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SSH(Secure Shell)
ssh의 개요
- 원격 시스템에 로그인하여 명령을 실행 하는 프로그램
- 기본적인 사용법은 telnet 과 유사하다.
- telnet과 ssh의 비교
- telnet : 클라이언트와 서버간의 데이터 전송 시, 일반 텍스트 형태를 패킷으로 전달하여 패킷의 내용이 유출당할 수 있다.
- ssh : 패킷 전송 시 암호화시키므로 안전하게 패킷을 전송할 수 있다.
- 버전
- ssh1
- ssh2
- ssh1을 개선한 것
- ssh과 호환은 되지만, 기본적으로 호환성을 포함하고 있는 것이 아니므로 ssh1을 지원하려면 ssh1 서버를 같이 운영해야 한다.
- 이중 암호화 RSA 키 교환을 비롯하여 다양한 키 교환 방법 지원
ssh의 특징
① 패킷을 암호화하여 다른 원격 프로그램인 telnet이나 rlogin에 비해 안전하다.
② rlogin처럼 패스워드 입력 없이 로그인이 가능하다.
③ rsh처럼 원격 셸을 지원한다.
④ 원격 복사(scp)를 지원한다.
⑤ 안전한 파일 전송(sftp)를 지원한다.
ssh 설치
- 리눅스에서는 ssh의 공개 버전인 openssh를 사용한다.
- SSH 서버 와 SSH 클라이언트 프로그램으로 나누어져 있다.
- 레드햇 계열 리눅스에서는 3개의 패키지로 구분해서 배포하고 있다.
| 패키지명 | 내용 |
| openssh | openssh 서버와 openssh 클라이언트에 모두 필요한 핵심적인 파일들이 들어있는 패키지 ssh-keygen 명령이 들어있음. |
| openssh-server | openssh 서버 관련 패키지 데몬인 sshd, sftp 서버 등이 들어 있음. |
| openssh-clients | openssh 클라이언트 관련 패키지 ssh, scp, sftp 등의 명령어가 들어 있음. |
ssh 서버 설정
- ssh 서버 환경 설정 파일 : /etc/ssh/sshd_config
- 실행 명령 : systemctl start sshd.service
sshd_config 의 주요 항목
# cat /etc/ssh/sshd_config더보기
# $OpenBSD: sshd_config,v 1.100 2016/08/15 12:32:04 naddy 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
# 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_dsa_key
HostKey /etc/ssh/ssh_host_ecdsa_key
HostKey /etc/ssh/ssh_host_ed25519_key
# Ciphers and keying
#RekeyLimit default none
# 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 Red Hat Enterprise Linux and may cause several
# problems.
UsePAM yes
#AllowAgentForwarding yes
#AllowTcpForwarding yes
#GatewayPorts no
X11Forwarding yes
#X11DisplayOffset 10
#X11UseLocalhost yes
#PermitTTY yes
#PrintMotd yes
#PrintLastLog yes
#TCPKeepAlive yes
#UseLogin no
#UsePrivilegeSeparation sandbox
#PermitUserEnvironment no
#Compression delayed
#ClientAliveInterval 0
#ClientAliveCountMax 3
#ShowPatchLevel no
#UseDNS yes
#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- Port 22
- ssh 서버가 사용하는 포트 지정
- #AddressFamily any
- 접속하는 IP 주소 버전을 지정하는 옵션
- any : IPv4 와 IPv6 주소 체계 모두 접속 가능
- inet : IPv4 주소 체계만 접속 가능
- #ListenAddress 0.0.0.0, #ListenAddress ::
- sshd 데몬이 허가하는 주소를 설정하는 영역
- 여러 개의 네트워크 인터페이스가 있을 경우, IP 대역과 포트를 지정할 수 있음.
- ListenAddress 192.168.1.1:22 형태로 지정하면 됨.
- 콜론(:)은 IPv6 주소 체계에서 사용하면 됨.
- Protocol 2
- ssh 프로토콜의 버전
- HostKey /etc/ssh/ssh_host_key
- SSH2의 RSA 암호화 방식의 호스트키 위치 지정
- HostKey /etc/ssh/ssh_host_dsa_key
- SSH2의 DSA 암호화 방식의 호스트키 위치 지정
- KeyRegenerationInterval 1h
- 서버의 키는 한번 접속이 이루어진 뒤에 자동적으로 다시 만들어짐.
- 다시 만드는 목적
- 나중에 호스트의 세션에 있는 키를 캡처해서 암호를 해독하거나 훔친 키를 사용하지 못하도록 하기 위함.
- 다시 만드는 목적
- 값이 0이면 키는 다시 만들어지지 않음을 의미함.
- 기본 단위 : 초(Second)
- m : 분(Minute), h : 시(Hour)
- 서버의 키는 한번 접속이 이루어진 뒤에 자동적으로 다시 만들어짐.
- ServerKeyBits 1024
- 서버 키의 비트수 정의
- SyslogFacility AUTHPRIV
- syslog와 관련하여 facility를 지정하는 항목
- ssh를 통한 접속은 AUTHPRIV로 처리하여 기록함.
- LogLevel INFO
- 로그 레벨을 지정 하는 항목
- 기본값 : INFO
- 그 외의 값
- QUIET : 기록하지 않음.
- FATAL : 치명적인 오류
- ERROR
- VERBOSE
- DEBUGS
- LoginGraceTime 2m
- 유저가 로그인에 실패했을 경우 서버가 연결을 끊는 시간
- 0 : 제한 없음
- PermitRootLogin yes
- root 로그인 허용 여부를 결정하는 항목
- yes : root으로 접속이 가능
- 보안을 강화하기 위해서는 no로 설정해야 함.
