shorewall-nesting(5)
NAME
nesting - Shorewall Nested Zones
SYNOPSIS
child-zone[:parent-zone[,parent-zone]...]
DESCRIPTION
In shorewall-zones[1](5), a zone may be declared to be a sub-zone of one or more other zones using the above syntax. The child-zone may be neither the firewall zone nor a vserver zone. The firewall zone may not appear as a parent zone, although all vserver zones are handled as sub-zones of the firewall zone. Where zones are nested, the CONTINUE policy in shorewall-policy[2](5) allows hosts that are within multiple zones to be managed under the rules of all of these zones.
EXAMPLE
/etc/shorewall/zones:
#ZONE TYPE OPTION
fw firewall
net ipv4
sam:net ipv4
loc ipv4
/etc/shorewall/interfaces:
#ZONE INTERFACE BROADCAST OPTIONS
- eth0 detect dhcp,norfc1918
loc eth1 detect
/etc/shorewall/hosts:
#ZONE HOST(S) OPTIONS
net eth0:0.0.0.0/0
sam eth0:206.191.149.197
/etc/shorewall/policy:
#SOURCE DEST POLICY LOG LEVEL
loc net ACCEPT
sam all CONTINUE
net all DROP info
all all REJECT info
The second entry above says that when Sam is the client, connection
requests should first be processed under rules where the source zone is
sam and if there is no match then the connection request should be
treated under rules where the source zone is net. It is important that
this policy be listed BEFORE the next policy (net to all). You can have
this policy generated for you automatically by using the
IMPLICIT_CONTINUE option in shorewall.conf[3](5).
Partial /etc/shorewall/rules:
#ACTION SOURCE DEST PROTO DPORT
...
DNAT sam loc:192.168.1.3 tcp ssh
DNAT net loc:192.168.1.5 tcp www
...
Given these two rules, Sam can connect to the firewall's internet
interface with ssh and the connection request will be forwarded to
192.168.1.3. Like all hosts in the net zone, Sam can connect to the
firewall's internet interface on TCP port 80 and the connection request
will be forwarded to 192.168.1.5. The order of the rules is not
significant. Sometimes it is necessary to suppress port forwarding for
a sub-zone. For example, suppose that all hosts can SSH to the firewall
and be forwarded to 192.168.1.5 EXCEPT Sam. When Sam connects to the
firewall's external IP, he should be connected to the firewall itself.
Because of the way that Netfilter is constructed, this requires two
rules as follows:
#ACTION SOURCE DEST PROTO DPORT
...
ACCEPT+ sam $FW tcp ssh
DNAT net loc:192.168.1.3 tcp ssh
...
The first rule allows Sam SSH access to the firewall. The second rule
says that any clients from the net zone with the exception of those in
the "sam" zone should have their connection port forwarded to
192.168.1.3. If you need to exclude more than one zone, simply use
multiple ACCEPT+ rules. This technique also may be used when the ACTION
is REDIRECT.
Care must be taken when nesting occurs as a result of the use of
wildcard interfaces (interface names ends in '+').
Here's an example. /etc/shorewall/zones:
/etc/shorewall/interfaces:
#ZONE INTERFACE BROADCAST OPTIONS
net ppp0
loc eth1
loc ppp+
dmz eth2
Because the net zone is declared before the loc zone, net is an
implicit sub-zone of loc and in the absence of a net->... CONTINUE
policy, traffic from the net zone will not be passed through loc->...
rules. But DNAT and REDIRECT rules are an exception!
* DNAT and REDIRECT rules generate two Netfilter rules: a 'nat' table
rule that rewrites the destination IP address and/or port number,
and a 'filter' table rule that ACCEPTs the rewritten connection.
* Policies only affect the 'filter' table.
As a consequence, the following rules will have unexpected behavior:
#ACTION SOURCE DEST PROTO DPORT
ACCEPT net dmz tcp 80
REDIRECT loc 3128 tcp 80
The second rule is intended to redirect local web requests to a proxy
running on the firewall and listening on TCP port 3128. But the 'nat'
part of that rule will cause all connection requests for TCP port 80
arriving on interface ppp+ (including ppp0!) to have their destination
port rewritten to 3128. Hence, the web server running in the DMZ will
be inaccessible from the web.
The above problem can be corrected in several ways.
The preferred way is to use the ifname pppd option to change the 'net'
interface to something other than ppp0. That way, it won't match ppp+.
If you are running Shorewall version 4.1.4 or later, a second way is to
simply make the nested zones explicit:
#ZONE TYPE OPTION
fw firewall
loc ipv4
net:loc ipv4
dmz ipv4
If you take this approach, be sure to set IMPLICIT_CONTINUE=No in
shorewall.conf.
When using other Shorewall versions, another way is to rewrite the DNAT
rule (assume that the local zone is entirely within 192.168.2.0/23):
#ACTION SOURCE DEST PROTO DPORT
ACCEPT net dmz tcp 80
REDIRECT loc:192.168.2.0/23 3128 tcp 80
Another way is to restrict the definition of the loc zone:
/etc/shorewall/interfaces:
#ZONE INTERFACE BROADCAST OPTIONS
net ppp0
loc eth1
- ppp+
dmz eth2
/etc/shorewall/hosts:
#ZONE HOST(S) OPTIONS
loc ppp+:192.168.2.0/23
FILES
/etc/shorewall/zones /etc/shorewall/interfaces /etc/shorewall/hosts /etc/shorewall/policy /etc/shorewall/rules
SEE ALSO
shorewall(8), shorewall-accounting(5), shorewall-actions(5), shorewall-blacklist(5), shorewall-hosts(5), shorewall_interfaces(5), shorewall-ipsets(5), shorewall-maclist(5), shorewall-masq(5), shorewall-nat(5), shorewall-netmap(5), shorewall-params(5), shorewall-policy(5), shorewall-providers(5), shorewall-proxyarp(5), shorewall-rtrules(5), shorewall-routestopped(5), shorewall-rules(5), shorewall.conf(5), shorewall-secmarks(5), shorewall-tcclasses(5), shorewall-tcdevices(5), shorewall-mangle(5), shorewall-tos(5), shorewall-tunnels(5), shorewall-zones(5)
NOTES
1. shorewall-zones
http://www.shorewall.net/manpages/shorewall-zones.html
2. shorewall-policy
http://www.shorewall.net/manpages/shorewall-policy.html
3. shorewall.conf
http://www.shorewall.net/manpages/shorewall.conf.html
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