Update to reflect new ipfw syntax.
This commit is contained in:
Alexander Langer
parent
705a564dad
commit
4255ccc7df
Notes:
svn2git
2020-12-08 03:00:23 +00:00
svn path=/head/; revision=377
1 changed files with 141 additions and 143 deletions
|
|
@ -1,9 +1,9 @@
|
||||||
<!-- $Id: firewalls.sgml,v 1.4 1996-01-31 19:02:57 mpp Exp $ -->
|
<!-- $Id: firewalls.sgml,v 1.5 1996-06-25 03:15:17 alex Exp $ -->
|
||||||
<!-- The FreeBSD Documentation Project -->
|
<!-- The FreeBSD Documentation Project -->
|
||||||
|
|
||||||
<sect><heading>Firewalls<label id="firewalls"></heading>
|
<sect><heading>Firewalls<label id="firewalls"></heading>
|
||||||
|
|
||||||
<p><em>Contributed by &a.gpalmer;.<newline>4th of October 1995</em>
|
<p><em>Contributed by &a.gpalmer; and &a.alex;.</em>
|
||||||
|
|
||||||
Firewalls are an area of increasing interest for people who are
|
Firewalls are an area of increasing interest for people who are
|
||||||
connected to the Internet, and are even finding applications on
|
connected to the Internet, and are even finding applications on
|
||||||
|
|
@ -96,7 +96,7 @@ can be set up so that you can limit which users can talk to which
|
||||||
destination machine. Again, what facilities are available depends
|
destination machine. Again, what facilities are available depends
|
||||||
largely on what proxy software you choose.
|
largely on what proxy software you choose.
|
||||||
|
|
||||||
<sect1><heading>What does <tt>IPFW</tt> allow me to do?</heading>
|
<sect1><heading>What does IPFW allow me to do?</heading>
|
||||||
|
|
||||||
<p><tt>IPFW</tt>, the software supplied with FreeBSD, is a packet
|
<p><tt>IPFW</tt>, the software supplied with FreeBSD, is a packet
|
||||||
filtering and accounting system which resides in the kernel, and has a
|
filtering and accounting system which resides in the kernel, and has a
|
||||||
|
|
@ -117,7 +117,7 @@ incoming and outgoing connections. This is a special case of the more
|
||||||
general use of <tt>IPFW</tt>, and the same commands and techniques
|
general use of <tt>IPFW</tt>, and the same commands and techniques
|
||||||
should be used in this situation.
|
should be used in this situation.
|
||||||
|
|
||||||
<sect1><heading>Enabling <tt>IPFW</tt> on FreeBSD</heading>
|
<sect1><heading>Enabling IPFW on FreeBSD</heading>
|
||||||
|
|
||||||
<p>As the main part of the <tt>IPFW</tt> system lives in the kernel, you will
|
<p>As the main part of the <tt>IPFW</tt> system lives in the kernel, you will
|
||||||
need to add one or more options to your kernel configuration
|
need to add one or more options to your kernel configuration
|
||||||
|
|
@ -137,35 +137,57 @@ packets through <tt>syslogd(8)</tt>. Without this option, even if you
|
||||||
specify that packets should be logged in the filter rules, nothing
|
specify that packets should be logged in the filter rules, nothing
|
||||||
will happen.
|
will happen.
|
||||||
|
|
||||||
<tag/options IPACCT/ Turns on the IP accounting facilities.
|
<tag/options "IPFIREWALL_VERBOSE_LIMIT=10"/ Limits the number of
|
||||||
|
packets logged through <tt>syslogd(8)</tt> on a per entry basis.
|
||||||
|
You may wish to use this option in hostile environments in which
|
||||||
|
you want to log firewall activity, but do not want to be open to
|
||||||
|
a denial of serivce attack via syslog flooding.
