Shorewall FAQsShorewall CommunityTomEastep2001-2016Thomas M. EastepPermission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License, Version
1.2 or any later version published by the Free Software Foundation; with
no Invariant Sections, no Front-Cover Texts, and no Back-Cover
Texts. A copy of the license is included in the section entitled GNU Free Documentation License.This article applies to Shorewall 4.4 and
later. If you are running a version of Shorewall earlier than Shorewall
4.4.0 then please see the documentation for that
release.Installing ShorewallWhere do I find Step by Step Installation and Configuration
Instructions?Answer: Check out the QuickStart Guides.(FAQ 92) There are lots of Shorewall packages; which one(s) do I
install?Answer: When first installing
Shorewall 4.4.0 or later, you must install the shorewall package. If you want to configure an
IPv6 firewall, you must also install shorewall6. Beginning with Shorewall 4.5, you
must first install the shorewall-core
package.(FAQ 92a) Someone once told me to install shorewall-perl;
anything to that?Answer: That was good advice in
Shorewall 4.2 and earlier. In those releases, there were two packages
that provided the basic firewalling functionality: shorewall-shell and shorewall-perl. Beginning with Shorewall 4.4.0,
shorewall-shell is discontinued and
shorewall-perl is renamed shorewall.(FAQ 37) I just installed Shorewall on Debian and the
/etc/shorewall directory is almost empty!!!Answer:Once you have installed the .deb package and before you attempt
to configure Shorewall, please heed the advice of Lorenzo Martignoni,
former Shorewall Debian Maintainer:For more information about Shorewall usage on Debian
system please look at /usr/share/doc/shorewall-common/README.Debian
provided by [the] shorewall-common Debian package.If you install using the .deb, you will find that your /etc/shorewall directory is almost empty.
This is intentional. The released configuration file skeletons may be
found on your system in the directory /usr/share/doc/shorewall-common/default-config.
Simply copy the files you need from that directory to /etc/shorewall and modify the
copies.(FAQ 37a) I just installed Shorewall on Debian and I can't find
the sample configurations.Answer: Beginning with
Shorewall 4.4, the samples are in the shorewall package and are
installed in /usr/share/doc/shorewall/examples/.(FAQ 14) I can't find the Shorewall 4.4 shorewall-common,
shorewall-shell and shorewall-perl packages? Where are they?Answer:In Shorewall 4.4, the
shorewall-shell package was discontinued. The
shorewall-common and
shorewall-perl packages were combined to form a
single shorewall package. In Shorewall 4.5, the
shorewall-core package was added and all of the
other packages depend on shorewall-core.(FAQ 1.5) After installing the latest version (> 5.1.10.1) of
Shorewall, when I change my configuration and 'shorewall reload' or
'shorewall restart', my changes aren't in the running ruleset. Why is
that happening?Answer: This happens when:You use INCLUDE (?INCLUDE).The included files are in a subdirectory of /etc/shorewall[6]
or in a separate directory.You have AUTOMAKE=Yes in shorewall[6].conf(5).When AUTOMAKE=Yes, the compiler looks for files in each directory
in CONFIG_PATH for files that are newer that the last-generated firewall
script. If none are found, the old script is used as is. Prior to
version 5.1.10.2, that search was recursive so changes in
sub-directories of /etc/shorewall[6] were automatically searched. This
had performance implications if directories on the CONFIG_PATH were
deeply nested. So, beginning with version 5.1.10.2, only the directories
themselves are searched. You can restore the pre-5.1.10.2 behavior by
setting AUTOMAKE=recursive, or
AUTOMAKE=integer, where integer specifies the
search depth. If your included files are in a separate directory, then
that directory must be added to CONFIG_PATH in order to allow AUTOMAKE
to work correctly.Upgrading Shorewall(FAQ 66) I'm trying to upgrade to Shorewall 4.x or later; which
of these packages do I need to install?Answer: Please see the upgrade issues.(FAQ 34) I am trying to upgrade to Shorewall 4.4 or later and I
can't find the shorewall-common, shorewall-shell and shorewall-perl
packages? Where are they?Answer:In Shorewall 4.4, the
shorewall-shell package was discontinued. The
shorewall-common and
shorewall-perl packages were combined to form a
single shorewall package. For further
information, please see the upgrade
issues..(FAQ 34a) I am trying to upgrade to Shorewall 4.4 and I'm getting
errors when I try to start Shorewall. Where can I find information about
these issues?Answer: Please see the upgrade issues.(FAQ 34b) I am trying to upgrade to Shorewall 4.4 and I'm seeing
warning messages when I try to start Shorewall. Where can I find
information about these issues?Answer: Please see the upgrade issues.(FAQ 76) I just upgraded my system and now masquerading doesn't
work? What happened?Answer: This happens to people
who ignore our advice and
allow the installer to replace their working
/etc/shorewall/shorewall.conf with one that has
default settings. Failure to forward traffic (such as during masqueraded
net access from a local network) usually means that /etc/shorewall/shorewall.conf
contains the default setting IP_FORWARDING=Keep; it should be
IP_FORWARDING=On.Update: Beginning with Shorewall
4.4.21, there is a shorewall update
command that does a smart merge of your existing shorewall.conf and the
new one.(FAQ 2 .6) After upgrading to the latest version (> 5.1.10.1)
of Shorewall, when I change my configuration and 'shorewall reload' or
'shorewall restart', my changes aren't in the running ruleset. Why is
that happening?Answer: See FAQ 1.5.Port Forwarding (Port Redirection)(FAQ 1) I want to forward UDP port 7777 to my personal PC with IP
address 192.168.1.5. I've looked everywhere and can't find how to do
it.Answer: The format of a
port-forwarding rule from the net to a local system
is as follows:#ACTION SOURCE DEST PROTO DPORT
DNAT net loc:local-IP-address[:local-port] protocolport-numberSo to forward UDP port 7777 to internal system 192.168.1.5, the
rule is:#ACTION SOURCE DEST PROTO DPORT
DNAT net loc:192.168.1.5 udp 7777If you want to forward requests directed to a particular address (
external-IP ) on your firewall to an internal
system:#ACTION SOURCE DEST PROTO DPORT SPORT ORIGDEST
DNAT net loc:local-IP-address>[:local-port] protocolport-number - external-IPIf you want to forward requests from a particular Internet address
( address ):#ACTION SOURCE DEST PROTO DPORT SPORT ORIGDEST
DNAT net:address loc:local-IP-address[:local-port] protocolport-number -Finally, if you need to forward a range of ports, in the DEST PORT
column specify the range as
low-port:high-port.The above does not work for forwarding
from the local network. If you want to do that, see FAQ 2.(FAQ 1a) Okay -- I followed those instructions but it doesn't
workAnswer: That is usually the
result of one of five things:You are trying to redirect a UDP port and there is already a
conntrack table entry for the flow, created via an ACCEPT
rule.Example: DNAT loc:192.168.0.2 dmz:192.168.1.3 udp 53Assuming that you have installed the
conntrack package, you can delete all such
conntrack table entries using: conntrack -D -s 192.168.0.2 -p udp --dport 53You are trying to test from inside your firewall (no, that
won't work -- see ).You have a more basic problem with your local system (the
one that you are trying to forward to) such as an incorrect
default gateway (it must be set to the IP address of your
firewall's internal interface; if that isn't possible for some
reason, see FAQ 1f).Your ISP is blocking that particular port inbound or, for
TCP, your ISP is dropping the outbound SYN,ACK response.You are running Mandriva Linux prior to 10.0 final and have
configured Internet Connection Sharing. In that case, the name of
your local zone is 'masq' rather than 'loc' (change all instances
of 'loc' to 'masq' in your rules). You may want to consider
re-installing Shorewall in a configuration which matches the
Shorewall documentation. See the two-interface QuickStart Guide for
details.(FAQ 1b) I'm still having problems with port forwardingAnswer: To further diagnose
this problem:As root, type shorewall reset ("shorewall-lite reset", if you are
running Shorewall Lite). This clears all Netfilter
counters.Try to connect to the redirected port from an external
host.As root type shorewall show nat ("shorewall-lite show nat", if you are
running Shorewall Lite).Locate the appropriate DNAT rule. It will be in a chain
called <source zone>_dnat
(net_dnat in the above examples).Is the packet count in the first column non-zero? If so, the
connection request is reaching the firewall and is being
redirected to the server. In this case, the problem is usually a
missing or incorrect default gateway setting on the local system
(the system you are trying to forward to -- its default gateway
must be the IP address of the firewall's interface to that system
unless you use the hack described in FAQ
1f).If the packet count is zero:the connection request is not reaching your server
(possibly it is being blocked by your ISP); oryou are trying to connect to a secondary IP address on
your firewall and your rule is only redirecting the primary IP
address (You need to specify the secondary IP address in the
ORIG. DEST. column in your DNAT rule);
oryour DNAT rule doesn't match the connection request in
some other way. In that case, you may have to use a packet
sniffer such as tcpdump or Wireshark to further diagnose the
problem.The traffic is entering your firewall on a different
interface (interfaces reversed in
/etc/shorewall/interfaces?).If the packet count is non-zero, check your log to see if
the connection is being dropped or rejected. If it is, then you
may have a zone definition problem such that the server is in a
different zone than what is specified in the DEST column. At a
root prompt, type "shorewall show zones"
("shorewall-lite show zones") then be sure that
in the DEST column you have specified the first zone in the list that matches
OUT=<dev> and DEST= <ip>from the REJECT/DROP log
message.If everything seems to be correct according to these tests
but the connection doesn't work, it may be that your ISP is
blocking SYN,ACK responses. This technique allows your ISP to
detect when you are running a server (usually in violation of your
service agreement) and to stop connections to that server from
being established.(FAQ 1c) From the Internet, I want to connect to port 1022 on
my firewall and have the firewall forward the connection to port 22 on
local system 192.168.1.3. How do I do that?Answer:In
/etc/shorewall/rules:#ACTION SOURCE DEST PROTO DPORT
DNAT net loc:192.168.1.3:22 tcp 1022(FAQ 1d) I have a web server in my DMZ and I use port
forwarding to make that server accessible from the Internet. That
works fine but when my local users try to connect to the server using
the Firewall's external IP address, it doesn't work.Answer: See FAQ 2b.(FAQ 1e) In order to discourage brute force attacks I would
like to redirect all connections on a non-standard port (4104) to port
22 on the router/firewall. I notice that setting up a REDIRECT rule
causes the firewall to open both ports 4104 and 22 to connections from
the net. Is it possible to only redirect 4104 to the localhost port 22
and have connection attempts to port 22 from the net dropped?On systems with the "Extended Conntrack Match"
(NEW_CONNTRACK_MATCH) capability (see the output of
shorewall show capabilities), port 22 is opened
only to connections whose original destination port is 4104 and this
FAQ does not apply.Answer courtesy of Ryan: Assume
that the IP address of your local firewall interface is 192.168.1.1.
