evdns(3)

evdns(3) | Library Functions Manual | evdns(3)

# NAME

evdns -

Welcome, gentle reader.

# SYNOPSIS

#include <event2/dns.h>
#include <event2/util.h>

## Macros

#define DNS_ERR_CANCEL 69
The request was canceled via a call to evdns_cancel_request. #define DNS_ERR_FORMAT 1
The name server was unable to interpret the query. #define DNS_ERR_NODATA 70
There were no answers and no error condition in the DNS packet. #define DNS_ERR_NONE 0
Error codes 0-5 are as described in RFC 1035. #define DNS_ERR_NOTEXIST 3
The domain name does not exist. #define DNS_ERR_NOTIMPL 4
The name server does not support the requested kind of query. #define DNS_ERR_REFUSED 5
The name server refuses to reform the specified operation for policy reasons. #define DNS_ERR_SERVERFAILED 2
The name server was unable to process this query due to a problem with the name server. #define DNS_ERR_SHUTDOWN 68
The request was canceled because the DNS subsystem was shut down. #define DNS_ERR_TIMEOUT 67
Communication with the server timed out. #define DNS_ERR_TRUNCATED 65
The reply was truncated or ill-formatted. #define DNS_ERR_UNKNOWN 66
An unknown error occurred. #define DNS_IPv4_A 1
#define DNS_IPv6_AAAA 3
#define DNS_NO_SEARCH DNS_QUERY_NO_SEARCH
#define DNS_OPTION_HOSTSFILE 8
#define DNS_OPTION_MISC 4
#define DNS_OPTION_NAMESERVERS 2
#define DNS_OPTION_SEARCH 1
#define DNS_OPTIONS_ALL 15
#define DNS_PTR 2
#define DNS_QUERY_NO_SEARCH 1
#define EVDNS_ADDITIONAL_SECTION 2
#define EVDNS_ANSWER_SECTION 0
#define EVDNS_AUTHORITY_SECTION 1
#define EVDNS_CLASS_INET 1
#define EVDNS_FLAGS_AA 0x400
#define EVDNS_FLAGS_RD 0x080
#define EVDNS_QTYPE_ALL 255
#define EVDNS_QTYPE_AXFR 252
#define EVDNS_TYPE_A 1
#define EVDNS_TYPE_AAAA 28
#define EVDNS_TYPE_CNAME 5
#define EVDNS_TYPE_MX 15
#define EVDNS_TYPE_NS 2
#define EVDNS_TYPE_PTR 12
#define EVDNS_TYPE_SOA 6
#define EVDNS_TYPE_TXT 16

## Typedefs

typedef void(* evdns_callback_type )(int result, char type, int count, int ttl, void *addresses, void *arg)
The callback that contains the results from a lookup. typedef void(* evdns_debug_log_fn_type )(int is_warning, const char *msg)
A callback that is invoked when a log message is generated. typedef void(* evdns_getaddrinfo_cb )(int result, struct evutil_addrinfo *res, void *arg)
Callback for evdns_getaddrinfo. typedef void(* evdns_request_callback_fn_type )(struct evdns_server_request *, void *)
A callback to implement a DNS server.

