A dialin server is used for remote access of a mainframe via modem communications. The server waits for the modem to issue the 'RING' command and then proceeds to answer and negotiate communications with the client.

     The most popular application for a dialin server is to allow network access for clients. The initial prompt will allow shell account access but the client may also envoke the pppd daemon for PPP communications. The PPP protocol will allow the client to browse the network intranet or the external internet.

     If your setup will allow the PPP protocol to browse the internet, it is also recommended that you setup your own DNS server to cache hostname resolutions. This will maximize bandwidth on the network gateway.


Dialin Applications:
The following programs can be used for dialin access:

	mgetty		- Supports fax and voice. Less features that uugetty.
	vgetty		- Part of mgetty, the voice support application.
	uugetty		- No support for fax or voicemail.

Mingetty only supports console logins and is used by most linux systems.

You can obtain information on mgetty at:
	http://www.leo.org/~doering/mgetty/
	http://alpha.greenie.net/mgetty/

Mgetty Overview:

     getty is usually started in the bootstrap phase. When another modem calls your modem, either the modem can answer the ring or getty can send a signal to the modem instructing it to answer the call. By setting the S0 register on the modem to 3, the modem will automatically answer the call on the 3rd ring. If S0 is set to 0, the modem will not answer. The modem will then have to be instructed by getty with the ATA command to answer the call. If getty instructs the modem to answer the call in place of the modem automatically answering, it is called a "manual answer".

     The manual answer approach is considered a good choice because if something is wrong with the computer but the modem is still operational (mostly with external modems), the modem will answer the call but no login prompt will appear. This is not desirable when long-distance charges may apply or the user may wait hopelessly for the login prompt.


Mgetty Setup:

     Mgetty cannot be called with a simple shell command or startup script as in /etc/rc.d. Mgetty must be called by the init process and configured accordingly in the /etc/inittab configuration file.

     During the bootstrap phase, the linux operating system calls the init process which is responsible for making sure that the gettys are running on the appropriate i/o device (virutal terminals, serial lines, and modems).

     When mgetty is started, it checks if a valid lock file held by another process is present. If it is, this means the port is in use, and mgetty will wait until the lock file goes away. Once the lock file is removed, mgetty creates it's own lock file, initializes the modem and removes its lock file. Mgetty then watches the port for data by using the poll() or select() method.

     The two types of port activities mgetty will detect are a dialout or a dialin. In the first case, a dialout will have created a lock file and mgetty will wait for it to go away, and then exit. Init will start a fresh mgetty process and then wait for a call. In the second case, when there is no lock file, mgetty creates a lock file and reads the characters available. If it finds a 'RING', it picks up the phone by sending 'ATA' and waits for the 'CONNECT' message. If the call is a fax, it saves the file in the 'FAX_SPOOL_IN' (usually '/var/spool/fax/incoming') and exits. If the call is a modem, it prints '/etc/issue' and displays a login prompt. Once it has received a login string, it calls '/bin/login' and lets it handle things from here.

     A typical lock file for serial port devices are in a file called '/usr/spool/uucp/LCK..ttyxx', containing the process ID (PID) of the process currently using the device.


The /etc/inittab Entry:

The typical 'inittab' entry for mgetty looks like this:
	tt:rlevel:respawn|off:/usr/local/sbin/mgetty [options] device
	F1a:23:respawn:/usr/local/sbin/mgetty -x 3 ttyS1
Once the /etc/inittab file is updated, you must restart init with the a HUP signal (kill -1 1).
	Options:
		-x level	- Sets the debuging level.
		-s speed	- Sets the port speed.
		-k space	- Minimum kb required on an incoing fax directory.
		-m 'rx tx'	- Sets the modem initialization sequence. (-r).
		-p 	- Sets the login prompt (escapes are allowed).
		-n rings	- Sets the number of ring before answer.
		-R n		- Enables ringback mode. Call back in n seconds.
		-i '/etc/issue'	- Specifies the issue file to display.
		-S faxdoc	- G3 document sent to polling fax machines (full path).

To Deny Logins:

     Mgetty will not answer the phone is a file is created in the /etc directory. This file looks like this: '/etc/nologin.ttyS2'. This will prevent answers when login processes are not allowed or when the line is being used as a voice line. You can configure cron jobs to automate desired timing and parameter configurations.


Direct Serial Lines:

     To use a direct serial line between two unix machines, or a unix Machine and some other machine, use the -r flag. This will prevent mgetty from trying to initialize the modem or wait for RINGs. This setup may be useful for leased line modems also.