If you configure SSHD to only listen on that address and add the
following rule, then you will have access on port 4104 from the net
and on port 22 from your LAN.#ACTION SOURCE DEST PROTO DPORT
DNAT net fw:192.168.1.1:22 tcp 4104(FAQ 1f) Why must the server that I port forward to have it's
default gateway set to my Shorewall system's IP address?Answer: Let's take an example.
Suppose thatYour Shorewall firewall's external IP address is
206.124.146.176 (eth0) and its internal IP address is 192.168.1.1
(eth1).You have another gateway router with external IP address
130.252.100.109 and internal IP address 192.168.1.254.You have an FTP server behind both routers with IP address
192.168.1.4The FTP server's default gateway is through the second
router (192.168.1.254).You have this rule on the Shorewall system:#ACTION SOURCE DEST PROTO DPORT SPORT ORIGDEST
DNAT net loc:192.168.1.4 tcp 21 - 206.124.146.176Internet host 16.105.221.4 issues the command ftp
206.124.146.176This results in the following sequence of events:16.105.221.4 sends a TCP SYN packet to 206.124.146.176
specifying destination port 21.The Shorewall box rewrites the destination IP address to
192.168.1.4 and forwards the packet.The FTP server receives the packet and accepts the
connection, generating a SYN,ACK packet back to 16.105.221.4.
Because the server's default gateway is through the second router,
it sends the packet to that router.At this point, one of two things can happen. Either the second
router discards or rejects the packet; or, it rewrites the source IP
address to 130.252.100.109 and forwards the packet back to
16.105.221.4. Regardless of which happens, the connection is doomed.
Clearly if the packet is rejected or dropped, the connection will not
be successful. But even if the packet reaches 16.105.221.4, that host
will reject it since it's SOURCE IP address (130.252.100.109) doesn't
match the DESTINATION IP ADDRESS (206.124.146.176) of the original SYN
packet.The best way to work around this problem is to change the
default gateway on the FTP server to the Shorewall system's internal
IP address (192.168.1.1). But if that isn't possible, you can work
around the problem with the following ugly hack in
/etc/shorewall/masq:#INTERFACE SOURCE ADDRESS PROTO PORT
eth1:192.168.1.4 0.0.0.0/0 192.168.1.1 tcp 21When running Shorewall 5.0.14 or later, the eqivalent
/etc/shorewall/snat file is:#ACTION SOURCE DEST PROTO PORT
SNAT(192.168.1.1) 0.0.0.0/0 eth1:192.168.1.4 tcp 21This rule has the undesirable side effect of making all FTP
connections from the net appear to the FTP server as if they
originated on the Shorewall system. But it will force the FTP server
to reply back through the Shorewall system who can then rewrite the
SOURCE IP address in the responses properly.(FAQ 1g) I would like to redirect port 80 on my public IP
address (206.124.146.176) to port 993 on Internet host
66.249.93.111Answer: This requires a vile
hack similar to the one in FAQ 2. Assuming
that your Internet zone is named net and connects
on interface eth0:In /etc/shorewall/rules:#ACTION SOURCE DEST PROTO DPORT SPORT ORIGDEST
?SECTION ALL
?SECTION ESTABLISHED
?SECTION RELATED
?SECTION INVALID
?SECTION UNTRACKED
?SECTION NEW
DNAT net net:66.249.93.111:993 tcp 80 - 206.124.146.176In /etc/shorewall/interfaces, specify the
routeback option on
eth0:?FORMAT 2
#ZONE INTERFACE OPTIONS
net eth0 routeback/etc/shorewall/masq;#INTERFACE SOURCE ADDRESS PROTO PORT
eth0:66.249.93.111 0.0.0.0/0 206.124.146.176 tcp 993When running Shorewall 5.0.14 or later, the equivalent
/etc/shorewall/snat file is:#ACTION SOURCE DEST PROTO PORT
SNAT(206.124.146.176) 0.0.0.0/0 eth0:66.249.93.111 tcp 993and in
/etc/shorewall/shorewall.conf:IP_FORWARDING=OnLike the hack in FAQ 2, this one results in all forwarded
connections looking to the server (66.249.93.11) as if they originated
on your firewall (206.124.146.176).(FAQ 1h) How do I set shorewall to allow ssh on port 9022 from
net? SSHD is listening on port 22.Answer: Use this rule.#ACTION SOURCE DEST PROTO DPORT
REDIRECT net 22 tcp 9022Note that the above rule will also allow connections from the
net on TCP port 22. If you don't want that, see FAQ 1e.(FAQ 1j) Why doesn't this DNAT rule work?I added this rule but I'm still seeing the log message
belowRULE:
DNAT scnet:172.19.41.2 dmz0:10.199.198.145 udp 2055
LOG:
Sep 21 12:55:37 fw001 kernel: [10357687.114928] Shorewall:scnet2fw:DROP:IN=eth2 OUT=
MAC=00:26:33:dd:aa:05:00:24:f7:19:ce:44:08:00 SRC=172.19.41.2 DST=172.19.1.1 LEN=1492
TOS=0x00 PREC=0x00 TTL=63 ID=23035 PROTO=UDP SPT=6376 DPT=2055 LEN=1472Answer: There was already a
conntrack entry for the failing connection before you added the rule.
Install the conntrack utility program
and use it to delete the entry.conntrack -D -s 172.19.41.2 -d 172.19.1.1 -p udp -sport 6367 -dport 2055 (FAQ 30) I'm confused about when to use DNAT rules and when to
use ACCEPT rules.Answer: It would be a good idea
to review the QuickStart
Guide appropriate for your setup; the guides cover this topic in
a tutorial fashion. DNAT rules should be used for connections that need
to go the opposite direction from SNAT/MASQUERADE. So if you masquerade
or use SNAT from your local network to the Internet then you will need
to use DNAT rules to allow connections from the Internet to your local
network.If you use both 1:1 NAT and SNAT/MASQUERADE, those connections
that are subject to 1:1 NAT should use ACCEPT rather than DNAT.
Note, however, that DNAT can be used to override 1:1 NAT so as to
redirect a connection to a different internal system or port than
would be the case using 1:1 NAT. You also want to use DNAT rules when you intentionally want to
rewrite the destination IP address or port number. In all other cases,
you use ACCEPT unless you need to hijack connections as they go through
your firewall and handle them on the firewall box itself; in that case,
you use a REDIRECT rule.The preceding answer should not be
interpreted to mean that DNAT can only be used in conjunction with
SNAT. But in common configurations using private local addresses, that
is the most common usage.(FAQ 8) I have several external IP addresses and use
/etc/shorewall/nat to associate them with systems in my DMZ. When I add
a DNAT rule, say for ports 80 and 443, Shorewall redirects connections
on those ports for all of my addresses. How can I restrict DNAT to only
a single address?Answer: Specify the external
address that you want to redirect in the ORIGDEST column.Example:#ACTION SOURCE DEST PROTO DPORT SPORT ORIGDEST
DNAT net net:192.168.4.22 tcp 80,443 - 206.124.146.178(FAQ 38) Where can I find more information about DNAT?Answer: Ian Allen has written a
Paper about DNAT and
Linux.(FAQ 48) How do I Set up a Transparent HTTP Proxy with
Shorewall?Answer: See Shorewall_Squid_Usage.html.DNS and Port Forwarding/NAT(FAQ 2) I port forward www requests to www.mydomain.com (IP
130.151.100.69) to system 192.168.1.5 in my local network. External
clients can browse http://www.mydomain.com but internal clients
can't.Answer: I have two objections to
this setup.Having an Internet-accessible server in your local network is
like raising foxes in the corner of your hen house. If the server is
compromised, there's nothing between that server and your other
internal systems. For the cost of another NIC and a cross-over
cable, you can put your server in a DMZ such that it is isolated
from your local systems - assuming that the Server can be located
near the Firewall, of course :-)The accessibility problem is best solved using
Split DNS (either use a separate DNS server for local
clients or use Bind
Version 9 views on your main name server)
such that www.mydomain.com resolves to 130.141.100.69 externally and
192.168.1.5 internally. I use a separate DNS server (dnsmasq) here
at shorewall.net.So the best and most secure way to solve this problem is to move
your Internet-accessible server(s) to a separate LAN segment with it's
own interface to your firewall and follow FAQ
2b. That way, your local systems are still safe if your server
gets hacked and you don't have to run a split DNS configuration
(separate server or Bind 9 views).If physical limitations make it impractical to segregate your
servers on a separate LAN, the next best solution it to use Split DNS.
