I am using EC2 instances with amazon linux installed (with amazon dns server settings, which comes from DHCP), as well as an RDS database. The EC2 instances are behind ELB and get high traffic. The application that I use is coded with PHP.
The problem is when PHP tries to connect to the RDS database, sometimes it returns the following error:
PHP Warning: mysqli_connect(): (HY000/2005): Unknown MySQL server host ...
It doesn't happen a lot but sometimes it is geting worse; I'm getting thousands of error events with that message.
Is there any suggestion for diagnosing the problem? I was thinking about dumping all DNS traffic to a file and checking it but servers get really high traffic so it will be hard to track from that file.
Ip: 197171459 total packets received 1 with invalid addresses 0 forwarded 0 incoming packets discarded 197171458 incoming packets delivered 175015443 requests sent out Icmp: 12528 ICMP messages received 0 input ICMP message failed. ICMP input histogram: destination unreachable: 188 echo requests: 12340 12559 ICMP messages sent 0 ICMP messages failed ICMP output histogram: destination unreachable: 219 echo replies: 12340 IcmpMsg: InType3: 188 InType8: 12340 OutType0: 12340 OutType3: 219 Tcp: 5231380 active connections openings 3978862 passive connection openings 881 failed connection attempts 6420 connection resets received 17 connections established 191630575 segments received 200105352 segments send out 2797151 segments retransmited 0 bad segments received. 6910 resets sent Udp: 5577451 packets received 219 packets to unknown port received. 0 packet receive errors 5577700 packets sent UdpLite: TcpExt: 172 invalid SYN cookies received 808 resets received for embryonic SYN_RECV sockets 7176788 TCP sockets finished time wait in fast timer 507 packets rejects in established connections because of timestamp 448055 delayed acks sent 2927 delayed acks further delayed because of locked socket Quick ack mode was activated 2433 times 94865861 packets directly queued to recvmsg prequeue. 16611185 packets directly received from backlog 54150864749 packets directly received from prequeue 2158966 packets header predicted 79141174 packets header predicted and directly queued to user 40780030 acknowledgments not containing data received 56946553 predicted acknowledgments 84 times recovered from packet loss due to SACK data Detected reordering 4 times using FACK Detected reordering 11 times using SACK Detected reordering 69 times using time stamp 70 congestion windows fully recovered 1241 congestion windows partially recovered using Hoe heuristic TCPDSACKUndo: 13 2491 congestion windows recovered after partial ack 0 TCP data loss events 220 timeouts after SACK recovery 104 fast retransmits 99 forward retransmits 7 retransmits in slow start 2792531 other TCP timeouts 22 times receiver scheduled too late for direct processing 2423 DSACKs sent for old packets 2785871 DSACKs received 5162 connections reset due to unexpected data 921 connections reset due to early user close 135 connections aborted due to timeout TCPDSACKIgnoredOld: 533 TCPDSACKIgnoredNoUndo: 393 TCPSackShifted: 477 TCPSackMerged: 536 TCPSackShiftFallback: 2709 TCPBacklogDrop: 46 TCPDeferAcceptDrop: 3906058 IpExt: InOctets: 69400712361 OutOctets: 94841399143