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Full Discussion: iptables - formatting icmp rules
Special Forums IP Networking iptables - formatting icmp rules Post 303017697 by CrazyDave on Sunday 20th of May 2018 09:04:31 PM
Old 05-20-2018
iptables - formatting icmp rules
Hi, I am relatively new to firewalls and netfilter. I have a Debian Stretch router box running dnsmasq, connected to a VPN. Occasionally dnsmasq polls all of the desired DNS servers to select the fastest. When it does this it responds to replies of the non-selected DNS servers with a icmp type three or "host unreachable". My firewall is very strict (I was hacked) and I am controlling sockets. I would like to respond to the DNS servers with this icmp message. I have tried many, many ways but none work, the message keeps on getting dropped. Here is an example rule set for one of the DNS servers:

Code:
# Owner: cryptostorm DNS in Langley in CA
-A OUTPUT -o tun0 -m state --state ESTABLISHED,NEW -p tcp --dport 53 -d 162.221.207.228 -j good_out_ips_accept
-A OUTPUT -o tun0 -m state --state ESTABLISHED,NEW -p udp --dport 53 -d 162.221.207.228 -j good_out_ips_accept
-A OUTPUT -m state --state ESTABLISHED,NEW -p icmp -m icmp --icmp-type 3 -d 162.221.207.228 -j good_out_ips_accept
-A OUTPUT -o tun0 -d 162.221.207.228 -j good_out_ips_drop

Here is the rule script:

Code:
-N good_out_ips_accept
-N good_out_ips_drop

-- many ips and ranges like above ----

-A good_out_ips_accept -j ACCEPT
-A good_out_ips_drop -j LOG  --log-level info --log-prefix "GOOD O/P IPs -- DROP :"
-A good_out_ips_drop -j DROP

Here is the resulting script from the firewall log:

Code:
May 20 16:24:21 gate kernel: [73690.667828] GOOD O/P IPs -- DROP :IN= OUT=tun0 SRC=10.7.7.88 DST=162.221.207.228 LEN=152 TOS=0x00 PREC=0xC0 TTL=64 ID=54071 PROTO=ICMP TYPE=3 CODE=3 [SRC=162.221.207.228 DST=10.7.7.88 LEN=124 TOS=0x00 PREC=0x00 TTL=57 ID=58899 DF PROTO=UDP SPT=53 DPT=50934 LEN=104 ]

To me the firewall is not seeing the icmp rule for some reason. Can anyone see the problem? Thanks for you help!

---------- Post updated at 06:04 PM ---------- Previous update was at 05:36 PM ----------

Well, I'm replying to my own post 10 minutes after writing it. All I needed was a "RELATED" on the state. I was hesitant to use this state as it seems to open a can of worms on some web sites...
 

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LEARN ABOUT DEBIAN

shorewall6-tcfilters

SHOREWALL6-TCFILTER(5)						  [FIXME: manual]					    SHOREWALL6-TCFILTER(5)

NAME

       tcfilters - shorewall6 u32 classifier rules file

SYNOPSIS

       /etc/shorewall6/tcfilters

DESCRIPTION

       Entries in this file cause packets to be classified for traffic shaping.

       Beginning with Shorewall 4.4.15, the file may contain entries for both IPv4 and IPv6. By default, all rules apply to IPv6 but that can be
       changed by inserting a line as follows:

       IPV4
	   Following entries apply to IPv4.

       IPV6
	   Following entries apply to IPv6

       ALL
	   Following entries apply to both IPv4 and IPv6. Each entry is processed twice; once for IPv4 and once for IPv6.

       The columns in the file are as follows (where the column name is followed by a different name in parentheses, the different name is used in
       the alternate specification syntax).

       CLASS - interface:class
	   The name or number of an interface defined in shorewall6-tcdevices[1](5) followed by a class number defined for that interface in
	   shorewall6-tcclasses[2](5).

       SOURCE - {-|address}
	   Source of the packet. May be a host or network address. DNS names are not allowed.

       DEST - {-|address}}
	   Destination of the packet. May be a host or network address. DNS names are not allowed.

       PROTO - {-|protocol-number|protocol-name|all}
	   Protocol.

       DEST PORT (dport) - [-|port-name-or-number]
	   Optional destination Ports. A Port name (from services(5)) or a port number; if the protocol is icmp, this column is interpreted as the
	   destination icmp-type(s).

       SOURCE PORT (sport) - [-|port-name-or-number]
	   Optional source port.

       TOS - [-|tos]
	   Optional - specifies the value of the TOS field. The tos value can be any of the following:

	   o   tos-minimize-delay

	   o   tos-maximuze-throughput

	   o   tos-maximize-reliability

	   o   tos-minimize-cost

	   o   tos-normal-service

	   o   hex-number

	   o   hex-number/hex-number

	   The hex-numbers must be exactly two digits (e.g., 0x04)x.

       LENGTH - [-|number]
	   Optional. Must be a power of 2 between 32 and 8192 inclusive. Packets with a total length that is strictly less than the specified
	   number will match the rule.

EXAMPLE

       Example 1:
	   Place all 'ping' traffic on interface 1 in class 10. Note that ALL cannot be used because IPv4 ICMP and IPv6 ICMP are two different
	   protocols.

		       #CLASS	 SOURCE    DEST 	PROTO	DEST
		      # 				       PORT

		      IPV4

		      1:10	0.0.0.0/0 0.0.0.0/0    icmp    echo-request
		      1:10	0.0.0.0/0 0.0.0.0/0    icmp    echo-reply

		      IPV6

		      1:10	::/0	  ::/0	       icmp6   echo-request
		      1:10	::/0	  ::/0	       icmp6   echo-reply

FILES

       /etc/shorewall6/tcfilters

SEE ALSO

       http://shorewall.net/traffic_shaping.htm

       http://shorewall.net/MultiISP.html

       http://shorewall.net/PacketMarking.html

NOTES

	1. shorewall6-tcdevices
	   http://www.shorewall.net/manpages6/shorewall6-tcdevices.html

	2. shorewall6-tcclasses
	   http://www.shorewall.net/manpages6/shorewall6-tcclasses.html

[FIXME: source] 						    06/28/2012						    SHOREWALL6-TCFILTER(5)
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