[Opendnssec-user] TTL clamped to minimum 3600?

Havard Eidnes he at uninett.no
Wed Jan 20 17:03:48 UTC 2016


I jumped the gun on this one, in both cases; now that I've had
time to look at it a bit more calmly, I think I can formulate a
better question, and I'm now back to thinking there's a bug
in OpenDNSSEC.

In a zone we sign, we try to set the TTL on a CNAME record to
300s.  On the hidden master, when queried for the name, we get:

login.uninett.no.       300     IN      CNAME   login2.uninett.no.

which is the intended TTL.

The zone data (or rather, an incremental change) passes through
OpenDNSSEC, and if I ask for the data for this record including
DNSSEC data from the name server at the other end, I get

login.uninett.no.       3600    IN      CNAME   login2.uninett.no.
login.uninett.no.       300     IN      RRSIG   CNAME 8 3 300 20160210142813 20160120135441 46194 uninett.no. fatEO2XW4afwWolA1hfRRhI678GYEhe1WGKNKKfs1yUHsO+UYZXfmKV+ RN6a5CLwRDjfm3Nel7bhRDXZydYGjuyMZyotvQXHBbdon101Dmt23Ofl V4xaieMS+aWFq6gr6tM1WCueZHgUwDmllqAT81i3JxLRSNQnxumiv2nj EO8=

Notice that the TTL on the CNAME record has been, umm...,
"adjusted" to 3600, while the RRSIG TTL and the original-ttl
field in the RRSIG record has the original TTL from the hidden
master from the original CNAME record.

If you ask a validating resolver for this record, it will of
course set the TTL to 300 and start decrementing the TTL from
there, as the specs dictate.

However, if you ask a non-validating resolver, it will start to
count from 3600, and that's not what we intended.

The really strange thing is that we have other records in this
zone which have a lower TTL than 3600, and which show up in the
signed version of the zone with this identical lower-than-3600
TTL, e.g. the address records at the zone apex.

Are we hitting some bug which only hits data transferred using
incremental zone transfers, so that if the zone is transferred
using AXFR, the original TTL is preserved, while it isn't with
IXFR transfers?  That's what it looks like from here, but I've
not dug into the code to answer that question...

Config as discussed earlier: zone transfer in, zone transfer out,
patched OpenDNSSEC 1.4.7 (ref. the earlier fixes discussed here).


- Håvard

More information about the Opendnssec-user mailing list