SSH 的一些安全小技巧
作者﹕網中人 <netman@study-area.org>
版本：v0.03
日期：2007-11-26
* 版本歷程：
1) 2005-09-16 v0.01
- 初版
2) 2006-06-02 v0.02
- 修改 sshopen.sh
3) 2007-11-26 v0.03
- deny unmatched ip
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一, 前言
關於 ssh 的好處, 相信不用我多說了吧?
簡而言之, 之前的 rpc command 與 telnet 都全可用 ssh 代替.
比方如下的這些常見功能:
- 遠端登錄
ssh user@remote.machine
- 遠端執行
ssh user@remote.machine 'command ...'
- 遠端複制
scp user@remote.machine:/remote/path /local/path
scp /local/path user@remote.machine:/remote/path
- X forward
ssh -X user@remote.machine
xcommand ...
- Tunnel / Portforward
ssh -L 1234:remote.machine:4321 user@remote.machine
ssh -R 1234:local.machine:4321 user@remote.machine
ssh -L 1234:other.machine:4321 user@remote.machine
至於詳細的用法, 我這就不說了. 請讀者自行研究吧.
我這裡要說的, 是針對 ssh 服務為大家介紹一些安全技巧, 希望大家用得更安心些.
二, 實作
(實作以 RedHat 9 為範例)
--------------------------------------------------
轉往 client 端:
$ ssh-keygen -t rsa
* 按三下 enter 完成﹔不需設密碼，除非您會用 ssh-agent .
$ scp ~/.ssh/id_rsa.pub user1@server.machine:id_rsa.pub
* 若是 windows client, 可用 puttygen.exe 產生 public key,
然後複制到 server 端後修改之, 使其內容成為單一一行.
* 如果 server 端已經禁止密碼登入, 那請用其它放法復製 publick key.
---------------------------------------------------
登入 server 端:
1) 禁止 root 登錄
# vi /etc/ssh/sshd_config
PermitRootLogin no
2) 廢除密碼登錄, 強迫使用 RSA 驗證(假設 ssh 帳戶為 user1 )
# vi /etc/ssh/sshd_config
RSAAuthentication yes
PubkeyAuthentication yes
AuthorizedKeysFile .ssh/authorized_keys
PasswordAuthentication no
# service sshd restart
# su - user1
$ mkdir ~/.ssh 2>/dev/null
$ chmod 700 ~/.ssh
$ touch ~/.ssh/authorized_keys
$ chmod 644 ~/.ssh/authorized_keys
$ cat ~/id_rsa.pub >> ~/.ssh/authorized_keys
$ rm ~/id_rsa.pub
$ exit
3) 限制 su / sudo 名單:
# vi /etc/pam.d/su
auth required /lib/security/$ISA/pam_wheel.so use_uid
# visudo
%wheel ALL=(ALL) ALL
# gpasswd -a user1 wheel
4) 限制 ssh 使用者名單
# vi /etc/pam.d/sshd
auth required pam_listfile.so item=user sense=allow file=/etc/ssh_users
# echo user1 >> /etc/ssh_users
5) 封鎖 ssh 連線並改用 web 控管清單
# iptables -I INPUT -p tcp --dport 22 -j DROP
# mkdir /var/www/html/ssh_open
# cat > /var/www/html/ssh_open/.htaccess <<END
AuthName "ssh_open"
AuthUserFile /var/www/html/ssh_open/.htpasswd
AuthType basic
require valid-user
END
# htpasswd -c /var/www/html/ssh_open/.htpasswd user1
(最好還將 SSL 設起來, 或只限 https 連線更佳, 我這裡略過 SSL 設定, 請讀者自補.)
(如需控制連線來源, 那請再補 Allow/Deny 項目, 也請讀者自補.)
# cat > /var/www/html/ssh_open/ssh_open.php <<END
<?
