Cannot handle MTU-reklated timeouts?
From one vantage point, Knot resolver cannot get answers with DNS+DNSSEC, from any of the name servers. It cannot retry without EDNS because it would break DNSSEC but shouldn't it retry automatically with TCP?
% dig A www.afnic.fr
; <<>> DiG 9.11.2-P1-1-Debian <<>> A www.afnic.fr
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 6594
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags: do; udp: 4096
;; QUESTION SECTION:
;www.afnic.fr. IN A
;; Query time: 2066 msec
;; SERVER: 192.168.2.254#53(192.168.2.254)
;; WHEN: Thu Jan 25 15:44:33 UTC 2018
;; MSG SIZE rcvd: 41
% dig @ns2.nic.fr A www.afnic.fr
; <<>> DiG 9.11.2-P1-1-Debian <<>> @ns2.nic.fr A www.afnic.fr
; (2 servers found)
;; global options: +cmd
;; connection timed out; no servers could be reached
% dig +nodnssec @ns2.nic.fr A www.afnic.fr
; <<>> DiG 9.11.2-P1-1-Debian <<>> +nodnssec @ns2.nic.fr A www.afnic.fr
; (2 servers found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 3789
;; flags: qr aa rd; QUERY: 1, ANSWER: 3, AUTHORITY: 5, ADDITIONAL: 10
;; WARNING: recursion requested but not available
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
; COOKIE: 4bbe8d53108364eabca57f7a5a69fbee7ec3de6b2d63de14 (good)
;; QUESTION SECTION:
;www.afnic.fr. IN A
;; ANSWER SECTION:
www.afnic.fr. 600 IN CNAME www.nic.fr.
www.nic.fr. 600 IN CNAME lb01-1.nic.fr.
lb01-1.nic.fr. 600 IN A 192.134.5.24
;; AUTHORITY SECTION:
nic.fr. 172800 IN NS ns2.nic.fr.
nic.fr. 172800 IN NS ns1.nic.fr.
nic.fr. 172800 IN NS ns3.nic.fr.
nic.fr. 172800 IN NS ns6.ext.nic.fr.
nic.fr. 172800 IN NS ns1.ext.nic.fr.
;; ADDITIONAL SECTION:
ns1.nic.fr. 172800 IN AAAA 2001:67c:2218:2::4:1
ns2.nic.fr. 172800 IN AAAA 2001:660:3005:1::1:2
ns3.nic.fr. 172800 IN AAAA 2001:660:3006:1::1:1
ns6.ext.nic.fr. 172800 IN AAAA 2001:620:0:ff::2f
ns1.ext.nic.fr. 172800 IN A 193.51.208.13
ns1.nic.fr. 172800 IN A 192.134.4.1
ns2.nic.fr. 172800 IN A 192.93.0.4
ns3.nic.fr. 172800 IN A 192.134.0.49
ns6.ext.nic.fr. 172800 IN A 130.59.31.29
;; Query time: 7 msec
;; SERVER: 2001:660:3005:1::1:2#53(2001:660:3005:1::1:2)
;; WHEN: Thu Jan 25 15:46:54 UTC 2018
;; MSG SIZE rcvd: 414
% dig +tcp @ns2.nic.fr A www.afnic.fr
; <<>> DiG 9.11.2-P1-1-Debian <<>> +tcp @ns2.nic.fr A www.afnic.fr
; (2 servers found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 25722
;; flags: qr aa rd; QUERY: 1, ANSWER: 6, AUTHORITY: 6, ADDITIONAL: 19
;; WARNING: recursion requested but not available
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags: do; udp: 4096
; COOKIE: 55f9b6f5342698c35f05f3875a69fbfc73e77c5cf608ac42 (good)
;; QUESTION SECTION:
;www.afnic.fr. IN A
;; ANSWER SECTION:
www.afnic.fr. 600 IN CNAME www.nic.fr.
www.afnic.fr. 600 IN RRSIG CNAME 8 3 600 (
20180213225107 20180115074143 64491 afnic.fr.
OrzJ7+UmRiFpyz33DA7VvL7ZQADDd862bR6PSWrCuSIv
U4eE6BHKDjXkzDpp2TYUI+n+NSmkkOkmsD4tlOhfo8ao
573RhBnmoPVO3dnqrUWZf8omi4KfpS9sZmybV30DzN8m
lB2UbO5FtvN5p3L8kVWmqcxZyW/fNO07IUf7bwoIAoaS
XzJQfAPfoa7i93kbfleI5OTpjwmfHHEdcnl+a6bpcv2i
Uquo34c5SfD3aWThw54mS+zQXEhaW38Oxbnfsz0tSy4V
0ng4o4PVFqF/wpWt1x1NxfCe6wg6ODFhutuzDu7EQdH0
OI6Btn9lsMEl5hNEJGksudyjlvV9SkYyug== )
www.nic.fr. 600 IN CNAME lb01-1.nic.fr.
www.nic.fr. 600 IN RRSIG CNAME 8 3 600 (
20180222212642 20180123144323 64491 nic.fr.
