Regarding CNAME Chains

Warren Kumari warren at kumari.net
Fri May 28 15:03:20 UTC 2010


On May 28, 2010, at 8:55 AM, Ashwin wrote:

>
>  Hi,
>
>   From the server I get a response like
>
>             aaa CNAME bbb
>             ccc CNAME ddd
>             bbb CNAME ccc
>
> The ordering of the CNAME chain is incorrect, ideally it should be  
> like
>
>             aaa CNAME bbb
>             bbb CNAME ccc
>             ccc CNAME ddd
>
> Is there some RFC which prohibits the configuration of CNAME chains  
> as given in the first example, ie CNAME chains not in order.

Nope[0]


>
> I tried to search on the internet, and could come up with this para  
> in RFC 2308: DNS NCACHE
>
> "QNAME" - the name in the query section of an answer, or where this  
> resolves to a CNAME, or CNAME chain, the data field of the last  
> CNAME. The last CNAME in this sense is that which contains a value  
> which does not resolve to another CNAME. Implementations should note  
> that including CNAME records in responses in order, so that the  
> first has the label from the query section, and then each in  
> sequence has the label from the data section of the previous (where  
> more than one CNAME is needed) allows the sequence to be processed  
> in one pass, and considerably eases the task of the receiver.
>
> It would be a great help if it can be confirmed that resolvers can  
> expect CNAME chains to be in order.


Nope. resolvers can expect whatever they would like, but there is no  
guarantee that the their expectations will be met. It seems that most  
things do provide them in the "friendly" order, but I'm sure I have  
seen them not as well...

W

>
> Thanks & Regards
> Ashwin
>
>
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-- 
American Non-Sequitur Society;
we don't make sense, but we do like pizza!


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