The DNS Seed List Documentation does not clearly indicate whether the DNS seedlist will work for mongos hosts. The documentation should clearly indicate that the DNS seedlist should work for both mongod and mongos hosts.

Here is is some background research that might help clarify this further:

The dns seed-list connection format is based on the mongodb+srv protocol. Here is some more information on this mongodb+srv protocol:

• The connection string parser in the driver is extended with a new protocol mongodb+srv as a logical pre-processing step.
• In this preprocessing step, the driver will query the DNS server for SRV records on {hostname}.{domainname}, prefixed with _mongodb._tcp.: _mongodb._tcp.{hostname}

  .

  {domainname}

  .

• This DNS query is expected to respond with one or more SRV records.
• From the DNS result, the driver now must behave the same as if an mongodb:// URI was provided with all the host names and port numbers that were returned as part of the DNS SRV query result.
  The detailed specification for the "Initial DNS Seedlist Discovery" can be found here: https://github.com/mongodb/specifications/blob/master/source/initial-dns-seedlist-discovery/initial-dns-seedlist-discovery.rst

Now the ChangeLog for this specification indicates:

_2017-10-04 — 1.1.1
Removed from Future Work the line about multiple MongoS discovery. The current specification already allows for it, as multiple host names which are all MongoS servers is already allowed under SDAM. And this specification does not modify SDAM.
SDAM stands for Server Discovery And Monitoring, and SDAM specification defines how a MongoDB client discovers and monitors one or more servers. And the SDAM specification defines a "server" as a mongod or mongos process._

Therefore, based on the above, it is safe to conclude that DNS seedlist should work for mongod as well as mongos hosts. And our docs on DNS Seed List should mention this clearly.