deadwood - Online in the Cloud

PROGRAM:

NAME

deadwood - A fully recursive caching DNS resolver

DESCRIPTION

Deadwood is a fully recursive DNS cache. This is a DNS server with the following features:

* Full support for both DNS recursion and DNS forwarding caching

* Small size and memory footprint suitable for embedded systems

* Simple and clean codebase

* Secure design

* Spoof protection: Strong cryptography used to determine the Query ID and source port

* Ability to read and write the cache to a file

* Dynamic cache that deletes entries not recently used

* Ability to use expired entries in the cache when it is impossible to contact upstream
DNS servers.

* IPv6 support can be compiled in if desired

* Both DNS-over-UDP and DNS-over-TCP are handled by the same daemon

* Built-in dnswall functionality

COMMAND LINE ARGUMENTS

Deadwood has a single optional command line argument: The location of the configuration
file that Deadwood uses, specified with the "-f" flag. If this is not defined, Deadwood
uses the file "/etc/maradns/deadwood/dwood3rc" as the configuration file.

In other words, invoking Deadwood as deadwood will cause Deadwood to use
/etc/maradns/deadwood/dwood3rc as the configuration file; invoking Deadwood as deadwood -f
foobar will cause Deadwood to use the file "foobar" in the current working directory (the
directory one is in when starting Deadwood) as the configuration file.

CONFIGURATION FILE FORMAT

The Deadwood configuration file is modeled after Python 2's syntax. Any valid Deadwood
configuration file should also correctly parse in both Python 2.4.3 and Python 2.6.6. If
any configuration file does correctly parse in Deadwood but raises a syntax error in
Python, this is a bug that should be fixed.

This in mind, whitespace is significant; Deadwood parameters must be in the leftmost
column with no leading whitespace. This is a valid line (as long as there are no spaces to
its left):

recursive_acl = "127.0.0.1/16"

The following line, however, will raise a parse error:

recursive_acl = "127.0.0.1/16"

Observe the space to the left of the "recusive_acl" string in the incorrectly formatted
line.

PARAMETER TYPES

Deadwood has three different parameter types:

* Numeric parameters. Numeric parameters must not be surrounded by quotes, such as this
example:

filter_rfc1918 = 0

If a numeric parameter is surrounded by quotes, the error message "Unknown dwood3rc
string parameter" will appear.

* String parameters. String parameters must be surrounded by quotes, such as in this
example:

bind_address = "127.0.0.1"

* Dictionary parameters. All dictionary parameters must be initialized before use, and
dictionary parameters must have both the dictionary index and the value for said index
surrounded by quotes, such as in this example:

upstream_servers = {}
upstream_servers["."]="8.8.8.8, 8.8.4.4"

All dwood3rc parameters except the following are numeric parameters:

* bind_address (string)

* cache_file (string)

* chroot_dir (string)

* ip_blacklist (string)

* ipv4_bind_addresses (string)

* random_seed_file (string)

* recursive_acl (string)

* root_servers (dictionary)

* upstream_servers (dictionary)

SUPPORTED PARAMETERS

The Deadwood configuration file supports the following parameters:

bind_address

This is the IP (or possibly IPv6) address we bind to.

cache_file

This is the filename of the file used for reading and writing the cache to disk; this
string can have lowercase letters, the '-' symbol, the '_' symbol, and the '/' symbol (for
putting the cache in a subdirectory). All other symbols become a '_' symbol.

This file is read and written as the user Deadwood runs as.

chroot_dir

This is the directory the program will run from.

deliver_all

This affects behavior in Deadwood 2.3, but has no effect in Deadwood 3. This variable is
only here so Deadwood 2 rc files can run in Deadwood 3.

dns_port

This is the port Deadwood binds to and listens on for incoming connections. The default
value for this is the standard DNS port: port 53

filter_rfc1918

When this has a value of 1, a number of different IP ranges are not allowed to be in DNS A
replies:

* 192.168.x.x

* 172.[16-31].x.x

* 10.x.x.x

* 127.x.x.x

* 169.254.x.x

* 224.x.x.x

* 0.0.x.x

If one of the above IPs is detected in a DNS reply, and filter_rfc1918 has a value of 1,
Deadwood will return a synthetic "this host does not reply" response (a SOA record in the
NS section) instead of the A record.

