Hardening OpenBSD Internet Servers
Packet Filter and IP Filter on Non Firewalls
Traditionally firewall software has run on computers with two or
more network interfaces to control the flow of traffic between them.
Increasingly firewall software is run on a single machine which it
protects. I've tried to cover everything you need to know to use IP
Filter or Packet Filter as a single host firewall on this page and
discuss when such use may or may not be appropriate.
Packet Filter and IP Filter
Firewall software such as IP Filter and its 3.0 replacement, Packet
Filter, has traditionally been used to create firewalls on computers
with two or more network interfaces. In this configuration it
protects computers on the inside from unauthorized access from the
outside, usually the Internet. A firewall may also control the
types of network traffic that are allowed to leave the local
network. Adequate coverage of the issues in setting up a firewall
would take a book. If you are
going to use Packet Filter, it's probably best to read the
OpenBSD documentation at
If for some reason you want to use the older IP Filter firewall
you should visit its home page
IP Filter home page
which has a concise set of
that cover most of IP Filter's capabilities. There was an
"ipf HOWTO" but that seems to have disappeared. When I wrote this
PF had only recently replaced IP Filter and documentation was
somewhat sparse. After more than a decade as OpenBSD's official
firewall PF is well documented and accepted. I certainly would not
wish to switch to an older product that has not been an integral
part of OpenBSD for more than decade. When the change was made I
went with what came with OpenBSD. Specific adjustments
were necessary. Some changes were not documented; see the
OpenBSD 2.9 to 3.0 page for details.
The OpenBSD developers were quite helpful when I asked about the
issues I encountered.,
The concepts and filter rules are very similar but not identical
and the handling of state related rules is entirely different; the
controlling programs and options are also very different.
Depending on the OpenBSD version, Packet Filter or IP Filter can also
be used as a more flexible and powerful replacement for TCP Wrappers
protecting only the computer on which it runs. The advantage of a
firewall is that it allows complete control of network traffic before
it reaches any IP port. Incoming UDP and ICMP traffic as well as any
outgoing traffic can be controlled. TCP Wrappers cannot control
anything but incoming TCP connections and then only to sshd, Sendmail,
and services that are started by inetd. The disadvantage of a firewall
is that its rules are much more complex than TCP Wrapper's simple
hosts.allow and hosts.deny files.
In 2.9 and earlier, if you are using the GENERIC kernel or a custom
kernel with the IPFILTER and IPFILTER_LOG options left on, IP Filter
is in your kernel and ready to go. You can turn it on from the
command line with "# ipf -Fa -f /etc/ipf.rules -E". In 3.0, the kernel
pseudo-device pf 1 # packet filter
pseudo-device pflog 1 # pf log
The 3.0 command line equivalent is "#pfctl -Fa -R /etc/pf.conf -e". In
both cases, the first time you don't need the "-Fa", flush all, option
but making this a habit insures that any rules you load in the
specified file will be the only active rules. With Packet Filter "-
Fr" would flush only the rules and keep NAT rules, state and
statistics. /etc/ipf.rules is the default location for IP Filter rules
and /etc/pf.conf for Packet Filter rules. In 2.9 and earlier, if you
change "ipfilter=NO" in /etc/rc.conf to "ipfilter=YES", or in 3.0
change "pf=NO" to "pf= YES", after rebooting you will have exactly the
same active ruleset as the command line examples.
I've included an
example firewall rule set suitable for
use on a single NIC web server. The examples include a backslash to
indicate a continued long line; this will not work with
either Packet Filter or IP Filter but the sample rules, except as
noted in the comments, will work with both. I'll cover the rules in
the order they were developed, not their current physical order in
the file. The first rules were
pass in quick on lo0 all
pass out quick on lo0 all
which were added to the automatically installed, default rules,
before the original pass in and out from any to any rules. Both
Packet Filter and IP Filter's normal processing order is to evaluate
all the rules and apply the last rule that matches a packet being
checked. The "quick" keyword causes rule evaluation to stop as soon
as the packet matches a rule with the quick keyword. The lo0 is the
loopback interface. Soon rules that block everything not explicitly
allowed will be added. Unintentionally blocking loopback traffic can
The purpose of using a firewall, is to control network traffic
between the local computer and any other networked computers.
