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Suzanne Goldlust 2020-06-01 14:46:24 +00:00 committed by Ondřej Surý
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@ -28,18 +28,18 @@ BIND 9 Security Considerations
Access Control Lists Access Control Lists
-------------------- --------------------
Access Control Lists (ACLs) are address match lists that you can set up Access Control Lists (ACLs) are address match lists that can be set up
and nickname for future use in ``allow-notify``, ``allow-query``, and nicknamed for future use in ``allow-notify``, ``allow-query``,
``allow-query-on``, ``allow-recursion``, ``blackhole``, ``allow-query-on``, ``allow-recursion``, ``blackhole``,
``allow-transfer``, ``match-clients``, etc. ``allow-transfer``, ``match-clients``, etc.
Using ACLs allows you to have finer control over who can access your ACLs give users finer control over who can access the
name server, without cluttering up your config files with huge lists of name server, without cluttering up config files with huge lists of
IP addresses. IP addresses.
It is a *good idea* to use ACLs, and to control access to your server. It is a *good idea* to use ACLs, and to control access.
Limiting access to your server by outside parties can help prevent Limiting access to the server by outside parties can help prevent
spoofing and denial of service (DoS) attacks against your server. spoofing and denial of service (DoS) attacks against the server.
ACLs match clients on the basis of up to three characteristics: 1) The 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 client's IP address; 2) the TSIG or SIG(0) key that was used to sign the
@ -50,7 +50,7 @@ Here is an example of ACLs based on client addresses:
:: ::
// Set up an ACL named "bogusnets" that will block // Set up an ACL named "bogusnets" that blocks
// RFC1918 space and some reserved space, which is // RFC1918 space and some reserved space, which is
// commonly used in spoofing attacks. // commonly used in spoofing attacks.
acl bogusnets { acl bogusnets {
@ -77,9 +77,9 @@ Here is an example of ACLs based on client addresses:
allow-query { any; }; allow-query { any; };
}; };
This allows authoritative queries for "example.com" from any address, This allows authoritative queries for ``example.com`` from any address,
but recursive queries only from the networks specified in "our-nets", but recursive queries only from the networks specified in ``our-nets``,
and no queries at all from the networks specified in "bogusnets". and no queries at all from the networks specified in ``bogusnets``.
In addition to network addresses and prefixes, which are matched against In addition to network addresses and prefixes, which are matched against
the source address of the DNS request, ACLs may include ``key`` the source address of the DNS request, ACLs may include ``key``
@ -87,32 +87,32 @@ elements, which specify the name of a TSIG or SIG(0) key.
When BIND 9 is built with GeoIP support, ACLs can also be used for When BIND 9 is built with GeoIP support, ACLs can also be used for
geographic access restrictions. This is done by specifying an ACL geographic access restrictions. This is done by specifying an ACL
element of the form: ``geoip db database field value`` element of the form: ``geoip db database field value``.
The ``field`` indicates which field to search for a match. Available fields The ``field`` parameter indicates which field to search for a match. Available fields
are "country", "region", "city", "continent", "postal" (postal code), are ``country``, ``region``, ``city``, ``continent``, ``postal`` (postal code),
"metro" (metro code), "area" (area code), "tz" (timezone), "isp", ``metro`` (metro code), ``area`` (area code), ``tz`` (timezone), ``isp``,
"asnum", and "domain". ``asnum``, and ``domain``.
``value`` is the value to search for within the database. A string may be quoted ``value`` is the value to search for within the database. A string may be quoted
if it contains spaces or other special characters. An "asnum" search for if it contains spaces or other special characters. An ``asnum`` search for
autonomous system number can be specified using the string "ASNNNN" or the autonomous system number can be specified using the string "ASNNNN" or the
integer NNNN. When "country" search is specified with a string is two characters integer NNNN. If a ``country`` search is specified with a string that is two characters
long, then it must be a standard ISO-3166-1 two-letter country code; otherwise long, it must be a standard ISO-3166-1 two-letter country code; otherwise
it is interpreted as the full name of the country. Similarly, if this is a it is interpreted as the full name of the country. Similarly, if
"region" search and the string is two characters long, then it treated as a ``region`` is the search term and the string is two characters long, it is treated as a
standard two-letter state or province abbreviation; otherwise it treated as the standard two-letter state or province abbreviation; otherwise, it is treated as the
full name of the state or province. full name of the state or province.
