TC flag

[Kevin Darcy]

hank at interall.co.il wrote:

> Can a DNS server be informed to respond via TCP rather than UDP via  the TC
> (truncated) flag in the reply to udp?

As the RFC states, if the answer is truncated (as indicated by the TC flag),
the requestor should retry using TCP. The most the *server* can do is set TC;
the rest is up to the *client*, and since the RFC says "SHOULD" rather than
"MUST", the client has the option to just fail the query if it's truncated.

> RFC 1123 Section 6.1.3.2 states:
>
>     6.1.3.2  Transport Protocols
>
>        DNS resolvers and recursive servers MUST support UDP, and
>        SHOULD support TCP, for sending (non-zone-transfer) queries.
>        Specifically, a DNS resolver or server that is sending a
>        non-zone-transfer query MUST send a UDP query first.  If the
>        Answer section of the response is truncated and if the
>        requester supports TCP, it SHOULD try the query again using
>        TCP.
>
>        DNS servers MUST be able to service UDP queries and SHOULD
>        be able to service TCP queries.  A name server MAY limit the
>        resources it devotes to TCP queries, but it SHOULD NOT
>        refuse to service a TCP query just because it would have
>        succeeded with UDP.
>
> Are there any known DNS implementations that do not support the TC flag?

I'm not sure what you mean by "support". And when you say "implementations" do
you mean DNS servers or resolvers? Servers are required to set TC if the
response doesn't fit in the packet. Any server that fails to do so is
*broken*; it's not really a matter of "support" but rather correctness or
brokenness. Similarly, any resolver that ignores the TC flag, treating an
incomplete answer as a complete one, is just simply broken and needs to be
fixed. The only "support" issue is whether the resolver opts to retry
truncated answers using TCP or not. I'm not aware of any that don't TCP retry
as default behavior (but this can be configurable, see for example the
"ignore" option of "dig").


- Kevin