前一阵子发现服务会有偶发的服务不可用的情况，记录一下这个问题的排查过程。
现象是这样的：每天到了某个时间点，就会出现服务不稳定的情况，偶发接口调不通。
线上业务使用了lvs-nginx-tomcat三层结构，首先查看tomcat监控，没有什么特别异常的情况，响应时间和错误码没发现有什么异常，CPU、IO等等指标也都正常。
再查看nginx上的监控，发现在某个时刻这个服务的5xx报错突增，大概7、8秒之后又恢复了。
继续在nginx服务器上找线索，发现Nginx在那个时间点会出现报错：
2015/12/24 10:30:38 [error] 134330: check time out with peer: 10.79.40.1xx:80
线上nginx会每秒探测后端所有服务器的某个uri，如果返回的http状态码是200则认为正常，连续3次探测失败则摘除探测失败的服务器，直到探测成功再恢复。
从日志中可以发现nginx在出问题的时间点对于后端所有tomcat的探测请求都出现了问题，导致摘除了所有后端服务器，在这段时间里请求会报502异常。
从nginx上的日志可以看到探测请求没有返回，那么请求实际发到tomcat了没有？线上业务中的探测频率是1s/次，于是到tomcat的访问日志里查找线索，过滤一个nginx对tomcat的所有探测请求：
可用看出从7:00:10-7:00:40左右的探测请求是有丢失的。
前端机的负载并不高，于是我们第一时间认为这可能是nginx到tomcat服务器的网络有问题。统计了一下线上日志，出问题的机器集中在某个网段，并且集中在一天之内的某几个时间点，这似乎也进一步印证了我们的猜测。
但到此为止仅仅是怀疑，为了证明我们的猜测，我们尝试去复现问题。我们在nginx上部署了一个简单的脚本，用curl命令对同样的tomcat发起每秒一次的请求，但结果比较诡异：
监测方式监测地址http版本频率所在服务器目的服务器问题
nginx/1.01snginxtomcat有
curl/1.01snginxtomcat无
这跟我们之前的猜测不一致，没办法，尝试在两端抓包查看网络状况，
tomcat抓包：
nginx抓包：
tomcat服务器在7:00:10已经接收了请求并且回复了ACK，7:00:13 nginx超时主动断开连接，7:00:15时tomcat才返回数据，网络的问题被排除了。
那么接下来的重点就是tomcat本身，在接收问题请求的时候，tomcat服务究竟做了什么？
还是通过简单的脚本，在容易出问题的时间段连续使用jstack打印线程栈，查找出问题时处于RUNNABLE状态的catalina线程，发现这里有一句很可疑：
这个服务用的还是比较古老的tomcat6.0.32，查看源码，可以发现在tomcat对请求header做完解析之后会调用这个函数：
MessageBytes valueMB = headers.getValue(“host”);
// Check host header
if (http11 && (valueMB == null)) {
error = true;
// 400 – Bad request
if (log.isDebugEnabled()) {
log.debug(sm.getString(“http11processor.request.prepare”)+
” host header missing”);
}
response.setStatus(400);
adapter.log(request, response, 0);
}
parseHost(valueMB);
….
/**
* Parse host.
*/
public void parseHost(MessageBytes valueMB) {
if (valueMB == null || valueMB.isNull()) {
// HTTP/1.0
// Default is what the socket tells us. Overriden if a host is
// found/parsed
request.setServerPort(socket.getLocalPort());
InetAddress localAddress = socket.getLocalAddress();
// Setting the socket-related fields. The adapter doesnt know
// about socket.
request.serverName().setString(localAddress.getHostName());
return;
}
也就是说，如果request请求的header里没有设置host，那么tomcat会使用自己服务器的hostname作为request对象的host属性。
再对比线上nginx探测的请求和curl发出的请求，可以看出nginx的探测请求确实没有带任何header，而curl请求默认是带了3个header的：
curl:
GET / HTTP/1.0
Host: localhost:8080
User-Agent: curl/7.43.0
Accept: */*
nginx:
GET / HTTP/1.0
到这里可以确认，如果请求的header里没有带Host的话就有可能出现问题。找到了hang住的位置，那么接下来的问题就是，为什么这里会hang住？
第一个问题：这个getHostByAddr在做什么？翻出jvm源码，这个函数的定义在
jdk/src/share/classes/java/net/Inet4AddressImpl.java
String getHostByAddr(byte[] addr) throws UnknownHostException;
继续研究getHostByAddr，对应的实现位于jdk/src/solaris/native/java/net/Inet6AddressImpl.c：
/*
* Class: java_net_Inet6AddressImpl
* Method: getHostByAddr
* Signature: (I)Ljava/lang/String;
*/
JNIEXPORT jstring JNICALL
Java_java_net_Inet6AddressImpl_getHostByAddr(JNIEnv *env, jobject this,jbyteArray addrArray) {
jstring ret = NULL;
ifdef AF_INET6
char host[NI_MAXHOST+1];
int error = 0;
int len = 0;
jbyte caddr[16];
if (NET_addrtransAvailable()) {
struct sockaddr_in him4;
struct sockaddr_in6 him6;
struct sockaddr *sa;
/*
* For IPv4 addresses construct a sockaddr_in structure.
