1. 安裝OpenVPN , 使用路由模式(原生android client不支援 TAP橋接模式)
# 安裝EPEL軟體庫
dnf install https://dl.fedoraproject.org/pub/epel/epel-release-latest-8.noarch.rpm
dnf config-manager --set-enabled PowerTools
dnf repolist epel
# 安裝 git
dnf -y install git
cd ~
# 從github下載安裝命令檔
git clone https://github.com/Nyr/openvpn-install.git
cd openvpn-install/
chmod +x openvpn-install.sh
# 執行安裝檔 ./openvpn-install.sh
接下來依指示輸入你的設定值, 完成後自動產生設定檔

修改 /etc/openvpn/server/server.conf 讓OpenVPN支援PAM
local 10.0.0.10 #主機IP
port 1194
proto udp
dev tun
ca ca.crt
cert server.crt
key server.key
dh dh.pem
auth SHA512
tls-crypt tc.key
topology subnet
server 10.8.0.0 255.255.255.0
ifconfig-pool-persist ipp.txt
push "redirect-gateway def1 bypass-dhcp"
push "dhcp-option DNS 10.0.0.1"
keepalive 10 120
cipher AES-256-CBC
user nobody
group nobody
persist-key
persist-tun
status openvpn-status.log
verb 3
crl-verify crl.pem
explicit-exit-notify
plugin /usr/lib64/openvpn/plugins/openvpn-plugin-auth-pam.so login #啟用PAM驗證, 使用預設設定檔/etc/pam.d/login
重新啟動openvpn
systemctl restart openvpn-server@server
修改Client 端的預設值, 由於用戶端要使用與主機不同網段10.1.0.0/24的資源故需要增加路由
/etc/openvpn/server/client-common.txt
client dev tun
proto udp
remote 1.2.3.4 1194 # 外部實體IP
resolv-retry infinite
nobind
persist-key
persist-tun
remote-cert-tls server
auth SHA512
cipher AES-256-CBC
ignore-unknown-option block-outside-dns
block-outside-dns
auth-user-pass # 除了openvpn本身的金鑰驗證, 另外開啟AD使用者帳號密碼驗證
verb 3
route 10.1.0.0 255.255.255.0 10.0.0.253 # 增加通往10.1.0.0/24網斷路由, 網關為10.0.0.253
更改完成後重新執行安裝命令產生新的client端 .ovpn 設定檔
./openvpn-install.sh
過程中選 Add a new user, 至於第一次產生的user設定可以選擇 Revoke an existing user
移除掉. 產生的 .ovpn 檔會在/root資料夾下

2. 取得網域憑證伺服器的Public key ,通常會在以下位址且檔名類似
\\caserver\CertEnroll\caserver_MYDOMAIN Root Certification Authority.crt
使用cifs工具掛載目錄後 下載到centos8上並轉為PEM格式
# 掛載cifs, server2003使用smb1.0所以參數須加上vers=1.0
mount.cifs //ca_server/CertEnroll /mnt/cifs -o user=administrator,pass=password,dom=mydomain,vers=1.0
cp /mnt/cifs/xxxx.crt xxxx.crt
# 將der 轉換為 pem
openssl x509 -inform der -in xxxx.crt -out xxxx.pem
# 搬移到 trust資料夾
mv xxxx.pem /etc/pki/ca-trust/source/anchors
# 將憑證更新至主機信任憑證裡
update-ca-trust 
3. 設定sssd 連接 Active Directory
修改/etc/sssd/sssd.conf
[sssd]
services = nss, pam, ssh
config_file_version = 2
domains = mydomain

[sudo]

[nss]

[pam]
offline_credentials_expiration = 60

[domain/mydomain]
cache_credentials = True
ldap_search_base = dc=mydomain,dc=com
id_provider = ldap
ldap_uri = ldaps://dc.mydomain.com:636   #AD主機, 使用ssl協定連線
ldap_schema = AD
ldap_default_bind_dn = cn=administrator,cn=users,dc= mydomain,dc=com  # 這裡為了方便使用管理員帳號, 為安全起見可以使用唯讀的網域帳號
ldap_default_authtok = password_for_administrator  # 管理員密碼
ldap_tls_reqcert = demand
ldap_tls_cacert = /etc/pki/tls/certs/ca-bundle.crt
ldap_tls_cacertdir = /etc/pki/tls/certs
ldap_search_timeout = 50
ldap_network_timeout = 60
ldap_id_mapping = True
ldap_referrals = false

enumerate = False
fallback_homedir = /home/%u
default_shell = /bin/bash
修改 /etc/openlap/ldap.conf
URI ldaps://dc.mydomain.com:636
base    dc=mydomain,dc=com
TLS_CACERT  /etc/pki/tls/certs/ca-bundle.crt
SASL_NOCANON    on
強制啟用sssd
authselect select sssd –force
systemctl enable --now sssd
systemctl restart sssd
由於Windows 2003 CA仍使用舊版SHA1, 因此需要改crypto政策
update-crypto-policies --set LEGACY
完成後重新開機

