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| author | Laurent Bercot <ska-skaware@skarnet.org> | 2014-12-10 03:05:47 +0000 |
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| committer | Laurent Bercot <ska-skaware@skarnet.org> | 2014-12-10 03:05:47 +0000 |
| commit | 416ef5e2bf59bb2e45066a1d5d91ac677c0f48e5 (patch) | |
| tree | 1c746d673dcec7a8488c6ac51db8245411034376 /doc/getaddrinfo.html | |
| download | s6-dns-416ef5e2bf59bb2e45066a1d5d91ac677c0f48e5.tar.xz | |
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diff --git a/doc/getaddrinfo.html b/doc/getaddrinfo.html new file mode 100644 index 0000000..da0469c --- /dev/null +++ b/doc/getaddrinfo.html @@ -0,0 +1,94 @@ +<html> + <head> + <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> + <meta http-equiv="Content-Language" content="en" /> + <title>s6-dns: the problem with getaddrinfo()</title> + <meta name="Description" content="s6-dns: the problem with getaddrinfo()" /> + <meta name="Keywords" content="s6-dns client library getaddrinfo API interface" /> + <!-- <link rel="stylesheet" type="text/css" href="http://skarnet.org/default.css" /> --> + </head> +<body> + +<p> +<a href="index.html">s6-dns</a><br /> +<a href="http://skarnet.org/software/">Software</a><br /> +<a href="http://skarnet.org/">skarnet.org</a> +</p> + +<h1> The problem with getaddrinfo() </h1> + +<p> + The standard C library provides an API to perform name +resolution: +<a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/getaddrinfo.html">getaddrinfo()</a>, +formerly <tt>gethostbyname()</tt>. However, for DNS resolution as well as +implementation in the libc, this interface is very impractical, to the point of +being unusable. Here are a few reasons why. +</p> + +<h2> getaddrinfo() performs NSS resolution, not DNS resolution. </h2> + +<p> + I explained this point in a message to the +<a href="http://busybox.net/">Busybox</a> mailing-list. You can +<a href="http://lists.busybox.net/pipermail/busybox/2012-July/078123.html">read +the post here</a>. +(There is a mistake in that post about <tt>/etc/nsswitch.conf</tt> and +<tt>/etc/host.conf</tt> syntax; the following two messages in the thread +correct that mistake.) +</p> + +<p> + TLDR: depending on the machine configuration, it is possible that +<tt>getaddrinfo()</tt> will not use DNS at all. +</p> + +<h2> It is unboundedly synchronous. </h2> + +<p> + DNS resolution performs network I/O, which can take a non-negligible +amount of time. <tt>getaddrinfo()</tt> is a blocking call and there is +no way to specify a timeout to make it return early, so it may block +indefinitely. This is bad design. +</p> + +<p> + Also, network operations being asynchronous by nature, even a +synchronous API should provide a way to perform several queries at +once and return when one of them, or all of them, get an answer. +<tt>getaddrinfo()</tt> does not even offer that. +</p> + +<h2> It focuses on the wrong details. </h2> + +<p> + Because it's generic, <tt>getaddrinfo()</tt> is cumbersome to use. + The data structures are impractical, requiring the user to fill in +information that is irrelevant to DNS resolution. The details of the +network transport protocols are of no interest to the user who just +wants answers to his DNS queries! +</p> + +<p> + But at the same time, <tt>getaddrinfo()</tt> does not allow the +user to provide the details he wants or refine his search. It's a very +basic and monolithic entry point, with no DNS-specific knobs. For +instance, only A and AAAA queries are supported, which is clearly +insufficient. +</p> + +<h2> Conclusion </h2> + +<p> +<tt>getaddrinfo()</tt> is a toy interface. For any half-serious DNS work, +another API must be used. +</p> + +<p> + Most people who need a real DNS client library use BIND's libresolv. +<a href="libresolv.html">This page</a> explains what is wrong with it, +and what s6-dns tries to do better. +</p> + +</body> +</html> |