Shorewall Community Tom Eastep 2004-03-04 2001-2004 Thomas M. Eastep Permission 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, with no Front-Cover, and with no Back-Cover Texts. A copy of the license is included in the section entitled GNU Free Documentation License.

Answer: Check out the QuickStart Guides.

Answer: The first example in the rules file documentation shows how to do port forwarding under Shorewall. The format of a port-forwarding rule to a local system is as follows: #ACTION SOURCE DEST PROTO DEST PORT DNAT net loc:<local IP address>[:<local port>] <protocol> <port #> So to forward UDP port 7777 to internal system 192.168.1.5, the rule is: #ACTION SOURCE DEST PROTO DEST PORT DNAT net loc:192.168.1.5 udp 7777 If you want to forward requests directed to a particular address ( <external IP> ) on your firewall to an internal system: #ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL # PORT DEST. DNAT net loc:<local IP address>[:<local port>] <protocol> <port #> - <external IP> Finally, if you need to forward a range of ports, in the PORT column specify the range as <low-port>:<high-port>.

Answer: That is usually the result of one of four things: You 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 should be set to the IP address of your firewall's internal interface). Your ISP is blocking that particular port inbound. You are running Mandrake Linux 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.

Answer: To further diagnose this problem: As root, type iptables -t nat -Z. This clears the NetFilter counters in the nat table. Try to connect to the redirected port from an external host. As root type shorewall show nat 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 should be the IP address of the firewall's interface to that system). If the packet count is zero: the connection request is not reaching your server (possibly it is being blocked by your ISP); or you 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); or your 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 ethereal to further diagnose the problem.

In /etc/shorewall/rules: #ACTION SOURCE DEST PROTO DEST PORT DNAT net loc:192.168.3:22 tcp 1022

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. 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.

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 Bind Version 9 views (or using a separate DNS server for local clients) such that www.mydomain.com resolves to 130.141.100.69 externally and 192.168.1.5 internally. That's what I do here at shorewall.net for my local systems that use one-to-one NAT. If you insist on an IP solution to the accessibility problem rather than a DNS solution, then 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. If you are running Shorewall 1.4.0 or earlier see the 1.3 FAQ for instructions suitable for those releases. If you are running Shorewall 1.4.1 or Shorewall 1.4.1a, please upgrade to Shorewall 1.4.2 or later. Otherwise:In this configuration, all loc->loc traffic will look to the server as if it came from the firewall rather than from the original client! In /etc/shorewall/interfaces: #ZONE INTERFACE BROADCAST OPTIONS loc eth1 detect routeback In /etc/shorewall/rules: #ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL # PORT DEST. DNAT loc loc:192.168.1.5 tcp www - 130.151.100.69:192.168.1.254 That rule only works of course if you have a static external IP address. If you have a dynamic IP address and are running Shorewall 1.3.4 or later then include this in /etc/shorewall/init: ETH0_IP=`find_interface_address eth0` and make your DNAT rule: #ACTION SOURCE DEST PROTO DEST PORT SOURCE ORIGINAL # PORT DEST. DNAT loc loc:192.168.1.5 tcp www - $ETH0_IP:192.168.1.254 Using this technique, you will want to configure your DHCP/PPPoE client to automatically restart Shorewall each time that you get a new IP address.

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 hosts'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=0 Answer: This is another problem that is best solved using Bind Version 9 views. 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 routing all Z->Z traffic through your firewall then: Set the Z->Z policy to ACCEPT. Masquerade Z to itself. Set the routeback option on the interface to Z. Set the ALL INTERFACES column in the nat file to Yes. In this configuration, all Z->Z traffic will look to the server as if it came from the firewall rather than from the original client! I DO NOT RECOMMEND THIS SETUP. Zone: dmz Interface: eth2 Subnet: 192.168.2.0/24 In /etc/shorewall/interfaces: #ZONE INTERFACE BROADCAST OPTIONS loc eth2 192.168.2.255 routeback In /etc/shorewall/policy: #SOURCE DESTINATION POLICY LIMIT:BURST dmz dmz ACCEPT In /etc/shorewall/masq: #INTERFACE SUBNET ADDRESS eth2 192.168.2.0/24 In /etc/shorewall/nat, be sure that you have Yes in the ALL INTERFACES column.

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.

