About DHCP Options

Dynamic Host Configuration Protocol (DHCP) is a network protocol that assigns network configuration parameters, like IPs, through DHCP servers. Nets enable you to set DHCP options for virtual machines (VMs) you create in them.

Nets and DHCP Options

VMs you create in a Net receive a private DNS name per private IP, and if you attach a public IP to them, a public DNS name. These DNS names are configured by a domain name server specified in the DHCP options set associated with the Net.

When creating a Net, a default set of DHCP options is created and automatically associated with the Net. You can also create your own with the options of your choice and associate it with your Net, which replaces the default one. For more information about the default DHCP options set and the default values applied to your Net, see the Default DHCP Options Set section below.

When creating a DHCP options set, you can specify the following elements:

  • The domain name server (DNS) and the domain name to use for the VMs created in your Net.
    In the domain-name-servers option, you can specify the IPs of one or more domain name servers.
    If you specify a custom domain name server, you can also specify a custom DNS hostname for your VMs using the domain-name option.

    To use the OUTSCALE domain name server in a custom DHCP options set, you need to specify the following values:
    - domain-name-servers: OutscaleProvidedDNS
    - domain-name: <region_name>.compute.internal
    For more information, see the Default DHCP Options Set section below.

  • The NTP server to use to synchronize these VMs with machines in another network, for example through a Net peering, a VPN connection, or a DirectLink connection.
    In the ntp-servers option, you can specify the IPs of one or more NTP servers.

    • If the NTP servers you specify are not in a network that is already routed in the route tables of your Net, you need to add a route to their IP. For more information, see Creating a Route.

    • If you do not specify any NTP server and if you created the VM using an official OUTSCALE machine image (OMI), a default OUTSCALE NTP server is used. This NTP server is configured within the official OMI and you do not need to add any route for it.

  • The log server to use to store the logs. In the log-servers option, you can specify the IPs of one or more log servers.

    If the log servers you specify are not in a network that is already routed in the route tables of your Net, you need to add a route to their IP. For more information, see Creating a Route.

The DHCP options sets you create are available for your account and you can associate them with one or more of your Nets. However, a Net can have one associated DHCP options set only at a time.

You cannot modify a DHCP options set after its creation. You can however replace the DHCP options set used by your Net with either another custom one that you created or the default one. For more information, see Modifying the DHCP Options Set Associated with a Net.

When you modify the DHCP options set associated with a Net, the options it contains are automatically applied to both existing VMs and VMs that you create afterward. Existing VMs apply these options within a few hours after you modify the DHCP options set, depending on their DHCP lease renewal frequency. You do not need to restart the VMs. You can however force the renewal of your DHCP lease using the dhclient command for Linux VMs, or the ipconfig/renew command for Windows VMs.

You can delete a DHCP options set you created at any time, but you cannot delete the default one for your Net. For more information, see Deleting a DHCP Options Set.

Default DHCP Options Set

The default DHCP options set contains the following options:

  • domain-name-servers: OutscaleProvidedDNS, mapping to 3DS OUTSCALE’s domain name server.

  • domain-name: The domain name for your Region, which follows the <region_name>.compute.internal format. For example, a VM with the 10.0.1.5 private IP in the eu-west-2 Region has the hostname ip-10-0-1-5.eu-west-2.compute.internal.

The default DHCP options set does not contain any NTP server, but one is automatically configured within official OMIs. To use another NTP server, you need to create a DHCP options set including the ntp-servers option, and add the corresponding route to the server if needed. For more information, see the Nets and DHCP Options section above.

The OutscaleProvidedDNS domain name server is owned by 3DS OUTSCALE. The first IP of the CIDR block of your Net is reserved for this server. For example, in a Net with the 10.0.0.0/16 CIDR block, the IP of the server is 10.0.0.1.

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