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

NAME
secmarks - Shorewall6 file SYNOPSIS
/etc/shorewall6/secmarks DESCRIPTION
Important Unlike rules in the shorewall6-rules[1](5) file, evaluation of rules in this file will continue after a match. So the final secmark for each packet will be the one assigned by the LAST rule that matches. The secmarks file is used to associate an SELinux context with packets. It was added in Shorewall6 version 4.4.13. 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). SECMARK - {SAVE|RESTORE|context|COMMENT comment} SAVE If an SELinux context is associated with the packet, the context is saved in the connection. Normally, the remaining columns should be left blank. RESTORE If an SELinux context is not currently associated with the packet, then the saved context (if any) is associated with the packet. Normally, the remaining columns should be left blank. context An SELinux context. COMMENT The remainder of the line is treated as a comment which is attached to subsequent rules until another COMMENT line is found or until the end of the file is reached. To stop adding comments to rules, use a line with only the word COMMENT. CHAIN - {P|I|F|O|T}[:{N|I|NI|E|ER}] P - PREROUTING I - INPUT F - FORWARD O - OUTPUT T - POSTROUTING It may be optionally followed by a colon and an indication of the Netfilter connection state(s) at which the context is to be applied: :N - NEW connection :I - INVALID connection :NI - New or INVALID connection :E - ESTABLISHED connection :ER - ESTABLISHED or RELATED connection SOURCE - {-interface|[interface:]address-or-range[,address-or-range]...}[exclusion] May be: 1. An interface name - matches traffic entering the firewall on the specified interface. May not be used in classify rules or in rules using the T in the CHAIN column. 2. A comma-separated list of host or network IP addresses or MAC addresses. 3. An interface name followed by a colon (":") followed by a comma-separated list of host or network IP addresses or MAC addresses. MAC addresses must be prefixed with "~" and use "-" as a separator. Example: ~00-A0-C9-15-39-78 You may exclude certain hosts from the set already defined through use of an exclusion (see shorewall6-exclusion[2](5)). DEST - {-|{interface|[interface:]address-or-range[,address-or-range]...}[exclusion] May be: 1. An interface name. May not be used in the PREROUTING or INPUT chains. The interface name may be optionally followed by a colon (":") and an IP address list. 2. A comma-separated list of host or network IP addresses. The list may include ip address ranges if your kernel and iptables include iprange support. You may exclude certain hosts from the set already defined through use of an exclusion (see shorewall6-exclusion[3](5)). PROTO - {-|tcp:syn|ipp2p|ipp2p:udp|ipp2p:all|protocol-number|protocol-name|all} Protocol - ipp2p requires ipp2p match support in your kernel and iptables. PORT(S) (dport) - [-|port-name-number-or-range[,port-name-number-or-range]...] Optional destination Ports. A comma-separated list of Port names (from services(5)), port numbers or port ranges; if the protocol is icmp, this column is interpreted as the destination icmp-type(s). ICMP types may be specified as a numeric type, a numberic type and code separated by a slash (e.g., 3/4), or a typename. See http://www.shorewall.net/configuration_file_basics.htm#ICMP. If the protocol is ipp2p, this column is interpreted as an ipp2p option without the leading "--" (example bit for bit-torrent). If no PORT is given, ipp2p is assumed. This column is ignored if PROTOCOL = all but must be entered if any of the following field is supplied. In that case, it is suggested that this field contain "-" SOURCE PORT(S) (sport) - [-|port-name-number-or-range[,port-name-number-or-range]...] Optional source port(s). If omitted, any source port is acceptable. Specified as a comma-separated list of port names, port numbers or port ranges. USER - [!][user-name-or-number][:group-name-or-number] This optional column may only be non-empty if the SOURCE is the firewall itself. When this column is non-empty, the rule applies only if the program generating the output is running under the effective user and/or group specified (or is NOT running under that id if "!" is given). Examples: joe program must be run by joe :kids program must be run by a member of the 'kids' group !:kids program must not be run by a member of the 'kids' group MARK - [!]value[/mask][:C] Defines a test on the existing packet or connection mark. The rule will match only if the test returns true. If you don't want to define a test but need to specify anything in the following columns, place a "-" in this field. ! Inverts the test (not equal) value Value of the packet or connection mark. mask A mask to be applied to the mark before testing. :C Designates a connection mark. If omitted, the packet mark's value is tested. EXAMPLE
Mark the first incoming packet of a connection on the loopback interface and destined for address ::1 and tcp port 3306 with context system_u:object_r:mysqld_t:s0 and save that context in the conntrack table. On subsequent input packets in the connection, set the context from the conntrack table. /etc/shorewall6/interfaces: #ZONE INTERFACE BROADCAST OPTIONS - lo - ignore /etc/shorewall6/secmarks: #SECMARK CHAIN: SOURCE DEST PROTO DEST SOURCE USER/ MARK # STATE PORT(S) PORT(S) GROUP system_u:object_r:mysqld_packet_t:s0 I:N lo ::1 tcp 3306 SAVE I:N RESTORE I:ER FILES
/etc/shorewall6/secmarks SEE ALSO
http://james-morris.livejournal.com/11010.html http://shorewall.net/configuration_file_basics.htm#Pairs shorewall6(8), shorewall6-actions(5), shorewall6-blacklist(5), shorewall6-hosts(5), shorewall6-interfaces(5), shorewall6-maclist(5), shoewall6-netmap(5),shorewall6-params(5), shorewall6-policy(5), shorewall6-providers(5), shorewall6-rtrules(5), shorewall6-routestopped(5), shorewall6-rules(5), shorewall6.conf(5), shorewall6-tcclasses(5), shorewall6-tcdevices(5), shorewall6-tcrules(5), shorewall6-tos(5), shorewall6-tunnels(5), shorewall6-zones(5) NOTES
1. shorewall6-rules http://www.shorewall.net/manpages6/shorewall6-rules.html 2. shorewall6-exclusion http://www.shorewall.net/manpages6/shorewall6-exclusion.html 3. shorewall6-exclusion http://www.shorewall.net/manpages6/shorewall-exclusion.html [FIXME: source] 06/28/2012 SHOREWALL6-SECMARKS(5)
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