Answer: The common.def included with version 1.3.x always rejects connection requests on TCP port 113 rather than dropping them. This is necessary to prevent outgoing connection problems to services that use the Auth mechanism for identifying requesting users. Shorewall also rejects TCP ports 135, 137, 139 and 445 as well as UDP ports 137-139. These are ports that are used by Windows (Windows can be configured to use the DCE cell locator on port 135). Rejecting these connection requests rather than dropping them cuts down slightly on the amount of Windows chatter on LAN segments connected to the Firewall. If you are seeing port 80 being closed, that's probably your ISP preventing you from running a web server in violation of your Service Agreement. You can change the default behavior of Shorewall through use of an /etc/shorewall/common file. See the Extension Script Section. Beginning with Shorewall 1.4.9, Shorewall no longer rejects the Windows SMB ports (135-139 and 445) by default and silently drops them instead.

Answer: Take a deep breath and read the nmap man page 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 do the nmap UDP scan again.

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 firerwall will block that attempt.

Here's a writeup on a nice integration of Shorewall and PortSentry.

Answer: If you want your firewall to be totally open for ping, Create /etc/shorewall/common if it doesn't already exist. Be sure that the first command in the file is . /etc/shorewall/common.def Add the following to /etc/shorewall/common run_iptables -A icmpdef -p ICMP --icmp-type echo-request -j ACCEPT For a complete description of Shorewall ping management, see this page.

Answer: 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. 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 firewall to the internet.

See the Shorewall and FTP page.

Answer: Most likely, you need to set CLAMPMSS=Yes in /etc/shorewall/shorewall.conf.

Answer: Add the routeback option to br0 in /etc/shorewall/interfaces.

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 messaged 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). By default, older versions of Shorewall ratelimited log messages through settings in /etc/shorewall/shorewall.conf -- If you want to log all messages, set: LOGLIMIT="" LOGBURST="" Beginning with Shorewall version 1.3.12, you can set up Shorewall to log all of its messages to a separate file.

Answer: Here are several links that may be helpful: http://www.shorewall.net/pub/shorewall/parsefw/ http://www.fireparse.com http://cert.uni-stuttgart.de/projects/fwlogwatch http://www.logwatch.org http://gege.org/iptables http://home.regit.org/ulogd-php.html I personally use Logwatch. It emails me a report each day from my various systems with each report summarizing the logged activity on the corresponding system.

Temporarily add the following rule: DROP net fw udp 10619

Jan 8 15:50:48 norcomix kernel: Shorewall:net2all:DROP:IN=eth0 OUT= MAC=00:40:c7:2e:09:c0:00:01:64:4a:70:00:08:00 SRC=208.138.130.16 DST=24.237.22.45 LEN=53 TOS=0x00 PREC=0x00 TTL=251 ID=8288 DF PROTO=UDP SPT=53 DPT=40275 LEN=33 Answer: There are two possibilities: They are late-arriving replies to DNS queries. They are corrupted reply packets. You can distinguish the difference by setting the logunclean option (/etc/shorewall/interfaces) on your external interface (eth0 in the above example). If they get logged twice, they are corrupted. I solve this problem by using an /etc/shorewall/common file like this: # # Include the standard common.def file # . /etc/shorewall/common.def # # The following rule is non-standard and compensates for tardy # DNS replies # run_iptables -A common -p udp --sport 53 -mstate --state NEW -j DROP The above file is also include in all of my sample configurations available in the Quick Start Guides and in the common.def file in Shorewall 1.4.0 and later.

What is labeled as the MAC address in a 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) MAC=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)

Answer: If you are running Shorewall version 1.4.4 or 1.4.4a then check the errata. Otherwise: Find where klogd is being started (it will be from one of the files in /etc/init.d -- sysklogd, klogd, ...). Modify that file or the appropriate configuration file so that klogd is started with -c <n> where <n> is a log level of 5 or less; or See the dmesg man page (man dmesg). You must add a suitable dmesg command to your startup scripts or place it in /etc/shorewall/start. Under RedHat and Mandrake, the max log level that is sent to the console is specified in /etc/sysconfig/init in the LOGLEVEL variable. Set LOGLEVEL=5 to suppress info (log level 6) messages on the console. Under Debian, you can set KLOGD=-c 5 in /etc/init.d/klogd to suppress info (log level 6) messages on the console. Under SuSE, add -c 5 to KLOGD_PARAMS in /etc/sysconfig/syslog to suppress info (log level 6) messages on the console.

