故障现象：
Error relocating /usr/bin/tcpdump: pcap_open: symbol not found
Error relocating /usr/bin/tcpdump: pcap_findalldevs_ex: symbol not found
折腾了几个小时，google上都找不到结果。
准备上恩山求助的时候，自己突然意识到问题原因了。
Package: tcpdump
Version: 4.99.1-1
Depends: libc, libpcap1
Status: install user installed
Section: net
Architecture: x86_64
Size: 363238
Filename: tcpdump_4.99.1-1_x86_64.ipk
Description: Network monitoring and data acquisition tool
注意到：Depends: libc, libpcap1这一行。有两个依赖包。
opkg install libc
opkg install libpcap
处理成功。