|
||||||
|
|
||||||
|
<p>When a chain entry reaches the packet limit specified, logging
|
||||||
|
is turned off for that particular entry. To resume logging, you
|
||||||
|
will need to reset the associated counter using the <tt>ipfw(8)</tt>
|
||||||
|
utility:
|
||||||
|
|
||||||
|
<tscreen><verb>
|
||||||
|
ipfw zero 4500
|
||||||
|
</verb></tscreen>
|
||||||
|
|
||||||
|
Where 4500 is the chain entry you wish to continue logging.
|
||||||
|
|
||||||
</descrip>
|
</descrip>
|
||||||
|
|
||||||
<sect1><heading>Configuring <tt>IPFW</tt></heading>
|
Previous versions of FreeBSD contained an <tt>IPFIREWALL_ACCT</tt>
|
||||||
|
option. This is now obsolete as the firewall code automatically
|
||||||
|
includes accounting facilities.
|
||||||
|
|
||||||
|
<sect1><heading>Configuring IPFW</heading>
|
||||||
|
|
||||||
<p>The configuration of the <tt>IPFW</tt> software is done through the
|
<p>The configuration of the <tt>IPFW</tt> software is done through the
|
||||||
<tt>ipfw(8)</tt> utility. The syntax for this command looks
|
<tt>ipfw(8)</tt> utility. The syntax for this command looks
|
||||||
quite complicated, but it is relatively simple once you understand
|
quite complicated, but it is relatively simple once you understand
|
||||||
its structure.
|
its structure.
|
||||||
|
|
||||||
<p>There are currently two different command line formats for the
|
<p>There are currently four different command categories used by the
|
||||||
utility, depending on what you are doing. The first form is used when
|
utility: addition/deletion, listing, flushing, and clearing.
|
||||||
adding/deleting entries from the firewall or accounting chains, or
|
Addition/deletion is used to build the rules that control how packets
|
||||||
when clearing the counters for an entry on the accounting chain. The
|
are accepted, rejected, and logged. Listing is used to examine the
|
||||||
second form is used for more general actions, such as flushing the
|
contents of your rule set (otherwise known as the chain) and packet
|
||||||
rule chains, listing the rule chains or setting the default policy.
|
counters (accounting). Flushing is used to remove all entries from
|
||||||
|
the chain. Clearing is used to zero out one or more accounting
|
||||||
|
entries.
|
||||||
|
|
||||||
<sect2><heading>Altering the <tt>IPFW</tt> rules</heading>
|
<sect2><heading>Altering the IPFW rules</heading>
|
||||||
|
|
||||||
<p>The syntax for this form of the command is:
|
<p>The syntax for this form of the command is:
|
||||||
<tscreen>
|
<tscreen>
|
||||||
ipfw [-n] <em>command</em> <em>action</em> <em>protocol</em> <em>addresses</em>
|
ipfw [-N] <em>command</em> [index] <em>action</em>
|
||||||
|
<em>protocol</em> <em>addresses</em> [options]
|
||||||
</tscreen>
|
</tscreen>
|
||||||
|
|
||||||
<p>There is one valid flag when using this form of the command:
|
<p>There is one valid flag when using this form of the command:
|
||||||
|
|
||||||
<descrip>
|
<descrip>
|
||||||
<tag/-n/Do not attempt to resolve given addresses.
|
<tag/-N/Resolve addresses and service names.
|
||||||
</descrip>
|
</descrip>
|
||||||
|
|
||||||
The <em>command</em> given can be shortened to the shortest unique
|
The <em>command</em> given can be shortened to the shortest unique
|
||||||
|
|
@ -173,69 +195,39 @@ form. The valid <em>commands</em> are:
|
||||||
|
|
||||||
<descrip>
|
<descrip>
|
||||||
|
|
||||||
<tag/addfirewall/Add an entry to the firewall rule list
|
<tag/add/Add an entry to the firewall/accounting rule list
|
||||||
|
|
||||||
<tag/delfirewall/Delete an entry from the firewall rule list
|
<tag/delete/Delete an entry from the firewall/accounting rule list
|
||||||
|
|
||||||
<tag/addaccounting/Add an entry to the accounting rule list
|
|
||||||
|
|
||||||
<tag/delaccounting/Delete an entry from the accounting rule list
|
|
||||||
|
|
||||||
<tag/clraccounting/Clear the counters for an accounting rule entry.
