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5.4

Odd IPTABLES behaviour

Asked by duncan_roe in IP Tables/IP Chains, Linux, Linux Networking

Tags: iptables firewall Linux

I forgot to allow bootps requests from bootpc but it worked anyway. The packet is logged as about to be dropped (that is the next rule), but the bootpc server responds so must have got it.
This is contrary to my understanding of what should happen (and makes it harder for me to sell management on iptables as a firewall solution).
Does anyone have any idea what could be going on?

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             #!/bin/sh
set -x
exec >/tmp/iptables-filter-mintec.out 2>&1
 
# Set up a firewall: drop all incoming UDP & connects except DNS UDP:-
 
#Don't set up these rules twice
/usr/sbin/iptables -L -n|grep ppptab >/dev/null ||
{
 
  # A chain to log & drop a packet, except don't log FIN pkts
  /usr/sbin/iptables -N logdrop
  /usr/sbin/iptables -A logdrop -p tcp -m tcp --tcp-flags FIN FIN -j DROP
  # Comment out logging if too much stuff gets logged
  /usr/sbin/iptables -A logdrop -j LOG --log-level debug
  /usr/sbin/iptables -A logdrop -j DROP
 
  # A chain to inspect incoming (to this box) packets from ppp connection
  /usr/sbin/iptables -N ppptab
  # Allow icmp but not too many
  /usr/sbin/iptables -A ppptab -p icmp -m limit --limit 5/second -j ACCEPT
  # Allow DNS replies and queries
  /usr/sbin/iptables -A ppptab -p udp --source-port 53 -j ACCEPT
  /usr/sbin/iptables -A ppptab -p udp --destination-port 53 -j ACCEPT
  # Allow reply packets to an outbound telnet
  /usr/sbin/iptables -A ppptab -p tcp --source-port 23 -j ACCEPT
  # Drop everything else
  /usr/sbin/iptables -A ppptab -j logdrop
 
  # Firewall rule - check incoming (to this box) packets from ppp connection
  /usr/sbin/iptables -A INPUT -i ppp0 -j ppptab
  /usr/sbin/iptables -A INPUT -i eth1 -j ppptab
}

           
         

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Odd IPTABLES behaviour

Asked by duncan_roe in IP Tables/IP Chains, Linux, Linux Networking

Tags: iptables firewall Linux

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