iptables - formatting icmp rules

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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:

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

Here is the rule script:

-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:

May 20 16:24:21 gate kernel: [73690.667828] GOOD O/P IPs -- DROP :IN= OUT=tun0 SRC= DST= LEN=152 TOS=0x00 PREC=0xC0 TTL=64 ID=54071 PROTO=ICMP TYPE=3 CODE=3 [SRC= DST= 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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RESOLVCONF.CONF(5)					      System Manager's Manual						RESOLVCONF.CONF(5)

resolvconf.conf -- resolvconf configuration file DESCRIPTION
resolvconf.conf is the configuration file for resolvconf(8). The resolvconf.conf file is a shell script that is sourced by resolvconf(8), meaning that resolvconf.conf must contain valid shell commands. Listed below are the standard resolvconf.conf variables that may be set. After updating this file, you may wish to run resolvconf -u to apply the new configuration. RESOLVCONF OPTIONS
interface_order These interfaces will always be processed first. If unset, defaults to the following:- lo lo[0-9]* dynamic_order These interfaces will be processed next, unless they have a metric. If unset, defaults to the following:- tap[0-9]* tun[0-9]* vpn vpn[0-9]* ppp[0-9]* ippp[0-9]* search_domains Prepend search domains to the dynamically generated list. search_domains_append Append search domains to the dynamically generated list. domain_blacklist A list of domains to be removed from consideration. To remove a domain, you can use foo.* To remove a sub domain, you can use *.bar name_servers Prepend name servers to the dynamically generated list. You should set this to if you use a local name server other than libc. name_servers_append Append name servers to the dynamically generated list. name_server_blacklist A list of name servers to be removed from consideration. The default is as some faulty routers send it via DHCP. To remove a block, you can use 192.168.* private_interfaces These interfaces name servers will only be queried for the domains listed in their resolv.conf. Useful for VPN domains. This is equivalent to the resolvconf -p option. state_dir Override the default state directory of /run/resolvconf. This should not be changed once resolvconf is in use unless the old direc- tory is copied to the new one. LIBC OPTIONS
The following variables affect resolv.conf(5) directly:- resolv_conf Defaults to /etc/resolv.conf if not set. resolv_conf_options A list of libc resolver options, as specified in resolv.conf(5). resolv_conf_passthrough When set to YES the latest resolv.conf is written to resolv_conf without any alteration. resolv_conf_sortlist A libc resolver sortlist, as specified in resolv.conf(5). resolv_conf_local_only If a local nameserver is configured then the default is just to specify that and ignore all other entries as they will be configured for the local nameserver. Set this to YES to list them instead, if you need working DNS and the local nameserver stops functioning at the expense of duplicated server queries. SUBSCRIBER OPTIONS
openresolv ships with subscribers for the name servers dnsmasq(8), named(8), pdnsd(8) and unbound(8). Each subscriber can create configura- tion files which should be included in in the subscribers main configuration file. dnsmasq_conf This file tells dnsmasq which nameservers to use for specific domains. dnsmasq_resolv This file tells dnsmasq which nameservers to use for global lookups. Example resolvconf.conf for dnsmasq: nameservers= dnsmasq_conf=/etc/dnsmasq-conf.conf dnsmasq_resolv=/etc/dnsmasq-resolv.conf Example dnsmasq.conf: listen-address= conf-file=/etc/dnsmasq-conf.conf resolv-file=/etc/dnsmasq-resolv.conf named_options Include this file in the named options block. This file tells named which nameservers to use for global lookups. named_zones Include this file in the named global scope, after the options block. This file tells named which nameservers to use for specific domains. Example resolvconf.conf for named: nameservers= named_options=/etc/named-options.conf named_zones=/etc/named-zones.conf Example named.conf: options { listen-on {; }; include /etc/named-options.conf; }; include /etc/named-zones.conf; pdnsd_conf This is the main pdnsd configuration file which we modify to add our forward domains to. If this variable is not set then we rely on the pdnsd configuration file setup to read pdnsd_resolv as documented below. pdnsd_resolv This file tells pdnsd about global nameservers. If this variable is not set then it's written to pdnsd_conf. Example resolvconf.conf for pdnsd: nameservers= pdnsd_conf=/etc/pdnsd.conf # pdnsd_resolv=/etc/pdnsd-resolv.conf Example pdnsd.conf: global { server_ip =; status_ctl = on; } server { # A server definition is required, even if emtpy. label="empty"; proxy_only=on; # file="/etc/pdnsd-resolv.conf"; } unbound_conf This file tells unbound about specific and global nameservers. Example resolvconf.conf for unbound: nameservers= unbound_conf=/etc/unbound-resolvconf.conf Example unbound.conf: include: /etc/unbound-resolvconf.conf SEE ALSO
resolv.conf(5) and resolvconf(8). AUTHORS
Roy Marples <roy@marples.name> BUGS
Please report them to http://roy.marples.name/projects/openresolv BSD
March 19, 2012 BSD

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