TUVAhMpnA+WtxiisXpBtVf+MkkwLm1vApGmzy23lc9+m
/Mi6yBuiRaXVuWUZ/M8CoOzm8vL7maU4hI5tgqwccMem
nAnrgM34m93rKf5AUnYLw85OHPgf7d6Kr79O6UOI5tUX
R5l4pyZv3HpYPNsOmSUxkXYURZrByuF6IvXV2J+yzvDV
k28e5mTAMc2sTrDZJ/7ZGF6vi6FoGp/Qe/gujPv3GBLw
CMCNd9FXYyLP8I22iLoPPxcgOhulUBNHuzGjM3X4GIAm
9D3Jjz/Hrm2G6mFgcsyt0CUOSK+ClZx4temUrEI73KPK
I6VWzvgLVV2pzdpDDNaZUJHnDY0ovfxtgg== )
lb01-1.nic.fr. 600 IN A 192.134.5.24
lb01-1.nic.fr. 600 IN RRSIG A 8 3 600 (
20180220192248 20180122044323 64491 nic.fr.
JoKmE6E79tkrh4elgng92IJ4lErXM1yMX8pu6jsh/6Gc
hrY6KsCh9YLAab4qKqDGOEjoqH74op3HWw0GY3usAjPN
k4+FtZoGfZGLLfREOn0oZSgSSHjiga+VcnVFS7Fw2v8s
WI+TzCsOYRyDOtUSMielnanfdxvi4HtNieh0u9m+tz5G
BwVsQAdMD3o34P48DBZASjXCNvcoMpTCI5E0r6wXhnqI
vNAAW2JS1pHLXUxxqme22p/g8HWcZ+cW4uLTBAlZW8Xe
6uLQYMircoT7EhPOndQMrzHB0rqjrHYdXyJe/htJEcfP
XR5sQqOGQFMuTiPXXv1uP0v0wme1161kkA== )
;; AUTHORITY SECTION:
nic.fr. 172800 IN NS ns3.nic.fr.
nic.fr. 172800 IN NS ns2.nic.fr.
nic.fr. 172800 IN NS ns1.ext.nic.fr.
nic.fr. 172800 IN NS ns6.ext.nic.fr.
nic.fr. 172800 IN NS ns1.nic.fr.
nic.fr. 172800 IN RRSIG NS 8 2 172800 (
20180210074028 20180111115419 64491 nic.fr.
EcoBebipzdTzUEP8TGx5m72+NlQV7a57S9aXZyYLxmiI
1n8w23ScIWE5w5aBUwZdlg9wDvkBSRX17VUxG37wxEgR
VxD+qH0vLIl45GjRUSaWbWXPLjWTuAoy+5euDKsdT/Ej
x1wOu0KcUPxKBur6eTEj0TaNETSD7h4b5eTouff/JoWV
T727DCkr9lYSMqrC7jFav6YiJRlv7zXMWRFMPlD4hssC
ejLQ2E3OlPE+0SoqpIVJI0TqzNJYEiPnSJ7AaDuI2dji
fT6E4iArnU8XSRtXjMrtBJd6oRNJn+v9qH73oIiMpZC1
5AEbfH1q9tj8MT1FCa43iEePOr36+WFUbg== )
;; ADDITIONAL SECTION:
ns1.nic.fr. 172800 IN AAAA 2001:67c:2218:2::4:1
ns2.nic.fr. 172800 IN AAAA 2001:660:3005:1::1:2
ns3.nic.fr. 172800 IN AAAA 2001:660:3006:1::1:1
ns6.ext.nic.fr. 172800 IN AAAA 2001:620:0:ff::2f
ns1.ext.nic.fr. 172800 IN A 193.51.208.13
ns1.nic.fr. 172800 IN A 192.134.4.1
ns2.nic.fr. 172800 IN A 192.93.0.4
ns3.nic.fr. 172800 IN A 192.134.0.49
ns6.ext.nic.fr. 172800 IN A 130.59.31.29
ns1.ext.nic.fr. 172800 IN RRSIG A 8 4 172800 (
20180218001157 20180119024323 64491 nic.fr.
