> I'm looking at the spec for Via: processing.  
> RFC 3261 clearly shows that a Via: line can 
> contain DNS names, as well as textual
> representations of network addresses.
> 
> How many products actually implement support 
> for resolving DNS names provided in Via: lines?  

Not very many support DNS resolution of a
Via entry.  However many support the concept
of adding the Via's received parameter when
it does not match the Via's sent-by address.

> This seems...  well, it seems unnecessary,
> potentially insecure (allowing for easy DDoS 
> and potentially amplification attacks), and 
> just a general nuisance.  Is this kind of
> thing ever actually used for a good purpose?  

Some people have toyed with the idea of
advancing response addresses based upon
rfc3263 during failure situations.

> Does any product do anything *but* just put 
> real network addresses in Via: lines in
> requests?

Yes.

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