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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/libresolv.html | |
| download | s6-dns-416ef5e2bf59bb2e45066a1d5d91ac677c0f48e5.tar.xz | |
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diff --git a/doc/libresolv.html b/doc/libresolv.html new file mode 100644 index 0000000..7a9fd31 --- /dev/null +++ b/doc/libresolv.html @@ -0,0 +1,140 @@ +<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 libresolv</title> + <meta name="Description" content="s6-dns: the problem with libresolv" /> + <meta name="Keywords" content="s6-dns client library libresolv BIND 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 libresolv </h1> + +<p> + The BIND name server software comes with its own client library, +named <em>libresolv</em>. +</p> + +<p> + As can be expected from an ISC product, libresolv is not good software. +Here are a few reasons why. +</p> + +<h2> libresolv's security model is flawed. </h2> + +<p> + The same people who wrote BIND wrote libresolv. That is the amount of +trust you can place in libresolv. Ten years ago, the security status +of libresolv looked like +<a href="http://cr.yp.to/djbdns/res-disaster.html">this</a>. I am not +confident that is has improved: bugs in the software may have been +fixed, but new ones will appear, and most importantly, the security +management policy at ISC is still the same: security holes will be +denied instead of acknowledged and worked upon. +</p> + +<p> + If you find a bug, and <em>a fortiori</em> a security hole, in +s6-dns, you can be sure it will be fixed promptly with apologies +from the author. skarnet.org doesn't do obfuscation, and never lets +politics get in the way of technical quality. +</p> + +<h2> libresolv is unboundedly synchronous. </h2> + +<p> + You'd expect a real DNS client library to do better in this aspect +than <a href="getaddrinfo.html">getaddrinfo()</a>, but no: libresolv's +function calls are still purely synchronous and may uncontrollably +block if the network is unresponsive. +</p> + +<p> + Additionally, libresolv <em>only</em> provides a synchronous +interface to clients. Despite the fundamentally asynchronous nature +of DNS, and the need to implement asynchronous primitives +internally, only blocking calls are made available in the API. +This forces users to stack yet another piece of software on top +of their dependencies if they need asynchronous DNS resolution. +</p> + +<p> + libs6dns provides several layers of asynchronous interfaces. +The user has access to +<a href="libs6dns/s6dns_engine.html">low-level packet sending +and receiving</a>, to +<a href="libs6dns/s6dns_resolve.html#parallel">synchronous +resolution of several queries at once</a>, and to a +<a href="skadns/">real high-level asynchronous DNS library</a>. +</p> + +<h2> It is too big for what it does. </h2> + +<p> + The <tt>libresolv-2.13.so</tt> binary file compiled for an i386 +Debian Linux system is roughly 71k bytes. The <tt>libs6dns.so.2.0</tt> +file, for the same system, is roughly 41k bytes, while offering +more functionality. libresolv does not do any high-level answer +parsing, so the user still has to do some work after the libresolv +calls. s6-dns tries to be small and still provide the user comfortable +interfaces. +</p> + +<h2> The API is cumbersome to use. </h2> + +<p> + Some examples of less-than-ideal interfaces for the users: +</p> + +<ul> + <li> There is a flag in libresolv's global state structure that +determines if queries are to be sent recursive or iterative. Which +means that a program that needs to make both recursive and iterative +queries must duplicate this state structure and use one for recursive +queries, one for iterative queries. Why this complexity ? The +recursive flag should be given for every query, not be a part +of the resolver state. </li> + <li> When a libresolv answer arrives but doesn't fit into the +user-supplied buffer, the query is discarded and has to be retried. +Does "network efficiency" ring a bell ? Also, the length of +the original answer is returned by the first call, which is a good +thing: the user can now provide a large enough buffer so the call +succeeds the next time, right ? Wrong. The next answer can +be different from the first, and in particular, longer, which +means that the next query can <em>still fail</em>. </li> +</ul> + +<p> + There is a reason why system calls and local functions take +user-supplied buffers as arguments. They are relatively fast, it is +not too costly to call them again if the buffer is too small the +first time, and the result is consistent, i.e. after the first call, +the right buffer length is <em>known</em>. But functions making +network exchanges with variable-length results from one call to +another ? Those <em>need</em> heap-allocated storage. It is +good design to avoid using the heap whenever possible, but it is +not good design to waste network round-trips to save a <tt>malloc()</tt>. +</p> + +<h2> Conclusion </h2> + +<p> + Like many other "standards", and C library interfaces in particular, +<tt>libresolv</tt> is at best a mediocre one, that people use because +there has been nothing better so far. +</p> + +<p> + s6-dns tries to be an alternative solution - not as ambitious, +but based on solid design principles and a reliable code base. +</p> + +</body> +</html> |