On Mon, Jul 24, 2017 at 04:19:10PM +0200, Paolo Abeni wrote:
> Once that a system enter the buggy status, do the packets reach the
> relevant socket's queue?
> 
> ss -u
> nstat |grep -e Udp -e Ip
> 
> will help checking that.

I now have the issue on one machine, a Xen guest acting as authoritative
nameserver for my domains. Here are the outputs during normal use, with
artificial queries coming in on eth0:

[9/1075]mh@impetus:~ $ ss -u
Recv-Q Send-Q                                     Local Address:Port            
                                          Peer Address:Port                
0      0                                              127.0.0.1:56547           
                                             127.0.0.1:domain               
0      0                                         216.231.132.60:27667           
                                            198.41.0.4:domain               
0      0                                         216.231.132.60:44121           
                                               8.8.8.8:domain               
0      0                                         216.231.132.60:29814           
                                            198.41.0.4:domain               
[10/1076]mh@impetus:~ $ ss -u
Recv-Q Send-Q                                     Local Address:Port            
                                          Peer Address:Port                
[11/1076]mh@impetus:~ $ ss -u
Recv-Q Send-Q                                     Local Address:Port            
                                          Peer Address:Port                
[12/1076]mh@impetus:~ $ ss -u
Recv-Q Send-Q                                     Local Address:Port            
                                          Peer Address:Port                
[13/1076]mh@impetus:~ $ ss -u
Recv-Q Send-Q                                     Local Address:Port            
                                          Peer Address:Port                
[14/1076]mh@impetus:~ $ nstat  | grep -e Udp -e Ip
IpInReceives                    400688             0.0
IpInAddrErrors                  18567              0.0
IpInUnknownProtos               3                  0.0
IpInDelivers                    330634             0.0
IpOutRequests                   283637             0.0
UdpInDatagrams                  145860             0.0
UdpNoPorts                      1313               0.0
UdpInErrors                     9356               0.0
UdpOutDatagrams                 153093             0.0
UdpIgnoredMulti                 34148              0.0
Ip6InReceives                   161178             0.0
Ip6InNoRoutes                   8                  0.0
Ip6InDelivers                   73841              0.0
Ip6OutRequests                  77575              0.0
Ip6InMcastPkts                  87332              0.0
Ip6OutMcastPkts                 109                0.0
Ip6InOctets                     21880674           0.0
Ip6OutOctets                    9633059            0.0
Ip6InMcastOctets                9371483            0.0
Ip6OutMcastOctets               6636               0.0
Ip6InNoECTPkts                  161202             0.0
Ip6InECT1Pkts                   15                 0.0
Ip6InECT0Pkts                   11                 0.0
Ip6InCEPkts                     4                  0.0
Udp6InDatagrams                 11725              0.0
Udp6NoPorts                     2                  0.0
Udp6InErrors                    1989               0.0
Udp6OutDatagrams                14483              0.0
IpExtInBcastPkts                34148              0.0
IpExtInOctets                   47462716           0.0
IpExtOutOctets                  31262696           0.0
IpExtInBcastOctets              7476059            0.0
IpExtInNoECTPkts                400178             0.0
IpExtInECT1Pkts                 22                 0.0
IpExtInECT0Pkts                 481                0.0
IpExtInCEPkts                   14                 0.0
[15/1077]mh@impetus:~ $ nstat  | grep -e Udp -e Ip
IpInReceives                    25                 0.0
IpInDelivers                    25                 0.0
IpOutRequests                   16                 0.0
UdpInDatagrams                  1                  0.0
UdpInErrors                     24                 0.0
UdpOutDatagrams                 16                 0.0
Ip6InReceives                   15                 0.0
Ip6InDelivers                   14                 0.0
Ip6OutRequests                  12                 0.0
Ip6InMcastPkts                  1                  0.0
Ip6InOctets                     1219               0.0
Ip6OutOctets                    4384               0.0
Ip6InMcastOctets                131                0.0
Ip6InNoECTPkts                  15                 0.0
IpExtInOctets                   11779              0.0
IpExtOutOctets                  1023               0.0
IpExtInNoECTPkts                25                 0.0
[16/1077]mh@impetus:~ $ nstat  | grep -e Udp -e Ip
IpInReceives                    24                 0.0
IpInDelivers                    24                 0.0
IpOutRequests                   18                 0.0
UdpInErrors                     22                 0.0
UdpOutDatagrams                 16                 0.0
Ip6InReceives                   15                 0.0
Ip6InDelivers                   12                 0.0
Ip6OutRequests                  10                 0.0
Ip6InMcastPkts                  3                  0.0
Ip6InOctets                     1160               0.0
Ip6OutOctets                    2456               0.0
Ip6InMcastOctets                216                0.0
Ip6InNoECTPkts                  15                 0.0
IpExtInOctets                   8612               0.0
IpExtOutOctets                  1127               0.0
IpExtInNoECTPkts                24                 0.0
[17/1077]mh@impetus:~ $ nstat  | grep -e Udp -e Ip
IpInReceives                    5                  0.0
IpInDelivers                    4                  0.0
IpOutRequests                   3                  0.0
UdpNoPorts                      1                  0.0
UdpInErrors                     2                  0.0
UdpOutDatagrams                 1                  0.0
Ip6InReceives                   12                 0.0
Ip6InDelivers                   12                 0.0
Ip6OutRequests                  10                 0.0
Ip6InOctets                     944                0.0
Ip6OutOctets                    2364               0.0
Ip6InNoECTPkts                  12                 0.0
IpExtInOctets                   429                0.0
IpExtOutOctets                  226                0.0
IpExtInNoECTPkts                5                  0.0
[18/1077]mh@impetus:~ $

