linux - DDNS resource records TTL

Where are the TTL of dynamically added records configured in bind?

I have a DNS-zone with standart $TTL set to 10 minutes. When the zone is updated, the hosts added get their own TTLs that differ from the zone default. The DHCP lease is configured to 24h.

$ORIGIN .
$TTL 600        ; 10 minutes
example                 IN SOA  firefly.example. postmaster.example. (

                                2013102334 ; serial
                                28800      ; refresh (8 hours)
                                7200       ; retry (2 hours)
                                2419200    ; expire (4 weeks)
                                86400      ; minimum (1 day)
                                )
                        NS      firefly.example.
$ORIGIN mqmedia.
$TTL 18000      ; 5 hours
Andreass-MBP            A       192.168.1.79

                        TXT     "31fdf53482504a9965e4e7a210ebfe6080"
$TTL 600        ; 10 minutes
atom-builder            A       192.168.1.13
$TTL 86400      ; 1 day
axis-00031c200982       A       192.168.1.47
                        TXT     "311b67a2c49ae34eb511fa989cb22c9e65"
axis-00031c205143       A       192.168.1.208
                        TXT     "319dfebd7e94f89fa7ebda8e87d6702499"
axis-00031c2d0067       A       192.168.1.45
                        TXT     "31f70f97c582af3ace2ec4773a19edd451"

$TTL 18000      ; 5 hours
axis-00408c99d062       A       192.168.1.168
                        TXT     "3141073fe7f825c3c3a9daa1b864ab6afd"
axis-00408cac034d       A       192.168.1.209
                        TXT     "317e6bffaf74e74855f944b1e18ba6aac8"
axis-00408cd5b244       A       192.168.1.69
                        TXT     "31d7bebe14245b2f42f8d601554eb284d5"
axis-00408cdb9d15       A       192.168.1.213
                        TXT     "31aaa279559559302bf51db4cb8e2b69f8"

Answer

The TTL is always configured at the zone file. It may be set as default, using $TTL on the beginning of the file; as individual $TTL statements spread through the file, and influencing the records below it; or even on the record itself.

When the server does a nsupdate to add the record for your host, bind will check to see if there is any TTL already present, marking $TTL 86400, and add the record below it, if not, it will add the $TTL for the specified time and the record below it.