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c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe<hr>
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe</div>
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe<div class="chapter">
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe<div class="titlepage"><div><div><h1 class="title">
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe<a name="Bv9ARM.ch07"></a>Chapter�7.�<acronym class="acronym">BIND</acronym> 9 Security Considerations</h1></div></div></div>
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe<div class="toc">
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe<p><b>Table of Contents</b></p>
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe<dl class="toc">
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe<dt><span class="section"><a href="Bv9ARM.ch07.html#Access_Control_Lists">Access Control Lists</a></span></dt>
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe<dt><span class="section"><a href="Bv9ARM.ch07.html#chroot_and_setuid"><span class="command"><strong>Chroot</strong></span> and <span class="command"><strong>Setuid</strong></span></a></span></dt>
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe<dd><dl>
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe<dt><span class="section"><a href="Bv9ARM.ch07.html#chroot">The <span class="command"><strong>chroot</strong></span> Environment</a></span></dt>
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe<dt><span class="section"><a href="Bv9ARM.ch07.html#setuid">Using the <span class="command"><strong>setuid</strong></span> Function</a></span></dt>
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov</dl></dd>
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe<dt><span class="section"><a href="Bv9ARM.ch07.html#dynamic_update_security">Dynamic Update Security</a></span></dt>
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe</dl>
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe</div>
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe <div class="section">
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe<div class="titlepage"><div><div><h2 class="title" style="clear: both">
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe<a name="Access_Control_Lists"></a>Access Control Lists</h2></div></div></div>
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe <p>
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe Access Control Lists (ACLs) are address match lists that
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe you can set up and nickname for future use in
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe <span class="command"><strong>allow-notify</strong></span>, <span class="command"><strong>allow-query</strong></span>,
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe <span class="command"><strong>allow-query-on</strong></span>, <span class="command"><strong>allow-recursion</strong></span>,
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe <span class="command"><strong>blackhole</strong></span>, <span class="command"><strong>allow-transfer</strong></span>,
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe <span class="command"><strong>match-clients</strong></span>, etc.
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe </p>
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe <p>
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe Using ACLs allows you to have finer control over who can access
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe your name server, without cluttering up your config files with huge
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe lists of IP addresses.
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe </p>
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe <p>
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe It is a <span class="emphasis"><em>good idea</em></span> to use ACLs, and to
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe control access to your server. Limiting access to your server by
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe outside parties can help prevent spoofing and denial of service
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe (DoS) attacks against your server.
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe </p>
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe <p>
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe ACLs match clients on the basis of up to three characteristics:
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe 1) The client's IP address; 2) the TSIG or SIG(0) key that was
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe used to sign the request, if any; and 3) an address prefix
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe encoded in an EDNS Client Subnet option, if any.
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe </p>
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe <p>
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe Here is an example of ACLs based on client addresses:
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe </p>
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe<pre class="programlisting">
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe// Set up an ACL named "bogusnets" that will block
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe// RFC1918 space and some reserved space, which is
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe// commonly used in spoofing attacks.
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Loweacl bogusnets {
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe 0.0.0.0/8; 192.0.2.0/24; 224.0.0.0/3;
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe 10.0.0.0/8; 172.16.0.0/12; 192.168.0.0/16;
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe};
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe// Set up an ACL called our-nets. Replace this with the
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe// real IP numbers.
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankovacl our-nets { x.x.x.x/24; x.x.x.x/21; };
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankovoptions {
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov ...
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov ...
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov allow-query { our-nets; };
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe allow-recursion { our-nets; };
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov ...
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov blackhole { bogusnets; };
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe ...
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov};
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankovzone "example.com" {
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov type master;
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov file "m/example.com";
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov allow-query { any; };
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov};
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov</pre>
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov <p>
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe This allows authoritative queries for "example.com" from any
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov address, but recursive queries only from the networks specified
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe in "our-nets", and no queries at all from the networks
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov specified in "bogusnets".
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe </p>
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov <p>
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe In addition to network addresses and prefixes, which are
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov matched against the source address of the DNS request, ACLs
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov may include <code class="option">key</code> elements, which specify the
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov name of a TSIG or SIG(0) key, or <code class="option">ecs</code>
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov elements, which specify a network prefix but are only matched
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov if that prefix matches an EDNS client subnet option included
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe in the request.
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov </p>
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov <p>
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov The EDNS Client Subnet (ECS) option is used by a recursive
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov resolver to inform an authoritative name server of the network
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov address block from which the original query was received, enabling
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov authoritative servers to give different answers to the same
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov resolver for different resolver clients. An ACL containing
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov an element of the form
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe <span class="command"><strong>ecs <em class="replaceable"><code>prefix</code></em></strong></span>
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov will match if a request arrives in containing an ECS option
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe encoding an address within that prefix. If the request has no
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov ECS option, then "ecs" elements are simply ignored. Addresses
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov in ACLs that are not prefixed with "ecs" are matched only
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov against the source address.