Answer: Logging occurs out of a number of chains (as indicated in the log message) in Shorewall: man1918 or logdrop The destination address is listed in /etc/shorewall/rfc1918 with a logdrop target -- see /etc/shorewall/rfc1918. rfc1918 or logdrop The source address is listed in /etc/shorewall/rfc1918 with a logdrop target -- see /etc/shorewall/rfc1918. all2<zone>, <zone>2all or all2all You 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. <zone1>2<zone2> Either 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. <interface>_mac The packet is being logged under the maclist interface option. logpkt The packet is being logged under the logunclean interface option. badpkt The packet is being logged under the dropunclean interface option as specified in the LOGUNCLEAN setting in /etc/shorewall/shorewall.conf. blacklst The packet is being logged because the source IP is blacklisted in the /etc/shorewall/blacklist file. newnotsyn The packet is being logged because it is a TCP packet that is not part of any current connection yet it is not a syn packet. Options affecting the logging of such packets include NEWNOTSYN and LOGNEWNOTSYN in /etc/shorewall/shorewall.conf. INPUT or FORWARD The packet has a source IP address that isn't in any of your defined zones (shorewall check and look at the printed zone definitions) or the chain is FORWARD and the destination IP isn't in any of your defined zones. Also see for another cause of packets being logged in the FORWARD chain. logflags The packet is being logged because it failed the checks implemented by the tcpflags interface option. Jun 27 15:37:56 gateway kernel: Shorewall:all2all:REJECT:IN=eth2 OUT=eth1 SRC=192.168.2.2 DST=192.168.1.3 LEN=67 TOS=0x00 PREC=0x00 TTL=63 ID=5805 DF PROTO=UDP SPT=1803 DPT=53 LEN=47 Let's look at the important parts of this message: all2all:REJECT This packet was REJECTed out of the all2all chain -- the packet was rejected under the all->all REJECT policy ( above). IN=eth2 the packet entered the firewall via eth2. If you see IN= with no interface name, the packet originated on the firewall itself. OUT=eth1 if 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. SRC=192.168.2.2 the packet was sent by 192.168.2.2 DST=192.168.1.3 the packet is destined for 192.168.1.3 PROTO=UDP UDP Protocol DPT=53 The destination port is 53 (DNS) For additional information about the log message, see http://logi.cc/linux/netfilter-log-format.php3. 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

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=ICMP TYPE=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 LAN Answer: While most people associate the Internet Control Message Protocol (ICMP) with ping, ICMP is a key piece of the internet. 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 a lot of broken implementations. That is what you are seeing with these messages. 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. I have also seen cases where the source IP in the ICMP itself isn't set back to the external IP of the remote NAT gateway; that causes your firewall to log and drop the packet out of the rfc1918 chain because the source IP is reserved by RFC 1918.

