在 Ubuntu 16.04 上，我安装了 ufw 并对其进行了配置，使其具有以下状态（sudo ufw status verbose）：
Status: active
Logging: on (full)
Default: deny (incoming), allow (outgoing), deny (routed)
New profiles: skip
To Action From
-- ------ ----
80,443/tcp (Nginx Full) ALLOW IN Anywhere
995/tcp (Dovecot Secure POP3) ALLOW IN Anywhere
993/tcp (Dovecot Secure IMAP) ALLOW IN Anywhere
22/tcp (OpenSSH) ALLOW IN Anywhere
25/tcp (Postfix) ALLOW IN Anywhere
465/tcp (Postfix SMTPS) ALLOW IN Anywhere
9522/tcp (hinext) ALLOW IN Anywhere
9522,9523/tcp (hinext) ALLOW IN Anywhere
9524/tcp (test) ALLOW IN Anywhere
9522/tcp (hinext (v6)) ALLOW IN Anywhere (v6)
9522,9523/tcp (hinext (v6)) ALLOW IN Anywhere (v6)
9524/tcp (test (v6)) ALLOW IN Anywhere (v6)
从中可以看出，端口 8822 不在列表中，因此应该被默认策略（针对deny链incoming）阻止。
但是：我可以从外部世界打开到端口 8822 的 SSH 连接，到监听端口 22 和 8822 的 SSH 服务器。
为什么到端口 8822 的流量可以穿过 ufw 防火墙而不会被丢弃？
如需进一步的诊断信息，iptables-save -c请参见：
# Generated by iptables-save v1.6.0 on Tue Apr 24 23:55:19 2018
*raw
:PREROUTING ACCEPT [622500:111511726]
:OUTPUT ACCEPT [631989:135819596]
COMMIT
# Completed on Tue Apr 24 23:55:19 2018
# Generated by iptables-save v1.6.0 on Tue Apr 24 23:55:19 2018
*mangle
:PREROUTING ACCEPT [622500:111511726]
:INPUT ACCEPT [622500:111511726]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [631989:135819596]
:POSTROUTING ACCEPT [631989:135819596]
COMMIT
# Completed on Tue Apr 24 23:55:19 2018
# Generated by iptables-save v1.6.0 on Tue Apr 24 23:55:19 2018
*nat
:PREROUTING ACCEPT [46994:2923568]
:POSTROUTING ACCEPT [7607:511281]
:OUTPUT ACCEPT [7607:511281]
COMMIT
# Completed on Tue Apr 24 23:55:19 2018
# Generated by iptables-save v1.6.0 on Tue Apr 24 23:55:19 2018
*filter
:INPUT ACCEPT [63:5355]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
:VZ_FORWARD - [0:0]
:VZ_INPUT - [0:0]
:VZ_OUTPUT - [0:0]
:ufw-after-forward - [0:0]
:ufw-after-input - [0:0]
:ufw-after-logging-forward - [0:0]
:ufw-after-logging-input - [0:0]
:ufw-after-logging-output - [0:0]
:ufw-after-output - [0:0]
:ufw-before-forward - [0:0]
:ufw-before-input - [0:0]
:ufw-before-logging-forward - [0:0]
:ufw-before-logging-input - [0:0]
:ufw-before-logging-output - [0:0]
:ufw-before-output - [0:0]
:ufw-logging-allow - [0:0]
:ufw-logging-deny - [0:0]
:ufw-not-local - [0:0]
:ufw-reject-forward - [0:0]
:ufw-reject-input - [0:0]
:ufw-reject-output - [0:0]
:ufw-skip-to-policy-forward - [0:0]
:ufw-skip-to-policy-input - [0:0]
:ufw-skip-to-policy-output - [0:0]
:ufw-track-forward - [0:0]
:ufw-track-input - [0:0]
:ufw-track-output - [0:0]
:ufw-user-forward - [0:0]
:ufw-user-input - [0:0]
:ufw-user-limit - [0:0]
:ufw-user-limit-accept - [0:0]
:ufw-user-logging-forward - [0:0]
:ufw-user-logging-input - [0:0]
:ufw-user-logging-output - [0:0]
:ufw-user-output - [0:0]
[622500:111511726] -A INPUT -j VZ_INPUT
[491972:96179570] -A INPUT -j ufw-before-logging-input
[491972:96179570] -A INPUT -j ufw-before-input
[21445:1425920] -A INPUT -j ufw-after-input
[17022:1199401] -A INPUT -j ufw-after-logging-input
[17022:1199401] -A INPUT -j ufw-reject-input
[17022:1199401] -A INPUT -j ufw-track-input
[0:0] -A FORWARD -j VZ_FORWARD
[0:0] -A FORWARD -j ufw-before-logging-forward
[0:0] -A FORWARD -j ufw-before-forward
[0:0] -A FORWARD -j ufw-after-forward
[0:0] -A FORWARD -j ufw-after-logging-forward
[0:0] -A FORWARD -j ufw-reject-forward
[0:0] -A FORWARD -j ufw-track-forward