|
|
||||||
|
|
||||||
</descrip>
|
</descrip>
|
||||||
|
|
||||||
If no command is given, it will default <bf>addfirewall</bf> or
|
Previous versions of <tt>IPFW</tt> used separate firewall and
|
||||||
<bf>addaccounting</bf> depending on the arguments given.
|
accounting entries. The present version provides packet accounting
|
||||||
|
with each firewall entry.
|
||||||
|
|
||||||
<p>Currently, the firewall support in the kernel applies a set of
|
<p>If an <tt>index</tt> value is supplied, it used to place the entry
|
||||||
weights to the rule being added. This means that the rules will
|
at a specific point in the chain. Otherwise, the entry is placed at
|
||||||
<em>not</em> be evaluated in the order that they are given to the
|
the end of the chain at an index 100 greater than the last chain
|
||||||
system. The weighting system is designed so that rules which are very
|
entry (this does not include the default policy, rule 65535, deny).
|
||||||
specific are evaluated first, and rules which cover very large ranges
|
|
||||||
are evaluated last. In other words, a rule which applies to a specific
|
|
||||||
port on a specific host will have a higher priority than a rule which
|
|
||||||
applies to that same port, but on a range of hosts, or that host on a
|
|
||||||
range of ports.
|
|
||||||
|
|
||||||
<p>The weighting system is not perfect, however, and can lead to
|
Valid <em>actions</em> are:
|
||||||
problems. The best way to see what order it has put your rules in is
|
|
||||||
to use the <bf>list</bf> command, as that command lists the rules in
|
|
||||||
the order that they are evaluated, not the order that they were fed to
|
|
||||||
the system.
|
|
||||||
|
|
||||||
<p>The <em>actions</em> available depend on which rule chain the
|
|
||||||
entry is destined for. For the firewall chain, valid
|
|
||||||
<em>actions</em> are:
|
|
||||||
|
|
||||||
<descrip>
|
<descrip>
|
||||||
|
|
||||||
<tag/reject/Drop the packet, and send an ICMP HOST_UNREACHABLE or
|
<tag/reject/Drop the packet, and send an ICMP host or port
|
||||||
ICMP PORT_UNREACHABLE (as appropriate) packet to the source.
|
unreachable (as appropriate) packet to the source.
|
||||||
|
|
||||||
<tag/lreject/As <bf>reject</bf>, but also log the packet details.
|
<tag/allow/Pass the packet on as normal.
|
||||||
|
|
||||||
<tag/deny/Drop the packet.
|
<tag/deny/Drop the packet. The source is not notified via an ICMP
|
||||||
|
message (thus it appears that the packet never arrived at the
|
||||||
|
destination).
|
||||||
|
|
||||||
<tag/ldeny/As <bf>deny</bf>, but also log the packet details.
|
<tag/count/Update packet counters but do not allow/deny the packet
|
||||||
|
based on this rule. The search continues with the next chain entry.
|
||||||
|
|
||||||
<tag/log/Log the packets details and pass it on as normal.
|
<tag/reject/Discard the packet, sending an ICMP host/port unreachable
|
||||||
|
message back to the source.
|
||||||
<tag/accept/Pass the packet on as normal.
|
|
||||||
|
|
||||||
<tag/pass/Synonym for <bf>accept</bf>.
|
|
||||||
|
|
||||||
</descrip>
|
|
||||||
|
|
||||||
For the accounting chain, valid <em>actions</em> are:
|
|
||||||
|
|
||||||
<descrip>
|
|
||||||
|
|
||||||
<tag/single/Count packets matching the address specifier.
|
|
||||||
|
|
||||||
<tag/bidirectional/Count packets matching the address specifier, and
|
|
||||||
also packets traveling in the opposite direction (i.e. those going
|
|
||||||
from ``destination'' to ``source'').
