Hello there,
For the past couple of days i've been looking into setting up an old pc as a
firewall/router for a couple of students.
To do so i enabled iptables and started looking into configuration issues.
Eventually i came up with a config that worked. haha. Then i realised this
config was fishy and started deleting lines as i went along. Now almost no
lines are left and the darned thing still works even after reboots,
re-loads, restarts.
Apparently there's something i'm not getting thru my thick skull about
packet filtering. Could someone explain to me in text (no diagrams) how a
packet is evaluated and then processed tru the chains, also what is done and
not-done any more after a packet has passed thru a chain. Somehow i have the
idea this config works from the lan to the outside but not from the outside
to the lan or something.
Any good resources, tips, explanations are welcome. I'm to dumb for this i
guess.
Regards,
Joris
The 'final edition ?' is the version i stripped as much as possible.
The 'first edition' is the non-stripped version.
-------------------------------------------
--------- Final edition ?-------------------
-------------------------------------------
kotwall:~# iptables -L -v
Chain INPUT (policy DROP 0 packets, 0 bytes)
pkts bytes target prot opt in out source
destination
0 0 ACCEPT tcp -- eth1 any kotwall anywhere
tcp dpt:webcache
0 0 ACCEPT tcp -- eth1 any kotwall anywhere
tcp dpt:www
0 0 ACCEPT tcp -- eth1 any kotwall anywhere
tcp dpt:https
23 1865 ACCEPT tcp -- eth0 any 192.168.1.0/24 kotwebb
tcp dpt:www
0 0 ACCEPT tcp -- eth1 any 195.130.132.0/24
192.168.1.0/24 tcp spt:pop3
0 0 ACCEPT tcp -- eth1 any 195.130.132.0/24
192.168.1.0/24 tcp spt:smtp
139 13284 ACCEPT all -- any any anywhere anywhere
state NEW,RELATED,ESTABLISHED
0 0 ACCEPT tcp -- eth0 any anywhere anywhere
tcp dpt:10000
0 0 ACCEPT tcp -- eth0 any anywhere anywhere
tcp dpt:20000
Chain FORWARD (policy DROP 0 packets, 0 bytes)
pkts bytes target prot opt in out source
destination
74 5199 ACCEPT all -- eth0 eth1 192.168.1.0/24 anywhere
83 24686 ACCEPT all -- eth1 eth0 anywhere
192.168.1.0/24
Chain OUTPUT (policy DROP 142 packets, 10072 bytes)
pkts bytes target prot opt in out source
destination
0 0 ACCEPT tcp -- any eth1 192.168.1.0/24
195.130.132.0/24 tcp dpt:pop3
0 0 ACCEPT tcp -- any eth1 192.168.1.0/24
195.130.132.0/24 tcp dpt:smtp
195 46559 ACCEPT all -- any any anywhere anywhere
state RELATED,ESTABLISHED
kotwall:~#
-------------------------------------------
----------- works as well -----------------
----------- First edition -----------------
-------------------------------------------
kotwall:~# iptables -L -v --line-numbers
Chain INPUT (policy DROP 0 packets, 0 bytes)
num pkts bytes target prot opt in out source destination
1 0 0 ACCEPT tcp -- eth1 any kotwall anywhere tcp dpt:webcache
2 0 0 ACCEPT tcp -- eth1 any kotwall anywhere tcp dpt:www
3 0 0 ACCEPT tcp -- eth1 any kotwall anywhere tcp dpt:https
4 42 6054 ACCEPT tcp -- eth0 any 192.168.1.0/24 kotwebb tcp dpt:www
5 0 0 ACCEPT tcp -- eth1 any 195.130.132.0/24 192.168.1.0/24 tcp spt:pop3
6 0 0 ACCEPT tcp -- eth1 any 195.130.132.0/24 192.168.1.0/24 tcp spt:smtp
7 555 55369 ACCEPT all -- any any anywhere anywhere state
NEW,RELATED,ESTABLISHED
8 0 0 ACCEPT tcp -- eth0 any anywhere anywhere tcp dpt:10000
9 0 0 ACCEPT tcp -- eth0 any anywhere anywhere tcp dpt:20000
Chain FORWARD (policy DROP 0 packets, 0 bytes)
num pkts bytes target prot opt in out source destination
1 327 28253 ACCEPT all -- eth0 eth1 192.168.1.0/24 anywhere
2 0 0 DROP all -- eth0 eth0 192.168.1.0/24 192.168.1.0/24
3 0 0 DROP all -- eth0 eth1 192.168.1.0/24 192.168.1.0/24
4 343 167K ACCEPT all -- eth1 eth0 anywhere 192.168.1.0/24
Chain OUTPUT (policy ACCEPT 1 packets, 76 bytes)
num pkts bytes target prot opt in out source destination
1 0 0 ACCEPT all -- any any kotwall kotwall
2 0 0 ACCEPT tcp -- any eth1 kotwall anywhere tcp dpt:webcache
3 0 0 ACCEPT tcp -- any eth1 kotwall anywhere tcp dpt:www
4 0 0 ACCEPT tcp -- any eth1 kotwall anywhere tcp dpt:https
5 0 0 ACCEPT tcp -- any eth1 192.168.1.0/24 195.130.132.0/24 tcp dpt:pop3
6 0 0 ACCEPT tcp -- any eth1 192.168.1.0/24 195.130.132.0/24 tcp dpt:smtp
7 0 0 ACCEPT tcp -- any eth0 anywhere anywhere tcp spt:10000
8 0 0 ACCEPT tcp -- any eth0 anywhere anywhere tcp spt:20000
9 671 92915 ACCEPT all -- any any anywhere anywhere state
RELATED,ESTABLISHED
kotwall:~#
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