KNOW-HOW CONNECTIVITY
Turning PDAs into Linux terminals
BRAINSWAP
THE
SECOND
CLEMENS RUDOLPH
PDAs are undoubtedly invaluable gadgets and have all sorts of uses.
Using Linux as a gateway, they can even be used for surfing the Web,
reading emails or administering a server via Telnet. This feature will tell
you exactly how to do all this using a Psion PDA. However, the basic
principles we ll cover will point you in the right direction when it
comes to using other types of client computer too.
PDAs worth their are exactly what we ll reveal in this feature. Almost
salt have a serial interface everything we ll cover can be applied to any system
of some kind that lets them capable of connecting to a Linux box using a serial
communicate with the outside world. link, including old Atari, Amiga and even Intel
Most modern PDAs also include a range of Internet 80286-based systems.
applications as standard, so all you need to get
online is a modem. Alternatively, you can simply link
Preparing the Psion
them via a null modem cable to a Linux system that
has an Internet connection. You ll need to know the Before we do anything else, we have to make the
secrets of mgetty and pppd in order to do so, but client computer in our case the Psion organiser
these (and how a Psion PDA can make use of them) Internet-capable. A glance in the Psion s system
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CONNECTIVITY KNOW-HOW
Fig. 1:
Communication
menus in the Psion s
control panel
control panel (see Figure 1) reveals three icons used
to configure connections with the outside world:
Dialling, Modems and Internet. But since we ll be
using a fixed connection, we ll ignore Dialling.
We ll start with the Modem configuration
option. All we need to do is select Direct
connection for the Current modem option.
Next, turning to the Internet configuration option,
we need to use the New option to make a new
profile based on Default settings , then give it an
appropriate name we ll use Intranet , but
anything will do. The settings that appear when you
select the Edit option are shown in the
screenshots in Figure 2. These are pretty
straightforward to configure as needed.
At this point there is just one other step to
complete on the Psion; as paradoxical as it may sound,
in the PDA s main menu, under Extras , we have to
change the Link option to OFF in the
Communication section. This is the only way to get
the little organiser to perform purely TCP
communication and to stop it acting as a PsiWin client.
The next step is to start configuring our Linux box
Configuration of the gateway
Two basic types of connection are possible between a
client and our Linux system. The first is a simple console
connection, in which only a log-in (thus a console) is
opened. The second type of connection makes use of
the Point-to-Point Protocol, or PPP for short.
Using these two types of connection, the link
between the Psion and the Internet can be made, the
Psion s Comms terminal program making use of the
console connection, and its Web and Mail
programs making use of a Point-to-Point connection.
Port monitoring with mgetty
In order to build our communications framework,
we first
n
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KNOW-HOW CONNECTIVITY
Fig. 2:
Configuring an
Internet service
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Espionage with logwin
When using a high debuglevel, as we have done in our examples, it makes sense to keep a
close eye on the outputs of the services in order to see if anything is wrong. To do so, when
working under X, you can use xterm (or similar) and an appropriate console to look at any
interesting logfiles.
To achieve this for xterm (and its variants) we need to change to root mode using su -l , and
then type
xterm -e tail -f /var/log/messages /var/log/warn /var/log/mgetty.ttyS[0-9]
The regular expression [0-9] at the end is necessary to capture only files ending in a
number and not, for example, an old log file already automatically grabbed by the system.
This would obviously cause some confusion in the case of tail.
Figure 3: Console log-in
with Comms
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KNOW-HOW CONNECTIVITY
Transport of configuration files
One option for transferring the configuration files required to give a Psion Internet access
from one PC onto another would be to save the data to floppy disk. However, a much more
elegant solution is to store the configuration onto the Psion itself and transfer it to the host
as required. This does necessitate a program to be run on the host that allows a serial data
transfer for files. This is where the almost ubiquitous minicom, which acts as a terminal
emulator, comes in handy. By linking this with the Psion s Comms program it is very simple
to exchange files between the two systems. The y-modem protocol comes in vary handy
here. A few tips on how all this works can be found at http://www.mda.de/homes/tron5/
psilink.html.
Table 1: The pppd-options and their effect
pppd-Option Description
crtscts This activates hardware flow control.
lock This parameter serves to hang a UUCP-conform lock in front of the device in use. In other words an exclusive user
right to the COM-port is established for the process accessing it at that moment (/var/lock/...).
noauth With this option, authentication (which in our case would cause interference) is suppressed.
noccp This option ensures that no CCP (Compression Control Protocol) negotiation occurs during the connection. The
Linux machine and the Psion negotiate about compression at start of the connection process (whether wanted or
not), but the pppd, which stems from a time in which telephone lines were even more unreliable than they are
today, normally tries to keep negotiating every 10 seconds.
nopersist This parameter is not absolutely necessary, as it applies by default. We have included it for completeness and
security reasons though (see Read sequence of configs of pppd in the text). In any case, it makes sure that the
pppd stops when a serial connection (from the Psion) is closed. This, of course, only makes sense if an existing
connection is checked by LCP (Line Control Protocol)
silent Silent serves to ensure that the daemon continues to wait patiently until something stirs on the line. It instructs
the daemon to wait, silently for whatever comes in and do nothing until then.
proxyarp Using this parameter, an entry containing the IP address that we need for the Psion is added to the ARP table (or
Address Resolution Protocol Table to give its proper name). Effectively, it means something like It is now one of us .
local This tells the daemon that it doesn t need to worry about a modem on the serial line, and so
it
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About the author
Clemens Rudolph is a
programmer for an ISP, and
specializes in PHP and Perl. He
never tires of poking around
in the bowels of his system.
Comms versus Hermes
Hermes, a Telnet client and terminal emulation program for the Psion (available from
http://www.iota.demon.co.uk/psion/hermes/hermes.html), is a really amazing application that
can provide a pure serial console connection as easily as a PPP-connection. In contrast, Comms
is only a terminal and so can only be persuaded to work with the likes of login or minicom.
This means that Hermes is really great when you need to log in to a Linux box on which you
don t have root privileges. You can start pppd as normal user, provided the rights have been
set accordingly and on call up the path is given as /usr/sbin/pppd .... You will need to make
some configuration changes in Hermes in order to do so though. In the Connection menu,
look for the Connection option and switch it to TCP-Connection . Now all you need to do
is enter the IP address of the destination system and you ll have a Telnet log-in in seconds.
5 · 2001 LINUX MAGAZINE 85
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