*/
if ((*env)->GetArrayLength(env, addrArray) == 4) {
jint addr;
(*env)->GetByteArrayRegion(env, addrArray, 0, 4, caddr);
addr = ((caddr[0]<<24) & 0xff000000);
addr |= ((caddr[1] <<16) & 0xff0000);
addr |= ((caddr[2] <<8) & 0xff00);
addr |= (caddr[3] & 0xff);
memset((void *) &him4, 0, sizeof(him4));
him4.sin_addr.s_addr = (uint32_t) htonl(addr);
him4.sin_family = AF_INET;
sa = (struct sockaddr *) &him4;
len = sizeof(him4);
} else {
/*
* For IPv6 address construct a sockaddr_in6 structure.
*/
(*env)->GetByteArrayRegion(env, addrArray, 0, 16, caddr);
memset((void *) &him6, 0, sizeof(him6));
memcpy((void *)&(him6.sin6_addr), caddr, sizeof(struct in6_addr) );
him6.sin6_family = AF_INET6;
sa = (struct sockaddr *) &him6 ;
len = sizeof(him6) ;
}
error = (*getnameinfo_ptr)(sa, len, host, NI_MAXHOST, NULL, 0,
NI_NAMEREQD);
if (!error) {
ret = (*env)->NewStringUTF(env, host);
}
}
endif /* AF_INET6 */
if (ret == NULL) {
JNU_ThrowByName(env, JNU_JAVANETPKG “UnknownHostException”, NULL);
}
return ret;
}
getnameinfo_ptr的定义位于jdk/src/solaris/native/java/net/net_util_md.c：
getnameinfo_ptr = (getnameinfo_f)
JVM_FindLibraryEntry(RTLD_DEFAULT, “getnameinfo”);
实际是调用了glibc库函数，man一下getnameinfo
DESCRIPTION
The getnameinfo() function is the inverse of getaddrinfo(3): it converts a socket address to a corresponding host and service,
in a protocol-independent manner. It combines the functionality of gethostbyaddr(3) and getservbyport(3), but unlike those
functions, getaddrinfo(3) is reentrant and allows programs to eliminate IPv4-versus-IPv6 dependencies.
The sa argument is a pointer to a generic socket address structure (of type sockaddr_in or sockaddr_in6) of size salen that
holds the input IP address and port number. The arguments host and serv are pointers to caller-allocated buffers (of size
hostlen and servlen respectively) into which getnameinfo() places null-terminated strings containing the host and service names
respectively.
The caller can specify that no hostname (or no service name) is required by providing a NULL host (or serv) argument or a zero
hostlen (or servlen) argument. However, at least one of hostname or service name must be requested.
结合man page说明和调用的上下文可以推测出这个函数可以通过ip查host，但是是怎么查的呢？继续查找代码，首先要确定操作系统用的glibc版本：
随便在机器上编译一个c程序，使用ldd命令查看它的依赖库路径：
[root@localhost test] ldd a.out
linux-vdso.so.1 => (0x00007fff595ff000)
libc.so.6 => /lib64/libc.so.6 (0x0000003e60e00000)
/lib64/ld-linux-x86-64.so.2 (0x0000003e60600000)
[root@localhost test] /lib64/libc.so.6
GNU C Library stable release version 2.12, by Roland McGrath et al.
Copyright (C) 2010 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE.
Compiled by GNU CC version 4.4.7 20120313 (Red Hat 4.4.7-4).
Compiled on a Linux 2.6.32 system on 2013-11-21.