Before you complain "It's too hard to set up split DNS!", check
here.If you really want to route traffic between two internal systems
through your firewall, then proceed as described below.All traffic redirected through use of this technique will look
to the server as if it originated on the firewall rather than on the
original client! So the server's access logs will be useless for
determining which local hosts are accessing the server.Assuming that your external interface is eth0 and your internal
interface is eth1 and that eth1 has IP address 192.168.1.254 with subnet
192.168.1.0/24, then:In /etc/shorewall/interfaces:?FORMAT 2
#ZONE INTERFACE OPTIONS
loc eth1 routebackIn /etc/shorewall/masq:#INTERFACE SOURCE ADDRESS PROTO PORT
eth1:192.168.1.5 192.168.1.0/24 192.168.1.254 tcp wwwWhen running Shorewall 5.0.14 or later, the corresponding
/etc/shorewall/snat file is:#ACTION SOURCE DEST PROTO PORT
SNAT(192.168.1.254) 192.168.1.0/24 eth1:192.168.1.5 tcp wwwNote: The technique described here is known as
hairpinning NAT and is described in section 6
of RFC
4787. In that RFC, it is required that the
external IP address be used as the
source:#INTERFACE SOURCE ADDRESS PROTO PORT
eth1:192.168.1.5 192.168.1.0/24 130.151.100.69 tcp wwwEquivalent /etc/shorewall/snat:#ACTION SOURCE DEST PROTO PORT
SNAT(130.151.100.69) 192.168.1.0/24 eth1:192.168.1.5 tcp wwwIn /etc/shorewall/rules:#ACTION SOURCE DEST PROTO DPORT SPORT ORIGDEST
?SECTION ALL
?SECTION ESTABLISHED
?SECTION RELATED
?SECTION INVALID
?SECTION UNTRACKED
?SECTION NEW
DNAT loc loc:192.168.1.5 tcp www - 130.151.100.69That rule (and the second one in the previous bullet) only
works of course if you have a static external IP address. If you
have a dynamic IP address then make your DNAT rule:#ACTION SOURCE DEST PROTO DPORT SPORT ORIGDEST
?SECTION ALL
?SECTION ESTABLISHED
?SECTION RELATED
?SECTION INVALID
?SECTION UNTRACKED
?SECTION NEW
DNAT loc loc:192.168.1.5 tcp www - ð0Using this technique, you will want to configure your
DHCP/PPPoE/PPTP/… client to automatically reload Shorewall each time
that you get a new IP address.If your local interface is a bridge, see FAQ 2e for additional configuration
steps.(FAQ 2a) I have a zone <quote>Z</quote> with an RFC1918 subnet
and I use one-to-one NAT to assign non-RFC1918 addresses to hosts in
Z. Hosts in Z cannot communicate with each other using their external
(non-RFC1918 addresses) so they can't access each other using their
DNS names.If the ALL INTERFACES column in /etc/shorewall/nat is empty or
contains Yes, you will also see log messages like the
following when trying to access a host in Z from another host in Z
using the destination host's public address:Oct 4 10:26:40 netgw kernel:
Shorewall:FORWARD:REJECT:IN=eth1 OUT=eth1 SRC=192.168.118.200
DST=192.168.118.210 LEN=48 TOS=0x00 PREC=0x00 TTL=127 ID=1342 DF
PROTO=TCP SPT=1494 DPT=1491 WINDOW=17472 RES=0x00 ACK SYN URGP=0Answer: This is another problem
that is best solved using split DNS. It allows both external and
internal clients to access a NATed host using the host's DNS
name.Another good way to approach this problem is to switch from
one-to-one NAT to Proxy ARP. That way, the hosts in Z have non-RFC1918
addresses and can be accessed externally and internally using the same
address.If you don't like those solutions and prefer to route all
Z->Z traffic through your firewall then:Set the routeback option on the interface to Z.Set the ALL INTERFACES column in the nat file to
Yes.Example:Zone: dmz, Interface: eth2, Subnet: 192.168.2.0/24, Address of server 192.168.2.2In /etc/shorewall/interfaces:?FORMAT 2
#ZONE INTERFACE OPTIONS
dmz eth2 routebackIn /etc/shorewall/masq:#INTERFACE SOURCE
eth2:192.168.1.2 192.168.2.0/24When running Shorewall 5.0.14 or later, the equivalent
/etc/shorewall/snat is:#ACTION SOURCE DEST PROTO PORT
MASQUERADE 192.168.1.0/24 eth2:192.168.1.2 tcp wwwIn /etc/shorewall/nat, be sure that you
have Yes in the ALL INTERFACES column.(FAQ 2b) I have a web server in my DMZ and I use port
forwarding to make that server accessible from the Internet as
www.mydomain.com. That works fine but when my local users try to
connect to www.mydomain.com, it doesn't work.Answer: Let's assume the
following:External IP address is 206.124.146.176 on eth0 (www.mydomain.com).Server's IP address is 192.168.2.4You can enable access to the server from your local network
using the firewall's external IP address by adding this rule:#ACTION SOURCE DEST PROTO DPORT SPORT ORIGDEST
?SECTION ALL
?SECTION ESTABLISHED
?SECTION RELATED
?SECTION INVALID
?SECTION UNTRACKED
?SECTION NEW
DNAT loc dmz:192.168.2.4 tcp 80 - 206.124.146.176If your external IP address is dynamic, then you must make your
DNAT rule:#ACTION SOURCE DEST PROTO DPORT SPORT ORIGDEST
?SECTION ALL
?SECTION ESTABLISHED
?SECTION RELATED
?SECTION INVALID
?SECTION UNTRACKED
?SECTION NEW
DNAT loc dmz:192.168.2.4 tcp 80 - ð0With dynamic IP addresses, you probably don't want to use
shorewall[-lite]
save and shorewall[-lite]
restore.(FAQ 2c) I tried to apply the answer to FAQ 2 to my external
interface and the net zone and it didn't work. Why?Answer: Did you set IP_FORWARDING=On in
shorewall.conf?(FAQ 2d) Does Shorewall support hairpinning NAT?Answer: Yes.In the case of simple masquerade/SNAT, see FAQ 2.For one-to-one (static), NAT, simply place 'Yes' in the ALL
INTERFACES column of each entry in /etc/shorewall/nat.(FAQ 2e) I have the situation in FAQ 2 but my local interface
is a bridge and the solution in FAQ 2 doesn't workAnswer: Assume that the bridge
is br0 and that eth2 is the bridge port that connects to the LAN
containing 192.168.1.5In addition to the steps in FAQ 2 (replacing eth1 with br0), you
also need to:Set the hairpin option on
eth2.brctl hairpin br0 eth2 onOn Debian and derivitives, you can place that command in
/etc/network/interfaces as a post-up command:auto br0
iface br0 inet static
bridge_ports eth2
bridge_fd 0
bridge_maxwait 0
address 192.168.1.1
netmask 255.255.255.0
post-up /sbin/brctl hairpin br0 eth2 onInstall ebtables if it is not already installed.Be sure that all traffic going out of eth2 has the correct
MAC address.ebtables -t nat -A POSTROUTING -o eth2 -j snat --to-source br0-MAC-addresswhere br0-MAC-address is the MAC address of br0.Here's a working example of /etc/shorewall/start that
executes the above command.if [ $(ebtables -t nat -L POSTROUTING | wc -l) -lt 4 ]; then
ebtables -t nat -A POSTROUTING -o eth2 -j snat --to-source 0:19:21:d0:61:65
fiBlacklisting(FAQ 63) I just blacklisted IP address 206.124.146.176 and I can
still ping it. What did I do wrong?Answer: Nothing.Blacklisting an IP address blocks incoming traffic from that IP
address. And if you set BLACKLISTNEWONLY=Yes in
shorewall.conf, then only new connections from that address are disallowed; traffic from
that address that is part of an established connection (such as ping
replies) is allowed.Beginning with Shorewall 4.4.13, you can use the
option in /etc/shorewall/interfaces
to implement blacklisting by destination IP address.Beginning with Shorewall 4.4.26, you can use /etc/shorewall/blrules
to implement arbitrary blacklist rules.(FAQ 84) I put some IPs in the blacklist file in /etc/shorewall
to block the ips but i'm still getting reports from PSAD from those ips
saying they're port scanning. Shouldn't being on the blacklist drop all
packets from those ips?Answer: You probably forgot to
specify the blacklist option for your
external interface(s) in /etc/shorewall/interfaces.Netmeeting/MSN(FAQ 3) I want to use Netmeeting or MSN Instant Messenger with
Shorewall. What do I do?Answer: There is an H.323
connection tracking/NAT module that helps with
Netmeeting.Look here for a solution for MSN IM
but be aware that there are significant security risks involved with
this solution. Also check the Netfilter mailing list archives at http://www.netfilter.org.Open Ports(FAQ 100) With Shorewall started, the output of 'iptables -L'
looks like my firewall is wide open!Answer: The problem here is that
a bare iptables -L command produces totally useless
output. Use shorewall show instead.The shorewall show command is a wrapper
around iptables -L -n -v.(FAQ 51) How do I Open Ports in Shorewall?Answer: No one who has installed
Shorewall using one of the Quick Start Guides should
have to ask this question.Regardless of which guide you used, all outbound communication is
open by default. So you do not need to 'open ports' for output.For input:If you installed using the Standalone Guide, then please
re-read this
section.If you installed using the Two-interface Guide, then please
re-read these sections: Port
Forwarding (DNAT), and Other ConnectionsIf you installed using the Three-interface Guide, then please
re-read these sections: Port
Forwarding (DNAT) and Other ConnectionsIf you installed using the Shorewall Setup Guide then
you had better read the guide again -- you clearly missed a
lot.Also please see the Port Forwarding
section of this FAQ.(FAQ 4) I just used an online port scanner to check my firewall
and it shows some ports as <quote>closed</quote> rather than
<quote>blocked</quote>. Why?Answer: The default Shorewall
setup invokes the Drop action prior to
enforcing a DROP policy and the default policy to all zones from the
Internet is DROP. The Drop action is defined in
/usr/share/shorewall/action.Drop which in turn
invokes the Auth macro (defined in
/usr/share/shorewall/macro.Auth) specifying the
REJECT action (i.e., Auth(REJECT)). This is necessary to prevent
outgoing connection problems to services that use the
Auth mechanism for identifying requesting users. That is
the only service which the default setup rejects.If you are seeing closed TCP ports other than 113 (auth) then
either you have added rules to REJECT those ports or a router outside of
your firewall is responding to connection requests on those
ports.If you would prefer to 'stealth' port 113, then:If you are running Shorewall 4.4.20 or earlier, copy
/usr/share/shorewall/action.Drop to
/etc/shorewall/ and modify the invocation of
Auth to Auth(DROP).If you are running Shorewall 4.4.21 or later, in
shorewall.conf, set DROP_DEFAULT="Drop(-,DROP)". See the Action HOWTO to learn how that magic
works.(FAQ 4a) I just ran an nmap UDP scan of my firewall and it
showed 100s of ports as open!!!!Answer: Take a deep breath and
read the nmap manpage section about UDP scans. If nmap gets nothing back from your firewall then it reports
the port as open. If you want to see which UDP ports are really open,
temporarily change your net->all policy to REJECT, restart
Shorewall and run the nmap UDP scan again.(FAQ 4b) I have a port that I can't close no matter how I
change my rules.I had a rule that allowed telnet from my local network to my
firewall; I removed that rule and restarted Shorewall but my telnet
