Configuring a FreeBSD system for dial-in service is similar to configuring terminals, except that modems are used instead of terminal devices. FreeBSD supports both external and internal modems.
External modems are more convenient because they often can be configured via parameters stored in non-volatile RAM and they usually provide lighted indicators that display the state of important RS-232 signals, indicating whether the modem is operating properly.
Internal modems usually lack non-volatile RAM, so their configuration may be limited to setting DIP switches. If the internal modem has any signal indicator lights, they are difficult to view when the system's cover is in place.
When using an external modem, a proper cable is needed. A standard RS-232C serial cable should suffice.
FreeBSD needs the RTS and CTS signals for flow control at speeds above 2400 bps, the CD signal to detect when a call has been answered or the line has been hung up, and the DTR signal to reset the modem after a session is complete. Some cables are wired without all of the needed signals, so if a login session does not go away when the line hangs up, there may be a problem with the cable. Refer to Section 25.2.1, “Serial Cables and Ports” for more information about these signals.
Like other UNIX(R)-like operating systems, FreeBSD uses the hardware signals to find out when a call has been answered or a line has been hung up and to hangup and reset the modem after a call. FreeBSD avoids sending commands to the modem or watching for status reports from the modem.
FreeBSD supports the NS8250, NS16450, NS16550, and NS16550A-based RS-232C (CCITT V.24) communications interfaces. The 8250 and 16450 devices have single-character buffers. The 16550 device provides a 16-character buffer, which allows for better system performance. Bugs in plain 16550 devices prevent the use of the 16-character buffer, so use 16550A devices if possible. Because single-character-buffer devices require more work by the operating system than the 16-character-buffer devices, 16550A-based serial interface cards are preferred. If the system has many active serial ports or will have a heavy load, 16550A-based cards are better for low-error-rate communications.
The rest of this section demonstrates how to configure a modem to receive incoming connections, how to communicate with the modem, and offers some troubleshooting tips.
As with terminals,
init spawns a
getty process for each configured serial
port used for dial-in connections. When a user dials the
modem's line and the modems connect, the “Carrier
Detect” signal is reported by the modem. The kernel
notices that the carrier has been detected and instructs
getty to open the port and display a
login: prompt at the specified initial line
speed. In a typical configuration, if garbage characters are
received, usually due to the modem's connection speed being
different than the configured speed,
tries adjusting the line speeds until it receives reasonable
characters. After the user enters their login name,
which completes the login process by asking for the user's
password and then starting the user's shell.
There are two schools of thought regarding dial-up modems. One configuration method is to set the modems and systems so that no matter at what speed a remote user dials in, the dial-in RS-232 interface runs at a locked speed. The benefit of this configuration is that the remote user always sees a system login prompt immediately. The downside is that the system does not know what a user's true data rate is, so full-screen programs like Emacs will not adjust their screen-painting methods to make their response better for slower connections.
The second method is to configure the
RS-232 interface to vary its speed based on
the remote user's connection speed. Because
getty does not understand any particular
modem's connection speed reporting, it gives a
login: message at an initial speed and
watches the characters that come back in response. If the
user sees junk, they should press Enter until
they see a recognizable prompt. If the data rates do not
getty sees anything the user types
as junk, tries the next speed, and gives the
login: prompt again. This procedure normally
only takes a keystroke or two before the user sees a good
prompt. This login sequence does not look as clean as the
locked-speed method, but a user on a low-speed connection
should receive better interactive response from full-screen
When locking a modem's data communications rate at a
particular speed, no changes to
/etc/gettytab should be needed. However,
for a matching-speed configuration, additional entries may be
required in order to define the speeds to use for the modem.
