Setting up a local firewall

/dev/rob0 rob0 at gmx.co.uk
Mon Aug 1 14:54:55 CEST 2005


Bryan Christ wrote:
> I locked myself out of  my server until I rebooted it.  My goal was to

BTDT :)

> lock down everything and allow only SSH connectivity.  Can anyone show 
> me where my logic went wrong?  Here was the fatal script which I wrote:
> 
> /sbin/iptables -F INPUT
> /sbin/iptables -A INPUT -s 0/0 -j DROP
> /sbin/iptables -A INPUT -s 0/0 -m state  --state NEW,ESTABLISHED -p tcp 
> --dport 22 -j ACCEPT

Rules are evaluated in order. Everything matches your first rule. 
Nothing reaches the second one.

> My guess is that I missed accepting syn packets, but I'm not ready to 
> "try" again.

iptables -F INPUT
iptables -P INPUT DROP
# Let in replies to the connections you initiate
iptables -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -A INPUT -i lo -j ACCEPT # allow loopback
iptables -A INPUT -p tcp --dport 22 -j ACCEPT
# Everything else falls on the default policy.

Strictly speaking the loopback line is not needed; it just means that 
the system will be able to talk to itself. Similarly the --state line is 
extra; it just means that when you ssh in you can have useful network 
connectivity. I put that one in its own chain ...

iptables -F ; iptables -X
iptables -N State
iptables -A State -m state --state INVALID -j DROP
iptables -A State -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -I INPUT -j State
iptables -I FORWARD -j State

See, that's the first rule in both INPUT and FORWARD. Order is 
important! That's why we have -A|--append and -I|--insert.
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