samba - Mar 2018 - dhcp lease time hardware in script at dhcpd/Bind/Samba wiki page

There seems to be a slight issue with the instructions at the following 
wiki page:

https://wiki.samba.org/index.php/Configure_DHCP_to_update_DNS_records_with_BIND9

The /usr/local/bin/dhcp-dyndns.sh script seems to use a hardwired value 
for the lease expiry time of 3600 - independent of whatever is 
configured in dhcpd.conf. With the examples provided, it should work, as 
the example dhcpd.conf uses 1800 for lease expiry time - so dhcpd will 
try to renew the lease before it actually expires. However, when I 
implemented the script, I didn't notice that the lease time is hardwired 
in the dhcp-dyndns.sh script - so I've set a higher value in dhcpd.conf. 
This seems to have caused all sorts of strange behaviour, as 
Bind/samba_dlz were expiring leases and dhcpd wasn't renewing them - 
thinking they are still valid.

Would it be possible to pass the max lease time value from dhcpd.conf to 
the dhcp-dyndns.sh script somehow? I would have made a suggestion 
myself, but for the life of me I can't find anywhere on the net a 
reference of standard variable which can be used in dhcpd.conf.

If the above is not possible, maybe a max-lease-time variable should be 
placed at the top of the dhcp-dyndns.sh script. so that at least it is 
obvious that this is set again in this script, and not carried over from 
dhcpd.conf?

On Thu, 29 Mar 2018 15:01:26 +0100
Sebastian Arcus via samba <samba at lists.samba.org> wrote:
> There seems to be a slight issue with the instructions at the
> following wiki page:
> 
>
https://wiki.samba.org/index.php/Configure_DHCP_to_update_DNS_records_with_BIND9
> 
> The /usr/local/bin/dhcp-dyndns.sh script seems to use a hardwired
> value for the lease expiry time of 3600 - independent of whatever is 
> configured in dhcpd.conf. With the examples provided, it should work,
> as the example dhcpd.conf uses 1800 for lease expiry time - so dhcpd
> will try to renew the lease before it actually expires. However, when
> I implemented the script, I didn't notice that the lease time is
> hardwired in the dhcp-dyndns.sh script - so I've set a higher value
> in dhcpd.conf. This seems to have caused all sorts of strange
> behaviour, as Bind/samba_dlz were expiring leases and dhcpd wasn't
> renewing them - thinking they are still valid.
> 
> Would it be possible to pass the max lease time value from dhcpd.conf
> to the dhcp-dyndns.sh script somehow? I would have made a suggestion 
> myself, but for the life of me I can't find anywhere on the net a 
> reference of standard variable which can be used in dhcpd.conf.
> 
> If the above is not possible, maybe a max-lease-time variable should
> be placed at the top of the dhcp-dyndns.sh script. so that at least
> it is obvious that this is set again in this script, and not carried
> over from dhcpd.conf?
> 
You could try adding something like this near to the top of
dhcp-dyndns.sh:

TTL=$(cat /etc/dhcp/dhcpd.conf | grep 'max-lease-time' | awk -F
';'
'{print $1}' | awk '{print $NF}')

Then replace the '3600' with '$TTL'

Rowland

On 29/03/18 15:52, Rowland Penny via samba wrote:
> On Thu, 29 Mar 2018 15:01:26 +0100
> Sebastian Arcus via samba <samba at lists.samba.org> wrote:
> 
>> There seems to be a slight issue with the instructions at the
>> following wiki page:
>>
>>
https://wiki.samba.org/index.php/Configure_DHCP_to_update_DNS_records_with_BIND9
>>
>> The /usr/local/bin/dhcp-dyndns.sh script seems to use a hardwired
>> value for the lease expiry time of 3600 - independent of whatever is
>> configured in dhcpd.conf. With the examples provided, it should work,
>> as the example dhcpd.conf uses 1800 for lease expiry time - so dhcpd
>> will try to renew the lease before it actually expires. However, when
>> I implemented the script, I didn't notice that the lease time is
>> hardwired in the dhcp-dyndns.sh script - so I've set a higher value
>> in dhcpd.conf. This seems to have caused all sorts of strange
>> behaviour, as Bind/samba_dlz were expiring leases and dhcpd wasn't
>> renewing them - thinking they are still valid.
>>
>> Would it be possible to pass the max lease time value from dhcpd.conf
>> to the dhcp-dyndns.sh script somehow? I would have made a suggestion
>> myself, but for the life of me I can't find anywhere on the net a
>> reference of standard variable which can be used in dhcpd.conf.
>>
>> If the above is not possible, maybe a max-lease-time variable should
>> be placed at the top of the dhcp-dyndns.sh script. so that at least
>> it is obvious that this is set again in this script, and not carried
>> over from dhcpd.conf?
>>
> 
> You could try adding something like this near to the top of
> dhcp-dyndns.sh:
> 
> TTL=$(cat /etc/dhcp/dhcpd.conf | grep 'max-lease-time' | awk -F
';'
> '{print $1}' | awk '{print $NF}')
> 
> Then replace the '3600' with '$TTL'
Thank you for that! I take it there is no corresponding variable, which 
can be used as an argument in dhcpd.conf when calling dhcp-dyndns.sh?

Also, I discovered another typo on the wiki page - which was actually a 
bit of a pain to track down. On the following line:

rzone=$(echo ${ip} | awk -F '.' '{print
$3"."$2"."$1".in-addr.arpa"}’)

The last quote should be a single quote, but it is actually a backtick.

I can report that on Slackware-current with dhcpd 4.4.1, the extra 
setting suggested to set the rzone is necessary - otherwise Bind tries 
to use a B class reverse zone instead of a C one and complains that it 
isn't authoritative for it:

updating zone '168.192.IN-ADDR.ARPA/IN': update failed: not 
authoritative for update zone (NOTAUTH)

I have other Slackware servers with dhcpd 4.3.6 and 4.3.4 - and those 
are happy without the rzone line.

Hi Rowland,
> You could try adding something like this near to the top of
> dhcp-dyndns.sh:
> 
> TTL=$(cat /etc/dhcp/dhcpd.conf | grep 'max-lease-time' | awk -F
';'
> '{print $1}' | awk '{print $NF}')
> 
> Then replace the '3600' with '$TTL'
Users like comments in their config files. Your script:

cat /etc/dhcp/dhcpd.conf | grep 'max-lease-time' | awk -F ';'
'{print $1}' | awk '{print $NF}'
7200
7200


better is this:

cat /etc/dhcp/dhcpd.conf | egrep '^\s*max-lease-time' | awk -F
';' '{print $1}' | awk '{print $NF}'
7200

So this matches:

max-lease-time
 max-lease-time
	max-lease-time

but not:
# max-lease-time

btw
cheaper is sed and i am a sed fan (keep it simple):

time cat /etc/dhcp/dhcpd.conf|sed -nre
's/^\s*max-lease-time\s*([0-9]+).*/\1/p'
7200

real	0m0,004s
user	0m0,003s
sys 	0m0,004s


time cat /etc/dhcp/dhcpd.conf | grep 'max-lease-time' | awk -F
';' '{print $1}' | awk '{print $NF}'
7200
7200

real	0m0,006s
user	0m0,004s
sys 	0m0,012s

> Rowland

-- 

Gruss
	Harry Jede