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content/blog/multicastreceivescript.md

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+---
+title: "Multicast Receive Script"
+date: 2020-11-18T10:09:15-05:00
+draft: false
+tags: []
+---
+
+I use `socat` to debug mutlicast traffic, though the syntax for it is complicated to learn. Here is the command that I normally use to debug multicast traffic.
+
+```bash
+socat UDP4-RECVFROM:"$port",ip-add-membership="$multicast_address":0.0.0.0,fork -
+```
+
+This says to:
+
+- Listen to UDP traffic from `$port`.
+- Subscribe to `$multicast_address`.
+- `0.0.0.0` means to do it from the interface that matches the routing table for the multicast address.
+- The rest makes it print the traffic to standard out.
+
+To make life easier I created a little script called `mrecv` that takes a multicast address and port and forms the socat command for me.
+
+```bash
+#!/bin/bash
+
+show_usage() {
+    echo "Usage: mrecv [multicast_address] [port]"
+    exit 1
+}
+
+contains_help_flag() {
+    if [ "$1" = "-h" ] || [ "$1" = "--help" ]; then
+        return 0
+    fi
+    return 1
+}
+
+if [ "$#" -ne 2 ] ||
+   contains_help_flag "$1" ||
+   contains_help_flag "$2"; then
+    show_usage
+fi
+
+multicast_address="$1"
+port="$2"
+
+socat UDP4-RECVFROM:"$port",ip-add-membership="$multicast_address":0.0.0.0,fork -
+```
+