[Pdns-users] greater number of truncated replies with pdns_recursor?

Tony Adams aadams at esnet.com
Fri Mar 24 19:15:54 UTC 2006

I recently upgraded from snapshot 563 to release 2.9.20.  This was done
primarily to fix the round robin mx lookup issue that I had previously
reported.  In the time between reporting that issue and upgrading I was
forwarding recursive queries to verizon's dns servers (tacky, I know).

Since I upgraded to 2.9.20 I've been using the pdns_recursor.  Evidently
I get more truncated UDP replies via this method than I was getting with
Verizon's servers.  In just over a week's time I've seen multiple
Microsoft mail servers start exhibiting the behavior described in this
KB article

The work around has been to enable the answering of TCP queries, which I
don't specifically have a problem with.  I suppose this is more of a
report than a particular request for assistance.

C:\dig>dig earthlink.net mx @ns2
;; Truncated, retrying in TCP mode.    <<<<  See what I mean?

; <<>> DiG 9.2.3 <<>> earthlink.net mx @ns2
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 18467
;; flags: qr rd ra; QUERY: 1, ANSWER: 20, AUTHORITY: 0, ADDITIONAL: 20

;earthlink.net.                 IN      MX

earthlink.net.          1744    IN      MX      5 mx9.earthlink.net.
earthlink.net.          1744    IN      MX      5 mxf.earthlink.net.
earthlink.net.          1744    IN      MX      5 mxb.earthlink.net.

Earthlink's MX record is the only one that I can reliably reproduce the
truncated reply situation with.

This 'truncated' reply causes MS SMTP (IIS/Exchange) to retry the
request in TCP mode.  Now that I'm allowing TCP connections to my DNS
servers the behavior should cease.  I guess my question becomes, why
does the pdns recursor seem to produce more truncated replies than the
(presumably) bind server run by verizon ( to whom I was sending
recursive requests before I upgraded to pdns 2.9.20?

Tony Adams
Systems Engineer
E Solutions Corporation

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