Copyright © 2005, 2006, 2007 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”.
2007/08/22
Table of Contents
Warning
Make sure you are running a current, vendor-supported distribution, before attempting to perform this setup, older distributions do not meet the minimum requirements,and you will need to recompile iptables, kernel and other software on your system. If you don't follow this advice,we will not be able to help you.
Warning
Reading just Shorewall documentation is probably not going to give you enough background to use this material. Shorewall may make iptables easy but the Shorewall team doesn't have the resources to be able to spoon-feed Linux policy routing to you (please remember that the user's manual for a tractor doesn't teach you to grow corn either). You will likely need to refer to the following additional information:
The LARTC HOWTO: http://www.lartc.org
Output of man ip
Output of ip route help and ip rule help
Beginning with Shorewall 2.3.2, limited support is included for multiple internet connections. Limitations of this support are as follows:
It utilizes static routing configuration. As such, there is no provision for reacting to the failure of either of the uplinks.
The routing changes are made and the route cache is purged when Shorewall is started and when Shorewall is restarted (unless you specify the "-n" option to shorewall restart). Ideally, restarting the packet filter should have no effect on routing.
Prior to Shorewall 3.4.0, the routes and route rules added by this support were not completely removed during shorewall stop, shorewall clear or shorewall restart.
Let's assume that a firewall is connected via two separate ethernet interfaces to two different ISPs as in the following diagram.
 |
eth0 connects to ISP1. The IP address of eth0 is 206.124.146.176 and the ISP's gateway router has IP address 206.124.146.254.
eth1 connects to ISP 2. The IP address of eth1 is 130.252.99.27 and the ISP's gateway router has IP address 130.252.99.254.
eth2 connects to the local LAN. Its IP configuration is not relevant to this discussion.
Each of these providers is described in an
entry in the file /etc/shorewall/providers.
Entries in /etc/shorewall/providers can
specify that outgoing connections are to be load-balanced between the
two ISPs. Entries in /etc/shorewall/tcrules can be
used to direct particular outgoing connections to one ISP or the other.
Use of /etc/shorewall/tcrules is not required for
/etc/shorewall/providers to work, but you must
select a unique MARK value for each provider so Shorewall can set up the
correct marking rules for you.
When you use the track option in
/etc/shorewall/providers, connections from the
internet are automatically routed back out of the correct interface and
through the correct ISP gateway. This works whether the connection is
handled by the firewall itself or if it is routed or port-forwarded to a
system behind the firewall.
Shorewall will set up the routing and will update the
/etc/iproute2/rt_tables to include the table names
and numbers of the tables that it adds.
Caution
This feature uses packet
marking to control the routing. As a consequence, there are
some restrictions concerning entries in
/etc/shorewall/tcrules:
Packet marking for traffic control purposes may not be done in the PREROUTING table for connections involving providers with 'track' specified (see below).
You may not use the SAVE or RESTORE options.
You may not use connection marking.
The /etc/shorewall/providers file can also be
used in other routing scenarios. See the Squid documentation for an
example.
Entries in this file have the following columns. As in all Shorewall configuration files, enter "-" in a column if you don't want to enter any value.
- NAME
The provider name. Must begin with a letter and consist of letters and digits. The provider name becomes the name of the generated routing table for this provider.
- NUMBER
A number between 1 and 252. This becomes the routing table number for the generated table for this provider.
- MARK
A mark value used in your /etc/shorewall/tcrules file to direct packets to this provider. Shorewall will also mark connections that have seen input from this provider with this value and will restore the packet mark in the PREROUTING CHAIN. Mark values must be in the range 1-255.
Beginning with Shorewall version 3.2.0 Beta 6, you may use may set HIGH_ROUTE_MARKS=Yes in
/etc/shorewall/shorewall.conf. This allows you to:Use connection marks for traffic shaping, provided that you assign those marks in the FORWARD chain.
Use mark values > 255 for provider marks in this column. These mark values must be a multiple of 256 in the range 256-65280 (hex equivalent 0x100 - 0xFF00 with the low-order 8 bits being zero).
- DUPLICATE
Gives the name or number of a routing table to duplicate. May be 'main' or the name or number of a previously declared provider. For most applications, you want to specify 'main' here.
- INTERFACE
The name of the interface to the provider.
Caution
The Shorewall implementation of Multi-ISP support assumes that each provider has its own interface.