- StrictModes yes
- 로그인을 허용하기 전에 다음의 파일들을 SSH 데몬 이 체크할 수 있도록 해주는 설정
- 파일 모드
- 사용자 홈 디렉터리 소유권
- 원격 호스트의 파일들
- 로그인을 허용하기 전에 다음의 파일들을 SSH 데몬 이 체크할 수 있도록 해주는 설정
- MaxAuthTries 6
- 접속할 때 재시도할 수 있는 최대 횟수를 지정하는 항목
- MaxSessions 10
- 네트워크 주소 당 최대 연결할 수 있는 세션 수를 지정하는 항목
- RSAAuthentication yes
- RSA 인증을 사용할 것인지를 설정하는 항목
- PubkeyAuthentication yes
- 공개키를 통한 인증을 허가할 것인지를 설정하는 항목
- AuthorizedKeysFile .ssh/authorized_keys
- 클라이언트에서 생성한 공개키를 저장할 파일명을 설정하는 항목
- RhostsRSAAuthentication no
- 프로토콜 1 기반 RSA 를 이용할 때, rhost나 /etc/hosts.equiv 파일이 있으면 이것을 사용해 인증할 수 있도록 하는 항목
- 기본값 : no
- HostbasedAuthentication no
- rhost나 /etc/hosts.equiv 를 이용하여 접속할 때 공개키를 이용하여 접속을 허가할 것인지를 설정할 때 사용
- 기본값 : no
- IgnoreUserKnownHosts no
- RhostsRSAAuthentication이나 HostbasedAuthentication을 사용한 인증 시, 각 사용자의 홈 디렉터리에 있는 .ssh/known_hosts를 무시할 것인지를 설정할 때 사용
- 기본값 : no
- IgnoreRhosts yes
- RhostsRSAAuthentication이나 HostbasedAuthentication을 사용한 인증 시, .rhosts 파일을 무시할 것인지를 설정할 때 사용
- 기본값 : yes (.rhosts 파일 무시)
- PasswordAuthentication yes
- 패스워드를 이용한 인증을 허용할 때 사용
- 기본값 : yes
- PermitEmptyPasswords no
- 패스워드 인증할 때 서버가 비어있는 패스워드를 인정하는 것과 관련된 설정
- 기본값 : no
- ChallengeResponseAuthentication yes
- Challenge-Response 관련 인증 설정
- # KerberosAuthentication no, # KerberosOrLocalPasswd yes, # KerberosTicketCleanup yes, # KerberosGetAFSToken no, #KerberosUseKuserok yes
- Kerberos 인증 관련 설정
- GSSAPIAuthentication yes, GSSAPICleanupCredentials no
- GSSAPI 인증 관련 설정
- UsePAM yes
- PAM 을 통한 제어를 할 것인지 설정
- # AllowTcpForwarding yes
- TCP 를 통한 포워딩을 허가할 것인지 설정
- # GatewayPorts no
- 포워딩된 포트에 대하여 클라이언트의 접속을 허가할 것인지 설정
- X11Forwarding yes
- 원격에서 X11 포워딩을 허용할 지 설정
- yes : xhost 보다 안전한 방법으로 원격에 있는 X 프로그램을 사용할 수 있음.
- # X11DisplayOffset 10
- X11 로 포워딩될 때 디스플레이 번호를 지정해주는 설정
- # X11UseLocalhost yes
- X11 로 포워딩 서버에 접속할 때 localhost 관련 설정
- # PrintMotd yes
- SSH 로그인 시, /etc/motd 파일의 내용을 프린트 되도록 설정
- # PrintLastLog yes
- 로그인 시, 지난번 로그인 기록을 보여줄 것인지 설정
- # TCPKeepAlive yes
- 클라이언트의 접속이 끊어졌는지 체크하기 위해 일정 시간 메시지를 전달하도록 하는 설정
- # UseLogin no
- 로그인 세션을 사용할 것인지 설정
- # UsePrivilegeSeparation sandbox
- 네트워크 트래픽 발생 시 권한이 없는 자식 프로세스에 의해 생성된 권한을 분리할 것인지 설정
- # PermitUserEnvironment no
- 사용자의 홈 디렉터리의 .ssh/environment나 .ssh/authorized_keys 안의 environment-옵션을 사용하게 할 것인지 설정
- # Compression delayed
- 압축을 허가할 것인지 설정
- # ClientAliveInterval 0
- 클라이언트로부터 전송되는 데이터가 없을 경우, 서버는 메시지를 보내게 되는데 이때의 시간을 지정함.
- 0 : 메시지를 보내지 않음.
- # ClientAliveCountMax 3
- 클라이언트에게 전달하는 메시지 횟수 지정
- # UseDNS yes
- 클라이언트 호스트 주소를 아이피로 해석할 것인지 설정
- # PidFile /var/run/sshd.pid
- ssh 데몬의 PID 를 기록하는 파일
- # Banner none
- 배너 관련으로 로그인 전에 보여주는 메시지 파일을 지정하는 부분
- 예) 텔넷
- /etc/issue, /etc/issue.net 파일을 사용
- Subsystem sftp /usr/libexec/openssh/sftp-server
- sftp 서버를 사용하기 위한 항목 sftp
- 프로토콜 버전 2에서 사용되는 것
- ssh와 같이 ftp의 보안을 강화하기 위해 사용되는 ftp 프로그램
- sftp 서버를 사용하기 위한 항목 sftp
관련 명령어
① ssh 클라이언트 명령어 : ssh
- 리눅스에서는 openssh-clients 패키지를 설치하면 ssh 명령을 이용해서 접속할 수 있다.
- 윈도우에는 SecureCRT, XShell 등과 같은 프로그램을 이용해야 한다.
사용법
방법 1
- ssh는 서버로 접속할 때, 같은 계정으로 접속을 시도하므로 클라이언트와 서버의 계정이 같으면 생략해도 된다.
$ ssh [option] 호스트명 or IP주소
방법 2
- -l 옵션 대신에 사용하는 방법
- 클라이언트 계정과 서버의 계정이 다르면 계정 이름을 명시해야 한다.
$ ssh 계정 이름@호스트네임
방법 3
- rsh와 같이 원격 셸을 사용할 수 있음.
- 접속 없이 명령만 내릴 때 유용하다.
$ ssh 호스트네임 명령
주요 옵션
| 옵션 | 설명 |
| -l | - 다른 계정으로 접속할 때 사용 - 이 옵션 대신 서버 주소 앞에 @를 붙여 사용할 수도 있음. |
| -p | ssh 서버의 포트 번호가 22번이 아닌 경우, -p 옵션을 사용해서 바뀐 포트를 지정할 때 사용함. |
사용 예
- 203.247.40.246 서버로 클라이언트의 계정과 같은 계정인 starrykss 로 접속을 시도함.
- 관련 키 설정을 위해 yes라고 입력하고, 원격지 서버의 패스워드를 입력하면 접속됨.
[starrykss@www ~] $ ssh 203.247.40.246
...
Are you sure you want to continue connecting (yes/no)? yes
...
starrykss@203.247.40.246's password:
Last login: ...
- 192.168.1.1 서버로 클라이언트의 계정과 다른 계정인 yuloje로 접속을 시도함.
[starrykss@www ~] $ ssh -l yuloje 192.168.1.1
- 192.168.1.1 서버로 yuloje라는 계정으로 접속을 시도함.
[starrykss@www ~] $ ssh yuloje@192.168.1.1
- 192.168.1.1 서버의 ssh 포트 번호가 180인 경우에 사용
[starrykss@www ~] $ ssh -p 180 192.168.1.1
- 원격에서 starrykss.org에 yuloje라는 계정으로 접속하여 data라는 디렉터리를 생성함.
[starrykss@www ~] $ ssh -l yuloje starrykss.org mkdir data
② 인증키를 이용한 SSH 서버 접속 : ssh-keygen
- SSH 서버를 접속할 때, 패스워드를 입력하여 접속하지만, 인증키를 이용하여 접속할 수도 있다.
- SSH 클라이언트에서 ssh-keygen 명령을 이용해서 비밀키와 공개키 를 생성하고 SSH 서버에 공개키를 복사하면 된다.
