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Hi CCIE3000, You did not miss anything. Instead, you have an extra command on the router that works as DNS server. Please remove the “ip name-server” from the server, and you should be fine. Here’s the working config, and scrolling down, you will see the debugging message showing you why the router crashes. The DNS server router crashes only when you try to send a query for an invalid hostname, which causes the server to go into a loop.
Client R1# ———- ip name-server 2.2.2.2 ! ip domain-lookup is enabled by default ip domain-lookup Server R2# ———- ip dns server ! ip domain-lookup is enabled by default ip domain-lookup ip host R2 2.2.2.2 ip host R1 1.1.1.1 ! DO NOT point name-server to itself ! NO ip name-server 2.2.2.2 R1#ping R2 Translating “R2″…domain server (2.2.2.2) [OK] Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 155.1.0.5, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 56/58/60 ms R1# *Apr 7 11:35:07.163: Domain: query for R2 type 1 to 2.2.2.2 *Apr 7 11:35:07.195: DOM: dom2cache: hostname is R2, RR type=1, class=1, ttl=1, n=4Reply received ok
R2#debug domain Domain Name System debugging is on R2# *Apr 7 02:18:27.683: DNS: Incoming UDP query (id#2) *Apr 7 02:18:27.683: DNS: Type 1 DNS query (id#2) for host ‘R2′ from 12.0.0.1(58198) *Apr 7 02:18:27.683: DNS: Query for my own hostname: R2 *Apr 7 02:18:27.683: DNS: Spoofing reply to query (id#2) *Apr 7 02:18:27.683: DNS: Finished processing query (id#2) in 0.004 secs
REPRODUCE ROUTER CRASH ———————- R2#c Enter configuration commands, one per line. End with CNTL/Z. R2(config)#ip name-server 2.2.2.2 R2(config)# R2# R2# R2# ! ! The R2 successfully serves the DNS queuries ! for valid hostnames (When “ping R2″ is issued on R1 router) R1#ping R2 Translating “R2″…domain server (2.2.2.2) [OK] Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 155.1.0.5, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 56/58/60 ms *Apr 7 11:35:58.539: Domain: query for R2 type 1 to 2.2.2.2 *Apr 7 11:35:58.571: DOM: dom2cache: hostname is R2, RR type=1, class=1, ttl=1, n=4Reply received ok
R2# *Apr 7 02:19:19.059: DNS: Incoming UDP query (id#3) *Apr 7 02:19:19.059: DNS: Type 1 DNS query (id#3) for host ‘R2′ from 12.0.0.1(54174) *Apr 7 02:19:19.059: DNS: Query for my own hostname: R2 *Apr 7 02:19:19.059: DNS: Spoofing reply to query (id#3) *Apr 7 02:19:19.059: DNS: Finished processing query (id#3) in 0.000 secs R2# R2# ! The R2 crashes when “ping R3″ is issued on R1)
R1#ping R3 Translating “R3″…domain server (2.2.2.2) *Apr 7 11:36:22.991: Domain: query for R3 type 1 to 2.2.2.2 % Unrecognized host or address, or protocol not running. timed out *Apr 7 11:36:55.459: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/1, changed state to down *Apr 7 11:36:55.459: %OSPF-5-ADJCHG: Process 1, Nbr 12.0.0.2 on Serial0/1 from FULL to DOWN, Neighbor Down: Interface down or detached