[631989:135819596] -A OUTPUT -j VZ_OUTPUT
[478124:111192792] -A OUTPUT -j ufw-before-logging-output
[478124:111192792] -A OUTPUT -j ufw-before-output
[4466:322671] -A OUTPUT -j ufw-after-output
[4466:322671] -A OUTPUT -j ufw-after-logging-output
[4466:322671] -A OUTPUT -j ufw-reject-output
[4466:322671] -A OUTPUT -j ufw-track-output
[23:1823] -A VZ_INPUT -p tcp -m tcp --dport 80 -j ACCEPT
[5136:565736] -A VZ_INPUT -p tcp -m tcp --dport 22 -j ACCEPT
[4:172] -A VZ_INPUT -p tcp -m tcp --dport 25 -j ACCEPT
[4:172] -A VZ_INPUT -p tcp -m tcp --dport 110 -j ACCEPT
[6:304] -A VZ_INPUT -p tcp -m tcp --dport 53 -j ACCEPT
[2:115] -A VZ_INPUT -p udp -m udp --dport 53 -j ACCEPT
[410:19580] -A VZ_INPUT -p tcp -m tcp --dport 32768:65535 -j ACCEPT
[39:3651] -A VZ_INPUT -p udp -m udp --dport 32768:65535 -j ACCEPT
[1:44] -A VZ_INPUT -p tcp -m tcp --dport 8880 -j ACCEPT
[3:152] -A VZ_INPUT -p tcp -m tcp --dport 8443 -j ACCEPT
[8:470] -A VZ_INPUT -s 127.0.0.1/32 -d 127.0.0.1/32 -p tcp -j ACCEPT
[0:0] -A VZ_INPUT -s 127.0.0.1/32 -d 127.0.0.1/32 -p udp -j ACCEPT
[17:2105] -A VZ_OUTPUT -p tcp -m tcp --sport 80 -j ACCEPT
[4940:995587] -A VZ_OUTPUT -p tcp -m tcp --sport 22 -j ACCEPT
[4:214] -A VZ_OUTPUT -p tcp -m tcp --sport 25 -j ACCEPT
[4:192] -A VZ_OUTPUT -p tcp -m tcp --sport 110 -j ACCEPT
[6:240] -A VZ_OUTPUT -p tcp -m tcp --sport 53 -j ACCEPT
[0:0] -A VZ_OUTPUT -p udp -m udp --sport 53 -j ACCEPT
[3888:279384] -A VZ_OUTPUT -p tcp -j ACCEPT
[39:2831] -A VZ_OUTPUT -p udp -j ACCEPT
[0:0] -A VZ_OUTPUT -p tcp -m tcp --sport 8880 -j ACCEPT
[0:0] -A VZ_OUTPUT -p tcp -m tcp --sport 8443 -j ACCEPT
[0:0] -A VZ_OUTPUT -s 127.0.0.1/32 -d 127.0.0.1/32 -p tcp -j ACCEPT
[0:0] -A VZ_OUTPUT -s 127.0.0.1/32 -d 127.0.0.1/32 -p udp -j ACCEPT
[5:391] -A ufw-after-input -p udp -m udp --dport 137 -j ufw-skip-to-policy-input
[0:0] -A ufw-after-input -p udp -m udp --dport 138 -j ufw-skip-to-policy-input
[16:700] -A ufw-after-input -p tcp -m tcp --dport 139 -j ufw-skip-to-policy-input
[1936:99244] -A ufw-after-input -p tcp -m tcp --dport 445 -j ufw-skip-to-policy-input
[0:0] -A ufw-after-input -p udp -m udp --dport 67 -j ufw-skip-to-policy-input
[0:0] -A ufw-after-input -p udp -m udp --dport 68 -j ufw-skip-to-policy-input
[0:0] -A ufw-after-input -m addrtype --dst-type BROADCAST -j ufw-skip-to-policy-input
[0:0] -A ufw-after-logging-forward -j LOG --log-prefix "[UFW BLOCK] "
[63:5355] -A ufw-after-logging-input -j LOG --log-prefix "[UFW BLOCK] "
[0:0] -A ufw-after-logging-output -j LOG --log-prefix "[UFW ALLOW] "
[0:0] -A ufw-before-forward -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
[0:0] -A ufw-before-forward -p icmp -m icmp --icmp-type 3 -j ACCEPT
[0:0] -A ufw-before-forward -p icmp -m icmp --icmp-type 4 -j ACCEPT
[0:0] -A ufw-before-forward -p icmp -m icmp --icmp-type 11 -j ACCEPT
[0:0] -A ufw-before-forward -p icmp -m icmp --icmp-type 12 -j ACCEPT
[0:0] -A ufw-before-forward -p icmp -m icmp --icmp-type 8 -j ACCEPT
[0:0] -A ufw-before-forward -j ufw-user-forward
[10789:9641505] -A ufw-before-input -i lo -j ACCEPT
[252164:53646696] -A ufw-before-input -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
[3048:131944] -A ufw-before-input -m conntrack --ctstate INVALID -j ufw-logging-deny
[3048:131944] -A ufw-before-input -m conntrack --ctstate INVALID -j DROP
[0:0] -A ufw-before-input -p icmp -m icmp --icmp-type 3 -j ACCEPT
