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On 01/08/2015 11:21, Meino.Cramer@×××.de wrote: |
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> Hi, |
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> |
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> With ufw I want temporary block any access from my Gentoo PC to certain |
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> domains. Since domain names change IP addresses I dont want to block |
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> on base of the IP only. |
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> |
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> Is this possible with ufw? |
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That is really not a good idea, which is why packet filtering firewalls |
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seldom attempt it. |
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It means that *every*single*packet* involves a reverse DNS lookup to get |
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the (unreliable) DNS name (which might not even be listed at all), do a |
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string comparison and make a block decision based on that. All of which |
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is probably an order of magnitude more resource use that simply sending |
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the packet out. There are optimizations of course, such as caching the |
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results of previous lookups, but there's still a considerable overhead. |
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There's a few ways around it: |
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1. Rethink your firewalling policy. Maybe you really don't need to block |
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stuff and just think you do. |
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2. Do a DNS lookup and check the TTL. If it's high, say 86400 then it |
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cannot change more than once a day. So you only need to do a lookup once |
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a day. Write or get a script that looks up your banned domains every so |
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often, gets the new IP if it changed and reload a new netfilter rule set. |
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#2 is the correct approach for large firewalls with many users but does |
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involves a quite sophisticated codebase, probably way more than you need |
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for your 1 pc. Which brings us back to #1 |
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-- |
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Alan McKinnon |
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alan.mckinnon@×××××.com |
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