! As R2 cannot resolve IP for hostname R3, it tries to forward the queury to ! the next DNS server with IP address of itself. And the request keeps looping within R2. ! ! R2# DNS: Re-sending DNS query (type 1, id#4) to 2.2.2.2 DNS: Incoming UDP query (id#4) DNS: Type 1 DNS query (id#4) for host ‘R3′ from 2.2.2.2(53) DNS: Re-sending DNS query (type 1, id#4) to 2.2.2.2 DNS: Incoming UDP query (id#4) DNS: Type 1 DNS query (id#4) for host ‘R3′ from 2.2.2.2(53) DNS: Re-sending DNS query (type 1, id#4) to 2.2.2.2 DNS: Incoming UDP query (id#4) DNS: Type 1 DNS query (id#4) for host ‘R3′ from 2.2.2.2(53) DNS: Re-sending DNS query (type 1, id#4) to 2.2.2.2 DNS: Incoming UDP query (id#4) DNS: Type 1 DNS query (id#4) for host ‘R3′ from 2.2.2.2(53) DNS: Re-sending DNS query (type 1, id#4) to 2.2.2.2 DNS: Incoming UDP query (id#4) DNS: Type 1 DNS query (id#4) for host ‘R3′ from 2.2.2.2(53) DNS: Re-sending DNS query (type 1, id#4) to 2.2.2.2 DNS: Incoming UDP query (id#4) DNS: Type 1 DNS query (id#4) for host ‘R3′ from 2.2.2.2(53) DNS: Re-sending DNS query (type 1, id#4) to 2.2.2.2 DNS: Incoming UDP query (id#4) DNS: Type 1 DNS query (id#4) for host ‘R3′ from 2.2.2.2(53) DNS: Re-sending DNS query (type 1, id#4) to 2.2.2.2 DNS: Incoming UDP query (id#4) DNS: Type 1 DNS query (id#4) for host ‘R3′ from 2.2.2.2(53) DNS: Re-sending DNS query (type 1, id#4) to 2.2.2.2 DNS: Incoming UDP query (id#4) DNS: Type 1 DNS query (id#4) for host ‘R3′ from 2.2.2.2(53) DNS: Re-sending DNS query (type 1, id#4) to 2.2.2.2 DNS: Incoming UDP query (id#4) DNS: Type 1 DNS query (id#4) for host ‘R3′ from 2.2.2.2(53) DNS: Re-sending DNS query (type 1, id#4) to 2.2.2.2 DNS: Incoming UDP query (id#4) DNS: Type 1 DNS query (id#4) for host ‘R3′ from 12.0.0.1(49206) DNS: Re-sending DNS query (type 1, id#4) to 2.2.2.2 DNS: Incoming UDP query (id#4) DNS: Type 1 DNS query (id#4) for host ‘R3′ from 2.2.2.2(53) DNS: Re-sending DNS query (type 1, id#4) to 2.2.2.2 DNS: Incoming UDP query (id#4) DNS: Type 1 DNS query (id#4) for host ‘R3′ from 2.2.2.2(53) DNS: Re-sending DNS query (type 1, id#4) to 2.2.2.2 DNS: Incoming UDP query (id#4) DNS: Type 1 DNS query (id#4) for host ‘R3′ from 2.2.2.2(53) DNS: Re-sending DNS query (type 1, id#4) to 2.2.2.2 DNS: Incoming UDP query (id#4) DNS: Type 1 DNS query (id#4) for host ‘R3′ from 2.2.2.2(53) DNS: Re-sending DNS query (type 1, id#4) to 2.2.2.2 DNS: Incoming UDP query (id#4) DNS: Type 1 DNS query (id#4) for host ‘R3′ from 2.2.2.2(53) DNS: Re-sending DNS query (type 1, id#4) to 2.2.2.2 DNS: Incoming UDP query (id#4) DNS: Type 1 DNS query (id#4) for host ‘R3′ from 2.2.2.2(53) DNS: Re-sending DNS query (type 1, id#4) to 2.2.2.2 DNS: Incoming UDP query (id#4) DNS: Type 1 DNS query (id#4) for host ‘R3′ from 2.2.2.2(53) DNS: Re-sending DNS query (type 1, id#4) to 2.2.2.2 DNS: Incoming UDP query (id#4) DNS: Type 1 DNS query (id#4) for host ‘R3′ from 2.2.2.2(53) DNS: Re-sending DNS query (type 1, id#4) to 2.2.2.2

— On Sat, 9/6/08, CCIE3000 wrote:
From: CCIE3000 Subject: Router acting as DNS server To: “Cisco certification” Date: Saturday, September 6, 2008, 1:42 AM
Hi Guys,
Am I missing something?? Whenever I try and configure a router to act as a DNS server it always ends up crashing due to memory problems.
I configure
ip dns server
and then enter my ip host commands.
I also configure ip domain-name lookup and ip name-server commands on all the routers.
Is there something else I’m missing that if I configured it would stop the router dieing on me ?/
Thanks, Si.
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