|
|
||||||
|
|
||||||
</descrip>
|
</descrip>
|
||||||
|
|
||||||
|
|
@ -254,30 +246,22 @@ The <em>protocols</em> which can be specified are:
|
||||||
|
|
||||||
<tag/udp/Matches UDP packets
|
<tag/udp/Matches UDP packets
|
||||||
|
|
||||||
<tag/syn/Matches the TCP SYN (synchronization) packet used during TCP
|
|
||||||
connection negotiation. You can use this to block ``incoming'' TCP
|
|
||||||
connections, but allow ``outgoing'' TCP connections.
|
|
||||||
</descrip>
|
</descrip>
|
||||||
|
|
||||||
<p>The <em>address</em> specification is:
|
<p>The <em>address</em> specification is:
|
||||||
<tscreen>
|
<tscreen>
|
||||||
[<bf>from</bf> <<em>address/mask</em>>[<em>port</em>]] [<bf>to</bf>
|
<bf>from</bf> <<em>address/mask</em>>[<em>port</em>] <bf>to</bf>
|
||||||
<<em>address/mask</em>>[<em>port</em>]] [<bf>via</bf> <<em>interface</em>>]
|
<<em>address/mask</em>>[<em>port</em>&rsqb [<bf>via</bf> <<em>interface</em>>]
|
||||||
</tscreen>
|
</tscreen>
|
||||||
|
|
||||||
<p>You can only specify <em>port</em> in conjunction with
|
<p>You can only specify <em>port</em> in conjunction with
|
||||||
<em>protocols</em> which support ports (UDP, TCP and SYN).
|
<em>protocols</em> which support ports (UDP and TCP).
|
||||||
|
|
||||||
<p>The order of the <bf>from</bf>, <bf>to</bf>, and
|
|
||||||
<bf>via</bf> keywords is unimportant. Any of them can be omitted,
|
|
||||||
in which case a default entry for that keyword will be supplied which
|
|
||||||
matches everything.
|
|
||||||
|
|
||||||
<p>The <bf>via</bf> is optional and may specify the IP address or
|
<p>The <bf>via</bf> is optional and may specify the IP address or
|
||||||
domain name of a local IP interface, or an interface name (e.g.
|
domain name of a local IP interface, or an interface name (e.g.
|
||||||
<tt>ed0</tt>) to match only packets coming through this interface. The
|
<tt>ed0</tt>) to match only packets coming through this interface.
|
||||||
keyword <bf>via</bf> can be substituted by <bf>on</bf>, for
|
Interface unit numbers can be specified with an optional wildcard.
|
||||||
readability reasons.
|
For example, <tt>ppp*</tt> would match all kernel PPP interfaces.
|
||||||
|
|
||||||
<p>The syntax used to specify an <tt><address/mask></tt> is:
|
<p>The syntax used to specify an <tt><address/mask></tt> is:
|
||||||
<tscreen>
|
<tscreen>
|
||||||
|
|
@ -310,14 +294,50 @@ to specify either a single port or a list of ports, or
|
||||||
<tscreen><verb>
|
<tscreen><verb>
|
||||||
port:port
|
port:port
|
||||||
</verb></tscreen>
|
</verb></tscreen>
|
||||||
to specify a range of ports. The name of a service (from
|
to specify a range of ports.
|
||||||
<em>/etc/services</em>) can be used instead of a numeric port value.
|
|
||||||
|
|
||||||
<sect2><heading>Listing/flushing the <tt>IPFW</tt> rules</heading>
|
<p>The <em>options</em> available are:
|
||||||
|
|
||||||
|
<descrip>
|
||||||
|
|
||||||
|
<tag/frag/Matches if the packet is the first fragment of the datagram.
|
||||||
|
|
||||||
|
<tag/in/Matches if the packet is on the way in.
|
||||||
|
|
||||||
|
<tag/out/Matches if the packet is on the way out.
|
||||||
|
|
||||||
|
<tag/ipoptions <em>spec</em>/Matches if the IP header contains the
|
||||||
|
comma separated list of options specified in <em>spec</em>. The
|
||||||
|
supported list of IP options are: <bf>ssrr</bf> (strict source route),
|
||||||
|
<bf>lsrr</bf> (loose source route), <bf>rr</bf> (record packet route),
|
||||||
|
and <bf>ts</bf> (timestamp). The absence of a particular option may
|
||||||
|
be denoted with a leading '!'.
