Siprox is an proxy/masquerading daemon for the SIP protocol.
It handles registrations of SIP clients on a private IP network
and performs rewriting of the SIP message bodies to make SIP
connections possible via an masquerading firewall.
It allows SIP clients (like kphone, linphone) to work behind
an IP masquerading firewall or router.

General Overview:

• SIP (RFC3261) Proxy for SIP based softphones hidden behind a masquerading firewall
• works with "dial-up" conenctions (dynamic IP addresses)
• Multiple local users/hosts can be masqueraded simultaneously
• Access control (IP based) for incoming traffic
• Proxy Authentication for registration of local clients (User Agents) with individual passwords for each user
• May be used as pure Outbound proxy (registration of local UAs to a 3rd party registrar)
• Fli4l OPT_SIP (still experimental) available, check FLI4L OPT Paket
• supports Linux and FreeBSD (other BSD derivates not yet tested)
• Full duplex RTP data stream proxy for *incoming* and *outgoing* audio data - no firewall masquerading entries needed
• Port range to be used for RTP traffic is configurable (-> easy to set up apropriate firewall rules for RTP traffic)
• RTP proxy can handle multiple RTP streams (eg. audio + video) within a single SIP session.
• Supports running in a chroot jail and changing user-ID after startup
• All configuration done via one simple ascii configuration file
• Logging to syslog in daemon mode
• RPM package
• The host part of UA registration entries can be masqueraded (mask_host, masked_host config items). Some Siemens SIP phones seem to need this 'feature'.

Written in C.