session still works!!!Answer: Rules only govern the
establishment of new connections. Once a connection is established
through the firewall it will be usable until disconnected (tcp) or
until it times out (other protocols). If you stop telnet and try to
establish a new session your firewall will block that attempt.(FAQ 4c) How do I use Shorewall with PortSentry?Answer: Here's a writeup describing a
nice integration of Shorewall and PortSentry.Connection ProblemsWhy are these packets being Dropped/Rejected? How do I decode
Shorewall log messages?Please see FAQ 17.(FAQ 5) I've installed Shorewall and now I can't ping through the
firewallAnswer: For a complete
description of Shorewall ping management, see this page.(FAQ 15) My local systems can't see out to the netAnswer: Every time I read
systems can't see out to the net, I wonder where the
poster bought computers with eyes and what those computers will
see when things are working properly :-). That aside, the
most common causes of this problem are:The default gateway on each local system isn't set to the IP
address of the local firewall interface. You can test this
by:At a root shell prompt, type 'shorewall clear'.From a local system, attempt to ping the IP address of the
Shorewall system's internet (external) interface. If that
doesn't work, then the default gateway on the system from which
you pinged is not set correctly.Be sure to 'shorewall start' after the test.The entry for the local network in the
/etc/shorewall/masq file is wrong or
missing.The DNS settings on the local systems are wrong or the user is
running a DNS server on the firewall and hasn't enabled UDP and TCP
port 53 from the local net to the firewall or from the firewall to
the Internet.Forwarding is not enabled (This is often the problem for
Debian users). Enter this command:cat /proc/sys/net/ipv4/ip_forwardIf the value displayed is 0 (zero) then set IP_FORWARDING=On in
/etc/shorewall/shorewall.conf and restart
Shorewall.(FAQ 29) FTP Doesn't WorkAnswer: See the Shorewall and FTP page.(FAQ 33) From clients behind the firewall, connections to some
sites fail. Connections to the same sites from the firewall itself work
fine. What's wrong?Answer: Most likely, you need to
set CLAMPMSS=Yes in /etc/shorewall/shorewall.conf.(FAQ 35) I have two Ethernet interfaces to my local network which
I have bridged. When Shorewall is started, I'm unable to pass traffic
through the bridge. I have defined the bridge interface (br0) as the
local interface in <filename>/etc/shorewall/interfaces</filename>; the
bridged Ethernet interfaces are not defined to Shorewall. How do I tell
Shorewall to allow traffic through the bridge?Answer: Add the
option to br0 in /etc/shorewall/interfaces.For more information on this type of configuration, see the Shorewall Simple Bridge
documentation.(FAQ 64) I just upgraded my kernel to 2.6.20 (or later) and my
bridge/firewall stopped working. What is wrong?Answer: In kernel 2.6.20, the
Netfilter physdev match feature was changed such
that it is no longer capable of matching the output device of
non-bridged traffic. You will see messages such as the following in your
log:Apr 20 15:03:50 wookie kernel: [14736.560947] physdev match: using --physdev-out in the OUTPUT, FORWARD and POSTROUTING chains for
non-bridged traffic is not supported anymore.This kernel change, while necessary, means that Shorewall zones
may no longer be defined in terms of bridge ports. See the Shorewall-perl bridging
documentation for information about how to configure
bridge/firewalls.Following the instructions in the new bridging documentation
will not prevent the above message from being issued.(FAQ 85) Shorewall is rejecting connections from my local lan
because it thinks they are coming from the 'net' zone.I'm seeing this in my log:Aug 31 16:51:24 fw22 kernel: Shorewall:net2fw:DROP:IN=eth5 OUT= MAC=00:0c:29:74:9c:0c:08:00:20:b2:5f:db:08:00
SRC=10.1.50.14 DST=10.1.50.7 LEN=57 TOS=0x00 PREC=0x00 TTL=255 ID=32302 DF
PROTO=UDP SPT=53289 DPT=53 LEN=37Answer: This occurs when the
external interface and an internal interface are connected to the same
switch or hub. See this article
for details. The solution is to never connect more than one firewall
interface to the same hub or switch (an obvious exception is that when
you have a switch that supports VLAN tagging and the interfaces are
associated with different VLANs).Logging(FAQ 91) I changed the shorewall.conf file in /etc/shorewall/ to
spit out logs to /var/log/shorewall.log and it's not happening after I
restart shorewall. LOGFILE=/var/log/shorewall.log <-- that should be
the correct line, right?Answer: No, that is not correct.
The LOGFILE setting tells Shorewall where to find the log; it does not
determine where messages are written. See the next
FAQ.(FAQ 6) Where are the log messages written and how do I change
the destination?Answer: NetFilter uses the
kernel's equivalent of syslog (see man syslog) to log
messages. It always uses the LOG_KERN (kern) facility (see man
openlog) and you get to choose the log level (again, see
man syslog) in your policies and
rules. The
destination for messages logged by syslog is controlled by
/etc/syslog.conf (see man
syslog.conf). When you have changed
/etc/syslog.conf, be sure to restart syslogd (on a
RedHat system, service syslog restart).It is also possible to set up
Shorewall to log all of Netfilter's messages to a separate
file.(FAQ 6a) Are there any log parsers that work with
Shorewall?Answer: Here are several links
that may be helpful:https://shorewall.org/pub/shorewall/parsefw/http://aaron.marasco.com/linux.htmlhttp://cert.uni-stuttgart.de/projects/fwlogwatchhttp://www.logwatch.orgI personally use fwlogwatch.
It emails me a report each day from my various systems with each
report summarizing the logged activity on the corresponding system;
here's a sample:
fwlogwatch summary
Generated Tuesday March 02 08:14:37 PST 2010 by root.
362 (and 455 older than 86400 seconds) of 817 entries in the file "/var/log/ulog/syslogemu.log" are packet logs, 138 have unique characteristics.
First packet log entry: Mar 01 08:16:06, last: Mar 02 08:06:21.
All entries were logged by the same host: "gateway".
All entries have the same target: "-".
Only entries with a count of at least 5 are shown.
net-dmz DROP eth2 36 packets from 61.158.162.9 to 206.124.146.177
net-fw DROP eth0 21 packets from 89.163.162.13 to 76.104.233.98
net-fw DROP eth0 19 packets from 61.184.101.46 to 76.104.233.98
net-fw DROP eth0 12 packets from 81.157.214.103 to 76.104.233.98
net-fw DROP eth0 11 packets from 174.37.159.222 to 76.104.233.98
net-fw DROP eth0 10 packets from 221.195.73.86 to 76.104.233.98
net-dmz DROP eth2 9 packets from 202.199.158.6 to 206.124.146.177
net-fw DROP eth2 9 packets from 202.199.158.6 to 206.124.146.176
net-dmz DROP eth2 9 packets from 202.199.158.6 to 206.124.146.178
net-fw DROP eth0 6 packets from 221.192.199.35 to 76.104.233.98
net-fw DROP eth2 5 packets from 61.158.162.9 to 206.124.146.177
Fwlogwatch contains a built-in web server that allows monitoring
recent activity in summary fashion.(FAQ 6b) DROP messages on port 10619 are flooding the logs with
their connect requests. Can I exclude these error messages for this
port temporarily from logging in Shorewall?Answer: Temporarily add the
following rule:#ACTION SOURCE DEST PROTO DPORT
?SECTION ALL
?SECTION ESTABLISHED
?SECTION RELATED
?SECTION INVALID
?SECTION UNTRACKED
?SECTION NEW
DROP net $FW udp 10619Alternatively, if you do not set BLACKLIST_LOGLEVEL you can
blacklist the port. In
/etc/shorewall/blrules:#ACTION SOURCE DEST PROTO DPORT
DROP net $FW udp 10619(FAQ 6d) Why is the MAC address in Shorewall log messages so
long? I thought MAC addresses were only 6 bytes in length.Answer: What is labeled as the
MAC address in a Netfilter (Shorewall) log message is actually the
Ethernet frame header. It contains:the destination MAC address (6 bytes)the source MAC address (6 bytes)the Ethernet frame type (2 bytes)ExampleMAC=00:04:4c:dc:e2:28:00:b0:8e:cf:3c:4c:08:00Destination MAC address = 00:04:4c:dc:e2:28Source MAC address = 00:b0:8e:cf:3c:4cEthernet Frame Type = 08:00 (IP Version 4)(FAQ 16) Shorewall is writing log messages all over my console
making it unusable!Answer:Just to be clear, it is not Shorewall that is writing all over
your console. Shorewall issues a single log message during each
start, restart,
stop, etc. It is rather your logging daemon that is
writing messages to your console. Shorewall itself has no control over
where a particular class of messages are written. See the Shorewall logging
documentation.The max log level to be sent to the console is available in
/proc/sys/kernel/printk:teastep@ursa:~$ cat /proc/sys/kernel/printk
6 6 1 7
teastep@ursa:~$ The first number determines the maximum log
level (syslog priority) sent to the console. Messages with priority
less than this number are sent to the
console. On the system shown in the example above, priorities 0-5 are
sent to the console. Since Shorewall defaults to using 'info' (6), the
Shorewall-generated Netfilter rule set will generate log messages that
will not appear on the console.The second number is the default log level for kernel printk()
calls that do not specify a log level.The third number specifies the minimum console log level while the
fourth gives the default console log level.If, on your system, the first number is 7 or greater, then the
default Shorewall configurations will cause messages to be written to
your console. The simplest solution is to add this to your
/etc/sysctl.conf file:kernel.printk = 4 4 1 7thensysctl -p /etc/sysctl.conf(FAQ 16a) cat /proc/sys/kernel/prink returns '4 4 1 7' and
still I get dmesg filled upAnswer: While we would argue
that 'dmesg filled up' is not necessarily a problem, the only way to
eliminate that is to set up
Shorewall to log all of Netfilter's messages to a separate
file.(FAQ 16b) Why can't I see any Shorewall messages in
/var/log/messages?Some people who ask this question report that the only Shorewall
messages that they see in /var/log/messages are
'started', 'restarted' and 'stopped' messages.Answer: First of all, it is
important to understand that Shorewall itself does not control where
Netfilter log messages are written. The LOGFILE setting in
shorewall.conf simply tells the
/sbin/shorewall[-lite] program where to look for
the log. Also, it is important to understand that a log level of
"debug" will generally cause Netfilter messages to be written to fewer
files in /var/log than a log
level of "info". The log level does not control the number of log
messages or the content of the messages.The actual log file where Netfilter messages are written is not
standardized and will vary by distribution and distribution version.
