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Man Pages

Manual Reference Pages  -  GDNSD.CONFIG (5)

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gdnsd.config - gdnsd configuration file



  options => {
    log_stats => 86400
    tcp_timeout => 15 ; zonefile-style comment
    include_optional_ns => true
    listen => [, ]

  # shell-style comment

  service_types => {
    foosvc => { plugin => http_status, vhost =>, url_path => "/checkme" }
    barsvc => $include{bar-svc.cfg}

  plugins => {
    null => {}


This man page describes the syntax of the primary gdnsd configuration file. The primary config file is always the the file named config in the configuration directory. The default configuration directory is /usr/local/etc/gdnsd, but this can be overridden by the -c commandline option.

The lower-level syntax and structure of the configuration language is described in detail at the end of this document, but it should be fairly intuitive from the example above. It is effectively a generic data structure language allowing arbitrarily-nested ordered hashes, ordered arrays, and scalar values. Double-quotes are used to quote scalars containing whitespace or various ambiguous metacharacters.

The top-level implicit hash of a gdnsd configuration file allows only 3 legal keys: options, service_types, and plugins.

Any of them which are present must have a Hash as their value.

All of them are optional, as is the configuration file itself. If you’re happy with an all-default configuration, you can simply not have a config file at all.


These options control the overall behavior of gdnsd(8).
username String, defaults to gdnsd. This is the username the daemon drops privileges to the uid/gid of on startup if started as root.
weaker_security Boolean, default false. When false, the daemon may take additional privilege-preventing measures beyond the basic drop of the uid/gid of the process. These may be platform specific and evolve over time, and may impose limits that break some users’ configurations in corner cases.

This option exists as an escape hatch to get things working again, but the name of the option is intended to pressure you to find another way to accomplish your goal without requiring reduced security.

At this time, the only security feature this controls is setting the Linux-specific prctl() flag PR_SET_NO_NEW_PRIVS on kernels 3.5 and higher. When this is set, it immutably prevents the process and all descendants from ever gaining new privileges again. This is done regardless of whether the daemon initially started as root and voluntarily dropped its own privileges or was started as a regular user.

Note that PR_SET_NO_NEW_PRIVS could break plugin_extmon configurations which execute binaries that need escalated privileges via set[ug]id bits and/or capabilities bits. A classic example of such a binary is ping.

zones_default_ttl Integer seconds, default 86400. This is the global default time-to-live for any record in any zonefile. It can be overridden with a more specific default within zone files themselves via the $TTL directive (see gdnsd.zonefile(5)).
max_ttl Integer seconds, default 3600000 (~42 days), range 3600 - 268435455 (2^28-1, ~8.5 years). This is the global maximum TTL. Any TTL found in a zone which exceeds this value will be clamped to this value with a warning. Note that the default maximum value is what the Internet’s root nameservers currently use for A-record TTLs, and those are arguably the most stable records in the whole system. It’s hard to imagine good reasons to raise this value in practice.
min_ttl Integer seconds, default 5, range 1 - 86400 (1 day). This is the global minimum TTL. Any TTL found in a zone which is below this value will be clamped to this value with a warning, including the minimum TTLs of DYN[AC] records and SOA ncache TTLs. This value must be less than or equal to max_ttl.
max_ncache_ttl Integer seconds, default 10800, range 10 - 86400. This is the global maximum for the SOA negative-cache TTL field. Values above this will be clamped with a warning. This value must be greater than or equal to min_ttl.
dns_port Integer port, 1-65535, default 53. This is the global default port number for DNS listener addresses which do not specify port numbers themselves.
http_port Integer port, 1-65535, default 3506. This is the default HTTP port number for stats listener addresses which do not specify port numbers themselves.
listen The listen option specifies the socket addresses the server listens on for DNS requests.

A listen-address specification is an IP (v4 or v6) address specified as a numeric string with standard formatting (anything numeric that getaddrinfo() supports on your platform), optionally followed by a colon and a port number. If no port number is specified, it defaults to the value from dns_port, which defaults to 53.

Due to various parsing ambiguities, if you wish to specify a non-default port number for an IPv6 listen address, you will have to enclose the address part in square brackets, and then enclose the entire string in double-quotes.

