OpenSSH Config File Usage Introduction

OpenSSH是使用SSH協議進行遠程連接的工具，當前最新版本是OpenSSH 7.7，於 April 03, 2018 釋出。

OpenSSH可分爲 client 和 server 端。其中 client 的配置文件有２個，一個是全局配置文件/etc/ssh/ssh_config，另一個是用戶配置文件~/.ssh/config，默認後者中的設置會覆蓋前者(全局配置文件)。通過配置~/.ssh/config，可實現SSH的自定義操作。

本文主要介紹~/.ssh/config的配置、使用，實驗主機爲Digital Ocean的VPS主機。

Preparation

在Digital Ocean中創建2台VPS，系統選用Debian Stretch 9.5，一台作爲跳板機，一台作爲內網主機，只能通過跳板機訪問，通過SSH keygen進行認證連接。

VPS信息如下

Hostname	Intern IP	Public IP	Port
front	`10.128.29.79`	`162.243.63.4`	22
secret	`10.128.24.198`	`162.243.86.165`	22

在主機front中創建ssh genkey，key的類型選擇ed25519，將公鑰中的內容添加到主機secret的~/.ssh/authorized_key中。此處假設已經進行防火牆配置，只允許通過主機front訪問主機secret。

爲方便進行實驗，將在主機front生成的密鑰對保存到本地主機/tmp目錄下。

測試過程如下

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[email protected]:~$ ssh front
Last login: Thur Jul  26 22:05:54 2018 from 116.235.185.156
[email protected]:~# ssh [email protected]
Last login: Thur Jul  26 22:13:19 2018 from 10.128.29.79
[email protected]:~# exit
logout

Connection to 10.128.24.198 closed.
[email protected]:~#

Introduction

ssh_config的說明可通過如下命令查看

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man ssh_config

也可參閱文檔 SSH Config File for OpenSSH Client

通過~/.ssh/config配置的主機，可以通過ssh、sftp等命令進行連接。

Parameters List

詳細配置參數清單如下

parameter	detail
Host
Match
AddKeysToAgent
AddressFamily
BatchMode
BindAddress
CanonicalDomains
CanonicalizeFallbackLocal
CanonicalizeHostname
CanonicalizeMaxDots
CanonicalizePermittedCNAMEs
CertificateFile
ChallengeResponseAuthentication
CheckHostIP
Cipher
Ciphers
ClearAllForwardings
Compression
CompressionLevel
ConnectionAttempts
ConnectTimeout
ControlMaster
ControlPath
ControlPersist
DynamicForward
EnableSSHKeysign
EscapeChar
ExitOnForwardFailure
FingerprintHash
ForwardAgent
ForwardX11
ForwardX11Timeout
ForwardX11Trusted
GatewayPorts
GlobalKnownHostsFile
GSSAPIAuthentication
GSSAPIKeyExchange
GSSAPIClientIdentity
GSSAPIServerIdentity
GSSAPIDelegateCredentials
GSSAPIRenewalForcesRekey
GSSAPITrustDns
HashKnownHosts
HostbasedAuthentication
HostbasedKeyTypes
HostKeyAlgorithms
HostKeyAlias
HostName
IdentitiesOnly
IdentityAgent
IdentityFile
IgnoreUnknown
Include
IPQoS
KbdInteractiveAuthentication
KbdInteractiveDevices
KexAlgorithms
LocalCommand
LocalForward
LogLevel
MACs
NoHostAuthenticationForLocalhost
NumberOfPasswordPrompts
PasswordAuthentication
PermitLocalCommand
PKCS11Provider
Port
PreferredAuthentications
Protocol
ProxyCommand
ProxyJump
ProxyUseFdpass
PubkeyAcceptedKeyTypes
PubkeyAuthentication
RekeyLimit
RemoteForward
RequestTTY
RevokedHostKeys
RhostsRSAAuthentication
RSAAuthentication
SendEnv
ServerAliveCountMax
ServerAliveInterval
StreamLocalBindMask
StreamLocalBindUnlink
StrictHostKeyChecking
TCPKeepAlive
Tunnel
TunnelDevice
UpdateHostKeys
UsePrivilegedPort
User
UserKnownHostsFile
VerifyHostKeyDNS
VisualHostKey
XAuthLocation

Format

在文件~/.ssh/config中配置的格式如下

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Host front
	HostName 162.243.63.4
	IdentityFile ~/.ssh/id_ed25519
	Port 22
	User root

Host ...
    ...
    ...
    ...