But anytime you see no logging, it's time to look outside the
Shorewall configuration for the cause. As an example, recent
SUSE releases use syslog-ng by default and
write Shorewall messages to
/var/log/firewall.Please see the Shorewall
logging documentation for further information.(FAQ 16c) Shorewall messages are flooding the output of
'dmesg'; how to I stop that?Answer: Switch to using ulogd.(FAQ 16d) I set LOGFILE=/var/log/shorewall but log messages are
still going to /var/log/messages.Answer: See the answer to FAQ 16b above.(FAQ 17) Why are these packets being Dropped/Rejected? How do I
decode Shorewall log messages?Answer: Logging of
dropped/rejected packets occurs out of a number of chains (as indicated
in the log message) in Shorewall:zone2all,
zone-all,
all2zone,
all-zone, all2all or
all-allYou have a policy
that specifies a log level and this packet is being logged under
that policy. If you intend to ACCEPT this traffic then you need a
rule to that
effect.Packets logged out of these chains may have a source and/or
destination that is not in any defined zone (see the output of
shorewall[-lite] show zones). Remember that
zone membership involves both a firewall interface and an ip
address.zone12zone2
or zone1-zone2Either you have a policy for
zone1 to zone2 that
specifies a log level and this packet is being logged under that
policy or this packet matches a rule that includes a
log level.@zone12zone2
or
@zone1-zone2You have a policy for traffic from
zone1 to
zone2 that specifies TCP connection
rate limiting (value in the LIMIT column). The logged packet
exceeds that limit and was dropped. Note that these log messages
themselves are severely rate-limited so that a syn-flood won't
generate a secondary DOS because of excessive log message. These
log messages were added in Shorewall 2.2.0 Beta 7.zone12zone2~,
zone1-zone2~
or ~blacklistnnThese are the result of entries in the /etc/shorewall/blrules
file.interface_mac or
interface_recThe packet is being logged under the maclistinterface
option.blacklistThe packet is being logged because the source IP is
blacklisted in the /etc/shorewall/blacklist file.INPUT or FORWARDThe packet has a source IP address that isn't in any of your
defined zones (shorewall[-lite] show
zones and look at the printed zone definitions)
or the chain is FORWARD and the destination IP isn't in any of
your defined zones. If the chain is FORWARD and the IN and OUT
interfaces are the same or they match the same wildcard entry in
/etc/shorewall/interfaces,
then you probably need the routeback option on that interface
in/etc/shorewall/interfaces, you need the routeback option in the relevant entry in
/etc/shorewall/hosts
or you've done something silly like define a default route out of
an internal interface.With OPTIMIZE=1 in shorewall.conf, such
packets may also be logged out of a <zone>2all chain or the
all2all chain.OUTPUTThe packet has a destination IP address that isn't in any of
your defined zones(shorewall[-lite] show zones
and look at the printed zone definitions).With OPTIMIZE=1 in shorewall.conf, such
packets may also be logged out of the fw2all chain or the all2all
chain.logflagsThe packet is being logged because it failed the checks
implemented by the tcpflagsinterface
option.sfilterOn systems running Shorewall 4.4.20 or later, either the
packet matched the interface option
or it is being routed out of the same interface on which it
arrived and the interface does not have the
or interface
option.Here is an example:Jun 27 15:37:56 gateway kernel:
Shorewall:all2all:REJECT:IN=eth2OUT=eth1SRC=192.168.2.2DST=192.168.1.3 LEN=67 TOS=0x00 PREC=0x00 TTL=63 ID=5805 DF PROTO=UDP
SPT=1803 DPT=53 LEN=47Let's look at the important parts of this message:all2all:REJECTThis packet was REJECTed out of the all2all chain -- the packet was rejected
under the all->all REJECT
policy (all2all above).IN=eth2the packet entered the firewall via eth2. If you see
IN= with no interface name, the packet originated
on the firewall itself.OUT=eth1if accepted, the packet would be sent on eth1. If you see
OUT= with no interface name, the packet would be
processed by the firewall itself.When a DNAT rule is logged, there will never be an OUT=
shown because the packet is being logged before it is routed.
Also, DNAT logging will show the original
destination IP address and destination port number. When a
REDIRECT rule is logged, the message will also show the
original destination IP address and port number.SRC=192.168.2.2the packet was sent by 192.168.2.2DST=192.168.1.3the packet is destined for 192.168.1.3PROTO=UDPUDP ProtocolDPT=53The destination port is 53 (DNS)In this case, 192.168.2.2 was in the dmz zone and
192.168.1.3 is in the loc zone. I was missing the
rule:ACCEPT dmz loc udp 53(FAQ 21) I see these strange log entries occasionally; what are
they?Nov 25 18:58:52 linux kernel:
Shorewall:net2all:DROP:IN=eth1 OUT=
MAC=00:60:1d:f0:a6:f9:00:60:1d:f6:35:50:08:00 SRC=206.124.146.179
DST=192.0.2.3 LEN=56 TOS=0x00 PREC=0x00 TTL=110 ID=18558 PROTO=ICMPTYPE=3 CODE=3 [SRC=192.0.2.3 DST=172.16.1.10 LEN=128 TOS=0x00 PREC=0x00
TTL=47 ID=0 DF PROTO=UDP SPT=53 DPT=2857 LEN=108 ]192.0.2.3 is external on my firewall... 172.16.0.0/24 is my
internal LANAnswer: First of all, please note
that the above is a very specific type of log message dealing with ICMP
port unreachable packets (PROTO=ICMP TYPE=3 CODE=3). Do not read this
answer and assume that all Shorewall log messages have something to do
with ICMP (hint -- see FAQ 17).While most people associate the Internet Control Message Protocol
(ICMP) with ping, ICMP is a key piece of IP. ICMP is used
to report problems back to the sender of a packet; this is what is
happening here. Unfortunately, where NAT is involved (including SNAT,
DNAT and Masquerade), there are many broken implementations. That is
what you are seeing with these messages. When Netfilter displays these
messages, the part before the "[" describes the ICMP packet and the part
between the "[" and "]" describes the packet for which the ICMP is a
response.Here is my interpretation of what is happening -- to confirm this
analysis, one would have to have packet sniffers placed a both ends of
the connection.Host 172.16.1.10 behind NAT gateway 206.124.146.179 sent a UDP DNS
query to 192.0.2.3 and your DNS server tried to send a response (the
response information is in the brackets -- note source port 53 which
marks this as a DNS reply). When the response was returned to to
206.124.146.179, it rewrote the destination IP TO 172.16.1.10 and
forwarded the packet to 172.16.1.10 who no longer had a connection on
UDP port 2857. This causes a port unreachable (type 3, code 3) to be
generated back to 192.0.2.3. As this packet is sent back through
206.124.146.179, that box correctly changes the source address in the
packet to 206.124.146.179 but doesn't reset the DST IP in the original
DNS response similarly. When the ICMP reaches your firewall (192.0.2.3),
your firewall has no record of having sent a DNS reply to 172.16.1.10 so
this ICMP doesn't appear to be related to anything that was sent. The
final result is that the packet gets logged and dropped in the all2all
chain.(FAQ 52) When I blacklist an IP address with "shorewall[-lite]
drop www.xxx.yyy.zzz", why does my log still show REDIRECT and DNAT
entries from that address?I blacklisted the address 130.252.100.59 using shorewall
drop 130.252.100.59 but I am still seeing these log
messages:Jan 30 15:38:34 server Shorewall:net_dnat:REDIRECT:IN=eth1 OUT= MAC=00:4f:4e:14:97:8e:00:01:5c:23:24:cc:08:00
SRC=130.252.100.59 DST=206.124.146.176 LEN=64 TOS=0x00 PREC=0x00 TTL=43 ID=42444 DF
PROTO=TCP SPT=2215 DPT=139 WINDOW=53760 RES=0x00 SYN URGP=0Answer: Please refer to the
Shorewall Netfilter
Documentation. Logging of REDIRECT and DNAT rules occurs in the
nat table's PREROUTING chain where the original destination IP address
is still available. Blacklisting occurs out of the filter table's INPUT
and FORWARD chains which aren't traversed until later.(FAQ 81) logdrop and logreject don't log.I love the ability to type 'shorewall logdrop ww.xx.yy.zz' and
completely block a particular IP address. However, the log part doesn't
happen. When I look in the logdrop chain, there is no LOG prefix.Answer: You haven't set a value
for BLACKLIST_LOGLEVEL in shorewall.conf (5).(FAQ 36) My log is filling up with these BANDWIDTH
messages!Dec 15 16:47:30 heath-desktop kernel: [17182740.184000] BANDWIDTH_IN:IN=eth1 OUT= MAC=ff:ff:ff:ff:ff:ff:00:01:5c:23:79:02:08:00
SRC=10.119.248.1 DST=255.255.255.255 LEN=328 TOS=0x00 PREC=0x00 TTL=64
ID=62081 PROTO=UDP SPT=67 DPT=68 LEN=308
Dec 15 16:47:30 heath-desktop last message repeated 2 times
Dec 15 16:47:30 heath-desktop kernel: [17182740.188000] BANDWIDTH_IN:IN=eth1 OUT= MAC=ff:ff:ff:ff:ff:ff:00:01:5c:23:79:02:08:00
SRC=10.112.70.1 DST=255.255.255.255 LEN=328 TOS=0x00 PREC=0x00 TTL=64
ID=62082 PROTO=UDP SPT=67 DPT=68 LEN=308
Dec 15 16:47:30 heath-desktop last message repeated 2 timesAnswer: The Webmin 'bandwidth'
module adds commands to /etc/shorewall/start that
creates rules to log every packet to/from/through the firewall.
DON'T START THE BANDWIDTH SERVICE IN
WEBMIN!To correct this situation once it occurs, edit
/etc/shorewall/start and insert 'return 0' prior to
the BANDWIDTH rules.Routing(FAQ 32) My firewall has two connections to the Internet from two
different ISPs. How do I set this up in Shorewall?Answer: See this article about Shorewall and Multiple
ISPs.(FAQ 49) When I start Shorewall, my routing table gets blown
away. Why does Shorewall do that?Answer: This is usually the
consequence of a one-to-one nat configuration blunder:Specifying the primary IP address for an interface in the
EXTERNAL column of /etc/shorewall/nat even
though the documentation (and the comments in the file) warn you not
to do that.Specifying ADD_IP_ALIASES=Yes and RETAIN_ALIASES=No in
/etc/shorewall/shorewall.conf.This combination causes Shorewall to delete the primary IP address
from the network interface specified in the INTERFACE column which
usually causes all routes out of that interface to be deleted. The
solution is to not specify the primary IP address
of an interface in the EXTERNAL column.Starting and Stopping(FAQ 94) After I start Shorewall, ps doesn't show any shorewall
process running. What is the Shorewall daemon called?Answer: Shorewall is not a
daemon. It is a configuration tool that configures your kernel based on
the contents of /etc/shorewall/. Once the
start command completes, Shorewall has done its job
and there are no Shorewall processes remaining in the system.(FAQ 7) When I stop Shorewall using <quote>shorewall[-lite]
stop</quote>, I can't connect to anything. Why doesn't that command
work?Answer: The
stop command places the firewall in a safe state;
connections that are allowed are governed by the setting of
ADMINISABSENTMINDED in shorewall.conf (5) and the
contents of shorewall-stoppedrules
(5). To totally open the firewall, use the clear
command.(FAQ 9) Why can't Shorewall detect my interfaces properly at
startup?I just installed Shorewall and when I issue the
start command, I see the following:Processing /etc/shorewall/params ...