This example configures a 14.4 Kbps modem with a top
interface speed of 19.2 Kbps using 8-bit, no parity
connections. It configures
getty to start
the communications rate for a V.32bis connection at
19.2 Kbps, then cycles through 9600 bps,
2400 bps, 1200 bps, 300 bps, and back to
19.2 Kbps. Communications rate cycling is implemented
nx= (next table) capability. Each
line uses a
tc= (table continuation) entry
to pick up the rest of the settings for a particular data
# # Additions for a V.32bis Modem # um|V300|High Speed Modem at 300,8-bit:\ :nx=V19200:tc=std.300: un|V1200|High Speed Modem at 1200,8-bit:\ :nx=V300:tc=std.1200: uo|V2400|High Speed Modem at 2400,8-bit:\ :nx=V1200:tc=std.2400: up|V9600|High Speed Modem at 9600,8-bit:\ :nx=V2400:tc=std.9600: uq|V19200|High Speed Modem at 19200,8-bit:\ :nx=V9600:tc=std.19200:
For a 28.8 Kbps modem, or to take advantage of compression on a 14.4 Kbps modem, use a higher communications rate, as seen in this example:
# # Additions for a V.32bis or V.34 Modem # Starting at 57.6 Kbps # vm|VH300|Very High Speed Modem at 300,8-bit:\ :nx=VH57600:tc=std.300: vn|VH1200|Very High Speed Modem at 1200,8-bit:\ :nx=VH300:tc=std.1200: vo|VH2400|Very High Speed Modem at 2400,8-bit:\ :nx=VH1200:tc=std.2400: vp|VH9600|Very High Speed Modem at 9600,8-bit:\ :nx=VH2400:tc=std.9600: vq|VH57600|Very High Speed Modem at 57600,8-bit:\ :nx=VH9600:tc=std.57600:
For a slow CPU or a heavily loaded system without 16550A-based serial ports, this configuration may produce sio “silo” errors at 57.6 Kbps.
The configuration of
similar to Example 25.1, “Configuring Terminal Entries”, but a different
argument is passed to
dialup is used for the terminal type.
xxx with the process
init will run on the device:
xxx" dialup on
dialup terminal type can be
changed. For example, setting
vt102 as the
default terminal type allows users to use
VT102 emulation on their remote
For a locked-speed configuration, specify the speed with
a valid type listed in
This example is for a modem whose port speed is locked at
ttyu0 "/usr/libexec/getty std.
19200" dialup on
In a matching-speed configuration, the entry needs to
reference the appropriate beginning “auto-baud”
/etc/gettytab. To continue the
example for a matching-speed modem that starts at
19.2 Kbps, use this entry:
ttyu0 "/usr/libexec/getty V19200" dialup on
/etc/ttys, wait until
the modem is properly configured and connected before
kill -HUP 1
High-speed modems, like V.32,
V.32bis, and V.34
modems, use hardware (
stty to set the hardware flow
control flag for the modem port. This example sets the
crtscts flag on
dial-in and dial-out initialization devices:
stty -f /dev/ttyu1.init crtscts
stty -f /dev/cuau1.init crtscts
This section provides a few tips for troubleshooting a dial-up modem that will not connect to a FreeBSD system.
Hook up the modem to the FreeBSD system and boot the system.
If the modem has status indication lights, watch to see
whether the modem's DTR indicator lights
login: prompt appears on the
system's console. If it lights up, that should mean that FreeBSD
has started a
getty process on the
appropriate communications port and is waiting for the modem
to accept a call.
If the DTR indicator does not light,
login to the FreeBSD system through the console and type
ps ax to see if FreeBSD is running a
getty process on the correct port:
114 ?? I 0:00.10 /usr/libexec/getty V19200
If the second column contains a
instead of a
?? and the modem has not
accepted a call yet, this means that
has completed its open on the communications port. This could
indicate a problem with the cabling or a misconfigured modem
getty should not be able to open
the communications port until the carrier detect signal has
been asserted by the modem.
getty processes are waiting to
open the port, double-check that the entry for the port is
/etc/ttys. Also, check
/var/log/messages to see if there are
any log messages from
Next, try dialing into the system. Be sure to use 8 bits,
no parity, and 1 stop bit on the remote system. If a prompt
does not appear right away, or the prompt shows garbage, try
pressing Enter about once per second. If
there is still no
try sending a
BREAK. When using a
high-speed modem, try dialing again after locking the
dialing modem's interface speed.
If there is still no
login: prompt, check
/etc/gettytab again and double-check
The initial capability name specified in the entry in
/etc/ttysmatches the name of a capability in
nx=entry matches another
tc=entry matches another
If the modem on the FreeBSD system will not answer, make sure that the modem is configured to answer the phone when DTR is asserted. If the modem seems to be configured correctly, verify that the DTR line is asserted by checking the modem's indicator lights.
If it still does not work, try sending an email to the FreeBSD general questions mailing list describing the modem and the problem.