And here, hopefully a bit more helpful:

[19/1078]mh@impetus:~ $ ss -u ; nstat  | grep -e Udp -e Ip ; dig +time=2 
@8.8.8.8 zugschlus.de mx ; ss -u ; nstat  | grep -e Udp -e Ip 
Recv-Q Send-Q                                     Local Address:Port            
                                          Peer Address:Port                
0      0                                         216.231.132.60:27333           
                                            198.41.0.4:domain               
0      0                                         216.231.132.60:38101           
                                            198.41.0.4:domain               
0      0                                         216.231.132.60:15836           
                                            198.41.0.4:domain               
0      0                                         216.231.132.60:50655           
                                               8.8.8.8:domain               
0      0                                         216.231.132.60:41953           
                                            198.41.0.4:domain               
0      0                                         216.231.132.60:6888            
                                            198.41.0.4:domain               
0      0                                         216.231.132.60:51441           
                                            198.41.0.4:domain               
0      0                                         216.231.132.60:42503           
                                            198.41.0.4:domain               
0      0                                         216.231.132.60:12575           
                                            198.41.0.4:domain               
0      0                                         216.231.132.60:13857           
                                            198.41.0.4:domain
0      0                                         216.231.132.60:16419           
                                         192.36.148.17:domain
0      0                                         216.231.132.60:39227           
                                            198.41.0.4:domain
0      0                                              127.0.0.1:54608           
                                             127.0.0.1:domain
0      0                                         216.231.132.60:20818           
                                            198.41.0.4:domain
0      0                                         216.231.132.60:56662           
                                            198.41.0.4:domain
0      0                                         216.231.132.60:48259           
                                         192.36.148.17:domain
0      0                                         216.231.132.60:37803           
                                            198.41.0.4:domain
IpInReceives                    59                 0.0
IpInAddrErrors                  1                  0.0
IpInDelivers                    56                 0.0
IpOutRequests                   50                 0.0
UdpInDatagrams                  1                  0.0
UdpInErrors                     50                 0.0
UdpOutDatagrams                 47                 0.0
UdpIgnoredMulti                 1                  0.0
Ip6InReceives                   75                 0.0
Ip6InDelivers                   73                 0.0
Ip6OutRequests                  64                 0.0
Ip6InMcastPkts                  2                  0.0
Ip6InOctets                     7837               0.0
Ip6OutOctets                    11876              0.0
Ip6InMcastOctets                279                0.0
Ip6InNoECTPkts                  75                 0.0
Udp6InErrors                    3                  0.0
IpExtInBcastPkts                1                  0.0
IpExtInOctets                   18447              0.0
IpExtOutOctets                  3478               0.0
IpExtInBcastOctets              183                0.0
IpExtInNoECTPkts                59                 0.0

; <<>> DiG 9.10.3-P4-Debian <<>> +time=2 @8.8.8.8 zugschlus.de mx
; (1 server found)
;; global options: +cmd
;; connection timed out; no servers could be reached
Recv-Q Send-Q                                     Local Address:Port            
                                          Peer Address:Port
0      0                                         216.231.132.60:7879            
                                          202.12.27.33:domain
0      0                                         216.231.132.60:32711           
                                          202.12.27.33:domain
0      0                                         216.231.132.60:54238           
                                          202.12.27.33:domain
0      0                                         216.231.132.60:30948           
                                        192.228.79.201:domain
0      0                                         216.231.132.60:4106            
                                          202.12.27.33:domain
0      0                                         216.231.132.60:6667            
                                          202.12.27.33:domain
0      0                                         216.231.132.60:2090            
                                        192.228.79.201:domain
0      0                                         216.231.132.60:60459           
                                        192.228.79.201:domain
0      0                                         216.231.132.60:16427           
                                          202.12.27.33:domain
0      0                                         216.231.132.60:9019            
                                          202.12.27.33:domain
0      0                                         216.231.132.60:2113            
                                          202.12.27.33:domain
0      0                                         216.231.132.60:34907           
                                          202.12.27.33:domain
0      0                                         216.231.132.60:34654           
                                          202.12.27.33:domain
0      0                                         216.231.132.60:47725           
                                          202.12.27.33:domain
0      0                                         216.231.132.60:35774           
                                          202.12.27.33:domain
IpInReceives                    38                 0.0
IpInDelivers                    38                 0.0
IpOutRequests                   38                 0.0
UdpInDatagrams                  2                  0.0
UdpInErrors                     34                 0.0
UdpOutDatagrams                 36                 0.0
Ip6InReceives                   14                 0.0
Ip6InDelivers                   13                 0.0
Ip6OutRequests                  13                 0.0
Ip6InMcastPkts                  1                  0.0
Ip6InOctets                     1046               0.0
Ip6OutOctets                    6277               0.0
Ip6InMcastOctets                133                0.0
Ip6InNoECTPkts                  13                 0.0
Ip6InECT0Pkts                   1                  0.0
Udp6InDatagrams                 1                  0.0
Udp6OutDatagrams                1                  0.0
IpExtInOctets                   15963              0.0
IpExtOutOctets                  2397               0.0
IpExtInNoECTPkts                37                 0.0
IpExtInECT0Pkts                 1                  0.0
[20/1079]mh@impetus:~ $

I am afraid I cannot keep this state for much longer than a few
additional hours as this is an authoritative name server...

Greetings
Marc

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