4. 防火牆上設定允許連向1.2.3.4 UDP1194的流量並引導至內部10.0.0.10

5. 用戶端安裝OpenVPN Client軟體後匯入.ovpn檔
Windows7/8/10 Client: OpenVPN-GUI (嘗試過OpenVPN Connect for Windows無法匯入.ovpn)
iOS/Android Client: OpenVPN Connect

參考資料:
OpenVPN Server Configuration on RHEL / Rocky / Alma

https://www.redhat.com/en/blog/consistent-security-crypto-policies-red-hat-enterprise-linux-8
https://medium.com/jerrynotes/linux-authentication-windows-ad-without-join-domain-7963c3fd44c5

〈OpenVPN + PAM + SSSD + Active Directory〉這篇文章最早發佈於《MIS 的流水年華》。

/var/log/messages

Jan 26 07:14:12 fedora saslauthd[27264]: do_auth         : auth failure: [user=user1] [service=smtp] [realm=] [mech=pam] [reason=PAM auth error]
Jan 26 07:14:18 fedora saslauthd[27267]: do_auth         : auth failure: [user=user1] [service=smtp] [realm=] [mech=pam] [reason=PAM auth error]
Jan 26 07:14:22 fedora saslauthd[27262]: do_auth         : auth failure: [user=user1] [service=smtp] [realm=] [mech=pam] [reason=PAM auth error]
Jan 26 07:14:26 fedora saslauthd[27265]: do_auth         : auth failure: [user=user1] [service=smtp] [realm=] [mech=pam] [reason=PAM auth error]
Jan 26 07:14:32 fedora saslauthd[27267]: do_auth         : auth failure: [user=user1] [service=smtp] [realm=] [mech=pam] [reason=PAM auth error]

/var/log/maillog

Jan 26 07:14:13 fedora sendmail[29536]: s0PNE8Zh029536: [78.93.245.133] did not issue MAIL/EXPN/VRFY/ETRN during connection to MTA
Jan 26 07:14:18 fedora sendmail[29537]: s0PNED8R029537: [78.93.245.133] did not issue MAIL/EXPN/VRFY/ETRN during connection to MTA
Jan 26 07:14:23 fedora sendmail[29538]: s0PNEItR029538: [78.93.245.133] did not issue MAIL/EXPN/VRFY/ETRN during connection to MTA
Jan 26 07:14:28 fedora sendmail[29539]: s0PNENjN029539: [78.93.245.133] did not issue MAIL/EXPN/VRFY/ETRN during connection to MTA
Jan 26 07:14:32 fedora sendmail[29540]: s0PNESYI029540: [78.93.245.133] did not issue MAIL/EXPN/VRFY/ETRN during connection to MTA

/var/log/secure

Jan 26 07:14:10 fedora saslauthd[27264]: pam_unix(smtp:auth): check pass; user unknown
Jan 26 07:14:10 fedora saslauthd[27264]: pam_unix(smtp:auth): authentication failure; logname= uid=0 euid=0 tty= ruser= rhost=
Jan 26 07:14:10 fedora saslauthd[27264]: pam_succeed_if(smtp:auth): error retrieving information about user chi
Jan 26 07:14:15 fedora saslauthd[27267]: pam_unix(smtp:auth): check pass; user unknown
Jan 26 07:14:15 fedora saslauthd[27267]: pam_unix(smtp:auth): authentication failure; logname= uid=0 euid=0 tty= ruser= rhost=
Jan 26 07:14:15 fedora saslauthd[27267]: pam_succeed_if(smtp:auth): error retrieving information about user chi
Jan 26 07:14:21 fedora saslauthd[27262]: pam_unix(smtp:auth): check pass; user unknown
Jan 26 07:14:21 fedora saslauthd[27262]: pam_unix(smtp:auth): authentication failure; logname= uid=0 euid=0 tty= ruser= rhost=
Jan 26 07:14:21 fedora saslauthd[27262]: pam_succeed_if(smtp:auth): error retrieving information about user chi
Jan 26 07:14:25 fedora saslauthd[27265]: pam_unix(smtp:auth): check pass; user unknown
Jan 26 07:14:25 fedora saslauthd[27265]: pam_unix(smtp:auth): authentication failure; logname= uid=0 euid=0 tty= ruser= rhost=
Jan 26 07:14:25 fedora saslauthd[27265]: pam_succeed_if(smtp:auth): error retrieving information about user chi
Jan 26 07:14:30 fedora saslauthd[27267]: pam_unix(smtp:auth): check pass; user unknown
Jan 26 07:14:30 fedora saslauthd[27267]: pam_unix(smtp:auth): authentication failure; logname= uid=0 euid=0 tty= ruser= rhost=
Jan 26 07:14:30 fedora saslauthd[27267]: pam_succeed_if(smtp:auth): error retrieving information about user chi