The structure of the listen option as a whole can take one of three basic forms. In its simplest form, it is just a single listen-address specification as a string, such as:

  options => { listen = }

It can also take the form of an array of such addresses, as in:

  options => {
    listen = [,,

It can also be a hash where the keys are listen addresses, and the values are per-address options, as in:

  options => {
    listen => { => {
        tcp_timeout = 7
      }, => {
        udp_threads = 5

The per-address options (which are identical to, and locally override, the global option of the same name) are tcp_threads, tcp_timeout, tcp_clients_per_thread, udp_threads, udp_recv_width, udp_rcvbuf, and udp_sndbuf.

Finally, it can also be set to the special string value any, as in:

  options => { listen => any }

This is the default mode if no explicit listen option is provided. In this mode, the daemon will listen on the dns_port port (default 53) on the IPv4 and IPv6 ANY addresses and ::. gdnsd’s ANY-address sockets should correctly handle sending outgoing datagrams via the interface they were received on with a source address matching the destination address of the request.

http_listen Basically like listen above, but used for the HTTP listener (serving stats information), and defaulting to port 3506. The hash form isn’t supported as there are no per-address options, and the any/all options don’t exist here. The default is to listen on the IPv4 and IPv6 any-addresses ( and ::).

It makes common sense to restrict access to this service via firewall rules, as the data served leaks information about the rate and nature of your DNS traffic. This is mostly intended for your own internal monitoring purposes.

tcp_threads Integer, default 1, min 0, max 1024. This is the number of separate TCP listening sockets and corresponding listener threads that will be created for each DNS listener address. On a multi-core host, increasing this parameter (up to at most a small multiple of the CPU core count) may increase overall performance. Note that on hosts without SO_REUSEPORT support (notably Linux < 3.9, Solaris), any setting greater than 1 will be forced to 1 with a warning, as support multiple sockets/threads per-address are not supported without SO_REUSEPORT.
udp_threads Exactly like tcp_threads, but for UDP sockets per DNS listening address.
tcp_clients_per_thread Integer, default 128, min 1, max 65535. This is maximum number of tcp DNS connections gdnsd will allow to occur in parallel per listening tcp thread. Once this limit is reached by a given thread, no new connections will be allowed to that thread until one of the existing ones closes or times out. Note that sockets map 1:m to threads, and thus the total client limit for connecting to a given socket address would be tcp_clients_per_thread * tcp_threads.
tcp_timeout Integer seconds, default 5, min 3, max 60. TCP DNS connections will be forcibly shut down if they go idle without receiving and responding to a valid query for this many seconds. gdnsd(8) allows multiple requests per connection, and this idle timeout applies to the time between requests as well.
udp_recv_width Integer, default 8, min 1, max 64. On supported Linux kernels this setting tunes the use of more efficient interfaces to receive and send multiple packets with a single syscall. Higher values reduce syscall overhead and generally give the server higher throughput and better efficiency under high loads.

I believe that this is basically always a win under load when supported, but values much larger than necessary do have a chance to increase average response latency very slightly. The optimal setting is highly dependent on local hardware, software configuration, and network load conditions.

Setting this to a value of 1 will completely disable this code, as if we were running on a platform that didn’t support it. On platforms that don’t support it, this option has no effect and is ignored. On Linux if we don’t detect a 3.0 or higher kernel at runtime, we fall back to the same code as other platforms that don’t support it.

udp_rcvbuf Integer, min 4096, max 1048576. If set, this value will be used to set the SO_RCVBUF socket option on the UDP listening socket(s). Most users do not need to tune this value. If left unset, the code takes a somewhat heuristic approach, trying to raise the value only if the OS-supplied default seems too low, and multiplying it a bit in the case of udp_recv_width > 1.
udp_sndbuf Integer, min 4096, max 1048576. If set, this value will be used to set the SO_SNDBUF socket option on the UDP listening socket(s). Tuning advice mirrors the above.
max_http_clients Integer, default 128, min 1, max 65535. Maximum number of HTTP connections to allow in parallel at any given time. Once this number is reached, no more new connections will be answered until an existing connection closes or times out.
http_timeout Integer seconds, default 5, min 3, max 60. HTTP connections will be forcibly shut down if they go idle for more than this many seconds.
zones_strict_data Boolean, default false

If false (the default), reporting of many less-serious errors in zone data are emitted as mere logged warnings, and the zone data is still loaded and served.