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe </p>
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov<h3 class="title">Note</h3>
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov <p>
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe (Note: The authoritative ECS implementation in
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov <span class="command"><strong>named</strong></span> is based on an early version of the
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov specification, and is known to have incompatibilities with
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov other implementations. It is also inefficient, requiring
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov a separate view for each client subnet to be sent different
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe answers, and it is unable to correct for overlapping subnets in
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov the configuration. It can be used for testing purposes, but is
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe not recommended for production use.)
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov </p>
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov </div>
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov <p>
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe When <acronym class="acronym">BIND</acronym> 9 is built with GeoIP support,
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe ACLs can also be used for geographic access restrictions.
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe This is done by specifying an ACL element of the form:
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov <span class="command"><strong>geoip [<span class="optional">db <em class="replaceable"><code>database</code></em></span>] <em class="replaceable"><code>field</code></em> <em class="replaceable"><code>value</code></em></strong></span>
c10c16dec587a0662068f6e2991c29ed3a9db943Richard Lowe </p>
a9478106a12424322498e53cf7cd75bd8a4d6004Yuri Pankov <p>
The <em class="replaceable"><code>field</code></em> indicates which field
to search for a match. Available fields are "country",
"region", "city", "continent", "postal" (postal code),
"metro" (metro code), "area" (area code), "tz" (timezone),
"isp", "org", "asnum", "domain" and "netspeed".
</p>
<p>
<em class="replaceable"><code>value</code></em> is the value to search
for within the database. A string may be quoted if it
contains spaces or other special characters. If this is
an "asnum" search, then the leading "ASNNNN" string can be
used, otherwise the full description must be used (e.g.
"ASNNNN Example Company Name"). If this is a "country"
search and the string is two characters long, then it must
be a standard ISO-3166-1 two-letter country code, and if it
is three characters long then it must be an ISO-3166-1
three-letter country code; otherwise it is the full name
of the country. Similarly, if this is a "region" search
and the string is two characters long, then it must be a
standard two-letter state or province abbreviation;
otherwise it is the full name of the state or province.
</p>
<p>
The <em class="replaceable"><code>database</code></em> field indicates which
GeoIP database to search for a match. In most cases this is
unnecessary, because most search fields can only be found in
a single database. However, searches for country can be
answered from the "city", "region", or "country" databases,
and searches for region (i.e., state or province) can be
answered from the "city" or "region" databases. For these
search types, specifying a <em class="replaceable"><code>database</code></em>
will force the query to be answered from that database and no
other. If <em class="replaceable"><code>database</code></em> is not
specified, then these queries will be answered from the "city",
database if it is installed, or the "region" database if it is
installed, or the "country" database, in that order.
</p>
<p>
By default, if a DNS query includes an EDNS Client Subnet (ECS)
option which encodes a non-zero address prefix, then GeoIP ACLs
will be matched against that address prefix. Otherwise, they
are matched against the source address of the query. To
prevent GeoIP ACLs from matching against ECS options, set
the <span class="command"><strong>geoip-use-ecs</strong></span> to <code class="literal">no</code>.
</p>
<p>
Some example GeoIP ACLs:
</p>
<pre class="programlisting">geoip country US;
geoip country JAP;
geoip db country country Canada;
geoip db region region WA;
geoip city "San Francisco";
geoip region Oklahoma;
geoip postal 95062;
geoip tz "America/Los_Angeles";
geoip org "Internet Systems Consortium";
</pre>
<p>
ACLs use a "first-match" logic rather than "best-match":
if an address prefix matches an ACL element, then that ACL
is considered to have matched even if a later element would
have matched more specifically. For example, the ACL
<span class="command"><strong> { 10/8; !10.0.0.1; }</strong></span> would actually
match a query from 10.0.0.1, because the first element
indicated that the query should be accepted, and the second
element is ignored.