사용법
$ ssh-keygen [option]주요 옵션
| 옵션 | 설명 |
| -t | - 사용할 암호화 알고리즘을 지정하는 옵션 - rsa, dsa 등을 사용할 수 있음. - ssh2 버전에서 지정하지 않으면 rsa를 사용함. |
사용 예
- DSA를 이용해서 인증키 새성
$ ssh-keygen -t dsa
- RSA를 이용해서 인증키 새성
$ ssh-keygenGenerating public/private dsa key pair.
Enter file in which to save the key (/home/starrykss/.ssh/id_dsa):
Created directory '/home/starrykss/.ssh'.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/starrykss/.ssh/id_dsa.
Your public key has been saved in /home/starrykss/.ssh/id_dsa.pub.
The key fingerprint is:
SHA256:X+3OzWtcnwXAKEHISaBCC7hY6ObxRoKZwOEAE1FHkWs starrykss@localhost.localdomain
The key's randomart image is:
+---[DSA 1024]----+
|@Bo.=*.+o. o |
|Oooo. + . . o |
|*B. . . . |
|** .E .. |
|o =. S . .. |
| . o . . . o|
| . . o =|
| o *.|
| +.+|
+----[SHA256]-----+- 2개의 파일이 생성된다.
- id_rsa (비밀키)
- id_rsa.pub (공개키)
- 패스워드를 별도로 설정하지 않으면 서버에 접속할 때, 패스워드 없이 로그인이 가능하다.
- 생성된 파일 중에 공개키 파일에 해당하는 id_rsa.pub 파일을 원격지 서버의 홈 디렉터리 안에 .ssh/authorized_keys 로 복사해두면 된다.
(참고) 원격 셸 및 scp를 이용해서 작업하기
- 원격지에 .ssh 디렉터리를 생성하고 scp 를 이용해서 공개키를 복사한다.
$ ssh 203.247.40.246 mkdir .ssh
starrykss@203.247.40.246's password:
$ scp .ssh/id_rsa.pub 203.247.40.246:.ssh/authorized_keys
starrykss@203.247.40.246's password:
id_rsa.pub 100% 392 0.4KB/s 00:00- ssh 명령으로 접속 시도
$ ssh ssh 203.247.40.246
Last login: ...
문제 해결 전략
- ssh 또는 ssh-keygen 명령어와 관련된 문제가 출제 되었을 경우,man ssh 또는 man ssh-keygen 명령을 사용하여 메뉴얼을 확인해가면서 문제를 해결한다.
$ man ssh
$ man ssh-keygen
- sshd_config 파일 설정과 관련된 문제가 출제 되었을 경우, man sshd_config 명령을 사용하여 메뉴얼을 확인해가면서 문제를 해결한다.
$ man sshd_config더보기
SSHD_CONFIG(5) BSD File Formats Manual SSHD_CONFIG(5)
NAME
sshd_config — OpenSSH SSH daemon configuration file
SYNOPSIS
/etc/ssh/sshd_config
DESCRIPTION
sshd(8) reads configuration data from /etc/ssh/sshd_config (or the file specified with -f on the
command line). The file contains keyword-argument pairs, one per line. Lines starting with ‘#’ and
empty lines are interpreted as comments. Arguments may optionally be enclosed in double quotes (")
in order to represent arguments containing spaces.
The possible keywords and their meanings are as follows (note that keywords are case-insensitive and
arguments are case-sensitive):
AcceptEnv
Specifies what environment variables sent by the client will be copied into the session's
environ(7). See SendEnv in ssh_config(5) for how to configure the client. The TERM envi‐
ronment variable is always sent whenever the client requests a pseudo-terminal as it is
required by the protocol. Variables are specified by name, which may contain the wildcard
characters ‘*’ and ‘?’. Multiple environment variables may be separated by whitespace or
spread across multiple AcceptEnv directives. Be warned that some environment variables
could be used to bypass restricted user environments. For this reason, care should be taken
in the use of this directive. The default is not to accept any environment variables.
AddressFamily
Specifies which address family should be used by sshd(8). Valid arguments are any (the
default), inet (use IPv4 only), or inet6 (use IPv6 only).
AllowAgentForwarding
Specifies whether ssh-agent(1) forwarding is permitted. The default is yes. Note that dis‐
abling agent forwarding does not improve security unless users are also denied shell access,
as they can always install their own forwarders.
AllowGroups
This keyword can be followed by a list of group name patterns, separated by spaces. If
specified, login is allowed only for users whose primary group or supplementary group list
matches one of the patterns. Only group names are valid; a numerical group ID is not recog‐
nized. By default, login is allowed for all groups. The allow/deny directives are pro‐
cessed in the following order: DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.
All of the specified user and group tests must succeed, before user is allowed to log in.
See PATTERNS in ssh_config(5) for more information on patterns.
AllowStreamLocalForwarding
Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted. The available
options are yes (the default) or all to allow StreamLocal forwarding, no to prevent all
StreamLocal forwarding, local to allow local (from the perspective of ssh(1)) forwarding
only or remote to allow remote forwarding only. Note that disabling StreamLocal forwarding
does not improve security unless users are also denied shell access, as they can always
install their own forwarders.
AllowTcpForwarding
Specifies whether TCP forwarding is permitted. The available options are yes (the default)
or all to allow TCP forwarding, no to prevent all TCP forwarding, local to allow local (from
the perspective of ssh(1)) forwarding only or remote to allow remote forwarding only. Note
that disabling TCP forwarding does not improve security unless users are also denied shell
access, as they can always install their own forwarders.
AllowUsers
This keyword can be followed by a list of user name patterns, separated by spaces. If spec‐
ified, login is allowed only for user names that match one of the patterns. Only user names
are valid; a numerical user ID is not recognized. By default, login is allowed for all
users. If the pattern takes the form USER@HOST then USER and HOST are separately checked,
restricting logins to particular users from particular hosts. HOST criteria may addition‐
ally contain addresses to match in CIDR address/masklen format. The allow/deny directives
are processed in the following order: DenyUsers, AllowUsers, DenyGroups, and finally
AllowGroups. All of the specified user and group tests must succeed, before user is allowed
to log in.
See PATTERNS in ssh_config(5) for more information on patterns.
AuthenticationMethods
Specifies the authentication methods that must be successfully completed for a user to be
granted access. This option must be followed by one or more comma-separated lists of
authentication method names, or by the single string any to indicate the default behaviour
of accepting any single authentication method. If the default is overridden, then success‐
ful authentication requires completion of every method in at least one of these lists.
For example, "publickey,password publickey,keyboard-interactive" would require the user to
complete public key authentication, followed by either password or keyboard interactive
authentication. Only methods that are next in one or more lists are offered at each stage,
so for this example it would not be possible to attempt password or keyboard-interactive
authentication before public key.
