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# mDNS Repeater
Starting with VyOS 1.2 a {abbr}`mDNS (Multicast DNS)` repeater functionality is
provided. Additional information can be obtained from
<https://en.wikipedia.org/wiki/Multicast_DNS>.
Multicast DNS uses the reserved address `224.0.0.251`, which is
"administratively scoped" and does not leave the subnet. mDNS repeater
retransmits mDNS packets from one interface to other interfaces. This enables
support for devices using mDNS discovery (like network printers, Apple Airplay,
Chromecast, various IP based home-automation devices etc) across multiple VLANs.
Since the mDNS protocol sends the {abbr}`AA(Authoritative Answer)` records in
the packet itself, the repeater does not need to forge the source address.
Instead, the source address is of the interface that repeats the packet.
:::{note}
You can not run this in a VRRP setup, if multiple mDNS repeaters
are launched in a subnet you will experience the mDNS packet storm death!
:::
## Configuration
```{cfgcmd} set service mdns repeater interface \<interface\>
To enable mDNS repeater you need to configure at least two interfaces so that
all incoming mDNS packets from one interface configured here can be
re-broadcasted to any other interface(s) configured under this section.
```
```{cfgcmd} set service mdns repeater disable
mDNS repeater can be temporarily disabled without deleting the service using
```
```{cfgcmd} set service mdns repeater ip-version \<ipv4 | ipv6 | both\>
mDNS repeater can be enabled either on IPv4 socket or on IPv6 socket or both
to re-broadcast. By default, mDNS repeater will listen on both IPv4 and IPv6.
```
```{cfgcmd} set service mdns repeater allow-service \<service\>
mDNS repeater can be configured to re-broadcast only specific services. By
default, all services are re-broadcasted.
```
```{cfgcmd} set service mdns repeater browse-domain \<domain\>
Allow listing additional custom domains to be browsed (in addition to the
default ``local``) so that they can be reflected.
```
```{cfgcmd} set service mdns repeater cache-entries \<entries\>
Specify how many resource records are cached per interface. Bigger values
allow mDNS work correctly in large LANs but also increase memory consumption.
Defaults to: 4096
```
## Firewall recommendations
Unlike typical routed traffic, mDNS packets relayed between interfaces do not
traverse the FORWARD hook chain in the firewall. Instead, they are processed
through the following hooks:
> - **INPUT**: For packets received by the local system
> - **OUTPUT**: For packets sent from the local system
To control or allow mDNS packet forwarding via the relay, you must define
appropriate rules in the INPUT and OUTPUT directions. Rules in the FORWARD
direction will have no effect on mDNS relay traffic.
```none
set firewall ipv4 input filter rule 10 action 'accept'
set firewall ipv4 input filter rule 10 destination address '224.0.0.251'
set firewall ipv4 input filter rule 10 destination port '5353'
set firewall ipv4 input filter rule 10 protocol 'udp'
set firewall ipv4 output filter rule 10 action 'accept'
set firewall ipv4 output filter rule 10 destination address '224.0.0.251'
set firewall ipv4 output filter rule 10 destination port '5353'
set firewall ipv4 output filter rule 10 protocol 'udp'
```
## Example
To listen on both `eth0` and `eth1` mDNS packets and also repeat packets
received on `eth0` to `eth1` (and vice-versa) use the following commands:
```none
set service mdns repeater interface 'eth0'
set service mdns repeater interface 'eth1'
```
To allow only specific services, for example `_airplay._tcp` or `_ipp._tcp`,
(instead of all services) to be re-broadcasted, use the following command:
```none
set service mdns repeater allow-service '_airplay._tcp'
set service mdns repeater allow-service '_ipp._tcp'
```
To allow listing additional custom domain, for example
`openthread.thread.home.arpa`, so that it can reflected in addition to the
default `local`, use the following command:
```none
set service mdns repeater browse-domain 'openthread.thread.home.arpa'
```
## Operation
```{opcmd} restart mdns repeater
Restart mDNS repeater service.
```
```{opcmd} show log mdns repeater
Show logs for mDNS repeater service.
```
```{opcmd} monitor log mdns repeater
Follow the logs for mDNS repeater service.
```
[multicast dns]: <https://en.wikipedia.org/wiki/Multicast_DNS>
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