I just read this message by Mark Andrews on the BIND mailing list. It explains the possible issues with DNSSEC and over-protective firewalls, giving test-commands to verify your setup. This post is also interesting for regular DNS traffic, since a firewall doesn’t know the difference.

Here is the most interesting part:

First, you should verify that you can talk to L.ROOT-SERVERS.NET using plain DNS. This will ensure that failures in the subsequent tests are meaningful.

e.g.

dig +nodnssec +norec +ignore ns . @L.ROOT-SERVERS.NET

; <<>> DiG 9.3.6-P1 <<>> +nodnssec +norec +ignore ns . @L.ROOT-SERVERS.NET
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 39974
;; flags: qr aa; QUERY: 1, ANSWER: 13, AUTHORITY: 0, ADDITIONAL: 15

;; QUESTION SECTION:
;.				IN	NS

;; ANSWER SECTION:
.			518400	IN	NS	a.root-servers.net.
.			518400	IN	NS	b.root-servers.net.
.			518400	IN	NS	c.root-servers.net.
.			518400	IN	NS	d.root-servers.net.
.			518400	IN	NS	e.root-servers.net.
.			518400	IN	NS	f.root-servers.net.
.			518400	IN	NS	g.root-servers.net.
.			518400	IN	NS	h.root-servers.net.
.			518400	IN	NS	i.root-servers.net.
.			518400	IN	NS	j.root-servers.net.
.			518400	IN	NS	k.root-servers.net.
.			518400	IN	NS	l.root-servers.net.
.			518400	IN	NS	m.root-servers.net.

;; ADDITIONAL SECTION:
a.root-servers.net.	518400	IN	A	198.41.0.4
b.root-servers.net.	518400	IN	A	192.228.79.201
c.root-servers.net.	518400	IN	A	192.33.4.12
d.root-servers.net.	518400	IN	A	128.8.10.90
e.root-servers.net.	518400	IN	A	192.203.230.10
f.root-servers.net.	518400	IN	A	192.5.5.241
g.root-servers.net.	518400	IN	A	192.112.36.4
h.root-servers.net.	518400	IN	A	128.63.2.53
i.root-servers.net.	518400	IN	A	192.36.148.17
j.root-servers.net.	518400	IN	A	192.58.128.30
k.root-servers.net.	518400	IN	A	193.0.14.129
l.root-servers.net.	518400	IN	A	199.7.83.42
m.root-servers.net.	518400	IN	A	202.12.27.33
a.root-servers.net.	518400	IN	AAAA	2001:503:ba3e::2:30
f.root-servers.net.	518400	IN	AAAA	2001:500:2f::f

;; Query time: 189 msec
;; SERVER: 2001:500:3::42#53(2001:500:3::42)
;; WHEN: Mon Feb  8 13:05:49 2010
;; MSG SIZE  rcvd: 492

Next we will see whether you can receive an answer that is greater than 512 bytes. This test simulates how named makes its initial queries. Most signed responses fit between 512 bytes and 1500 bytes and are returned in a single un-fragmented UDP packet. This test is designed to check this case.

e.g.

dig +dnssec +norec +ignore ns . @L.ROOT-SERVERS.NET

; <<>> DiG 9.3.6-P1 <<>> +dnssec +norec +ignore ns . @L.ROOT-SERVERS.NET
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 381
;; flags: qr aa; QUERY: 1, ANSWER: 14, AUTHORITY: 0, ADDITIONAL: 21