For keyboard interactive authentication it is also possible to restrict authentication to a
specific device by appending a colon followed by the device identifier bsdauth, pam, or
skey, depending on the server configuration. For example, "keyboard-interactive:bsdauth"
would restrict keyboard interactive authentication to the bsdauth device.
If the publickey method is listed more than once, sshd(8) verifies that keys that have been
used successfully are not reused for subsequent authentications. For example,
"publickey,publickey" requires successful authentication using two different public keys.
Note that each authentication method listed should also be explicitly enabled in the config‐
uration.
AuthorizedKeysCommand
Specifies a program to be used to look up the user's public keys. The program must be owned
by root, not writable by group or others and specified by an absolute path. Arguments to
AuthorizedKeysCommand accept the tokens described in the TOKENS section. If no arguments
are specified then the username of the target user is used.
The program should produce on standard output zero or more lines of authorized_keys output
(see AUTHORIZED_KEYS in sshd(8)). If a key supplied by AuthorizedKeysCommand does not suc‐
cessfully authenticate and authorize the user then public key authentication continues using
the usual AuthorizedKeysFile files. By default, no AuthorizedKeysCommand is run.
AuthorizedKeysCommandUser
Specifies the user under whose account the AuthorizedKeysCommand is run. It is recommended
to use a dedicated user that has no other role on the host than running authorized keys com‐
mands. If AuthorizedKeysCommand is specified but AuthorizedKeysCommandUser is not, then
sshd(8) will refuse to start.
AuthorizedKeysFile
Specifies the file that contains the public keys used for user authentication. The format
is described in the AUTHORIZED_KEYS FILE FORMAT section of sshd(8). Arguments to
AuthorizedKeysFile accept the tokens described in the TOKENS section. After expansion,
AuthorizedKeysFile is taken to be an absolute path or one relative to the user's home direc‐
tory. Multiple files may be listed, separated by whitespace. Alternately this option may
be set to none to skip checking for user keys in files. The default is
".ssh/authorized_keys .ssh/authorized_keys2".
AuthorizedPrincipalsCommand
Specifies a program to be used to generate the list of allowed certificate principals as per
AuthorizedPrincipalsFile. The program must be owned by root, not writable by group or oth‐
ers and specified by an absolute path. Arguments to AuthorizedPrincipalsCommand accept the
tokens described in the TOKENS section. If no arguments are specified then the username of
the target user is used.
The program should produce on standard output zero or more lines of AuthorizedPrincipalsFile
output. If either AuthorizedPrincipalsCommand or AuthorizedPrincipalsFile is specified,
then certificates offered by the client for authentication must contain a principal that is
listed. By default, no AuthorizedPrincipalsCommand is run.
AuthorizedPrincipalsCommandUser
Specifies the user under whose account the AuthorizedPrincipalsCommand is run. It is recom‐
mended to use a dedicated user that has no other role on the host than running authorized
principals commands. If AuthorizedPrincipalsCommand is specified but
AuthorizedPrincipalsCommandUser is not, then sshd(8) will refuse to start.
AuthorizedPrincipalsFile
Specifies a file that lists principal names that are accepted for certificate authentica‐
tion. When using certificates signed by a key listed in TrustedUserCAKeys, this file lists
names, one of which must appear in the certificate for it to be accepted for authentication.
Names are listed one per line preceded by key options (as described in AUTHORIZED_KEYS FILE
FORMAT in sshd(8)). Empty lines and comments starting with ‘#’ are ignored.
Arguments to AuthorizedPrincipalsFile accept the tokens described in the TOKENS section.
After expansion, AuthorizedPrincipalsFile is taken to be an absolute path or one relative to
the user's home directory. The default is none, i.e. not to use a principals file – in this
case, the username of the user must appear in a certificate's principals list for it to be
accepted.
Note that AuthorizedPrincipalsFile is only used when authentication proceeds using a CA
listed in TrustedUserCAKeys and is not consulted for certification authorities trusted via
~/.ssh/authorized_keys, though the principals= key option offers a similar facility (see
sshd(8) for details).
Banner The contents of the specified file are sent to the remote user before authentication is
allowed. If the argument is none then no banner is displayed. By default, no banner is
displayed.
ChallengeResponseAuthentication
Specifies whether challenge-response authentication is allowed (e.g. via PAM or through
authentication styles supported in login.conf(5)) The default is yes.
ChrootDirectory
Specifies the pathname of a directory to chroot(2) to after authentication. At session
startup sshd(8) checks that all components of the pathname are root-owned directories which
are not writable by any other user or group. After the chroot, sshd(8) changes the working
directory to the user's home directory. Arguments to ChrootDirectory accept the tokens
described in the TOKENS section.
The ChrootDirectory must contain the necessary files and directories to support the user's
session. For an interactive session this requires at least a shell, typically sh(1), and
basic /dev nodes such as null(4), zero(4), stdin(4), stdout(4), stderr(4), and tty(4)
devices. For file transfer sessions using SFTP no additional configuration of the environ‐
ment is necessary if the in-process sftp-server is used, though sessions which use logging
may require /dev/log inside the chroot directory on some operating systems (see
sftp-server(8) for details).
For safety, it is very important that the directory hierarchy be prevented from modification
by other processes on the system (especially those outside the jail). Misconfiguration can
lead to unsafe environments which sshd(8) cannot detect.
The default is none, indicating not to chroot(2).
Ciphers
Specifies the ciphers allowed. Multiple ciphers must be comma-separated. If the specified
value begins with a ‘+’ character, then the specified ciphers will be appended to the
default set instead of replacing them.
The supported ciphers are:
3des-cbc
aes128-cbc
aes192-cbc
aes256-cbc
aes128-ctr
aes192-ctr
aes256-ctr
aes128-gcm@openssh.com
aes256-gcm@openssh.com
arcfour
arcfour128
arcfour256
blowfish-cbc
cast128-cbc
chacha20-poly1305@openssh.com
The default is:
chacha20-poly1305@openssh.com,
aes128-ctr,aes192-ctr,aes256-ctr,
aes128-gcm@openssh.com,aes256-gcm@openssh.com,
aes128-cbc,aes192-cbc,aes256-cbc,
blowfish-cbc,cast128-cbc,3des-cbc
The list of available ciphers may also be obtained using "ssh -Q cipher".
ClientAliveCountMax
Sets the number of client alive messages which may be sent without sshd(8) receiving any
messages back from the client. If this threshold is reached while client alive messages are
being sent, sshd will disconnect the client, terminating the session. It is important to
note that the use of client alive messages is very different from TCPKeepAlive. The client
alive messages are sent through the encrypted channel and therefore will not be spoofable.
The TCP keepalive option enabled by TCPKeepAlive is spoofable. The client alive mechanism
is valuable when the client or server depend on knowing when a connection has become inac‐
tive.
The default value is 3. If ClientAliveInterval is set to 15, and ClientAliveCountMax is
left at the default, unresponsive SSH clients will be disconnected after approximately 45
seconds.