主要原因是因為 cyrus-sasl 當初設計只紀錄四個欄位不包含遠端 ip , 網路上有 patch 可以增加 rhost 欄 , 但是因為它會影響其他軟體目前並未被納入正式的 package. 相關資訊可以參考以下討論串
https://bugzilla.cyrusimap.org/show_bug.cgi?id=3468
https://bugzilla.redhat.com/show_bug.cgi?id=683797

rhost 為空值的話 fail2ban 就無法阻擋 smtp-auth 密碼猜測的攻擊. 測試版的 fail2ban 0.9 增加了一樣令人興奮的功能 , 可以在過濾器裡進行多行比對!!如此一來只要增加 sendmail 的 log level (預設為 9) , 就可以看出遠端 ip 及認證錯誤的關聯性再利用 fail2ban 加以阻擋.

首先修改 sendmail.mc 檔, 變更以下行

dnl define(`confLOG_LEVEL’, `9′)dnl

改為

define(`confLOG_LEVEL’, `10‘)dnl

執行指令後並重新啟動 sendmail

[root@mailhost /]# m4 /etc/mail/senmail.mc > /etc/mail/sendmail.cf
[root@mailhost /]# service sendmail restart

此時檢視 /var/log/maillog 會發現 sasl 認證錯誤時訊息如下

Jan 26 19:30:01 fedora sendmail[2370]: NOQUEUE: connect from 223.59.18.218.broad.sz.gd.dynamic.163data.com.cn [218.18.59.223] (may be forged)
Jan 26 19:30:05 fedora sendmail[2370]: s0QBU1Ir002370: AUTH failure (LOGIN): authentication failure (-13) SASL(-13): authentication failure: checkpass failed
Jan 26 19:30:06 fedora sendmail[2370]: s0QBU1Ir002370: 223.59.18.218.broad.sz.gd.dynamic.163data.com.cn [218.18.59.223] (may be forged) did not issue MAIL/EXPN/VRFY/ETRN during connection to MTA

這時紀錄了 pid 及 mail ID 及驗證錯誤相關訊息 . 原來的 Log Level 只會紀錄最後一行, 但是該行錯誤在某些為了快速發信的軟體或者電子報等的不嚴謹的軟體連接伺服器時都可能發生, 因此光靠該行無法明確判斷是否為垃圾郵件. 現在在本例中我們可以確認它是在使用PLAIN TEXT驗證後才發生該錯誤 . 注意在繁忙的 server 中 ,該錯誤並不一定會連在一起, 因為server同時會處理其他服務如dovecot , 所以我們該使用 pid 或者 mail ID 來關聯這些錯誤.

接下來就是在 fail2ban 的 git 下載 0.9 版並安裝(略)
https://github.com/fail2ban/fail2ban/tarball/0.9

0.9版設定上有些差異 , 說明有提到盡量將各別設定或者自訂部份放在 /etc/fail2ban/jail.local 或者 /etc/fail2ban/jail.d 下

/etc/fail2ban/fail2ban.conf

[Definition]
loglevel = 3
logtarget = /var/log/fail2ban.log #個人比較喜歡紀錄在個別檔案中(預設值 SYSLOG)
socket = /var/run/fail2ban/fail2ban.sock
pidfile = /var/run/fail2ban/fail2ban.pid

/etc/fail2ban/jail.conf  這邊只列一些預設項目和重要設定, logpath部份注意一下, 當然也可以在jail.d下面覆蓋預設值.其他要新增的設定我們會加到/etc/fail2ban/jail.d/local.conf

