If any file requires a passphrase, ssh-add asks for the passphrase from the user. The passphrase is read from the user's tty. ssh-add retries the last passphrase if multiple identity files are given.
The authentication agent must be running and the SSH_AUTH_SOCK environment variable must contain the name of its socket for ssh-add to work.
The options are as follows:
Destination constraints of the form `[user@]dest-hostname' permit use of the key only from the origin host (the one running ssh-agent1) to the listed destination host, with optional user name.
Constraints of the form `src-hostname>[user@]dst-hostname' allow a key available on a forwarded ssh-agent1 to be used through a particular host (as specified by `src-hostname' to authenticate to a further host, specified by `dst-hostname'
Multiple destination constraints may be added when loading keys. When attempting authentication with a key that has destination constraints, the whole connection path, including ssh-agent1 forwarding, is tested against those constraints and each hop must be permitted for the attempt to succeed. For example, if key is forwarded to a remote host, `host-b' and is attempting authentication to another host, `host-c' then the operation will be successful only if `host-b' was permitted from the origin host and the subsequent `host-b>host-c' hop is also permitted by destination constraints.
Hosts are identified by their host keys, and are looked up from known hosts files by ssh-add. Wildcards patterns may be used for hostnames and certificate host keys are supported. By default, keys added by ssh-add are not destination constrained.
Destination constraints were added in OpenSSH release 8.9. Support in both the remote SSH client and server is required when using destination-constrained keys over a forwarded ssh-agent1 channel.
It is also important to note that destination constraints can only be enforced by ssh-agent1 when a key is used, or when it is forwarded by a cooperating ssh(1). Specifically, it does not prevent an attacker with access to a remote SSH_AUTH_SOCK from forwarding it again and using it on a different host (but only to a permitted destination).
SSH_ASKPASS_REQUIRE allows further control over the use of an askpass program. If this variable is set to ``never'' then ssh-add will never attempt to use one. If it is set to ``prefer'' then ssh-add will prefer to use the askpass program instead of the TTY when requesting passwords. Finally, if the variable is set to ``force'' then the askpass program will be used for all passphrase input regardless of whether DISPLAY is set.
Identity files should not be readable by anyone but the user. Note that ssh-add ignores identity files if they are accessible by others.