ClientAliveInterval
Sets a timeout interval in seconds after which if no data has been received from the client,
sshd(8) will send a message through the encrypted channel to request a response from the
client. The default is 0, indicating that these messages will not be sent to the client.
Compression
Specifies whether compression is enabled after the user has authenticated successfully. The
argument must be yes, delayed (a legacy synonym for yes) or no. The default is yes.
DenyGroups
This keyword can be followed by a list of group name patterns, separated by spaces. Login
is disallowed for users whose primary group or supplementary group list matches one of the
patterns. Only group names are valid; a numerical group ID is not recognized. By default,
login is allowed for all groups. The allow/deny directives are processed in the following
order: DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups. All of the specified
user and group tests must succeed, before user is allowed to log in.
See PATTERNS in ssh_config(5) for more information on patterns.
DenyUsers
This keyword can be followed by a list of user name patterns, separated by spaces. Login is
disallowed for user names that match one of the patterns. Only user names are valid; a
numerical user ID is not recognized. By default, login is allowed for all users. If the
pattern takes the form USER@HOST then USER and HOST are separately checked, restricting
logins to particular users from particular hosts. HOST criteria may additionally contain
addresses to match in CIDR address/masklen format. The allow/deny directives are processed
in the following order: DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups. All of
the specified user and group tests must succeed, before user is allowed to log in.
See PATTERNS in ssh_config(5) for more information on patterns.
DisableForwarding
Disables all forwarding features, including X11, ssh-agent(1), TCP and StreamLocal. This
option overrides all other forwarding-related options and may simplify restricted configura‐
tions.
ExposeAuthenticationMethods
When using SSH2, this option controls the exposure of the list of successful authentication
methods to PAM during the authentication and to the shell environment via the SSH_USER_AUTH
variable. See the description of this variable for more details. Valid options are: never
(Do not expose successful authentication methods), pam-only (Only expose them to PAM during
authentication, not afterwards), pam-and-env (Expose them to PAM and keep them in the shell
environment). The default is never.
FingerprintHash
Specifies the hash algorithm used when logging key fingerprints. Valid options are: md5 and
sha256. The default is sha256.
ForceCommand
Forces the execution of the command specified by ForceCommand, ignoring any command supplied
by the client and ~/.ssh/rc if present. The command is invoked by using the user's login
shell with the -c option. This applies to shell, command, or subsystem execution. It is
most useful inside a Match block. The command originally supplied by the client is avail‐
able in the SSH_ORIGINAL_COMMAND environment variable. Specifying a command of
internal-sftp will force the use of an in-process SFTP server that requires no support files
when used with ChrootDirectory. The default is none.
GatewayPorts
Specifies whether remote hosts are allowed to connect to ports forwarded for the client. By
default, sshd(8) binds remote port forwardings to the loopback address. This prevents other
remote hosts from connecting to forwarded ports. GatewayPorts can be used to specify that
sshd should allow remote port forwardings to bind to non-loopback addresses, thus allowing
other hosts to connect. The argument may be no to force remote port forwardings to be
available to the local host only, yes to force remote port forwardings to bind to the wild‐
card address, or clientspecified to allow the client to select the address to which the for‐
warding is bound. The default is no.
GSSAPIAuthentication
Specifies whether user authentication based on GSSAPI is allowed. The default is no.
GSSAPICleanupCredentials
Specifies whether to automatically destroy the user's credentials cache on logout. The
default is yes.
GSSAPIKeyExchange
Specifies whether key exchange based on GSSAPI is allowed. GSSAPI key exchange doesn't rely
on ssh keys to verify host identity. The default is “no”.
GSSAPIEnablek5users
Specifies whether to look at .k5users file for GSSAPI authentication access control. Further
details are described in ksu(1). The default is no.
GSSAPIStrictAcceptorCheck
Determines whether to be strict about the identity of the GSSAPI acceptor a client authenti‐
cates against. If set to yes then the client must authenticate against the host service on
the current hostname. If set to no then the client may authenticate against any service key
stored in the machine's default store. This facility is provided to assist with operation
on multi homed machines. The default is yes.
GSSAPIStoreCredentialsOnRekey
Controls whether the user's GSSAPI credentials should be updated following a successful con‐
nection rekeying. This option can be used to accepted renewed or updated credentials from a
compatible client. The default is “no”.
GSSAPIKexAlgorithms
The list of key exchange algorithms that are accepted by GSSAPI key exchange. Possible val‐
ues are
gss-gex-sha1-,
gss-group1-sha1-,
gss-group14-sha1-
The default is “gss-gex-sha1-,gss-group1-sha1-,gss-group14-sha1-”. This option only applies
to protocol version 2 connections using GSSAPI.
HostbasedAcceptedKeyTypes
Specifies the key types that will be accepted for hostbased authentication as a comma-sepa‐
rated pattern list. Alternately if the specified value begins with a ‘+’ character, then
the specified key types will be appended to the default set instead of replacing them. The
default for this option is:
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
ssh-ed25519-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ssh-dss-cert-v01@openssh.com,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
ssh-ed25519,ssh-rsa,ssh-dss
The list of available key types may also be obtained using "ssh -Q key".
HostbasedAuthentication
Specifies whether rhosts or /etc/hosts.equiv authentication together with successful public
key client host authentication is allowed (host-based authentication). The default is no.
HostbasedUsesNameFromPacketOnly
Specifies whether or not the server will attempt to perform a reverse name lookup when
matching the name in the ~/.shosts, ~/.rhosts, and /etc/hosts.equiv files during
HostbasedAuthentication. A setting of yes means that sshd(8) uses the name supplied by the
client rather than attempting to resolve the name from the TCP connection itself. The
default is no.
HostCertificate
Specifies a file containing a public host certificate. The certificate's public key must
match a private host key already specified by HostKey. The default behaviour of sshd(8) is
not to load any certificates.
HostKey
Specifies a file containing a private host key used by SSH. The defaults are
/etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_ecdsa_key, /etc/ssh/ssh_host_ed25519_key and
/etc/ssh/ssh_host_rsa_key.
Note that sshd(8) will refuse to use a file if it is group/world-accessible and that the
HostKeyAlgorithms option restricts which of the keys are actually used by sshd(8).
It is possible to have multiple host key files. It is also possible to specify public host
key files instead. In this case operations on the private key will be delegated to an
ssh-agent(1).
HostKeyAgent
Identifies the UNIX-domain socket used to communicate with an agent that has access to the
private host keys. If the string "SSH_AUTH_SOCK" is specified, the location of the socket
will be read from the SSH_AUTH_SOCK environment variable.
HostKeyAlgorithms
Specifies the host key algorithms that the server offers. The default for this option is:
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
ssh-ed25519-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ssh-dss-cert-v01@openssh.com,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
ssh-ed25519,ssh-rsa,ssh-dss
The list of available key types may also be obtained using "ssh -Q key".
IgnoreRhosts
Specifies that .rhosts and .shosts files will not be used in HostbasedAuthentication.
