[me@risby ~]$ grep urd /etc/services
urd 465/tcp smtps # URL Rendesvous Directory for SSM / SMTP over SSL (TLS)
Esta documentação do CISCO sugere que
URD is the designated transition solution for content providers that want to use not (yet) SSM enabled receiver applications. URD allows to enable existing applications to be SSM capable without modifying any receiver host software (application nor operating system) - as long as the application is started via a web browser. URD stands for "URL Rendezvous Directory" and relies on the last-hop router towards a SSM receiver host to intercept URLs from a webserver to detect the source address of the SSM channel.
e além disso,
Source Specific Multicast (SSM) is an extension of IP Multicast in which datagram traffic is forwarded to receivers from only those multicast sources for which the receivers have explicitly expressed interest.
Este é (um dos motivos) porque você deve sempre, sempre usar iptables -L -n -v
para listar as regras!