Host *
    Protocol 2
    StrictHostKeyChecking no
	HashKnownHosts yes
	UserKnownHostsFile /dev/null

以Host開頭，配額選項以key val形式設置，縮進1個Tab(或4個空格)。

Host *表示通用配置，適用於~/.ssh/config所配置的主機。Host front則表示針對主機front進行配置，如果通用配置和特定主機中的配置選項重複，則自動忽略通用配置中的選項。

使用SSH登錄遠程主機，需要提供遠程主機的IP、端口號、用戶名、密碼或key。則ssh_config中對應的參數選項如下

item	para
IP	`HostName`
端口號	`Port`
用戶名	`User`
key	`IdentityFile`

在最簡配置中，只要指定這4個選項即可。

如主機front的配置

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Host front
	HostName 162.243.63.4
	Port 22
	User root
	IdentityFile ~/.ssh/id_ed25519

處於安全等考慮，還需進行其他參數的配置。

Common Configuration

其中某些參數的配置，可參考Mozilla的Security/Guidelines/OpenSSH。

如果想通過socket進行通信，可使用指令ControlMaster、ControlPath、ControlPersist，創建目錄~/.ssh/sockets/存放生成的socket文件。

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ControlMaster auto
ControlPath ~/.ssh/sockets/%[email protected]%h:%p
#ControlPath ~/.ssh/sockets/%[email protected]%h
ControlPersist 300

通用配置如下

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Host *
	Protocol 2
	Port 22
	User root
	StrictHostKeyChecking no
	VerifyHostKeyDNS no
	AddressFamily inet
	HashKnownHosts yes
	UserKnownHostsFile /dev/null
	LogLevel QUIET
	IdentityFile ~/.ssh/id_ed25519
	ServerAliveCountMax 5
	ServerAliveInterval 120
	ControlMaster auto
	ControlPath ~/.ssh/sockets/%[email protected]%h:%p
	#ControlPath ~/.ssh/sockets/%[email protected]%h
	ControlPersist 300
   	#PreferredAuthentications gssapi-with-mic,hostbased,publickey,keyboard-interactive,password
   	PreferredAuthentications publickey,keyboard-interactive,password
	HostKeyAlgorithms [email protected],[email protected],ssh-ed25519,ssh-rsa,[email protected],[email protected],[email protected],ecdsa-sha2-nistp521,ecdsa-sha2-nistp384,ecdsa-sha2-nistp256
	KexAlgorithms [email protected],ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256,diffie-hellman-group-exchange-sha256
	MACs [email protected],[email protected],[email protected],hmac-sha2-512,hmac-sha2-256,[email protected]
	Compression yes
	#Cipher aes256-ctr
	Cipher [email protected]
	Ciphers [email protected],[email protected],[email protected],aes256-ctr,aes192-ctr,aes128-ctr

Visit Intern Host Via Front

配置通過主機front訪問內網主機secret(10.128.24.198)，通過參數ProxyCommand實現

In the command string, any occurrence of %h will be substituted by the host name to connect, %p by the port, and %r by the remote user name.

在ProxyCommand中，%h指代要連接的主機名，%p指代端口號，%r指代用戶名，SSH會自動進行替換。

Method 1

方式1通過ssh front -W %h:%p %r實現

注意

ssh後的 front 是跳板主機的主機名稱，即Host front中的front；
主機secret指定的key是主機front連接secret時使用的key；

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Host front
	HostName 162.243.63.4
	IdentityFile ~/.ssh/id_ed25519

Host secret
	HostName 162.243.86.165
	IdentityFile /tmp/id_ed25519
    ProxyCommand ssh front -W %h:%p %r

演示過程如下

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[email protected]:~$ ssh secret

Last login: Thur Jul  26 22:16:03 2018 from 10.128.29.79
[email protected]:~# exit
logout

[email protected]:~$

Method 2 nc

方式2通過ssh front nc %h %p %r實現，但是有前提，命令中的nc必須已經安裝在主機front中，否則會報錯，無法正常連接。

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Host front
	HostName 162.243.63.4
	IdentityFile ~/.ssh/id_ed25519

