--- name: "evdns" section: "3" platform: "linux" package: "libevent-dev" version: "1.4.13-stable-1" release: "Debian 6.0 (Squeeze)" platforms: ["linux"] url: "https://new.carta.tech/evdns" --- # evdns(3) asynchronous functions for DNS resolution. ## NAME **evdns\_init** **evdns\_shutdown** **evdns\_err\_to\_string** **evdns\_nameserver\_add** **evdns\_count\_nameservers** **evdns\_clear\_nameservers\_and\_suspend** **evdns\_resume** **evdns\_nameserver\_ip\_add** **evdns\_resolve\_ipv4** **evdns\_resolve\_reverse** **evdns\_resolv\_conf\_parse** **evdns\_config\_windows\_nameservers** **evdns\_search\_clear** **evdns\_search\_add** **evdns\_search\_ndots\_set** **evdns\_set\_log\_fn** – asynchronous functions for DNS resolution. ## SYNOPSIS **#include \** **#include \** **#include \** *int*\ **evdns\_init**(); *void*\ **evdns\_shutdown**(*int fail\_requests*); *const char \**\ **evdns\_err\_to\_string**(*int err*); *int*\ **evdns\_nameserver\_add**(*unsigned long int address*); *int*\ **evdns\_count\_nameservers**(); *int*\ **evdns\_clear\_nameservers\_and\_suspend**(); *int*\ **evdns\_resume**(); *int*\ **evdns\_nameserver\_ip\_add(const**(*char*, *\*ip\_as\_string);*); *int*\ **evdns\_resolve\_ipv4**(*const char \*name*, *int flags*, *evdns\_callback\_type callback*, *void \*ptr*); *int*\ **evdns\_resolve\_reverse**(*struct in\_addr \*in*, *int flags*, *evdns\_callback\_type callback*, *void \*ptr*); *int*\ **evdns\_resolv\_conf\_parse**(*int flags*, *const char \**); *void*\ **evdns\_search\_clear**(); *void*\ **evdns\_search\_add**(*const char \*domain*); *void*\ **evdns\_search\_ndots\_set**(*const int ndots*); *void*\ **evdns\_set\_log\_fn**(*evdns\_debug\_log\_fn\_type fn*); *int*\ **evdns\_config\_windows\_nameservers**(); ## DESCRIPTION Welcome, gentle reader Async DNS lookups are really a whole lot harder than they should be, mostly stemming from the fact that the libc resolver has never been very good at them. Before you use this library you should see if libc can do the job for you with the modern async call getaddrinfo\_a (see http://www.imperialviolet.org/page25.html#e498). Otherwise, please continue. This code is based on libevent and you must call event\_init before any of the APIs in this file. You must also seed the OpenSSL random source if you are using OpenSSL for ids (see below). This library is designed to be included and shipped with your source code. You statically link with it. You should also test for the existence of strtok\_r and define HAVE\_STRTOK\_R if you have it. The DNS protocol requires a good source of id numbers and these numbers should be unpredictable for spoofing reasons. There are three methods for generating them here and you must define exactly one of them. In increasing order of preference: - DNS\_USE\_GETTIMEOFDAY\_FOR\_ID Using the bottom 16 bits of the usec result from gettimeofday. This is a pretty poor solution but should work anywhere. - DNS\_USE\_CPU\_CLOCK\_FOR\_ID Using the bottom 16 bits of the nsec result from the CPU's time counter. This is better, but may not work everywhere. Requires POSIX realtime support and you'll need to link against -lrt on glibc systems at least. - DNS\_USE\_OPENSSL\_FOR\_ID Uses the OpenSSL RAND\_bytes call to generate the data. You must have seeded the pool before making any calls to this library. The library keeps track of the state of nameservers and will avoid them when they go down. Otherwise it will round robin between them. Quick start guide:\ \#include "evdns.h"\ void callback(int result, char type, int count, int ttl,\ void \*addresses, void \*arg);\ evdns\_resolv\_conf\_parse(DNS\_OPTIONS\_ALL, "/etc/resolv.conf");\ evdns\_resolve("www.hostname.com", 0, callback, NULL); When the lookup is complete the callback function is called. The first argument will be one of the DNS\_ERR\_\* defines in evdns.h. Hopefully it will be DNS\_ERR\_NONE, in which case type will be DNS\_IPv4\_A, count will be the number of IP addresses, ttl is the time which the data can be cached for (in seconds), addresses will point to an array of uint32\_t's and arg will be whatever you passed to evdns\_resolve. Searching: In order for this library to be a good replacement for glibc's resolver it supports searching. This involves setting a list of default domains, in which names will be queried for. The number of dots in the query name determines the order in which this list is used. Searching appears to be a single lookup from the point of view of the API, although many DNS queries may be generated from a single call to evdns\_resolve. Searching can also drastically slow down the resolution of names. To disable searching: 1. Never set it up. If you never call **evdns\_resolv\_conf\_parse,**() **evdns\_init,**() or **evdns\_search\_add**() then no searching will occur. 2. If you do call **evdns\_resolv\_conf\_parse**() then don't pass *DNS\_OPTION\_SEARCH* (or *DNS\_OPTIONS\_ALL,* which implies it). 