I need help with "CONNMARK --set-mark"

Adrian Turcu adrian.turcu at gmvinteractive.com
Mon Apr 11 12:34:19 CEST 2005

Hello list,

I experience some intermittent problems with CONNMARK chain when try to 
conserve the connection mark. Sometimes it works just fine, but sometimes it 
seems that it stops working for whatever reason. Let me explain my setup 
(it's a test environment):

outside world | ---- | eth1 Linux_box eth0 | ---- | server

The traffic to the server has to be marked in order to shape it with tc 
(iproute2). Well, I envisage that the rules in iptables will be growing fast 
for the production environment, so I took advise from someone (Wang Jiang) on 
LARTC who suggested to use CONNMARK to skip processing the long rules-list 
for each packet enters/output my Linux box (which is really cool stuff). The 
discussion was there on a different subject, but the problems I have are 
related (I believe) with CONNMARK behaviour.

So, I have the rules as follows:

# match 'n' skip already marked traffic
/sbin/iptables -t mangle -A PREROUTING -m connmark --mark 0x8000/0x8000 \
	-j CONNMARK --restore-mark --mask 0xffff
/sbin/iptables -t mangle -A PREROUTING -m connmark --mark 0x8000/0x8000 \
# Assign different marks to different traffic:
# is my server IP address and
# the traffic refers to Ingress (Egress will be similar)

# create a class :3280x
/sbin/iptables -t mangle -N class_3280x
/sbin/iptables -t mangle -A PREROUTING -i eth1 \
	-d ! -j class_3280x
# mark the traffic matchin this class and return 
# class :32801	(section 1)
/sbin/iptables -t mangle -A class_3280x -j CONNMARK --set-mark 0x8021/0xFFFF
/sbin/iptables -t mangle -A class_3280x -j RETURN

# create a class: 6550x
/sbin/iptables -t mangle -N class_6550x
/sbin/iptables -t mangle -A PREROUTING -i eth1 \
	-d -j class_6550x
# mark the traffic matching this class and return
# class 65501 (section 2)
/sbin/iptables -t mangle -A class_6550x -j CONNMARK --set-mark 0xFFDD/0xFFFF
/sbin/iptables -t mangle -A class_6550x -j RETURN

# iprtoute2 tc stuff to regulate the traffic
# Ingress
/sbin/tc qdisc add dev eth1 handle ffff: ingress

# class :32801 (0x8021)
/sbin/tc filter add dev eth1 parent ffff: protocol ip prio 10 \
        u32 match mark 0x8021 0xffff \
        police rate 512kbit burst 512kbit mtu 1500 drop flowid :1

# class :65501  (0xFFDD)
/sbin/tc filter add dev eth1 parent ffff: protocol ip prio 10 \
        u32 match mark 0xFFDD 0xffff \
        police rate 256kbit burst 256kbit mtu 1500 drop flowid :1

Well, I generate some traffic on the network of eth1 to match the above rules. 
I can see first packets (SYN if the rules are applied before the connections 
establishes) matching the iptables rules for each class and then the flow is 
matched by the first 2 rules only (which is what I intended to do - skip the 
rules if traffic already marked). Now, I can also see the marked connections 

cat /proc/net/ip_conntrack

which is fine.

When this setup works the traffic matches the tc rules, no probs, and it is 
assigned the bandwidth I want. When it doesn't work... the traffic matches 
the iptables rules in the same way when it's working, I can still track the 
connections marks in /proc/net/ip_conntrack, but the tc rules are never 
applied (I can see the stats for tc intercepting the traffic but not matching 
the marks, it's like they are not there).

Some will say that this is related with iproute2 tools, but I have came across 
with this situation: instead of CONNTRACK marking, I do normal MARK and save 
the marks with CONNTRACK (just before RETURN), which means that in sections 1 
& 2 identified above (for iptables rules) I rewrite the rules like this:

/sbin/iptables -t mangle -A class_3280x -j MARK --set-mark 0x8021
/sbin/iptables -t mangle -A class_3280x -j CONNMARK --save-mark
/sbin/iptables -t mangle -A class_3280x -j RETURN


/sbin/iptables -t mangle -A class_6550x -j MARK --set-mark 0xFFDD
/sbin/iptables -t mangle -A class_3280x -j CONNMARK --save-mark
/sbin/iptables -t mangle -A class_6550x -j RETURN

This works always for me, I have try it dozens of time with no problem, whilst 
the --set-mark in CONNTRACK works sometime 1 out of 10 tries or never. It 
looks that setting the mark per connection is not that accurate and I need to 
remark every packet if unmarked? Am I doing something wrong?

I'm using custom built kernel and iptables 1.3.1. This rather strange 
behaviour occurs (for at least I could tell) when restarting the linux box 
and reaplying the rules or just deleting the rules and reaplying them again 
or inserting new matching rules.

Any help will be much appreciated,
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