packet size

Maria Iano bind-lists at iano.org
Wed Sep 11 16:24:02 UTC 2013


What does it mean when the edns0 response to a dig says the overall packet size will be one value but the message size reported is different. For example in this reponse the OPT PSEUDOSECTION says udp: 4096 but at the end it says MSG SIZE  rcvd: 275.

$ dig www.google.com

; <<>> DiG 9.9.3-P2-gci-9.9.3-P2-1.P2.gci.el6 <<>> www.google.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 18023
;; flags: qr rd ra; QUERY: 1, ANSWER: 6, AUTHORITY: 4, ADDITIONAL: 5

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;www.google.com.			IN	A

;; ANSWER SECTION:
www.google.com.		113	IN	A	74.125.131.147
www.google.com.		113	IN	A	74.125.131.103
www.google.com.		113	IN	A	74.125.131.99
www.google.com.		113	IN	A	74.125.131.105
www.google.com.		113	IN	A	74.125.131.104
www.google.com.		113	IN	A	74.125.131.106

;; AUTHORITY SECTION:
google.com.		95419	IN	NS	ns4.google.com.
google.com.		95419	IN	NS	ns3.google.com.
google.com.		95419	IN	NS	ns2.google.com.
google.com.		95419	IN	NS	ns1.google.com.

;; ADDITIONAL SECTION:
ns1.google.com.		95419	IN	A	216.239.32.10
ns2.google.com.		95419	IN	A	216.239.34.10
ns3.google.com.		95419	IN	A	216.239.36.10
ns4.google.com.		95419	IN	A	216.239.38.10

;; Query time: 0 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Wed Sep 11 11:56:32 EDT 2013
;; MSG SIZE  rcvd: 275

I want to know because I have been asked what the impact to DNS in our environment would be if they dropped udp fragments at the border. It seems to me I would have to configure our name servers to drop back to tcp when a packet is over 1500. I'm trying to understand just how much that would impact the servers.

Thanks,
Maria


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