Siproxd is a proxy/masquerading daemon for SIP (Session Initiation
Protocol), which is used in IP telephony.
It handles registrations of SIP clients on a private IP network
and performs rewriting of the SIP message bodies to make SIP
connections work via a masquerading firewall (NAT).
It allows SIP software clients (like kphone, linphone) or SIP
hardware clients (Voice over IP phones which are SIP-compatible,
such as those from Cisco, Grandstream or Snom) to work behind
an IP masquerading firewall or NAT router.

SIP (Session Initiation Protocol, RFC3261) is the protocol of
choice for most VoIP (Voice over IP) phones to initiate
communication. By itself, SIP does not work via masquerading
firewalls as the transfered data contains IP addresses and
port numbers. There do exist other solutions to traverse NAT existing
(like STUN, or SIP aware NAT routers), but such a solutions has its
disadvantages or may not be applied to a given situation.
Siproxd does not aim to be a replacement for these solutions,
however in some situations siproxd may bring advantages.