Linux基础服务sshd简介

Posted 白-胖-子

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sshd由OpenSSH来提供

  • SSH 协议：Secure Shell，安全的shell协议。
  • SSH 为建立在应用层和传输层基础上的安全协议。
  • sshd服务使用SSH协议可以用来进行远程控制， 或在计算机之间传送文件。
  • sshd使用加密传输，较之使用明文传输的telnet传输文件要安全很多。

sshd配置文件

/etc/ssh/sshd_config

  • 如果井号开头的和后面参数没有空格的，表示默认值,是生效的
  • 如果井号开头和后面有空格的，表示纯注释

调优参数

端口

  • 默认端口22，外网生产环境需要修改
    Port 22
  • 使用参数指定端口连接非22默认端口[-p port]
    ssh -p 1234 root@10.0.0.88

监听地址

  • 默认缺省值为所有网卡的所有地址
  • 可以修改为指定IP
#ListenAddress 0.0.0.0
#ListenAddress ::

登陆等待时间

  • 从输入用户名到敲入密码的登陆等待时间
  • 默认是2分钟，可以调的小一些
    #LoginGraceTime 2m

允许root登录

  • 生产环境应该禁止root直接登录
  • Debian系列例如Ubuntu是默认禁止root登陆的
    PermitRootLogin yes

默认认证公钥文件

AuthorizedKeysFile .ssh/authorized_keys

是否允许使用密码认证登录

  • 做过密钥认证以后可关闭密码认证登录，防止暴力破解
    PasswordAuthentication yes

打印登陆提示信息和最后登录日志

  • 发现被黑线索
    PrintMotd no
    #PrintLastLog yes
[14:56:49 root@C8-88[ ~]#ssh 10.0.0.189
Last login: Sat Jul  3 22:00:12 2021 from 10.0.0.88

使用DNS反向解析

  • 如果敲完密码一直卡着，半天才进系统，可以将此项改为no
    #UseDNS no

修改登录提示

  • 创建或修改motd文件
    /etc/motd
  • 在文件中添加需要登录显示的内容
  • 修改配置文件打开PrintMotd
    sed -ri.bak 's/(PrintMotd )no/\\1yes/' /etc/ssh/sshd_config

fail2ban防止暴力破解

  • 监控日志系统，匹配日志信息，将过分的ip加入到ipatble中
  • python写的用python装py包
  • 主配置文件jail.conf
  • 模板服务文件在源码包的files中，fail2ban.service,redhat-initd
  • 查找古时候的启动文件,文件内容带有chkconfig相关字样
    grep chkconfig ./* -R --color
  • 老版本使用chkconfig添加启动项
    chekconfig --add fail2ban
  • 登录日志

相关主要文件说明：

  • jail [dʒeɪl]监狱
/etc/fail2ban/action.d #动作文件夹，内含默认文件。iptables以及mail等动作配置
/etc/fail2ban/fail2ban.conf    #定义了fai2ban日志级别、日志位置及sock文件位置
/etc/fail2ban/filter.d                     #条件文件夹，内含默认文件。过滤日志关键内容设置
/etc/fail2ban/jail.conf     #主要配置文件，模块化。主要设置启用ban动作的服务及动作阀值

应用案例

  • 远程ssh用户5分钟三次失败则禁用1小时
  • 防止频繁试密码
  • 修改[ssh-iptables]
  • 先启用功能
    enabled = ture
  • 指定日志
    logpath = /var/log/secure
  • 发一批邮件改sentmail，系统中需要已启用sentmail
  • 如果ssh服务器不是22端口，则需要改配置文件，
  • iptables配置文件也要改

查看ban的状态

fail2ban-client status ssh-iptable

使用shell脚本实现fail2ban功能

  • 利用定时任务查看安全日志
  • 将超过阈值的IP加入系统黑名单hosts.deny中去
#!/bin/bash
cat var/log/secure | awk '/Failed/{print $(NF-3)}' | sort | uniq -c | awk '{print $2"="$1;}' > /root/satools/black.txt
DEFINE='10'
for i in `cat /root/satools/black.txt`;do
    IP=`echo $i | awk -F= '{print $1}'`
    NUM=`echo $i | awk -F= '{print $2}'`

    if [ $NUM -gt $DEFINE ];then
        grep $IP /etc/hosts.deny > /dev/null
        if [ $? -gt 0 ];then
            echo "sshd:$IP" >> /etc/hosts/deny
        fi
    fi
done

利用denyhosts实现

  • epel中的denyhosts简单实现

系统再带pam模块提供防护功能

  • 系统本身体统pam模块功能
    /etc/pam.d/sshd
  • 添加错误尝试次数，和锁定时间
  • 在第一行下面添加一条规则
    auth required pam_tally2.so deny=3 unlock_time=600 even_deny_root root_unlock_time=1200

pam模块命令

  • 查看用户登录次数
    手动解除锁定：
    查看某一用户错误登陆次数：
    pam_tally –-user
    例如，查看work用户的错误登陆次数：
    pam_tally –-user work
    清空某一用户错误登陆次数：
    pam_tally –-user –-reset
    例如，清空 work 用户的错误登陆次数，
    pam_tally –-user work –-reset

xshell等客户端免密登录服务器

  • 使用xshell等客户端本地生成公钥文件
  • 将生成的公钥文件放到服务器对应的用户家目录的.ssh目录中去

==========================================================
配置文件内容：

#	$OpenBSD: sshd_config,v 1.103 2018/04/09 20:41:22 tj Exp $

# This is the sshd server system-wide configuration file.  See
# sshd_config(5) for more information.

# This sshd was compiled with PATH=/usr/local/bin:/usr/bin:/usr/local/sbin:/usr/sbin

# The strategy used for options in the default sshd_config shipped with
# OpenSSH is to specify options with their default value where
# possible, but leave them commented.  Uncommented options override the
# default value.