If this is set to true, such warnings will be upgraded and treated the same as the more-serious class of zone data errors which prevent successful loading of zone data. The consequences of this are variable: on initial startup or checkconf, this results in a failed zonefile, which may either be ignored or abort execution, depending on zones_strict_startup below. During a runtime zone data reload, any existing good copy of the zone would continue to be served until the error is corrected in the source.

zones_strict_startup Boolean, default true

If true (the default), on daemon startup (via start or restart) if any zone fails to load correctly, the daemon will abort. If false, the daemon will simply ignore the failed zone and continue operations.

Runtime reloads via SIGUSR1 and/or periodic/inotify scanning always treat bad zone data non-fatally (leaving any existing good copy intact in memory for lookups).

This also affects the checkconf action. It will only fail in terms of exit value on bad zonefiles if this is true (although it will note any failures to stderr regardless).

zones_rfc1035_auto Boolean, default true.

If auto is enabled (the default), the daemon will detect changes to zone data automatically at runtime and apply them as they appear. In the general case this is done by periodically scanning lstat() data on the contents of the zones directory and looking for metadata changes since last check.

On modern Linux systems, the daemon may also use inotify() to detect filesystem modifications in realtime. In these cases it will not usually run the periodic lstat() scans.

Regardless of whether this setting is true or false, you can always manually trigger a rescan of the zones directory for new data by sending the daemon a SIGUSR1 (or executing the reload-zones command, which sends SIGUSR1 for you).

zones_rfc1035_auto_interval Integer seconds, default 31, min 10, max 600. Only applies when zones_rfc1035_auto is true.

Sets the time interval for periodically checking the zonefile directory for changes. On systems which support inotify(), however, the automatic mode will almost always use that mechanism instead for even faster detection with less overhead. In the inotify() case, the interval is used only occasionally as a fallback mechanism to recover a consistent state after temporary inotify() failures due to inotify queue overflows or the zones directory itself being moved/deleted, etc.

zones_rfc1035_quiesce Floating-point seconds, default 3.0, min 1.02, max 60.0

Regardless of whether you’re using zones_rfc1035_auto and/or explicit zone reloads, this interval defines a quiescence delay timer that’s commonly used to coalesce multiple updates to the same file, avoid race conditions with zonefile writers, and potentially avoid filesystem timestamp issues. This timer value is also used as the delay to retry loading a zonefile indefinitely if it fails to load when we first detected a change due to e.g. permissions or locking issues (as opposed to parse failure).

The timer doesn’t generally apply in the inotify() case unless there are multiple nearly-simultaneous events for the same file, or (usually) when the file is modified in-place, or again if there’s a filesystem-level rather than parser-level issue loading the zonefile.

It is highly recommended that whatever tools or scripts you use to manage zonefile updates use atomic operations (in commandline terms: mv, rm and ln (without -s!); in syscall terms: rename(), unlink(), and link()) to replace them regardless of whether your system supports inotify() and regardless of whether you’re using zones_rfc1035_auto or not. The scanner ignores subdirectories and dotfiles; feel free to use those to write out the file initially before atomically putting data into view.

Performing non-atomic operations (e.g. in-place writes) on an active zonefile is inherently racy, especially if more than one update occurs in less time than the timestamp accuracy of the filesystem. The daemon makes some accommodations for handling these races, but there will always be ugly corner cases. It may help slightly if the in-place updater acquires an fcntl() advisory writelock. In-place writes will be especially unreliable if you overwrite a file while the daemon is scanning the directory during its initial startup, as no quiescence timers or other anti-race mechanisms are used during startup (as these would necessarily delay service availability).

Note that in the general case if a zone file never goes the full quiescence period without having yet another update applied to it, the new data may never actually be reloaded, as the daemon will constantly be trying to wait for a full period of quiescence on the file before loading it.

lock_mem Boolean, default false. Causes the daemon to do mlockall(MCL_CURRENT|MCL_FUTURE), which effectively locks all daemon memory into RAM, unable to be swapped. Possibly helpful in some production cases to ensure swap-in doesn’t affect DNS latency.

When started as root with lock_mem set to true, the daemon will remove any ulimits on locked memory before dropping privileges. When started as a regular user it may not be able to do so, and those limits could cause the server to abort execution at any time if they are set too low.

priority Signed integer, range -20 to +20, lower values are higher priority. If explicitly set, gdnsd will attempt setpriority() to this value on startup. If left unset and gdnsd is started as a normal user, no setpriority() call will be made. If left unset and gdnsd is started as root, it will default to calling setpriority() with the value -11.
disable_text_autosplit Boolean, default false. On the wire, TXT records are encoded as discrete chunks of up to 255 characters per chunk. The relevant RFCs state that multiple chunks should be treated by clients as if they are concatenated. That is to say, it should make no difference to a client whether the TXT data is sent as two 16-byte chunks or one 32-byte chunk.