3. When calling **evdns\_resolve,**() pass the *DNS\_QUERY\_NO\_SEARCH* flag. The order of searches depends on the number of dots in the name. If the number is greater than the ndots setting then the names is first tried globally. Otherwise each search domain is appended in turn. The ndots setting can either be set from a resolv.conf, or by calling evdns\_search\_ndots\_set. For example, with ndots set to 1 (the default) and a search domain list of \["myhome.net"\]:\ Query: www\ Order: www.myhome.net, www. Query: www.abc\ Order: www.abc., www.abc.myhome.net ## API reference - *int* **evdns\_init**() Initializes support for non-blocking name resolution by calling **evdns\_resolv\_conf\_parse**() on UNIX and **evdns\_config\_windows\_nameservers**() on Windows. - *int* **evdns\_nameserver\_add**(*unsigned long int address*) Add a nameserver. The address should be an IP address in network byte order. The type of address is chosen so that it matches in\_addr.s\_addr. Returns non-zero on error. - *int* **evdns\_nameserver\_ip\_add**(*const char \*ip\_as\_string*) This wraps the above function by parsing a string as an IP address and adds it as a nameserver. Returns non-zero on error - *int* **evdns\_resolve**(*const char \*name*, *int flags*, *evdns\_callback\_type callback*, *void \*ptr*) Resolve a name. The name parameter should be a DNS name. The flags parameter should be 0, or DNS\_QUERY\_NO\_SEARCH which disables searching for this query. (see defn of searching above). The callback argument is a function which is called when this query completes and ptr is an argument which is passed to that callback function. Returns non-zero on error - *void* **evdns\_search\_clear**() Clears the list of search domains - *void* **evdns\_search\_add**(*const char \*domain*) Add a domain to the list of search domains - *void* **evdns\_search\_ndots\_set**(*int ndots*) Set the number of dots which, when found in a name, causes the first query to be without any search domain. - *int* **evdns\_count\_nameservers**(*void*) Return the number of configured nameservers (not necessarily the number of running nameservers). This is useful for double-checking whether our calls to the various nameserver configuration functions have been successful. - *int* **evdns\_clear\_nameservers\_and\_suspend**(*void*) Remove all currently configured nameservers, and suspend all pending resolves. Resolves will not necessarily be re-attempted until evdns\_resume() is called. - *int* **evdns\_resume**(*void*) Re-attempt resolves left in limbo after an earlier call to evdns\_clear\_nameservers\_and\_suspend(). - *int* **evdns\_config\_windows\_nameservers**(*void*) Attempt to configure a set of nameservers based on platform settings on a win32 host. Preferentially tries to use GetNetworkParams; if that fails, looks in the registry. Returns 0 on success, nonzero on failure. - *int* **evdns\_resolv\_conf\_parse**(*int flags*, *const char \*filename*) Parse a resolv.conf like file from the given filename. See the man page for resolv.conf for the format of this file. The flags argument determines what information is parsed from this file: - DNS\_OPTION\_SEARCH domain, search and ndots options - DNS\_OPTION\_NAMESERVERS nameserver lines - DNS\_OPTION\_MISC timeout and attempts options - DNS\_OPTIONS\_ALL all of the above The following directives are not parsed from the file:\ sortlist, rotate, no-check-names, inet6, debug Returns non-zero on error: - 0 no errors - 1 failed to open file - 2 failed to stat file - 3 file too large - 4 out of memory - 5 short read from file ## Internals: Requests are kept in two queues. The first is the inflight queue. In this queue requests have an allocated transaction id and nameserver. They will soon be transmitted if they haven't already been. The second is the waiting queue. The size of the inflight ring is limited and all other requests wait in waiting queue for space. This bounds the number of concurrent requests so that we don't flood the nameserver. Several algorithms require a full walk of the inflight queue and so bounding its size keeps thing going nicely under huge (many thousands of requests) loads. If a nameserver loses too many requests it is considered down and we try not to use it. After a while we send a probe to that nameserver (a lookup for google.com) and, if it replies, we consider it working again. If the nameserver fails a probe we wait longer to try again with the next probe. ## SEE ALSO [event(3)](/event/3.md), [gethostbyname(3)](/gethostbyname/3.md), [resolv.conf(5)](/resolv.conf/5.md) ## HISTORY The **evdns** API was developed by Adam Langley on top of the **libevent** API. The code was integrate into **Tor** by Nick Mathewson and finally put into **libevent** itself by Niels Provos. ## AUTHORS The **evdns** API and code was written by Adam Langley with significant contributions by Nick Mathewson. ## BUGS This documentation is neither complete nor authoritative. If you are in doubt about the usage of this API then check the source code to find out how it works, write up the missing piece of documentation and send it to me for inclusion in this man page.