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;.				IN	NS

;; ANSWER SECTION:
.			518400	IN	NS	a.root-servers.net.
.			518400	IN	NS	b.root-servers.net.
.			518400	IN	NS	c.root-servers.net.
.			518400	IN	NS	d.root-servers.net.
.			518400	IN	NS	e.root-servers.net.
.			518400	IN	NS	f.root-servers.net.
.			518400	IN	NS	g.root-servers.net.
.			518400	IN	NS	h.root-servers.net.
.			518400	IN	NS	i.root-servers.net.
.			518400	IN	NS	j.root-servers.net.
.			518400	IN	NS	k.root-servers.net.
.			518400	IN	NS	l.root-servers.net.
.			518400	IN	NS	m.root-servers.net.
.			518400	IN	RRSIG	NS 8 0 518400 20100214000000 20100206230000 23763 . JWEPSBVBz37WV/cvjxtlBGGLFe88ojhBM3ZZW+ZC2umpQq5lMSnE/3vO WGVWq8gOFs/mlmXttk80WxwZfUvgDXddNtqbNoNWZaGPbH9F2O+B6yDX n6jE4EcMRcjoL752uZTDMZ8WHEiEorjxsVYr1ae1MYZO5K0CqzX/qkUW 1DY=

;; ADDITIONAL SECTION:
a.root-servers.net.	518400	IN	A	198.41.0.4
b.root-servers.net.	518400	IN	A	192.228.79.201
c.root-servers.net.	518400	IN	A	192.33.4.12
d.root-servers.net.	518400	IN	A	128.8.10.90
e.root-servers.net.	518400	IN	A	192.203.230.10
f.root-servers.net.	518400	IN	A	192.5.5.241
g.root-servers.net.	518400	IN	A	192.112.36.4
h.root-servers.net.	518400	IN	A	128.63.2.53
i.root-servers.net.	518400	IN	A	192.36.148.17
j.root-servers.net.	518400	IN	A	192.58.128.30
k.root-servers.net.	518400	IN	A	193.0.14.129
l.root-servers.net.	518400	IN	A	199.7.83.42
m.root-servers.net.	518400	IN	A	202.12.27.33
a.root-servers.net.	518400	IN	AAAA	2001:503:ba3e::2:30
f.root-servers.net.	518400	IN	AAAA	2001:500:2f::f
h.root-servers.net.	518400	IN	AAAA	2001:500:1::803f:235
j.root-servers.net.	518400	IN	AAAA	2001:503:c27::2:30
k.root-servers.net.	518400	IN	AAAA	2001:7fd::1
l.root-servers.net.	518400	IN	AAAA	2001:500:3::42
m.root-servers.net.	518400	IN	AAAA	2001:dc3::35

;; Query time: 191 msec
;; SERVER: 2001:500:3::42#53(2001:500:3::42)
;; WHEN: Mon Feb  8 12:51:28 2010
;; MSG SIZE  rcvd: 801

If you get a response like this then your firewall passes UDP responses greater than 512 bytes.

If you did not get a response like this, you need to fix your firewall.

Next we will test to see whether you can get a response greater than 1500 bytes. Such responses are normally fragmented, and this test will find out whether your firewall will pass fragmented UDP packets. Failure to pass such responses will force named to fall back to using queries which are likely to trigger the use of TCP, which should be avoided. Failure to pass such answers will also slow up the resolution process.

e.g.

dig +dnssec +norec +ignore any . @L.ROOT-SERVERS.NET

; <<>> DiG 9.3.6-P1 <<>> +dnssec +norec +ignore any . @L.ROOT-SERVERS.NET
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 57084
;; flags: qr aa; QUERY: 1, ANSWER: 21, AUTHORITY: 0, ADDITIONAL: 21

