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manuel220 08-17-2011 09:31 PM

What can be blocking the DNS response going back to my server?
 
1 Attachment(s)
I have a server with a hosting company, but not sure where but it started failing on every DNS request.

I stopped iptables, I see that I can get response from the DNS when using tcp but not udp. What else can be blocking the response from DNS on default udp??


I've requested network team of the hoster to allow udp to my server from the specific DNS server's ip.


Code:

cat /etc/resolv.conf
nameserver 8.8.8.8
nameserver 66.96.80.194
nameserver 66.96.80.43


Code:

[root@armadaazteca ~]# iptables -L
Chain INPUT (policy ACCEPT)
target    prot opt source              destination

Chain FORWARD (policy ACCEPT)
target    prot opt source              destination

Chain OUTPUT (policy ACCEPT)
target    prot opt source              destination



Code:

[root@armadaazteca ~]# route -n
Kernel IP routing table
Destination    Gateway        Genmask        Flags Metric Ref    Use Iface
68.233.243.0    0.0.0.0        255.255.255.0  U    0      0        0 eth0
169.254.0.0    0.0.0.0        255.255.0.0    U    0      0        0 eth0
0.0.0.0        68.233.243.1    0.0.0.0        UG    0      0        0 eth0

dig default using udp

Code:

[root@armadaazteca ~]# dig www.yahoo.com

; <<>> DiG 9.3.6-P1-RedHat-9.3.6-16.P1.el5 <<>> www.yahoo.com
;; global options:  printcmd
;; connection timed out; no servers could be reached

dig using tcp

Code:

[root@armadaazteca ~]# dig +tcp www.yahoo.com

; <<>> DiG 9.3.6-P1-RedHat-9.3.6-16.P1.el5 <<>> +tcp www.yahoo.com
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 63578
;; flags: qr rd ra; QUERY: 1, ANSWER: 4, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;www.yahoo.com.                IN      A

;; ANSWER SECTION:
www.yahoo.com.          258    IN      CNAME  fp3.wg1.b.yahoo.com.
fp3.wg1.b.yahoo.com.    19      IN      CNAME  any-fp3-lfb.wa1.b.yahoo.com.
any-fp3-lfb.wa1.b.yahoo.com. 259 IN    CNAME  any-fp3-real.wa1.b.yahoo.com.
any-fp3-real.wa1.b.yahoo.com. 19 IN    A      209.191.122.70

;; Query time: 34 msec
;; SERVER: 8.8.8.8#53(8.8.8.8)
;; WHEN: Wed Aug 17 21:26:04 2011
;; MSG SIZE  rcvd: 128

finaly a small packet capture

Attachment 7818

bathory 08-18-2011 02:46 AM

Hi,

You may try to reduce edns to 512 bytes and see if it helps
Code:

max-udp-size 512;
edns-udp-size 512;

Regards


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