Processing /etc/shorewall/shorewall.conf ...
Starting Shorewall...
Loading Modules...
Initializing...
Determining Zones...
Zones: net loc
Validating interfaces file...
Validating hosts file...
Determining Hosts in Zones...
Net Zone: eth0:0.0.0.0/0
Local Zone: eth1:0.0.0.0/0
Deleting user chains...
Creating input Chains...
...Why can't Shorewall detect my interfaces properly?Answer: The above output is
perfectly normal. The Net zone is defined as all hosts that are
connected through eth0 and the
local zone is defined as all hosts connected through eth1. You can set the routefilter option on an internal interface if
you wish to guard against 'Martians' (a Martian
is a packet with a source IP address that is not routed out of the
interface on which the packet was received). If you do that, it is a
good idea to also set the logmartians
option.(FAQ 22) I have some iptables commands that I want to run when
Shorewall starts. Which file do I put them in?Answer:You can place these
commands in one of the Shorewall Extension
Scripts. Be sure that you look at the contents of the chain(s)
that you will be modifying with your commands so that the commands will
do what is intended. Many iptables commands published in HOWTOs and
other instructional material use the -A command which adds the rules to
the end of the chain. Most chains that Shorewall constructs end with an
unconditional DROP, ACCEPT or REJECT rule and any rules that you add
after that will be ignored. Check man iptables and look
at the -I (--insert) command.(FAQ 43) I just installed the Shorewall RPM and Shorewall doesn't
start at boot time.Answer: When you install using
the "rpm -U" command, Shorewall doesn't run your distribution's tool for
configuring Shorewall startup. You will need to run that tool (insserv,
chkconfig, run-level editor, …) to configure Shorewall to start in the
the default run-levels of your firewall system.(FAQ 59) After I start Shorewall, there are lots of unused
Netfilter modules loaded. How do I avoid that?Answer: Copy
/usr/share/shorewall[-lite]/modules to
/etc/shorewall/modules and modify the copy to
include only the modules that you need. An alternative is to set
LOAD_HELPERS_ONLY=Yes in shorewall.conf (5).(FAQ 68) I have a VM under an OpenVZ system. I can't get rid of
the following message:ERROR: Command "/sbin/iptables -A FORWARD -m state --state
ESTABLISHED,RELATED -j ACCEPT" failed.Answer: See the Shorewall OpenVZ article.(FAQ 73) When I stop Shorewall, the firewall is wide open. Isn't
that a security risk?It is important to understand that the scripts in /etc/init.d are generally provided by your
distribution and not by the Shorewall developers. These scripts must
meet the requirements of the distribution's packaging system which may
conflict with the requirements of a tight firewall. So when you say
"…when I stop Shorewall…" it is necessary to distinguish between the
commands /sbin/shorewall stop and
/etc/init.d/shorewall stop./sbin/shorewall stop places the firewall in a
safe state, the details of which depend on your
/etc/shorewall/stoppedrules file (shorewall-stoppedrules(5))
and on the setting of ADMINISABSENTMINDED in
/etc/shorewall/shorewall.conf (shorewall.conf(5))./etc/init.d/shorewall stop may or may not do
the same thing. In the case of Debian systems for
example, that command actually executes /sbin/shorewall
clear which opens the firewall completely. In other words, in
the init script, stop reverses the effect of
start.Beginning with Shorewall 4.4, when the Shorewall tarballs are
installed on a Debian (or derivative) system, the
/etc/init.d/shorewall file is the same as would be
installed by the .deb. The behavior of /etc/init.d/shorewall
stop is controlled by the setting of SAFESTOP in
/etc/default/shorewall. When set to 0 (the
default), the firewall is cleared; when set to 1, the firewall is placed
in a safe state.(FAQ 78) After restart and bootup of my Debian firewall, all
traffic is blocked for hosts behind the firewall trying to connect out
onto the net or through the vpn (although i can reach the internal
firewall interface and obtain dumps etc). Once I issue 'shorewall clear'
followed by 'shorewall start' it then works, despite the config not
changingAnswer: Set IP_FORWARDING=On in
/etc/shorewall/shorewall.conf.(FAQ 86) My distribution (Ubuntu) uses NetworkManager to manage
my interfaces. I want to specify the upnpclient option for my interfaces
which requires them to be up and configured when Shorewall starts but
Shorewall is being started before NetworkManager.Answer: I faced a similar problem which I solved as
follows:Don't start Shorewall at boot time (Debian and Ubuntu users
may simply set startup=0 in
/etc/default/shorewall) or disable in systemd
using systemctl disable shorewall.service.In /etc/network/ip-up.d, I added a
shorewall script as follows:#!/bin/sh
shorewall status > /dev/null 2>&1 || shorewall start # Start Shorewall if it isn't already runningBe sure to secure the script for execute access.Update:Beginning with Shorewall 4.4.10, there is a new Shorewall Init Package
that is designed to handle this case.(FAQ 90) Shorewall starts fine but after several minutes, it
stops. Why is it doing that?Answer: Shorewall uses the
presence of a chain named shorewall to indicate
whether is started or stopped. That chain is created during execution of
a successful start, restart or restore command and is removed during stop and clear.
If shorewall status indicates that
Shorewall is stopped, then something has deleted that chain. Look at the
output of shorewall status; if it looks
like this:
gateway:~# shorewall status
Shorewall-4.4.11 Status at gateway - Wed Jul 21 13:21:41 PDT 2010
Shorewall is stopped
State:Started (Tue Jul 20 16:01:49 PDT 2010)
gateway:~#
then it means that something outside of Shorewall has deleted the
chain. This usually means that you were running another firewall package
before you installed Shorewall and that other package has replaced
Shorewall's Netfilter configuration with its own. You must remove (or at
least disable) the other firewall package and restart Shorewall.
gateway:~# shorewall status
Shorewall-4.4.11 Status at gateway - Wed Jul 21 13:26:29 PDT 2010
Shorewall is stopped
State:Stopped (Wed Jul 21 13:26:26 PDT 2010)
gateway:~#
then a shorewall stop command has
been executed (if the State shown in the output is Cleared, then a shorewall
clear command was executed). Most likely, you have installed
and configured the shorewall-init package and a
required interface has gone down.(FAQ 99) My /var/lib/shorewall-init.log shows that Shorewall is
running at boot but after boot 'iptables -L' shows an empty
configurationAnswer: This is caused by your
failure to disable your distributions default iptables configuration
tool when you installed Shorewall. Look for a service called 'iptables'
that is being started after Shorewall and disable it.(FAQ 101) How can I speed up 'shorewall start' and 'shorewall
restart' on my slow hardware?Answer: There are several steps
that you can take:If your kernel supports module autoloading (and distribution
default kernels almost always do), then set LOAD_HELPERS_ONLY=Yes in
shorewall.conf.Set AUTOMAKE=Yes in shorewall.conf. This will avoid the
compilation phase in cases where the configuration has not changed
since the last time that the configuration was compiled.Don't set optimization option 8. For example, if you currently
set OPTIMIZE=31, then change that to OPTIMIZE=23. Optimization
option 8 combines identical chains which can result in a smaller
ruleset, but it slows down the compilation of large rulesets.Rather than restart, use
reload. With the default setting of
RESTART=restart, restart performs
stop then start, while
reload avoids the stop
part.Use a capabilities file:Run shorewall show -f capabilties >
/etc/shorewall/capabilities Rerun that command each time you install a new kernel or a
new version of shorewall.(FAQ 103) Shorewall fails to start at boot but will start
immediately afterAnswer: This is usually
associated with SELinux. Here
is an example.(FAQ 104) I see <emphasis>kernel</emphasis> messages in my log
when I start or restart Shorewall or Shorewall6Example:> Oct 1 13:04:39 deb kernel: [ 9570.619744] xt_addrtype: ipv6 does not support BROADCAST matching
Answer: These are harmless.
Shorewall attempts to execute various commands to determine the
capabiities of your system. If you system doesn't support a command, it
will generally issue a kernel log message.(FAQ 106) Shorewall is not starting at boot on Debian
with systemdAnswer: To enable start at boot,
run systemctl enable shorewall.serviceMultiple ISPs(FAQ 57) I configured two ISPs in Shorewall but when I try to use
the second one, it doesn't work.Answer: The Multi-ISP
Documentation strongly recommends that you use the balance option on all providers even if you want
to manually specify which ISP to use. If you don't do that so that your
main routing table only has one default route, then you must disable
route filtering. Do not specify the routefilter option on the other interface(s) in
/etc/shorewall/interfaces and disable any
IP Address Spoofing protection that your
distribution supplies.(FAQ 58) But if I specify 'balance' then won't Shorewall balance
the traffic between the interfaces? I don't want that!Answer: Suppose that you want all
traffic to go out through ISP1 (mark 1) unless you specify otherwise.