The ``database`` field indicates which GeoIP database to search for a match. In The ``database`` 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 most cases this is unnecessary, because most search fields can only be found in
a single database. However, searches for "continent" or "country" can be a single database. However, searches for ``continent`` or ``country`` can be
answered from either the "city" or "country" databases, so for these search answered from either the ``city`` or ``country`` databases, so for these search
types, specifying a ``database`` will force the query to be answered from that types, specifying a ``database`` forces the query to be answered from that
database and no other. If ``database`` is not specified, then these queries database and no other. If a ``database`` is not specified, these queries
will be answered from the "city", database if it is installed, or the "country" are first answered from the ``city`` database if it is installed, and then from the ``country``
database if it is installed, in that order. Valid database names are "country", database if it is installed. Valid database names are ``country``,
"city", "asnum", "isp", and "domain". ``city``, ``asnum``, ``isp``, and ``domain``.
Some example GeoIP ACLs: Some example GeoIP ACLs:
@ -132,11 +132,11 @@ 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 prefix matches an ACL element, then that ACL is considered to have
matched even if a later element would have matched more specifically. matched even if a later element would have matched more specifically.
For example, the ACL ``{ 10/8; !10.0.0.1; }`` would actually match a For example, the ACL ``{ 10/8; !10.0.0.1; }`` would actually match a
query from 10.0.0.1, because the first element indicated that the query query from 10.0.0.1, because the first element indicates that the query
should be accepted, and the second element is ignored. should be accepted, and the second element is ignored.
When using "nested" ACLs (that is, ACLs included or referenced within 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 other ACLs), a negative match of a nested ACL tells the containing ACL to
continue looking for matches. This enables complex ACLs to be continue looking for matches. This enables complex ACLs to be
constructed, in which multiple client characteristics can be checked at constructed, in which multiple client characteristics can be checked at
the same time. For example, to construct an ACL which allows queries the same time. For example, to construct an ACL which allows queries
@ -148,11 +148,11 @@ signed with a particular key, use:
allow-query { !{ !10/8; any; }; key example; }; allow-query { !{ !10/8; any; }; key example; };
Within the nested ACL, any address that is *not* in the 10/8 network Within the nested ACL, any address that is *not* in the 10/8 network
prefix will be rejected, and this will terminate processing of the ACL. prefix is rejected, which terminates processing of the ACL.
Any address that *is* in the 10/8 network prefix will be accepted, but Any address that *is* in the 10/8 network prefix is accepted, but
this causes a negative match of the nested ACL, so the containing ACL 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 continues processing. The query is accepted if it is signed by
the key "example", and rejected otherwise. The ACL, then, will only the key ``example``, and rejected otherwise. The ACL, then, only
matches when *both* conditions are true. matches when *both* conditions are true.
.. _chroot_and_setuid: .. _chroot_and_setuid:
@ -160,12 +160,12 @@ matches when *both* conditions are true.
``Chroot`` and ``Setuid`` ``Chroot`` and ``Setuid``
------------------------- -------------------------
On UNIX servers, it is possible to run BIND in a *chrooted* environment On Unix servers, it is possible to run BIND in a *chrooted* environment
(using the ``chroot()`` function) by specifying the ``-t`` option for (using the ``chroot()`` function) by specifying the ``-t`` option for
``named``. This can help improve system security by placing BIND in a ``named``. This can help improve system security by placing BIND in a
"sandbox", which will limit the damage done if a server is compromised. "sandbox," which limits the damage done if a server is compromised.
