xref: /libevent-2.1.12/evdns.3 (revision ea52d9fd)

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.Dd October 7, 2006
.Dt EVDNS 3
.Os
.Sh NAME
.Nm evdns_init
.Nm evdns_shutdown
.Nm evdns_err_to_string
.Nm evdns_nameserver_add
.Nm evdns_count_nameservers
.Nm evdns_clear_nameservers_and_suspend
.Nm evdns_resume
.Nm evdns_nameserver_ip_add
.Nm evdns_resolve_ipv4
.Nm evdns_resolve_reverse
.Nm evdns_resolv_conf_parse
.Nm evdns_config_windows_nameservers
.Nm evdns_search_clear
.Nm evdns_search_add
.Nm evdns_search_ndots_set
.Nm evdns_set_log_fn
.Nd asynchronous functions for DNS resolution.
.Sh SYNOPSIS
.Fd #include <sys/time.h>
.Fd #include <event.h>
.Fd #include <evdns.h>
.Ft int
.Fn evdns_init
.Ft void
.Fn evdns_shutdown "int fail_requests"
.Ft "const char *"
.Fn evdns_err_to_string "int err"
.Ft int
.Fn evdns_nameserver_add "unsigned long int address"
.Ft int
.Fn evdns_count_nameservers
.Ft int
.Fn evdns_clear_nameservers_and_suspend
.Ft int
.Fn evdns_resume
.Ft int
.Fn evdns_nameserver_ip_add(const char *ip_as_string);
.Ft int
.Fn evdns_resolve_ipv4 "const char *name" "int flags" "evdns_callback_type callback" "void *ptr"
.Ft int
.Fn evdns_resolve_reverse "struct in_addr *in" "int flags" "evdns_callback_type callback" "void *ptr"
.Ft int
.Fn evdns_resolv_conf_parse "int flags" "const char *"
.Ft void
.Fn evdns_search_clear
.Ft void
.Fn evdns_search_add "const char *domain"
.Ft void
.Fn evdns_search_ndots_set "const int ndots"
.Ft void
.Fn evdns_set_log_fn "evdns_debug_log_fn_type fn"
.Ft int
.Fn evdns_config_windows_nameservers
.Sh DESCRIPTION
Welcome, gentle reader
.Pp
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.
.Pp
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).
.Pp
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.
.Pp
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:
.Pp
.Bl -tag -width "DNS_USE_GETTIMEOFDAY_FOR_ID" -compact -offset indent
.It 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.
.It 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.
.It 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.
.El
.Pp
The library keeps track of the state of nameservers and will avoid
them when they go down. Otherwise it will round robin between them.
.Pp
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);
.Pp
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.
.Pp
Searching:
.Pp
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.
.Pp
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.
.Pp
To disable searching:
.Bl -enum -compact -offset indent
.It
Never set it up. If you never call
.Fn evdns_resolv_conf_parse,
.Fn evdns_init,
or
.Fn evdns_search_add
then no searching will occur.
.It
If you do call
.Fn evdns_resolv_conf_parse
then don't pass
.Va DNS_OPTION_SEARCH
(or
.Va DNS_OPTIONS_ALL,
which implies it).
.It
When calling
.Fn evdns_resolve,
pass the
.Va DNS_QUERY_NO_SEARCH
flag.
.El
.Pp
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.
.Pp
The ndots setting can either be set from a resolv.conf, or by calling
evdns_search_ndots_set.
.Pp
For example, with ndots set to 1 (the default) and a search domain list of
["myhome.net"]:
 Query: www
 Order: www.myhome.net, www.
.Pp
 Query: www.abc
 Order: www.abc., www.abc.myhome.net
.Pp
.Sh API reference
.Pp
.Bl -tag -width 0123456
.It Ft int Fn evdns_init
Initializes support for non-blocking name resolution by calling
.Fn evdns_resolv_conf_parse
on UNIX and
.Fn evdns_config_windows_nameservers
on Windows.
.It Ft int Fn 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.
.It Ft int Fn 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
.It Ft int Fn 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).
.Pp
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.
.Pp
Returns non-zero on error
.It Ft void Fn evdns_search_clear
Clears the list of search domains
.It Ft void Fn evdns_search_add "const char *domain"
Add a domain to the list of search domains
.It Ft void Fn 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.
.It Ft int Fn 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.
.It Ft int Fn 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.
.It Ft int Fn evdns_resume "void"
Re-attempt resolves left in limbo after an earlier call to
evdns_clear_nameservers_and_suspend().
.It Ft int Fn 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.
.It Ft int Fn evdns_resolv_conf_parse "int flags" "const char *filename"
Parse a resolv.conf like file from the given filename.
.Pp
See the man page for resolv.conf for the format of this file.
The flags argument determines what information is parsed from
this file:
.Bl -tag -width "DNS_OPTION_NAMESERVERS" -offset indent -compact -nested
.It DNS_OPTION_SEARCH
domain, search and ndots options
.It DNS_OPTION_NAMESERVERS
nameserver lines
.It DNS_OPTION_MISC
timeout and attempts options
.It DNS_OPTIONS_ALL
all of the above
.El
.Pp
The following directives are not parsed from the file:
  sortlist, rotate, no-check-names, inet6, debug
.Pp
Returns non-zero on error:
.Bl -tag -width "0" -offset indent -compact -nested
.It 0
no errors
.It 1
failed to open file
.It 2
failed to stat file
.It 3
file too large
.It 4
out of memory
.It 5
short read from file
.El
.El
.Sh 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.
.Pp
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.
.Pp
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.
.Sh SEE ALSO
.Xr event 3 ,
.Xr gethostbyname 3 ,
.Xr resolv.conf 5
.Sh HISTORY
The
.Nm evdns
API was developed by Adam Langley on top of the
.Nm libevent
API.
The code was integrate into
.Nm Tor
by Nick Mathewson and finally put into
.Nm libevent
itself by Niels Provos.
.Sh AUTHORS
The
.Nm evdns
API and code was written by Adam Langley with significant
contributions by Nick Mathewson.
.Sh 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.