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Generate Hostname if not Sent by Client

Authority
Posts: 21
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Why does this option exist, why would people turn it on, what is handy about it, etc?

 

When turned off, does the DCHP server then simply vend IP address (skips creation of related DNS records)?

 

This option is enabled in our environment and is related to some transient issues (DNS churn between generated hostname and client-provided hostname). Tempted to disable this functionality, but first want to understand what benefits it may have and what behavior is like without...if we would miss it for any reason.

 

Thanks,

C

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Re: Generate Hostname if not Sent by Client

Expert
Posts: 269
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I have never had to enable that option (yet!). There are some clients that do not send a host name in their DHCP requests, so I can see this might be useful in cases where clients are trying to access an application that requires the clients to be registered in DNS (e.g. either just for a reverse lookup, or for a reverse and then forward lookup and match check). I have come across applications in the past where it was mandatory for clients to be registered in DNS.

 

Network monitoring systems also tend to issue a lot of reverse lookups, so it could be handy to have a client name displayed in the NMS GUI rather than an IP address, e.g. to differentiate between something that has obtained an address via DHCP as opposed to some static device that is discovered.

 

In your case it's difficult to make a recommendation as something could break - rather than unsetting it at the grid level you could try unsetting it on the individual ranges or subnets that are having problems and see if you get away with it! :-)

 

Good luck!

 

Paul

 

Paul Roberts
PCN (UK) Ltd

All opinions expressed are my own and not representative of PCN Inc./PCN (UK) Ltd. E&OE
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Re: Generate Hostname if not Sent by Client

Adviser
Posts: 138
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This option exists to automatically generate a corresponding DNS record for a device that receives an IP address via DHCP only if that device did not provide a hostname in the DHCP transaction. 

 

With the option to generate a hostname disabled, the DHCP process will simply hand out the lease but can still generate a DDNS update with the clients hostname and FQDN depending on if DDNS is enabled and how it is configured along with DHCP Option 81 handling. 

 

Here is a small excerpt from the Admin Guide that may provide some additional guidance:

 

About the Client FQDN Option

 

When an IPv4 DHCP client sends DHCP DISCOVER and DHCP REQUEST messages, it can include option 81, the Client FQDN option. An IPv6 DHCP client can include option 39, the Client FQDN option, when it sends Solicit and Request messages.

 

The Client FQDN option contains the FQDN (fully qualified domain name) of the client and instructions on whether the client or the server performs DDNS updates. You can configure the appliance to replace the FQDN in the option by defining a hostname rewrite policy. For information about adding and enabling a hostname rewrite policy, see Replacing Host Names for DDNS Updates on page 868.

The DHCP server can support option 81 for IPv4 and IPv6 clients, and use the host name or FQDN that the client provides for the update. It can also allow or deny the client’s request to update DNS, according to the administrative policies of your organization. The DHCP server indicates its response in the DHCP OFFER message it sends back to an IPv4 client, and in the Reply message it sends back to an IPv6 client.

Sending Updates with the FQDN Option Enabled

When you enable the DHCP server to support the FQDN option, it uses the information provided by the IPv4 or IPv6 client to update DNS as follows:

  • When an IPv4 or IPv6 DHCP client sends a DHCP request with the FQDN option, it can include either its FQDN or only its host name.

    • —  If the request includes the FQDN, the DHCP server uses this FQDN to update DNS. You can specify a list of forward-mapping zones to be updated for IPv4 and IPv6 clients using the FQDN option. For information, see Sending Updates for DHCP Clients Using the FQDN Option on page 872.

    • —  If the request includes the host name, the DHCP server provides the domain name. It combines the host name of the client and the domain name to create an FQDN for the client. It then updates DNS with the FQDN it created. (You can enter the domain name in the General page of the DHCP Properties window. For information, see Enabling DDNS for IPv4 and IPv6 DHCP Clients on page 863.)

