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Taikun OCP Guide

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DNS resolution for instances

The Networking service offers several methods to configure name
resolution (DNS) for instances. Most deployments should implement case 1
or 2a. Case 2b requires security considerations to prevent leaking
internal DNS information to instances.

Note

All of these setups require the configured DNS resolvers to be
reachable from the virtual network in question. So unless the resolvers
are located inside the virtual network itself, this implies the need for
a router to be attached to that network having an external gateway
configured.

Case 1:
Each virtual network uses unique DNS resolver(s)

In this case, the DHCP agent offers one or more unique DNS resolvers
to instances via DHCP on each virtual network. You can configure a DNS
resolver when creating or updating a subnet. To configure more than one
DNS resolver, repeat the option multiple times.

  • Configure a DNS resolver when creating a subnet.

    $ openstack subnet create --dns-nameserver DNS_RESOLVER

    Replace DNS_RESOLVER with the IP address of a DNS
    resolver reachable from the virtual network. Repeat the option if you
    want to specify multiple IP addresses. For example:

    $ openstack subnet create --dns-nameserver 203.0.113.8 --dns-nameserver 198.51.100.53

    Note

    This command requires additional options outside the scope of this
    content.

  • Add a DNS resolver to an existing subnet.

    $ openstack subnet set --dns-nameserver DNS_RESOLVER SUBNET_ID_OR_NAME

    Replace DNS_RESOLVER with the IP address of a DNS
    resolver reachable from the virtual network and
    SUBNET_ID_OR_NAME with the UUID or name of the subnet. For
    example, using the selfservice subnet:

    $ openstack subnet set --dns-nameserver 203.0.113.9 selfservice
  • Remove all DNS resolvers from a subnet.

    $ openstack subnet set --no-dns-nameservers SUBNET_ID_OR_NAME

    Replace SUBNET_ID_OR_NAME with the UUID or name of the
    subnet. For example, using the selfservice subnet:

    $ openstack subnet set --no-dns-nameservers selfservice

    Note

    You can use this option in combination with the previous one in order
    to replace all existing DNS resolver addresses with new ones.

You can also set the DNS resolver address to 0.0.0.0 for
IPv4 subnets, or :: for IPv6 subnets, which are special
values that indicate to the DHCP agent that it should not announce any
DNS resolver at all on the subnet.

Note

When DNS resolvers are explicitly specified for a subnet this way,
that setting will take precedence over the options presented in case
2.

Case 2:
DHCP agents forward DNS queries from instances

In this case, the DHCP agent offers the list of all DHCP agent’s IP
addresses on a subnet as DNS resolver(s) to instances via DHCP on that
subnet.

The DHCP agent then runs a masquerading forwarding DNS resolver with
two possible options to determine where the DNS queries are sent to.

Note

The DHCP agent will answer queries for names and addresses of
instances running within the virtual network directly instead of
forwarding them.

Case
2a: Queries are forwarded to an explicitly configured set of DNS
resolvers

In the dhcp_agent.ini file, configure one or more DNS
resolvers. To configure more than one DNS resolver, use a comma between
the values.

[DEFAULT]
dnsmasq_dns_servers = DNS_RESOLVER

Replace DNS_RESOLVER with a list of IP addresses of DNS
resolvers reachable from all virtual networks. For example:

[DEFAULT]
dnsmasq_dns_servers = 203.0.113.8, 198.51.100.53

Note

You must configure this option for all eligible DHCP agents and
restart them to activate the values.

Case
2b: Queries are forwarded to DNS resolver(s) configured on the host

In this case, the DHCP agent forwards queries from the instances to
the DNS resolver(s) configured in the resolv.conf file on
the host running the DHCP agent. This requires these resolvers being
reachable from all virtual networks.

In the dhcp_agent.ini file, enable using the DNS
resolver(s) configured on the host.

[DEFAULT]
dnsmasq_local_resolv = True

Note

You must configure this option for all eligible DHCP agents and
restart them to activate this setting.

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