The reason for this is to provide a "dnswall" that protects users for some kinds of
attacks, as described at http://crypto.stanford.edu/dns/

Please note that Deadwood only provides IPv4 "dnswall" functionality and does not help
protect against IPv6 answers. If protection against certain IPv6 AAAA records is needed,
either disable all AAAA answers by setting reject_aaaa to have a value of 1, or use an
external program to filter undesired IPv4 answers (such as the dnswall program).

The default value for this is 1

handle_noreply

When this is set to 0, Deadwood sends no reply back to the client (when the client is a
TCP client, Deadwood closes the TCP connection) when a UDP query is sent upstream and the
upstream DNS never sends a reply.

When this is set to 1, Deadwood sends a SERVER FAIL back to the client when a UDP query is
sent upstream and the upstream DNS never sends a reply.

The default value for this is 1

handle_overload

When this has a value of 0, Deadwood sends no reply when a UDP query is sent and the
server is overloaded (has too many pending connections); when it has a value of 1,
Deadwood sends a SERVER FAIL packet back to the sender of the UDP query. The default value
for this is 1.

hash_magic_number

This used to be used for Deadwood's internal hash generator to keep the hash generator
somewhat random and immune to certain types of attacks. In Deadwood 3.0, entropy for the
hash function is created by looking at the contents of /dev/urandom (secret.txt on Windows
machines) and the current timestamp. This parameter is only here so older configuration
files do not break in Deadwood 3.0.

ip_blacklist

This is a list of IPs that we do not allow to be in the answer to a DNS request. The
reason for this is to counteract the practice some ISPs have of converting a "this site
does not exist" DNS answer in to a page controlled by the ISP; this results in possible
security issues.

This parameter only accepts individual IPs, and does not use netmasks.

maradns_uid

The user-id Deadwood runs as. This can be any number between 10 and 65535; the default
value is 99 (nobody on RedHat-derived Linux distributions). This value is not used on
Windows systems.

maradns_gid

The group-id Deadwood runs as. This can be any number between 10 and 65535; the default
value is 99. This value is not used on Windows systems.

max_ar_chain

Whether resource record rotation is enabled. If this has a value of 1, resource record
rotation is enabled, otherwise resource record rotation is disabled.

Resource record rotation is usually desirable, since it allows DNS to act like a crude
load balancer. However, on heavily loaded systems it may be desirable to disable it to
reduce CPU usage.

The reason for the unusual name for this variable is to retain compatibility with MaraDNS
mararc files.

The default value is 1: Resource record rotation enabled.

max_inflights

The maximum number of simultaneous clients we process at the same time for the same query.

If, while processing a query for, say, "example.com.", another DNS client sends to
Deadwood another query for example.com, instead of creating a new query to process
example.com, Deadwood will attach the new client to the same query that is already "in
flight", and send a reply to both clients once we have an answer for example.com.

This is the number of simultaneous clients a given query can have. If this limit is
exceeded, subsequents clients with the same query are refused until an answer is found. If
this has a value of 1, we do not merge multiple requests for the same query, but give each
request its own connection.

The default value is 8.

max_ttl

The maximum amount of time we will keep an entry in the cache, in seconds (also called
"Maximum TTL").

This is the longest we will keep an entry cached. The default value for this parameter is
86400 (one day); the minimum value is 300 (5 minutes) and the maximum value this can have
is 7776000 (90 days).

The reason why this parameter is here is to protect Deadwood from attacks which exploit
there being stale data in the cache, such as the "Ghost Domain Names" attack.

maximum_cache_elements

The maximum number of elements our cache is allowed to have. This is a number between 32
and 16,777,216; the default value for this is 1024. Note that, if writing the cache to
disk or reading the cache from disk, higher values of this will slow down cache
reading/writing.