Setting this up in Shorewall is easy; setting up the routing is a bit harder. Assuming that eth0 and eth1 are the interfaces to the two ISPs then: /etc/shorewall/interfaces: #ZONE INTERFACE BROADCAST OPTIONS net eth0 detect net eth1 detect /etc/shorewall/policy: #SOURCE DESTINATION POLICY LIMIT:BURST net net DROP If you have masqueraded hosts, be sure to update /etc/shorewall/masq to masquerade to both ISPs. For example, if you masquerade all hosts connected to eth2 then: #INTERFACE SUBNET ADDRESS eth0 eth2 eth1 eth2 There was an article in SysAdmin covering this topic. It may be found at http://www.samag.com/documents/s=1824/sam0201h/ The following information regarding setting up routing for this configuration is reproduced from the LARTC HOWTO and has not been verified by the author. If you have questions or problems with the instructions given below, please post to the LARTC mailing list. A common configuration is the following, in which there are two providers that connect a local network (or even a single machine) to the big Internet. ________ +------------+ / | | | +-------------+ Provider 1 +------- __ | | | / ___/ \_ +------+-------+ +------------+ | _/ \__ | if1 | / / \ | | | | Local network -----+ Linux router | | Internet \_ __/ | | | \__ __/ | if2 | \ \___/ +------+-------+ +------------+ | | | | \ +-------------+ Provider 2 +------- | | | +------------+ \________ There are usually two questions given this setup. Split access The first is how to route answers to packets coming in over a particular provider, say Provider 1, back out again over that same provider. Let us first set some symbolical names. Let $IF1 be the name of the first interface (if1 in the picture above) and $IF2 the name of the second interface. Then let $IP1 be the IP address associated with $IF1 and $IP2 the IP address associated with $IF2. Next, let $P1 be the IP address of the gateway at Provider 1, and $P2 the IP address of the gateway at provider 2. Finally, let $P1_NET be the IP network $P1 is in, and $P2_NET the IP network $P2 is in. One creates two additional routing tables, say T1 and T2. These are added in /etc/iproute2/rt_tables. Then you set up routing in these tables as follows: ip route add $P1_NET dev $IF1 src $IP1 table T1 ip route add default via $P1 table T1 ip route add $P2_NET dev $IF2 src $IP2 table T2 ip route add default via $P2 table T2 Nothing spectacular, just build a route to the gateway and build a default route via that gateway, as you would do in the case of a single upstream provider, but put the routes in a separate table per provider. Note that the network route suffices, as it tells you how to find any host in that network, which includes the gateway, as specified above. Next you set up the main routing table. It is a good idea to route things to the direct neighbour through the interface connected to that neighbour. Note the `src' arguments, they make sure the right outgoing IP address is chosen. ip route add $P1_NET dev $IF1 src $IP1 ip route add $P2_NET dev $IF2 src $IP2 Then, your preference for default route: ip route add default via $P1 Next, you set up the routing rules. These actually choose what routing table to route with. You want to make sure that you route out a given interface if you already have the corresponding source address: ip rule add from $IP1 table T1 ip rule add from $IP2 table T2 This set of commands makes sure all answers to traffic coming in on a particular interface get answered from that interface. 'If $P0_NET is the local network and $IF0 is its interface, the following additional entries are desirable: ip route add $P0_NET dev $IF0 table T1 ip route add $P2_NET dev $IF2 table T1 ip route add 127.0.0.0/8 dev lo table T1 ip route add $P0_NET dev $IF0 table T2 ip route add $P1_NET dev $IF1 table T2 ip route add 127.0.0.0/8 dev lo table T2 Now, this is just the very basic setup. It will work for all processes running on the router itself, and for the local network, if it is masqueraded. If it is not, then you either have IP space from both providers or you are going to want to masquerade to one of the two providers. In both cases you will want to add rules selecting which provider to route out from based on the IP address of the machine in the local network. Load balancing The second question is how to balance traffic going out over the two providers. This is actually not hard if you already have set up split access as above. Instead of choosing one of the two providers as your default route, you now set up the default route to be a multipath route. In the default kernel this will balance routes over the two providers. It is done as follows (once more building on the example in the section on split-access): ip route add default scope global nexthop via $P1 dev $IF1 weight 1 \ nexthop via $P2 dev $IF2 weight 1 This will balance the routes over both providers. The weight parameters can be tweaked to favor one provider over the other. balancing will not be perfect, as it is route based, and routes are cached. This means that routes to often-used sites will always be over the same provider. Furthermore, if you really want to do this, you probably also want to look at Julian Anastasov's patches at http://www.ssi.bg/~ja/#routes , Julian's route patch page. They will make things nicer to work with.

The stop command is intended to place your firewall into a safe state whereby only those hosts listed in /etc/shorewall/routestopped' are activated. If you want to totally open up your firewall, you must use the shorewall clear command.

Answer: The output you will see looks something like this: /lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o: init_module: Device or resource busy Hint: insmod errors can be caused by incorrect module parameters, including invalid IO or IRQ parameters /lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o: insmod /lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o failed /lib/modules/2.4.17/kernel/net/ipv4/netfilter/ip_tables.o: insmod ip_tables failed iptables v1.2.3: can't initialize iptables table `nat': iptables who? (do you need to insmod?) Perhaps iptables or your kernel needs to be upgraded. This problem is usually corrected through the following sequence of commands service ipchains stop chkconfig --delete ipchains rmmod ipchains Also, be sure to check the errata for problems concerning the version of iptables (v1.2.3) shipped with RH7.2.

Answer: This is usually cured by the sequence of commands shown above in .

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. If you are running Shorewall 1.4.10 or later, you can consider setting the detectnets interface option on your local interface (eth1 in the above example). That will cause Shorewall to restrict the local zone to only those networks routed through that interface.

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 to be sure that the commands will do what they are 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.

Using a light-weight shell such as ash can dramatically decrease the time required to start or restart Shorewall. See the SHOREWALL_SHELL variable in shorewall.conf.

Shorewall works with any GNU/Linux distribution that includes the proper prerequisites.

Answer: See the Shorewall Feature List.

Answer: Yes. Shorewall support is included in Webmin 1.060 and later versions. See http://www.webmin.com

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 must more commonly used.

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.

At the shell prompt, type: /sbin/shorewall version

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: Shorewall can be configured to do that using the blacklisting facility. 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 protection Answer: Shorewall has facilities for limiting SYN and ICMP packets. Netfilter as included in standard Linux kernels doesn't support per-remote-host limiting except by explicit rule that specifies the host IP address; that form of limiting is supported by Shorewall.