//Set dir path for ip list
$dir_path=".";
//Set filename for ip list
$ip_list="ssh_open.txt";
//Get client ip
$user_ip=$_SERVER['REMOTE_ADDR'];
//allow specifying ip if needed
if (@$_GET['myip']) {
$user_ip=$_GET['myip'];
}
//checking IP format
if ($user_ip==long2ip(ip2long($user_ip))) {
//Put client ip to a file
if(@!($file = fopen("$dir_path/$ip_list","w+")))
{
echo "Permission denied!!<br>";
echo "Pls Check your rights to dir $dir_path or file $ip_list";
}
else
{
fputs($file,"$user_ip");
fclose($file);
echo "client ip($user_ip) has put into $dir_path/$ip_list";
}
} else {
echo "Invalid IP format!!<br>ssh_open.txt was not changed.";
}
?>
END
# touch /var/www/html/ssh_open/ssh_open.txt
# chmod 640 /var/www/html/ssh_open/*
# chgrp apache /var/www/html/ssh_open/*
# chmod g+w /var/www/html/ssh_open/ssh_open.txt
# chmod o+t /var/www/html/ssh_open
# service httpd restart
# mkdir /etc/iptables
# cat > /etc/iptables/sshopen.sh <<END
#!/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
list_dir=/var/www/html/ssh_open
list_file=$list_dir/allow_ssh.txt
bad_list=$list_dir/bad_ip.txt
auth_log=$list_dir/xinetd.log
trusted_ip="127.0.0.1 4.3.2.1"
chain_name=ssh_rules
mail_to=root
# clear chain if exits, or create chain.
iptables -L -n | /bin/grep -q "^Chain $chain_name" && {
iptables -F $chain_name
true
} || {
iptables -N $chain_name
iptables -I INPUT -p tcp --dport 22 -j $chain_name
}
# clear chain on demand
[ "$1" = clear ] && {
iptables -F $chain_name
cat /dev/null > $list_file
exit 0
}
# do nothing while list is empty
[ -s $list_file ] || exit 1
# deny connection if host dosn't math to list
host_ip=$(grep 'myssh from=' $auth_log | tail -1 | awk -F'=' '{print $NF}')
list_ip=$(cat $list_file)
if [ -n "$host_ip" -a "$host_ip" != "$list_ip" ]; then
echo -e "${trusted_ip/ /\n}" | grep -q "$host_ip" || {
/sbin/iptables-save | grep -q "INPUT -s $host_IP -j DROP$" || {
/sbin/iptables -I INPUT -s $host_ip -j DROP
echo $host_ip >> $bad_list
echo "$host_ip is blocked by $0 on $(date)" | mail -s "block
ip" $mail_to
}
}
exit 2
fi
# add rule
iptables -A $chain_name -p tcp --dport 22 -s $(< $list_file) -j ACCEPT && \
echo "ssh opened to $(< $list_file) on $(date)" | \
mail -s "sshopen" $mail_to
exit 0
END
# chmod +x /etc/iptables/sshopen.sh
# echo -e 'sshopen\t\t1234/tcp' >> /etc/services
# cat > /etc/xinetd.d/sshopen <<END
service sshopen
{
log_type = FILE /studyarea/www/phorum/xinetd.log
log_on_success = HOST
log_on_failure = HOST
disable = no
socket_type = stream
protocol = tcp
wait = no
user = root
server = /etc/iptables/sshopen.sh
}
# iptables -I INPUT -p tcp --dport 1234 -j ACCEPT
# cat > /etc/cron.d/sshopen <<END
*/5 * * * * root /etc/iptables/sshopen.sh clear
END
---------------------------
轉往 client 端
在 browser URL 輸入:
http://server.machine/ssh_open/ssh_open.php?myip=1.2.3.4
(若不指定 ?myip=1.2.3.4 則以 client 當時 IP 為準, 若沒經 proxy 的話.)
如此, server 端的 ssh_open.txt 只有單一記錄, 每次蓋寫.