Conceptually it's simpler to assure all loopback traffic is
allowed and focus our attention on the connection between the
local computer and the network, i.e., the NIC, which happens to be
the dc0 interface in this example.
pass in quick on dc0 proto tcp from 220.127.116.11/32 to
18.104.22.168/32 port = 22 flags S keep state
is added. In our example we are pretending to be on the 22.214.171.124
network subnetted as a class C or slash 24 ("/24") network. The
local machine that the IP Filter rules are being created for is a
public web server at 126.96.36.199; the /32 notation indicates a
single host. "/31" would be two machines, "/30" four and "/29"
eight, etc. The management workstation from which we are working is
188.8.131.52. "port = 22" allows traffic to the sshd server running
on the web server. If telnet were being still being used, the port
would be 23 instead.
The "flags S" in this rule matches a packet that is the begining of
a new TCP session, i.e., only the SYN flag is set. The "keep state"
causes all subsequent packets that are part of the same session in
both directions to be matched as well. The firewall's understanding
of IP headers allows it to determine which packets are part of the
same session. With "keep state" there is no need to figure out the
reverse rules, i.e., those that allow reply traffic. This rule will
not match packets that are part of an already existing SSH (or
telnet) session between these two machines. It only allows inbound
connections from one machine and does not allow outbound SSH
In the example we are using, a host with a single NIC, and not a real
firewall with multiple network interfaces, Packet Filter and IP Filter
will handle state rules in essentially the same manner. The same is
not true when two or more network interfaces are present. With IP
Filter, a state rule applied to the firewall as a whole and once a
stream of packets was allowed by a state rule on any interface it
would pass through other interfaces, even if there were rules that
otherwise would have blocked them. Packet Filter state rules are
interface specific. A state rule on the inner interface of a dual
homed firewall will allow return packets back through the inner
interface but the inner interface will never see the packets if the
outer interface has a rule that blocks them.
One place this will surely cause problems switching from 2.9 to 3.0,
is bridged firewalls. IP Filter did not allow "out" rules to be
defined so "in" rules had to be used on both or all interfaces to
control the flow of traffic. A simple mechanical transformation will
allow an IP Filter bridged firewall rule set to be adapted for Packet
Filter. Simply change all "in" rules on an inner interface to "out"
rules on the outside interface. The net results will be the same
except that it is possible some packets might pass into (but not out
of) the firewall that would formerly have been blocked on the inner
interface before entering the firewall. You could add a rule on the
inner interface to block these but before doing so be sure the rule
won't block return packets that are part of a stateful connection
established on the outer interface.
The rule above was set up first so that we would have an open path for
SSH. As long as this is available, we can work remotely and do all
additional configuration from our normal workstation. This rule was
set up remotely. Once set up, if it's right, we should be able to
open a new SSH session anytime we need one from our management
workstation, regardless of any other rules that may be added (unless
we put a more general and contradictory rule with the "quick" keyword
before this rule).
One disadvantage to the preceding rule is that each time a new
rule set is loaded for testing, any open SSH connections will be
broken and we will need to log into the web server again. This
can be solved with a pair of rules:
IP Filter Logs and Ipmon
pass in quick on dc0 proto tcp from 184.108.40.206/32 to
220.127.116.11/32 port = 22
pass in quick on dc0 proto tcp from 18.104.22.168/32 port = 22 to
These allow any packets between the management workstation and
sshd on the web server so SSH sessions may be kept active even
when the active rule set is replaced, as long as they are kept in
the new active rule set, before any other rule blocks this traffic.
This web page was first written when IP Filter was the current
OpenBSD firewall. For the logging, I'll cover IP Filter logs first
and then cover the same issues for Packet Filter. When the above
rule was first added, the keyword "log" immediately followed "pass
in" and preceded "quick". This allowed verification, by checking
the ipf logs, that the new SSH (or telnet) session was actually
using this rule, and not the still present pass all to all rules.
When debugging a new IP Filter rule set, it can be very helpful to
use the ipf logs as a debugging aid. By turning on logging for
the new rules regardless of whether they are pass or block, you
can see exactly what the rule or rules are doing. Leaving or
turning logging off all other rules, helps isolate the results
of the rules you are working on. When the rule set nears completion,
the "log" keyword can easily be added or removed as appropriate to log only
the traffic you wish to log.
The default ipmon setup makes reading the logs unnecessarily
difficult. Adding about 35 unnecessary bytes to the begining of
each log entry, causes most log entries to be just about two full
lines on standard text displays, making it very hard to tell where
one log entry ends and the next begins. Syslogd adds date and time,
hostname, process name and process ID to every log entry. Ipmon
already includes complete date and time data. Syslog's extra
information makes sense when it is being used to send logs to a
remote logging host or combining logs from multiple programs in
the same log file. Syslogd is however set up to use a dedicated
log file for ipmon, /var/log/ipflog.
