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chrisdb

Member

From: Belgium

Registered: 2018-05-16

Posts: 155

[SOLVED] Iptables simple statefull firewall

Hello,

I've been looking into iptables and I had some questions regarding the following:
https://wiki.archlinux.org/index.php/Si … rules_file

I was wondering why the following rules are necessary:

# iptables -A INPUT -p udp -m conntrack --ctstate NEW -j UDP
# iptables -A INPUT -p tcp --syn -m conntrack --ctstate NEW -j TCP

Doesn't the first rule already allow this?

-A INPUT -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT

I Mean if I initiated a connection like f.e. with a web browser, there's already a connection created. So I would guess the first rule Will pass and there's no need for the ones I mentioned above.

Thanks for any clarifications :-)

Last edited by chrisdb (2019-01-21 11:49:26)

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loqs

Member

Registered: 2014-03-06

Posts: 18,299

Re: [SOLVED] Iptables simple statefull firewall

These match a new unrelated connection.

# iptables -A INPUT -p udp -m conntrack --ctstate NEW -j UDP
# iptables -A INPUT -p tcp --syn -m conntrack --ctstate NEW -j TCP

This covers connections already allowed or a new connection that is related to an existing one such as an ICMP error.

# iptables -A INPUT -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT

chrisdb

Member

From: Belgium

Registered: 2018-05-16

Posts: 155

Re: [SOLVED] Iptables simple statefull firewall

These match a new unrelated connection.

# iptables -A INPUT -p udp -m conntrack --ctstate NEW -j UDP
# iptables -A INPUT -p tcp --syn -m conntrack --ctstate NEW -j TCP

So this would pass unrelated connections to the UDP or TCP  chain? And what happens then? Are all rules re-evaluated then?

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Piri

Member

Registered: 2018-06-02

Posts: 70

Re: [SOLVED] Iptables simple statefull firewall

I Mean if I initiated a connection like f.e. with a web browser, there's already a connection created. So I would guess the first rule Will pass and there's no need for the ones I mentioned above.

The TCP/UDP chains are for incoming connections that are not initiated by yourself (e.g. someone SSHs to your machine)

See the "The TCP and UDP chains" section where they explicitly open up SSH,HTTP,HTTPS, etc.

So this would pass unrelated connections to the UDP or TCP  chain? And what happens then? Are all rules re-evaluated then?

No this just attaches the TCP/UDP chains to NEW TCP/UDP connections.

chrisdb

Member

From: Belgium

Registered: 2018-05-16

Posts: 155

Re: [SOLVED] Iptables simple statefull firewall

No this just attaches the TCP/UDP chains to NEW TCP/UDP connections.

Are they all allowed then? So every incoming unrelated connection is accepted?

Sorry I'm just trying to understand

Thx

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loqs

Member

Registered: 2014-03-06

Posts: 18,299

Re: [SOLVED] Iptables simple statefull firewall

Please reread Simple_stateful_firewall#The_INPUT_chain

chrisdb

Member

From: Belgium

Registered: 2018-05-16

Posts: 155

Re: [SOLVED] Iptables simple statefull firewall

Ok, so if I understand correctly and following the example (Resulting iptables.rules file), nothing will happen because there are no specific TCP or UDP chain  rules specified.

So basically the packet will be dropped?

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loqs

Member

Registered: 2014-03-06

Posts: 18,299

Re: [SOLVED] Iptables simple statefull firewall

Simple_stateful_firewall#The_INPUT_chain

Similar to the previous chains, we set the default policy for the INPUT chain to DROP in case something somehow slips by our rules. Dropping all traffic and specifying what is allowed is the best way to make a secure firewall.

Warning: If you are logged in via SSH, the following will immediately disconnect the SSH session. To avoid it: (1) add the first INPUT chain rule below (it will keep the session open), (2) add a regular rule to allow inbound SSH (to be able to reconnect in case of a connection drop) and (3) set the policy.

# iptables -P INPUT DROP

Lone_Wolf

Administrator

From: Netherlands, Europe

Registered: 2005-10-04

Posts: 13,342

Re: [SOLVED] Iptables simple statefull firewall

If your internet connection support ipv4 and ipv6 , you'll have to create 2 sets of rules .
You might want to use nftables instead of iptables then.

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chrisdb

Member

From: Belgium

Registered: 2018-05-16

Posts: 155

Re: [SOLVED] Iptables simple statefull firewall

Thx for the answers

I will also have a look at nftables :-)

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