## Functions

struct evdns_server_port * evdns_add_server_port_with_base (struct event_base *base, evutil_socket_t socket, int flags, evdns_request_callback_fn_type callback, void *user_data)
Create a new DNS server port. int evdns_base_clear_nameservers_and_suspend (struct evdns_base *base)
Remove all configured nameservers, and suspend all pending resolves. int evdns_base_count_nameservers (struct evdns_base *base)
Get the number of configured nameservers. void evdns_base_free (struct evdns_base *base, int fail_requests)
Shut down the asynchronous DNS resolver and terminate all active requests. int evdns_base_load_hosts (struct evdns_base *base, const char *hosts_fname)
Load an /etc/hosts-style file from 'hosts_fname' into 'base'. int evdns_base_nameserver_add (struct evdns_base *base, unsigned long int address)
Add a nameserver. int evdns_base_nameserver_ip_add (struct evdns_base *base, const char *ip_as_string)
Add a nameserver by string address. int evdns_base_nameserver_sockaddr_add (struct evdns_base *base, const struct sockaddr *sa, ev_socklen_t len, unsigned flags)
Add a nameserver by sockaddr. struct evdns_base * evdns_base_new (struct event_base *event_base, int initialize_nameservers)
Initialize the asynchronous DNS library. int evdns_base_resolv_conf_parse (struct evdns_base *base, int flags, const char *const filename)
Parse a resolv.conf file. struct evdns_request * evdns_base_resolve_ipv4 (struct evdns_base *base, const char *name, int flags, evdns_callback_type callback, void *ptr)
Lookup an A record for a given name. struct evdns_request * evdns_base_resolve_ipv6 (struct evdns_base *base, const char *name, int flags, evdns_callback_type callback, void *ptr)
Lookup an AAAA record for a given name. struct evdns_request * evdns_base_resolve_reverse (struct evdns_base *base, const struct in_addr *in, int flags, evdns_callback_type callback, void *ptr)
Lookup a PTR record for a given IP address. struct evdns_request * evdns_base_resolve_reverse_ipv6 (struct evdns_base *base, const struct in6_addr *in, int flags, evdns_callback_type callback, void *ptr)
Lookup a PTR record for a given IPv6 address. int evdns_base_resume (struct evdns_base *base)
Resume normal operation and continue any suspended resolve requests. void evdns_base_search_add (struct evdns_base *base, const char *domain)
Add a domain to the list of search domains. void evdns_base_search_clear (struct evdns_base *base)
Obtain nameserver information using the Windows API. void evdns_base_search_ndots_set (struct evdns_base *base, const int ndots)
Set the 'ndots' parameter for searches. int evdns_base_set_option (struct evdns_base *base, const char *option, const char *val)
Set the value of a configuration option. void evdns_cancel_request (struct evdns_base *base, struct evdns_request *req)
Cancels a pending DNS resolution request. void evdns_close_server_port (struct evdns_server_port *port)
Close down a DNS server port, and free associated structures. const char * evdns_err_to_string (int err)
Convert a DNS error code to a string. struct evdns_getaddrinfo_request * evdns_getaddrinfo (struct evdns_base *dns_base, const char *nodename, const char *servname, const struct evutil_addrinfo *hints_in, evdns_getaddrinfo_cb cb, void *arg)
Make a non-blocking getaddrinfo request using the dns_base in 'dns_base'. void evdns_getaddrinfo_cancel (struct evdns_getaddrinfo_request *req)
int evdns_server_request_add_a_reply (struct evdns_server_request *req, const char *name, int n, const void *addrs, int ttl)
int evdns_server_request_add_aaaa_reply (struct evdns_server_request *req, const char *name, int n, const void *addrs, int ttl)
int evdns_server_request_add_cname_reply (struct evdns_server_request *req, const char *name, const char *cname, int ttl)
int evdns_server_request_add_ptr_reply (struct evdns_server_request *req, struct in_addr *in, const char *inaddr_name, const char *hostname, int ttl)
int evdns_server_request_add_reply (struct evdns_server_request *req, int section, const char *name, int type, int dns_class, int ttl, int datalen, int is_name, const char *data)
int evdns_server_request_drop (struct evdns_server_request *req)
Free a DNS request without sending back a reply. int evdns_server_request_get_requesting_addr (struct evdns_server_request *_req, struct sockaddr *sa, int addr_len)
Get the address that made a DNS request. int evdns_server_request_respond (struct evdns_server_request *req, int err)
Send back a response to a DNS request, and free the request structure. void evdns_server_request_set_flags (struct evdns_server_request *req, int flags)
Sets some flags in a reply we're building. void evdns_set_log_fn (evdns_debug_log_fn_type fn)
Set the callback function to handle DNS log messages. void evdns_set_random_bytes_fn (void(*fn)(char *, size_t))
Set a callback used to generate random bytes. void evdns_set_transaction_id_fn (ev_uint16_t(*fn)(void))
Set a callback that will be invoked to generate transaction IDs.

# Detailed 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.

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 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

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.

# Macro Definition Documentation

## #define DNS_ERR_NODATA 70

There were no answers and no error condition in the DNS packet. This can happen when you ask for an address that exists, but a record type that doesn't.

## #define DNS_ERR_NONE 0

Error codes 0-5 are as described in RFC 1035.

## #define DNS_ERR_SHUTDOWN 68

The request was canceled because the DNS subsystem was shut down.

