shorewall-names - Man Page

When you define an object in Shorewall (Zone^[1], Logical Interface, ipsets^[2], Actions^[3], etc., you give it a name. Shorewall names start with a letter and consist of letters, digits or underscores ("_"). Except for Zone names, Shorewall does not impose a limit on name length.

When an ipset is referenced, the name must be preceded by a plus sign ("+").

The last character of an interface may also be a plus sign to indicate a wildcard name.

Physical interface names match names shown by 'ip link ls'; if the name includes an at sign ("@"), do not include that character or any character that follows. For example, "sit1@NONE" is referred to as simply 'sit1".

For a pair of zones, Shorewall creates two Netfilter chains; one for connections in each direction. The names of these chains are formed by separating the names of the two zones by either "2" or "-".

Example: Traffic from zone A to zone B would go through chain A2B (think "A to B") or "A-B".

In Shorewall 4.6, the default separator is "-" but you can override that by setting ZONE_SEPARATOR="2" in shorewall.conf^[4] (5).

Zones themselves have names that begin with a letter and are composed of letters, numerals, and "_". The maximum length of a name is dependent on the setting of LOGFORMAT in shorewall.conf^[4] (5). See shorewall-zones^[1] (5) for details.

Host addresses in Shorewall configuration files may be specified as either IP addresses or DNS Names.

DNS names in iptables rules aren't nearly as useful as they first appear. When a DNS name appears in a rule, the iptables utility resolves the name to one or more IP addresses and inserts those addresses into the rule. So changes in the DNS->IP address relationship that occur after the firewall has started have absolutely no effect on the firewall's rule set.

For some sites, using DNS names is very risky. Here's an example:

teastep@ursa:~$ dig pop.gmail.com

; <<>> DiG 9.4.2-P1 <<>> pop.gmail.com
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 1774
;; flags: qr rd ra; QUERY: 1, ANSWER: 3, AUTHORITY: 7, ADDITIONAL: 0

;; QUESTION SECTION:
;pop.gmail.com.               IN A

;; ANSWER SECTION:
pop.gmail.com.          300   IN CNAME gmail-pop.l.google.com.
gmail-pop.l.google.com. 300   IN A     209.85.201.109
gmail-pop.l.google.com. 300   IN A     209.85.201.111

Note that the TTL is 300 -- 300 seconds is only 5 minutes. So five minutes later, the answer may change!

So this rule may work for five minutes then suddently stop working:

#ACTION        SOURCE               DEST              PROTO             DPORT
POP(ACCEPT)    loc                  net:pop.gmail.com

There are two options in shorewall[6].conf(5)^[4] that affect the use of DNS names in Shorewall[6] config files:

• DEFER_DNS_RESOLUTION - When set to No, DNS names are resolved at compile time; when set to Yes, DNS Names are resolved at runtime.
• AUTOMAKE - When set to Yes, start, restart and reload only result in compilation if one of the files on the CONFIG_PATH has changed since the the last compilation.

So by setting AUTOMAKE=Yes, and DEFER_DNS_RESOLUTION=No, compilation will only take place at boot time if a change had been make to the config but no restart or reload had taken place. This is clearly spelled out in the shorewall.conf manpage. So with these settings, so long as a 'reload' or 'restart' takes place after the Shorewall configuration is changes, there should be no DNS-related problems at boot time.

If your firewall rules include DNS names then, even if DEFER_DNS_RESOLUTION=No and AUTOMAKE=Yes:

• If your /etc/resolv.confis wrong then your firewall may not start.
• If your /etc/nsswitch.conf is wrong then your firewall may not start.
• If your Name Server(s) is(are) down then your firewall may not start.
• If your startup scripts try to start your firewall before starting your DNS server then your firewall may not start.
• Factors totally outside your control (your ISP's router is down for example), can prevent your firewall from starting.
• You must bring up your network interfaces prior to starting your firewall, or the firewall may not start.

Each DNS name must be fully qualified and include a minimum of two periods (although one may be trailing). This restriction is imposed by Shorewall to insure backward compatibility with existing configuration files.

Example 1. Valid DNS Names

• mail.shorewall.net
• shorewall.net. (note the trailing period).

Example 2. Invalid DNS Names

• mail (not fully qualified)
• shorewall.net (only one period)

DNS names may not be used as:

• The server address in a DNAT rule (/etc/shorewall/rules file)
• In the ADDRESS column of an entry in /etc/shorewall/masq.

• In the /etc/shorewall/nat file.

These restrictions are imposed by Netfilter and not by Shorewall.

When dealing with a complex configuration, it is often awkward to use physical interface names in the Shorewall configuration.

• You need to remember which interface is which.
• If you move the configuration to another firewall, the interface names might not be the same.

Beginning with Shorewall 4.4.4, you can use logical interface names which are mapped to the actual interface using the physical option in shorewall-interfaces^[5] (5).

Here is an example:

#ZONE  INTERFACE  OPTIONS
net    COM_IF     dhcp,blacklist,tcpflags,optional,upnp,routefilter=0,nosmurfs,logmartians=0,physical=eth0
net    EXT_IF     dhcp,blacklist,tcpflags,optional,routefilter=0,nosmurfs,logmartians=0,proxyarp=1,physical=eth2
loc    INT_IF     dhcp,logmartians=1,routefilter=1,tcpflags,nets=172.20.1.0/24,physical=eth1
dmz    VPS_IF     logmartians=1,routefilter=0,routeback,physical=venet0
loc    TUN_IF     physical=tun+

In this example, COM_IF is a logical interface name that refers to Ethernet interface eth0, EXT_IF is a logical interface name that refers to Ethernet interface eth2, and so on.

Here are a couple of more files from the same configuration:

shorewall-masq^[6] (5):

#INTERFACE SOURCE                    ADDRESS

COMMENT Masquerade Local Network
COM_IF     0.0.0.0/0
EXT_IF     !206.124.146.0/24         206.124.146.179:persistent

shorewall-providers^[7] (5)

#NAME   NUMBER   MARK    DUPLICATE  INTERFACE  GATEWAY         OPTIONS               COPY
Avvanta 1        0x10000 main       EXT_IF     206.124.146.254 loose,fallback        INT_IF,VPS_IF,TUN_IF
Comcast 2        0x20000 main       COM_IF     detect          balance               INT_IF,VPS_IF,TUN_IF

Note in particular that Shorewall translates TUN_IF to tun* in the COPY column.

1. Zone
   https://shorewall.org/manpages/shorewall-zones.html
2. ipsets
   https://shorewall.org/manpages/ipsets.html
3. Actions
   https://shorewall.org/manpages/Actions.html
4. shorewall.conf
   https://shorewall.org/manpages/shorewall.conf.html
5. shorewall-interfaces
   https://shorewall.org/manpages/shorewall-interfaces.html
6. shorewall-masq
   https://shorewall.org/manpages/shorewall-masq.html
7. shorewall-providers
   https://shorewall.org/manpages/shorewall-providers.html

shorewall(8).