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getaddrinfo with AF_CANONNAME making PTR lookups

This is spun off from discussion in mozilla bug 239358 [1].

The bug is that with recent glibc code (e.g. that in Fedora Core 2), many DNS lookups take a long time and sites like and take a long time to load.

This is because recently the behaviour of the AI_CANONNAME flag in getaddrinfo was changed. Previously, glibc used to return as the canonical name the name at the end of the DNS CNAME chain. But the current code uses reverse DNS (PTR) lookups to get the canonical name. This has several disadvantages: see my comment in the mozilla bug [2].

From discussion with Ulrich Drepper in the bug it appears that glibc does this to comply with POSIX. However, nothing in RFC 3493 [3] nor POSIX [4] suggests that the canonical name should be obtained using reverse lookups. In fact, POSIX strongly seems to suggest that the canonical name should be obtained by using CNAMEs.

Both the RFC and POSIX state:

   If the AI_CANONNAME flag is specified and the nodename argument is
   not null, the function shall attempt to determine the canonical name
   corresponding to nodename (for example, if nodename is an alias or
   shorthand notation for a complete name).

which does not say anything about how the canonical name should be obtained. However, POSIX adds the following:

The term ``canonical name'' is misleading; it is taken from the Domain
Name System (RFC 2181). It should be noted that the canonical name is a
result of alias processing, and not necessarily a unique attribute of a
host, address, or set of addresses. See RFC 2181 for more discussion of
this in the Domain Name System context.

It seems clear to me that if the canonical name is to be obtained by "alias processing", then it must be obtained by following the CNAME chain, which is a chain of aliases. Furthermore, no other platform that I know of uses PTR lookups. I and others in the IPv6 community have tested Solaris 9, FreeBSD 4.9 (with the stock, KAME and BIND resolvers), 4.10 and 5.2, and Windows (and of course previous versions of glibc) and none of them do this.

We can work around this in mozilla (a patch has been written and is awaiting review), but we do not want to add workarounds unless it is necessary. And it seems to me that this is incorrect behaviour on glibc's part and should be fixed in glibc.

Can we expect this to be the case or do we need to work around it?

Thanks, Lorenzo


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