The amount of memory each cache entry uses is variable depending on the operating system
used and the size of memory allocation pages assigned. In Windows XP, for example, each
entry uses approximately four kilobytes of memory and Deadwood has an overhead of
approximately 512 kilobytes. So, if there are 512 cache elements, Deadwood uses
approximately 2.5 megabytes of memory, and if there are 1024 cache elements, Deadwood uses
approximately 4.5 megabytes of memory. Again, these numbers are for Windows XP and other
operating systems will have different memory allocation numbers.

Please note that each root_servers and upstream_servers entry takes up space in Deadwood's
cache and that maximum_cache_elements will need to be increased to store a large number of
these entries.

maxprocs

This is the maximum number of pending remote UDP connections Deadwood can have. The
default value for this is 1024.

max_tcp_procs

This is the number of allowed open TCP connections. Default value: 8

num_retries

The number of times we retry to send a query upstream before giving up. If this is 0, we
only try once; if this is 1, we try twice, and so on, up to 32 retries. Note that each
retry takes timeout_seconds seconds before we retry again. Default value: 5

ns_glueless_type

The RR type we send to resolve glueless records. This should be 1 (A) when mainly using
IPv4 to resolve records. If glueless NS records have AAAA but not A records, and IPv6 is
enabled, it may make sense to give this a value of 255 (ANY). If IPv4 ever stops being
used on a large scale, it may eventually become possible to make this have a value of 28
(AAAA).

The default value is 1: An A (IPv4 IP) record. This parameter has not been tested; use at
your own risk.

random_seed_file

This is a file that contains random numbers, and is used as a seed for the
cryptographically strong random number generator. Deadwood will try to read 256 bytes
from this file (the RNG Deadwood uses can accept a stream of any arbitrary length).

Note that the hash compression function obtains some of its entropy before parsing the
mararc file, and is hard-coded to get entropy from /dev/urandom (secret.txt on Windows
systems). Most other entropy used by Deadwood comes from the file pointed to by
random_seed_file.

recurse_min_bind_port

The lowest numbered port Deadwood is allowed to bind to; this is a random port number used
for the source port of outgoing queries, and is not 53 (see dns_port above). This is a
number between 1025 and 32767, and has a default value of 15000. This is used to make DNS
spoofing attacks more difficult.

recurse_number_ports

The number of ports Deadwood binds to for the source port for outgoing connections; this
is a power of 2 between 256 and 32768. This is used to make DNS spoofing attacks more
difficult. The default value is 4096.

recursive_acl

This is a list of who is allowed to use Deadwood to perform DNS recursion, in "ip/mask"
format. Mask must be a number between 0 and 32 (for IPv6, between 0 and 128). For example,
"127.0.0.1/8" allows local connections.

reject_aaaa

If this has a value of 1, a bogus SOA "not there" reply is sent whenever an AAAA query is
sent to Deadwood. In other words, every time a program asks Deadwood for an IPv6 IP
address, instead of trying to process the request, when this is set to 1, Deadwood
pretends the host name in question does not have an IPv6 address.

This is useful for people who aren't using IPv6 but use applications (usually *NIX command
like applications like "telnet") which slow things down trying to find an IPv6 address.

This has a default value of 0. In other words, AAAA queries are processed normally unless
this is set.

reject_mx

When this has the default value of 1, MX queries are silently dropped with their IP
logged. A MX query is a query that is only done by a machine if it wishes to be its own
mail server sending mail to machines on the internet. This is a query an average desktop
machine (including one that uses Outlook or another mail user agent to read and send
email) will never make.

Most likely, if a machine is trying to make a MX query, the machine is being controlled by
a remote source to send out undesired "spam" email. This in mind, Deadwood will not allow
MX queries to be made unless reject_mx is explicitly set with a value of 0.

