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Recently, I decided to DROP packets that want to go out through the port 80. It seams my configuration has a problem, because some unwanted packets are dropped.

Excerpt of my configuration:

iptables -A OUTPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A OUTPUT -p tcp -s [PUBLIC IP OF MY SERVER] --sport 80 --dport 1024:65535 -m state --state ESTABLISHED -j ACCEPT

Question 1: isn't the second rule useless, since I already said in the first one that I accept all packets with "ESTABLISHED" state?

Question 2: Why aren't these two rules enough to accept the following dropped packets:

Jul 14 18:47:18 [HOSTNAME] kernel: iptables output: IN= OUT=eth0 SRC=[PUBLIC IP OF MY SERVER] DST=[A WWW CLIENT PUBLIC IP] LEN=40 TOS=0x00 PREC=0x00 TTL=64 ID=8408 DF PROTO=TCP SPT=80 DPT=50085 WINDOW=123 RES=0x00 ACK FIN URGP=0 
Jul 14 18:47:53 [HOSTNAME] kernel: iptables output: IN= OUT=eth0 SRC=[PUBLIC IP OF MY SERVER] DST=[A WWW CLIENT PUBLIC IP] LEN=40 TOS=0x00 PREC=0x00 TTL=64 ID=8409 DF PROTO=TCP SPT=80 DPT=50085 WINDOW=123 RES=0x00 ACK FIN URGP=0 
Jul 14 18:48:08 [HOSTNAME] kernel: iptables output: IN= OUT=eth0 SRC=[PUBLIC IP OF MY SERVER] DST=[A WWW CLIENT PUBLIC IP] LEN=40 TOS=0x00 PREC=0x00 TTL=64 ID=54091 DF PROTO=TCP SPT=80 DPT=25780 WINDOW=16616 RES=0x00 ACK FIN URGP=0

N.B:

  • There is no rule above those in the chain that drops packets.
  • The default policy is DROP.

EDIT I looked at this post, and also enabled logging of INVALID packets by the kernel:

echo 255 >/proc/sys/net/netfilter/nf_conntrack_log_invalid

Now it seams I have several kinds of errors:

Jul 14 22:00:40 [HOSTNAME] kernel: nf_ct_tcp: invalid RST IN= OUT= SRC=[ONE_CLIENT_IP] DST=[SERVER_IP] LEN=40 TOS=0x00 PREC=0x00 TTL=49 ID=47149 PROTO=TCP SPT=993 DPT=51364 SEQ=1043042446 ACK=0 WINDOW=0 RES=0x00 RST URGP=0 
Jul 14 21:57:11 [HOSTNAME] kernel: nf_ct_tcp: invalid packet ignored in state ESTABLISHED IN= OUT= SRC=[SERVER_IP] DST=[ONE_CLIENT_IP] LEN=48 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=TCP SPT=80 DPT=3782 SEQ=474588492 ACK=2243291425 WINDOW=14600 RES=0x00 ACK SYN URGP=0 OPT (020405B401010402)
Jul 14 21:57:25 [HOSTNAME] kernel: nf_ct_tcp: invalid packet ignored in state LAST_ACK IN= OUT= SRC=[SERVER_IP] DST=[ONE_CLIENT_IP] LEN=48 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=TCP SPT=80 DPT=3782 SEQ=474588492 ACK=2243291425 WINDOW=14600 RES=0x00 ACK SYN URGP=0 OPT (020405B401010402) 
Jul 14 21:57:41 [HOSTNAME] kernel: nf_ct_tcp: invalid packet ignored in state TIME_WAIT IN= OUT= SRC=[SERVER_IP] DST=[ONE_CLIENT_IP] LEN=48 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=TCP SPT=80 DPT=3782 SEQ=474588492 ACK=2243291425 WINDOW=14600 RES=0x00 ACK SYN URGP=0 OPT (020405B401010402)
Jul 14 21:58:52 [HOSTNAME] kernel: nf_ct_tcp: invalid packet ignored in state SYN_RECV IN= OUT= SRC=[SERVER_IP] DST=[ONE_CLIENT_IP] LEN=48 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=TCP SPT=80 DPT=50488 SEQ=3804975135 ACK=229029122 WINDOW=14600 RES=0x00 ACK SYN URGP=0 OPT (020405B401010402) 
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This post on UL seems similar to yours, and another on SO may help too. –  Paulo Almeida Jul 14 '13 at 17:25