- GATEWAY
The IP address of the provider's Gateway router.
You can enter detect here and Shorewall will attempt to automatically determine the gateway IP address.
Hint: "detect" is appropriate for use in cases where the interface named in the INTERFACE column is dynamically configured via DHCP etc.
- OPTIONS
A comma-separated list from the following:
- track
If specified, connections FROM this interface are to be tracked so that responses may be routed back out this same interface.
You want to specify 'track' if internet hosts will be connecting to local servers through this provider. Any time that you specify 'track', you will also want to specify 'balance' (see below).
Use of this feature requires that your kernel and iptables support CONNMARK target and connmark match support. It does not require the ROUTE target extension.
Warning
A bug in Shorewall versions 3.2.0-3.2.10, 3.4.0-3.4.6 and 4.0.0-4.0.2 prevents proper handling of PREROUTING marks when HIGH_ROUTE_MARKS=No and the track option is specified. Patches are available to correct this problem:
Shorewall version 3.2.0-3.4.3: http://www1.shorewall.net/pub/shorewall/3.2/shorewall-3.2.10/errata/patches/Shorewall/patch-3.2.10-2.diff
Shorewall version 3.4.4-3.4.6: http://www1.shorewall.net/pub/shorewall/3.4/shorewall-3.4.6/errata/patches/Shorewall/patch-3.4.6-1.diff
Shorewall-shell version 4.0.0-4.0.2: http://www1.shorewall.net/pub/shorewall/4.0/shorewall-4.0.2/errata/patches/Shorewall-shell/patch-shell-4.0.2-2.diff
Important
If you are using
/etc/shorewall/providersbecause you have multiple internet connections, we recommend that you specify 'track' even if you don't need it. It helps maintain long-term connections in which there are significant periods with no traffic.- balance
The providers that have 'balance' specified will get outbound traffic load-balanced among them. 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.
By default, each provider is given the same weight (1) . You can change the weight of a given provider by following balance with "=" and the desired weight (e.g., balance=2). The weights reflect the relative bandwidth of the providers connections and should be small numbers since the kernel actually creates additional default routes for each weight increment.
Important
If you are using
/etc/shorewall/providersbecause you have multiple internet connections, we recommend that you specify 'balance' even if you don't need it. You can still use entries in/etc/shorewall/tcrulesto force all traffic to one provider or another.Important
If you specify 'balance' and still find that all traffic is going out through only one provider, you may need to install a kernel built with CONFIG_IP_ROUTE_MULTIPATH_CACHED=n. Several users have reported that this change has corrected similar problems.
The SuSE 10.0 kernel is subject to this problem, and a kernel oops may result in this circumstance. SUSE 10.1 and SLES 10 have CONFIG_IP_ROUTE_MULTIPATH_CACHED=n set by default. The source of the problem seems to be an incompatibility between the LARTC patches and CONFIG_IP_ROUTE_MULTIPATH_CACHED.
- loose
Do not include routing rules that force traffic whose source IP is an address of the INTERFACE to be routed to this provider. Useful for defining providers that are to be used only when the appropriate packet mark is applied. Should not be specified together with balance.
- optional (added in Shorewall 3.2.2)
Shorewall will determine of this interface is up and has a configured IPv4 address. If it is not, a warning is issued and this provider is not configured.
Note
'optional' is designed to detect interface states that will cause shorewall start or shorewall restart to fail; just because an interface is in a state that Shorewall can [re]start without error doesn't mean that traffic can actually be sent through the interface.
For those of you who are termnally confused between track and balance:
track governs incoming connections.
balance governs outgoing connections.
- COPY
When you specify an existing table in the DUPLICATE column, Shorewall copies all routes through the interface specified in the INTERFACE column plus the interfaces listed in this column. At a minumum, you should list all interfaces on your firewall in this column except those internet interfaces specified in the INTERFACE column of entries in this file.
Adding another entry in the providers file simply creates an alternate routing table for you. In addition:
Unless loose is specified, an ip rule is generated for each IP address on the INTERFACE that routes traffic from that address through the associated routing table.