Before disabling this, please keep in mind that Deadwood is optimized to be used for web
surfing, not as a DNS server for a mail hub. In particular, the IPs for MX records are
removed from Deadwood's replies and Deadwood needs to perform additional DNS queries to
get the IPs corresponding to MX records, and Deadwood's testing is more geared for web
surfing (almost 100% A record lookup) and not for mail delivery (extensive MX record
lookup).

reject_ptr

If this has a value of 1, a bogus SOA "not there" reply is sent whenever a PTR query is
sent to Deadwood. In other words, every time a program asks Deadwood for "reverse DNS
lookup" -- the hostname for a given IP address -- instead of trying to process the
request, when this is set to 1, Deadwood pretends the IP address in question does not have
a hostname.

This is useful for people who are getting slow DNS timeouts when trying to perform a
reverse DNS lookups on IPs.

This has a default value of 0. In other words, PTR queries are processed normally unless
this is set.

resurrections

If this is set to 1, Deadwood will try to send an expired record to the user before giving
up. If it is 0, we don't. Default value: 1

root_servers

This is a list of root servers; its syntax is identical to upstream_servers (see below).
This is the type of DNS service ICANN, for example, runs. These are servers used that do
not give us complete answers to DNS questions, but merely tell us which DNS servers to
connect to to get an answer closer to our desired answer.

Please note that each root_servers entry takes up space in Deadwood's cache and that
maximum_cache_elements will need to be increased to store a large number of these entries.

tcp_listen

In order to enable DNS-over-TCP, this variable must be set and have a value of 1. Default
value: 0

timeout_seconds

This is how long Deadwood will wait before giving up and discarding a pending UDP DNS
reply. The default value for this is 1, as in 1 second, unless Deadwood was compiled with
FALLBACK_TIME enabled.

timeout_seconds_tcp

How long to wait on an idle TCP connection before dropping it. The default value for this
is 4, as in 4 seconds.

ttl_age

Whether TTL aging is enabled; whether entries in the cache have their TTLs set to be the
amount of time the entries have left in the cache.

If this has a value of 1, TTL entries are aged. Otherwise, they are not. The default
value for this is 1.

upstream_port

This is the port Deadwood uses to connect or send packets to the upstream servers. The
default value for this is 53; the standard DNS port.

upstream_servers

This is a list of DNS servers that the load balancer will try to contact. This is a
dictionary variable (array indexed by a string instead of by a number) instead of a simple
variable. Since upstream_servers is a dictionary variable, it needs to be initialized
before being used.

Deadwood will look at the name of the host that it is trying to find the upstream server
for, and will match against the longest suffix it can find.

For example, if someone sends a query for "www.foo.example.com" to Deadwood, Deadwood will
first see if there is an upstream_servers variable for "www.foo.example.com.", then look
for "foo.example.com.", then look for "example.com.", then "com.", and finally ".".

Here is an example of upstream_servers:

upstream_servers = {} # Initialize dictionary variable
upstream_servers["foo.example.com."] = "192.168.42.1"
upstream_servers["example.com."] = "192.168.99.254"
upstream_servers["."] = "10.1.2.3, 10.1.2.4"

In this example, anything ending in "foo.example.com" is resolved by the DNS server at
192.168.42.1; anything else ending in "example.com" is resolved by 192.168.99.254; and
anything not ending in "example.com" is resolved by either 10.1.2.3 or 10.1.2.4.

Important: the domain name upstream_servers points to must end in a "." character. This is
OK:

upstream_servers["example.com."] = "192.168.42.1"

But this is not OK:

upstream_servers["example.com"] = "192.168.42.1"

The reason for this is because BIND engages in unexpected behavior when a host name
doesn't end in a dot, and by forcing a dot at the end of a hostname, Deadwood doesn't have
to guess whether the user wants BIND's behavior or the "normal" behavior.

