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I am using Apple's mDNSResponder. It works in single network. Is there a way to make it work even if there is router between computer that browses for services and computer that is service provider?

I am not good in networking, but it seems to me that usually multicast packets don't go through router.

So, i have two possible options in mind:

  1. There might be some key in some network protocol that forces router to retranslate multicast packet to other network.
  2. There's some program you can install on router to do that (if no, is it even possible to write one?).
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There are methods for multicast routing, yes. The currently preferred protocol is PIM, which replaces the older DVMRP. It's even supported on various academic networks like GÉANT. (For example, here's an article about multicast across the CUDN.)

(I haven't tried either myself; the routers on networks I manage lack support.)

Alternatively, there is IGMP proxying. It's more widely supported and simpler to set up... But on the other hand, it is directional (probably designed for use with IPTV) – instead of proper routing, you must configure one network interface as "upstream" (the source of multicast data) and the rest as "downstream" (the passive recipients).

So, it might work for peer-to-peer scenarios like mDNS, but I wouldn't waste time.

Besides, some mDNS implementations use TTL=1, so routers would discard that anyway. (This is a problem with both routing and IGMP proxying.)

So your third option is mDNS proxying. If the router runs Linux with Avahi, you can find the "reflector" options in avahi-daemon.conf:

[reflector]
enable-reflector=yes
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