# Typedef Documentation

## typedef void(* evdns_callback_type)(int result, char type, int count, int ttl, void *addresses, void *arg)

The callback that contains the results from a lookup. .IP "•" 2 result is one of the DNS_ERR_* values (DNS_ERR_NONE for success)

•
  type is either DNS_IPv4_A or DNS_PTR or DNS_IPv6_AAAA

•
  count contains the number of addresses of form type

•
  ttl is the number of seconds the resolution may be cached for.

•
  addresses needs to be cast according to type. It will be an array of 4-byte sequences for ipv4, or an array of 16-byte sequences for ipv6, or a nul-terminated string for PTR.

## typedef void(* evdns_debug_log_fn_type)(int is_warning, const char *msg)

A callback that is invoked when a log message is generated. Parameters:
is_warning indicates if the log message is a 'warning'
msg the content of the log message

## typedef void(* evdns_getaddrinfo_cb)(int result, struct evutil_addrinfo *res, void *arg)

Callback for evdns_getaddrinfo.

## typedef void(* evdns_request_callback_fn_type)(struct evdns_server_request *, void *)

A callback to implement a DNS server. The callback function receives a DNS request. It should then optionally add a number of answers to the reply using the evdns_server_request_add_*_reply functions, before calling either evdns_server_request_respond to send the reply back, or evdns_server_request_drop to decline to answer the request.

Parameters:

req A newly received request
user_data A pointer that was passed to evdns_add_server_port_with_base().

# Function Documentation

## struct evdns_server_port* evdns_add_server_port_with_base (struct event_base *base, evutil_socket_tsocket, intflags, evdns_request_callback_fn_typecallback, void *user_data) [read]

Create a new DNS server port. Parameters:
base The event base to handle events for the server port.
socket A UDP socket to accept DNS requests.
flags Always 0 for now.
callback A function to invoke whenever we get a DNS request on the socket.
user_data Data to pass to the callback.

Returns:

an evdns_server_port structure for this server port.

## int evdns_base_clear_nameservers_and_suspend (struct evdns_base *base)

Remove all configured nameservers, and suspend all pending resolves. Resolves will not necessarily be re-attempted until evdns_base_resume() is called.

Parameters:

base the evdns_base to which to apply this operation

Returns:

0 if successful, or -1 if an error occurred

See Also:

evdns_base_resume()

## int evdns_base_count_nameservers (struct evdns_base *base)

Get the number of configured nameservers. This returns 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.

Parameters:

base the evdns_base to which to apply this operation

Returns:

the number of configured nameservers

See Also:

evdns_base_nameserver_add()

## void evdns_base_free (struct evdns_base *base, intfail_requests)

Shut down the asynchronous DNS resolver and terminate all active requests. If the 'fail_requests' option is enabled, all active requests will return an empty result with the error flag set to DNS_ERR_SHUTDOWN. Otherwise, the requests will be silently discarded.

Parameters:

evdns_base the evdns base to free
fail_requests if zero, active requests will be aborted; if non-zero, active requests will return DNS_ERR_SHUTDOWN.

See Also:

evdns_base_new()

## int evdns_base_load_hosts (struct evdns_base *base, const char *hosts_fname)

Load an /etc/hosts-style file from 'hosts_fname' into 'base'. If hosts_fname is NULL, add minimal entries for localhost, and nothing else.

Note that only evdns_getaddrinfo uses the /etc/hosts entries.

Return 0 on success, negative on failure.

## int evdns_base_nameserver_add (struct evdns_base *base, unsigned long intaddress)

Add a nameserver. The address should be an IPv4 address in network byte order. The type of address is chosen so that it matches in_addr.s_addr.

Parameters:

base the evdns_base to which to add the name server
address an IP address in network byte order

Returns:

0 if successful, or -1 if an error occurred

See Also:

evdns_base_nameserver_ip_add()

## int evdns_base_nameserver_ip_add (struct evdns_base *base, const char *ip_as_string)

Add a nameserver by string address. This function parses a n IPv4 or IPv6 address from a string and adds it as a nameserver. It supports the following formats:

•
  [IPv6Address]:port

•
  [IPv6Address]

•
  IPv6Address

•
  IPv4Address:port

•
  IPv4Address

If no port is specified, it defaults to 53.