接著:
$ telnet server.machine 1234
然後你有最多 5 分鐘時間用 ssh 連線 server !
---------------------------
此步驟的基本構思如下:
5.1) 將 sshd 的 firewall 連線全部 block 掉.
5.2) 然後在 httpd 那設一個 directory, 可設 ssl+htpasswd+allow/deny control,
然後在目錄內寫一個 php 將 browser ip 記錄於一份 .txt 文字檔裡.
視你的轉寫能力, 你可自動抓取 browser 端的 IP, 也可讓 browser 端傳入參數來指定.
文字檔只有單一記錄, 每次蓋寫, 定期清空.
5.3) 修改 /etc/services , 增加一個新項目(如 xxx), 並指定一個新 port(如 1234)
5.4) 再用 xinetd 監聽該 port , 並啟動令一隻 script,
設定 iptables , 從 step2 的清單裡取得 IP, 為之打開 ssh 連線.
5.5) 設 crontab 每數分中清理 iptables 關於 ssh 連線的規則及清空記錄.
這並不影響既有連線, 若逾時再連, 則重複上述.
6) 要是上一步驟沒設定, 你或許會擔心過多的人來 try 你的 ssh 服務的話:
# cat > /etc/iptables/sshblock.sh <<END
#!/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
LOG_FILE=/var/log/secure
KEY_WORD="Illegal user"
KEY_WORD1="Failed password for root"
PERM_LIST=/etc/firewall/bad.list.perm
LIMIT=5
MAIL_TO=root
IPT_SAV="$(iptables-save)"
bad_list=$(egrep "$KEY_WORD" $LOG_FILE | awk '{print $NF}' | xargs)
bad_list1=$(egrep "$KEY_WORD1" $LOG_FILE | awk '{print $11}' | xargs)
bad_list="$bad_list $bad_list1"
for i in $(echo -e "${bad_list// /\n}" | sort -u)
do
hit=$(echo $bad_list | egrep -o "$i" | wc -l)
[ "$hit" -ge "$LIMIT" ] && {
echo "$IPT_SAV" | grep -q "$i .*-j DROP" || {
echo -e "\n$i was dropped on $(date)\n" | mail -s "DROP by ${0##*/}: $i" $MAIL_TO
iptables -I INPUT -s $i -j DROP
}
egrep -q "^$i$" $PERM_LIST || echo $i >> $PERM_LIST
}
done
END
# chmod +x /etc/firewall/sshblock.sh
# cat >> /etc/hosts.allow <<END
sshd: ALL: spawn ( /etc/firewall/sshblock.sh )& : ALLOW
END
這樣, 那些亂 try SSH 的家夥, 頂多能試 5 次(LIMIT 可調整), 然後就給 BLOCK 掉了.
此外, 在 PERM_LIST 的 ip, 也可提供給 iptables 的初始 script , 來個永久性封閉:
for i in $(< $PERM_LIST)
do
/sbin/iptables -I INPUT -s $i -j DROP
done
7) 還有, 你想知道有哪些人對你做 full range port scan 的話:
# iptables -I INPUT -p tcp --dport 79 -j ACCEPT
cat > /etc/xinetd.d/finger <<END
service finger
{
socket_type = stream
wait = no
user = nobody
server = /usr/sbin/in.fingerd
disable = no
}
END
# cat >> /etc/hosts.allow <<END
in.fingerd: ALL : spawn ( echo -e "\nWARNING %a was trying finger.\n$(date)" | mail -s "finger from %a" root ) & : DENY
END
這裡, 我只是設為發信給 root.
事實上, 你可修改為起動 firewall 將 %a 這個傳回值給 ban 掉也行.
不過, 對方要是有選擇性的做 port scan , 沒掃到 finger 的話, 那當然就沒用了...
三, 結語
security 有蠻多挺好玩的小技巧, 有空再跟大家做分享... ^_^
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