Available extensions:
The C stubs add-on version 2.1.2.
crypt add-on version 2.1 by Michael Glad and others
GNU Libidn by Simon Josefsson
Native POSIX Threads Library by Ulrich Drepper et al
BIND-8.2.3-T5B
RT using linux kernel aio
libc ABIs: UNIQUE IFUNC
For bug reporting instructions, please see:
https://www.gnu.org/software/libc/download.html
去gnu官网下载对应版本的glibc源代码，查看源码，可以看出getnameinfo中调用了gethostbyaddr：
while (__gethostbyaddr_r ((const void *) &(((const struct sockaddr_in *)sa)->sin_addr),
sizeof(struct in_addr), AF_INET,
&th, tmpbuf, tmpbuflen,
&h, &herrno))
if (herrno == NETDB_INTERNAL && errno == ERANGE)
tmpbuf = extend_alloca (tmpbuf, tmpbuflen, 2 * tmpbuflen);
else
break;
}
在gethostbyaddr函数中有这么一段：
switch (af) {
case AF_INET:
(void) sprintf(qbuf, “%u.%u.%u.%u.in-addr.arpa”,
(uaddr[3] & 0xff),
(uaddr[2] & 0xff),
(uaddr[1] & 0xff),
(uaddr[0] & 0xff));
break;
case AF_INET6:
qp = qbuf;
for (n = IN6ADDRSZ – 1; n >= 0; n–) {
qp += SPRINTF((qp, “%x.%x.”,
uaddr[n] & 0xf,
(uaddr[n] >> 4) & 0xf));
}
strcpy(qp, “ip6.arpa”);
break;
default:
abort();
}
这里把ip地址按8位翻转之后，加了一个.in-addr.arpa后缀，之后就通过通用的函数发出dns query请求，最终会调用res_mkquery，man一下这个函数(man 3 res_mkquery)：
The res_mkquery() function constructs a query message in buf of length buflen for the domain name dname. The query type op is usually QUERY, but can be any of the types defined in
http://linux.die.net/man/3/res_mkquery
跟dns请求相关的实现略复杂，这里不再展开。
这里可以走一个小捷径，我们写一个最简单的c程序来查看getnameinfo都大致做了什么事情：
[root@localhost test] gcc test.c
include
include
int main() {
struct sockaddr_in ip;
const char *ipstr = “127.0.0.1”;
int err;
char host[NI_MAXHOST+1];
if (!inet_aton(ipstr, &ip))
errx(1, “cant parse IP address %s”, ipstr);
ip.sin_family = AF_INET;
printf(“noop\n”);
err = getnameinfo(&ip,sizeof(ip),host,NI_MAXHOST,NULL,0 ,NI_NAMEREQD);
printf(“start\n”);
err = getnameinfo(&ip,sizeof(ip),host,NI_MAXHOST,NULL,0 ,NI_NAMEREQD);
printf(“end\n”);
}
然后使用strace来跟踪系统调用：
[root@localhost test] strace ./a.out
execve(“./a.out”, [“./a.out”], [/* 26 vars */]) = 0
brk(0) = 0x17d5000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ff894dd7000
access(“/etc/ld.so.preload”, R_OK) = -1 ENOENT (No such file or directory)
open(“/etc/ld.so.cache”, O_RDONLY) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=59784, …}) = 0
mmap(NULL, 59784, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7ff894dc8000
close(3) = 0
open(“/lib64/libc.so.6”, O_RDONLY) = 3
read(3, “\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0000\356\341`>\0\0\0″…, 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=1926800, …}) = 0
mmap(0x3e60e00000, 3750152, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x3e60e00000
mprotect(0x3e60f8b000, 2093056, PROT_NONE) = 0
mmap(0x3e6118a000, 20480, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x18a000) = 0x3e6118a000
mmap(0x3e6118f000, 18696, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x3e6118f000
close(3) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ff894dc7000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ff894dc6000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ff894dc5000
arch_prctl(ARCH_SET_FS, 0x7ff894dc6700) = 0
mprotect(0x3e6118a000, 16384, PROT_READ) = 0
mprotect(0x3e6081f000, 4096, PROT_READ) = 0
munmap(0x7ff894dc8000, 59784) = 0
fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 0), …}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ff894dd6000
write(1, “noop\n”, 5noop
) = 5
socket(PF_FILE, SOCK_STREAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0) = 3
connect(3, {sa_family=AF_FILE, path=”/var/run/nscd/socket”}, 110) = -1 ENOENT (No such file or directory)
close(3) = 0
socket(PF_FILE, SOCK_STREAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0) = 3
connect(3, {sa_family=AF_FILE, path=”/var/run/nscd/socket”}, 110) = -1 ENOENT (No such file or directory)
close(3) = 0
brk(0) = 0x17d5000
brk(0x17f6000) = 0x17f6000
open(“/etc/nsswitch.conf”, O_RDONLY) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=1688, …}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ff894dd5000
read(3, “\n /etc/nsswitch.conf\n\n An ex”…, 4096) = 1688
read(3, “”, 4096) = 0
close(3) = 0
munmap(0x7ff894dd5000, 4096) = 0
open(“/etc/ld.so.cache”, O_RDONLY) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=59784, …}) = 0
mmap(NULL, 59784, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7ff894db6000
