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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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FIREWALLD.ZONE(5)						  firewalld.zone						 FIREWALLD.ZONE(5)

NAME
firewalld.zone - firewalld zone configuration files SYNOPSIS
/etc/firewalld/zones/zone.xml /usr/lib/firewalld/zones/zone.xml DESCRIPTION
A firewalld zone configuration file contains the information for a zone. These are the zone description, services, ports, icmp-blocks, masquerade, forward-ports and rich language rules in an XML file format. The file name has to be zone_name.xml where length of zone_name is currently limited to 17 chars. This is the structure of a zone configuration file: <?xml version="1.0" encoding="utf-8"?> <zone [version="versionstring"] [target="ACCEPT|%%REJECT%%|DROP"]> [ <short>short description</short> ] [ <description>description</description> ] [ <interface name="string"/> ] [ <source address="address[/mask]"/> ] [ <service name="string"/> ] [ <port port="portid[-portid]" protocol="tcp|udp"/> ] [ <icmp-block name="string"/> ] [ <masquerade/> ] [ <forward-port port="portid[-portid]" protocol="tcp|udp" [to-port="portid[-portid]"] [to-addr="ipv4address"]/> ] [ <rule [family="ipv4|ipv6"]> [ <source address="address[/mask]" [invert="bool"]/> ] [ <destination address="address[/mask]" [invert="bool"]/> ] [ <service name="string"/> | <port port="portid[-portid]" protocol="tcp|udp"/> | <protocol value="protocol"/> | <icmp-block name="icmptype"/> | <masquerade/> | <forward-port port="portid[-portid]" protocol="tcp|udp" [to-port="portid[-portid]"] [to-addr="address"]/> ] [ <log [prefix="prefixtext"] [level="emerg|alert|crit|err|warn|notice|info|debug"]/> [<limit value="rate/duration"/>] </log> ] [ <audit> [<limit value="rate/duration"/>] </audit> ] [ <accept/> | <reject [type="rejecttype"]/> | <drop/> ] </rule> ] </zone> The config can contain these tags and attributes. Some of them are mandatory, others optional. zone The mandatory zone start and end tag defines the zone. This tag can only be used once in a zone configuration file. There are optional attributes for zones: version="string" To give the zone a version. target="ACCEPT|%%REJECT%%|DROP" Can be used to accept, reject or drop every packet. The ACCEPT target is used in the trusted zone, every packet will be accepted. The %%REJECT%% target is used in the block zone, every packet will be rejected with the default firewalld reject type. The DROP target is used in the drop zone, every packet will be dropped. The default target is {chain}_ZONE_{zone} and will be used if the target is not specified. If other than the default target is used, all settings except interface and source are ignored, because the first rule created in firewall for this zone is 'jump to target'. short Is an optional start and end tag and is used to give a zone a more readable name. description Is an optional start and end tag to have a description for a zone. interface Is an optional empty-element tag and can be used several times. It can be used to bind an interface to a zone. An interface entry has exactly one attribute: name="string" The name of the interface to be bound to the zone. source Is an optional empty-element tag and can be used several times. It can be used to bind a source address or source address range to a zone. A source entry has exactly one attribute: address="address[/mask]" The source to be bound to the zone. The source is either an IP address or a network IP address with a mask for IPv4 or IPv6. The network family (IPv4/IPv6) will be automatically discovered. For IPv4, the mask can be a network mask or a plain number. For IPv6 the mask is a plain number. The use of host names is not supported. service Is an optional empty-element tag and can be used several times to have more than one service entry enabled. A service entry has exactly one attribute: name="string" The name of the service to be enabled. To get a list of valid service names firewall-cmd --list=services can be used. port Is an optional empty-element tag and can be used several times to have more than one port entry. All attributes of a port entry are mandatory: port="portid[-portid]" The port can either be a single port number portid or a port range portid-portid. protocol="tcp|udp" The protocol can either be tcp or udp. icmp-block Is an optional empty-element tag and can be used several times to have more than one icmp-block entry. Each icmp-block tag has exactly one mandatory attribute: name="string" The name of the Internet Control Message Protocol (ICMP) type to be blocked. To get a list of valid ICMP types firewall-cmd --list=icmptypes can be used. masquerade Is an optional empty-element tag. It can be used only once in a zone configuration and is not usable for IPv6. If it's present masquerading is enabled for the zone. If you want to enable masquerading, you should enable it in the zone bound to the external interface. forward-port Is an optional empty-element tag and can be used several times to have more than one port or packet forward entry. This is for IPv4 only. Use rich language rules for IPv6. There are mandatory and also optional attributes for forward ports: Mandatory attributes: The local port and protocol to be forwarded. port="portid[-portid]" The port can either be a single port number portid or a port range portid-portid. protocol="tcp|udp" The protocol can either be tcp or udp. Optional attributes: The destination of the forward. For local forwarding add to-port only. For remote forwarding add to-addr and use to-port optionally if the destination port on the destination machine should be different. to-port="portid[-portid]" The destination port or port range to forward to. If omitted, the value of the port= attribute will be used altogether with the to-addr attribute. to-addr="address" The destination IPv4 IP address. rule Is an optional element tag and can be used several times to have more than one rich language rule entry. The general rule structure: <rule [family="ipv4|ipv6"]/> [ <source address="address[/mask]" [invert="bool"]/> ] [ <destination address="address[/mask]" [invert="bool"]/> ] [ <service name="string"/> | <port port="portid[-portid]" protocol="tcp|udp"/> | <protocol value="protocol"/> | <icmp-block name="icmptype"/> | <masquerade/> | <forward-port port="portid[-portid]" protocol="tcp|udp" [to-port="portid[-portid]"] [to-addr="address"]/> ] [ <log [prefix="prefixtext"] [level="emerg|alert|crit|err|warn|notice|info|debug"]/> [<limit value="rate/duration"/>] </log> ] [ <audit> [<limit value="rate/duration"/>] </audit> ] [ <accept/> | <reject [type="rejecttype"]/> | <drop/> ] </rule> Rule structure for source black or white listing: <rule [family="ipv4|ipv6"]/> <source address="address[/mask]" [family="bool"]/> [ <log [prefix="prefixtext"] [level="emerg|alert|crit|err|warn|notice|info|debug"]/> [<limit value="rate/duration"/>] </log> ] [ <audit> [<limit value="rate/duration"/>] </audit> ] <accept/> | <reject [type="rejecttype"]/> | <drop/> </rule> For a full description on rich language rules, please have a look at firewalld.richlanguage(5). SEE ALSO
firewall-applet(1), firewalld(1), firewall-cmd(1), firewall-config(1), firewalld.conf(5), firewalld.direct(5), firewalld.icmptype(5), firewalld.lockdown-whitelist(5), firewall-offline-cmd(1), firewalld.richlanguage(5), firewalld.service(5), firewalld.zone(5), firewalld.zones(5) NOTES
firewalld home page at fedorahosted.org: http://fedorahosted.org/firewalld/ More documentation with examples: http://fedoraproject.org/wiki/FirewallD AUTHORS
Thomas Woerner <twoerner@redhat.com> Developer Jiri Popelka <jpopelka@redhat.com> Developer firewalld 0.3.9 FIREWALLD.ZONE(5)
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