HP X.25/9000 User's Guide HP-UX 11i v3 (5900-1523, August 2011)

Call acceptance mechanism and system security
The x29server process operates in conjunction with devices declared in the set of pad_spt
(PAD support) entries contained in the x29hosts file. It continually “listens” to the network and
initializes communications channels at the user level when a call request is received. x29server
can handle a large number of calls because each communication channel is managed by an
independent process.
The x29server process employs Berkeley Sockets (at X.25 level 3) to set up dedicated listening
sockets that only handle calls arriving at a particular subsystem. Addressing information contained
in incoming call request packets enables the listening process to determine if the call should be
accepted.
When a call request packet arrives x29server looks in the pad_spt entries to verify the following:
• Legal PID in the incoming call request packet – The PID must conform to the X.29 standard to
ensure support on all X.25 networks. x29server only verifies that the PID begins with 01
and that it is 4 bytes in length as illustrated below.
• Correct interface_name – If there is an entry in the interface_name field in the pad_spt
section (optional), it must contain the name of the interface over which the call has arrived.
• Correct address – If there is an entry in the local_x121 field in the pad_spt section
(optional), this entry must match the called address specified in the call request packet.
Furthermore, the local_x121 field entry must contain the complete address (that is, both the
address and subaddress of the local interface).
If all of the above conditions are met, the call is connected and x29server does the following:
◦ parses the x29hosts file to find the first previously accepted pad_spt entry whose
remote_x121 field matches the calling address contained in the call request packet.
◦ and, if there is a pty_slave_fname field in the selected pad_spt entry, x29server
verifies that the corresponding PTY master/slave pair exists and is available.
If the last two items are completed successfully, the call is accepted; otherwise, x29server rejects
the call.
NOTE: See the man pages for x29server for its syntax and parameters.
Supported remote PAD terminals
• 700/41, 700/92, 700/94, 700/22
• 2392A
• 2393A
• Vectra with AdvanceLink Terminal Emulation
Configuring remote PAD support
You can configure remote PAD support with SMH or by editing configurable parameters in the
/etc/x25/x29hosts and /etc/x25/x3config files. The x29hosts file contains general
96 PAD services