/etc/hosts.equiv and /etc/ssh/shosts.equiv are still used. The default is yes.
IgnoreUserKnownHosts
Specifies whether sshd(8) should ignore the user's ~/.ssh/known_hosts during
HostbasedAuthentication. The default is no.
IPQoS Specifies the IPv4 type-of-service or DSCP class for the connection. Accepted values are
af11, af12, af13, af21, af22, af23, af31, af32, af33, af41, af42, af43, cs0, cs1, cs2, cs3,
cs4, cs5, cs6, cs7, ef, lowdelay, throughput, reliability, or a numeric value. This option
may take one or two arguments, separated by whitespace. If one argument is specified, it is
used as the packet class unconditionally. If two values are specified, the first is auto‐
matically selected for interactive sessions and the second for non-interactive sessions.
The default is lowdelay for interactive sessions and throughput for non-interactive ses‐
sions.
KbdInteractiveAuthentication
Specifies whether to allow keyboard-interactive authentication. The argument to this key‐
word must be yes or no. The default is to use whatever value
ChallengeResponseAuthentication is set to (by default yes).
KerberosAuthentication
Specifies whether the password provided by the user for PasswordAuthentication will be vali‐
dated through the Kerberos KDC. To use this option, the server needs a Kerberos servtab
which allows the verification of the KDC's identity. The default is no.
KerberosGetAFSToken
If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire an AFS token before
accessing the user's home directory. The default is no.
KerberosOrLocalPasswd
If password authentication through Kerberos fails then the password will be validated via
any additional local mechanism such as /etc/passwd. The default is yes.
KerberosTicketCleanup
Specifies whether to automatically destroy the user's ticket cache file on logout. The
default is yes.
KerberosUseKuserok
Specifies whether to look at .k5login file for user's aliases. The default is yes.
KexAlgorithms
Specifies the available KEX (Key Exchange) algorithms. Multiple algorithms must be comma-
separated. Alternately if the specified value begins with a ‘+’ character, then the speci‐
fied methods will be appended to the default set instead of replacing them. The supported
algorithms are:
curve25519-sha256
curve25519-sha256@libssh.org
diffie-hellman-group1-sha1
diffie-hellman-group14-sha1
diffie-hellman-group-exchange-sha1
diffie-hellman-group-exchange-sha256
ecdh-sha2-nistp256
ecdh-sha2-nistp384
ecdh-sha2-nistp521
The default is:
curve25519-sha256,curve25519-sha256@libssh.org,
ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
diffie-hellman-group-exchange-sha256,
diffie-hellman-group14-sha1,
diffie-hellman-group1-sha1
The list of available key exchange algorithms may also be obtained using "ssh -Q kex".
ListenAddress
Specifies the local addresses sshd(8) should listen on. The following forms may be used:
ListenAddress host|IPv4_addr|IPv6_addr
ListenAddress host|IPv4_addr:port
ListenAddress [host|IPv6_addr]:port
If port is not specified, sshd will listen on the address and all Port options specified.
The default is to listen on all local addresses. Multiple ListenAddress options are permit‐
ted.
LoginGraceTime
The server disconnects after this time if the user has not successfully logged in. If the
value is 0, there is no time limit. The default is 120 seconds.
LogLevel
Gives the verbosity level that is used when logging messages from sshd(8). The possible
values are: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. The
default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify higher
levels of debugging output. Logging with a DEBUG level violates the privacy of users and is
not recommended.
MACs Specifies the available MAC (message authentication code) algorithms. The MAC algorithm is
used for data integrity protection. Multiple algorithms must be comma-separated. If the
specified value begins with a ‘+’ character, then the specified algorithms will be appended
to the default set instead of replacing them.
The algorithms that contain "-etm" calculate the MAC after encryption (encrypt-then-mac).
These are considered safer and their use recommended. The supported MACs are:
hmac-md5
hmac-md5-96
hmac-ripemd160
hmac-sha1
hmac-sha1-96
hmac-sha2-256
hmac-sha2-512
umac-64@openssh.com
umac-128@openssh.com
hmac-md5-etm@openssh.com
hmac-md5-96-etm@openssh.com
hmac-ripemd160-etm@openssh.com
hmac-sha1-etm@openssh.com
hmac-sha1-96-etm@openssh.com
hmac-sha2-256-etm@openssh.com
hmac-sha2-512-etm@openssh.com
umac-64-etm@openssh.com
umac-128-etm@openssh.com
The default is:
umac-64-etm@openssh.com,umac-128-etm@openssh.com,
hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
hmac-sha1-etm@openssh.com,
umac-64@openssh.com,umac-128@openssh.com,
hmac-sha2-256,hmac-sha2-512,hmac-sha1,
hmac-sha1-etm@openssh.com
The list of available MAC algorithms may also be obtained using "ssh -Q mac".
Match Introduces a conditional block. If all of the criteria on the Match line are satisfied, the
keywords on the following lines override those set in the global section of the config file,
until either another Match line or the end of the file. If a keyword appears in multiple
Match blocks that are satisfied, only the first instance of the keyword is applied.
The arguments to Match are one or more criteria-pattern pairs or the single token All which
matches all criteria. The available criteria are User, Group, Host, LocalAddress,
LocalPort, and Address. The match patterns may consist of single entries or comma-separated
lists and may use the wildcard and negation operators described in the PATTERNS section of
ssh_config(5).
The patterns in an Address criteria may additionally contain addresses to match in CIDR
address/masklen format, such as 192.0.2.0/24 or 2001:db8::/32. Note that the mask length
provided must be consistent with the address - it is an error to specify a mask length that
is too long for the address or one with bits set in this host portion of the address. For
example, 192.0.2.0/33 and 192.0.2.0/8, respectively.
Only a subset of keywords may be used on the lines following a Match keyword. Available
keywords are AcceptEnv, AllowAgentForwarding, AllowGroups, AllowStreamLocalForwarding,
AllowTcpForwarding, AllowUsers, AuthenticationMethods, AuthorizedKeysCommand,
AuthorizedKeysCommandUser, AuthorizedKeysFile, AuthorizedPrincipalsCommand,
AuthorizedPrincipalsCommandUser, AuthorizedPrincipalsFile, Banner, ChrootDirectory,
ClientAliveCountMax, ClientAliveInterval, DenyGroups, DenyUsers, ForceCommand, GatewayPorts,
GSSAPIAuthentication, HostbasedAcceptedKeyTypes, HostbasedAuthentication,
HostbasedUsesNameFromPacketOnly, IPQoS, KbdInteractiveAuthentication,
KerberosAuthentication, KerberosUseKuserok, MaxAuthTries, MaxSessions,
PasswordAuthentication, PermitEmptyPasswords, PermitOpen, PermitRootLogin, PermitTTY,
PermitTunnel, PermitUserRC, PubkeyAcceptedKeyTypes, PubkeyAuthentication, RekeyLimit,
RevokedKeys, StreamLocalBindMask, StreamLocalBindUnlink, TrustedUserCAKeys,
X11DisplayOffset, X11MaxDisplays, X11Forwarding and X11UseLocalHost.