If neither root_servers nor upstream_servers are set, Deadwood sets root_servers to use
the default ICANN root servers, as follows:

198.41.0.4 a.root-servers.net (VeriSign)
192.228.79.201 b.root-servers.net (ISI)
192.33.4.12 c.root-servers.net (Cogent)
199.7.91.13 d.root-servers.net (UMaryland)
192.203.230.10 e.root-servers.net (NASA Ames)
192.5.5.241 f.root-servers.net (ISC)
192.112.36.4 g.root-servers.net (DOD NIC)
128.63.2.53 h.root-servers.net (ArmyRU)
192.36.148.17 i.root-servers.net (NORDUnet)
192.58.128.30 j.root-servers.net (VeriSign)
193.0.14.129 k.root-servers.net (Reseaux)
199.7.83.42 l.root-servers.net (IANA)
202.12.27.33 m.root-servers.net (WIDE)

This list is current as of February 9, 2015, and was last changed on January 3, 2013.

Please note that each upstream_servers entry takes up space in Deadwood's cache and that
maximum_cache_elements will need to be increased to store a large number of these entries.

verbose_level

This determines how many messages are logged on standard output; larger values log more
messages. The default value for this is 3.

ip/mask format of IPs

Deadwood uses a standard ip/netmask formats to specify IPs. An ip is in dotted-decimal
format, e.g. "10.1.2.3" (or in IPv6 format when IPv6 support is compiled in).

The netmask is used to specify a range of IPs. The netmask is a single number between 1
and 32 (128 when IPv6 support is compiled in), which indicates the number of leading "1"
bits in the netmask.

10.1.1.1/24 indicates that any ip from 10.1.1.0 to 10.1.1.255 will match.

10.2.3.4/16 indicates that any ip from 10.2.0.0 to 10.2.255.255 will match.

127.0.0.0/8 indicates that any ip with "127" as the first octet (number) will match.

The netmask is optional, and, if not present, indicates that only a single IP will match.

DNS over TCP

DNS-over-TCP needs to be explicitly enabled by setting tcp_listen to 1.

Deadwood extracts useful information from UDP DNS packets marked truncated which almost
always removes the need to have DNS-over-TCP. However, Deadwood does not cache DNS packets
larger than 512 bytes in size that need to be sent using TCP. In addition, DNS-over-TCP
packets which are "incomplete" DNS replies (replies which a stub resolver can not use,
which can be either a NS referral or an incomplete CNAME reply) are not handled correctly
by Deadwood.

Deadwood has support for both DNS-over-UDP and DNS-over-TCP; the same daemon listens on
both the UDP and TCP DNS port.

Only UDP DNS queries are cached. Deadwood does not support caching over TCP; it handles
TCP to resolve the rare truncated reply without any useful information or to work with
very uncommon non-RFC-compliant TCP-only DNS resolvers. In the real world, DNS-over-TCP is
almost never used.

Parsing other files

It is possible to have Deadwood, while parsing the dwood3rc file, read other files and
parse them as if they were dwood3rc files.

This is done using execfile. To use execfile, place a line like this in the dwood3rc
file:

execfile("path/to/filename")

Where path/to/filename is the path to the file to be parsed like a dwood3rc file.

All files must be in or under the directory /etc/maradns/deadwood/execfile. Filenames can
only have lower-case letters and the underscore character ("_"). Absolute paths are not
allowed as the argument to execfile; the filename can not start with a slash ("/")
character.

If there is a parse error in the file pointed to by execfile, Deadwood will report the
error as being on the line with the execfile command in the main dwood3rc file. To find
where a parse error is in the sub-file, use something like "Deadwood -f
/etc/maradns/deadwood/execfile/filename" to find the parse error in the offending file,
where "filename" is the file to to parsed via execfile.

IPV6 support

This server can also be optionally compiled to have IPv6 support. In order to enable IPv6
support, add '-DIPV6' to the compile-time flags. For example, to compile this to make a
small binary, and to have IPv6 support:

export FLAGS='-Os -DIPV6'
make

SECURITY

Deadwood is a program written with security in mind.

