The mongostat documentation says "The mongostat utility provides a quick overview of the status of a currently running mongod or mongos instance" and, for --discover, "When connected to a mongos, mongostat will return data from all shards in the cluster."

Instead, when run against a mongos, mongostat just gives the error message "Failed: error connecting to db server: no reachable servers"

$ mongostat
2016-06-16T11:11:15.345-0400 Failed: error connecting to db server: no reachable servers
$ mongostat --discover
2016-06-16T11:11:22.690-0400 Failed: error connecting to db server: no reachable servers
$ mongostat --discover -vvvvv
2016-06-16T11:11:26.593-0400 adding new host to monitoring: localhost:27017
2016-06-16T11:11:26.593-0400 polling server: localhost:27017
2016-06-16T11:11:26.594-0400 getting session on server: localhost:27017
2016-06-16T11:11:30.097-0400 got error getting session to server localhost:27017
2016-06-16T11:11:30.097-0400 successfully got statline from host: localhost:27017
2016-06-16T11:11:30.097-0400 Failed: error connecting to db server: no reachable servers

But when just using mongo, we can connect to the local mongos just fine and view the hosts in the shards:

$ mongo --host localhost --port 27017
MongoDB shell version: 3.2.7
connecting to: localhost:27017/test
mongos> sh.status()
— Sharding Status —
sharding version:

{ "_id" : 1, "minCompatibleVersion" : 5, "currentVersion" : 6, "clusterId" : ObjectId("54e7a697c43b6b7af43aef7b") }

shards:

{ "_id" : "damshard1", "host" : damshard1/host1:27017,host2:27017" } { "_id" : "damshard2", "host" : damshard2/host3:27017,host4:27017" } { "_id" : "damshard3", "host" : damshard3/host5:27017,host6:27017" }

active mongoses:
"3.2.7" : 3
balancer:
Currently enabled: yes
Currently running: no
Balancer active window is set between 01:00 and 03:00 server local time
Failed balancer rounds in last 5 attempts: 0
Migration Results for the last 24 hours:
52 : Success
databases:

{ "_id" : "admin", "partitioned" : false, "primary" : "config" } { "_id" : "dam", "partitioned" : true, "primary" : "damshard1" }

dam.fs.chunks
shard key:

{ "files_id" : 1 }

unique: false
balancing: true
chunks:
damshard1 2640
damshard2 2669
damshard3 2640
too many chunks to print, use verbose if you want to force print

{ "_id" : "test", "partitioned" : false, "primary" : "damshard1" }

and mongostat can connect, from the same host where mongos is running, to a replica set:

$ mongostat --host=damshard1/host1:27017,host2:27017

insert query update delete getmore command flushes mapped vsize res faults qr|qw ar|aw netIn netOut conn set repl time
host1:27017 *0 *0 *0 *0 0 2|0 0 197.9G 396.8G 46.5G 0 0|0 0|0 246b 13k 12 damshard1 SEC 2016-06-16T11:18:57-04:00
host2:27017 *0 6 *0 *0 1 4|0 0 197.9G 397.2G 11.3G 0 0|0 0|0 2k 20k 67 damshard1 PRI 2016-06-16T11:18:57-04:00