[DEFAULT]
ignoreip = 127.0.0.1/8 10.0.18.0/24 10.160.8.0/23 #內部 ip 不阻擋
ignorecommand =
bantime  = 1200 #擋20分鐘
findtime  = 600
maxretry = 5 #出現五次紀錄即觸發
backend = auto
usedns = warn
logencoding = auto
enabled = false #預設不啟用任何服務 要啟用各別服務請在 jail.d下設定
filter = %(__name__)s
destemail = root@localhost
sender = root@localhost
mta = sendmail
protocol = tcp
chain = INPUT
port = 0:65535
banaction = iptables-multiport
action_ = %(banaction)s[name=%(__name__)s, port=”%(port)s”, protocol=”%(protocol)s”, chain=”%(chain)s”]
action_mw = %(banaction)s[name=%(__name__)s, port=”%(port)s”, protocol=”%(protocol)s”, chain=”%(chain)s”]
%(mta)s-whois[name=%(__name__)s, dest=”%(destemail)s”, protocol=”%(protocol)s”, chain=”%(chain)s”]
action_mwl = %(banaction)s[name=%(__name__)s, port=”%(port)s”, protocol=”%(protocol)s”, chain=”%(chain)s”]
%(mta)s-whois-lines[name=%(__name__)s, dest=”%(destemail)s”, logpath=%(logpath)s, chain=”%(chain)s”]
action_xarf = %(banaction)s[name=%(__name__)s, port=”%(port)s”, protocol=”%(protocol)s”, chain=”%(chain)s”]
xarf-login-attack[service=%(__name__)s, sender=”%(sender)s”, logpath=%(logpath)s, port=”%(port)s”]
action_blocklist_de  = blocklist_de[email=”%(sender)s”, service=%(filter)s, apikey=”%(blocklist_de_apikey)s”]
action = %(action_)s
[sshd]
port    = ssh
logpath = /var/log/secure
[sshd-ddos]
port    = ssh
logpath = /var/log/secure
[dovecot]
port    = pop3,pop3s,imap,imaps,submission,465,sieve
logpath = /var/log/maillog
[dovecot-auth]
filter  = dovecot
port    = pop3,pop3s,imap,imaps,submission,465,sieve
logpath = /var/log/secure

新增我們的過濾規則 /etc/fail2ban/filter.d/sendmail-sasl.conf

[Definition]
#比對規則 msgid 為多行比對中的關聯值 也就是mail ID
failregex = sendmail.*: (?P<msgid>[^:]+): AUTH failure.*\n(?:.*\n)*?.+ sendmail.*: (?P=msgid): .*\[<HOST>\] .*to MTA
ignoreregex =
[Init]
#設定LOG一次讀取到buffer的行數, 如果sever很忙碌導致符合的行中間插入很多其它的log , 你可以增加buffer行數值
maxlines = 10

啟用我們要檢查的服務 /etc/fail2ban/jail.d/local.conf

[DEFAULT]
[sshd]
enabled  = true
filter   = sshd
action   = iptables-allports[name=FTP, port=ftp, protocol=tcp]
logpath  = /var/log/secure
[vsftpd]
enabled  = true
filter   = vsftpd
action   = iptables-allports[name=FTP, port=ftp, protocol=tcp]
logpath  = /var/log/secure
[sendmail]
enabled  = true
filter   = sendmail
action   = iptables-allports[name=SENDMAIL, port=smtp, protocol=tcp]
logpath  = /var/log/maillog
[dovecot]
enabled  = true
filter   = dovecot
action   = iptables-allports[name=DOVECOT, port=”pop3,pop3s,imap,imaps”, protocol=tcp]
logpath  = /var/log/maillog
[dovecot-auth]
enabled  = true
filter  = dovecot
action   = iptables-allports[name=DOVECOT, port=”pop3,pop3s,imap,imaps”, protocol=tcp]
logpath = /var/log/secure
[sendmail-sasl]
enabled  = true
filter   = sendmail-sasl
action   = iptables-allports[name=SENDMAIL, port=smtp, protocol=tcp]
logpath  = /var/log/maillog

開始測試過濾規則 , 由於繁忙的server maillog 相當大 , 建議先複製小部份到測試用的檔案

[root@mailhost /]# fail2ban-regex /tmp/test.log .conf /etc/fail2ban/filter.d/sendmail-sasl.conf

若有符合的結果則顯示如下

Running tests
=============
Use   failregex file : sendmail-sasl.conf
Use         maxlines : 10
Use         log file : test.log
Use         encoding : UTF-8
Results
=======
Failregex: 21 total
|-  #) [# of hits] regular expression
|   1) [21] sendmail.*: (?P<msgid>[^:]+): AUTH failure.*\n(?:.*\n)*?.+ sendmail.*: (?P=msgid): .*\[<HOST>\] .*to MTA
`-
Ignoreregex: 0 total
Date template hits:
|- [# of hits] date format
|  [102] MON Day 24hour:Minute:Second
`-
Lines: 102 lines, 0 ignored, 42 matched, 60 missed
Missed line(s):: too many to print.  Use –print-all-missed to print all 60 lines

最後啟動 fail2ban 服務

[root@mailhost /]# service fail2ban start

接下來就是觀察/var/log/fail2ban.log , 看看是否有 BAN 掉惡意的 ip 來源囉

〈SASL + PAM 無法紀錄 rhost 以配合 fail2ban 的解法〉這篇文章最早發佈於《MIS 的流水年華》。