Since syslogd's facilities are not being used to advantage, it makes
better sense to have ipmon log directly to a file. Replacing the
default flags, "ipmon_flags=-Ds", in /etc/rc.conf with "ipmon_flags="-
aD /var/log/ipflog" will eliminate the syslogd additions and make the
logs much more readable. You can stop logging at any time with "#
kill -TERM `cat /var/run/ipmon.pid`" and restart it with "# ipmon -aD
/var/log/ipflog". You could use "-Do I" instead of "-aD" if you
didn't want to see state entries in the logs but only normal IP Filter
entries. The state entries are helpful in understanding what's going
on when you're setting up rules with state. To truncate (clear) the
log file while ipmon is running you can use "# echo> /var/log/ipflog".
This makes it easy to see the effects of just changed rules without
scrolling through old entries. It's not recommended once IP Filter is
being used in a real environment.
If you use IP Filter you should know about "# ipfstat -hion".
This shows all active rules, not the contents of the rules file.
Each rule is preceeded by a hit ("h") count so you can quickly
see which rules are being matched and which not. The "i" and "o"
specify that both in and out rules be listed. The "n" causes the
rule number to be listed. The listed rules are always grouped
into out rules and in rules with out rules first and shown in the
order the individual rules are applied. Output and input each
have there own rule numbers, starting with 1 and preceeded by an
at sign ("@"). These rule numbers match the rule numbers listed
in the logs. Most real rule sets will require piping through
Packet Filter and pflogd
The most significant change between IP Filter and Packet Filter is
logging. I don't think it's an exaggeration to say everything
related to logging is different between the two products. IP Filter
logged to a highly structured text format that focused mainly on
what the firewall did with the packet and contained only a few key
pieces of information describing key packet header settings with
no actual packet contents. Packet Filter, instead logs to a
standard tcpdump binary format. This includes the entire packet
contents or at least the first 96 bytes by default, if the packet
is longer. The length of packet data logged can be changed. The
format also includes the time the firewall processed the packet
and the same rule number, disposition (passed or blocked) and
interface information that IP Filter includes in its logs.
To see the log content in a human readable form you need to use
tcpdump with some format options (discussed below) to interpret
the binary file. Most of the key information content can be the
same as IP Filter, with the right format options, but the actual
format is very different.
The logging daemon ipmon, has been replaced by pflogd. Where ipmon
logged by default through syslogd and only optionally to a file,
pflogd always logs to a file. If you want standard log rotation,
newsyslog will work with pflogd. In fact the rotation is already
set up as part of the default install. If you use log rotation,
the standard setup replaces a log when it exceeds 250K and keeps
only 4 generations. On many firewalls this would keep only 4 hours
of logs because with aggressive logging and the binary format, the 250K
can easily be exceeded in an hour, the default interval for newsyslog.
In my opinion, the most valuable content on a firewall is the firewall
logs. Controlling network traffic is only half the firewall's job.
Understanding what the firewall has responded to, is equally important
and only the logs will reveal that. My /var filesystems are 2 - 8GB
to allow a significant amount of logs to be readily available for
online analysis. Further, new logs are transferred daily to a remote
machine where they are written to CD-Rs.
Newsyslog is a friend to any overworked system administrator and
OpenBSD's install defaults pretty much assure there will be no disk
space problems if the defaults are not changed. Any administrator
who reviews or uses scripts to analyze logs will likely want to
change or increase newsyslog.conf defaults depending on what they
regard as important and disk space availability. Newsyslog allows
logs to be rotated based on both size and or interval since the
previous rotation. Unfortunately it has no provision for rotating
logs at specific times such as midnight or midnight each Sunday or
the first day of the month. For logs that I care about, which
include firewall and web server logs in addition to some custom logs
I create with cron and scripts, I do not use newsyslog but write my
own scripts that include the date as part of the log's filename.