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;.				IN	ANY

;; ANSWER SECTION:
.			86400	IN	NSEC	ac. NS SOA RRSIG NSEC DNSKEY
.			86400	IN	RRSIG	NSEC 8 0 86400 20100214000000 20100206230000 23763 . haTtgLwOQ9Bm2F9BRqMtAzahIuUWrjcmRjFGI5s5jGUVpjgq/MOl7wRi IJ1nLQkXThzc8hn6b3faXXIhHE/8MShzOG4wFbHwJyltx8IT9E8XP4P5 Fz9TuE3EEElNE6GZNAg8UM4r8hyv/PSM8e7offdh7pg32kfW6fgoLsHy 8yQ=
.			86400	IN	DNSKEY	256 3 8 AwEAAa1Lh++++++++++++++++THIS/IS/AN/INVALID/KEY/AND/SHOU LD/NOT/BE/USED/CONTACT/ROOTSIGN/AT/ICANN/DOT/ORG/FOR/MOR E/INFORMATION+++++++++++++++++++++++++++++++++++++++++++ +++++++8
.			86400	IN	DNSKEY	257 3 8 AwEAAawBe++++++++++++++++THIS/IS/AN/INVALID/KEY/AND/SHOU LD/NOT/BE/USED/CONTACT/ROOTSIGN/AT/ICANN/DOT/ORG/FOR/MOR E/INFORMATION+++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++8=
.			86400	IN	RRSIG	DNSKEY 8 0 86400 20100214235959 20100131000000 19324 . v2DVoP16w3dqsOooCxAb393ExF6p1t3d3qJsYPkeV96/t3HIuVLnxpbV 02Wx+BR7dwLiURASmebvEhZrR4gNqO15M5gerrzDdY0IXA0q0xVAUj/J NvkdiniXjoQYGUwjJsdfqxvD7NQPtSz4YTuOvMlVffV1F2Bc6Woid7AK JGkb24MeQlAMy/gQqcLPs6c3a9RvZEwofMul66bUswGS+YsL8x9A6Cbt 1bdyhRUNYSl7AifA4++Pu+0MLpbrxH7DLI8O9ZfCA3LsEQUOFjYA+2jJ mzgFqZAU0HvxeQyStnLF3/bf7qifRegrn6+cTKjKtUZ52/kUFiaqgT2t 9TemTg==
.			518400	IN	NS	a.root-servers.net.
.			518400	IN	NS	b.root-servers.net.
.			518400	IN	NS	c.root-servers.net.
.			518400	IN	NS	d.root-servers.net.
.			518400	IN	NS	e.root-servers.net.
.			518400	IN	NS	f.root-servers.net.
.			518400	IN	NS	g.root-servers.net.
.			518400	IN	NS	h.root-servers.net.
.			518400	IN	NS	i.root-servers.net.
.			518400	IN	NS	j.root-servers.net.
.			518400	IN	NS	k.root-servers.net.
.			518400	IN	NS	l.root-servers.net.
.			518400	IN	NS	m.root-servers.net.
.			518400	IN	RRSIG	NS 8 0 518400 20100214000000 20100206230000 23763 . JWEPSBVBz37WV/cvjxtlBGGLFe88ojhBM3ZZW+ZC2umpQq5lMSnE/3vO WGVWq8gOFs/mlmXttk80WxwZfUvgDXddNtqbNoNWZaGPbH9F2O+B6yDX n6jE4EcMRcjoL752uZTDMZ8WHEiEorjxsVYr1ae1MYZO5K0CqzX/qkUW 1DY=
.			86400	IN	SOA	a.root-servers.net. nstld.verisign-grs.com. 2010020701 1800 900 604800 86400
.			86400	IN	RRSIG	SOA 8 0 86400 20100214000000 20100206230000 23763 . KA46XFSIJT3xKdvlo2av5FmeFl5R8etArvA9PLJb4JUz2jioqYTjhDbT 6L5kJQaiavMF1Lic5spulaHlCHmVy+gLetI49Nc8htnd0QPWTn/MG3do isDlv9nh6uCR6cJj5W/anIkubiLHBmO11QLwVNa1IybTgTCKHNwefxG0 i/M=