  • When a DHCP client sends a DHCP request with its hostname, the DHCP server adds the domain name you specified to create an FQDN for the client. It then updates DNS with the FQDN it created. For information about entering the domain name, see Enabling DDNS for IPv4 and IPv6 DHCP Clients on page 863.

  • When a DHCP client does not send a host name, the DHCP server provides a lease but does not update DNS. You can configure the DHCP server to generate a host name and update DNS as described in Generating Host Names for DDNS Updates on page 867.

  • If multiple DHCP clients specify the same FQDN or host name, the DHCP server allocates leases to the clients, but updates DNS only for the client that first sent the request. When it tries to update DNS for the succeeding clients, the update fails.

Check out our new Tech docs website at http://docs.infobox.com for latest documentation on Infoblox products
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Re: Generate Hostname if not Sent by Client

Techie
Posts: 2
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Can you please also clarify the behaviour regarding the PTR records. When the client doesn't send hostname and Infoblox is asked to update DNS, why is a PTR record not created.

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Re: Generate Hostname if not Sent by Client

[ Edited ]
Moderator
Moderator
Posts: 148
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Hello,

 

When the client does not send a hostname, Infoblox DHCP server generates a hostname for the client based on the IP address leased out and also adds a prefix to the hostname 'dhcp-' - IF "Generate Hostname if not Sent by Client" option is enabled. Below is dhcpd.conf configuration statement for the same.

ddns-hostname = pick ( option fqdn.hostname,option host-name,
concat ("dhcp-",binary-to-ascii(10,8,"-", leased-address)))

 

As far as DDNS is concerned, it should update both forward and reverse mapping zones alike (ie; It should create A and PTR records).

 

Based on your DNS architecture and DDNS configuration, there could be several contributing factors as to why PTR updates fail. Since there is no single easy answer, I would suggest verifying the below.

 

1. Do you have a reverse mapping DNS zone where the PTR record can be updated?
Example: For a leased out IP address 10.192.33.73, do you have either 33.192.10.in-addr.arpa or 192.10.in-addr.arpa or 10.in-addr.arpa zone in DNS?

 

2. If one of the above reverse mapping zones exist, are they authoritive primary on the same Infoblox grid or do they reside outside the grid such as on a Linux BIND or Microsoft Windows DNS?

 

3. If you open your DHCP server's configuration file (dhcpd.conf), by navigating to "Data Management-->DHCP-->Members-->View DHCP Configuration", are you able to find a zone statement and the correct primary server IP address there. 

Example:

zone "33.192.10.in-addr.arpa." {
	primary 10.192.33.200;
	key DHCP_UPDATER_default;
}

 
4. When A record updates succeed and PTR updates don't, do you find any relevant failure logs in the DHCP server's syslog?

 

5. If your 'x.x.in-addr-arpa' reverse mapping zone exist on an external server such as MS DNS or BIND, do you have that 'Zone Name' and 'DNS Server Address' correctly defined inside 'Configure DDNS', 'DNS Updates to External Zones' section?

 

Since your scenario requires active troubleshooting, I would recommend opening a case with Infoblox Support.

 

Best Regards,
Bibin Thomas

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Re: Generate Hostname if not Sent by Client

[ Edited ]
Techie
Posts: 2
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Hi,

 

Thanks for the detailed reply. I somehow was not notified and didn't see it earlier.

 

1. Reverse mapping zone exists

2. They are authoritive primary on the same Infoblox grid

3. Probably yes. Need to check with the team as I am not the admin

4. No related erros seen

5. Not applicable

 

I guess we will open a ticket then unless you have any other idea.

 

Thanks again.

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Re: Generate Hostname if not Sent by Client

Techie
Posts: 1
7255     0

How do you disable this feature?

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Re: Generate Hostname if not Sent by Client

Community Manager
Community Manager
Posts: 356
7255     0

For everyones benefit, new questions should be submitted as a new thread. For your reference- the setting can be found in the DHCP properties under the Advanced tab for IPv4 DDNS.

 

Regards,

Tony


@wreedmh wrote:

How do you disable this feature?


 

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