Depending on the log I use one of four date formats: YYMM, YYMMDD,
With ipmon's simple text format, when you wanted to quickly clear a
log while debugging new rules, you could simply truncate the
existing log with "#echo> /var/log/ipflog" at any time. This
corrupts the tcpdump binary format written by pflogd, making the log
unusable. I have not found any way to clear Packet Filter logs
except by stopping pflogd, moving or removing the existing
/var/log/pflog, and restarting pflogd. The following will clear
the current log:
#kill -TERM `cat /var/run/pflogd.pid`;rm pflog; pflogd -d 5
If you want to examine the old logs contents, you'd rename the
log instead of deleting it. The above could easily be put into
a simple shell script, adding a few lines of "cp pflog1 pflog2",
"cp pflog pflog1" to preserve as many "generations" of test
logs as desired. In the above example "-d 5" is used to set the
"delay" to 5 seconds, i.e., force pflogd to write the log to
disk every 5 seconds. For performance reasons, in a production
environment this my not be desirable but if you are using logs
as a debugging aid while developing new rules you don't want to
sit waiting 60 seconds (the default delay) at a time see results.
When pflogd terminates, it will flush the buffer to disk but if
you want to look at the live log, the 5 second delay will facilitate
To view either a saved Packet Filter log or the current one, you'll
need to use tcpdump. The example from the pflogd man page is a
good starting point:
tcpdump -n -e -ttt -r /var/log/pflog
or the similar:
tcpdump -netttvr /var/log/pflog
The second is the same as the first with the options run together and
the "-v", verbose, option added. "-netttvvr" with a second "v" will
provided some additional information for some packets. Once you
decide which format you prefer, I'd suggest putting this in a
shell script named pfl (list) or pfv (view) in /usr/local/sbin or
wherever you put your own system admin scripts. In any case, the
output will also need to be piped to less.
The "-n" option prevents tcpdump from doing a DNS lookup on IP
address in the logs. For firewall log analysis, besides adding an
enormous processing time overhead and adding significant network
traffic, DNS lookups serve no useful purpose. Except for idle
curiosity, who is sending packets is irrelevant. The IP addresses
for most real probes or attacks won't provide a DNS name in the
first place, and when they do, they will normally be some cable or
DSL connection or perhaps a dialup. In many cases these will be
compromised systems. You don't need DNS until after you have
already determined that you have significant activity that represents
an intense or prolonged probe or actual attack. I'd start with
whois or dig on the command line to determine the ISP or owner of
the netblock from which the attack is coming. If you don't provide
an alternate server these should go to your default DNS servers,
i.e., those provided by your ISP. If for some reason these do not
work you'll need to locate a DNS server that returns useful
information, possibly a free web whois service.
The "-e" option is probably essential as this causes the matching
rule set and number, the packet disposition (passed or blocked),
the interface name and the direction, to be included in the output.
The format is rather more verbose than IP Filter's counterparts.
The "-ttt" option causes the abbreviated month, day, and formated time
to start each log line. The default with no "t" option includes the
formated time but not the month or day. There is no year option.
"-tt" prints the unformatted time, i.e. the number of seconds since
midnight, Jan. 1, 1970. "-t" suppresses any time output in the logs.
A single "-v" adds additional information about the packets that may
be useful. A second "v" adds more information and changes the format
of the information generated by the first "v" for some packets. Many
packets have identical output whether one or two v's are used.
Once you settle on a time format and include the "-e" option, the text
output from tcpdump is roughly analogous to the IP Filter log format
as far as the source and destination IP addresses and ports. After
that everything changes. Not counting STATE log entries and certain
ICMP error messages, where IP Filter had a limited amount of information
about the packet in a very standardized format, the Packet Filter /
tcpdump output is highly variable depending on just what the packet
contained. Besides being highly variable, there tends to be a lot
more information, especially with the "v" options.
If you have reason to have Packet Filter logs scroll on a terminal
screen as "tail -f" would do for IP Filter logs, the following command
tcpdump -netttvi pflog0
This is the same as before except "r pflog" has been replaced by
"i pflog0" which causes tcpdump to listen on the pflog0 pseudo
interface instead of reading a file. Whatever is logged from Packet
Filter will appear on the screen.
After the stateful SSH or telnet connection is working,
block in log all
block out log all
replaced the previous default pass everything rules at the end of
the rules file. This will typically break the current telnet or SSH
session when first implemented but a new session can be started if
the previous rule for SSH or telnet is right. If logging is on, the
log file will start to grow. I watched what appeared and set up
rules to allow the traffic which I thought should be allowed.