MaxAuthTries
Specifies the maximum number of authentication attempts permitted per connection. Once the
number of failures reaches half this value, additional failures are logged. The default is
6.
MaxSessions
Specifies the maximum number of open shell, login or subsystem (e.g. sftp) sessions permit‐
ted per network connection. Multiple sessions may be established by clients that support
connection multiplexing. Setting MaxSessions to 1 will effectively disable session multi‐
plexing, whereas setting it to 0 will prevent all shell, login and subsystem sessions while
still permitting forwarding. The default is 10.
MaxStartups
Specifies the maximum number of concurrent unauthenticated connections to the SSH daemon.
Additional connections will be dropped until authentication succeeds or the LoginGraceTime
expires for a connection. The default is 10:30:100.
Alternatively, random early drop can be enabled by specifying the three colon separated val‐
ues start:rate:full (e.g. "10:30:60"). sshd(8) will refuse connection attempts with a prob‐
ability of rate/100 (30%) if there are currently start (10) unauthenticated connections.
The probability increases linearly and all connection attempts are refused if the number of
unauthenticated connections reaches full (60).
PasswordAuthentication
Specifies whether password authentication is allowed. The default is yes.
PermitEmptyPasswords
When password authentication is allowed, it specifies whether the server allows login to
accounts with empty password strings. The default is no.
PermitOpen
Specifies the destinations to which TCP port forwarding is permitted. The forwarding speci‐
fication must be one of the following forms:
PermitOpen host:port
PermitOpen IPv4_addr:port
PermitOpen [IPv6_addr]:port
Multiple forwards may be specified by separating them with whitespace. An argument of any
can be used to remove all restrictions and permit any forwarding requests. An argument of
none can be used to prohibit all forwarding requests. The wildcard ‘*’ can be used for host
or port to allow all hosts or ports, respectively. By default all port forwarding requests
are permitted.
PermitRootLogin
Specifies whether root can log in using ssh(1). The argument must be yes,
prohibit-password, without-password, forced-commands-only, or no. The default is yes.
If this option is set to prohibit-password or without-password, password and keyboard-inter‐
active authentication are disabled for root.
If this option is set to forced-commands-only, root login with public key authentication
will be allowed, but only if the command option has been specified (which may be useful for
taking remote backups even if root login is normally not allowed). All other authentication
methods are disabled for root.
If this option is set to no, root is not allowed to log in.
PermitTTY
Specifies whether pty(4) allocation is permitted. The default is yes.
PermitTunnel
Specifies whether tun(4) device forwarding is allowed. The argument must be yes,
point-to-point (layer 3), ethernet (layer 2), or no. Specifying yes permits both
point-to-point and ethernet. The default is no.
Independent of this setting, the permissions of the selected tun(4) device must allow access
to the user.
PermitUserEnvironment
Specifies whether ~/.ssh/environment and environment= options in ~/.ssh/authorized_keys are
processed by sshd(8). The default is no. Enabling environment processing may enable users
to bypass access restrictions in some configurations using mechanisms such as LD_PRELOAD.
PermitUserRC
Specifies whether any ~/.ssh/rc file is executed. The default is yes.
PidFile
Specifies the file that contains the process ID of the SSH daemon, or none to not write one.
The default is /var/run/sshd.pid.
Port Specifies the port number that sshd(8) listens on. The default is 22. Multiple options of
this type are permitted. See also ListenAddress.
PrintLastLog
Specifies whether sshd(8) should print the date and time of the last user login when a user
logs in interactively. The default is yes.
PrintMotd
Specifies whether sshd(8) should print /etc/motd when a user logs in interactively. (On
some systems it is also printed by the shell, /etc/profile, or equivalent.) The default is
yes.
PubkeyAcceptedKeyTypes
Specifies the key types that will be accepted for public key authentication as a comma-sepa‐
rated pattern list. Alternately if the specified value begins with a ‘+’ character, then
the specified key types will be appended to the default set instead of replacing them. The
default for this option is:
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
ssh-ed25519-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ssh-dss-cert-v01@openssh.com,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
ssh-ed25519,ssh-rsa,ssh-dss
The list of available key types may also be obtained using "ssh -Q key".
PubkeyAuthentication
Specifies whether public key authentication is allowed. The default is yes.
RekeyLimit
Specifies the maximum amount of data that may be transmitted before the session key is rene‐
gotiated, optionally followed a maximum amount of time that may pass before the session key
is renegotiated. The first argument is specified in bytes and may have a suffix of ‘K’,
‘M’, or ‘G’ to indicate Kilobytes, Megabytes, or Gigabytes, respectively. The default is
between ‘1G’ and ‘4G’, depending on the cipher. The optional second value is specified in
seconds and may use any of the units documented in the TIME FORMATS section. The default
value for RekeyLimit is default none, which means that rekeying is performed after the
cipher's default amount of data has been sent or received and no time based rekeying is
done.
RevokedKeys
Specifies revoked public keys file, or none to not use one. Keys listed in this file will
be refused for public key authentication. Note that if this file is not readable, then pub‐
lic key authentication will be refused for all users. Keys may be specified as a text file,
listing one public key per line, or as an OpenSSH Key Revocation List (KRL) as generated by
ssh-keygen(1). For more information on KRLs, see the KEY REVOCATION LISTS section in
ssh-keygen(1).
ShowPatchLevel
Specifies whether sshd will display the patch level of the binary in the identification
string. The patch level is set at compile-time. The default is “no”. This option applies
to protocol version 1 only.
StreamLocalBindMask
Sets the octal file creation mode mask (umask) used when creating a Unix-domain socket file
for local or remote port forwarding. This option is only used for port forwarding to a
Unix-domain socket file.
The default value is 0177, which creates a Unix-domain socket file that is readable and
writable only by the owner. Note that not all operating systems honor the file mode on
Unix-domain socket files.
StreamLocalBindUnlink
Specifies whether to remove an existing Unix-domain socket file for local or remote port
forwarding before creating a new one. If the socket file already exists and
StreamLocalBindUnlink is not enabled, sshd will be unable to forward the port to the Unix-
domain socket file. This option is only used for port forwarding to a Unix-domain socket
file.
The argument must be yes or no. The default is no.
StrictModes
Specifies whether sshd(8) should check file modes and ownership of the user's files and home
directory before accepting login. This is normally desirable because novices sometimes
accidentally leave their directory or files world-writable. The default is yes. Note that
this does not apply to ChrootDirectory, whose permissions and ownership are checked uncondi‐
tionally.
Subsystem
Configures an external subsystem (e.g. file transfer daemon). Arguments should be a subsys‐
tem name and a command (with optional arguments) to execute upon subsystem request.
The command sftp-server implements the SFTP file transfer subsystem.