Ordinarily, you may specify chunk(s) of a TXT record in gdnsd zonefiles as a string of any size up to the legal length (just short of 64K in practice), and gdnsd will auto-split the data into 255-byte chunks for transmission over the DNS protocol correctly. If you choose to manually break up your TXT record into multiple strings in the zonefile, gdnsd also honors these boundaries and will not attempt to merge them into larger chunks where possible.

If you set this option to true, the auto-splitting behavior is disabled, and any single character string specified in a zonefile as part of a TXT record which is larger than 255 bytes will be considered a syntax error.

include_optional_ns Boolean, default false. Causes the daemon to include the optional NS records in the Authority section of simple authoritative responses containing actual response data. Leaving this option in its default state results in smaller response packets and faster response packet generation in many common cases. This is similar in nature to (but not exactly like) BIND’s minimal-responses option, except that we default to the minimal mode.

Regardless of this setting, all *necessary* Authority-section records are always included, such as when they are necessary for delegation responses, NXDOMAIN responses, and NOERROR responses containing no RRsets in the answer section.

plugin_search_path A single string or an array of strings, default empty. Normally the daemon searches for plugins in the fixed path /usr/local/lib/gdnsd, using filenames of the form plugin_${name}.so. If you define this parameter, all paths in this list will be searched in the given order for plugins *before* trying the default, fixed search path.
realtime_stats Boolean, default false. Normally the daemon self-imposes a limit of not recalculating the daemon-wide statistics more often than once per second. This improves efficiency in the case that the polling traffic on our HTTP interface gets high.

For most uses the default should be fine. If you set this option to true, the stats will be recalculated on the spot for every stats request. The test suite uses this so that it can double-check statistics counters between every request it sends. I don’t imagine anyone else will need to use this option, and it could even be determinental to performance on SMP machines.

max_response Integer, default 16384, min 4096, max 64000. This number is used to size the per-I/O-thread buffers that we construct response packets in. For any sane, normal use of the DNS, the default value is far more than enough. For embedded or other low memory hosts, you might even consider setting this smaller than default to save a bunch of per-socket-context buffer space.

However, if you have strange DNS data that’s very large (giant RRsets, giant blobs of data in TXT records) which might generate response packets greater than the 16K default max here, you *must* set this parameter large enough to accommodate them or random very bad things will happen. It should be noted that the odds are high whatever you’re trying to do is misguided in the first place. You can size this by setting it to the max and running some test queries via dig (or a similar tool) to find your limit.

This number does not need to take into account UDP, IP, or any lower-level headers. Typically when probing your data for the largest response sizes you should do ANY queries and/or specific RR-type queries against the first CNAME in any CNAME chains leading to large RR-sets. Keep in mind that the include_optional_ns option will affect the sizing as well. Also keep in mind that wildcards and delegations can match any child name, including ones of maximal overall length.

max_edns_response Integer, default 1410, min 512, max 64000. This is the maximum size of a UDP edns response to a client, acting as a cap on the edns buffer size advertised by the client in its request.

The default of 1410 is the largest size suggested in RFC 6891 when falling back from the inability to deliver 4K-sized packets, and it seems very likely to be a successful size for unfragmented delivery on most networks today even given IPv6 and some reasonable tunneling.

The option obviously has no pragmatic effect if you do not have large response datasets in your zones in the first place.

This value will be capped at the configured (or default) value of max_response with a warning if configured above that value.

max_addtl_rrsets Integer, default 64, min 16, max 256. This is the maximum number of RR sets that will ever be added to the Additional section of a response packet. This sets a hard limit on the number of delegation glue NS records a subzone can have (which is checked at startup), and a runtime soft limit on other Additional section RR sets. When the limit is reached at runtime, the remaining potential additional RR sets are simply not added to the packet. Most users won’t need to raise this value, and users on low-memory/embedded hosts might want to lower it to save more memory.
max_cname_depth Integer, default 16, min 4, max 24. How deep CNAME -> CNAME chains are allowed to recurse within local data in a single zonefile. If a chain longer than this is detected between normal static CNAME entries in the authoritative data of a single zonefile, an error will be thrown when loading the zonefile.

