[tor-bugs] #8215 [Tor]: Simple Relay: random unknown UDP port in listen mode

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#8215: Simple Relay: random unknown UDP port in listen mode
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    Reporter:  elgo         |       Owner:                  
        Type:  enhancement  |      Status:  reopened        
    Priority:  normal       |   Milestone:  Tor: unspecified
   Component:  Tor          |     Version:                  
  Resolution:               |    Keywords:  tor-relay       
      Parent:               |      Points:                  
Actualpoints:               |  
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Comment(by nickm):

 So here's how DNS and UDP work together here.

 I want to make a DNS request.  I'm going to send a DNS request in a UDP
 packet to a DNS server.  That server is going to send me a response in
 another UDP packet.

 But UDP isn't like TCP: UDP deals with datagrams, not streams.  If I am
 going to receive any UDP response from the server, I need to have an open
 socket waiting for the UDP datagram to come back.

 That socket will show up as listening:  That's to be expected.  It's
 willing to accept replies from multiple DNS servers, because it's willing
 to send requests to them.

 I'm attaching a simple demonstration program in C.  It makes a DNS request
 to one or two of Google's public nameservers, waits for an answer, then
 sleeps for a while so you can run netstat or lsof before it exits.  If you
 run it as-is, then it uses "connect" to say that it only wants to talk to
 one nameserver.   If you add "#define TWO_REQUESTS" at the top of the
 file, then it'll use the same socket to send a request to two nameservers,
 and wait for replies from both.  In the first case, the program will show
 up as "connected"; in the second case, it will show up as "listening".
 But it's the same code in both cases: Send a UDP request, wait for a
 reply.

-- 
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/8215#comment:8>
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