Hello,
I can't understand clearly how packets traverse mangle table under
iptables (kernel 2.4.18). I would like to match all ACK and SYN packets. There is local
servies and masquerading. So, I wrote such filters:
iptables -t mangle -A OUTPUT -o $PPP -p tcp -m tcp --tcp-flags SYN SYN -j MARK --set-mark=200
iptables -t mangle -A INPUT -p tcp -m tcp --tcp-flags SYN SYN -j MARK --set-mark=200
iptables -t mangle -A PREROUTING -p tcp -m tcp --tcp-flags SYN SYN -j MARK --set-mark=200
iptables -t mangle -A OUTPUT -o $PPP -p tcp -m tcp --tcp-flags ACK ACK -j MARK --set-mark=200
iptables -t mangle -A INPUT -p tcp -m tcp --tcp-flags ACK ACK -j MARK --set-mark=200
iptables -t mangle -A PREROUTING -p tcp -m tcp --tcp-flags ACK ACK -j MARK --set-mark=200
iptablles -L -v -t mangle shows me:
########################
Chain PREROUTING (policy ACCEPT 8643 packets, 1826K bytes)
pkts bytes target prot opt in out source destination
5 300 MARK tcp -- any any anywhere anywhere tcp flags:SYN/SYN MARK set 0xc8
1263 215K MARK tcp -- any any anywhere anywhere tcp flags:ACK/ACK MARK set 0xc8
15 2610 MARK udp -- eth0 any anywhere anywhere udp spt:domain MARK set 0xc8
0 0 MARK udp -- any any anywhere anywhere udp dpt:domain MARK set 0xc8
1985 283K IMQ all -- eth0 any anywhere anywhere IMQ: todev 0
Chain INPUT (policy ACCEPT 8643 packets, 1826K bytes)
pkts bytes target prot opt in out source destination
5 300 MARK tcp -- any any anywhere anywhere tcp flags:SYN/SYN MARK set 0xc8
1263 215K MARK tcp -- any any anywhere anywhere tcp flags:ACK/ACK MARK set 0xc8
0 0 MARK tcp -- eth0 any proxy.takas.lt anywhere tcp spt:webcache MARK set 0x12c
0 0 MARK tcp -- eth0 any anywhere anywhere tcp spt:www MARK set 0x12c
15 2610 MARK udp -- eth0 any anywhere anywhere udp spt:domain MARK set 0xc8
Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
Chain OUTPUT (policy ACCEPT 4513 packets, 373K bytes)
pkts bytes target prot opt in out source destination
5 300 MARK tcp -- any eth0 anywhere anywhere tcp flags:SYN/SYN MARK set 0xc8
1843 155K MARK tcp -- any eth0 anywhere anywhere tcp flags:ACK/ACK MARK set 0xc8
0 0 MARK tcp -- any eth0 anywhere proxy.takas.lt tcp dpt:webcache MARK set 0x12c
0 0 MARK tcp -- any eth0 anywhere proxy.takas.lt tcp dpt:www MARK set 0x12c
15 1050 MARK udp -- any any anywhere anywhere udp dpt:domain MARK set 0xc8
Chain POSTROUTING (policy ACCEPT 4600 packets, 383K bytes)
pkts bytes target prot opt in out source destination
#####################
I noticed that SYN packet count is the same on all of the chains. ACK is the
same on the PREROUTING and and INPUT. So, what is the scheme?
By the way, what happens when packets enters IMQ target? Logically, it
is queued in the imq0 device and traverses all filters as new packet,
isn't it?
Bye,
Tadas
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