If the limit is exceeded at runtime (due to DYNC dynamic CNAME responses) the code will halt further recursive lookups for this request and return an empty NXDOMAIN response, and log a loud message to syslog on every single request for this broken domainname.

Note that this is the only thing preventing infinite CNAME loops caused by bad DYNC plugin configurations. Also note that even in the DYNC case, all of this applies only within a single zone. The gdnsd code never crosses the boundary between two distinct local zonefiles when processing queries.

edns_client_subnet Boolean, default true. Enables support for the edns-client-subnet option. gdnsd only includes this EDNS option in responses to queries which also contained the option. In the case of normal responses from static zone data, the scope mask will be set to zero. Dynamic response plugins have access to the query’s EDNS client-subnet data, and have full control over the response scope mask.

If the option is set to false, gdnsd will ignore the option in queries, never set it in its responses, and plugins will not have access to any data provided by any ignored edns-client-subnet option in queries.

Of the included standard plugins only reflect and geoip make use of edns-client-subnet information. The rest will leave the scope mask at zero as normal for client-location-agnostic static data.

Relevant links documenting edns-client-subnet:

<> <>

chaos_response String, default gdnsd. When gdnsd receives any query with the class CH (Chaos), as opposed to the normal IN (Internet), it will return a single response record of class CH and type TXT, which contains the string defined here. This is something like BIND’s version reporting, which responds to version.bind queries in the CH class, and is what a client will see if they use such a query against a gdnsd server.
log_stats Integer, default 3600, min 0, max 86400. The current stats counters will be emitted as log output (e.g. to syslog) every log_stats seconds. If set to zero, periodic stats logging is disabled. Regardless of this setting, stats counters are always emitted to the log once at the time of daemon shutdown.
run_dir String, defaults to /var/run/gdnsd. This is the directory which the daemon owns as its run directory. It will create this directory and/or modify the permissions and ownership of it on startup. If it does not exist and cannot be created, or the permissions and ownership cannot be set to acceptable values, the daemon will not start.

The contents of this directory are private to the daemon and shouldn’t be interfered with. This can live on a filesystem that’s volatile across reboots, and doesn’t require much disk space.

state_dir String, defaults to /var/db/gdnsd. This is the directory which the daemon owns as its state directory. It will create this directory and/or modify the permissions and ownership of it on startup. If it does not exist and cannot be created, or the permissions and ownership cannot be set to acceptable values, the daemon will not start.

The contents of this directory belong to the system administrator and are used to communicate persistent, stateful information to the daemon. This should live on a filesystem which is preserved across reboots.


service_types is used in conjunction with certain gdnsd plugins. If you are not using such a plugin, you can safely ignore this section and omit it from your configuration.

The service_types hash contains generic definitions for how to monitor a given types of service, independently of any specific address or hostname for that service.

There are two trivial service_types internally defined as the names up and down, which do no actual monitoring and simply set the monitored state permanently UP or DOWN. up is the default service_type when no service_type is specified.

Within the definition of a service_type there are several generic parameters related to timing and anti-flap, as well as plugin-specific parameters that vary per plugin.

A service type does not, however, specify a name or address for a specific instance of a service. Those would occur on a per-address basis in a resolving plugin’s configuration down in the plugins stanza, and the plugin’s configuration would then reference a named service type to be used when monitoring said address.

A service monitored through these mechanisms is always in either the UP or DOWN state at runtime from a monitoring perspective. The UP state is maintained in the face of intermittent or isolated failures until the anti-flap thresholds are crossed and the state moves to DOWN.

Any services monitored for plugins also have their state reported alongside the standard gdnsd statistics report, served by the built-in HTTP server (default port is 3506).

The following are the generic parameters for all service_types:
up_thresh Integer, default 20, min 1, max 65535. Number of monitoring requests which must succeed in a row without any failures to transition a given resource from the DOWN state to the UP state.
ok_thresh Integer, default 10, min 1, max 65535. See below.
down_thresh Integer, default 10, min 1, max 65535. The ok_thresh and down_thresh parameters control the transition from the UP state to the DOWN state while trying to prevent flappy behavior. Their behavior is best described in terms of an internal failure counter for a resource which is currently in the UP state. The failure counter starts at zero on state transition into the UP state.

Every state poll that results in a failed response, even if other successful responses are interleaved between them, increments the failure counter. If the failure counter reaches down_thresh the resource is transitioned to the DOWN state. However, if ok_thresh successes occur in a row with no failures between them, the failure counter is reset back to zero.