If you specify track, then connections which have had at least one packet arrive on the interface listed in the INTERFACE column have their connection mark set to the value in the MARK column. In the PREROUTING chain, packets with a connection mark have their packet mark set to the value of the associated connection mark; packets marked in this way bypass any prerouting rules that you create in
/etc/shorewall/tcrules. This ensures that packets associated with connections from outside are always routed out of the correct interface.If you specify balance, then Shorewall will replace the 'default' route with weight 100 in the 'main' routing table with a load-balancing route among those gateways where balance was specified. So if you configure default routes, be sure that their weight is less than 100 or the route added by Shorewall will not be used.
That's all that these entries do. You still have to follow the principle stated in the Shorewall Routing documentation:
Routing determines where packets are to be sent.
Once routing determines where the packet is to go, the firewall (Shorewall) determines if the packet is allowed to go there.
The bottom line is that if you want traffic to go out through a
particular provider then you must mark that traffic
with the provider's MARK value in
/etc/shorewall/tcrules and you must do that marking
in the PREROUTING chain; or, you must provide the appropriate rules in
/etc/shorewall/route_rules.
Warning
If you are running a Shorewall version prior to 3.4.0, entries
in /etc/shorewall/providers permanently alter
your firewall/gateway's routing; that is, the effect of these changes
is not reversed by shorewall stop or
shorewall clear. To restore routing to its original
state, you may have to restart your network. This can usually be done
by /etc/init.d/network restart or
/etc/init.d/networking restart. Check your
distribution's networking documentation.
Here are some additional things to consider:
You can mitigate the effect of the Shorewall-generated changes to your routing table by specifying a metric for each default route that you configure. Shorewall will generate a load-balancing default route (assuming that balance has been specified for some of the providers) that does not include a metric and that will therefore not replace any existing route that has a non-zero metric.
The -n option to shorewall restart and shorewall restore can be used to prevent the command from changing your routing.
The
/etc/shorewall/stoppedfile can also be used to restore routing when you stop Shorewall. With your firewall in its normal (single-table) routing configuration, you can capture the contents as follows:ip route ls > routes
Here's what the
routesfile looked like after I did that on my firewall:192.168.1.1 dev eth3 scope link 206.124.146.177 dev eth1 scope link 192.168.2.2 dev tun0 proto kernel scope link src 192.168.2.1 192.168.2.0/24 via 192.168.2.2 dev tun0 192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.254 206.124.146.0/24 dev eth3 proto kernel scope link src 206.124.146.176 169.254.0.0/16 dev eth0 scope link 127.0.0.0/8 dev lo scope link default via 206.124.146.254 dev eth3
Now edit the file as shown below:
ip route flush table main ip route add 192.168.1.1 dev eth3 scope link ip route add 206.124.146.177 dev eth1 scope link ip route add 192.168.2.2 dev tun0 proto kernel scope link src 192.168.2.1 ip route add 192.168.2.0/24 via 192.168.2.2 dev tun0 ip route add 192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.254 ip route add 206.124.146.0/24 dev eth3 proto kernel scope link src 206.124.146.176 ip route add 169.254.0.0/16 dev eth0 scope link ip route add 127.0.0.0/8 dev lo scope link ip route add default via 206.124.146.254 dev eth3 ip route flush cache
Now paste the contents of that file into
/etc/shorewall/stopped.You might also want to consider adding the following to the file:
ip rule ls | while read priority rule; do case ${priority%:} in 0|3276[67]) ;; *) ip rule del $rule ;; esac doneThat code will delete all but the default routing rules.
Given that Shorewall is simply a tool to configure Netfilter and does not run continuously in your system, entries in the providers file do not provide any automatic failover in the event of failure of one of your Internet connections.
One problem that often arises with Multi-ISP configuration is
'Martians'. If your internet interfaces are configured with the
routefilter option in
/etc/shorewall/interfaces (remember that if you set
that option, you should also select logmartians), then things may not work correctly
and you will see messages like this:
Feb 9 17:23:45 gw.ilinx kernel: martian source 206.124.146.176 from 64.86.88.116, on dev eth1 Feb 9 17:23:45 gw.ilinx kernel: ll header: 00:a0:24:2a:1f:72:00:13:5f:07:97:05:08:00
The above message is somewhat awkwardly phrased. The source IP in
this incoming packet was 64.86.88.116 and the destination IP address is
206.124.146.176. Another gotcha is that the incoming packet has already
had the destination IP address changed for DNAT or because the original
outgoing connection was altered by an entry in
/etc/shorewall/masq (SNAT or Masquerade). So the
destination IP address (206.124.146.176) may not have been the
destination IP address in the packet as it was initially
received.