close(3) = 0
open(“/lib64/libnss_files.so.2”, O_RDONLY) = 3
read(3, “\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\360!\0\0\0\0\0\0″…, 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=65928, …}) = 0
mmap(NULL, 2151824, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7ff894ba8000
mprotect(0x7ff894bb4000, 2097152, PROT_NONE) = 0
mmap(0x7ff894db4000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0xc000) = 0x7ff894db4000
close(3) = 0
mprotect(0x7ff894db4000, 4096, PROT_READ) = 0
munmap(0x7ff894db6000, 59784) = 0
getpid() = 28054
open(“/etc/resolv.conf”, O_RDONLY) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=50, …}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ff894dd5000
read(3, “nameserver 172.16.xx.xx\nnamese”…, 4096) = 50
read(3, “”, 4096) = 0
close(3) = 0
munmap(0x7ff894dd5000, 4096) = 0
uname({sys=”Linux”, node=”localhost.localdomain”, …}) = 0
open(“/etc/host.conf”, O_RDONLY) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=9, …}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ff894dd5000
read(3, “multi on\n”, 4096) = 9
read(3, “”, 4096) = 0
close(3) = 0
munmap(0x7ff894dd5000, 4096) = 0
open(“/etc/hosts”, O_RDONLY|O_CLOEXEC) = 3
fcntl(3, F_GETFD) = 0x1 (flags FD_CLOEXEC)
fstat(3, {st_mode=S_IFREG|0644, st_size=400, …}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ff894dd5000
read(3, “127.0.0.1 localhost localhost.”…, 4096) = 400
read(3, “”, 4096) = 0
close(3) = 0
munmap(0x7ff894dd5000, 4096) = 0
open(“/etc/ld.so.cache”, O_RDONLY) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=59784, …}) = 0
mmap(NULL, 59784, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7ff894db6000
close(3) = 0
open(“/lib64/libnss_dns.so.2”, O_RDONLY) = 3
read(3, “\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\0\20\0\0\0\0\0\0″…, 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=27424, …}) = 0
mmap(NULL, 2117880, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7ff8949a2000
mprotect(0x7ff8949a7000, 2093056, PROT_NONE) = 0
mmap(0x7ff894ba6000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x4000) = 0x7ff894ba6000
close(3) = 0
open(“/lib64/libresolv.so.2”, O_RDONLY) = 3
read(3, “\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\00009\240b>\0\0\0″…, 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=113952, …}) = 0
mmap(0x3e62a00000, 2202248, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x3e62a00000
mprotect(0x3e62a16000, 2097152, PROT_NONE) = 0
mmap(0x3e62c16000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x16000) = 0x3e62c16000
mmap(0x3e62c18000, 6792, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x3e62c18000
close(3) = 0
mprotect(0x3e62c16000, 4096, PROT_READ) = 0
mprotect(0x7ff894ba6000, 4096, PROT_READ) = 0
munmap(0x7ff894db6000, 59784) = 0
socket(PF_INET, SOCK_DGRAM|SOCK_NONBLOCK, IPPROTO_IP) = 3
connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr(“172.16.xx.xx”)}, 16) = 0
poll([{fd=3, events=POLLOUT}], 1, 0) = 1 ([{fd=3, revents=POLLOUT}])
sendto(3, “\36\247\1\0\0\1\0\0\0\0\0\0\0010\0010\0010\0010\7in-addr\4arp”…, 38, MSG_NOSIGNAL, NULL, 0) = 38
poll([{fd=3, events=POLLIN}], 1, 5000) = 1 ([{fd=3, revents=POLLIN}])
ioctl(3, FIONREAD, [73]) = 0
recvfrom(3, “\36\247\205\203\0\1\0\0\0\1\0\0\0010\0010\0010\0010\7in-addr\4arp”…, 1024, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr(“172.16.xx.xx”)}, [16]) = 73
close(3) = 0
write(1, “start\n”, 6start
) = 6
open(“/etc/hosts”, O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=400, …}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ff894dd5000
read(3, “127.0.0.1 localhost localhost.”…, 4096) = 400
read(3, “”, 4096) = 0
close(3) = 0
munmap(0x7ff894dd5000, 4096) = 0
socket(PF_INET, SOCK_DGRAM|SOCK_NONBLOCK, IPPROTO_IP) = 3
connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr(“172.16.xx.xx”)}, 16) = 0
poll([{fd=3, events=POLLOUT}], 1, 0) = 1 ([{fd=3, revents=POLLOUT}])
sendto(3, “~\223\1\0\0\1\0\0\0\0\0\0\0010\0010\0010\0010\7in-addr\4arp”…, 38, MSG_NOSIGNAL, NULL, 0) = 38
poll([{fd=3, events=POLLIN}], 1, 5000) = 1 ([{fd=3, revents=POLLIN}])
ioctl(3, FIONREAD, [73]) = 0
recvfrom(3, “~\223\205\203\0\1\0\0\0\1\0\0\0010\0010\0010\0010\7in-addr\4arp”…, 1024, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr(“172.16.xx.xx”)}, [16]) = 73
close(3) = 0
write(1, “end\n”, 4end
) = 4
exit_group(4) = ?
