Iptables packet flow diagram
Here is the diagram I keep stuck to my wall for reference when dealing with iptables.
I cannot remember what site I used as a reference when creating the diagram. The original was an ascii chart though. This is created in dia and exported to pdf too. Both attached for convenience. CC licensed as long as the original chart I copied allows it.
Packet flow examples:
All packets to or from localhost travel down the left side of the chart.
From external destination localhost: PREROUTING -> INPUT -> [Local Process] -> OUTPUT -> POSTROUTING.
From localhost destination external: OUTPUT -> POSTROUTING -> [External Host] -> PREROUTING -> INPUT.
All forwarded packets travel the right side of the chart and travel all three tables coming in and then again going out.
From external dest internal: PREROUTING -> FORWARD -> POSTROUTING.
Internal response to external: PREROUTING -> FORWARD -> POSTROUTING.
and so forth.
This is what caught me for a while. Forwarded packet travel the right side route and get out of (or in to) the network. The response then gets generated and does not start from FORWARD or POSTROUTING, but from PREROUTING again. So all NEW forwarded packets need to be marked in PREROUTING and the mark saved. Not new packets need that mark restored in PREROUTING, and all packets need that mark restored in POSTROUTING. (As demonstrated by my previous post).
Locally generated traffic only sees OUTPUT and POSTROUTING before hitting the network and needs to be marked before hitting POSTROUTING, hence the OUTPUT chain rules in my previous post.
I will accept changes to the chart too if anybody wants modifications made.
Multi gateway balancing with iptables
I have been testing load balancing via both iptables and ip route nexthop for a couple of days now. They both work pretty well too.
This only balances outgoing traffic as incoming traffic balanced via DNS RR and the firewall just returns the traffic on the interface it arrived on as per the previous post.
On the whole, I prefer the iptables solution. It seems to balance the traffic better. ip route balances outgoing connections based on nexthop of the route to that host is not already in it’s routing cache. While iptables balances traffic by alternate outgoing connections. The only downside I have seen is occasional connection drops to the BlackBerry servers.
After 24 hours of iptables balancing:
ppp0 Link encap:Point-to-Point Protocol RX bytes:1186783900 (1.1 GB) TX bytes:1290603327 (1.2 GB) ppp1 Link encap:Point-to-Point Protocol RX bytes:1109227490 (1.1 GB) TX bytes:1140565429 (1.1 GB)
This is using inclusion rules for determining balanced traffic. These are the rules that ended up on the production server:
# Load balancing rules (Split 50/50 between fwmark 1/2) iptables -t mangle -A balance1 -d 192.168.0.0/16 -j RETURN iptables -t mangle -A balance1 -d 10.0.0.0/8 -j RETURN iptables -t mangle -A balance1 -m connmark ! --mark 0 -j RETURN iptables -t mangle -A balance1 -m state --state ESTABLISHED,RELATED -j RETURN iptables -t mangle -A balance1 -m statistic --mode nth --every 2 --packet 0 -j CONNMARK --set-mark 1 iptables -t mangle -A balance1 -m statistic --mode nth --every 2 --packet 1 -j CONNMARK --set-mark 2 # Check to see if we have already marked a packet iptables -t mangle -A PREROUTING -m state --state ESTABLISHED,RELATED -j CONNMARK --restore-mark iptables -t mangle -A OUTPUT -m state --state ESTABLISHED,RELATED -j CONNMARK --restore-mark # Mark incoming connections to return on the interface they came in on iptables -t mangle -A PREROUTING -i ppp0 -m state --state NEW -j CONNMARK --set-mark 1 iptables -t mangle -A PREROUTING -i ppp1 -m state --state NEW -j CONNMARK --set-mark 2 # New outgoing packets iptables -t mangle -A PREROUTING -i eth0 -p tcp --dport 22 -m state --state NEW -j balance1 iptables -t mangle -A PREROUTING -i eth0 -p tcp --dport 25 -m state --state NEW -j balance1 iptables -t mangle -A PREROUTING -i eth0 -p tcp --dport 80 -m state --state NEW -j balance1 iptables -t mangle -A PREROUTING -i eth0 -p tcp --dport 443 -m state --state NEW -j balance1 iptables -t mangle -A OUTPUT -p tcp --dport 80 -m state --state NEW -j balance1 # Choose our route and save the mark iptables -t mangle -A PREROUTING -m connmark --mark 1 -j MARK --set-mark 1 iptables -t mangle -A PREROUTING -m connmark --mark 2 -j MARK --set-mark 2 iptables -t mangle -A PREROUTING -m state --state NEW -m connmark ! --mark 0 -j CONNMARK --save-mark
That’s all of the CONNMARK and MARK related rules I use.
The new outgoing packets section is where I choose what packets should be balanced and accounts for about 95% of our outgoing traffic.
The balance1 chain just has some checks at the beginning to catch further traffic that should not be balanced in case some rule gets messed up.
Of the new outgoing packets rules, the PREROUTE lines are for forwarded traffic and the OUTPUT rule is for traffic generated on that host by a transparent squid proxy.
Hope somebody finds that useful one day.
agentk
