Bind slave slow to update zones

Question

I have two Bind servers in a master/slave configuration. When the serial is incremented on the master zone file, it takes longer than expected for the zone to update on the slave. I have replicated the problem on two test servers. With a ttl of 5 secs, it takes serveral minutes for the slave to update. I am using Bind 9.8 and Ubuntu 12.04.

Here's my configuration for the master test server:

named.conf.local

zone "example.com" {
        type master;
        file "/var/lib/bind/db.example.com.zone";
        //forwarders {};
        // If we do not comment the ''forwarders'' "empty" clients of the local subnet     in my case don't have access to the upstream DNS ?
        //allow-update { key ns-example-com_rndc-key; };
        allow-update { key rndc-key; };
        //confusion between the file name to import (ns-example-com_rndc-key) and the     key label (rndc-key) ?
};
zone "0.168.192.in-addr.arpa" {
    type master;
    file "/var/lib/bind/db.example.com.inv.zone";
    //see comment below (zone "example.com")
    //forwarders {};
    //allow-update { key ns-example-com_rndc-key; };
    allow-update { key rndc-key; };
};

db.root.example.com

$TTL    5
@       IN      SOA   sid.example.com. root.example.com. (
               2007010416           ; Serial
                       5           ; Refresh [1m]
                      5           ; Retry   [10m]
                    5           ; Expire  [1d]
                      5 )         ; Negative Cache TTL [1h]
;
@       IN      NS      sid.example.com.
@       IN      MX      10 sid.example.com.

sid     IN      A       192.168.0.3
etch    IN      A       192.168.0.3

pop     IN      CNAME   sid
www     IN      CNAME   sid
mail    IN      CNAME   sid

db.example.com.inv.zone

@ IN SOA        sid.example.com. root.example.com. (
               2007010401           ; Serial
                     3600           ; Refresh [1h]
                      600           ; Retry   [10m]
                    86400           ; Expire  [1d]
                      600 )         ; Negative Cache TTL [1h]
;
@       IN      NS      sid.example.com.

1       IN      PTR     sid.example.com.
2       IN      PTR     etch.example.com.

Here's the configuration for the slave:

zone "example.com" {
    type slave;
    file "/var/cache/bind/db.example.com.zone";
    masters { 192.168.0.2; };
//forwarders {};
    // If we do not comment the ''forwarders'' "empty" clients of the local subnet in my case don't have access to the upstream DNS ?
    //allow-update { key ns-example-com_rndc-key; };
    allow-update { key rndc-key; };
    //confusion between the file name to import (ns-example-com_rndc-key) and the key label (rndc-key) ?
};
zone "0.168.192.in-addr.arpa" {
    type slave;
    file "/var/cache/bind/db.example.com.inv.zone";
    masters { 192.168.0.2; };
//see comment below (zone "example.com")
    //forwarders {};
    //allow-update { key ns-example-com_rndc-key; };
    allow-update { key rndc-key; };
};

Answer 1

From the copy-left guide DNS for Rocket Scientists:

By default BIND9 will send NOTIFY messages to all the target names (right-hand names) that appear in NS RRs for the zone

So you need to add your slave server as a NS entry in db.example.com.inv.zone.

Additionally, you may need to add the settings:

• In the master: notify yes; and allow-transfer { SLAVE_IP; };
• In the slave: allow-notify { MASTER_IP; };

Answer 2

When the serial number is changed on the master, it will notify the slave immediately. In other words, notify is enabled by default. BUT, the way notify works is, the master looks at the NS records of that particular domain in the zone file, and notifies the servers listed in the NS record, excluding itself. In your case, I don't see the slave server's hostname being listed as an NS record in the db.root.example.com zone file. Therefore, the slave is contacting the master, listed in the slave's config file - masters { 192.168.0.2; }; at the Refresh interval, 1 hour. That is why it is taking long time to update.

Solution - add the NS record for the slave, also include its IP address as an A record. Typical setup is as follows -

  @        IN      NS      ns1.example.com.
  @        IN      NS      ns2.example.com.
  ns1     IN      A       192.168.1.1
  ns2     IN      A       192.168.1.2

First and third line, for master. Second and fourth line for slave.