可以看出，首次查询时会读取/etc/nsswitch.conf，/etc/resolv.conf，后面的请求先读了/etc/hosts，找不到就向dns服务器发送了一个udp查询请求（SOCK_DGRAM），之后使用了poll等待返回结果，有返回的话使用recvfrom接收结果。
为了验证看代码得到的结果再次抓包，不过这次只过滤53端口的数据包（dns服务的端口为53）：
[root@79-40-151-yf-core logs] tcpdump -i eth1 port 53
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth1, link-type EN10MB (Ethernet), capture size 65535 bytes
18:26:42.135322 IP 10.79.40.xx.35611 > 10.71.16.xx.domain: 59106+ PTR? xx.40.79.10.in-addr.arpa. (43)
18:26:42.135348 IP 10.79.40.xx.35611 > 172.16.xx.xx.domain: 59106+ PTR? xx.40.79.10.in-addr.arpa. (43)
18:26:42.136024 IP 172.16.xx.xx.domain > 10.79.40.xx.35611: 59106 NXDomain 0/0/0 (43)
18:26:42.136258 IP 10.79.40.xx.43068 > 10.71.16.xx.domain: 32990+ PTR? xx.16.71.10.in-addr.arpa. (43)
18:26:42.136276 IP 10.79.40.xx.43068 > 172.16.xx.xx.domain: 32990+ PTR? xx.16.71.10.in-addr.arpa. (43)
跟前面推测的一样，这里确实在向dns服务器发送这种后缀是.in-addr.arpa的请求。
可用在wiki上找到这类查询的详细描述：https://en.wikipedia.org/wiki/Reverse_DNS_lookup
最后一个问题是这个查询会超时吗？超时时间是多少？根据man page结果，dns查询的超时是5秒：
$ man resolv.conf
…
timeout:n
sets the amount of time the resolver will wait for a response from a remote name server before retrying the query via a different name server. Measured
in seconds, the default is RES_TIMEOUT (currently 5, see
…
跟抓包结果一致，并且注意到strace中poll的最后一个参数是5000，和默认的超时时间一致。
我们可以在/etc/resolv.conf里增加关于超时时间的配置：
options timeout:1
再用strace试一下，果然poll的参数变成1000了。
但是线上机器配置了dnsmasq缓存，为什么缓存没有生效？
配置了dnsmasq后再次使用tcpdump，可用看到lo网卡和eth1网卡都有查询请求，由于反向dns查询不到主机名，dnsmasq无法缓存结果，只能每次都把请求转发给实际dns。
线上除了这个网段的机器还有其他机器，为什么其他机器没有问题？
没出问题的机器里/etc/hosts配置了本机ip对应的hostname，在hosts文件中查询到了就不会再去搜索dns。
DNS解析在那个时间为什么会消耗5秒？
由于udp协议本身传输不可靠没有重发的机制，在网络异常的时候只能默默的等待超时，具体网络的问题这里就不展开了。
如何解决这个问题？
首先第一反应是想到升级tomcat版本，查看新版tomcat代码，有问题的代码果然没有了，线上服务升级到tomcat8后也恢复了正常。
如果不能升级tomcat，可以在nginx的探测增加host header，避免前端机反向查询请求。
如果两者都不能做，那么可以在本机hosts中配置对应本机ip的hostname，可以避免通过dns服务器查询。
