We have plisten (protected listen) and plisten1 (protected listen1).
Both listen and listen1 are listeners for Plan9.

How To Protect

Both plisten and plisten1 check the IP of requester.
The steps are as follows:
(1) if it is burst access then reject
(2) if (it is not in accept_database) and (it is in reject_database) then reject
(3) start a subprocess for the connection
That is, step (1) and (2) are added in Plan9 official listen and listen1.

The definition of burst access:
trials more than maxconnect in a given time (10 seconds).
the maxconnect is given in command option.

For simplicity, directories /sys/log/accept and /sys/log/reject are used in place of accept_database and reject_database.

File names in these directories are the IPs to accept or reject.
Therefore you can register 202.250.160.40 to accept_database by

	touch /sys/log/accept/202.250.160.40

pop3 before connection

Pop3 is the only way for a remote (non Plan9) user to register himself to accept_database.
Many authentication failures will let him to be registered in reject_database.

The code below is tcp110:

#!/bin/rc

ifs='!
' r=`{cat $3/remote} l=`{cat $3/local} {ip_local=$l(1) ip=$r(1) p=$l(2)}

if(test -e /sys/log/reject/$ip){
	/usr/local/bin/386/logit -l pop3 Rejected $ip
	echo '-ERR Rejected'
	exit
}

if(test -e /sys/log/accept/$ip){
	/$cputype/bin/upas/pop3
	exit
}

w=`{tail -10 /sys/log/pop3 | grep 'Fail '$ip | wc}

if(test $w(1) -gt 5){
	touch /sys/log/reject/$ip
	/usr/local/bin/386/logit -l pop3 List $ip
	echo '-ERR Rejected'
	exit
}

/$cputype/bin/alarm 60 /$cputype/bin/upas/pop3

# /sys/log/pop3 is something like:
# old pop3 message:
# ar Apr  8 14:56:50 user arisawa logged in
# new pop3 message:
# ar Apr  8 14:56:50 user arisawa OK 202.250.160.166
a=`{tail -1 /sys/log/pop3}
if(~ $a(7) OK && ~ $a(8) $ip){
	touch /sys/log/accept/$ip
	exit
}

/usr/local/bin/386/logit -l pop3 Fail $ip

/rc/bin/service/tcp110

Other services are rejected unless the requesting IP is in accept_database.

#!/bin/rc

ifs='!
' r=`{cat $3/remote} l=`{cat $3/local} {ip=$r(1) p=$l(2)}

if(test -e /sys/log/accept/$ip){
	exec /bin/aux/sshserve -A 'tis password' `{cat $3/remote} >>[2]/sys/log/ssh
}

echo Rejected

/usr/local/bin/386/logit -l honeypot $p $ip

/rc/bin/service/tcp22

You need to change pop3.c so that the script tcp110 can work.

	if(newns(user, 0) < 0){
		senderr("newns failed: %r; server exiting");
		exits(nil);
	}
-	syslog(0, "pop3", "user %s logged in", user);
+	syslog(0, "pop3", "user %s OK %s", user, peeraddr);
	enableaddr();

/sys/src/cmd/upas/pop3/pop3.c