树莓派用tcpdump与socat抓包重传时遇Permission denied错误求助

你在树莓派上通过tcpdump捕获多播包、再用socat转发为广播包时，遇到了如下权限错误：

tcpdump: listening on eth0, link-type EN10MB (Ethernet), snapshot length 262144 bytes
2023/09/26 20:17:17 socat[2400] E sendto(5, 0x3fb000, 24, 0, AF=2 192.168.7.255:14555, 16): Permission denied

使用的脚本如下：

#!/bin/bash
# Define the multicast group and port to capture
multicast_group="224.1.1.1"  # Replace with your specific multicast group address
multicast_port=12345  # Replace with the multicast port number
# Define the broadcast address and port to retransmit
broadcast_address="192.168.xxx.xxx"  # Broadcast address for the target network
forward_port=54321  # Replace with the port number you want to use for forwarding
while true; do
# Start tcpdump to capture UDP multicast packets
sudo tcpdump -i eth0 -n -s0 -U -w - "udp and port $multicast_port" | \
sudo socat -u - UDP4-DATAGRAM:$broadcast_address:$forward_port
done

问题原因 #

这个错误并非因为缺少sudo权限，而是socat默认禁止发送UDP广播包，即使以root身份运行也需要显式开启广播允许选项。

解决步骤 #

1. 修改socat的参数，在UDP4-DATAGRAM目标后添加,broadcast选项，允许发送广播包：

   sudo tcpdump -i eth0 -n -s0 -U -w - "udp and port $multicast_port" | \
   sudo socat -u - UDP4-DATAGRAM:$broadcast_address:$forward_port,broadcast
   

2. 确保broadcast_address是当前子网的有效广播地址（比如子网掩码为255.255.255.0时，广播地址通常是192.168.x.255）。
3. 重新运行脚本（可直接用sudo ./your_script.sh执行，或确保脚本有可执行权限后再用sudo运行）。

内容的提问来源于stack exchange，提问作者dgholmes