Another useful feature in the UNIX version of BIND is the ability to run Another useful feature in the Unix version of BIND is the ability to run
the daemon as an unprivileged user ( ``-u`` user ). We suggest running the daemon as an unprivileged user ( ``-u`` user ). We suggest running
as an unprivileged user when using the ``chroot`` feature. as an unprivileged user when using the ``chroot`` feature.
@ -179,17 +179,17 @@ Here is an example command line to load BIND in a ``chroot`` sandbox,
The ``chroot`` Environment The ``chroot`` Environment
~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~
In order for a ``chroot`` environment to work properly in a particular For a ``chroot`` environment to work properly in a particular
directory (for example, ``/var/named``), you will need to set up an directory (for example, ``/var/named``), the
environment that includes everything BIND needs to run. From BIND's environment must include everything BIND needs to run. From BIND's
point of view, ``/var/named`` is the root of the filesystem. You will point of view, ``/var/named`` is the root of the filesystem;
need to adjust the values of options like ``directory`` and ``pid-file`` the values of options like ``directory`` and ``pid-file``
to account for this. must be adjusted to account for this.
Unlike with earlier versions of BIND, you typically will *not* need to Unlike with earlier versions of BIND,
compile ``named`` statically nor install shared libraries under the new ``named`` does *not* typically need to be compiled statically, nor do shared libraries need to be installed under the new
root. However, depending on your operating system, you may need to set root. However, depending on the operating system, it may be necessary to set
up things like ``/dev/zero``, ``/dev/random``, ``/dev/log``, and up locations such as ``/dev/zero``, ``/dev/random``, ``/dev/log``, and
``/etc/localtime``. ``/etc/localtime``.
.. _setuid: .. _setuid:
@ -199,13 +199,13 @@ Using the ``setuid`` Function
Prior to running the ``named`` daemon, use the ``touch`` utility (to Prior to running the ``named`` daemon, use the ``touch`` utility (to
change file access and modification times) or the ``chown`` utility (to change file access and modification times) or the ``chown`` utility (to
set the user id and/or group id) on files to which you want BIND to set the user id and/or group id) on files where BIND should
write. write.
.. note:: .. note::
If the ``named`` daemon is running as an unprivileged user, it will If the ``named`` daemon is running as an unprivileged user, it
not be able to bind to new restricted ports if the server is cannot bind to new restricted ports if the server is
reloaded. reloaded.
.. _dynamic_update_security: .. _dynamic_update_security:
@ -219,10 +219,10 @@ address of the host requesting the update, by listing an IP address or
network prefix in the ``allow-update`` zone option. This method is network prefix in the ``allow-update`` zone option. This method is
insecure since the source address of the update UDP packet is easily insecure since the source address of the update UDP packet is easily
forged. Also note that if the IP addresses allowed by the forged. Also note that if the IP addresses allowed by the
``allow-update`` option include the address of a slave server which ``allow-update`` option include the address of a secondary server which
performs forwarding of dynamic updates, the master can be trivially performs forwarding of dynamic updates, the primary can be trivially
attacked by sending the update to the slave, which will forward it to attacked by sending the update to the secondary, which forwards it to
the master with its own source IP address causing the master to approve the primary with its own source IP address - causing the primary to approve
it without question. it without question.
For these reasons, we strongly recommend that updates be For these reasons, we strongly recommend that updates be
@ -234,4 +234,4 @@ names, not IP addresses or network prefixes. Alternatively, the new
Some sites choose to keep all dynamically-updated DNS data in a Some sites choose to keep all dynamically-updated DNS data in a
subdomain and delegate that subdomain to a separate zone. This way, the subdomain and delegate that subdomain to a separate zone. This way, the
top-level zone containing critical data such as the IP addresses of top-level zone containing critical data such as the IP addresses of
public web and mail servers need not allow dynamic update at all. public web and mail servers need not allow dynamic updates at all.