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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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NF2CSV(1)						      General Commands Manual							 NF2CSV(1)

NAME
nf2csv - iptables to CSV data SYNOPSIS
nf2csv [options] DESCRIPTION
nf2csv Parses iptables log messages and generates comma-separate value formatted data. This is useful to provide input to the AfterGlow project (see http://afterglow.sourceforge.net) so iptables logs can be visualized graphically. An interesting application of nf2csv and AfterGlow is to parse and visualize the iptables logfiles made available by the Honeynet project in their Scan of the Month challenges. The Scan30 and Scan34 challenges (see http://www.honeynet.org/scans/scan30/ and http://www.honeynet.org/scans/scan34/) contain extensive iptables logfiles, and some graphical representations of these can be viewed here: http://www.cipherdyne.org/psad/honeynet/. The psad pro- gram also has the ability to generate CSV data from iptables logs with its --CSV mode. OPTIONS
-f, --fields <tokens> Specify the set of fields that should be printed from iptables log messages. The most common usage of this argument is SRC DST DPT to print the source and destination IP addresses, followed by the destination port number. Available fields to print include: SRC, SPT, DST, DPT, PROTO, LEN, IN, TOS, TTL, SEQ, ID, TYPE, CODE (and these can also be referred to as src, dst, sp, dp, proto, ip_len, intf, tos, and ttl). There are several additional fields that are not given specific tags within iptables log messages, and these can be included by specifying one of the following: flags, top_opts, ip_opts, chain, log_prefix, frag_bit, src_mac, dst_mac, and udp_len. Each of these fields accepts a search criteria in the form of a numeric comparison, string match, or IP match. See the EXAMPLES section below for more information. -u, --unique-lines Only print unique output lines. This can drastically reduce the output of nf2csv depending on the characteristics of the iptables logfile that is being parsed. -m, --max-lines <num> Specify the maximum number of output lines nf2csv will generate. This is useful for providing a limited set of data to AfterGlow in order to make visualizations more clear and less cluttered. -r, --regex <regex> Specify a regular expression that must match against the entire iptables log message in order for it to be included within the CSV output. This allows log messages to be included from the output with all of the flexibility of regular expressions. See the EXAM- PLES section below for more information. -n, --neg-regex <regex> Specify a regular expression that must not match against the iptables log message in order for it to be included within the CSV out- put. This allows log messages to be excluded from the output with all of the flexibility of regular expressions. See the EXAMPLES section below for more information. -s, --start-line <line> Specify the starting line where nf2csv begins to process iptables log data. If you are processing a huge file with thousands of iptables log messages this option can be useful to parse a specific chunk of this data. Also see the --end-line option below. -e, --end-line <line> Specify the last line of iptables log data that nf2csv will parse. EXAMPLES
The following examples illustrate the command line arguments that could be supplied to nf2csv in a few situations: Print source and destination IP addresses and the destination port number: $ nfcsv -f src dst dp Same as above, but now require that the source IP come from the 11.11.11.0/24 subnet: $ nfcsv -f src:11.11.11.0/24 dst dp Display instances of the MyDoom worm: $ nfcsv -f src dst dp:3127 Display packets that have low TTL values: $ nfcsv -f src dst ttl:<10 Display all traffic to or from the host 11.11.11.67 (this sets up an OR condition between the src and dst fields): $ nfcsv -f src dst dp -r 11.11.11.67 Display likely instances of Window Messanger popup spam attempts (note the use of the --regex argument to require minimal lengths on the UDP length field and source port, but the output contains the destination port of 1026): $ nfcsv -f src dst dp -r SPT={4}.*LEN=[4-9]{2} SEE ALSO
psad(8) AUTHOR
Michael Rash <mbr@cipherdyne.org> BUGS
Send bug reports to mbr@cipherdyne.org. Suggestions and/or comments are always welcome as well. DISTRIBUTION
nf2csv is distributed with the psad project (http://www.cipherdyne.org/psad/) under the GNU General Public License (GPL), and the latest version may be downloaded from http://www.cipherdyne.org/ Linux Jun, 2006 NF2CSV(1)
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