EbvmCLKgiLEojrvu+G9jNIWtAX0zuu/7NOpU1Tisjpop
Zx9hX7qYb1HLCH1qhU8s3iBhFedqSUXF6rtZHzz2qtJ+
h+G4UgNAWCyDsHVeH5lisXirUDx+Hp9+Z1EAoUb5Kf6F
0f2tNAt6MkJYBg+DjxcuRDRWoG5LBBphPz8mhgnoVp9r
Jh4C6XqfrT9T18k999ijdIcIfXwhP+ghmXhg0zW2PIUr
3B9KKkfo4YDWJOZ2hpH3wM8l0Ka/C6v9naU1RQ+j01Ay
izENN2xnevbaTCp4D6FYE2JVWHASuYlbZynqs0N4K6R/
EoGjHHkG2xgSgHTpeHbs0YYzQbdJo63m5w== )
ns1.nic.fr. 172800 IN RRSIG A 8 3 172800 (
20180221220313 20180123084323 64491 nic.fr.
Rnwhloou1+lYWji0oVvMrODRsank5EHRLDN85WVXv4JW
z2fHI+5IZLWaGlUProhkwqfobH0rjWFA/phPkhL0Bije
gfhTGpQNheD5BN8MfPEMxYYQgZ7O/V6bWGh3M+IezrZv
rBESOBpCnGbdadxJvUlZAmaYWJ2AL1wKYQ6OWP/viz0X
AfIE6qxWfGa4xG3LrEWSL7Y+RFrM41TaY+Dd2fEnOJbo
pUXtd6EC871zxdmzUQJbcpjWxKzXN4qysfSsk+T8H1my
R4OnbczneKb/EmAfII6BMblJyuTyVnHu+3Q0Yd8NaXtE
Sg1rL+bKwIowuIzW5wbnMRU8dQs5axgEGA== )
ns1.nic.fr. 172800 IN RRSIG AAAA 8 3 172800 (
20180219001645 20180120004323 64491 nic.fr.
ikRbTnsIqZ6s+i/QTyALVrOtdY68VtPZFyD56V0RzoTB
ZPBM25OCRERAPbNSBpStTnsUL8v8bSwV/L1kcL5dkZNz
C/TJyhZtSJHVUAUi2ngOF2BjzjF3o2L7+44Ke1vnCGmv
ObjJdjYMa9AK+/ybhvecLQ67+I84gN9v+srLmpGTeOIh
Zebzj4QKWe97GHWbzF2CroAynJg3efEaU/WTXf6lhHIW
/Tytwg1voUja/oWL55ABO6rjcMghYI/t81iBf/kwstmq
Mo6PxeiT9UKS9b18Kh2vOGmcsP3b5+L3Kz4OW1ujFV37
aklHXJbQCMcrFGH5VFO9cHoKlJxOIxSpSw== )
ns2.nic.fr. 172800 IN RRSIG A 8 3 172800 (
20180218140517 20180119224323 64491 nic.fr.
FLsCHaSy4D2o0ScOmuRb7jb2BuKuh5bXIokv/wHHSjcr
h95xA5HDToX4iP9vTplnkFPl1MNNMOKob7wB51+n6w5i
DX/WecFhktOm41M/a0RXt0FUv4YRCnWKWjGZ23pqpsO4
W7aQvk9MYVqQbjJacEyfGzeDjlYSzXwkiH4dOONFeNs6
VoE6ihnRm+9pOKReyyr50tro7PcMakIXI8S0B9wGT2y8
kRPLTcrsDobLnVCL+1H2bH0pA068n/YOPih1QZLa2Pw6
RzzypcGECRuSxm34spNSD06/gfyjRPao78cMPW/dN3Fc
1Yp7jbdX6gJ28TnmhTSFfj0xdZaBNDR0Ig== )
ns2.nic.fr. 172800 IN RRSIG AAAA 8 3 172800 (
20180216033932 20180117084323 64491 nic.fr.
QI1REwIzT+XKk1vfRT3vBeoOvJtc8raJO/8Gv4AJQQBF
MHEEvNuMiKYG7KCP4sBomh3YGgQdlKe/yWpoSIfdpg9b
naLoFXxl+aPMLEEOihgkONUIsp/D4f16xlR03vWRYPYr
vAUZCjblJ1Drb9ht9UoX5wiWuJIfhU8hjRm0kAE3duE4
mNAHbHbr2diCyFEVHiqXT9360gB9XET3PWprJoLzsn+R
N+fyTMAEruOeWVH8J3+MxrzdBKyxCS5dzUml0iPlcFr+
G5sAa4XdZK5ja2kMHuPfuIQj8C83GzPj1ZrRLpmjEpb1
DE9xnZO4obIkiAhmvB1J65CDi/2KEj5MeA== )
ns3.nic.fr. 172800 IN RRSIG A 8 3 172800 (
20180217182034 20180119004323 64491 nic.fr.