There a couple of common causes for these problems:
You have connected both of your external interfaces to the same hub/switch. Connecting multiple firewall interfaces to a common hub or switch is always a bad idea that will result in hard-to-diagnose problems.
You are specifying both the loose and balance options on your provider(s). This causes individual connections to ping-pong back and forth between the interfaces which is guaranteed to cause problems.
You are redirecting traffic from the local system out of one interface or the other using packet marking in your
/etc/shorewall/tcrulesfile. A better approach is to configure the application to use the appropriate local IP address (the IP address of the interface that you want the application to use). See below.
If all else fails, remove the routefilter option from your external interfaces. If you do this, you may wish to add rules to log and drop packets from the Internet that have source addresses in your local networks. For example, if the local LAN in the above diagram is 192.168.1.0/24, then you would add this rule:
#ACTION SOURCE DEST DROP:info net:192.168.1.0/24 all
Be sure the above rule is added before any other rules with net in the SOURCE column.
The configuration in the figure at the top of this section would
be specified in /etc/shorewall/providers as
follows.
#NAME NUMBER MARK DUPLICATE INTERFACE GATEWAY OPTIONS COPY ISP1 1 1 main eth0 206.124.146.254 track,balance eth2 ISP2 2 2 main eth1 130.252.99.254 track,balance eth2
Other configuration files go something like this:
/etc/shorewall/interfaces:
#ZONE INTERFACE BROADCAST OPTIONS net eth0 detect … net eth1 detect …
/etc/shorewall/policy:
#SOURCE DESTINATION POLICY LIMIT:BURST net net DROP
Regardless of whether you have masqueraded hosts or not, the
following entries are required in
/etc/shorewall/masq if you plan to redirect
connections from the firewall using entries in
/etc/shorewall/tcrules or if you specify balance on your providers.
#INTERFACE SUBNET ADDRESS eth0 130.252.99.27 206.124.146.176 eth1 206.124.146.176 130.252.99.27
Those entries ensure that traffic originating on the firewall and redirected via packet marks always has the source IP address corresponding to the interface that it is routed out of.
Note
If you have a Dynamic IP address on either of the interfaces,
you can use shell variables to construct the above rules. For example,
if eth0 had a dynamic IP
address, then:
/etc/shorewall/params:
ETH0_IP=$(find_first_interface_address eth0)
/etc/shorewall/masq:
#INTERFACE SUBNET ADDRESS eth0 130.252.99.27 $ETH0_IP eth1 $ETH0_IP 130.252.99.27
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 206.124.146.176 eth1 eth2 130.252.99.27
Warning
Entries in /etc/shorewall/masq have no
effect on which ISP a particular connection will be sent through. That
is rather the purpose of entries in
/etc/shorewall/tcrules or
/etc/shorewall/route_rules.
Now suppose that you want to route all outgoing SMTP traffic from your local network through ISP 2. You would make this entry in /etc/shorewall/tcrules (and if you are running a version of Shorewall earlier than 3.0.0, you would set TC_ENABLED=Yes in /etc/shorewall/shorewall.conf).
#MARK SOURCE DEST PROTO PORT(S) CLIENT USER TEST # PORT(S) 2:P <local network> 0.0.0.0/0 tcp 25
When there are more than two providers, you need to extend the two-provider case in the expected way:
For each external address, you need an entry in
/etc/shorewall/masqto handle the case where a connection using that address as the SOURCE is sent out of the interfaces other than the one that the address is configured on.For each external interface, you need to add an entry to
/etc/shorewall/masqfor each internal network that needs to be masqueraded (or use SNAT) through that interface.
If we extend the above example to add eth3 with IP address 16.105.78.4 with gateway 16.105.78.254, then:
/etc/shorewall/providers:
#NAME NUMBER MARK DUPLICATE INTERFACE GATEWAY OPTIONS COPY ISP1 1 1 main eth0 206.124.146.254 track,balance eth2 ISP2 2 2 main eth1 130.252.99.254 track,balance eth2 ISP3 3 3 main eth3 16.105.78.254 track,balance eth2
/etc/shorewall/masq:
#INTERFACE SUBNET ADDRESS eth0 130.252.99.27 206.124.146.176 eth3 130.252.99.27 16.105.78.4 eth1 206.124.146.176 130.252.99.27 eth3 206.124.146.176 16.105.78.4 eth0 16.106.78.4 206.124.146.176 eth1 16.106.78.4 130.252.99.27 eth0 eth2 206.124.146.176 eth1 eth2 130.252.99.27 eth3 eth2 16.105.78.4
Experience has shown that in some cases, problems occur with applications running on the firewall itself. This is especially true when you have specified routefilter on your external interfaces in /etc/shorewall/interfaces (see above). When this happens, it is suggested that you have the application use specific local IP addresses rather than 0.
