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NetFilter: How to set tcp flags?

+1 vote
172 views

How do i set tcp flags ? I tried

 nft add rule filter output tcp flags {syn,rst} counter

but failed ... also can some point me to valid syntaxes ..

posted Sep 25, 2013 by Mandeep Sehgal

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Is there a way to find out if there any iptables rules set on a machine ?

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I've done some digging but really can't find a good explanation of the syntax for matching TCP flags in nftables. In iptables a rule can be written like:

-A TEST_BADFLAGS -p tcp --tcp-flags SYN,FIN SYN,FIN -j LOG_BADFLAGS

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Is it possible to bind multiple address families in netfilter queue? I see IPv4 show up in my queue, but not ARP. With error code removed, here is how I'm calling nfq_bind:

netfilterqueue_handle = nfq_open();
netfilterqueue_queue = nfq_create_queue( netfilterqueue_handle, 0,

nfq_bind_pf( netfilterqueue_handle, AF_INET );
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I'm thinking the more likely possibility is the iptable rules I'm using to send traffic to the queue are too restrictive. Here are the rules I have:

# Generated by iptables-save v1.4.21 on Sat Feb 14 10:40:46 2015
*nat
:PREROUTING ACCEPT [161:14105]
:INPUT ACCEPT [56:4995]
:OUTPUT ACCEPT [56:4496]
:POSTROUTING ACCEPT [56:4496]
-A POSTROUTING -s 10.0.1.0/24 -o eth0 -j MASQUERADE
COMMIT
# Completed on Sat Feb 14 10:40:46 2015
# Generated by iptables-save v1.4.21 on Sat Feb 14 10:40:46 2015
*filter
:INPUT ACCEPT [1017:217421]
:FORWARD DROP [53:2307]
:OUTPUT ACCEPT [934:211104]
:MYRA - [0:0]
-A FORWARD -m conntrack --ctstate RELATED,ESTABLISHED -j MYRA
-A FORWARD -s 10.0.1.0/24 -o eth0 -m conntrack --ctstate NEW -j MYRA
-A MYRA -j NFQUEUE --queue-num 0 --queue-bypass
COMMIT
# Completed on Sat Feb 14 10:40:46 2015

Do I have to add another FORWARD line to get ARP to jump to MYRA? What would it look like?

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