Use aspas duplas:
iptables -A INPUT -i em1 -j LOG --log-prefix="iptables1: "
iptables -t nat -A PREROUTING -i em1 -j LOG --log-prefix="iptables2: "
Qual log-prefix
significa:
--log-prefix - When logging, put this text before the log message. Use double quotes around the text to use.