|
||||||
|
|
||||||
|
<tag/established/Matches if the packet is part of an already established
|
||||||
|
TCP connection (i.e. it has the RST or ACK bits set).
|
||||||
|
|
||||||
|
<tag/setup/Matches if the packet is an attempt to establish a TCP connection
|
||||||
|
(the SYN bit set is set but the ACK bit is not).
|
||||||
|
|
||||||
|
<tag/tcpflags <em>flags</em>/Matches if the TCP header contains
|
||||||
|
the comma separated list of <em>flags</em>. The supported flags
|
||||||
|
are <bf>fin</bf>, <bf>syn</bf>, <bf>rst</bf>, <bf>psh</bf>, <bf>ack</bf>,
|
||||||
|
and <bf>urg</bf>. The absence of a particular flag may be indicated
|
||||||
|
by a leading '!'.
|
||||||
|
|
||||||
|
<tag/icmptypes <em>types</em>/Matches if the ICMP type is present in
|
||||||
|
the list <em>types</em>. The list may be specified as any combination
|
||||||
|
of ranges and/or individual types separated by commas. Commonly used
|
||||||
|
ICMP types are: <bf>0</bf> echo reply (ping reply), <bf>5</bf>
|
||||||
|
redirect, and <bf>8</bf> echo request (ping request).
|
||||||
|
|
||||||
|
</descrip>
|
||||||
|
|
||||||
|
<sect2><heading>Listing the IPFW rules</heading>
|
||||||
|
|
||||||
<p>The syntax for this form of the command is:
|
<p>The syntax for this form of the command is:
|
||||||
<tscreen>
|
<tscreen>
|
||||||
ipfw [-ans] <em>command</em> [<em>argument</em>]
|
ipfw [-atN] l
|
||||||
</tscreen>
|
</tscreen>
|
||||||
|
|
||||||
<p>There are three valid flags when using this form of the command:
|
<p>There are three valid flags when using this form of the command:
|
||||||
|
|
@ -325,56 +345,39 @@ ipfw [-ans] <em>command</em> [<em>argument</em>]
|
||||||
<descrip>
|
<descrip>
|
||||||
|
|
||||||
<tag/-a/While listing, show counter values. This option is the only
|
<tag/-a/While listing, show counter values. This option is the only
|
||||||
way to see accounting counters. Works only with <bf>-s</bf>.
|
way to see accounting counters.
|
||||||
|
|
||||||
<tag/-n/Do not attempt to resolve given addresses.
|
<tag/-t/Display the last match times for each chain entry. The time
|
||||||
|
listing is incompatible with the input syntax used by the
|
||||||
|
<tt>ipfw(8)</tt> utility.
|
||||||
|
|
||||||
<tag/-s/Use short listing form. This should be used with <bf>-a</bf>
|
<tag/-N/Do not attempt to resolve given addresses.
|
||||||
to see accounting counters. The short form listing is incompatible
|
|
||||||
with the input syntax used by the <tt>ipfw(8)</tt> utility.
|
|
||||||
|
|
||||||
</descrip>
|
</descrip>
|
||||||
|
|
||||||
The <em>command</em> given can be shortened to the shortest unique
|
<sect2><heading>Flushing the IPFW rules</heading>
|
||||||
form. The valid <em>commands</em> are:
|
|
||||||
|
|
||||||
<descrip>
|
<p>The syntax for flushing the chain is:
|
||||||
|
<tscreen>
|
||||||
|
ipfw flush
|
||||||
|
</tscreen>
|
||||||
|
|
||||||
<tag/list/List the chain rule entries. Unless the <bf>-s</bf> flag is
|
<p>This causes all entries in the firewall chain to be removed except
|
||||||
given, the format is compatible with the command line syntax.
|
the fixed default policy enforced by the kernel (index 65535). Use
|
||||||
|
caution when flushing rules, the default deny policy will leave your
|
||||||
|
system cut off from the network until allow entries are added to the
|
||||||
|
chain.
