mongod(3):
......
--unixSocketPrefix <path>
Specifies a path for the UNIX socket. Unless this option has a
value, mongod and mongos, create a socket with the /tmp as a prefix.
......
You may do like this:
# install -d -m 0775 -o root -g mongodb /run/mongodb
# echo 'OPTIONS="--unixSocketPrefix=/run/mongodb"' > /etc/sysconfig/mongodb
# service mongodb restart
# ls /run/mongodb/*
/run/mongodb/mongodb-27017.sock