A domain name server implementing this specification is called a _DNS Seed_, and answers incoming DNS queries of type `A`, `AAAA`, or `SRV` as specified in RFCs 1035<sup>[1](#ref-1)</sup>, 3596<sup>[2](#ref-2)</sup> and 2782<sup>[3](#ref-3)</sup> respectively.
-`a`: address types, used to specify what address types should be returned for `SRV` queries. This is a bitfield that uses the types from [BOLT #7](07-routing-gossip.md) as bit index. This condition MAY only be used for `SRV` queries. (default value: 6, i.e. `2 || 4`, since bit 1 and bit 2 are set for IPv4 and IPv6, respectively)
Conditions are passed in the DNS seed query as individual, dot-separated subdomain components.
A query for `r0.a2.n10.lseed.bitcoinstats.com` would mean: Return 10 (`n10`) IPv4 (`a2`) records for nodes supporting Bitcoin (`r0`).
The DNS seed MUST evaluate the conditions from the _seed root domain_ and going up-the-tree, meaning right-to-left in a fully qualified domain name. In the example above, that would be: `n10`, then `a2`, then `r0`.
If a condition (key) is specified more than once, the DNS seed MUST discard any earlier value for that condition and use the new value instead. For `n5.r0.a2.n10.lseed.bitcoinstats.com`, the result is then: ~~`n10`~~, `a2`, `r0`, `n5`.
If the DNS seed does not implement filtering by a given condition it MAY ignore the condition altogether (i.e. the seed filtering is best effort only).
Queries distinguish between _wildcard_ queries and _node_ queries, depending on whether the `l`-key is set or not.
Upon receiving a wildcard query, the DNS seed MUST select a random subset of up to `n` IPv4 or IPv6 addresses of nodes that are listening for incoming connections.
The results are serialized in a reply with a query type matching the client's query type, i.e. `A` queries result in `A` replies, `AAAA` queries result in `AAAA` replies, and `SRV` queries result in `SRV` replies, but they may be augmented with additional records (e.g. to add `A` or `AAAA` records matching the returned `SRV` records).
The DNS seed MAY additionally return the corresponding `A` and `AAAA` records that indicate the IP address for the `SRV` entries in the Extra section of the reply.
Due to the large size of the resulting reply, the reply may be dropped by intermediate resolvers, hence the DNS seed MAY omit these additional records upon detecting a repeated query.
In accordance with the Bitcoin DNS Seed policy<sup>[4](#ref-4)</sup>, replies to random queries (i.e. queries to the seed root domain and to the `_nodes._tcp.` alias for `SRV` queries) MUST be random samples from the set of all known good nodes and MUST NOT be biased.