3 Answers 3

(actually I made this answer elsewhere, I thought it was the same site)

according to these:

http://www.tcpipguide.com/free/t_TCPConnectionTermination-2.htm

http://www.iptables.info/en/connection-state.html (not up to date)

It might be possible the (maybe mobile) client closed first, didn't wait for the final FIN/ACK and never sent its very final ACK, or perhaps the server answered too late and the client itself is firewalled or any other slow response case... So the server retries, beyond a timer (sysctl net.netfilter.nf_conntrack_tcp_timeout_last_ack) but the netfilter has dropped the state before the real tcp stack drops it.

you should take traces and see if you have duplicates packets for example.

The 2nd rule is a subset of the first rule, so it is useless. Try increasing values of various tcp_timeout settings (sysctl -w net.netfilter.... or echo XX > /proc/sys/net/netfilter/...) and see if those logs disappear. I set this for similar reasons in the past and it "solved" some mysterious netfilter logs. This might increase conntrack memory usage.

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1) Second one seems to be useless if you have first one.

2) Now. Why would you have DROP by default on OUTPUT at all? You do not trust yourself? I would leave it ACCEPT by default. Just have your rules applied to INPUT chain with DROP by default. With proper rules in INPUT chain and DROP by default iptables will do great job on securing your server. Creating such policies for OUTPUT seems to be too much if you are not doing very special security set up.

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I'm doing very specific security setup. A website on my server has been hacked, and on some backdoors, they used outgoing 80 port to communicate with the hacker. Thank for your answer to my first question. I still need an answer for the 2) question. I do not want to change the OUTPUT policy to "accept". –  Fox Jul 14 '13 at 19:14
    
I see... Take a look at this post. There is very similar problem which is probably your case as well. Look at best answer in the above post. –  Nikolay Jul 14 '13 at 19:29
    
Thank you for your answer. I just edited my post with more informations provided by kernel INVALID packets logging. –  Fox Jul 14 '13 at 20:04
    
Interesting. Is it happening with all clients or just sometimes with some of them? I am trying to see if it is clients problem (improper config, timeouts, etc...) –  Nikolay Jul 14 '13 at 20:47

Before the three-way TCP handshake is complete, the connection won't be "established" yet. The handshake requires that the client send a SYN packet, you return a SYN+ACK packet, and the client then return an ACK packet. However, you are going to block the SYN+ACK outgoing packet.

A better thing to do would be to allow all packets in OUTPUT which match -p tcp --sport 80 --dport 1024:65535. You could also limit the lower bound further; IANA recommends ports 41952 and above be used for random source ports, but as you may notice many clients are not compliant. A rule like this will allow any traffic that is a response to a port 80 connection where the port on the client is not a privileged port (and will prevent malware from calling home on 80 or 443).

If you prefer to use connection states for your rules, you could also allow outgoing packets on source port 80 with states ESTABLISHED,RELATED,SYN_RECV (in your configuration, you can just use SYN_RECV because the first rule will take care of it). This will allow the outgoing SYN+ACK to set up the outgoing connection.

The invalid packets seem to relate to the fact that they are SYN+ACK packets, but the connection isn't waiting to be established. Likely this is an artifact unrelated to the problem and nothing to worry about.

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Thank you very much for your answer. Unfortunately, "SYN_RECV" does not seem to exist in iptables documentation. So to make this work with iptables, would the syntax iptables -A OUTPUT -p tcp --tcp-flags ALL SYN,ACK --sport 80 --dport 1024:65535 -j ACCEPT work? –  Fox Jul 15 '13 at 18:43
    
Hum... I tried this, but dropped packets continue. Actually, why should I allowed SYN,ACK packets to go out? The one which are blocked by my firewall are ACK,FIN, not SYN,ACK. –  Fox Jul 15 '13 at 18:49
    
This is interesting because it means that the closing of the connection is what is blocked. However, if this is the problem, you can also add FIN_WAIT and LAST_ACK. –  Falcon Momot Jul 15 '13 at 19:30

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