Telit Wireless Solutions Easy GPRS - Manual (Page 21)

Easy GPRS User Guide
80000ST10028 Rev. 1 - 14/03/07
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Rights Reserved page 21 of 37
When packet incomes from the IP address <incoming IP>, the firewall chain rules will be scanned for
matching with the following criteria:
<incoming IP> & <net mask> = <ip_address> ?
if the result is yes, then the packet is accepted and the rule scan is finished, otherwise the next chain
is taken into account until the end of the rules when the packet is silently dropped if no matching was
found.
For example, let’s assume we want to accept connections only from our devices which are on the IP
addresses ranging from 197.158.1.1 to 197.158.255.255
We need to add the following chain to the firewall:
AT#FRWL=1,"197.158.1.1","255.255.0.0"
2.1.2.2 Request the socket connection to be opened in listen
With the AT command #SKTL (socket Listen) the TCP request to start listening for connection
requests is executed. The Telit module opens a listening socket on the port specified, waiting for
incoming TCP connections (depending on the parameter request) with the internet hosts
The parameters that shall be specified are the local port where packets shall be received, the type of
socket and the closing behaviour.
2.1.2.3 Receiving connection requests
Once the connection request is received, the module reports an indication of connection with an
unsolicited code
+CONN FROM: <remote address>
• then connection is accepted and once it is up the module reports the code:
CONNECT
From this moment the data incoming in the serial port is packet and sent to the Internet host, while the
data received from the host is serialised and flushed to the Terminal Equipment.
Note that the connections request are FIRST screened in the firewall, then if they are accepted they
pass to the listening socket; therefore only hosts that are in the ACCEPT chain rules of the firewall can
induce a connection request, the other host requests will be silently discarded without any indication to
the remote host (for security reasons).
Once the connection is received and closed, the socket is not anymore in listen. If the application
needs again to be in listen, then it shall send again the socket listen #SKTL command.
NOTE: before issuing this command the GPRS context should be activated with AT#GPRS=1. When
disconnecting the #SKTL command does not close the GPRS context, and leaves it active for the