FreeBSD / OpenBSD: PF Firewall Filter Large Number Of Subnets and IP Address

Q. How do I filter larger number of subnets and IPs using OpenBSD's pf firewall under FreeBSD 7.x server? How do I log all dropped packets from such ips? How do I block upto 10000 IPs or subnet without any performance penalty?

A. You can easily filter large number of IPs or subnets using pf firewall. PF provides tables to hold large number of IPv4 and IPv6 address. Lookups against a table are very fast and consume less memory and processor time. Tables are created in pf.conf file. Tables can also be populated from text files containing a list of IP addresses and networks.

How do I configure tables to drop large number of IPs?

Open pf.conf file, enter:
# vi /etc/pf.conf
Add following code:
table <blockedips> persist file "/etc/pf.blocked.ip.conf"
ext_if="em1" # interface connected to internet
Add following code to drop and log all ips / subnet listed in /etc/pf.blocked.ip.conf, file
block drop in log (all) quick on $ext_if from <blockedips> to any
Save and close the file. Now create file /etc/pf.blocked.ip.conf file using vi text editor, enter:
vi /etc/pf.blocked.ip.conf
Sample output:

192.168.1.0/24
202.54.1.5
# 202.54.4.5

The file /etc/pf.blocked.ip.conf should contain a list of IP addresses and/or CIDR network blocks, one per line. Any line beginning with # is treated as a comment and ignored by pf. To load new rules, simply type:
# pfctl -nf /etc/pf.conf
# pfctl -f /etc/pf.conf

How do I view all IP address listed in tables?

Type the following command
# pfctl -t blockedips -T show
Sample output:

   58.65.232.0/21
   58.83.12.0/22
   64.28.176.0/20
   64.255.128.0/19
   66.231.64.0/20
   67.213.128.0/20
   69.8.176.0/20

How do I add subnet called 91.196.232.0/22 on the fly?

Use pfctl command itself, to add CIDR or IP on fly, enter:
# pfctl -t blockedips -T add 202.54.11.11
# pfctl -t blockedips -T add 91.196.232.0/22

How do I delete subnet called 91.196.232.0/22 on the fly?

Type the command as follows:
# pfctl -t blockedips -T delete 91.196.232.0/22
Please note that all changes made using pfct are dynamic. You need to update your file on disk to save the changes.

How do I see statistics for each IP / CIDR?

The -v option can display statistics for each table entry (IP/CIDR), enter:
# pfctl -t blockedips -T show -v
Sample output:

   216.243.240.0/20
        Cleared:     Thu Jul 10 03:01:01 2008
        In/Block:    [ Packets: 0                  Bytes: 0                  ]
        In/Pass:     [ Packets: 0                  Bytes: 0                  ]
        Out/Block:   [ Packets: 0                  Bytes: 0                  ]
        Out/Pass:    [ Packets: 0                  Bytes: 0                  ]
   216.255.176.0/20
        Cleared:     Thu Jul 10 03:01:01 2008
        In/Block:    [ Packets: 0                  Bytes: 0                  ]
        In/Pass:     [ Packets: 0                  Bytes: 0                  ]
        Out/Block:   [ Packets: 0                  Bytes: 0                  ]
        Out/Pass:    [ Packets: 0                  Bytes: 0                  ]

How do I view log of dropped IP from default /var/log/pflog file?

Use tcpdump command to read a log file:
# tcpdump -n -e -ttt -r /var/log/pflog
# tcpdump -n -e -ttt -r /var/log/pflog port 80
# tcpdump -n -e -ttt -r /var/log/pflog and host 202.33.1.2
You can also view log in real time, enter:
# tcpdump -n -e -ttt -i pflog0
# tcpdump -n -e -ttt -i pflog0 port 80
# tcpdump -n -e -ttt -i pflog0 host 202.33.1.2