There is good bit of NTP
(Network Time Protocol for
synchronizing computer times) traffic on my network so NTP related
entries were among the first in the logs. NTP is a UDP protocol
that uses port 123 at both ends. The following rules were added
to allow NTP traffic:
pass in quick on dc0 proto udp from 22.214.171.124/24 port = 123
to 126.96.36.199/24 port = 123
pass out quick on dc0 proto udp from 188.8.131.52/24 port = 123
to 184.108.40.206/24 port = 123
Two rules covers everything both ways and no state information is
needed. It's sometimes considered poor practice to use a source
port as part of a filtering rule because whoever controls the
machine can typically change this at will and you cannot rely on the
expected protocols being on the standard ports. When you control
both ends of a connection there seems to be no reason not to specify
the standard port in the filter rules. If you happen to be running
a version of ntpd that has buffer overflow issues (spring 2001) and
an attacker gains control of one of your machines, no rule that
allows the local machine to perform its normal NTP communication
with the now compromised machine, can prevent the attacker from
taking advantage of the ntpd weakness.
Both Packet Filter and IP Filter can track "stateful" UDP
connections even though UDP is a stateless protocol. I chose not to
use stateful inspection in this case because my web servers
typically also act as NTP servers and thus are clients to some
computers and servers to others, so stateful connections would need to
be set up both directions. Since it's my intent to allow all NTP
packets in both directions there is no reason to examine state.
There is some ICMP traffic in connection with NTP. Also, on a
LAN it's handy to be able to ping other machines and so all local
ICMP traffic is allowed with the following two rules:
pass in quick on dc0 proto icmp from 220.127.116.11/24 to 18.104.22.168/24
pass out quick on dc0 proto icmp from 22.214.171.124/24 to 126.96.36.199/24
To allow the web server to ping anywhere and receive responses
replace the second rule with:
pass out quick on dc0 proto icmp from 188.8.131.52/24 to any keep state
The "keep state" rule will create temporary rules that allow any
response to a ICMP packet sent out the dc0 interface to come back in.
Outside attempts to probe the web server will be denied because the
don't have the state to match the second rule nor the correct address
to match the first and will be blocked when they don't match any other
explicitly allowed traffic.
In these ICMP rules, the second IP address and netmask in the
first rule and the first IP address and netmask in the second rule
could have been specific to the current host 184.108.40.206/32. Below
there are rules that allow specific services to make connections between
individual hosts on the local network and these use host specific
addresses. As both ICMP and NTP traffic are considered routine on our
LAN, these rules are written more generally so that this rule set can
be used on a different hosts without the need to reevaluate every
single rule and IP address.
There are periodic outbound transfers of backup and log data to
an FTP server at 220.127.116.11. To allow these the
following rules are created.
pass out quick on dc0 proto tcp from 18.104.22.168/32 port > 1024
to 22.214.171.124/32 port = 21 flags S keep state
pass in quick on dc0 proto tcp from 126.96.36.199 port = 20
to 188.8.131.52/32 port > 1024 flags S keep state
These are typical TCP high port to low port connections. The
standard or active FTP complication is added to the mix. The
client, in this case the local web server, makes a control
connection to the ftp server on port 21. For data transfer, the ftp
server then opens a connection from port 20 to a high numbered
port on the client that has been communicated via the control
connection. Here the "keep state" saves on the number of rules by
eliminating the need to specify the reverse connections.
After setting up these rules, the FTP transfers began working but I
found blocked high port to high port connections from the client to
the ftp server. These consistently came between the opening of the
control connection and the data connection. Apparently this
combination of client and server were defaulting to passive mode FTP
and when that failed reverted to active mode. After adding a rule
(next below) to allow connections from high ports on the web server
client to high ports on the ftp server, the active connections from
the ftp server's port 20 to high client ports stopped:
pass out quick on dc0 proto tcp from 184.108.40.206/32 port > 1024
to 220.127.116.11/32 port > 1024 flags S keep state
Allowing data to be transferred to the web server from the
management / development workstation via FTP required another set
of rules in which the web server was also the ftp server,
reversing the role performed by the web server in the
previous set of rules:
pass in quick on dc0 proto tcp from 18.104.22.168/32 port > 1024
to 22.214.171.124/32 port = 21 flags S keep state
pass out quick on dc0 proto tcp from 126.96.36.199 port = 20
to 188.8.131.52/32 port > 1024 flags S keep state
Note that all the FTP related rule are to and from specific machines
("/32") which are the only ones on the LAN to perform these
functions. After all rules to allow the various adminstrative
traffic were in place, a rule to enable the computer to serve as a
public web server was set up.
pass in quick on dc0 proto tcp from any
to 184.108.40.206/32 port = 80 flags S keep state
The preceding rule will allow new web connections from any computer
using any port to the local computer's port 80, standard HTTP port,
and all return traffic.