</p>
<p>
When using "nested" ACLs (that is, ACLs included or referenced
within other ACLs), a negative match of a nested ACL will
the containing ACL to continue looking for matches. This
enables complex ACLs to be constructed, in which multiple
client characteristics can be checked at the same time. For
example, to construct an ACL which allows queries only when
it originates from a particular network <span class="emphasis"><em>and</em></span>
only when it is signed with a particular key, use:
</p>
<pre class="programlisting">
allow-query { !{ !10/8; any; }; key example; };
</pre>
<p>
Within the nested ACL, any address that is
<span class="emphasis"><em>not</em></span> in the 10/8 network prefix will
be rejected, and this will terminate processing of the
ACL. Any address that <span class="emphasis"><em>is</em></span> in the 10/8
network prefix will be accepted, but this causes a negative
match of the nested ACL, so the containing ACL continues
processing. The query will then be accepted if it is signed
by the key "example", and rejected otherwise. The ACL, then,
will only matches when <span class="emphasis"><em>both</em></span> conditions
are true.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="chroot_and_setuid"></a><span class="command"><strong>Chroot</strong></span> and <span class="command"><strong>Setuid</strong></span>
</h2></div></div></div>
<p>
On UNIX servers, it is possible to run <acronym class="acronym">BIND</acronym>
in a <span class="emphasis"><em>chrooted</em></span> environment (using
the <span class="command"><strong>chroot()</strong></span> function) by specifying
the <code class="option">-t</code> option for <span class="command"><strong>named</strong></span>.
This can help improve system security by placing
<acronym class="acronym">BIND</acronym> in a "sandbox", which will limit
the damage done if a server is compromised.
</p>
<p>
Another useful feature in the UNIX version of <acronym class="acronym">BIND</acronym> is the
ability to run the daemon as an unprivileged user ( <code class="option">-u</code> <em class="replaceable"><code>user</code></em> ).
We suggest running as an unprivileged user when using the <span class="command"><strong>chroot</strong></span> feature.
</p>
<p>
Here is an example command line to load <acronym class="acronym">BIND</acronym> in a <span class="command"><strong>chroot</strong></span> sandbox,
<span class="command"><strong>/var/named</strong></span>, and to run <span class="command"><strong>named</strong></span> <span class="command"><strong>setuid</strong></span> to
user 202:
</p>
<p>
<strong class="userinput"><code>/usr/local/sbin/named -u 202 -t /var/named</code></strong>
</p>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="chroot"></a>The <span class="command"><strong>chroot</strong></span> Environment</h3></div></div></div>
<p>
In order for a <span class="command"><strong>chroot</strong></span> environment
to work properly in a particular directory (for example,
<code class="filename">/var/named</code>), you will need to set
up an environment that includes everything
<acronym class="acronym">BIND</acronym> needs to run. From
<acronym class="acronym">BIND</acronym>'s point of view,
<code class="filename">/var/named</code> is the root of the
filesystem. You will need to adjust the values of
options like <span class="command"><strong>directory</strong></span> and
<span class="command"><strong>pid-file</strong></span> to account for this.
</p>
<p>
Unlike with earlier versions of BIND, you typically will
<span class="emphasis"><em>not</em></span> need to compile <span class="command"><strong>named</strong></span>
statically nor install shared libraries under the new root.
However, depending on your operating system, you may need
to set up things like
<code class="filename">/dev/zero</code>,
<code class="filename">/dev/random</code>,
<code class="filename">/dev/log</code>, and
<code class="filename">/etc/localtime</code>.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="setuid"></a>Using the <span class="command"><strong>setuid</strong></span> Function</h3></div></div></div>
<p>
Prior to running the <span class="command"><strong>named</strong></span> daemon,
use
the <span class="command"><strong>touch</strong></span> utility (to change file
access and
modification times) or the <span class="command"><strong>chown</strong></span>
utility (to
set the user id and/or group id) on files
to which you want <acronym class="acronym">BIND</acronym>
to write.
</p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>
If the <span class="command"><strong>named</strong></span> daemon is running as an
unprivileged user, it will not be able to bind to new restricted
ports if the server is reloaded.
</p>
</div>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="dynamic_update_security"></a>Dynamic Update Security</h2></div></div></div>
<p>
Access to the dynamic
update facility should be strictly limited. In earlier versions of
<acronym class="acronym">BIND</acronym>, the only way to do this was
based on the IP
address of the host requesting the update, by listing an IP address
or
network prefix in the <span class="command"><strong>allow-update</strong></span>
zone option.
This method is insecure since the source address of the update UDP
packet
is easily forged. Also note that if the IP addresses allowed by the
<span class="command"><strong>allow-update</strong></span> option include the
address of a slave
server which performs forwarding of dynamic updates, the master can
be
trivially attacked by sending the update to the slave, which will
forward it to the master with its own source IP address causing the
master to approve it without question.
</p>
<p>
For these reasons, we strongly recommend that updates be
cryptographically authenticated by means of transaction signatures
(TSIG). That is, the <span class="command"><strong>allow-update</strong></span>
option should
list only TSIG key names, not IP addresses or network
prefixes. Alternatively, the new <span class="command"><strong>update-policy</strong></span>
option can be used.
</p>
<p>
Some sites choose to keep all dynamically-updated DNS data
in a subdomain and delegate that subdomain to a separate zone. This
way, the top-level zone containing critical data such as the IP
addresses
of public web and mail servers need not allow dynamic update at
all.
</p>
</div>
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