LOGIN_DUO(8)

NAME
login_duo - second-factor authentication via Duo login service

SYNOPSIS
login_duo [-d] [-c file] [-h host] [-f user] [command [args...]]

DESCRIPTION
login_duo provides secondary authentication via the Duo authentication service, executing the user's login shell or command only if successful.

The following options are available:

-c  Specify an alternate configuration file to load. Default is /etc/duo/login_duo.conf

-d  Debug mode; send logs to stderr instead of syslog.

-h  Specify the remote IP address for this login (normally taken from the SSH_CONNECTION environment variable, if set).

-f  Specify an alternate Duo user to authenticate as.

If login_duo is installed setuid root (the default), these options are only available to the super-user.

After successful Duo authentication, the user's login shell is invoked, or if an alternate command or SSH_ORIGINAL_COMMAND environment variable is specified, it will be executed via the user's shell with a -c option.

CONFIGURATION
The INI-format configuration file must have a “duo” section with the following options:

host
    Duo API host (required).

ikey
    Duo integration key (required).

skey
    Duo secret key (required).

groups
    If specified, Duo authentication is required only for users whose primary group or supplementary group list matches one of the space-separated pattern-lists (see PATTERNS below).

failmode
    On service or configuration errors that prevent Duo authentication, fail “safe” (allow access) or “secure” (deny access). Default is “safe”.

pushinfo
    Send command to be approved via Duo Push authentication. Default is “no”.

http_proxy
    Use the specified HTTP proxy, same format as the HTTP_PROXY environment variable.

autopush
    Upon successful first-factor authentication, automatically send a login request to the primary second-factor (usually Duo Push). Can be “yes” or “no”. Default is “no”.

motd
    Print the contents of /etc/motd to screen after a successful login. Either "yes" or "no." Default is "no".

prompts
    Number of login attempts a user gets. Default is 3. If using autopush, it is recommended to set prompts to 1.

accept_env_factor
    Look for factor selection or passcode in the DUO_PASSCODE environment variable, before prompting the user. Can override autopush. Default is "no".

fallback_local_ip
    If unable to determine the authentication users's IP address, fallback on the IP address of the server. Default is "no".

https_timeout
    Set to the number of seconds to wait for HTTPS responses from Duo Security. If Duo Security takes longer than the configured number of seconds to respond to the preauth API call, the configured failmode is triggered. Other network operations such as DNS resolution, TCP connection establishment, and the SSL handshake have their own independent timeout and retry logic. Default is 0, which disables the HTTPS timeout.

An example configuration file:

    [duo]
    host = api-deadbeef.duosecurity.com
    ikey = SI9F...53RI
    skey = 4MjR...Q2NmRiM2Q1Y
    pushinfo = yes
    autopush = yes

If installed setuid root (the default), login_duo performs Duo authentication as a dedicated privilege separation user, requiring that the configuration file be owned and readable only by this user.

PATTERNS
A pattern consists of zero or more non-whitespace characters, ‘*’ (a wildcard that matches zero or more characters), or ‘’? (a wildcard that matches exactly one character).

A pattern-list is a comma-separated list of patterns. Patterns within pattern-lists may be negated by preceding them with an exclamation mark (‘’)!. For example, to specify Duo authentication for all users (except those that are also admins), and for guests:

    groups = users,!wheel,!*admin guests

EXAMPLES
login_duo can be enabled system-wide by specifying its full path as a ForceCommand in sshd_config(5) to capture any SSH remote login (including subsystems, remote commands, and interactive login):

    ForceCommand /usr/local/sbin/login_duo

Similarly, a group of administrators could require two-factor authentication for login to a shared root account by specifying login_duo as the forced command for each public key in ~root/.ssh/authorized_keys:

    command="/usr/local/sbin/login_duo -f alice"
    ssh-rsa AAAAB2...19Q== alice@example.net
    command="/usr/local/sbin/login_duo -f bob"
    ssh-dss AAAAC3...51R== bob@example.net

A user without root access could configure their own account to require Duo authentication via the same ~/.ssh/authorized_keys forced command mechanism and a user-installed (non-setuid) login_duo.

FILES

/etc/duo/login_duo.conf
    Default configuration file path

AUTHORS
login_duo was written by Duo Security <support@duosecurity.com>

NOTES
When used to protect remote SSH access, only interactive sessions support interactive Duo login. For scp(1), sftp(1), rsync(1), and other ssh(1) remote commands, login_duo automatically tries the user's default out-of-band factor (smartphone push or voice callback) and disables real-time login progress reporting to provide a clean shell environment.