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put the evdns documentation into the header file. pathetic start of e…
…vdns manpage. a little bit more testing and debug output for the DNS regression test. add a BSD copyright to evdns.h with appropriate explanations. svn:r239
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.\" | ||
.\" Copyright (c) 2006 Niels Provos <provos@citi.umich.edu> | ||
.\" All rights reserved. | ||
.\" | ||
.\" Redistribution and use in source and binary forms, with or without | ||
.\" modification, are permitted provided that the following conditions | ||
.\" are met: | ||
.\" | ||
.\" 1. Redistributions of source code must retain the above copyright | ||
.\" notice, this list of conditions and the following disclaimer. | ||
.\" 2. Redistributions in binary form must reproduce the above copyright | ||
.\" notice, this list of conditions and the following disclaimer in the | ||
.\" documentation and/or other materials provided with the distribution. | ||
.\" 3. The name of the author may not be used to endorse or promote products | ||
.\" derived from this software without specific prior written permission. | ||
.\" | ||
.\" THIS SOFTWARE IS PROVIDED ``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 AUTHOR 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. | ||
.\" | ||
.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 | ||
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. | ||
.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: | ||
1. Never set it up. If you never call evdns_resolv_conf_parse or | ||
evdns_search_add then no searching will occur. | ||
.Pp | ||
2. If you do call evdns_resolv_conf_parse then don't pass | ||
DNS_OPTION_SEARCH (or DNS_OPTIONS_ALL, which implies it). | ||
.Pp | ||
3. When calling evdns_resolve, pass the DNS_QUERY_NO_SEARCH flag. | ||
.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 | ||
API reference: | ||
.Pp | ||
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. | ||
.Pp | ||
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 | ||
.Pp | ||
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). | ||
.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 | ||
.Pp | ||
void evdns_search_clear() | ||
Clears the list of search domains | ||
.Pp | ||
void evdns_search_add(const char *domain) | ||
Add a domain to the list of search domains | ||
.Pp | ||
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. | ||
.Pp | ||
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. | ||
.Pp | ||
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. | ||
.Pp | ||
int evdns_resume(void) | ||
Re-attempt resolves left in limbo after an earlier call to | ||
evdns_clear_nameservers_and_suspend(). | ||
.Pp | ||
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. | ||
.Pp | ||
int 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: | ||
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 | ||
.Pp | ||
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 | ||
.Pp | ||
Internals: | ||
.Pp | ||
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. |
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