In addition to use a buffer-overflow resistant string library and a coding style and SQA
process that checks for buffer overflows and memory leaks, Deadwood uses a strong
pseudo-random number generator (The 32-bit version of RadioGatun) to generate both the
query ID and source port. For the random number generator to be secure, Deadwood needs a
good source of entropy; by default Deadwood will use /dev/urandom to get this entropy. If
you are on a system without /dev/urandom support, it is important to make sure that
Deadwood has a good source of entropy so that the query ID and source port are hard to
guess (otherwise it is possible to forge DNS packets).

The Windows port of Deadwood includes a program called "mkSecretTxt.exe" that creates a
64-byte (512 bit) random file called "secret.txt" that can be used by Deadwood (via the
"random_seed_file" parameter); Deadwood also gets entropy from the timestamp when Deadwood
is started and Deadwood's process ID number, so it is same to use the same static
secret.txt file as the random_seed_file for multiple invocations of Deadwood.

Note that Deadwood is not protected from someone on the same network viewing packets sent
by Deadwood and sending forged packets as a reply.

To protect Deadwood from certain possible denial-of-service attacks, it is best if
Deadwood's prime number used for hashing elements in the cache is a random 31-bit prime
number. The program RandomPrime.c generates a random prime that is placed in the file
DwRandPrime.h that is regenerated whenever either the program is compiled or things are
cleaned up with make clean. This program uses /dev/urandom for its entropy; the file
DwRandPrime.h will not be regenerated on systems without /dev/urandom.

On systems without direct /dev/urandom support, it is suggested to see if there is a
possible way to give the system a working /dev/urandom. This way, when Deadwood is
compiled, the hash magic number will be suitably random.

If using a precompiled binary of Deadwood, please ensure that the system has /dev/urandom
support (on Windows system, please ensure that the file with the name secret.txt is
generated by the included mkSecretTxt.exe program); Deadwood, at runtime, uses
/dev/urandom (secret.txt in Windows) as a hardcoded path to get entropy (along with the
timestamp) for the hash algorithm.

DAEMONIZATION

Deadwood does not have any built-in daemonization facilities; this is handled by the
external program Duende or any other daemonizer.

Example configuration file

Here is an example dwood3rc configuration file:

# This is an example deadwood rc file
# Note that comments are started by the hash symbol

bind_address="127.0.0.1" # IP we bind to

# The following line is disabled by being commented out
#bind_address="::1" # We have optional IPv6 support

# Directory we run program from (not used in Win32)
chroot_dir = "/etc/maradns/deadwood"

# The following upstream DNS servers are Google's
# (as of December 2009) public DNS servers. For
# more information, see the page at
# http://code.google.com/speed/public-dns/
#
# If neither root_servers nor upstream_servers are set,
# Deadwood will use the default ICANN root servers.
#upstream_servers = {}
#upstream_servers["."]="8.8.8.8, 8.8.4.4"

# Who is allowed to use the cache. This line
# allows anyone with "127.0" as the first two
# digits of their IP to use Deadwood
recursive_acl = "127.0.0.1/16"

# Maximum number of pending requests
maxprocs = 2048

# Send SERVER FAIL when overloaded
handle_overload = 1

maradns_uid = 99 # UID Deadwood runs as
maradns_gid = 99 # GID Deadwood runs as

maximum_cache_elements = 60000

# If you want to read and write the cache from disk,
# make sure chroot_dir above is readable and writable
# by the maradns_uid/gid above, and uncomment the
# following line.
#cache_file = "dw_cache"

# If your upstream DNS server converts "not there" DNS replies
# in to IPs, this parameter allows Deadwood to convert any reply
# with a given IP back in to a "not there" IP. If any of the IPs
# listed below are in a DNS answer, Deadwood converts the answer
# in to a "not there"
#ip_blacklist = "10.222.33.44, 10.222.3.55"

# By default, for security reasons, Deadwood does not allow IPs in
# the 192.168.x.x, 172.[16-31].x.x, 10.x.x.x, 127.x.x.x,
# 169.254.x.x, 224.x.x.x, or 0.0.x.x range. If using Deadwood
# to resolve names on an internal network, uncomment the
# following line:
#filter_rfc1918 = 0

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