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AWS EC2 DNS 解析诊断

tag-icon • tag-icon domain-name-system amazon-ec2 amazon-rds rds
AWS EC2 DNS 解析诊断

我使用的是安装了 Amazon Linux 的 EC2 实例（带有 Amazon DNS 服务器设置，来自 DHCP），以及 RDS 数据库。EC2 实例位于 ELB 后面，流量很大。我使用的应用程序是用 PHP 编码的。

问题是当 PHP 尝试连接 RDS 数据库时，有时会返回以下错误：

PHP Warning:  mysqli_connect(): (HY000/2005): Unknown MySQL server host ...

这种情况并不常见，但有时情况会变得更糟；我收到了带有该消息的数千个错误事件。

有什么诊断问题的建议吗？我原本想将所有 DNS 流量转储到一个文件并进行检查，但服务器的流量非常大，因此很难从该文件进行跟踪。

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

答案1

有一个已知的 AWS 错误，会导致 DNS 解析偶尔失败：

https://forums.aws.amazon.com/thread.jspa?messageID=330465#330465

您可能需要使用持久连接进行测试，因为这将减少执行 DNS 解析的频率。

本地 DNS 缓存（例如pdns-递归器或者域名缓存) 会降低频率，但 RDS 主机名记录的 TTL 非常短（60 秒），所以这意味着问题发生的频率会大大降低，但每天仍会发生几次。

答案2

您提到流量很大。我想知道您是否遇到了网络问题。您是否已经在服务器上监控 SNMP 统计数据？您应该考虑对 IF-MIB 中的某些值进行趋势分析：

IF-MIB::ifInOctets.1 = Counter32: 117194642
IF-MIB::ifInOctets.2 = Counter32: 3406296104
IF-MIB::ifInOctets.3 = Counter32: 754235769
IF-MIB::ifInOctets.4 = Counter32: 0
IF-MIB::ifInUcastPkts.1 = Counter32: 112415844
IF-MIB::ifInUcastPkts.2 = Counter32: 352495427
IF-MIB::ifInUcastPkts.3 = Counter32: 588414566
IF-MIB::ifInUcastPkts.4 = Counter32: 0
IF-MIB::ifInNUcastPkts.1 = Counter32: 0
IF-MIB::ifInNUcastPkts.2 = Counter32: 5038722
IF-MIB::ifInNUcastPkts.3 = Counter32: 4835908
IF-MIB::ifInNUcastPkts.4 = Counter32: 0
IF-MIB::ifInDiscards.1 = Counter32: 0
IF-MIB::ifInDiscards.2 = Counter32: 0
IF-MIB::ifInDiscards.3 = Counter32: 0
IF-MIB::ifInDiscards.4 = Counter32: 0
IF-MIB::ifInErrors.1 = Counter32: 0
IF-MIB::ifInErrors.2 = Counter32: 0
IF-MIB::ifInErrors.3 = Counter32: 0
IF-MIB::ifInErrors.4 = Counter32: 0

有关更多信息：

http://www.oidview.com/mibs/0/IF-MIB.html

您还可以使用以下命令检查一些网络统计信息：

# netstat -s

不过，通常我认为，在生产中引用其他服务器时，在配置文件中使用 IP 是更好的选择。

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