Host secret
	HostName 162.243.86.165
	IdentityFile /tmp/id_ed25519
    ProxyCommand ssh front nc %h %p %r

Method 3 ProxyJump

OpenSSH 7.3中添加了ProxyJump選項，通過該選項可實現與ProxyCommand相同的功能，但設置更爲簡單。

注意：要想使用ProxyJump，須確保OpenSSH的版本至少爲7.3。

具體實例可參見What is new in OpenSSH 7.4 (in RHEL 7.4)?中的示例。

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Host front
	HostName 162.243.63.4
	IdentityFile ~/.ssh/id_ed25519

Host secret
	HostName 162.243.86.165
	IdentityFile /tmp/id_ed25519
	ProxyJump front

Port Forward

端口轉發分爲LocalForward、RemoteForward、DynamicForward這3種。

在主機front中進行操作

LocalForward

在主機front安裝Nginx，在瀏覽器中輸入外網IP162.243.63.4即能訪問Web內容，默認爲80端口。通過本地端口轉發，將主機front的 80 端口轉發到本機的 9999 端口，實現通過訪問本地127.0.0.1:9999獲取主機front中的Web頁面，即80端口顯示的內容。

配置如下內容

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Host front
	HostName 162.243.63.4
	IdentityFile ~/.ssh/id_ed25519
	LocalForward 9999 127.0.0.1:80

執行

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ssh -fNg front

使用ss -tnl可查看到本機已經在監聽9999端口。

注意：僅指定-fNg即可，不要添加L、R、D。如果設置爲-fNgL，會出現報錯

Bad local forwarding specification

提取Head信息比對

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# 162.243.63.4
[email protected]:~$ curl -I 162.243.63.4
HTTP/1.1 200 OK
Server: nginx/1.15.2
Date: Thur, 26 Jul 2018 15:12:04 GMT
Content-Type: text/html
Content-Length: 867
Last-Modified: Thur, 26 Jul 2018 15:06:10 GMT
Connection: keep-alive
ETag: "58c01de2-363"
Accept-Ranges: bytes

# 127.0.0.1:9999
[email protected]:~$ curl -I 127.0.0.1:9999
HTTP/1.1 200 OK
Server: nginx/1.15.2
Date: Thur, 26 Jul 2018 15:12:14 GMT
Content-Type: text/html
Content-Length: 867
Last-Modified: Thur, 26 Jul 2018 15:06:10 GMT
Connection: keep-alive
ETag: "58c01de2-363"
Accept-Ranges: bytes

[email protected]:~$

RemoteForward

本機安裝有Nginx，在瀏覽器中輸入127.0.0.1即能訪問Web內容，默認爲80端口。通過遠程端口轉發，將本機的 80 端口轉發到遠程主機front的 7777 端口。實現在主機front中通過訪問本地127.0.0.1:7777獲取本機中的Web頁面，即80端口顯示的內容。

注意：使用主機front進行端口轉發，須在主機front的/etc/ssh/sshd_config中將選項GatewayPorts設置爲yes

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GatewayPorts yes

然後重啓sshd服務

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systemctl restart ssh

配置如下內容

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Host front
	HostName 162.243.63.4
	IdentityFile ~/.ssh/id_ed25519
	RemoteForward 7777 127.0.0.1:80

注意：RemoteForward的第一個參數爲遠程主機front要啓用的端口號，第二個參數127.0.0.1:80爲本機的端口信息。

執行

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ssh -fNg front

可在主機front中看到已經監聽端口7777。

提取Head信息比對

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# local host
[email protected]:~$ curl -I 127.0.0.1:80
HTTP/1.1 200 OK
Server: nginx/1.15.2
Date: Thur, 26 Jul 2018 15:49:06 GMT
Content-Type: text/html
Content-Length: 640
Last-Modified: Tue, 07 Mar 2017 00:32:08 GMT
Connection: keep-alive
ETag: "58bdff88-280"
Accept-Ranges: bytes

[email protected]:~$


# front host
[email protected]:~# curl -I 127.0.0.1:7777
HTTP/1.1 200 OK
Server: nginx/1.15.2
Date: Thur, 26 Jul 2018 15:47:56 GMT
Content-Type: text/html
Content-Length: 640
Last-Modified: Tue, 07 Mar 2017 00:32:08 GMT
Connection: keep-alive
ETag: "58bdff88-280"
Accept-Ranges: bytes