Parameters:

base the evdns_base to which to apply this operation

Returns:

0 if successful, or -1 if an error occurred

See Also:

evdns_base_nameserver_add()

## struct evdns_base* evdns_base_new (struct event_base *event_base, intinitialize_nameservers) [read]

Initialize the asynchronous DNS library. This function initializes support for non-blocking name resolution by calling evdns_resolv_conf_parse() on UNIX and evdns_config_windows_nameservers() on Windows.

Parameters:

event_base the event base to associate the dns client with
initialize_nameservers 1 if resolve.conf processing should occur

Returns:

evdns_base object if successful, or NULL if an error occurred.

See Also:

evdns_base_free()

## int evdns_base_resolv_conf_parse (struct evdns_base *base, intflags, const char *constfilename)

Parse a resolv.conf file. The 'flags' parameter determines what information is parsed from the resolv.conf file. See the man page for resolv.conf for the format of this file.

The following directives are not parsed from the file: sortlist, rotate, no-check-names, inet6, debug.

If this function encounters an error, the possible return values are: 1 = failed to open file, 2 = failed to stat file, 3 = file too large, 4 = out of memory, 5 = short read from file, 6 = no nameservers listed in the file

Parameters:

base the evdns_base to which to apply this operation
flags any of DNS_OPTION_NAMESERVERS|DNS_OPTION_SEARCH|DNS_OPTION_MISC| DNS_OPTIONS_HOSTSFILE|DNS_OPTIONS_ALL
filename the path to the resolv.conf file

Returns:

0 if successful, or various positive error codes if an error occurred (see above)

See Also:

resolv.conf(3), evdns_config_windows_nameservers()

## struct evdns_request* evdns_base_resolve_ipv4 (struct evdns_base *base, const char *name, intflags, evdns_callback_typecallback, void *ptr) [read]

Lookup an A record for a given name. Parameters:
base the evdns_base to which to apply this operation
name a DNS hostname
flags either 0, or DNS_QUERY_NO_SEARCH to disable searching for this query.
callback a callback function to invoke when the request is completed
ptr an argument to pass to the callback function

Returns:

an evdns_request object if successful, or NULL if an error occurred.

See Also:

evdns_resolve_ipv6(), evdns_resolve_reverse(), evdns_resolve_reverse_ipv6(), evdns_cancel_request()

## struct evdns_request* evdns_base_resolve_ipv6 (struct evdns_base *base, const char *name, intflags, evdns_callback_typecallback, void *ptr) [read]

Lookup an AAAA record for a given name. Parameters:
base the evdns_base to which to apply this operation
name a DNS hostname
flags either 0, or DNS_QUERY_NO_SEARCH to disable searching for this query.
callback a callback function to invoke when the request is completed
ptr an argument to pass to the callback function

Returns:

an evdns_request object if successful, or NULL if an error occurred.

See Also:

evdns_resolve_ipv4(), evdns_resolve_reverse(), evdns_resolve_reverse_ipv6(), evdns_cancel_request()

## struct evdns_request* evdns_base_resolve_reverse (struct evdns_base *base, const struct in_addr *in, intflags, evdns_callback_typecallback, void *ptr) [read]

Lookup a PTR record for a given IP address. Parameters:
base the evdns_base to which to apply this operation
in an IPv4 address
flags either 0, or DNS_QUERY_NO_SEARCH to disable searching for this query.
callback a callback function to invoke when the request is completed
ptr an argument to pass to the callback function

Returns:

an evdns_request object if successful, or NULL if an error occurred.

See Also:

evdns_resolve_reverse_ipv6(), evdns_cancel_request()

## struct evdns_request* evdns_base_resolve_reverse_ipv6 (struct evdns_base *base, const struct in6_addr *in, intflags, evdns_callback_typecallback, void *ptr) [read]

Lookup a PTR record for a given IPv6 address. Parameters:
base the evdns_base to which to apply this operation
in an IPv6 address
flags either 0, or DNS_QUERY_NO_SEARCH to disable searching for this query.
callback a callback function to invoke when the request is completed
ptr an argument to pass to the callback function

Returns:

an evdns_request object if successful, or NULL if an error occurred.

See Also:

evdns_resolve_reverse_ipv6(), evdns_cancel_request()

## int evdns_base_resume (struct evdns_base *base)

Resume normal operation and continue any suspended resolve requests. Re-attempt resolves left in limbo after an earlier call to evdns_base_clear_nameservers_and_suspend().

