I was able to successfully install Snort on RT-AC87U Merlin build 378.55
I have documented the steps below for Entware.
http://www.snbforums.com/threads/step-by-step-snort-on-asusmerlin.27374/
Step by Step procedure for installing and configuring SNORT AsusMerlin
login into WebUI at 192.168.xxx.xxx
Adminstration -> System ->
Format JFFS partition at next boot = No.
Enable JFFS custom scripts and configs = Yes
Enable SSH = Yes
venkat@venkat:~$telnet 192.168.2.1
Trying...
Snort Logging
To Run Sort use the following command.
snort -A fast -d -D -c /opt/snort/conf/snort.conf -l /opt/snort/log
where /opt/snort/log is the directory created by me.
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Step by Step procedure for installing and configuring SNORT on TomatoUSB.