;; ADDITIONAL SECTION:
a.root-servers.net.	518400	IN	A	198.41.0.4
b.root-servers.net.	518400	IN	A	192.228.79.201
c.root-servers.net.	518400	IN	A	192.33.4.12
d.root-servers.net.	518400	IN	A	128.8.10.90
e.root-servers.net.	518400	IN	A	192.203.230.10
f.root-servers.net.	518400	IN	A	192.5.5.241
g.root-servers.net.	518400	IN	A	192.112.36.4
h.root-servers.net.	518400	IN	A	128.63.2.53
i.root-servers.net.	518400	IN	A	192.36.148.17
j.root-servers.net.	518400	IN	A	192.58.128.30
k.root-servers.net.	518400	IN	A	193.0.14.129
l.root-servers.net.	518400	IN	A	199.7.83.42
m.root-servers.net.	518400	IN	A	202.12.27.33
a.root-servers.net.	518400	IN	AAAA	2001:503:ba3e::2:30
f.root-servers.net.	518400	IN	AAAA	2001:500:2f::f
h.root-servers.net.	518400	IN	AAAA	2001:500:1::803f:235
j.root-servers.net.	518400	IN	AAAA	2001:503:c27::2:30
k.root-servers.net.	518400	IN	AAAA	2001:7fd::1
l.root-servers.net.	518400	IN	AAAA	2001:500:3::42
m.root-servers.net.	518400	IN	AAAA	2001:dc3::35

;; Query time: 191 msec
;; SERVER: 2001:500:3::42#53(2001:500:3::42)
;; WHEN: Mon Feb  8 13:15:19 2010
;; MSG SIZE  rcvd: 1906

If you get a reponse like this then your firewall passes UDP responses greater than 1500 bytes.

If you did not get a response like this, you need to fix your firewall.

Next we need to see whether your firewall passes outbound TCP queries. Even when using EDNS, some answers will not fit into a UDP packet. Such responses require queries to be performed over TCP.

e.g.

dig +dnssec +norec +vc any . @L.ROOT-SERVERS.NET

; <<>> DiG 9.3.6-P1 <<>> +dnssec +norec +vc any . @L.ROOT-SERVERS.NET
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 20036
;; flags: qr aa; QUERY: 1, ANSWER: 21, AUTHORITY: 0, ADDITIONAL: 21