The first release of Shorewall was in March of 2001. Shorewall 1.2.12 was released in May of 2002. It is now the year 2004 and soon Shorewall 2.0 will be available. Shorewall 1.2.12 is poorly documented and is missing many of the features that Shorewall users find essential today.

Is there any way it can add a rule before the rfc1918 blocking that will let all traffic to and from the 192.168.100.1 address of the modem in/out but still block all other rfc1918 addresses? Answer: If you are running a version of Shorewall earlier than 1.3.1, create /etc/shorewall/start and in it, place the following: run_iptables -I rfc1918 -s 192.168.100.1 -j ACCEPT If you are running version 1.3.1 or later, simply add the following to /etc/shorewall/rfc1918: Be sure that you add the entry ABOVE the entry for 192.168.0.0/16. #SUBNET TARGET 192.168.100.1 RETURN If you add a second IP address to your external firewall interface to correspond to the modem address, you must also make an entry in /etc/shorewall/rfc1918 for that address. For example, if you configure the address 192.168.100.2 on your firewall, then you would add two entries to /etc/shorewall/rfc1918: #SUBNET TARGET 192.168.100.1 RETURN 192.168.100.2 RETURN

The solution is the same as above. Simply substitute the IP address of your ISPs DHCP server.

Answer: Yes. See Shorewall and Aliased Interfaces.

You probably haven't set TC_ENABLED=Yes in /etc/shorewall/shorewall.conf so the contents of the tcrules file are simply being ignored.

Yes. Consult the QuickStart guide that you used during your initial setup for information about how to set up rules for your server.

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>,... ACCEPT net:192.0.2.16/28,192.0.2.44 fw tcp 22

Edit /etc/shorewall/shorewall.conf and change NEWNOTSYN=No to NEWNOTSYN=Yes then restart Shorewall.

Add this command to your /etc/shorewall/start file: run_iptables -D OUTPUT -p ! icmp -m state --state INVALID -j DROP

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.

The last few lines of a startup trace are these: + run_iptables2 -t nat -A eth0_masq -s 192.168.2.0/24 -d 0.0.0.0/0 -j MASQUERADE + '[' 'x-t nat -A eth0_masq -s 192.168.2.0/24 -d 0.0.0.0/0 -j MASQUERADE' = 'x-t nat -A eth0_masq -s 192.168.2.0/24 -d 0.0.0. 0/0 -j MASQUERADE' ']' + run_iptables -t nat -A eth0_masq -s 192.168.2.0/24 -d 0.0.0.0/0 -j MASQUERADE + iptables -t nat -A eth0_masq -s 192.168.2.0/24 -d 0.0.0.0/0 -j MASQUERADE iptables: Invalid argument + '[' -z '' ']' + stop_firewall + set +x Answer: Your new kernel contains headers that are incompatible with the ones used to compile your iptables utility. You need to rebuild iptables using your new kernel source.

Experimental Shorewall Bridging Firewall support is available — check here for details.

1.192004-03-05TEAdded link to Bridge.1.182004-02-27TEAdded FAQ 35.1.172004-02-15TEAdded FAQ 34.1.162004-02-11TEAdded FAQ 33.1.152004-01-25TEUpdated FAQ 32 to mention masquerading. Remove tables.1.142004-01-24TEAdded FAQ 27a regarding kernel/iptables incompatibility.1.132004-01-24TEAdd a note about the detectnets interface option in FAQ 9.1.122004-01-20TEImprove FAQ 16 answer.1.112004-01-14TECorrected broken link1.102004-01-09TEAdded a couple of more legacy FAQ numbers.1.92004-01-08TECorrected typo in FAQ 26a. Added warning to FAQ 2 regarding source address of redirected requests.1.82003-12-31TEAdditions to FAQ 4.1.72003-12-30TERemove dead link from FAQ 1.1.62003.12-18TEAdd external link reference to FAQ 17.1.52003-12-16TEAdded a link to a Sys Admin article about multiple internet interfaces. Added Legal Notice. Moved "abstract" to the body of the document. Moved Revision History to this Appendix.1.42003-12-13TECorrected formatting problems1.32003-12-10TEChanged the title of FAQ 171.22003-12-09TEAdded Copyright and legacy FAQ numbers1.12003-12-04MNConverted to Simplified DocBook XML1.02002-08-13TEInitial revision