|
||||||
|
|
||||||
<tag/flush/Flush the chain rule entries.
|
<sect2><heading>Clearing the IPFW packet counters</heading>
|
||||||
|
|
||||||
<tag/zero/Clear counters for the entire accounting chain.
|
<p>The syntax for clearing one or more packet counters is:
|
||||||
|
<tscreen>
|
||||||
|
ipfw zero [index]
|
||||||
|
</tscreen>
|
||||||
|
|
||||||
<tag/policy/Set or display the default policy for the firewall
|
<p>When used without an <em>index</em> argument, all packet counters
|
||||||
code. Without an argument, the current policy will be displayed.
|
are cleared. If an <em>index</em> is supplied, the clearing operation
|
||||||
|
only affects a specific chain entry.
|
||||||
</descrip>
|
|
||||||
|
|
||||||
The <bf>list</bf> and <bf>flush</bf> commands may optionally be passed
|
|
||||||
an <em>argument</em> to specify which chain to flush. Valid arguments are:
|
|
||||||
|
|
||||||
<descrip>
|
|
||||||
|
|
||||||
<tag/firewall/The packet filter chain.
|
|
||||||
|
|
||||||
<tag/accounting/The accounting chain.
|
|
||||||
|
|
||||||
</descrip>
|
|
||||||
|
|
||||||
<p>The <bf>policy</bf> command can be given one of two arguments:
|
|
||||||
|
|
||||||
<descrip>
|
|
||||||
|
|
||||||
<tag/accept/If a packet is not matched by any rule, pass it on.
|
|
||||||
|
|
||||||
<tag/deny/If a packet is not matched by any rule, do not pass it on.
|
|
||||||
|
|
||||||
</descrip>
|
|
||||||
|
|
||||||
As usual, the arguments can be shortened to the shortest unique form
|
|
||||||
(in this case, the first letter).
|
|
||||||
|
|
||||||
<sect1><heading>Example commands for ipfw</heading>
|
<sect1><heading>Example commands for ipfw</heading>
|
||||||
|
|
||||||
|
|
@ -383,7 +386,7 @@ As usual, the arguments can be shortened to the shortest unique form
|
||||||
<bf>nice.people.org</bf> by being forwarded by the router:
|
<bf>nice.people.org</bf> by being forwarded by the router:
|
||||||
|
|
||||||
<tscreen><verb>
|
<tscreen><verb>
|
||||||
ipfw addf deny tcp from evil.hacker.org to nice.people.org telnet
|
ipfw add deny tcp from evil.hacker.org to nice.people.org 23
|
||||||
</verb></tscreen>
|
</verb></tscreen>
|
||||||
|
|
||||||
<p>The next example denies and logs any TCP traffic from the entire
|
<p>The next example denies and logs any TCP traffic from the entire
|
||||||
|
|
@ -391,7 +394,7 @@ ipfw addf deny tcp from evil.hacker.org to nice.people.org telnet
|
||||||
machine (any port).
|
machine (any port).
