Ryfujk

I'm having a hard time finding proper docs on the order in which the OpenSSH client tries private keys for authenticating against a server, given that all of the following are present:

• key files with default names in ~/.ssh, e.g. ~/.ssh/id_rsa,


• key files with non-default names that are configured to be used with a specific host via ~/.ssh/config,


• keys managed by ssh-agent.

Are all of these locations always tried (provided that the host matches an entry in ~/.ssh/config), and in which order?

Background: I have ~/.ssh/id_rsa which can only be used to authenticate against server A, and I have another key managed by ssh-agent that can only be used to authenticate against server B. When trying to ssh to B, authentications fails, apparently because only ~/.ssh/id_rsa is tried. There does not seem to be a fallback to try the key managed by ssh-agent.

I know about the -v, -vv etc. options, but I'd like to know before I try how ssh is going to behave. There must be a saner method to find out than trial and error.

Use the source, Luke!

OpenSSH is open source so instead of trial-error, you can read the code to get better understanding what is going on there. ssh.c is a good place to start. It has a function load_public_identity_files(void), which takes care of this. In the first place, the keys from PKCS#11 (Smartcard, HSM) are used:

(nkeys = pkcs11_add_provider(options.pkcs11_provider, NULL,

and then the keys provided by options.identity_files:

for (i = 0; i < options.num_identity_files; i++) {

This variable is set in readconf.c:

if (options->num_identity_files == 0) {

    add_identity_file(options, "~/", _PATH_SSH_CLIENT_ID_RSA, 0);

    add_identity_file(options, "~/", _PATH_SSH_CLIENT_ID_DSA, 0);

#ifdef OPENSSL_HAS_ECC

    add_identity_file(options, "~/", _PATH_SSH_CLIENT_ID_ECDSA, 0);

#endif

    add_identity_file(options, "~/",

        _PATH_SSH_CLIENT_ID_ED25519, 0);

}

The real paths of the files are defined in pathnames.h:

#define _PATH_SSH_USER_DIR  ".ssh"

[...]

#define _PATH_SSH_CLIENT_ID_DSA     _PATH_SSH_USER_DIR "/id_dsa"

#define _PATH_SSH_CLIENT_ID_ECDSA   _PATH_SSH_USER_DIR "/id_ecdsa"

#define _PATH_SSH_CLIENT_ID_RSA     _PATH_SSH_USER_DIR "/id_rsa"

#define _PATH_SSH_CLIENT_ID_ED25519 _PATH_SSH_USER_DIR "/id_ed25519"

To the background question:

This is exactly why the IdentitiesOnly option exists and why you should use it in the ~/.ssh/config if you have more than one key to manage. The ssh-agent identities are used after the default ones.

If you want to see how SSH client tries private keys for authenticating against a server, you can run it with -v option.

In my case it looks like:

debug1: Trying private key: /Users/atolkachev/.ssh/id_rsa

debug1: Trying private key: /Users/atolkachev/.ssh/id_dsa

debug1: Trying private key: /Users/atolkachev/.ssh/id_ecdsa

debug1: Trying private key: /Users/atolkachev/.ssh/id_ed25519