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Project website docs.freebsd.org/en
Translation license BSD 2-Clause "Simplified" License
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File mask documentation/content/*/articles/filtering-bridges/_index.po
Translation file Download documentation/content/zh_CN/articles/filtering-bridges/_index.po
Last change April 10, 2021, 1:05 p.m.
Last author Hongyou Bai

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Total 61 2,591 15,287
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Failing checks 0% 0 0% 0 0% 0
Strings with suggestions 0% 0 0% 0 0% 0
Untranslated strings 93% 57 99% 2,584 99% 15,237
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New string to translate

Documentation / articles/filtering-bridges/_index — Chinese (Simplified) (zh_CN)

1 new string to translate appeared in the translation. 4 months ago
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Documentation / articles/filtering-bridges/_index — Chinese (Simplified) (zh_CN)

The "documentation/content/zh_CN/articles/filtering-bridges/_index.po" file was changed. 4 months ago
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String updated in the repository

Documentation / articles/filtering-bridges/_index — Chinese (Simplified) (zh_CN)

New in FreeBSD 4.0, is the concept of stateful filtering. This is a big improvement for UDP traffic, which typically is a request going out, followed shortly thereafter by a response with the same set of IP addresses and port numbers (but with source and destination reversed, of course). For firewalls that have no statekeeping, there is almost no way to deal with this sort of traffic as a single session. But with a firewall that can "remember" an outgoing UDP packet and, for the next few minutes, allow a response, handling UDP services is trivial. The following example shows how to do it. It is possible to do the same thing with TCP packets. This allows you to avoid some denial of service attacks and other nasty tricks, but it also typically makes your state table grow quickly in size.
 
4 months ago
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Documentation / articles/filtering-bridges/_index — Chinese (Simplified) (zh_CN)

7 new strings to translate appeared to the translation. a year ago
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Documentation / articles/filtering-bridges/_index — Chinese (Simplified) (zh_CN)

The "documentation/content/zh_CN/articles/filtering-bridges/_index.po" file was changed. a year ago
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Documentation / articles/filtering-bridges/_index — Chinese (Simplified) (zh_CN)

There are two rules for passing SMTP and DNS traffic towards the mail server and the name server, if you have them. Obviously the whole rule set should be flavored to personal taste, this is only a specific example (rule format is described accurately in the man:ipfw[8] man page). Note that for "relay" and "ns" to work, name service lookups must work _before_ the bridge is enabled. This is an example of making sure that you set the IP on the correct network card. Alternatively it is possible to specify the IP address instead of the host name (required if the machine is IP-less).
 
a year ago
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Documentation / articles/filtering-bridges/_index — Chinese (Simplified) (zh_CN)

Now it is time to create your own file with custom firewall rules, to secure the inside network. There will be some complication in doing this because not all of the firewall functionalities are available on bridged packets. Furthermore, there is a difference between the packets that are in the process of being forwarded and packets that are being received by the local machine. In general, incoming packets are run through the firewall only once, not twice as is normally the case; in fact they are filtered only upon receipt, so rules that use `out` or `xmit` will never match. Personally, I use `in via` which is an older syntax, but one that has a sense when you read it. Another limitation is that you are restricted to use only `pass` or `drop` commands for packets filtered by a bridge. Sophisticated things like `divert`, `forward` or `reject` are not available. Such options can still be used, but only on traffic to or from the bridge machine itself (if it has an IP address).
 
a year ago
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Documentation / articles/filtering-bridges/_index — Chinese (Simplified) (zh_CN)

At this point you should be able to insert the machine between two sets of hosts without compromising any communication abilities between them. If so, the next step is to add the `net.link.ether.bridge._[blah]_=_[blah]_` portions of these rows to the [.filename]#/etc/sysctl.conf# file, to have them execute at startup.
 
a year ago
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Documentation / articles/filtering-bridges/_index — Chinese (Simplified) (zh_CN)

There is another important thing to know. When running IP over Ethernet, there are actually two Ethernet protocols in use: one is IP, the other is ARP. ARP does the conversion of the IP address of a host into its Ethernet address (MAC layer). To allow the communication between two hosts separated by the bridge, it is necessary that the bridge will forward ARP packets. Such protocol is not included in the IP layer, since it exists only with IP over Ethernet. The FreeBSD firewall filters exclusively on the IP layer and therefore all non-IP packets (ARP included) will be forwarded without being filtered, even if the firewall is configured to not permit anything.
 
a year ago
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Documentation / articles/filtering-bridges/_index — Chinese (Simplified) (zh_CN)

Before rebooting to load the new kernel or the required modules (according to the previously chosen installation method), you have to make some changes to the [.filename]#/etc/rc.conf# configuration file. The default rule of the firewall is to reject all IP packets. Initially we will set up an `open` firewall, to verify its operation without any issue related to packet filtering (in case you are going to execute this procedure remotely, such configuration will avoid you to remain isolated from the network). Put these lines in [.filename]#/etc/rc.conf#:
 
a year ago
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