KmEbNRoN4hIRNIqoJGOTgUfviyVQ9120n21/Rs4/Io5u
snCdSWCTvf/D3Hha/LgZJorAmrREg7bBUxUaa3cnfIDh
YGOYFKR/2MX0X6Deto7SZQmE/v9US/4OEdw3Ljt3a+Wf
2wpEskAEghShiGyx6FxjC8e/u7k0+ezoj6LQ937EChmQ
8mbQ1lqGTjyNAYdjnMX4Xn+lMLZS/Pdakjfe8R2+v83b
r3kR/CaUDe83XyQcEquuZxc1ZAJRlxU9zf6eipIm2vaK
oQUhlfY9nY1lh26YWOieUOc3lRXc4WGUTJD2COhKqzAH
LXMilBwDaFyKzCKKjaEDYgXzBfzXbH+5Kg== )
ns3.nic.fr. 172800 IN RRSIG AAAA 8 3 172800 (
20180217223849 20180119024323 64491 nic.fr.
IHcI0GyJuHGaEVRKih40IEVtVRiAR6RcMvLoNVAhXfj4
7zjOdglQVSrRo0/S+rWCH7d09XLpr41lzcQH6OpDFlMT
Mvo7Qmznpq9Ji9UpZxVLvTmsjtE5jyeZLBzCct3BPz8J
ArszsygEpdKz4uKI8OM6xu7NMb/ALTaNgOo0zLYSe8xh
VRniLMmfr0yGN9iPD6Jex4Dx4YpyEQ0v4V0kxyHzY5M0
FS0naX7PdsF+0rnQ2J89LutYrT+v2P8lmGSSK1EWNbyM
dDvxn95aWmcEy797jjETaJb/fn/+ZjFf44mBYXX6e44I
rvGNUIN3M7+Xbim7AW0gcukrL3Z/S5/J9g== )
ns6.ext.nic.fr. 172800 IN RRSIG A 8 4 172800 (
20180217143618 20180118084323 64491 nic.fr.
D1KGzW/W3KxrqCw0w8t0u7sLVwwYc5xcrGkdpc85ww5z
EKEWKCx8VvfjAboeVaRgUK28Dz5YbNU7IAecGjY1I8+k
5uoVRXltm7M10Hj8lrsMn5CCnn+GUM86yBHX3d5TTYI7
fhyYOV5JBcA4HHX2i9DIUu3NucXFFIk3DeZtcDJO8G4H
/6SUQW3Dh/ro2SoVjYLwuCUv/Fxj38hSMjBhcOcTRsD9
Id+8nKSkwearfP1T6TwTCeyOFOyzpLD7F2voEgOP9hmc
IRHNPCEcBz5bG1lNK7rGlHRtX7AfXzxR7679Izu1KYfX
3cD+dhAzP+tX+O/UHEEf57gR2jKp1TzHcw== )
ns6.ext.nic.fr. 172800 IN RRSIG AAAA 8 4 172800 (
20180220151403 20180122004323 64491 nic.fr.
eao2DZ3WqSdUU+uY/etN6HD+Ior6+TbxMYyNvODVTJMy
j3+W01c1bufanNDDd1LGzLy3ZcFXzvywkD5gdh1+ehk0
8XfP2gVkguQT3ofzO5qhFQw4esmtFczVzpHc+Zkjc1uu
9cQ6MZvvVp6H+ndgAipKgB+siQvHQ1wnXhUaiL7RAU8b
ittpsr1yjweKDiKCF0NGKyhe3ALDqin7/PQRFZ40l87Y
vZxloZL9lwb+L/TiWr8lm+iGrHuqHkxCpXrIDKX7GwZL
cj4rggFVS+jwjFYZdeDYmVo9HamYL2b+PYMt+/PJlRBg
ccuvVmZyCiEyeLC4tjCoHfMWHjJZW+cAHQ== )
;; Query time: 11 msec
;; SERVER: 2001:660:3005:1::1:2#53(2001:660:3005:1::1:2)
;; WHEN: Thu Jan 25 15:47:08 UTC 2018
;; MSG SIZE rcvd: 4238(Same thing with the two other authoritative name servers.)