So with the default values, the expected behavior is that if an UP resource experiences 10 (possibly isolated or intermittent) monitor-polling failures over any length of time, without a string of 10 successes in a row somewhere within the sequence to reset the counter, it will transition to the DOWN state. Once DOWN, it will require 20 successes in a row before transitioning back to the UP state.

interval Integer seconds, default 10, min 1, max 255. Number of seconds between successive monitoring requests for a given resource.
timeout Integer seconds, default interval/2, min 1, max 255. Maximum time the monitoring code will wait for a successful response before giving up and considering the request to be a failure. Defaults to half of the interval, and must be less than interval.
plugin String, required. This indicates which specific plugin to use to execute the monitoring requests. Any parameters other than the generic ones listed here are consumed by the plugin.
There are six monitoring plugins included with gdnsd that can be used in a service_types definition, each of which may have additional, plugin-specific configuration options in addition to the generic ones above. Each of these is documented in detail in its own manpage e.g. gdnsd-plugin-FOO:
tcp_connect Checks TCP basic connectivity on a given port. Only supports address resources, not CNAMEs.
http_status Checks HTTP connectivity, with options for the port, URL, and vhost to use in the request, and the acceptable HTTP status codes in the response. Only supports address resources, not CNAMEs.
extmon Periodically executes a custom external commandline program to poll for the status of a resource. Supports both address and CNAME resources.
extfile Reads the contents of a file on disk to import state monitoring data from another source. Supports both address and CNAME resources.
static Configures a static monitoring result, mostly for testing / example code. Supports both address and CNAME resources.
null Configures an always-down static result, mostly for testing / example code. Supports both address and CNAME resources.


The plugins hash is optional, and contains one key for every dynamic resolution plugin you wish to load and use. The value must be a hash, and the contents of that hash are supplied to the plugin to use in configuring itself. If the plugin requires no configuration, the empty hash {} will suffice. It is up to the plugin to determine whether the supplied hash of configuration data is legal or not.

Monitoring-only plugins can also be given plugin-global level configuration here if the plugin author deemed it necessary.

gdnsd ships with eight different monitoring plugins, all of which have their own separate manpage documentation (e.g. man gdnsd-plugin-FOO):
reflect Reflects DNS client source IP and/or edns-client-subnet information back to the requestor as address data for debugging.
simplefo Simple primary->secondary failover of monitored addresses
multifo All-active failover of monitored round-robin address groups
weighted Weighted-round-robin responses with a variety of behavioral flavors, for both monitored addresses and CNAMEs.
metafo Static-ordered address(-group) meta-failover between ’datacenters’, which are resources defined in terms of other plugins. Supports both address and CNAME data.
geoip Combines metafo’s functionality with MaxMind GeoIP databases to select different datacenter address(-group) preference/failover orderings for different clients based on approximate geographic location. Supports both address and CNAME data.
null Returns all-zeros addresses or the CNAME invalid. - mostly for testing and as simple example code.
static Configures static mappings of resources names to IP addresses or CNAMEs - mostly for testing and as simple example code.
A configuration example showing the trivial plugins, as well as demonstrating the service_types described earlier:

  service_types => {
    corpwww_type => {
      plugin => http_status
      vhost =>
      url_path => /check_me
      down_thresh => 5
      interval => 5

  plugins => {
    null => {},
    reflect => {},
    static => {
      foo =
      bar =
      somehost =
    multifo => {
      web-lb =>
        service_types => [ corpwww_type, xmpp ],
        lb01 =>,
        lb02 =>,
        lb03 =>,

And then in your zonefile, you could have (among your other RRs):

  zeros 600 DYNA null
  reflect 10 DYNA reflect
  reflect-both 10 DYNA reflect!both
  pointless 42 DYNA static!foo
  acname 400 DYNC static!somehost
  www 300/45 DYNA multifo!web-lb


At the lowest level, the syntax of gdnsd config files roughly resembles an anonymous Perl data structure (using reference syntax). There are three basic data types for values: ordered hashes (associative arrays mapping keys to values), ordered arrays of values, and simple strings. Hashes and arrays can be nested to arbitrary depth. Generally speaking, whitespace is optional. Single-line comments in both shell (#) and DNS zonefile styles (;) are allowed. They run to the end of the current line and are considered to be whitespace by the parser.

