7. BIND 9 Security Considerations
7.1. Security Assumptions
BIND 9’s design assumes that access to the objects listed below is limited only to trusted parties. An incorrect deployment, which does not follow rules set by this section, cannot be the basis for CVE assignment or special security-sensitive handling of issues.
Unauthorized access can potentially disclose sensitive data, slow down server operation, etc. Unauthorized, unexpected, or incorrect writes to listed objects can potentically cause crashes, incorrect data handling, or corruption.
All files stored on disk - including zone files, configuration files, key files, temporary files, etc.
Clients communicating via control socket using configured keys
Access to statistics channels from untrusted clients
Sockets used for Dynamic Update Policies type external
Certain aspects of the DNS protocol are left unspecified, such as the handling of responses from DNS servers which do not fully conform to the DNS protocol. For such a situation, BIND implements its own safety checks and limits which are subject to change as the protocol and deployment evolve.
7.1.2. DNS Resolvers
By definition, DNS resolvers act as traffic amplifiers; during normal operation, a DNS resolver can legitimately generate more outgoing traffic (counted in packets or bytes) than the incoming client traffic that triggered it. The DNS protocol specification does not currently specify limits for this amplification, but BIND implements its own limits to balance interoperability and safety. As a general rule, if a traffic amplification factor for any given scenario is lower than 100 packets, ISC does not handle the given scenario as a security issue. These limits are subject to change as DNS deployment evolves.
All DNS answers received by the DNS resolver are treated as untrusted input and are subject to safety and correctness checks. However, protocol non-conformity might cause unexpected behavior. If such unexpected behavior is limited to DNS domains hosted on non-conformant servers, it is not deemed a security issue in BIND.
7.2. Access Control Lists
Access Control Lists (ACLs) are address match lists that can be set up
and nicknamed for future use in allow-notify, allow-query,
allow-query-on, allow-recursion, blackhole,
allow-transfer, match-clients, etc.
ACLs give users finer control over who can access the name server, without cluttering up configuration files with huge lists of IP addresses.
It is a good idea to use ACLs, and to control access. Limiting access to the server by outside parties can help prevent spoofing and denial of service (DoS) attacks against the 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:
// Set up an ACL named "bogusnets" that blocks
// RFC1918 space and some reserved space, which is
// commonly used in spoofing attacks.
acl bogusnets {
0.0.0.0/8; 192.0.2.0/24; 224.0.0.0/3;
10.0.0.0/8; 172.16.0.0/12; 192.168.0.0/16;
};
// 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 "example.com" {
type primary;
file "m/example.com";
allow-query { any; };
};
This allows authoritative queries for example.com 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 key
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
geographic access restrictions. This is done by specifying an ACL
element of the form: geoip db database field value.
The field parameter 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,
asnum, and domain.
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
autonomous system number can be specified using the string “ASNNNN” or the
integer NNNN. If a country search is specified with a string that is two characters
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
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 is treated as the
full name of the state or province.
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
a single database. However, searches for continent or country can be
answered from either the city or country databases, so for these search
types, specifying a database forces the query to be answered from that
database and no other. If a database is not specified, these queries
are first answered from the city database if it is installed, and then from the country
database if it is installed. Valid database names are country,
city, asnum, isp, and domain.
Some example GeoIP ACLs:
geoip country US;
geoip country JP;
geoip db country country Canada;
geoip 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 { 10/8; !10.0.0.1; } would actually match a
query from 10.0.0.1, because the first element indicates 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 tells 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 a query only when it originates from a particular network and only when it is signed with a particular key, use:
allow-query { !{ !10/8; any; }; key example; };
Within the nested ACL, any address that is not in the 10/8 network
prefix is rejected, which terminates the processing of the ACL.
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
continues processing. The query is accepted if it is signed by
the key example, and rejected otherwise. The ACL, then, only
matches when both conditions are true.
7.3. Chroot and Setuid
On Unix servers, it is possible to run BIND in a chrooted environment
(using the chroot() function) by specifying the -t option for
named. This can help improve system security by placing BIND in a
“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
the daemon as an unprivileged user (-u user). We suggest running
as an unprivileged user when using the chroot feature.
Here is an example command line to load BIND in a chroot sandbox,
/var/named, and to run named setuid to user 202:
/usr/local/sbin/named -u 202 -t /var/named
7.3.1. The chroot Environment
For a chroot environment to work properly in a particular
directory (for example, /var/named), the
environment must include everything BIND needs to run. From BIND’s
point of view, /var/named is the root of the filesystem;
the values of options like directory and pid-file
must be adjusted to account for this.
Unlike with earlier versions of BIND,
named does not typically need to be compiled statically, nor do shared libraries need to be installed under the new
root. However, depending on the operating system, it may be necessary to set
up locations such as /dev/zero, /dev/random, /dev/log, and
/etc/localtime.
7.3.2. Using the setuid Function
Prior to running the named daemon, use the touch utility (to
change file access and modification times) or the chown utility (to
set the user id and/or group id) on files where BIND should
write.
Note
If the named daemon is running as an unprivileged user, it
cannot bind to new restricted ports if the server is
reloaded.
7.4. Dynamic Update Security
Access to the dynamic update facility should be strictly limited. In
earlier versions of BIND, 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 allow-update 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
allow-update option include the address of a secondary server which
performs forwarding of dynamic updates, the primary can be trivially
attacked by sending the update to the secondary, which forwards it to
the primary with its own source IP address - causing the primary 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 allow-update option should list only TSIG key
names, not IP addresses or network prefixes. Alternatively, the
update-policy 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 addresses of public web and mail servers, need not allow dynamic updates at all.