ifconfig alias

# cat /etc/hostname.dc0
inet 192.168.0.2 255.255.255.0 NONE media 100baseTX
inet alias 192.168.0.3 255.255.255.255
inet alias 192.168.0.4 255.255.255.255
Once you've made this file, it just takes a reboot for it to take effect. You can, however, bring up the aliases by hand using the ifconfig(8) utility. To bring up the first alias you would use the command:

# ifconfig dc0 inet alias 192.168.0.3 netmask 255.255.255.255

REFERENCE:
http://www.openbsd.org/faq/faq6.html#Setup.aliases

pftop filter specific pattern

-f filter. for example:

pftop -f 'src host 172.30.26.34'

snmpd setup

It's pretty easy to enable snmpd on OpenBSD. Simply edit /etc/snmpd.conf to configure the interface which snmpd should listen on, and left other settings by default. Fireup snmpd and it will listen on udp port 161.

Then you can use snmp tools such as MRTG or Cacti to retrieve info via SNMP version 2.

get packages for current & previous version of OpenBSD

http://ftp.openbsd.org/pub/OpenBSD/ftplist

Only packages of current three versions available on the official FTP. Some of other mirror FTPs also have packages of very early versions.