** Description changed:
Description:
When the user is doing "ping -f myhostname.tld", ping does a ipaddr->hostname
request at EACH packet. Which is quite a lot when used with a -f option.
Moreover it is quite useless as when used with -f, ping doesnt display the
result of the ipaddr->hostname, but only a single dot per packet.
- I discovered this when implementing a nss plugin (/etc/nsswitch.conf kindof
stuff).
- I dunno the actual impact of this, just seems like very inefficient.
- But if the nss plugin doesn't have an request cache, this will become
quite a
denial of service for this service
+
+ -gjarboni's addition--
+
+ Actually, this can be a lot worse in certain cases. For example, try
+ pinging dnstest.bigjar.com. You can't reproduce this in text but what
+ happens is:
+
+ Pinging machine sends out ICMP echo request
+ Pinging machine sends out in-addr.arpa DNS query (ip address -> DNS name)
+ Pinging machine receives NXDOMAIN (no such record) from DNS server
+ ------ 5 to 10 seconds pass ------
+ Pinging machine prints:
+ 64 bytes from 12.173.97.1: icmp_seq=1 ttl=252 time=20.9 ms
+
+ This happens for every packet. If you interrupt ping during the 5 to 10
+ second period it will print the "64 bytes from ..." bit before returning
+ to the shell prompt.
+
+ I shudder to imagine someone trying to walk a user through testing
+ connectivity with this bug. It also occurs in Debian 4.0.
+
+ I have packet traces and a much more detailed write up if anyone is
+ interested. This bug doesn't happen with all IP addresses that don't
+ have a hostname associated with them, just certain ones, but it appears
+ to specific to certain name servers.
--
ping doing LOTS of useless dns request
https://bugs.launchpad.net/bugs/82720
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