A hash is surrounded by curly braces ({ and }). Keys are separated from their values by either => or = (at your stylistic discretion). Hash keys follow the same rules as simple string values. Hash values can be simple strings, arrays, or hashes. Key/value pairs can optionally have a trailing comma for stylistic clarity and separation.

An array is surrounded by square braces ([ and ]). Values can be simple strings, arrays, or hashes. Values can optionally have a trailing comma for style.

Strings (and thus keys) can be written in both quoted and unquoted forms. In the quoted form, the string is surrounded by double-quotes ("), and can contain any literal byte value (even binary/utf-8 stuff, or NUL) other than " or \. Those two characters must be escaped by \, i.e. \" and \\.

In the unquoted form, there are no surrounding quotes, and the allowed set of unescaped characters is further restricted. The following are not allowed: ][}{;#,"=\ (that is, square brackets, curly brackets, semicolons, octothorpes, commas, double quotes, equal signs, and backslashes). Additionally, the first character cannot be a $ (dollar sign).

Both forms use the same escaping rules, which are the same RFC-standard escaping rules used in zone files. The escapes always start with \. \ followed by any single byte other than a digit (0 - 9) is interepreted as that byte. \ followed by exactly 3 digits interprets those digits as the unsigned decimal integer value of the desired byte (the 3 digit value cannot exceed 255).

To illustrate the escaping and quoting, the following sets of example strings show different encodings of the same parsed value:



  white\ space
  "white space"



The top level of the config file is an implicit hash with no bracing by default, but can also be an array bounded by square brackets. This is not legal for the primary gdnsd configuration file, but could be useful in includefiles (see below).

As a general rule, anywhere the higher-level syntax allows an array of values, you can substitute a single value. The code will treat it as if it were an array of length 1.

When we refer in other sections above to a value as being an Integer (or other specific scalar type), we’re referring to constraints on the content of the character string value. All scalar values are character strings. Boolean values are characters strings which have the value true or false, in any mix of upper or lower case.

The following 3 example configuration files are identical in their parsed meanings, and should clarify anything miscommunicated above:

Example 1 (simple and clean):

  options = {
    listen = [, ],
    http_listen =,

Example 2 (fat arrows, no commas, some arbitrary quoting):

  "options" => {
    listen => [ ]
    http_listen => ""

Example 3 (compressed and ugly):



vscf now has a mechanism for config includefiles. The syntax is


where filename can use the same kinds of escaping and/or double-quoting as normal scalar string data. Whitespace between the filename and the surrounding brackets is optional. Whitespace between $include and the following { is not allowed. If the filename is relative (does not begin with /), it is interpreted as relative to the directory containing the parent file. Include files can nest other include files to arbitrary depth.

Keep in mind that at the top level of any given vscf file (even include files), the file must syntactically be either an implicit hash or an explicit, square-bracket-bounded, array.

The include statement can be used in two distinct contexts within the syntax structure of a config file:
Value Context The include statement can replace any whole value (that is, the right hand side of a hash map entry or a member of an array) with its own contents, which are either a hash or an array. Note that there is no mechanism for flattening an include-file’s array into the parent array (the whole included array would be a single array item within the parent array). Examples:

  main config:
    options => { listen => $include{foo} }
    [, ]

  main config:
    plugins => $include{ "bar" }
    geoip => { ... }
    extmon => { ... }

Hash-Merge Context The include statement can also appear in a hash where a key would normally be expected. In this case, the included file must be in hash (rather than array) form at the top level, and its contents are merged into the parent hash. The merge is shallow, and conflicting keys are not allowed. Example:

  main config:
    options => { ... },
    plugins => {
        extmon => { ... },
        metafo => { ... },
        simplefo => { ... }
    geoip => { ... },
    weighted => { ... }


gdnsd(8), gdnsd.zonefile(5), gdnsd-plugin-simplefo(8), gdnsd-plugin-multifo(8), gdnsd-plugin-weighted(8), gdnsd-plugin-metafo(8), gdnsd-plugin-geoip(8), gdnsd-plugin-extmon(8), gdnsd-plugin-extfile(8) gdnsd-plugin-api(3)

The gdnsd manual.


Copyright (c) 2012 Brandon L Black <>

This file is part of gdnsd.

gdnsd is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

gdnsd is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with gdnsd. If not, see <>.

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gdnsd 2.2.2 GDNSD.CONFIG (5) 2016-04-03

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