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I have an Openwrt 10.03.1 installed from: openwrt-ar71xx-wrt160nl-squashfs-factory.bin (firewall only customized with this: sed 's/REJECT/DROP/g' /etc/config/firewall) - so besides this, IT'S A DEFAULT OPENWRT INSTALL
I have 3 SSID's on it for 3 clients. It's a WRT160NL.
It connects to the internet via pppoe, so pppoe-wan is the WAN interface (in reality the "eth1" is the WAN interface, the cable is plugged into that port).
So I printed out the next things to paper:

iptables-save

root@OPENWRT:~# iptables-save
# Generated by iptables-save v1.4.6 on Wed Nov 21 16:59:23 2012
*nat
:PREROUTING ACCEPT [282:28098]
:POSTROUTING ACCEPT [12:748]
:OUTPUT ACCEPT [170:12487]
:nat_reflection_in - [0:0]
:nat_reflection_out - [0:0]
:postrouting_rule - [0:0]
:prerouting_lan - [0:0]
:prerouting_rule - [0:0]
:prerouting_wan - [0:0]
:zone_lan_nat - [0:0]
:zone_lan_prerouting - [0:0]
:zone_wan_nat - [0:0]
:zone_wan_prerouting - [0:0]
-A PREROUTING -j prerouting_rule 
-A PREROUTING -i br-lan -j zone_lan_prerouting 
-A PREROUTING -i pppoe-wan -j zone_wan_prerouting 
-A POSTROUTING -j postrouting_rule 
-A POSTROUTING -o br-lan -j zone_lan_nat 
-A POSTROUTING -o pppoe-wan -j zone_wan_nat 
-A postrouting_rule -j nat_reflection_out 
-A prerouting_rule -j nat_reflection_in 
-A zone_lan_prerouting -j prerouting_lan 
-A zone_wan_nat -j MASQUERADE 
-A zone_wan_prerouting -j prerouting_wan 
COMMIT
# Completed on Wed Nov 21 16:59:23 2012
# Generated by iptables-save v1.4.6 on Wed Nov 21 16:59:23 2012
*raw
:PREROUTING ACCEPT [88762:5585776]
:OUTPUT ACCEPT [32677:185865297]
:zone_lan_notrack - [0:0]
:zone_wan_notrack - [0:0]
-A PREROUTING -i br-lan -j zone_lan_notrack 
-A PREROUTING -i pppoe-wan -j zone_wan_notrack 
COMMIT
# Completed on Wed Nov 21 16:59:23 2012
# Generated by iptables-save v1.4.6 on Wed Nov 21 16:59:23 2012
*mangle
:PREROUTING ACCEPT [88762:5585776]
:INPUT ACCEPT [86059:4425898]
:FORWARD ACCEPT [2664:1156339]
:OUTPUT ACCEPT [32677:185865297]
:POSTROUTING ACCEPT [35338:187021465]
:zone_wan_MSSFIX - [0:0]
-A FORWARD -j zone_wan_MSSFIX 
-A zone_wan_MSSFIX -o pppoe-wan -p tcp -m tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu 
COMMIT
# Completed on Wed Nov 21 16:59:23 2012
# Generated by iptables-save v1.4.6 on Wed Nov 21 16:59:23 2012
*filter
:INPUT DROP [0:0]
:FORWARD DROP [0:0]
:OUTPUT ACCEPT [0:0]
:forward - [0:0]
:forwarding_lan - [0:0]
:forwarding_rule - [0:0]
:forwarding_wan - [0:0]
:input - [0:0]
:input_lan - [0:0]
:input_rule - [0:0]
:input_wan - [0:0]
:nat_reflection_fwd - [0:0]
:output - [0:0]
:output_rule - [0:0]
:reject - [0:0]
:syn_flood - [0:0]
:zone_lan - [0:0]
:zone_lan_ACCEPT - [0:0]
:zone_lan_DROP - [0:0]
:zone_lan_REJECT - [0:0]
:zone_lan_forward - [0:0]
:zone_wan - [0:0]
:zone_wan_ACCEPT - [0:0]
:zone_wan_DROP - [0:0]
:zone_wan_REJECT - [0:0]
:zone_wan_forward - [0:0]
-A INPUT -m state --state INVALID -j DROP 
-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT 
-A INPUT -i lo -j ACCEPT 
-A INPUT -p tcp -m tcp --tcp-flags FIN,SYN,RST,ACK SYN -j syn_flood 
-A INPUT -j input_rule 
-A INPUT -j input 
-A FORWARD -m state --state INVALID -j DROP 
-A FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT 
-A FORWARD -j forwarding_rule 
-A FORWARD -j forward 
-A OUTPUT -m state --state INVALID -j DROP 
-A OUTPUT -m state --state RELATED,ESTABLISHED -j ACCEPT 
-A OUTPUT -o lo -j ACCEPT 
-A OUTPUT -j output_rule 
-A OUTPUT -j output 
-A forward -i br-lan -j zone_lan_forward 
-A forward -i pppoe-wan -j zone_wan_forward 
-A forwarding_rule -j nat_reflection_fwd 
-A input -i br-lan -j zone_lan 
-A input -i pppoe-wan -j zone_wan 
-A output -j zone_lan_ACCEPT 
-A output -j zone_wan_ACCEPT 
-A reject -p tcp -j REJECT --reject-with tcp-reset 
-A reject -j REJECT --reject-with icmp-port-unreachable 
-A syn_flood -p tcp -m tcp --tcp-flags FIN,SYN,RST,ACK SYN -m limit --limit 25/sec --limit-burst 50 -j RETURN 
-A syn_flood -j DROP 
-A zone_lan -j input_lan 
-A zone_lan -j zone_lan_ACCEPT 
-A zone_lan_ACCEPT -o br-lan -j ACCEPT 
-A zone_lan_ACCEPT -i br-lan -j ACCEPT 
**-A zone_lan_DROP -o br-lan -j DROP**
**-A zone_lan_DROP -i br-lan -j DROP**
**-A zone_lan_REJECT -o br-lan -j reject**
**-A zone_lan_REJECT -i br-lan -j reject**
-A zone_lan_forward -j zone_wan_ACCEPT 
-A zone_lan_forward -j forwarding_lan 
-A zone_lan_forward -j zone_lan_DROP 
-A zone_wan -p udp -m udp --dport 68 -j ACCEPT 
-A zone_wan -p icmp -m icmp --icmp-type 8 -j DROP 
-A zone_wan -j input_wan 
-A zone_wan -j zone_wan_DROP 
-A zone_wan_ACCEPT -o pppoe-wan -j ACCEPT 
-A zone_wan_ACCEPT -i pppoe-wan -j ACCEPT 
-A zone_wan_DROP -o pppoe-wan -j DROP 
-A zone_wan_DROP -i pppoe-wan -j DROP 
-A zone_wan_REJECT -o pppoe-wan -j reject 
-A zone_wan_REJECT -i pppoe-wan -j reject 
-A zone_wan_forward -j forwarding_wan 
-A zone_wan_forward -j zone_wan_DROP 
COMMIT
# Completed on Wed Nov 21 16:59:23 2012
root@OPENWRT:~# 

