[20190211]简单测试端口是否打开.txt

--//昨天看一个链接,提到如果判断一个端口是否打开可以简单执行如下:
--//参考链接:https://dba010.com/2019/02/04/check-if-a-port-on-a-remote-system-is-reachable-without-telnet/

1.测试:
# cat < /dev/tcp/192.168.100.40/1521
# echo $?
0
# cat < /dev/tcp/192.168.100.40/1522
-bash: connect: Connection refused
-bash: /dev/tcp/192.168.100.40/1522: Connection refused

# alias zdate
alias zdate='date +'\''%Y/%m/%d %T'\'''
--//不过我的测试需要等待一分钟,不知道那个受那个内核参数的影响,不知道那位知道:
# zdate ;cat < /dev/tcp/192.168.100.40/1521 ; zdate
2019/02/11 08:42:59
2019/02/11 08:43:59

--//换另外一台机器测试:
# zdate ;cat < /dev/tcp/192.168.100.78/1521;zdate
2019/02/11 08:46:07
2019/02/11 08:46:17

# zdate ;cat < /dev/tcp/192.168.100.40/1521;zdate
2019/02/11 08:46:36
2019/02/11 08:47:36

--//视乎和远程服务器某个内核参数有关.测试100.40需要60秒(这台机器很老redhat 4.3的版本),而测试100.78需要10秒.
--//在100.78上使用tcpdump监测:
# tcpdump -i eth0 host 192.168.xxx.xx -nnn
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes
09:03:41.242607 IP 192.168.xxx.xx.48292 > 192.168.100.78.1521: S 226276375:226276375(0) win 29200 <mss 1460,sackOK,timestamp 3393435130 0,nop,wscale 7>
09:03:41.242785 IP 192.168.100.78.1521 > 192.168.xxx.xx.48292: S 995705425:995705425(0) ack 226276376 win 14480 <mss 1460,sackOK,timestamp 1354661055 3393435130,nop,wscale 7>
09:03:41.242852 IP 192.168.xxx.xx.48292 > 192.168.100.78.1521: . ack 1 win 229 <nop,nop,timestamp 3393435130 1354661055>
09:03:51.253449 IP 192.168.100.78.1521 > 192.168.xxx.xx.48292: F 1:1(0) ack 1 win 114 <nop,nop,timestamp 1354671066 3393435130>
09:03:51.253770 IP 192.168.xxx.xx.48292 > 192.168.100.78.1521: F 1:1(0) ack 2 win 229 <nop,nop,timestamp 3393445141 1354671066>
09:03:51.253797 IP 192.168.100.78.1521 > 192.168.xxx.xx.48292: . ack 2 win 114 <nop,nop,timestamp 1354671066 3393445141>

--//也可以看出有10秒的等待时间.在100.78上执行:
# echo /proc/sys/net/ipv4/* | xargs   -n 1  strings -1 -f | grep ": 10$"
/proc/sys/net/ipv4/cipso_cache_bucket_size: 10
/proc/sys/net/ipv4/igmp_max_msf: 10
/proc/sys/net/ipv4/inet_peer_gc_mintime: 10
/proc/sys/net/ipv4/tcp_keepalive_intvl: 10

*/
--//开始以为/proc/sys/net/ipv4/tcp_keepalive_intvl: 10有关,我查看100.40的设置/proc/sys/net/ipv4/tcp_keepalive_intvl=75,也不是这个
--//参数.我修改这个参数测试还是10秒.

--//实际上还有1个更快的测试方法,发送信息到/dev/tcp/ip_address/port,例子如下:

# echo a >  /dev/tcp/192.168.100.78/1521
# echo $?
0

# echo a >  /dev/tcp/192.168.100.78/1522
-bash: connect: 拒绝连接
-bash: /dev/tcp/192.168.100.78/1522: 拒绝连接

--//在100.78上使用tcpdump监测:
 # tcpdump -i eth0 host 192.168.xxx.xx -nnn
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes
09:12:41.312584 IP 192.168.xxx.xx.48331 > 192.168.100.78.1521: S 3309983125:3309983125(0) win 29200 <mss 1460,sackOK,timestamp 3393975215 0,nop,wscale 7>
09:12:41.312761 IP 192.168.100.78.1521 > 192.168.xxx.xx.48331: S 124881852:124881852(0) ack 3309983126 win 14480 <mss 1460,sackOK,timestamp 1355201125 3393975215,nop,wscale 7>
09:12:41.312850 IP 192.168.xxx.xx.48331 > 192.168.100.78.1521: . ack 1 win 229 <nop,nop,timestamp 3393975216 1355201125>
09:12:41.312895 IP 192.168.