Giacomo Vacca

If you've ever installed a package from command line on Linux, you must have noticed two main prompts related to package configuration: one asking what to do with installed configuration files with local changes, and one providing feedback right after the installation/upgrade/removal/purge has completed. Under Debian the latter was most probably the postinst script, one of the Package Maintainer Scripts which is executed after installation and configuration. These diagrams are very useful to understand what happens to the Maintainer Scripts in different circumstances: first installation, upgrade, removal, purge. Their name is probably self-explanatory: preinst , postinst , prerm , postrm . Each of them takes zero or more arguments depending on the scenario. It can help to understand that those are really just scripts executed by dpkg - and typically they are shell scripts, sometimes with interactive prompts. If you're building your own packages, you surely already h...

SIP is a relatively simple, text-based, human readable protocol that is now the standard de facto for VoIP signalling. The protocol though (in my opinion!) is a little tricky, where typically the tricks are: details . In this first post of a series of "Trip-wires for the SIP fans", I'll talk about CANCEL. The main concept is easy: a caller may decide to cancel a call before this is answered. To do this, it sends a CANCEL request to the called party. What's important to know is that: A CANCEL request relates to an INVITE request, and does not relate to the SIP dialog the request may have created (or will create). For this reason the To header tag must be the same as the INVITE request, even if meanwhile there's been a provisional response to the INVITE creating a dialog (e.g. a 180 with a tag in the To header). From RFC 3261, 9.1 : The following procedures are used to construct a CANCEL request.  The    Request-URI, Call-ID, To, the numeric part of ...