ifconfig

root@OPENWRT:~# ifconfig
br-lan    Link encap:Ethernet  HWaddr AA:AA:AA:AA:AA:AA
      inet addr:192.168.1.1  Bcast:192.168.1.255  Mask:255.255.255.0
      UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
      RX packets:87174 errors:0 dropped:0 overruns:0 frame:0
      TX packets:137186 errors:0 dropped:0 overruns:0 carrier:0
      collisions:0 txqueuelen:0 
      RX bytes:4532245 (4.3 MiB)  TX bytes:192952659 (184.0 MiB)

eth0      Link encap:Ethernet  HWaddr AA:AA:AA:AA:AA:AA
      UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
      RX packets:0 errors:0 dropped:0 overruns:0 frame:0
      TX packets:16 errors:0 dropped:0 overruns:0 carrier:0
      collisions:0 txqueuelen:1000 
      RX bytes:0 (0.0 B)  TX bytes:2578 (2.5 KiB)
      Interrupt:4 

eth1      Link encap:Ethernet  HWaddr BB:BB:BB:BB:BB:BB
      UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
      RX packets:3661 errors:0 dropped:0 overruns:0 frame:0
      TX packets:3447 errors:0 dropped:0 overruns:0 carrier:0
      collisions:0 txqueuelen:1000 
      RX bytes:1221049 (1.1 MiB)  TX bytes:224533 (219.2 KiB)
      Interrupt:5 

lo        Link encap:Local Loopback  
      inet addr:127.0.0.1  Mask:255.0.0.0
      UP LOOPBACK RUNNING  MTU:16436  Metric:1
      RX packets:24 errors:0 dropped:0 overruns:0 frame:0
      TX packets:24 errors:0 dropped:0 overruns:0 carrier:0
      collisions:0 txqueuelen:0 
      RX bytes:2088 (2.0 KiB)  TX bytes:2088 (2.0 KiB)

mon.wlan0 Link encap:UNSPEC  HWaddr CC-CC-CC-CC-CC-C1-00-00-00-00-00-00-00-00-00-00  
      UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
      RX packets:263 errors:0 dropped:0 overruns:0 frame:0
      TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
      collisions:0 txqueuelen:32 
      RX bytes:20929 (20.4 KiB)  TX bytes:0 (0.0 B)

pppoe-wan Link encap:Point-to-Point Protocol  
      inet addr:1.2.3.4  P-t-P:10.0.0.1  Mask:255.255.255.255
      UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1492  Metric:1
      RX packets:1646 errors:0 dropped:0 overruns:0 frame:0
      TX packets:1448 errors:0 dropped:0 overruns:0 carrier:0
      collisions:0 txqueuelen:3 
      RX bytes:1063783 (1.0 MiB)  TX bytes:132628 (129.5 KiB)

