Advanced Connections

The following guide contains information specific to certain types of MongoDB configurations.

For an example of connecting to a simple standalone server, see the Tutorial. To establish a connection with authentication options enabled, see the Authentication page.

Connecting to a Replica Set

Connecting to a replica set is much like connecting to a standalone MongoDB server. Simply specify the replica set name using the ?replicaSet=myreplset URI option.

#include <bson/bson.h>
#include <mongoc/mongoc.h>

main (int argc, char *argv[])
   mongoc_client_t *client;

   mongoc_init ();

   /* Create our MongoDB Client */
   client = mongoc_client_new (

   /* Do some work */
   /* TODO */

   /* Clean up */
   mongoc_client_destroy (client);
   mongoc_cleanup ();

   return 0;


Multiple hostnames can be specified in the MongoDB connection string URI, with a comma separating hosts in the seed list.

It is recommended to use a seed list of members of the replica set to allow the driver to connect to any node.

Connecting to a Sharded Cluster

To connect to a sharded cluster, specify the mongos nodes the client should connect to. The C Driver will automatically detect that it has connected to a mongos sharding server.

If more than one hostname is specified, a seed list will be created to attempt failover between the mongos instances.


Specifying the replicaSet parameter when connecting to a mongos sharding server is invalid.

#include <bson/bson.h>
#include <mongoc/mongoc.h>

main (int argc, char *argv[])
   mongoc_client_t *client;

   mongoc_init ();

   /* Create our MongoDB Client */
   client = mongoc_client_new ("mongodb://myshard01:27017/");

   /* Do something with client ... */

   /* Free the client */
   mongoc_client_destroy (client);

   mongoc_cleanup ();

   return 0;

Connecting to an IPv6 Address

The MongoDB C Driver will automatically resolve IPv6 addresses from host names. However, to specify an IPv6 address directly, wrap the address in [].

mongoc_uri_t *uri = mongoc_uri_new ("mongodb://[::1]:27017");

Connecting with IPv4 and IPv6

If connecting to a hostname that has both IPv4 and IPv6 DNS records, the behavior follows RFC-6555. A connection to the IPv6 address is attempted first. If IPv6 fails, then a connection is attempted to the IPv4 address. If the connection attempt to IPv6 does not complete within 250ms, then IPv4 is tried in parallel. Whichever succeeds connection first cancels the other. The successful DNS result is cached for 10 minutes.

As a consequence, attempts to connect to a mongod only listening on IPv4 may be delayed if there are both A (IPv4) and AAAA (IPv6) DNS records associated with the host.

To avoid a delay, configure hostnames to match the MongoDB configuration. That is, only create an A record if the mongod is only listening on IPv4.

Connecting to a UNIX Domain Socket

On UNIX-like systems, the C Driver can connect directly to a MongoDB server using a UNIX domain socket. Pass the URL-encoded path to the socket, which must be suffixed with .sock. For example, to connect to a domain socket at /tmp/mongodb-27017.sock:

mongoc_uri_t *uri = mongoc_uri_new ("mongodb://%2Ftmp%2Fmongodb-27017.sock");

Include username and password like so:

mongoc_uri_t *uri = mongoc_uri_new ("mongodb://user:pass@%2Ftmp%2Fmongodb-27017.sock");

Connecting to a server over TLS

These are instructions for configuring TLS/SSL connections.

To run a server locally (on port 27017, for example):

$ mongod --port 27017 --tlsMode requireTLS --tlsCertificateKeyFile server.pem --tlsCAFile ca.pem

Add /?tls=true to the end of a client URI.

mongoc_client_t *client = NULL;
client = mongoc_client_new ("mongodb://localhost:27017/?tls=true");

MongoDB requires client certificates by default, unless the --tlsAllowConnectionsWithoutCertificates is provided. The C Driver can be configured to present a client certificate using the URI option tlsCertificateKeyFile, which may be referenced through the constant MONGOC_URI_TLSCERTIFICATEKEYFILE.

mongoc_client_t *client = NULL;
mongoc_uri_t *uri = mongoc_uri_new ("mongodb://localhost:27017/?tls=true");
mongoc_uri_set_option_as_utf8 (uri, MONGOC_URI_TLSCERTIFICATEKEYFILE, "client.pem");

client = mongoc_client_new_from_uri (uri);

The client certificate provided by tlsCertificateKeyFile must be issued by one of the server trusted Certificate Authorities listed in --tlsCAFile, or issued by a CA in the native certificate store on the server when omitted.

See Configuring TLS for more information on the various TLS related options.

Compressing data to and from MongoDB

MongoDB 3.4 added Snappy compression support, zlib compression in 3.6, and zstd compression in 4.2. To enable compression support the client must be configured with which compressors to use:

mongoc_client_t *client = NULL;
client = mongoc_client_new ("mongodb://localhost:27017/?compressors=snappy,zlib,zstd");

The compressors option specifies the priority order of compressors the client wants to use. Messages are compressed if the client and server share any compressors in common.

Note that the compressor used by the server might not be the same compressor as the client used. For example, if the client uses the connection string compressors=zlib,snappy the client will use zlib compression to send data (if possible), but the server might still reply using snappy, depending on how the server was configured.

The driver must be built with zlib and/or snappy and/or zstd support to enable compression support, any unknown (or not compiled in) compressor value will be ignored. Note: to build with zstd requires cmake 3.12 or higher.

Additional Connection Options

The full list of connection options can be found in the mongoc_uri_t docs.

Certain socket/connection related options are not configurable:





TCP Keep Alive



How long a connection needs to remain idle before TCP starts sending keepalive probes

120 seconds


The time in seconds between TCP probes

10 seconds


How many probes to send, without acknowledgement, before dropping the connection

9 probes


Send packets as soon as possible or buffer small packets (Nagle algorithm)

Enabled (no buffering)