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 --------------------------------------- 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 onerr=fail # 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... ^_^ 祝大家: 中秋快樂! netman