UDP Broadcast Relay for Linux / FreeBSD / pfSense / OPNsense

( For Opensense a plugin is already available )

This program listens for packets on a specified UDP broadcast port. When a packet is received, it sends that packet to all specified interfaces but the one it came from as though it originated from the original sender.

The primary purpose of this is to allow devices or game servers on separated local networks (Ethernet, WLAN, VLAN) that use udp broadcasts to find each other to do so.

INSTALL

make
cp udpbroadcastrelay /some/where

USAGE

./udpbroadcastrelay \
    -id id \
    --port <udp-port> \
    --dev eth0 --dev eth1
    [--dev ethx...] \
    [--blockid id...] \
    [--blockcidr network-prefix/size] \
    [--allowcidr network-prefix/size] \
    [--msearch action[,search-term]] \   
    [--multicast 224.0.0.251] \
    [-s <spoof_source_ip>]
    [-t|--ttl-id] [-d] [-f]
    [-h|--help]

• udpbroadcastrelay must be run as root to be able to create a raw socket (necessary) to send packets as though they originated from the original sender.

• id must be unique number between instances with range 1 - 63. This is used to set the DSCP of outgoing packets to determine if a packet is an echo and should be discarded.

• blockid can be used to drop packets received from other instances of udpbroadcastrelay using the specified ID value.

• --blockcidr can be used to block packets from a range of IP source addresses, given in CIDR notation. This option can be specified multiple times to block more than one range. Where multiple overlapping CIDRs are specified with the --blockcidr and --allowcidr options the most specific match (longest prefix) will take effect.

• --allowcidr can be used to only allow packets from a range of IP source addresses, given in CIDR notation. This option can be specified multiple times to allow more than one range. Once this option is specified the default behaviour for packets which does not any CIDRs changes from Allow to Block.

• udp-port Destination udp port to listen to. Range 1 - 65535. Example values for common services are given below.

• -dev <ethx> specifies the name of an interface to receive and transmit packets on. This option needs to be specified at least twice for 2 separate interfaces otherwise this tool won't actually do anything!

• The tool can listen for and relay packets using multicast groups with --multicast <group address>.

• The source address for all packets can be modified with -s <ip>. This is unusual.

• A special source ip of -s 1.1.1.1 can be used to set the source ip to the address of the outgoing interface and the source UDP port to the same as the destination port. -s 1.1.1.2 does the same but leaves the UDP ports unchanged. These values are notably required to cater for the Chromecast system.

• Special SSDP processing can be turned on using the --msearch option. By default SSDP M-SEARCH packets are treated the same as any other packet. The action parameter changes this default:

  • block: drop the M-SEARCH packet.
  • fwd: forward the M-SEARCH packet like a regular packet (and the -s into account).
  • proxy: create a local proxy for the M-SEARCH request, send request out with subnet local address and proxy port. Received responses are sent back to original requester with no processing.
  • dial: perform full DIAL protocol processing on M-SEARCH request. Create proxies for M-SEARCH, Locator and REST services. Use this for Youtube app on Smart TVs

  When a search-term is also specified the given action will only apply to M-SEARCH packets containing this specific search term. --msearch can be specified multiple times to add more search terms. The value of -s affects normal and M-SEARCH packets with the forward action.

  The old -s 1.1.1.3 option should be replaced with --msearch dial.

• The original version of this tool marked the TTL of outgoing relayed packets to detect echos and preserved DSCP. This original behavior can be restored by setting the [-t|--ttl-id] parameter.

• -d will enable debugging output. Specify -d twice for extra debugging info

• -f will fork the application to the background and create a pid file at /var/run/udpbroadcastrelay_ID.pid

• -h|--help Display a detailed help dialog.

EXAMPLE

mDNS / Multicast DNS (Chromecast Discovery + Bonjour + More)

./udpbroadcastrelay --id 1 --port 5353 --dev eth0 --dev eth1 --multicast 224.0.0.251 -s 1.1.1.1

(Chromecast requires broadcasts to originate from an address on its subnet)

mDNS example which allows messages from hosts on 192.168.1.0/24 and 192.168.20.0/24 subnets but blocks host 192.168.20.20

./udpbroadcastrelay --id 1 --port 5353 --dev eth0 --dev eth1 --multicast 224.0.0.251 -s 1.1.1.1 --allowcidr 192.168.1.0/24 --allowcidr 192.168.20.0/24 --blockcidr 192.168.20.20/32

This will prevent relaying broadcast/multicast packets from host 192.168.20.20. It will not stop any unicast traffic from this host.

SSDP (Roku Discovery, DLNA Media, Sonos, UPnP + More)

./udpbroadcastrelay --id 1 --port 1900 --dev eth0 --dev eth1 --multicast 239.255.255.250

Youtube Application on Smart TV

./udpbroadcastrelay --id 1 --dev eth0 --dev eth1 --port 1900 --multicast 239.255.255.250 -s 1.1.1.2 --msearch dial

Youtube Application on Smart TV along with DLNA media playback

./udpbroadcastrelay --id 1 --dev eth0 --dev eth1 --port 1900 --multicast 239.255.255.250 -s 1.1.1.2 --msearch proxy,urn:schemas-upnp-org:device:MediaServer:1 --msearch dial

Lifx Bulb Discovery

./udpbroadcastrelay --id 1 --port 56700 --dev eth0 --dev eth1

Broadlink IR Emitter Discovery

./udpbroadcastrelay --id 1 --port 80 --dev eth0 --dev eth1

Warcraft 3 Server Discovery

./udpbroadcastrelay --id 1 --port 6112 --dev eth0 --dev eth1

Windows Network Neighborhood Discovery

NetBIOS Name Service (137), SMB Browser (138) and SSDP (1900). Windows Network Discovery across networks relies on relaying these three protocols all at once. To requires that three separate instances of udpbroadcastrelay run simultaneously so in this example we execute the command with the "-f" parameter in order to run the tool in the background. ./udpbroadcastrelay --id 1 --port 137 --dev eth0 --dev eth1 -f ./udpbroadcastrelay --id 2 --port 138 --dev eth0 --dev eth1 -f ./udpbroadcastrelay --id 3 --port 1900 --dev eth0 --dev eth1 --multicast 239.255.255.250 -f

Syncthing Discovery

./udpbroadcastrelay --id 1 --port 21027 --dev eth0 --dev eth1

Raknet Discovery (Minecraft)

./udpbroadcastrelay --id 1 --port 19132 --dev eth0 --dev eth1

Note about firewall rules

If you are running udpbroadcastrelay on a router, it can be an easy way to relay broadcasts between VLANs. However, beware that these broadcasts will not establish a RELATED firewall relationship between the source and destination addresses.

This means if you have strict firewall rules, the recipient may not be able to respond to the broadcaster. For instance, the SSDP protocol involves sending a broadcast packet to port 1900 to discover devices on the network. The devices then respond to the broadcast with a unicast packet back to the original sender. You will need to make sure that your firewall rules allow these response packets to make it back to the original sender.

Recent changes

• Added --blockcidr and --allowcidr options
• Print interface names instead of numbers in packet information messages
• Removed the -s 1.1.1.3 option and replaced it with a more general --msearch option, which allows finer control (see USAGE section)
• Updated the expiry time for M-SEARCH, Locator and REST proxies.
• Fixed build information not being printed when -d is specififed
• Introduced a second level of debug info, activated by specifying -d twice

Please note: Maintenance of this project is on a 'when I have a moment' basis, and that may be several months.