Finally, for IP Filter only, some standard rules from another
firewall are added to block all source routed packets:
block in log quick on dc0 all with opt lsrr
block in log quick on dc0 all with opt ssrr
The previous rules will cause syntax errors on Packet Filter.
These options are unnecessary as Packet Filter blocks all
IP packets with any options set, including all source routed
packets. A new "allow-opts" Packet Filter option will allow
packets with options set; this is less safe than the default.
Also the keywords "head" and "group" which were allowed by
IP Filter are gone and will cause errors with Packet Filter.
Port Scans With Nmap
Because the test web server with the
sample IP Filter ruleset has an active
publicly accessible web server, there is no way to hide the
existence of this computer. The defined rules do however hide
nearly all other information regarding the test web server. Nmap was
used from another computer on the same network segment to scan the
test web server. (This testing has not been repeated with OpenBSD
3.0 and Packet Filter.) No other firewall or protective device
intervened in any way. With the IP Filter rules turned off, nmap
accurately and almost immediately identifies all open ports
including NTP if a UDP scan is performed. Besides the public web
server, ftpd, telnetd and sshd servers were available. Once the rules
are turned on, the only port that nmap can reliably find is the
public web server on port 80.
A standard nmap ping scan (-sP) concludes the "Host seems down."
and suggests trying "-P0". "-sP -P0" correctly concludes the
host is up. Standard "-sT" and "-sS" scans report all TCP ports
except 80 as blocked; nmap cannot tell its user whether any other
TCP services are actually running or not.
A "-sU" UDP scan misses that 123 is open because nmap normally
uses high ports to scan from. As the rules only allow traffic
from 123 as well as to 123 the nmap probe is blocked. Without
the source port restriction nmap would likely find the open port.
Surprisingly, even using the source port option "-g 123", nmap
still fails to find the open NTP port. Attempts to limit the
range of ports searched, "-sU -p 123-127" repeatedly result in
false positive open port reports for all scanned ports.
This occurs whether or not a source port is specified or not and
whether a useful (123) or arbitrary high port is specified. In
the meantime, the computer from which nmap is running has an
active NTP connection with the test web server that's being probed.
The nmap machine was actually a time server for the test web
server but no combination of command line settings that I could
find, allowed nmap to find this ntpd server once the IP Filter
rules were active.
In short, nmap, generally acknowledged as the most sophisticated
network scanner available, is not able to learn more about the
test web server when it's protected by strong IP Filter rules
than any ordinary user with ping and a web browser could. Its
able to learn exactly what the IP Filter rules on the test web
server allow the machine from which nmap is running to know. Its
attempt at OS finger printing completely fails, listing 9
possibilities, none of which are even in the right OS family.
With the IP Filter rules disabled, nmap, determines the right OS
but is off by three version numbers.
Packet Filter or IP Filter, running on a single host, can provide
the same kind of strong firewall protection to that host, that one
would normally expect from a dedicated firewall using Packet Filter
or IP Filter.
If this were a standard firewall, additional rules to block all
the non routable address such as 10., 192.168., 127., etc. would
be added to prevent these invalid packets from coming in or
escaping out to the Internet. Most are already blocked by the
existing rules because there are no rules allowing them except
the "any" to the web server at port 80. Doing a dozen or so rules
that should already be in place on another firewall to protect
one port seems excessive.
The preceding paragraph makes a statement that is an assumption
that applies throughout the Hardening OpenBSD Internet Servers
section. It is that the steps discussed are being applied
to a machine that will become a dedicated firewall or be behind
a dedicated firewall.
A Single Home Machine
This does not need to be the case. There are at least two scenarios
where using techniques discussed here without a dedicated firewall
makes sense. One is a single home machine, perhaps as a dual or
multi-boot system with other operating systems. I expect that the
large majority of persons with an OpenBSD system at home have
multiple machines but there must be some with only one. There are
now several million active cable and DSL connections. The large
majority of these are connected to Windows machines. The most secure
of these are using Zone Alarm or another "personal" firewall. There
is no reason that a single OpenBSD machine using Packet Filter or IP
Filter should not be more secure.
