[Pdns-users] DNS answer don't fit UDP packet
pdns at vava.org
Fri Oct 10 16:18:58 UTC 2003
> For now I reduced my mx list to fit UDP.
> But is there any configuration that makes PDNS shrink ADDITIONAL SECTION
> in order to fit UDP and answer UDP query with UDP.
Some insightfull comments can be found in RFC1123
18.104.22.168 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
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.
Truncated responses MUST NOT be saved (cached) and later
used in such a way that the fact that they are truncated is
UDP is preferred over TCP for queries because UDP
queries have much lower overhead, both in packet count
and in connection state. The use of UDP is essential
for heavily-loaded servers, especially the root
servers. UDP also offers additional robustness, since
a resolver can attempt several UDP queries to different
servers for the cost of a single TCP query.
It is possible for a DNS response to be truncated,
although this is a very rare occurrence in the present
Internet DNS. Practically speaking, truncation cannot
be predicted, since it is data-dependent. The
dependencies include the number of RRs in the answer,
the size of each RR, and the savings in space realized
by the name compression algorithm. As a rule of thumb,
truncation in NS and MX lists should not occur for
answers containing 15 or fewer RRs.
RFC1123 SUPPORT SERVICES -- DOMAINS October 1989
Whether it is possible to use a truncated answer
depends on the application. A mailer must not use a
truncated MX response, since this could lead to mail
loops. Responsible practices can make
UDP suffice in the vast
majority of cases. Name servers must use compression
in responses. Resolvers must differentiate truncation
of the Additional section of a response (which only
loses extra information) from truncation of the Answer
section (which for MX records renders the response
unusable by mailers). Database administrators should
list only a reasonable number of primary names in lists
of name servers, MX alternatives, etc.
However, it is also clear that some new DNS record
types defined in the future will contain information
exceeding the 512 byte limit that applies to UDP, and
hence will require TCP. Thus, resolvers and name
servers should implement TCP services as a backup to
UDP today, with the knowledge that they will require
the TCP service in the future.
By the way it seems that truncating additional section is the
recommended strategy, but in this case this affect the caching
I have not already look at the code to see if it's possible to truncate
the answer just before sending the UDP answer or if this affect powerdns
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