How do I configure SSH so it doesn't try all the identity files automatically?

You can use the IdentitiesOnly=yes option along with IdentityFile (see ssh_config man page). That way, you can specify which file(s) it should look for.

In this example, ssh will only look in the identities given in the ssh_config files + the 4 ones listed on the command line (the identities provided by the agent will be ignored):

ssh -o IdentitiesOnly=yes \
    -o IdentityFile=id1.key \
    -o IdentityFile=id2.key \
    -i id3.key \
    -i id4.key \
    [email protected]

The forms -i and -o IdentityFile= are interchangeable.

In .ssh/config, you can include config like this:

Host example
User user123
Hostname example.com
IdentityFile ~/.ssh/id_rsa_example
IdentityFile ~/.ssh/id_rsa_example2
IdentitiesOnly yes

user76528's short answer is correct, but I just had this problem and thought some elaboration would be useful. You might also care about this solution if you've wondered "Why is ssh ignoring my identityfile configuration option"?

Firstly, unlike every other option in ssh_config, ssh does not use the first IdentityFile that it finds. Instead the IdentityFile option adds that file to a list of identities used. You may stack multiple IdentityFile options, and the ssh client will try them all until the server accepts one or rejects the connection.

Second, if you use an ssh-agent, ssh will automatically try to use the keys in the agent, even if you have not specified them with in ssh_config's IdentityFile (or -i) option. This is a common reason you might get the Too many authentication failures for user error. Using the IdentitiesOnly yes option will disable this behavior.

If you ssh as multiple users to multiple systems, I recommend putting IdentitiesOnly yes in your global section of ssh_config, and putting each IdentityFile within the appropriate Host subsections.

I generally do it like so:

$ ssh -o IdentitiesOnly=yes -F /dev/null -i ~/path/to/some_id_rsa [email protected]

The options are as follows:

• -o IdentitiesOnly=yes - tells SSH to only use keys that are provided via the CLI and none from the $HOME/.ssh or via ssh-agent
• -F /dev/null - disables the use of $HOME/.ssh/config
• -i ~/path/to/some_id_rsa - the key that you explicitly want to use for the connection

Example

$ ssh -v -o IdentitiesOnly=yes -F /dev/null -i ~/my_id_rsa [email protected]
OpenSSH_6.2p2, OSSLShim 0.9.8r 8 Dec 2011
debug1: Reading configuration data /dev/null
debug1: Connecting to someserver.mydom.com [10.128.12.124] port 22.
debug1: Connection established.
debug1: identity file /Users/sammingolelli/my_id_rsa type 1
debug1: identity file /Users/sammingolelli/my_id_rsa-cert type -1
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_6.2
debug1: Remote protocol version 2.0, remote software version OpenSSH_5.3
debug1: match: OpenSSH_5.3 pat OpenSSH_5*
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: server->client aes128-ctr hmac-md5 none
debug1: kex: client->server aes128-ctr hmac-md5 none
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024<1024<8192) sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP
debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
debug1: Server host key: RSA f5:60:30:71:8c:a3:da:a3:fe:b1:6d:0b:20:87:23:e1
debug1: Host 'someserver' is known and matches the RSA host key.
debug1: Found key in /Users/sammingolelli/.ssh/known_hosts:103
debug1: ssh_rsa_verify: signature correct
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: Roaming not allowed by server
debug1: SSH2_MSG_SERVICE_REQUEST sent
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey,gssapi-keyex,gssapi-with-mic,password
debug1: Next authentication method: publickey
debug1: Offering RSA public key: /Users/sammingolelli/my_id_rsa
debug1: Server accepts key: pkalg ssh-rsa blen 535
debug1: read PEM private key done: type RSA
debug1: Authentication succeeded (publickey).
Authenticated to someserver.mydom.com ([10.128.12.124]:22).
debug1: channel 0: new [client-session]
debug1: Requesting [email protected]
debug1: Entering interactive session.
Last login: Tue Dec  8 19:03:24 2015 from 153.65.219.15
someserver$

Notice in the above output that ssh has only identified the my_id_rsa private key via the CLI and that it uses it to connect to someserver.

Specifically these sections:

debug1: identity file /Users/sammingolelli/my_id_rsa type 1
debug1: identity file /Users/sammingolelli/my_id_rsa-cert type -1

and:

debug1: Next authentication method: publickey
debug1: Offering RSA public key: /Users/sammingolelli/my_id_rsa
debug1: Server accepts key: pkalg ssh-rsa blen 535
debug1: read PEM private key done: type RSA
debug1: Authentication succeeded (publickey).