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<tr><th colspan="3" align="center">Chapter 7. <acronym class="acronym">BIND</acronym> 9 Security Considerations</th></tr>
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<div class="chapter">
<div class="titlepage"><div><div><h1 class="title">
<a name="Bv9ARM.ch07"></a>Chapter 7. <acronym class="acronym">BIND</acronym> 9 Security Considerations</h1></div></div></div>
<div class="toc">
<p><b>Table of Contents</b></p>
<dl class="toc">
<dt><span class="section"><a href="Bv9ARM.ch07.html#Access_Control_Lists">Access Control Lists</a></span></dt>
<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>
<dt><span class="section"><a href="Bv9ARM.ch07.html#chroot">The <span class="command"><strong>chroot</strong></span> Environment</a></span></dt>
<dt><span class="section"><a href="Bv9ARM.ch07.html#setuid">Using the <span class="command"><strong>setuid</strong></span> Function</a></span></dt>
<dt><span class="section"><a href="Bv9ARM.ch07.html#dynamic_update_security">Dynamic Update Security</a></span></dt>

      <div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="Access_Control_Lists"></a>Access Control Lists</h2></div></div></div>

          Access Control Lists (ACLs) are address match lists that
          you can set up and nickname for future use in
          <span class="command"><strong>allow-notify</strong></span>, <span class="command"><strong>allow-query</strong></span>,
          <span class="command"><strong>allow-query-on</strong></span>, <span class="command"><strong>allow-recursion</strong></span>,
          <span class="command"><strong>blackhole</strong></span>, <span class="command"><strong>allow-transfer</strong></span>,
          <span class="command"><strong>match-clients</strong></span>, etc.
          Using ACLs allows you to have finer control over who can access
          your name server, without cluttering up your config files with huge
          lists of IP addresses.
          It is a <span class="emphasis"><em>good idea</em></span> to use ACLs, and to
          control access to your server. Limiting access to your server by
          outside parties can help prevent spoofing and denial of service
          (DoS) attacks against your server.
          ACLs match clients on the basis of up to three characteristics:
          1) The client's IP address; 2) the TSIG or SIG(0) key that was
          used to sign the request, if any; and 3) an address prefix
          encoded in an EDNS Client Subnet option, if any.
          Here is an example of ACLs based on client addresses:

<pre class="programlisting">
// Set up an ACL named "bogusnets" that will block
// RFC1918 space and some reserved space, which is
// commonly used in spoofing attacks.
acl bogusnets {;;;;;;

// Set up an ACL called our-nets. Replace this with the
// real IP numbers.
acl our-nets { x.x.x.x/24; x.x.x.x/21; };
options {
  allow-query { our-nets; };
  allow-recursion { our-nets; };
  blackhole { bogusnets; };

zone "" {
  type master;
  file "m/";
  allow-query { any; };

          This allows authoritative queries for "" from any
          address, but recursive queries only from the networks specified
          in "our-nets", and no queries at all from the networks
          specified in "bogusnets".
          In addition to network addresses and prefixes, which are
          matched against the source address of the DNS request, ACLs
          may include <code class="option">key</code> elements, which specify the
          name of a TSIG or SIG(0) key, or <code class="option">ecs</code>
          elements, which specify a network prefix but are only matched
          if that prefix matches an EDNS client subnet option included
          in the request.
          The EDNS Client Subnet (ECS) option is used by a recursive
          resolver to inform an authoritative name server of the network
          address block from which the original query was received, enabling
          authoritative servers to give different answers to the same
          resolver for different resolver clients.  An ACL containing
          an element of the form
          <span class="command"><strong>ecs <em class="replaceable"><code>prefix</code></em></strong></span>
          will match if a request arrives in containing an ECS option
          encoding an address within that prefix.  If the request has no
          ECS option, then "ecs" elements are simply ignored.  Addresses
          in ACLs that are not prefixed with "ecs" are matched only
          against the source address.
        <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
            (Note: The authoritative ECS implementation in
            <span class="command"><strong>named</strong></span> is based on an early version of the
            specification, and is known to have incompatibilities with
            other implementations.  It is also inefficient, requiring
            a separate view for each client subnet to be sent different
            answers, and it is unable to correct for overlapping subnets in
            the configuration.  It can be used for testing purposes, but is
            not recommended for production use.)
          When <acronym class="acronym">BIND</acronym> 9 is built with GeoIP support,
          ACLs can also be used for geographic access restrictions.
          This is done by specifying an ACL element of the form:
          <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>
          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".
          <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.
          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.
          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>.
          Some example GeoIP ACLs:
        <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";

          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; !; }</strong></span> would actually
          match a query from, because the first element
          indicated that the query should be accepted, and the second
          element is ignored.
          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:
        <pre class="programlisting">
allow-query { !{ !10/8; any; }; key example; };
          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.

      <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>

          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.
          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.
          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:
          <strong class="userinput"><code>/usr/local/sbin/named -u 202 -t /var/named</code></strong>

        <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>

            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.
            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>.

        <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>

            Prior to running the <span class="command"><strong>named</strong></span> daemon,
            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.
          <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
            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.

      <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>

          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
          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
          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
          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.

          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.

          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
          of public web and mail servers need not allow dynamic update at

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