[0:0] -A ufw-before-input -p icmp -m icmp --icmp-type 4 -j ACCEPT
[0:0] -A ufw-before-input -p icmp -m icmp --icmp-type 11 -j ACCEPT
[0:0] -A ufw-before-input -p icmp -m icmp --icmp-type 12 -j ACCEPT
[21:952] -A ufw-before-input -p icmp -m icmp --icmp-type 8 -j ACCEPT
[0:0] -A ufw-before-input -p udp -m udp --sport 67 --dport 68 -j ACCEPT
[22062:1348553] -A ufw-before-input -j ufw-not-local
[0:0] -A ufw-before-input -d 224.0.0.251/32 -p udp -m udp --dport 5353 -j ACCEPT
[0:0] -A ufw-before-input -d 239.255.255.250/32 -p udp -m udp --dport 1900 -j ACCEPT
[22062:1348553] -A ufw-before-input -j ufw-user-input
[0:0] -A ufw-before-logging-forward -j LOG --log-prefix "[UFW AUDIT] "
[327:39433] -A ufw-before-logging-input -j LOG --log-prefix "[UFW AUDIT] "
[10:3444] -A ufw-before-logging-output -j LOG --log-prefix "[UFW AUDIT] "
[10789:9641505] -A ufw-before-output -o lo -j ACCEPT
[277561:60253281] -A ufw-before-output -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
[1888:133952] -A ufw-before-output -j ufw-user-output
[0:0] -A ufw-logging-allow -j LOG --log-prefix "[UFW ALLOW] "
[29:1244] -A ufw-logging-deny -m conntrack --ctstate INVALID -j LOG --log-prefix "[UFW AUDIT INVALID] "
[29:1244] -A ufw-logging-deny -j LOG --log-prefix "[UFW BLOCK] "
[22062:1348553] -A ufw-not-local -m addrtype --dst-type LOCAL -j RETURN
[0:0] -A ufw-not-local -m addrtype --dst-type MULTICAST -j RETURN
[0:0] -A ufw-not-local -m addrtype --dst-type BROADCAST -j RETURN
[0:0] -A ufw-not-local -m limit --limit 3/min --limit-burst 10 -j ufw-logging-deny
[0:0] -A ufw-not-local -j DROP
[0:0] -A ufw-skip-to-policy-forward -j DROP
[1957:100335] -A ufw-skip-to-policy-input -j DROP
[0:0] -A ufw-skip-to-policy-output -j ACCEPT
[1:60] -A ufw-track-output -p tcp -m conntrack --ctstate NEW -j ACCEPT
[1746:126492] -A ufw-track-output -p udp -m conntrack --ctstate NEW -j ACCEPT
[526:29104] -A ufw-user-input -p tcp -m multiport --dports 80,443 -m comment --comment "\'dapp_Nginx%20Full\'" -j ACCEPT
[76:3832] -A ufw-user-input -p tcp -m tcp --dport 995 -m comment --comment "\'dapp_Dovecot%20Secure%20POP3\'" -j ACCEPT
[8:372] -A ufw-user-input -p tcp -m tcp --dport 993 -m comment --comment "\'dapp_Dovecot%20Secure%20IMAP\'" -j ACCEPT
[9724:581800] -A ufw-user-input -p tcp -m tcp --dport 22 -m comment --comment "\'dapp_OpenSSH\'" -j ACCEPT
[61:3500] -A ufw-user-input -p tcp -m tcp --dport 25 -m comment --comment "\'dapp_Postfix\'" -j ACCEPT
[10:456] -A ufw-user-input -p tcp -m tcp --dport 465 -m comment --comment "\'dapp_Postfix%20SMTPS\'" -j ACCEPT
[0:0] -A ufw-user-input -p tcp -m tcp --dport 9522 -m comment --comment "\'dapp_hinext\'" -j ACCEPT
[1:52] -A ufw-user-input -p tcp -m multiport --dports 9522,9523 -m comment --comment "\'dapp_hinext\'" -j ACCEPT
[5:256] -A ufw-user-input -p tcp -m tcp --dport 9524 -m comment --comment "\'dapp_test\'" -j ACCEPT
[0:0] -A ufw-user-limit -m limit --limit 3/min -j LOG --log-prefix "[UFW LIMIT BLOCK] "
[0:0] -A ufw-user-limit -j REJECT --reject-with icmp-port-unreachable
[0:0] -A ufw-user-limit-accept -j ACCEPT
COMMIT
# Completed on Tue Apr 24 23:55:19 2018
答案1
我使用 重启了防火墙sudo service ufw restart。之后，端口 8822 被 usw 阻止了 - 理所当然。
此外，iptables-save还告诉了我所期望的故事：请参阅此摘录：
*filter
:INPUT DROP [6:320]
:FORWARD DROP [0:0]
:OUTPUT ACCEPT [0:0]
请注意“DROP”默认策略，而在我的初始帖子中，我注意到一个我不理解的“ACCEPT”默认策略。
因此，重新启动 ufw 显然可以解决问题。