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;.				IN	ANY

;; ANSWER SECTION:
.			86400	IN	NSEC	ac. NS SOA RRSIG NSEC DNSKEY
.			86400	IN	RRSIG	NSEC 8 0 86400 20100214000000 20100206230000 23763 . haTtgLwOQ9Bm2F9BRqMtAzahIuUWrjcmRjFGI5s5jGUVpjgq/MOl7wRi IJ1nLQkXThzc8hn6b3faXXIhHE/8MShzOG4wFbHwJyltx8IT9E8XP4P5 Fz9TuE3EEElNE6GZNAg8UM4r8hyv/PSM8e7offdh7pg32kfW6fgoLsHy 8yQ=
.			86400	IN	DNSKEY	256 3 8 AwEAAa1Lh++++++++++++++++THIS/IS/AN/INVALID/KEY/AND/SHOU LD/NOT/BE/USED/CONTACT/ROOTSIGN/AT/ICANN/DOT/ORG/FOR/MOR E/INFORMATION+++++++++++++++++++++++++++++++++++++++++++ +++++++8
.			86400	IN	DNSKEY	257 3 8 AwEAAawBe++++++++++++++++THIS/IS/AN/INVALID/KEY/AND/SHOU LD/NOT/BE/USED/CONTACT/ROOTSIGN/AT/ICANN/DOT/ORG/FOR/MOR E/INFORMATION+++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++8=
.			86400	IN	RRSIG	DNSKEY 8 0 86400 20100214235959 20100131000000 19324 . v2DVoP16w3dqsOooCxAb393ExF6p1t3d3qJsYPkeV96/t3HIuVLnxpbV 02Wx+BR7dwLiURASmebvEhZrR4gNqO15M5gerrzDdY0IXA0q0xVAUj/J NvkdiniXjoQYGUwjJsdfqxvD7NQPtSz4YTuOvMlVffV1F2Bc6Woid7AK JGkb24MeQlAMy/gQqcLPs6c3a9RvZEwofMul66bUswGS+YsL8x9A6Cbt 1bdyhRUNYSl7AifA4++Pu+0MLpbrxH7DLI8O9ZfCA3LsEQUOFjYA+2jJ mzgFqZAU0HvxeQyStnLF3/bf7qifRegrn6+cTKjKtUZ52/kUFiaqgT2t 9TemTg==
.			518400	IN	NS	a.root-servers.net.
.			518400	IN	NS	b.root-servers.net.
.			518400	IN	NS	c.root-servers.net.
.			518400	IN	NS	d.root-servers.net.
.			518400	IN	NS	e.root-servers.net.
.			518400	IN	NS	f.root-servers.net.
.			518400	IN	NS	g.root-servers.net.
.			518400	IN	NS	h.root-servers.net.
.			518400	IN	NS	i.root-servers.net.
.			518400	IN	NS	j.root-servers.net.
.			518400	IN	NS	k.root-servers.net.
.			518400	IN	NS	l.root-servers.net.
.			518400	IN	NS	m.root-servers.net.
.			518400	IN	RRSIG	NS 8 0 518400 20100214000000 20100206230000 23763 . JWEPSBVBz37WV/cvjxtlBGGLFe88ojhBM3ZZW+ZC2umpQq5lMSnE/3vO WGVWq8gOFs/mlmXttk80WxwZfUvgDXddNtqbNoNWZaGPbH9F2O+B6yDX n6jE4EcMRcjoL752uZTDMZ8WHEiEorjxsVYr1ae1MYZO5K0CqzX/qkUW 1DY=
.			86400	IN	SOA	a.root-servers.net. nstld.verisign-grs.com. 2010020701 1800 900 604800 86400
.			86400	IN	RRSIG	SOA 8 0 86400 20100214000000 20100206230000 23763 . KA46XFSIJT3xKdvlo2av5FmeFl5R8etArvA9PLJb4JUz2jioqYTjhDbT 6L5kJQaiavMF1Lic5spulaHlCHmVy+gLetI49Nc8htnd0QPWTn/MG3do isDlv9nh6uCR6cJj5W/anIkubiLHBmO11QLwVNa1IybTgTCKHNwefxG0 i/M=

;; ADDITIONAL SECTION:
a.root-servers.net.	518400	IN	A	198.41.0.4
b.root-servers.net.	518400	IN	A	192.228.79.201
c.root-servers.net.	518400	IN	A	192.33.4.12
d.root-servers.net.	518400	IN	A	128.8.10.90
e.root-servers.net.	518400	IN	A	192.203.230.10
f.root-servers.net.	518400	IN	A	192.5.5.241
g.root-servers.net.	518400	IN	A	192.112.36.4
h.root-servers.net.	518400	IN	A	128.63.2.53
i.root-servers.net.	518400	IN	A	192.36.148.17
j.root-servers.net.	518400	IN	A	192.58.128.30
k.root-servers.net.	518400	IN	A	193.0.14.129
l.root-servers.net.	518400	IN	A	199.7.83.42
m.root-servers.net.	518400	IN	A	202.12.27.33
a.root-servers.net.	518400	IN	AAAA	2001:503:ba3e::2:30
f.root-servers.net.	518400	IN	AAAA	2001:500:2f::f
h.root-servers.net.	518400	IN	AAAA	2001:500:1::803f:235
j.root-servers.net.	518400	IN	AAAA	2001:503:c27::2:30
k.root-servers.net.	518400	IN	AAAA	2001:7fd::1
l.root-servers.net.	518400	IN	AAAA	2001:500:3::42
m.root-servers.net.	518400	IN	AAAA	2001:dc3::35

;; Query time: 380 msec
;; SERVER: 2001:500:3::42#53(2001:500:3::42)
;; WHEN: Mon Feb  8 13:18:19 2010
;; MSG SIZE  rcvd: 1906

If you did not receive a answer like this, you need to fix your firewall.

If all the above tests succeeded, then you should have no issues when all the root servers are responding with signed versions of the root zone.

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