Parameters:

base the evdns_base to which to apply this operation

Returns:

0 if successful, or -1 if an error occurred

See Also:

evdns_base_clear_nameservers_and_suspend()

## void evdns_base_search_add (struct evdns_base *base, const char *domain)

Add a domain to the list of search domains. Parameters:
domain the domain to be added to the search list

## void evdns_base_search_clear (struct evdns_base *base)

Obtain nameserver information using the Windows API. 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 if successful, or -1 if an error occurred

See Also:

evdns_resolv_conf_parse() Clear the list of search domains.

## void evdns_base_search_ndots_set (struct evdns_base *base, const intndots)

Set the 'ndots' parameter for searches. Sets the number of dots which, when found in a name, causes the first query to be without any search domain.

Parameters:

ndots the new ndots parameter

## int evdns_base_set_option (struct evdns_base *base, const char *option, const char *val)

Set the value of a configuration option. The currently available configuration options are:

ndots, timeout, max-timeouts, max-inflight, attempts, randomize-case, bind-to, initial-probe-timeout, getaddrinfo-allow-skew.

In versions before Libevent 2.0.3-alpha, the option name needed to end with a colon.

Parameters:

base the evdns_base to which to apply this operation
option the name of the configuration option to be modified
val the value to be set

Returns:

0 if successful, or -1 if an error occurred

## void evdns_cancel_request (struct evdns_base *base, struct evdns_request *req)

Cancels a pending DNS resolution request. Parameters:
base the evdns_base that was used to make the request
req the evdns_request that was returned by calling a resolve function

See Also:

evdns_base_resolve_ipv4(), evdns_base_resolve_ipv6, evdns_base_resolve_reverse

## void evdns_close_server_port (struct evdns_server_port *port)

Close down a DNS server port, and free associated structures.

## const char* evdns_err_to_string (interr)

Convert a DNS error code to a string. Parameters:
err the DNS error code

Returns:

a string containing an explanation of the error code

## struct evdns_getaddrinfo_request* evdns_getaddrinfo (struct evdns_base *dns_base, const char *nodename, const char *servname, const struct evutil_addrinfo *hints_in, evdns_getaddrinfo_cbcb, void *arg) [read]

Make a non-blocking getaddrinfo request using the dns_base in 'dns_base'. If we can answer the request immediately (with an error or not!), then we invoke cb immediately and return NULL. Otherwise we return an evdns_getaddrinfo_request and invoke cb later.

When the callback is invoked, we pass as its first argument the error code that getaddrinfo would return (or 0 for no error). As its second argument, we pass the evutil_addrinfo structures we found (or NULL on error). We pass 'arg' as the third argument.

Limitations:

•
  The AI_V4MAPPED and AI_ALL flags are not currently implemented.

•
  For ai_socktype, we only handle SOCKTYPE_STREAM, SOCKTYPE_UDP, and 0.

•
  For ai_protocol, we only handle IPPROTO_TCP, IPPROTO_UDP, and 0.

## void evdns_server_request_set_flags (struct evdns_server_request *req, intflags)

Sets some flags in a reply we're building. Allows setting of the AA or RD flags

## void evdns_set_log_fn (evdns_debug_log_fn_typefn)

Set the callback function to handle DNS log messages. If this callback is not set, evdns log messages are handled with the regular Libevent logging system.

Parameters:

fn the callback to be invoked when a log message is generated

## void evdns_set_random_bytes_fn (void(*)(char *, size_t)fn)

Set a callback used to generate random bytes. By default, we use the same function as passed to evdns_set_transaction_id_fn to generate bytes two at a time. If a function is provided here, it's also used to generate transaction IDs.

NOTE: This function has no effect in Libevent 2.0.4-alpha and later, since Libevent now provides its own secure RNG.

## void evdns_set_transaction_id_fn (ev_uint16_t(*)(void)fn)

Set a callback that will be invoked to generate transaction IDs. By default, we pick transaction IDs based on the current clock time, which is bad for security.

Parameters:

fn the new callback, or NULL to use the default.

NOTE: This function has no effect in Libevent 2.0.4-alpha and later, since Libevent now provides its own secure RNG.

# Author

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Wed Apr 10 2013 | libevent