool of Public Addresses

In very large LANs there comes a point where there are just too many LAN addresses to fit into a single public address. If a block of public IP addresses is available, these addresses can be used as a “pool”, and IPNAT may pick one of the public IP addresses as packet-addresses are mapped on their way out.

For example, instead of mapping all packets through a single public IP address, as in:

    map dc0 192.168.1.0/24 -> 204.134.75.1

A range of public IP addresses can be specified either with a netmask:

    map dc0 192.168.1.0/24 -> 204.134.75.0/255.255.255.0

or using CIDR notation:

    map dc0 192.168.1.0/24 -> 204.134.75.0/24

## 30.5.20 Port Redirection

A very common practice is to have a web server, email server, database server and DNS server each segregated to a different PC on the LAN. In this case the traffic from these servers still have to be NATed, but there has to be some way to direct the inbound traffic to the correct LAN PCs. IPNAT has the redirection facilities of NAT to solve this problem. For example, assuming a web server operating on LAN address `10.0.10.25` and using a single public IP address of `20.20.20.5` the rule would be coded as follows:

    rdr dc0 20.20.20.5/32 port 80 -> 10.0.10.25 port 80

or:

    rdr dc0 0.0.0.0/0 port 80 -> 10.0.10.25 port 80

or for a LAN DNS Server on LAN address of `10.0.10.33` that needs to receive public DNS requests:

    rdr dc0 20.20.20.5/32 port 53 -> 10.0.10.33 port 53 udp

## 30.5.21 FTP and NAT

FTP is a dinosaur left over from the time before the Internet as it is known today, when research universities were leased lined together and FTP was used to share files among research Scientists. This was a time when data security was not a consideration. Over the years the FTP protocol became buried into the backbone of the emerging Internet and its username and password being sent in clear text was never changed to address new security concerns. FTP has two flavors, it can run in active mode or passive mode. The difference is in how the data channel is acquired. Passive mode is more secure as the data channel is acquired by the ordinal ftp session requester. For a real good explanation of FTP and the different modes see http://www.slacksite.com/other/ftp.html.

### 30.5.21.1 IPNAT Rules

IPNAT has a special built in FTP proxy option which can be specified on the NAT map rule. It can monitor all outbound packet traffic for FTP active or passive start session requests and dynamically create temporary filter rules containing only the port number really in use for the data channel. This eliminates the security risk FTP normally exposes the firewall to from having large ranges of high order port numbers open.

This rule will handle all the traffic for the internal LAN:

    map dc0 10.0.10.0/29 -> 0/32 proxy port 21 ftp/tcp

This rule handles the FTP traffic from the gateway:

    map dc0 0.0.0.0/0 -> 0/32 proxy port 21 ftp/tcp

This rule handles all non-FTP traffic from the internal LAN:

    map dc0 10.0.10.0/29 -> 0/32

The FTP map rule goes before our regular map rule. All packets are tested against the first rule from the top. Matches on interface name, then private LAN source IP address, and then is it a FTP packet. If all that matches then the special FTP proxy creates temp filter rules to let the FTP session packets pass in and out, in addition to also NATing the FTP packets. All LAN packets that are not FTP do not match the first rule and fall through to the third rule and are tested, matching on interface and source IP, then are NATed.

### 30.5.21.2 IPNAT FTP Filter Rules

Only one filter rule is needed for FTP if the NAT FTP proxy is used.

Without the FTP Proxy, the following three rules will be needed:

    # Allow out LAN PC client FTP to public Internet
    # Active and passive modes
    pass out quick on rl0 proto tcp from any to any port = 21 flags S keep state

    # Allow out passive mode data channel high order port numbers
    pass out quick on rl0 proto tcp from any to any port > 1024 flags S keep state

    # Active mode let data channel in from FTP server
    pass in quick on rl0 proto tcp from any to any port = 20 flags S keep state

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