If you have two or more machines, I'd strongly recommend putting a 486
or low end Pentium to use as a dedicated firewall with NAT. Even a 486
should be quite adequate for a cable or DSL connection. It's much
easier to develop a tight rule set on dedicated firewall where there
is clear cut distinction between inside and outside than to set up
comparably tight rules on a machine where you want secure connections
with the outside world and fairly open connections between local
machines. Using dynamic NAT, where multiple internal IP addresses are
dynamically translated into a single valid external IP address, on a
dedicated firewall allows flexibility not possible otherwise. You can
put pretty much whatever you want behind NAT and those on the outside
are not likely to ever know.
Regarding services, unless this is purely a learning experiment and
the machine has no valuable or sensitive data, I'd stay away from SMTP
(accepting outside connections) and DNS given Sendmail and Bind's
histories and get these services from your ISP. I'd run web and ftp
servers only if they were part of the reason for having an Internet
connection. Running your own web server would let you do and learn
things not feasible on any hosted site. The more adventurous your web
experiments become, the more likely they are to expose your machine to
risks, at least if the web site is public or
semi-public. In the firewall rules, I'd block everything inbound,
unless you had a public web or ftp server.
Outbound, I'd make everything stateful and seriously think about
blocking everything except specific services you actually know you
use. If your IP address is 220.127.116.11 and your network
interface is xx0 the fast way to let anything out while
preserving some degree of safety is:
A Small Organization Web Server
pass out quick on xx0 proto tcp from 18.104.22.168/32 to any keep state
pass out quick on xx0 proto udp from 22.214.171.124/32 to any keep state
pass out quick on xx0 proto icmp from 126.96.36.199/32 to any keep state
block in quick on xx0 all
I'd be extremely wary of active FTP and would not use the
following rule though it's about as good as you can do to enable
active FTP to any potential site:
pass in quick on xx0 proto tcp from any port = 20 to 188.8.131.52/32 port > 1024 flags S keep state
This is an example of the kind of poor rule that depends on
trusting a source port. Any attacker who controls his own machine
can set the source port for almost anything to 20. It leaves
Xwindow and other software that uses higher ports exposed.
Instead, I'd wait until I encounterd a site that had something I
really wanted and did not support passive FTP and then create a
custom rule for that site. I'd also clear the rule later unless I
expected to reuse the site.
The other time that an Internet connected computer, not behind a
dedicated firewall, might make sense can occur at a small
organization. Such an organization may already have a permanent
Internet connection behind dedicated hardware. Examples would
include a Novell LAN behind an Instant Internet or any LAN behind
other Internet appliances. After evaluating the alternatives, it's
decided that available hosting options are not acceptable, and
that the organization needs to locally host its own web site.
Placing a public web server in the same network space as the
LAN, would be a very poor choice, since the LAN would be
unprotected if the web server were compromised. If the security
device supported three or more network segments, placing the
new web server on separate segment would make the best sense.
Assuming the security device lacks the ability to protect both the
LAN and the new web server while keeping them separate, it makes
better sense to put the web server outside the security device. If
there are two or more computers outside the security device, a
dedicated firewall will almost surely be appropriate. If there is
only one computer outside, running Packet Filter or IP Filter on the
web server, might make better sense than setting up a separate
machine as a dedicated firewall.
If there is only one outside computer and no dedicated firewall, then
taking all the steps discussed in these pages makes good sense. The
computer will be exposed to a full time Internet connection and
advertise its presence with only its own resources to protect it. I
would definitely take advantage of extensive file removal, immutable
files, including on the main system executable directories, and
security level 2. As the system might have content developers who may
not be counted on for strong passwords, very close attention should be
paid to file and directory permissions and users' group memberships.
Assigned passwords might be appropriate.
Clearly TCP Wrappers is not sufficient and Packet Filter or IP
Filter with a tight custom ruleset, a must. The rule set would look
a lot like the example above. Additional rules to cover all the non
routable addresses not actually used locally, should be included.
If the inner LAN is NATed, the web server would only see traffic
from outer interface of the router, firewall, NAT, machine and
not any inside LAN addresses.
With the right firewall rule set, an attack through the web
application itself should be the only avenue available to the
whole Internet. With a public web server, this path cannot be
closed. Thus, close attention should also be paid to the
security features of the web server, presumably Apache. Any
capabilities not actually used by the web applications should be
removed. Custom compilation including all necessary and only
necessary modules would be a good idea. Even if unwanted modules
can be excluding by editing httpd.conf, it's more secure if
unwanted modules are not be available for loading.