Alternately the name internal-sftp implements an in-process SFTP server. This may simplify
configurations using ChrootDirectory to force a different filesystem root on clients.
By default no subsystems are defined.
SyslogFacility
Gives the facility code that is used when logging messages from sshd(8). The possible val‐
ues are: DAEMON, USER, AUTH, AUTHPRIV, LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5,
LOCAL6, LOCAL7. The default is AUTH.
TCPKeepAlive
Specifies whether the system should send TCP keepalive messages to the other side. If they
are sent, death of the connection or crash of one of the machines will be properly noticed.
However, this means that connections will die if the route is down temporarily, and some
people find it annoying. On the other hand, if TCP keepalives are not sent, sessions may
hang indefinitely on the server, leaving "ghost" users and consuming server resources.
The default is yes (to send TCP keepalive messages), and the server will notice if the net‐
work goes down or the client host crashes. This avoids infinitely hanging sessions.
To disable TCP keepalive messages, the value should be set to no.
TrustedUserCAKeys
Specifies a file containing public keys of certificate authorities that are trusted to sign
user certificates for authentication, or none to not use one. Keys are listed one per line;
empty lines and comments starting with ‘#’ are allowed. If a certificate is presented for
authentication and has its signing CA key listed in this file, then it may be used for
authentication for any user listed in the certificate's principals list. Note that certifi‐
cates that lack a list of principals will not be permitted for authentication using
TrustedUserCAKeys. For more details on certificates, see the CERTIFICATES section in
ssh-keygen(1).
UseDNS Specifies whether sshd(8) should look up the remote host name, and to check that the
resolved host name for the remote IP address maps back to the very same IP address. The
default is “yes”.
If this option is set to no then only addresses and not host names may be used in
~/.ssh/authorized_keys from and sshd_config Match Host directives.
UsePAM Enables the Pluggable Authentication Module interface. If set to yes this will enable PAM
authentication using ChallengeResponseAuthentication and PasswordAuthentication in addition
to PAM account and session module processing for all authentication types.
Because PAM challenge-response authentication usually serves an equivalent role to password
authentication, you should disable either PasswordAuthentication or
ChallengeResponseAuthentication.
If UsePAM is enabled, you will not be able to run sshd(8) as a non-root user. The default
is no.
UsePrivilegeSeparation
Specifies whether sshd(8) separates privileges by creating an unprivileged child process to
deal with incoming network traffic. After successful authentication, another process will
be created that has the privilege of the authenticated user. The goal of privilege separa‐
tion is to prevent privilege escalation by containing any corruption within the unprivileged
processes. The argument must be yes, no, or sandbox. If UsePrivilegeSeparation is set to
sandbox then the pre-authentication unprivileged process is subject to additional restric‐
tions. The default is sandbox.
VersionAddendum
Optionally specifies additional text to append to the SSH protocol banner sent by the server
upon connection. The default is none.
X11DisplayOffset
Specifies the first display number available for sshd(8)'s X11 forwarding. This prevents
sshd from interfering with real X11 servers. The default is 10.
X11MaxDisplays
Specifies the maximum number of displays available for sshd(8)'s X11 forwarding. This pre‐
vents sshd from exhausting local ports. The default is 1000.
X11Forwarding
Specifies whether X11 forwarding is permitted. The argument must be yes or no. The default
is no.
When X11 forwarding is enabled, there may be additional exposure to the server and to client
displays if the sshd(8) proxy display is configured to listen on the wildcard address (see
X11UseLocalhost), though this is not the default. Additionally, the authentication spoofing
and authentication data verification and substitution occur on the client side. The secu‐
rity risk of using X11 forwarding is that the client's X11 display server may be exposed to
attack when the SSH client requests forwarding (see the warnings for ForwardX11 in
ssh_config(5)). A system administrator may have a stance in which they want to protect
clients that may expose themselves to attack by unwittingly requesting X11 forwarding, which
can warrant a no setting.
Note that disabling X11 forwarding does not prevent users from forwarding X11 traffic, as
users can always install their own forwarders.
X11UseLocalhost
Specifies whether sshd(8) should bind the X11 forwarding server to the loopback address or
to the wildcard address. By default, sshd binds the forwarding server to the loopback
address and sets the hostname part of the DISPLAY environment variable to localhost. This
prevents remote hosts from connecting to the proxy display. However, some older X11 clients
may not function with this configuration. X11UseLocalhost may be set to no to specify that
the forwarding server should be bound to the wildcard address. The argument must be yes or
no. The default is yes.
XAuthLocation
Specifies the full pathname of the xauth(1) program, or none to not use one. The default is
/usr/bin/xauth.
TIME FORMATS
sshd(8) command-line arguments and configuration file options that specify time may be expressed
using a sequence of the form: time[qualifier], where time is a positive integer value and qualifier
is one of the following:
⟨none⟩ seconds
s | S seconds
m | M minutes
h | H hours
d | D days
w | W weeks
Each member of the sequence is added together to calculate the total time value.
Time format examples:
600 600 seconds (10 minutes)
10m 10 minutes
1h30m 1 hour 30 minutes (90 minutes)
TOKENS
Arguments to some keywords can make use of tokens, which are expanded at runtime:
%% A literal ‘%’.
%F The fingerprint of the CA key.
%f The fingerprint of the key or certificate.
%h The home directory of the user.
%i The key ID in the certificate.
%K The base64-encoded CA key.
%k The base64-encoded key or certificate for authentication.
%s The serial number of the certificate.
%T The type of the CA key.
%t The key or certificate type.
%u The username.
AuthorizedKeysCommand accepts the tokens %%, %f, %h, %t, and %u.
AuthorizedKeysFile accepts the tokens %%, %h, and %u.
AuthorizedPrincipalsCommand accepts the tokens %%, %F, %f, %K, %k, %h, %i, %s, %T, %t, and %u.
AuthorizedPrincipalsFile accepts the tokens %%, %h, and %u.
ChrootDirectory accepts the tokens %%, %h, and %u.
FILES
/etc/ssh/sshd_config
Contains configuration data for sshd(8). This file should be writable by root only, but it
is recommended (though not necessary) that it be world-readable.
SEE ALSO
sftp-server(8), sshd(8)
AUTHORS
OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu Ylonen. Aaron Campbell,
Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt and Dug Song removed many bugs, re-added newer
features and created OpenSSH. Markus Friedl contributed the support for SSH protocol versions 1.5
and 2.0. Niels Provos and Markus Friedl contributed support for privilege separation.
BSD May 9, 2022 BSD
문제 유형
① sshd_config 파일의 내용 일부가 주어지고, 조건에 제시된 내용을 적는 문제
- man sshd_config 명령을 사용하여 메뉴얼을 확인해가면서 문제를 해결한다.
TCPKeepAlive yes
( PrintLastLog ) yes
X11Forwarding yes# vi ( /etc/ssh/sshd_config )
( PermitRootLogin no )728x90
그리드형(광고전용)
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