# If you want to change the port on a SELinux system, you have to tell
# SELinux about this change.
# semanage port -a -t ssh_port_t -p tcp #PORTNUMBER
#
#Port 22
#AddressFamily any
#ListenAddress 0.0.0.0
#ListenAddress ::

HostKey /etc/ssh/ssh_host_rsa_key
HostKey /etc/ssh/ssh_host_ecdsa_key
HostKey /etc/ssh/ssh_host_ed25519_key

# Ciphers and keying
#RekeyLimit default none

# System-wide Crypto policy:
# This system is following system-wide crypto policy. The changes to
# Ciphers, MACs, KexAlgoritms and GSSAPIKexAlgorithsm will not have any
# effect here. They will be overridden by command-line options passed on
# the server start up.
# To opt out, uncomment a line with redefinition of  CRYPTO_POLICY=
# variable in  /etc/sysconfig/sshd  to overwrite the policy.
# For more information, see manual page for update-crypto-policies(8).

# Logging
#SyslogFacility AUTH
SyslogFacility AUTHPRIV
#LogLevel INFO

# Authentication:

#LoginGraceTime 2m
PermitRootLogin yes
#StrictModes yes
#MaxAuthTries 6
#MaxSessions 10

#PubkeyAuthentication yes

# The default is to check both .ssh/authorized_keys and .ssh/authorized_keys2
# but this is overridden so installations will only check .ssh/authorized_keys
AuthorizedKeysFile	.ssh/authorized_keys

#AuthorizedPrincipalsFile none

#AuthorizedKeysCommand none
#AuthorizedKeysCommandUser nobody

# For this to work you will also need host keys in /etc/ssh/ssh_known_hosts
#HostbasedAuthentication no
# Change to yes if you don't trust ~/.ssh/known_hosts for
# HostbasedAuthentication
#IgnoreUserKnownHosts no
# Don't read the user's ~/.rhosts and ~/.shosts files
#IgnoreRhosts yes

# To disable tunneled clear text passwords, change to no here!
#PasswordAuthentication yes
#PermitEmptyPasswords no
PasswordAuthentication yes

# Change to no to disable s/key passwords
#ChallengeResponseAuthentication yes
ChallengeResponseAuthentication no

# Kerberos options
#KerberosAuthentication no
#KerberosOrLocalPasswd yes
#KerberosTicketCleanup yes
#KerberosGetAFSToken no
#KerberosUseKuserok yes

# GSSAPI options
GSSAPIAuthentication yes
GSSAPICleanupCredentials no
#GSSAPIStrictAcceptorCheck yes
#GSSAPIKeyExchange no
#GSSAPIEnablek5users no

# Set this to 'yes' to enable PAM authentication, account processing,
# and session processing. If this is enabled, PAM authentication will
# be allowed through the ChallengeResponseAuthentication and
# PasswordAuthentication.  Depending on your PAM configuration,
# PAM authentication via ChallengeResponseAuthentication may bypass
# the setting of "PermitRootLogin without-password".
# If you just want the PAM account and session checks to run without
# PAM authentication, then enable this but set PasswordAuthentication
# and ChallengeResponseAuthentication to 'no'.
# WARNING: 'UsePAM no' is not supported in Fedora and may cause several
# problems.
UsePAM yes

#AllowAgentForwarding yes
#AllowTcpForwarding yes
#GatewayPorts no
X11Forwarding yes
#X11DisplayOffset 10
#X11UseLocalhost yes
#PermitTTY yes

# It is recommended to use pam_motd in /etc/pam.d/sshd instead of PrintMotd,
# as it is more configurable and versatile than the built-in version.
PrintMotd no

#PrintLastLog yes
#TCPKeepAlive yes
#PermitUserEnvironment no
#Compression delayed
#ClientAliveInterval 0
#ClientAliveCountMax 3
#UseDNS no
#PidFile /var/run/sshd.pid
#MaxStartups 10:30:100
#PermitTunnel no
#ChrootDirectory none
#VersionAddendum none

# no default banner path
#Banner none

# Accept locale-related environment variables
AcceptEnv LANG LC_CTYPE LC_NUMERIC LC_TIME LC_COLLATE LC_MONETARY LC_MESSAGES
AcceptEnv LC_PAPER LC_NAME LC_ADDRESS LC_TELEPHONE LC_MEASUREMENT
AcceptEnv LC_IDENTIFICATION LC_ALL LANGUAGE
AcceptEnv XMODIFIERS

# override default of no subsystems
Subsystem	sftp	/usr/libexec/openssh/sftp-server

# Example of overriding settings on a per-user basis
#Match User anoncvs
#	X11Forwarding no
#	AllowTcpForwarding no
#	PermitTTY no
#	ForceCommand cvs server

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