Then simply add these two rules as the first marking rules in your
/etc/shorewall/mangle (was tcrules) file:#ACTION SOURCE DEST
MARK(1):P 0.0.0.0/0
MARK(1) $FW
other MARK rulesNow any traffic that isn't marked by one of your other MARK rules
will have mark = 1 and will be sent via ISP1. That will work whether
balance is specified or not!Using DNS Names(FAQ 79) Can I use DNS names in Shorewall configuration file
entries in place of IP addresses?Answer: Yes, but we advise
strongly against it.Traffic Shaping(FAQ 67) I just configured Shorewall's builtin traffic shaping
and now Shorewall fails to Start.The error I receive is as follows:RTNETLINK answers: No such file or directory
We have an error talking to the kernel
ERROR: Command "tc filter add dev eth2 parent ffff: protocol ip prio
50 u32 match ip src 0.0.0.0/0 police rate 500kbit burst 10k drop flowid
:1" FailedAnswer: This message indicates that your kernel
doesn't have 'traffic policing' support. If your kernel is modularized,
you may be able to resolve the problem by loading the act_police kernel module. Other kernel modules
that you will need include:cls_basiccls_fwcls_u32sch_htbsch_ingresssch_sfq(FAQ 97) I enable Shorewall traffic shaping and now my upload
rate is way below what I specifiedAnswer: This is likely due to TCP
Segmentation Offload (TSO) and/or Generic Segmentation Offload (GSO)
being enabled in the network adapter. To verify, install the
ethtool package and use the -k command:root@gateway:~# ethtool -k eth1
Offload parameters for eth1:
rx-checksumming: on
tx-checksumming: on
scatter-gather: on
tcp-segmentation-offload: on
udp-fragmentation-offload: off
generic-segmentation-offload: on
generic-receive-offload: off
large-receive-offload: off
ntuple-filters: off
receive-hashing: off
root@gateway:~#If that is the case, you can correct the problem by adjusting the
minburst setting in
/etc/shorewall/tcinterfaces (simple traffic shaping) or
/etc/shorewall/tcdevices (complex traffic shaping). We suggest starting
at 10-12kb and adjust as necessary. Example (simple traffic
shaping):#INTERFACE TYPE IN_BANDWIDTH OUT_BANDWIDTH
eth0 External 50mbit:200kb 5.0mbit:100kb:200ms:100mbit:10kbAlternatively, you can turn off TSO and GSO using this command in
/etc/shorewall/init:ethtool -K ethN tso off gso off(FAQ 97a) I enable Shorewall traffic shaping and now my download
rate is way below what I specifiedAnswer: This is likely due to
Generic Receive Offload (GRO) being enabled in the network adapter. To
verify, install the ethtool package and use the
-k command:root@gateway:/etc/shorewall# ethtool -k eth1
Offload parameters for eth1:
rx-checksumming: on
tx-checksumming: on
scatter-gather: on
tcp-segmentation-offload: on
udp-fragmentation-offload: off
generic-segmentation-offload: on
generic-receive-offload: on
large-receive-offload: off
ntuple-filters: off
receive-hashing: off
root@gateway:/etc/shorewall#
To work around the issue, use this command:ethtool -K ethN gro offBeginning with Shorewall 4.4.25, another option is available in
the form of a rate-estimated policing
filter.Example from /etc/shorewall/tcdevices:#INTERFACE IN_BANDWITH OUT_BANDWIDTH OPTIONS
1:COMB_IF ~20mbit:250ms:4sec ${UPLOAD}kbit hfsc,linklayer=ethernet,overhead=0To create a rate-estimated filter, precede the bandwidth with a
tilde ("~"). The optional interval and decay_interval determine how
often the rate is estimated and how many samples are retained for
estimating. Please see http://ace-host.stuart.id.au/russell/files/tc/doc/estimators.txt
for details.About Shorewall(FAQ 10) What Distributions does Shorewall work with?Answer: Shorewall works with any
GNU/Linux distribution that includes the proper prerequisites.(FAQ 11) What Features does Shorewall have?Answer: See the Shorewall Feature List.(FAQ 12) Is there a GUI?Answer: Yes! Shorewall support is
available in Webmin. See http://www.webmin.com. But beware of
the issue described in FAQ 36.(FAQ 13) Why do you call it <quote>Shorewall</quote>?Answer: Shorewall is a
concatenation of Shoreline (the city where I live) and
Firewall. The full name of the
product is actually Shoreline Firewall but
Shorewall is much more commonly used.(FAQ 23) Why do you use such ugly fonts on your web site?Answer: The Shorewall web site is
almost font neutral (it doesn't explicitly specify fonts except on a few
pages) so the fonts you see are largely the default fonts configured in
your browser. If you don't like them then reconfigure your
browser.(FAQ 25) How do I tell which version of Shorewall or Shorewall
Lite I am running?Answer: At the shell prompt,
type:/sbin/shorewall[-lite] version -a(FAQ 25a) It says 4.4.7.5; how do I know if it is
Shorewall-shell or Shorewall-perl?Answer: It is Shorewall-perl.
Shorewall-shell is discontinued in Shorewall 4.4.(FAQ 31) Does Shorewall provide protection against....IP Spoofing: Sending packets over the WAN interface using an
internal LAP IP address as the source address?Answer: Yes.Tear Drop: Sending packets that contain overlapping
fragments?Answer: This is the
responsibility of the IP stack, not the Netfilter-based firewall
since fragment reassembly occurs before the stateful packet filter
ever touches each packet.Smurf and Fraggle: Sending packets that use the WAN or LAN
broadcast address as the source address?Answer: Shorwall filters
these packets under the nosmurfs interface
option in /etc/shorewall/interfaces.Land Attack: Sending packets that use the same address as the
source and destination address?Answer: Yes, if the routefilter interface
option is selected.DOS: - SYN Dos - ICMP Dos - Per-host Dos protectionAnswer: Yes.(FAQ 65) How do I accomplish failover with Shorewall?Answer:This article
by Paul Gear should help you get started.Alias IP Addresses/Virtual Interfaces(FAQ 18) Is there any way to use aliased ip addresses with
Shorewall, and maintain separate rule sets for different IPs?Answer: Yes. See Shorewall and Aliased
Interfaces.(FAQ 83) Is there no way to nest the firewall zone or create
subzones? I've got a system with Linux-VServers, it's one interface
(eth0) with multiple IPsAnswer: Beginning with Shorewall
4.4.11 Beta 2, you can create vserver
zones that are nested within the firewall zone.Prior to 4.4.11 Beta 2, there is no way to create sub-zones of the
firewall zone. But you can use shell variables to make vservers easier
to deal with./etc/shorewall/params:VS1=fw:192.168.2.12
VS2=fw:192.168.2.13
VS3=fw:192.168.2.14/etc/shorewall/rules:#ACTION SOURCE DEST PROTO DPORT
?SECTION ALL
?SECTION ESTABLISHED
?SECTION RELATED
?SECTION INVALID
?SECTION UNTRACKED
?SECTION NEW
ACCEPT $VS1 net tcp 25
DNAT net $VS1 tcp 25
etc...Shorewall Lite(FAQ 53) What is Shorewall Lite?Answer: Shorewall Lite is a
companion product to Shorewall and is designed to allow you to maintain
all Shorewall configuration information on a single system within your
network. See the Compiled Firewall
script documentation for details.(FAQ 54) If I want to use Shorewall Lite, do I also need to
install Shorewall on the same system?Answer: No. In fact, we recommend
that you do NOT install Shorewall on
systems where you wish to use Shorewall Lite. You must have Shorewall
installed on at least one system within your network in order to use
Shorewall Lite.(FAQ 55) How do I decide which product to use - Shorewall or
Shorewall Lite?Answer: If you plan to have only
a single firewall system, then Shorewall is the logical choice. I also
think that Shorewall is the appropriate choice for laptop systems that
may need to have their firewall configuration changed while on the road.
In the remaining cases, Shorewall Lite will work very well. At
shorewall.net, the two laptop systems have the full Shorewall product
installed as does my personal Linux desktop system. All other Linux
systems that run a firewall use Shorewall Lite and have their
configuration directories on my desktop system.(FAQ 60) What are the compatibility restrictions between
Shorewall and Shorewall LiteAnswer: There are no
compatibility constraints between Shorewall and Shorewall-lite.VOIP(FAQ 77) Shorewall is eating my Asterisk egress traffic!Somehow, my firewall config is causing a one-way audio problem in
Asterisk. If a person calls into the PBX, they cannot hear me speaking,
but I can hear them. If I plug the Asterisk server directly into the
router, bypassing the firewall, the problem goes away.Answer: There are two things to
try when VOIP problems are encountered. Both begin with executing two
rmmod commands.If your kernel version is 2.6.20 or earlier:rmmod ip_nat_sip
rmmod ip_conntrack_sipIf your kernel version is 2.6.21 or
later:rmmod nf_nat_sip
rmmod nf_conntrack_sipThe first alternative seems to work for those running recent
kernels (2.6.26 or later):Copy /usr/share/shorewall/modules to
/etc/shorewall
(/usr/share/shorewall/helpers if you have
LOAD_HELPERS_ONLY in shorewall.conf).Edit the copy and change this line:
loadmodule nf_conntrack_sip
to
loadmodule nf_conntrack_sip sip_direct_media=0
shorewall restartThe second alternative is to not load the sip helpers:If you are running kernel 2.6.20 or earlier, then change the
DONT_LOAD specification in your shorewall.conf to:DONT_LOAD=ip_nat_sip,ip_conntrack_sipIf you are running kernel 2.6.21 or later, then change Then
change the DONT_LOAD specification in your shorewall.conf
to:DONT_LOAD=nf_nat_sip,nf_conntrack_sipIPv6(FAQ 80) Does Shorewall support IPV6?Answer: Shorewall IPv6
support is currently available in Shorewall 4.2.4 and
later.(FAQ 80a) Why does Shorewall lPv6 Support Require Kernel 2.6.24
or later?Answer: Shorewall implements a
stateful firewall which requires connection tracking be present in
ip6tables and in the kernel. Linux kernels before 2.6.20 didn't
support connection tracking for IPv6. So we could not even start to
develop Shorewall IPv6 support until 2.6.20 and there were significant
problems with the facility until at least kernel 2.6.23. When
distributions began offering IPv6 connection tracking support, it was
with kernel 2.6.25. So that is what we developed IPv6 support on and
that's all that we initially tested on. Subsequently, we have tested
Shorewall6 on Ubuntu Hardy with kernel 2.6.24. If you are running
2.6.20 or later, you can try to run
Shorewall6 by hacking
/usr/share/shorewall/prog.footer6 and changing the kernel
version test to check for your kernel version rather than 2.6.24
(20624). But after that, you are on your own.kernel=$(printf "%2d%02d%02d\n" $(echo $(uname -r) 2> /dev/null | sed 's/-.*//' | tr '.' ' ' ) | head -n1)
if [ $kernel -lt 20624 ]; then
error_message "ERROR: $PRODUCT requires Linux kernel 2.6.24 or later"
status=2
else
Update: The above logic is found in