An IP Filter Bug
Returning to the more general issues related to Packet Filter and IP
Filter, it should be clear that these firewalls are much more
powerful and flexible as a network access control tools than TCP
Wrappers. Should every OpenBSD computer connected to the Internet be
running Packet Filter or IP Filter, even if it is behind a strong
firewall with a custom rule set? Considering that one of the primary
arguments in favor of using firewalls is the ability to centralize a
major part of the security policy and the complexity Packet Filter
and IP Filter rules compared to TCP Wrappers, for most sites, I
expect the answer is no.
In some circumstances, two layers of IP Filter or other single firewall
can actually provide less security than TCP Wrappers behind IP
Consider the early April 2001, IP Filter fragment
caching vulnerability. This was described as a "serious
vulnerability." A careful reading of the details of the problem
suggest that it would be a major technical challenge to actually use
this in real life situation. In essence, if a stateful session
could be established with a computer, for example a public web
server, IP packets can be constructed that will allow the computer
with the connection to the web server to send those packets to any
other port on the web server or even switch UDP with TCP traffic.
With the right packets, firewall rules prohibiting connections to
other ports or with other protocols would be ignored.
Constructing the necessary packets was not a trivial technical
task. To actually do anything with the vulnerability, however,
also requires that the public web server be running other
services that have some vulnerability and that an exploit for
the second vulnerability exists and can be wrapped inside the
packets used to bypass the firewall rules. I've seen nothing
to suggest there were any real security breaches as a result
of the caching vulnerability.
My understanding of the vulnerability is that if a public
web server can be reached through an IP Filter firewall,
successfully constructing the right packets to take advantage
of the vulnerability would allow these packets to pass through
a second or third IP Filter firewall as easily as the first.
This bug or any other bug in IP Filter that allows
circumventing the firewall rules could very well negate any
value of stacked IP Filters. Since the vulnerability allows
reaching services that are by definition not public (they
were believed to be protected by IP Filter rules) if a second
IP Filter also denied access to the service, TCP Wrappers
placed at the same location would also be expected to deny
access to the service. The second IP Filter is likely to
have the same bugs as the first but TCP Wrappers being a
totally different product almost certainly would not.
Unless the attacking computer happened to be in TCP Wrappers
hosts.allow file, the attack would not be expected to be
How Much Is Too Much
In the case of bugs in Packet Filter or IP Filter, TCP Wrappers will
almost certainly provide better protection than a second layer of the
outer firewall. Of course there is nothing technical to stop TCP
Wrappers from running on the same computer as the second copy of
Packet Filter or IP Filter. Still more protection but at what cost.
Each of these protections makes a network more difficult and expensive
to change. Making change more difficult is the entire point of many
of these measures. Thus when an event such as hardware failure
requires rapid change, this is hindered. Suitable replacement
hardware might be available but the more customized each machine's
rule sets and configuration, the more work will be required to make a
Everything has a point of diminishing returns. Defense in depth
means taking multiple approaches to protecting resources, not
duplicating effort. No one puts two different makes of firewalls
serially in line with each other. Superficially this is more
secure but it doubles the work for at best a marginal security
improvement. In practice it's likely to hurt security. Staff
can't learn either firewall as well as they would if there were
only one. Unless at least one is a bridged firewall, back to back
firewalls create routing issues. It creates two back to back,
single points of failure. If both were routing firewalls,
removing one would require changes to the other before traffic
could flow. If either firewall is down the network is
disconnected. Instead of being careful with one rule set, staff
may rush changes with two because there is twice the work or they
may confuse syntax between different rule sets.
No one should confuse these hypothetical back to back firewalls with
the real and common situation where an outer firewall protects a DMZ
with public Internet servers and a second firewall, likely of the
same make as the first but with different rules and probably
supporting NAT, protects the LAN and DMZ from each other.
A proliferation of IP address or port restrictions around a network
looks more like stacking firewalls than a carefully planned mixture
of backups, user and password management, disabled services,
firewall, file access controls, etc., which complement but don't
duplicate each other. For myself, taking a few minutes to change a
few bytes in inetd.conf and add half a dozen short lines to
hosts.allow and deny is worth an extra layer of protection for
critical services on a few hosts in a DMZ. Widespread use of Packet
Filter or IP Filter on individual hosts already protected by a
strong custom rule set on a dedicated Packet Filter or IP Filter
firewall is overkill for most situations. Still, it is worth
knowing how to setup a firewall on a computer with a single
network interface for those situations where it is appropriate.
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