|
||||||
|
|
||||||
<tscreen><verb>
|
<tscreen><verb>
|
||||||
ipfw addf ldeny tcp from evil.hacker.org/24 to nice.people.org
|
ipfw add deny log tcp from evil.hacker.org/24 to nice.people.org
|
||||||
</verb></tscreen>
|
</verb></tscreen>
|
||||||
|
|
||||||
If you do not want people sending X sessions to your internal network
|
If you do not want people sending X sessions to your internal network
|
||||||
|
|
@ -399,23 +402,27 @@ If you do not want people sending X sessions to your internal network
|
||||||
filtering:
|
filtering:
|
||||||
|
|
||||||
<tscreen><verb>
|
<tscreen><verb>
|
||||||
ipfw addf deny syn to my.org/28 6000
|
ipfw add deny setup from any to my.org/28 6000
|
||||||
</verb></tscreen>
|
</verb></tscreen>
|
||||||
|
|
||||||
To allow access to the SUP server on <bf>sup.FreeBSD.ORG</bf>, use the
|
To allow access to the SUP server on <bf>sup.FreeBSD.ORG</bf>, use the
|
||||||
following command:
|
following command:
|
||||||
|
|
||||||
<tscreen><verb>
|
<tscreen><verb>
|
||||||
ipfw addf accept syn to sup.FreeBSD.ORG supfilesrv
|
ipfw addf accept syn to sup.FreeBSD.ORG 871
|
||||||
</verb></tscreen>
|
</verb></tscreen>
|
||||||
|
|
||||||
To see the accounting records:
|
To see the accounting records:
|
||||||
<tscreen><verb>
|
<tscreen><verb>
|
||||||
ipfw -sa list accounting
|
ipfw -a list
|
||||||
</verb></tscreen>
|
</verb></tscreen>
|
||||||
or in the short form
|
or in the short form
|
||||||
<tscreen><verb>
|
<tscreen><verb>
|
||||||
ipfw -sa l a
|
ipfw -a l
|
||||||
|
</verb></tscreen>
|
||||||
|
You can also see the last time a chain entry was matched with
|
||||||
|
<tscreen><verb>
|
||||||
|
ipfw -at l
|
||||||
</verb></tscreen>
|
</verb></tscreen>
|
||||||
|
|
||||||
<sect1><heading>Building a packet filtering firewall</heading>
|
<sect1><heading>Building a packet filtering firewall</heading>
|
||||||
|
|
@ -479,10 +486,11 @@ want to allow from the inside. Some general rules are:
|
||||||
where most of the security sensitive services are, like finger, SMTP
|
where most of the security sensitive services are, like finger, SMTP
|
||||||
(mail) and telnet.
|
(mail) and telnet.
|
||||||
|
|
||||||
<item>Block incoming SYN connections to ports between 1001 and 1024
|
<item>Block incoming SYN (<bf>setup</bf>) connections to ports
|
||||||
(this will allow internal users to rsh/rlogin to the outside). If you
|
between 1001 and 1024 (this will allow internal users to rsh/rlogin to
|
||||||
do not want to allow rsh/rlogin connections from the inside to the
|
the outside). If you do not want to allow rsh/rlogin connections from
|
||||||
outside, then extend the above suggestion to cover ports 1-1024.
|
the inside to the outside, then extend the above suggestion to cover
|
||||||
|
ports 1-1024.
|
||||||
|
|
||||||
<item>Block <bf>all</bf> incoming UDP traffic. There are very few
|
<item>Block <bf>all</bf> incoming UDP traffic. There are very few
|
||||||
useful services that travel over UDP, and what useful traffic there is
|
useful services that travel over UDP, and what useful traffic there is
|
||||||
|
|
@ -509,16 +517,6 @@ normally fall outside the 1-1024 range specified above.
|
||||||
|
|
||||||
</itemize>
|
</itemize>
|
||||||
|
|
||||||
<p>Of course, if you want to make sure that no un-authorized traffic
|
|
||||||
gets through the firewall, change the default policy to ``deny''. This
|
|
||||||
will mean that any traffic which is allowed through has to be
|
|
||||||
specified explicitly in an ``accept'' or ``allow'' filter rule. Which
|
|
||||||
ports you allow through is again something that you will have to
|
|
||||||
decide for yourself. If you do set the default policy to be deny, you
|
|
||||||
will probably want to install proxy servers, as no traffic will be
|
|
||||||
able to get OUT either unless you allow TCP SYN connections going form
|
|
||||||
the inside out.
|
|
||||||
|
|
||||||
<p>As I said above, these are only <em>guidelines</em>. You will have
|
<p>As I said above, these are only <em>guidelines</em>. You will have
|
||||||
to decide what filter rules you want to use on your firewall
|
to decide what filter rules you want to use on your firewall
|
||||||
yourself. I cannot accept ANY responsibility if someone breaks into
|
yourself. I cannot accept ANY responsibility if someone breaks into
|
||||||
|
|
|
||||||
Loading…
Reference in a new issue