Examples:
Squid: In
squid.conf, set tcp_outgoing_address to the IP address of the interface that you want Squid to use.In OpenVPN, set local (--local on the command line) to the IP address that you want the server to receive connections on.
The /etc/shorewall/route_rules file was added
in Shorewall version 3.2.0. The route_rules file
allows assigning certain traffic to a particular provider just as
entries in the tcrules file. The difference between
the two files is that entries in route_rules are
independent of Netfilter.
Warning
Prior to Shorewall version 3.4.0, entries in
/etc/shorewall/route_rules permanently alter your
firewall/gateway's routing; that is, the effect of these changes is
not reversed by shorewall stop or
shorewall clear. To restore routing to its original
state, you may have to restart your network. This can usually be done
by /etc/init.d/network restart or
/etc/init.d/networking restart. Check your
distribution's networking documentation.
Please also see the warning in the section above entitled What an Entry in the Providers File Does.
Routing rules are maintained by the Linux kernel and can be displayed using the ip rule ls command. When routing a packet, the rules are processed in turn until the packet is successfully routed.
gateway:~ # ip rule ls
0: from all lookup local <=== Local (to the firewall) IP addresses
10001: from all fwmark 0x1 lookup Blarg <=== This and the next rule are generated by the
10002: from all fwmark 0x2 lookup Comcast 'MARK' values in /etc/shorewall/providers.
20000: from 206.124.146.176 lookup Blarg <=== This and the next rule are generated unless
20256: from 24.12.22.33 lookup Comcast 'loose' is specified; based in the output of 'ip addr ls'
32766: from all lookup main <=== This is the routing table shown by 'iproute -n'
32767: from all lookup default <=== This table is usually empty
gateway:~ #In the above example, there are two providers: Blarg and Comcast with MARK 1 going to Blarg and mark 2 going to Comcast.
Columns in the file are:
- SOURCE (Optional)
An ip address (network or host) that matches the source IP address in a packet. May also be specified as an interface name optionally followed by ":" and an address. If the device 'lo' is specified, the packet must originate from the firewall itself.
- DEST (Optional)
An ip address (network or host) that matches the destination IP address in a packet.
If you choose to omit either SOURCE or DEST, place "-" in that column. Note that you may not omit both SOURCE and DEST.
- PROVIDER
The provider to route the traffic through. May be expressed either as the provider name or the provider number.
- PRIORITY
The rule's priority which determines the order in which the rules are processed.
1000-1999 Before Shorewall-generated 'MARK' rules
11000- 11999 After 'MARK' rules but before Shorewall-generated rules for ISP interfaces.
26000-26999 After ISP interface rules but before 'default' rule.
Rules with equal priority are applied in the order in which they appear in the file.
Example 1: You want all traffic entering the firewall on eth1 to be routed through Comcast.
#SOURCE DEST PROVIDER PRIORITY eth1 - Comcast 1000
With this entry, the output of ip rule ls would be as follows.
gateway:~ # ip rule ls
0: from all lookup local
1000: from all iif eth1 lookup Comcast
10001: from all fwmark 0x1 lookup Blarg
10002: from all fwmark 0x2 lookup Comcast
20000: from 206.124.146.176 lookup Blarg
20256: from 24.12.22.33 lookup Comcast
32766: from all lookup main
32767: from all lookup default
gateway:~ #Note that because we used a priority of 1000, the
test for eth1 is inserted
before the fwmark tests.
Example 2: You use OpenVPN (routed setup w/tunX) in combination with multiple providers. In this case you have to set up a rule to ensure that the OpenVPN traffic is routed back through the tunX interface(s) rather than through any of the providers. 10.8.0.0/24 is the subnet choosen in your OpenVPN configuration (server 10.8.0.0 255.255.255.0).
#SOURCE DEST PROVIDER PRIORITY - 10.8.0.0/24 main 1000