Eye r knot a netwurking gaei. mDNSResponder oddity.

Posted:
in Genius Bar edited January 2014
Anyone care to explain this console.log message that I'm seeing *constantly*?



Code:


Nov 13 19:26:48 dhcp23490 mDNSResponder[168]: mDNSPlatformSendUDP sendto failed to send packet on InterfaceID 01806E00 en1/4 to 224.0.0.251:5353 skt 8 error -1 errno 51 (Network is unreachable)







dig -x on the IP gives:



Code:


ddhcp23490:~ smithja$ dig -x 224.0.0.251



; <<>> DiG 9.2.2 <<>> -x 224.0.0.251

;; global options: printcmd

;; Got answer:

;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 65133

;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0



;; QUESTION SECTION:

;251.0.0.224.in-addr.arpa. IN PTR



;; AUTHORITY SECTION:

224.in-addr.arpa. 8563 IN SOA flag.ep.net. hostmaster.ep.net. 991010 86400 10800 1209600 86400



;; Query time: 74 msec

;; SERVER: 152.2.21.1#53(152.2.21.1)

;; WHEN: Thu Nov 13 19:56:33 2003

;; MSG SIZE rcvd: 100







ep.net is apparently a clearing house for 'exchange points', whatever those are. One's listed for Research Triangle Park, which is in my area.



A dig on the authoritative server flag.ep.net gives:



Code:


dhcp23490:~ smithja$ dig flag.ep.net



; <<>> DiG 9.2.2 <<>> flag.ep.net

;; global options: printcmd

;; Got answer:

;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 37163

;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 5



;; QUESTION SECTION:

;flag.ep.net. IN A



;; ANSWER SECTION:

flag.ep.net. 165940 IN A 198.32.4.13



;; AUTHORITY SECTION:

ep.net. 86107 IN NS dot.ep.net.

ep.net. 86107 IN NS flag.ep.net.



;; ADDITIONAL SECTION:

dot.ep.net. 165940 IN A 198.32.2.10

dot.ep.net. 86107 IN AAAA 2001:478:6::230:48ff:fe22:6a29

dot.ep.net. 86107 IN AAAA 3ffe::1:0:230:48ff:fe22:6a29

flag.ep.net. 165940 IN A 198.32.4.13

flag.ep.net. 86366 IN AAAA 3ffe:805::2d0:b7ff:fee8:c4d9



;; Query time: 21 msec

;; SERVER: 152.2.21.1#53(152.2.21.1)

;; WHEN: Thu Nov 13 20:02:32 2003

;; MSG SIZE rcvd: 193







I'm stumped. ?

Comments

  • Reply 1 of 2
    I'm not a network person either, but from memory 224.0.0.251:5353 is related to rendezvous (which would make sense cause rendezvous is multicast DNS (mDNS). Not sure what the problem is though...
  • Reply 2 of 2
    ok, i may be wrong about this, but i'm pretty sure that 224.0.0.251 is a version of a broadcast ip. I don't know how familiar you are with low level networking stuff, but if your on a subnet, and want to ping everyone on your subnet, you can send a ping to x.y.z.255, where x,y,z can be several things, x=127,y=z=0; x=first octect of ur ip,y=second octet, z=third;also, x=first octet OR'd with extra bits, y similarly, and similar for z. the last octect '255' is all 1's (binary). and actually, it could be several possibilities. overall the broadcast ip usually has to be your ip AND'd with your subnet.



    so basically, the router gets the ping request for x.y.z.w. It says, which of my computers will AND perfectly with x.y.z.w. It looks like 224.0.0.251 nearly grabs all the bits, so its sending a message to the router/switch saying basically "hit almost everyone on the subnet". (the reason for not doing 255 in the first or last octet has probably something to do with the protocol). Technically, your subnet is your ip OR'd with 255 on the last octet ('w' from above). Routers usually don't let u broadcast outside of your ip, and i think switches all restrict you like that. So thats why it didn't bother with the y or z octets.



    Now, as for why its saying "network unreachable" it probably is reachable, but no one (aside from yourself) is there. Try doing this: ping 224.0.0.251 in terminal. I'm guessing that it'll return no hits. Sometimes the router/switch is smart enough not to throw the request back at the sender. Some routers/switches aren't smart enough, so u may get hits from that ping. It's nothing to cause alarm. It just means that it couldn't hit anyone with that broadcast ip. Some routers/switches will drop broadcasts that don't strictly search the requestor's subnet, so maybe this broadcast ip is ur ip OR'd with some other stuff out of ur subnet.



    In general, just stay out of the console unless you are looking for something in particular. A lot of the stuff in there looks scary, but is really harmless.
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