/usr/share/shorewall/prog.footer in later
Shorewall releases.(FAQ 40) I have an interface that gets its IPv6 configuration
from radvd. When I start Shorewall6, I immediately loose my default
route. Why?Answer: You have configured
forwarding on the interface which disables autoconfiguration of the
interface. To retain autoconfiguration on the interface when Shorewall6
starts, specify forwarding=0 in the
OPTIONS column on the interface's entry in shorewall6-interfaces
(5).(FAQ 96) I am starting to use ipv6, but on my ipv4 FW,
when restarting Shorewall . it puts in ip6tables rules. How do i
dissable that ?Answer: This is a two-step process.Set DISABLE_IPV6=No in shorewall.conf (5) and
restart Shorewall.Execute these commands at a root shell prompt:ip6tables -P INPUT ACCEPTip6tables -P OUTPUT ACCEPTip6tables -P FORWARD ACCEPTYou will probably want to soon install Shorewall6 so that you have an IPv6
firewall as well as one for IPv4.Wifidog(FAQ 105) Can Shorewall work with Wifidog?Answer: Yes, with a couple of
restrictions:Wifidog must be started after Shorewall. If Shorewall is
restarted/reloaded, then wifidog must be restarted.FORWARD_CLEAR_MARK must be set to in
shorewall.conf.Miscellaneous(FAQ 20) I have just set up a server. Do I have to change
Shorewall to allow access to my server from the Internet?Answer: Yes. Consult the QuickStart guide that you
used during your initial setup for information about how to set up rules
for your server.(FAQ 24) How can I allow connections to, let's say, the ssh port
only from specific IP Addresses on the Internet?Answer: In the SOURCE column of
the rule, follow net by a colon and a list of the
host/subnet addresses as a comma-separated list.net:<ip1>,<ip2>,...Example:ACCEPT net:192.0.2.16/28,192.0.2.44 fw tcp 22(FAQ 26) When I try to use any of the SYN options in nmap on or
behind the firewall, I get <quote>operation not permitted</quote>. How
can I use nmap with Shorewall?Answer: Temporarily remove any
rejNotSyn, dropNotSyn, dropInvalid, NotSyn(...) and Invalid(...) rules from
/etc/shorewall/rules and restart Shorewall.(FAQ 27) I'm compiling a new kernel for my firewall. What should
I look out for?Answer: First take a look at the
Shorewall kernel configuration page. You
probably also want to be sure that you have selected the NAT of local connections (READ HELP) on the Netfilter Configuration menu. Otherwise, DNAT rules with
your firewall as the source zone won't work with your new kernel.(FAQ 28) How do I use Shorewall as a Bridging Firewall?Answer: Shorewall Bridging
Firewall support is available — check here for details.(FAQ 39) How do I block connections to a particular domain
name?I tried this rule to block Google's Adsense that you'll find on
everyone's site. Adsense is a Javascript that people add to their Web
pages. So I entered the rule:#ACTION SOURCE DEST PROTO
REJECT fw net:pagead2.googlesyndication.com allHowever, this also sometimes restricts access to "google.com". Why
is that? Using dig, I found these IPs for domain
googlesyndication.com:216.239.37.99
216.239.39.99And this for google.com:216.239.37.99
216.239.39.99
216.239.57.99So my guess is that you are not actually
blocking the domain, but rather the IP being called. So how in the world
do you block an actual domain name?Answer: Packet filters like
Netfilter base their decisions on the contents of the various protocol
headers at the front of each packet. Stateful packet filters (of which
Netfilter is an example) use a combination of header contents and state
created when the packet filter processed earlier packets. Netfilter (and
Shorewall's use of Netfilter) also consider the network interface(s)
where each packet entered and/or where the packet will leave the
firewall/router.When you specify a domain name in a
Shorewall rule, the iptables program resolves that name to one
or more IP addresses and the actual Netfilter rules that are created are
expressed in terms of those IP addresses. So the rule that you entered
was equivalent to:#ACTION SOURCE DEST PROTO
REJECT $FW net:216.239.37.99 all
REJECT $FW net:216.239.39.99 allGiven
that name-based multiple hosting is a common practice (another example:
lists.shorewall.net and www1.shorewall.net are both hosted on the same
system with a single IP address), it is not possible to filter
connections to a particular name by examination of protocol headers
alone. While some protocols such as FTP
require the firewall to examine and possibly modify packet payload,
parsing the payload of individual packets doesn't always work because
the application-level data stream can be split across packets in
arbitrary ways. This is one of the weaknesses of the 'string match'
Netfilter extension available in later Linux kernel releases. The only
sure way to filter on packet content is to proxy the connections in
question -- in the case of HTTP, this means running something like
Squid. Proxying allows
the proxy process to assemble complete application-level messages which
can then be accurately parsed and decisions can be made based on the
result.(FAQ 42) How can I tell which features my kernel and iptables
support?Answer: Use the
shorewall[-lite] show capabilities command at a root
prompt.gateway:~# shorewall show capabilities
Shorewall has detected the following iptables/netfilter capabilities:
NAT: Available
Packet Mangling: Available
Multi-port Match: Available
Extended Multi-port Match: Available
Connection Tracking Match: Available
Extended Connection Tracking Match Support: Available
Old Connection Tracking Match Syntax: Not available
Packet Type Match: Available
Policy Match: Available
Physdev Match: Available
Physdev-is-bridged Support: Available
Packet length Match: Available
IP range Match: Available
Recent Match: Available
Owner Match: Available
Ipset Match: Available
CONNMARK Target: Available
Extended CONNMARK Target: Available
Connmark Match: Available
Extended Connmark Match: Available
Raw Table: Available
IPP2P Match: Available
Old IPP2P Match Syntax: Not available
CLASSIFY Target: Available
Extended REJECT: Available
Repeat match: Available
MARK Target: Available
Extended MARK Target: Available
Mangle FORWARD Chain: Available
Comments: Available
Address Type Match: Available
TCPMSS Match: Available
Hashlimit Match: Available
Old Hashlimit Match: Not available
NFQUEUE Target: Available
Realm Match: Available
Helper Match: Available
Connlimit Match: Available
Time Match: Available
Goto Support: Available
LOGMARK Target: Available
IPMARK Target: Available
LOG Target: Available
Persistent SNAT: Available
gateway:~# (FAQ 19) How do I open the firewall for all traffic to/from the
LAN?Answer: Add these two
policies:#SOURCE DEST POLICY LOGLEVEL LIMIT CONNLIMIT
$FW loc ACCEPT
loc $FW ACCEPTYou should also delete any ACCEPT rules from $FW->loc and
loc->$FW since those rules are redundant with the above
policies.(FAQ 88) Can I run Snort with Shorewall?Answer: Yes. In Network
Intrusion Detection System (NIDS) mode, Snort is libpcap
based (like tcpdump) so it doesn't interfere with Shorewall. We have had
reports that users have also been successful in using Snort in
inline more with Shorewall, but no HOWTO exists at
this time.(FAQ 89) How do I connect to the web server in my aDSL modem from
my local LAN?Answer: Here's what I did:My local network is 172.20.1.0/24, so I set the IP address in
the modem to 172.20.1.2.The IP address of my firewall's interface to the LAN is
172.20.1.254. The logical name of the DSL interface is EXT_IF and my
LAN interface is INT_IF.I added the following two configuration entries:/etc/shorewall/masq:#INTERFACE SOURCE ADDRESS
?COMMENT DSL Modem
EXT_IF:172.20.1.2 0.0.0.0/0 172.20.1.254
When running Shorewall 5.0.14 or later, the equivalent
/etc/shorewall/snat is:#ACTION SOURCE DEST PROTO PORT
SNAT(172.20.1.254) 0.0.0.0/0 EXT_IF:192.168.1.2 tcp www/etc/shorewall/proxyarp:#ADDRESS INTERFACE EXTERNAL HAVEROUTE PERSISTENT
172.20.1.2 EXT_IF INT_IF no yes
If you can't change the IP address of your modem and its current
address isn't in your local network, then you need to change this
slightly; assuming that the modem IP address is 192.168.1.1:Do not include an entry in
/etc/shorewall/proxyarp.Add an IP address in 192.168.1.0/24 to your external interface
using your configuration's network management tools. For
Debian-based systems, that means adding this to the interface's
stanza in /etc/network/interfaces: post-up /sbin/ip addr add 192.168.1.254/24 dev external-interfaceYour entry in /etc/shorewall/masq would
then be:#INTERFACE SOURCE ADDRESS
COMMENT DSL Modem
EXT_IF:192.168.1.1 0.0.0.0/0 192.168.1.254
When running Shorewall 5.0.14 or later, the equivalent
/etc/shorewall/snat is:#ACTION SOURCE DEST PROTO PORT
SNAT(192.168.1.254) 0.0.0.0/0 EXT_IF:192.168.1.1 tcp www(FAQ 93) I'm not able to use Shorewall to manage a
bridge. I get the following error: ERROR: BRIDGING=Yes is not supported
by Shorewall 4.4.13.3.Answer: If you want to apply
firewall rules to the traffic passing between bridge ports, see https://shorewall.org/bridge-Shorewall-perl.html.
If you simply want to allow all traffic between ports, then see https://shorewall.org/SimpleBridge.html.(FAQ 95) What is this $FW that I see in the configuration files
and documentation?Answer: FW is a shell variable
that expands to the name that you gave to the firewall zone in shorewall-zones(5). The
default name for the firewall zone is fw:#ZONE TYPE OPTIONS
fw firewallSo, using the default or sample configurations, writing $FW is the same as writing fw. If you give the firewall zone a different
name, gate for example, then writing
$FW would be the same as writing
gate.#ZONE TYPE OPTIONS
gate firewallWhy was that done?Answer: The firewall zone has
special semantics, so having a way to refer to it in a
configuration-independent way makes writing the documentation,
examples, macros, etc. easier.(FAQ 98) How do I Unsubscribe from the Mailing ListAnswer: There are two
ways:On the webGo to https://lists.sourceforge.net/lists/listinfo/shorewall-users.
At the bottom of the form is a section entitled "Shorewall-users Subscribers". At the bottom
of that section find:
"To unsubscribe from
Shorewall-users, get a password reminder, or change your
subscription options enter your subscription
email address:".
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instructions in that email.(FAQ 102) What is 'qt'? I see it in some of the older
documentation.Answer: 'qt' stands for 'quiet';
qt() is a shell function that accepts a command with arguments as
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/dev/null. It is defined in the Shorewall-core shell library
lib.common.