The following new console features improve your ability to troubleshoot remote systems:
The consadm command enables you to select a serial device as an auxiliary (or remote) console. Using the consadm command, a system administrator can configure one or more serial ports to display redirected console messages and to host sulogin sessions when the system transitions between run levels. This feature enables you to dial in to a serial port with a modem to monitor console messages and participate in init state transitions. (For more information, see sulogin(1M) and the step-by-step procedures that follow.)
While you can log in to a system using a port configured as an auxiliary console, it is primarily an output device displaying information that is also displayed on the default console. If boot scripts or other applications read and write to and from the default console, the write output displays on all the auxiliary consoles, but the input is only read from the default console. (For more information on using the consadm command during an interactive login session, see Using the consadm Command During an Interactive Login Session.)
Console output now consists of kernel and syslog messages written to a new pseudo device, /dev/sysmsg. In addition, rc script startup messages are written to /dev/msglog. Previously, all of these messages were written to /dev/console.
Scripts that direct console output to /dev/console need to be changed to /dev/msglog if you want to see script messages displayed on the auxiliary consoles. Programs referencing /dev/console should be explicitly modified to use syslog() or strlog() if you want messages to be redirected to an auxiliary device.
The consadm command runs a daemon to monitor auxiliary console devices. Any display device designated as an auxiliary console that disconnects, hangs up or loses carrier, is removed from the auxiliary console device list and is no longer active. Enabling one or more auxiliary consoles does not disable message display on the default console; messages continue to display on /dev/console.
Keep the following in mind when using auxiliary console messaging during run level transitions:
Input cannot come from an auxiliary console if user input is expected for an rc script that is run when a system is booting. The input must come from the default console.
The sulogin program, invoked by init to prompt for the superuser password when transitioning between run levels, has been modified to send the superuser password prompt to each auxiliary device in addition to the default console device.
When the system is in single-user mode and one or more auxiliary consoles are enabled using the consadm command, a console login session runs on the first device to supply the correct superuser password to the sulogin prompt. When the correct password is received from a console device, sulogin disables input from all other console devices.
A message is displayed on the default console and the other auxiliary consoles when one of the consoles assumes single-user privileges. This message indicates which device has become the console by accepting a correct superuser password. If there is a loss of carrier on the auxiliary console running the single-user shell, one of two actions might occur:
If the auxiliary console represents a system at run level 1, the system proceeds to the default run level.
If the auxiliary console represents a system at run level S, the system displays the ENTER RUN LEVEL (0-6, s or S): message on the device where the init s or shutdown command had been entered from the shell. If there isn't any carrier on that device either, you will have to reestablish carrier and enter the correct run level. The init or shutdown command will not redisplay the run-level prompt.
If you are logged in to a system using a serial port, and an init or shutdown command is issued to transition to another run level, the login session is lost whether this device is the auxiliary console or not. This situation is identical to releases without auxiliary console capabilities.
Once a device is selected as an auxiliary console using the consadm command, it remains the auxiliary console until the system is rebooted or the auxiliary console is unselected. However, the consadm command includes an option to set a device as the auxiliary console across system reboots. (See the following procedure for step-by-step instructions.)
If you want to run an interactive login session by logging in to a system using a terminal that is connected to a serial port, and then using the consadm command to see the console messages from the terminal, note the following behavior:
If you use the terminal for an interactive login session while the auxiliary console is active, the console messages are sent to the /dev/sysmsg or /dev/msglog devices.
While you issue commands on the terminal, input goes to your interactive session and not to the default console (/dev/console).
If you run the init command to change run levels, the remote console software kills your interactive session and runs the sulogin program. At this point, input is accepted only from the terminal and is treated like it's coming from a console device. This allows you to enter your password to the sulogin program as described in Using Auxiliary Console Messaging During Run Level Transitions.
Then, if you enter the correct password on the (auxiliary) terminal, the auxiliary console runs an interactive sulogin session, locks out the default console and any competing auxiliary console. This means the terminal essentially functions as the system console.
From here you can change to run level 3 or go to another run level. If you change run levels, sulogin runs again on all console devices. If you exit or specify that the system should come up to run level 3, then all auxiliary consoles lose their ability to provide input. They revert to being display devices for console messages.
As the system is coming up, you must provide information to rc scripts on the default console device. After the system comes back up, the login program runs on the serial ports and you can log back into another interactive session. If you've designated the device to be an auxiliary console, you will continue to get console messages on your terminal, but all input from the terminal goes to your interactive session.
The consadm daemon does not start monitoring the port until after you add the auxiliary console with the consadm command. As a security feature, console messages are only redirected until carrier drops, or the auxiliary console device is unselected. This means carrier must be established on the port before you can successfully use the consadm command.
For more information on enabling an auxiliary console, see the consadm(1m) man page.
# consadm -a devicename
Example 15-3 Enabling an Auxiliary (Remote) Console
# consadm -a /dev/term/a # consadm /dev/term/a
# consadm -a -p devicename
This adds the device to the list of persistent auxiliary consoles.
Example 15-4 Enabling an Auxiliary (Remote) Console Across System Reboots
# consadm -a -p /dev/term/a # consadm /dev/term/a
Example 15-5 Disabling an Auxiliary (Remote) Console
# consadm -d /dev/term/a # consadm