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Old 08-17-2011, 10:31 PM   #1
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Question What can be blocking the DNS response going back to my server?

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.

cat /etc/resolv.conf

[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

[root@armadaazteca ~]# route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface   U     0      0        0 eth0     U     0      0        0 eth0         UG    0      0        0 eth0
dig default using udp

[root@armadaazteca ~]# dig

; <<>> DiG 9.3.6-P1-RedHat-9.3.6-16.P1.el5 <<>>
;; global options:  printcmd
;; connection timed out; no servers could be reached
dig using tcp

[root@armadaazteca ~]# dig +tcp

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

;                 IN      A

;; ANSWER SECTION:          258     IN      CNAME    19      IN      CNAME 259 IN     CNAME 19 IN     A

;; Query time: 34 msec
;; WHEN: Wed Aug 17 21:26:04 2011
;; MSG SIZE  rcvd: 128
finaly a small packet capture

Old 08-18-2011, 03:46 AM   #2
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You may try to reduce edns to 512 bytes and see if it helps
max-udp-size 512;
edns-udp-size 512;


dns failure, firewall, linux

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