On 04/11/18 07:27, Rick Stevens wrote: > I seem to recall the same thing, that iptables opens incoming UDP port > 53 for some period of time if it saw an outgoing UDP port 53 request. > And I, like you, can't recall what that period was--although I think > it was 60 seconds. That's still more than the the basic Linux resolver > library's limit.
That isn't how DNS requests work. When a client makes a DNS request the destination port is 53 and the source port is a random high port. Frame 3: 77 bytes on wire (616 bits), 77 bytes captured (616 bits) Ethernet II, Src: PcsCompu_16:b2:62 (08:00:27:16:b2:62), Dst: Synology_76:13:a8 (00:11:32:76:13:a8) Internet Protocol Version 4, Src: 192.168.1.191, Dst: 192.168.1.1 User Datagram Protocol, Src Port: 43629, Dst Port: 53 Domain Name System (query) The client then listens on that same random port and the sever response is from source port 53 Frame 4: 253 bytes on wire (2024 bits), 253 bytes captured (2024 bits) Ethernet II, Src: Synology_76:13:a8 (00:11:32:76:13:a8), Dst: PcsCompu_16:b2:62 (08:00:27:16:b2:62) Internet Protocol Version 4, Src: 192.168.1.1, Dst: 192.168.1.191 User Datagram Protocol, Src Port: 53, Dst Port: 43629 Domain Name System (response) -- Conjecture is just a conclusion based on incomplete information. It isn't a fact.
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