DNS and Authoritative Servers Post: 302878509

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Special Forums IP Networking DNS and Authoritative Servers Post 302878509 by DGPickett on Friday 6th of December 2013 12:17:39 PM

12-06-2013

Registered User

Want recursion is a client request attribute. A server can do with 'go ash him' but a basic DNS gethostbyname library routine just wants the final answer. I suppose a DNS server might exist that can be configured to forward requests with recursion wanted. It would have a more meager cache, but if it is sharing a slow connection, it gets a quicker answer from a better connected correspondent DNS server up the chain. An absolute root server (com, net,org) might refuse to do recursion -- they are too busy as it is. Google 'DNS Recursion' and lo and behold, http://technet.microsoft.com/en-us/l.../cc771738.aspx it is an anti-DenialOfService trick to not accept recursive requests. In this case, no simple clients can connect, just other DNS servers, so you need a ring of recursion-enabled DNS servers around your simple clients. A caching no-domain DNS server is a nice thing to have as locally as possible, so you can keep asking for the same host (or address or other query) and get a local, cached answer quickly. The DNS service to support your domain (or your addresses in their pseudo domain) is really a completely separate function from a DNS server to answer queries about other domains. Caching is a dual edged sword, though, as it can be poisoned. Hackers send an unsolicited packet with bad information as if responding to a request, and the DNS server accepts and saves it. That seems a worse problem then Denial Of Service from queries to domains with infinite loops of redirection.

If there are no cache hits, a query, recursive or not, will eventually go to the authoritative source. However, any DNS server on your query's path may have a non-authoritative answer in cache to any of the multiple queries needed to finally answer your query: a.b.c.d may go to d, c.d, b.c.d to get a.b.c.d in the end.

Last edited by DGPickett; 12-06-2013 at 01:24 PM..

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LEARN ABOUT DEBIAN

testmxlookup

TESTMXLOOKUP(1) 					      Double Precision, Inc.						   TESTMXLOOKUP(1)

NAME

       testmxlookup - Look up mail relays for a domain

SYNOPSIS

       testmxlookup [@ip-address] [--dnssec] [--udpsize n] {domain}

DESCRIPTION

       testmxlookup lists the names and IP addresses of mail relays that receive mail for the domain. This is useful in diagnosing mail delivery
       problems.

       testmxlookup sends a DNS MX query for the specified domain, followed by A/AAAA queries, if needed.  testmxlookup lists the hostname and the
       IP address of every mail relay, and its MX priority.

   DIAGNOSTICS
       The error message "Hard error" indicates that the domain does not exist, or does not have any mail relays. The error message "Soft error"
       indicates a temporary error condition (usually a network failure of some sorts, or the local DNS server is down).

   OPTIONS
       @ip-address
	   Specify the DNS server's IP address, where to send the DNS query to, overriding the default DNS server addresses read from
	   /etc/resolv.conf.

	   "ip-address" must be a literal, numeric, IP address.

       --dnssec
	   Enable the DNSSEC extension. If the DNS server has DNSSEC enabled, and the specified domain's DNS records are signed, the list of IP
	   addresses is suffixed by "(DNSSEC)", indicating a signed response.

	   This is a diagnostic option. Older DNS servers may respond with an error, to a DNSSEC query.

       --udpsize n
	   Specify that n is the largest UDP packet size that the DNS server may send. This option is only valid together with "--dnssec". If
	   "--dnssec" always returns an error, try "--udpsize 512" (the default setting is 1280 bytes, which is adequate for Ethernet, but other
	   kinds of networks may impose lower limits).

SEE ALSO

       courier(8)[1], RFC 1035[2].

AUTHOR

       Sam Varshavchik
	   Author

NOTES

	1. courier(8)
	   [set $man.base.url.for.relative.links]/courier.html

	2. RFC 1035
	   http://www.rfc-editor.org/rfc/rfc1035.txt

Courier Mail Server						    11/18/2011							   TESTMXLOOKUP(1)