Please read the file README.txt first; this file is the reference manual for Deadwood and does not describe how to install the Deadwood service. 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 CONFIGURATION FILE FORMAT The Deadwood configuration file, dwood3rc.txt, 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. 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 is set to 1, RFC1918 (private) IP addresses are not allowed to be in DNS replies. See http://crypto.stanford.edu/dns/ for why this improves security. 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. 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 simultanious clients we process at the same time for the same query. If, while processone 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. 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. Each cache entry uses approximately four kilobytes of memory. maxprocs This is the maximum number of pending remote UDP connections Deadwood can have. The default value for this is 32. 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 8 retries. Note that each retry takes timeout_seconds seconds before we retry again. Default value: 2 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, in "ip/mask" format. Mask must be a number between 0 and 32. 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). 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. 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 2, as in 2 seconds. 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 deoesn'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) 128.8.10.90 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 May 3, 2010, and was last changed on November 1, 2007. 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, 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 Deadwood has support for both DNS-over-UDP and DNS-over-TCP; the same daemon listens on both the UDP and TCP DNS port. DNS-over-TCP must be explicitly enabled by setting tcp_listen to 1. Only UDP DNS queries are cached. Deadwood does not support caching over TCP; it handles TCP to resolve the occasional truncated reply or handle the occasional non-RFC-compliant TCP-only DNS resolver. 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 Deadwood is run from. 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. 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. 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/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 = 8 # 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" BUGS DNS-over-TCP needs to be explicitly enabled. Note that DNS-over-TCP is almost never used. Also, 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, namely either a NS referral or an incomplete CNAME reply) are not handled correctly by Deadwood. Again, DNS-over-TCP is very rare. Deadwood can not process DNS resource record types with numbers between 65392 and 65407. These RR types are marked by the IANA for "private use"; Deadwood reserves these record types for internal use. This is only 16 record types out of the 65536 possible DNS record types (only 71 have actually been assigned by IANA, so this is a non-issue in the real world). It is not clear whether the DNS RFCs allow ASCII control characters in DNS names. Even if they were, Deadwood does not allow ASCII control characters (bytes with a value less then 32) in DNS names. Other characters (UTF-8, etc.) are allowed. LEGAL DISCLAIMER THIS SOFTWARE IS PROVIDED BY THE AUTHORS ''AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHORS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. AUTHORS Sam Trenholme (http://www.samiam.org) is responsible for this program and reference.