wlan0     Link encap:Ethernet  HWaddr CC:CC:CC:CC:CC:C1
      UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
      RX packets:0 errors:0 dropped:0 overruns:0 frame:0
      TX packets:16 errors:0 dropped:0 overruns:0 carrier:0
      collisions:0 txqueuelen:32 
      RX bytes:0 (0.0 B)  TX bytes:2850 (2.7 KiB)

wlan0-1   Link encap:Ethernet  HWaddr CC:CC:CC:CC:CC:C2
      UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
      RX packets:0 errors:0 dropped:0 overruns:0 frame:0
      TX packets:16 errors:0 dropped:0 overruns:0 carrier:0
      collisions:0 txqueuelen:32 
      RX bytes:0 (0.0 B)  TX bytes:2850 (2.7 KiB)

wlan0-2   Link encap:Ethernet  HWaddr CC:CC:CC:CC:CC:C3
      UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
      RX packets:0 errors:0 dropped:0 overruns:0 frame:0
      TX packets:16 errors:0 dropped:0 overruns:0 carrier:0
      collisions:0 txqueuelen:32 
      RX bytes:0 (0.0 B)  TX bytes:2850 (2.7 KiB)

brctl show

root@OPENWRT:~# brctl show
bridge name bridge id       STP enabled interfaces
br-lan      8000.129ce121c98e   no      eth0
                            wlan0
                            wlan0-1
                            wlan0-2
root@OPENWRT:~# 

...and take a closer look at it several times. I found a few things that I don't understand:

1)
These rules afaik completely USELESS, I'm sure about this, so there is no real question regarding this, at least a "fixme".
-A zone_lan_DROP -o br-lan -j DROP
-A zone_lan_DROP -i br-lan -j DROP
-A zone_lan_REJECT -o br-lan -j reject
-A zone_lan_REJECT -i br-lan -j reject

2)
The real question is... why are there soo many tables?
nat_reflection_in, nat_reflection_out, postrouting_rule, prerouting_lan, prerouting_rule, prerouting_wan, zone_lan_nat, zone_lan_prerouting, zone_wan_nat, zone_wan_prerouting, zone_lan, zone_lan_forward, zone_wan, zone_wan_forward, etc..
Couldn't the rules done without tables? OpenWrt routers are usually have small CPU, why use complex rules? Why not more simple? Without tables?

3)
...If no one can bring up with a good answer that why are these tables needed...... then afaik I will flush all the rules, etc., and do it by hand.. in a more simpler way..

share|improve this question
    
Good question, or rather, good point. While I don't think the rule complexity has a significant impact on the CPU (a bit of branching, not more), I am more concerned about the impact that kind of complexity has on the user's brain. At least on mine! :) OpenWrt is used in home routers, so the rules and rationale behind them should be understood by home users. Which is why I reframed your question, going a bit more into detail. –  Lumi Nov 29 '12 at 14:42
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1 Answer 1

up vote 4 down vote accepted

1) These rules afaik completely USELESS, I'm sure about this, so there is no real question regarding this, at least a "fixme".

No, these rules are useful. I'll tell you why if you tell me why you think they're useless.

Ok, I'm kidding, I'll tell you whether you want it or not. The purpose of these rules are to keep the design simple. Simplicity is not measured by the number of rules. There is method to these rules. Each table has a simple-to-understand purpose that is apparent in its name. It happens that in the default configuration some of the tables have a single rule. It would require substantially more complex code in OpenWRT to optimize away single-rule tables. It would also make it more difficult for a system administrator to tweak the rules without going through this hypothetical compiler.

2) The real question is... why are there soo many tables?

The tables correspond to features of the firewall setup of OpenWRT. You could have fewer rules, but then you'd lose features that are useful to some users.

Couldn't the rules done without tables? OpenWrt routers are usually have small CPU, why use complex rules? Why not more simple? Without tables?

You could undoubtedly make your own configuration with fewer tables (unless your firewall is extremely simple, you'll probably end up creating a few). OpenWRT is more flexible because it accommodates many users.

The number of rules is unrelated to the CPU speed or RAM size. The effect of the number of tables is pretty much uncorrelated with the time it takes to go through them — on the contrary, having more tables and fewer rules per table means that the path each packet goes through will be shorter (having a wider tree helps make it less deep). The impact on memory is negligible: a few hundred bytes per table vs a few megabytes of RAM.

share|improve this answer
2  
»There is method to these rules. Each table has a simple-to-understand purpose that is apparent in its name.« It is the multitude of the chains that makes this confusing to the newbie (like me). I can see this is system where you are supposed to file things in certain ways. Just how you would go about that is not apparent at all. In this I fully agree with the OP. If you happen to have any pointers to examples of how you're supposed to file in real-life use cases in this scheme, I would appreciate that. –  Lumi Nov 29 '12 at 0:48
    
zone_lan_notrack is one i've wondered about. There are several that don't make sense or have a clear purpose. Not to say there isn't one. –  Sneaky Wombat Apr 23 '13 at 17:20
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