[email protected]:~#

DynamicForward

動態端口轉發，創建SOCKS，在本機監聽6666端口

配置如下內容

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Host front
	HostName 162.243.63.4
	IdentityFile ~/.ssh/id_ed25519
	DynamicForward 127.0.0.1:6666

執行

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ssh -fNg front

使用ss -tnl可查看到本機已經在監聽6666端口。

X Forwarding

X is a popular window system for Unix workstations, and one of its best features is its transparency. Using X, you can run remote X applications that open their windows on your local display (and vice versa, running local applications on remote displays). Unfortunately, the inter-machine communication is insecure and wide open to snoopers. But there’s good news: SSH X forwarding makes the communication secure by tunneling the X protocol. – https://docstore.mik.ua/orelly/networking_2ndEd/ssh/ch09_03.htm

X是一種通信協議，通過該協議，可將遠程主機中的應用在本地窗口中打開，通信默認是不安全的，通過 SSH X Forwarding 可確保通信安全。

因X協議在認證請求連接的客戶端時，需要對客戶端進行身份認證，方式可分爲兩種Host-based X和Key-based X。後者通過程序xauth維護X的認證key，通常保存在文件~/.Xauthority中。

Step 1 xauth

故首先須確保在本地和遠程主機中安裝了xauth。

Step 2 sshd_config

然後在配置文件/etc/ssh/sshd_config中進行如下配置

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X11Forwarding yes
X11DisplayOffset 10
X11UseLocalhost yes

修改完成後重啓sshd服務。

如果X11UseLocalhost no爲no，則會出現如下報錯

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/usr/bin/xauth: (stdin):1:  bad display name "Malachite:10.0" in "remove" command
/usr/bin/xauth: (stdin):2:  bad display name "Malachite:10.0" in "add" command

其中Malachite:10.0的含義，可閱讀如下說明：

A central concept of X is the display, an abstraction for the screen managed by an X server. When an X client is invoked, it needs to know which display to use. Displays are named by strings of the form HOST:n.v, where:

HOST is the name of the machine running the X server controlling the display.
n is the display number, an integer, usually 0. X allows for multiple displays controlled by a single server; additional displays are numbered 1, 2, and so on.
v is the visual number, another integer. A visual is a virtual display. X supports multiple virtual displays on a single, physical display. If there’s only one virtual display (which is the most common scenario), you omit the “.v”, and the default is visual 0. – https://docstore.mik.ua/orelly/networking_2ndEd/ssh/ch09_03.htm

Step 3 DISPLAY

如果不使用X轉發，但想使使用遠程主機(通過SSH登錄)中的X，則必須手動設置變量DISPLAY，說明如下

SSH sets the DISPLAY variable automatically only if X forwarding is in effect. If you don’t use X forwarding but want to use X on a remote machine you logged into via SSH, remember that you have to set the DISPLAY variable yourself. You should only do this when the both machines are on the same, trusted network, as the X protocol by itself is quite insecure. – https://docstore.mik.ua/orelly/networking_2ndEd/ssh/ch09_03.htm

在本機(X client)中進行如下設置

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export DISPLAY=:0.0

也可将其写入文件~/.bashrc中。

Step 4 ssh -Y

操作完成後即可通過ssh -X或ssh -Y登錄遠程主機，啓用含有圖形化界面的程序，即可看到效果(在本地跳出新窗口，內容爲遠程主機中的圖形化程序)。

-X Enables X11 forwarding -Y Enables trusted X11 forwarding.

如果按照以上操作执行后仍报错，可退出当前用户登录，重新登入后尝试。

The Onion Router

The Onion Router簡稱Tor，是一個匿名網絡。可通過LocalForward進行端口轉發，在本機連入Tor網絡，實現網路匿名訪問。

在VPS中安裝tor，服務啓用後，監聽 9050 端口。

配置如下

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Host tor
	HostName 162.243.63.4
	IdentityFile ~/.ssh/id_ed25519
	LocalForward 3333 127.0.0.1:9050

執行

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ssh -fNg tor

操作成功後，會在本機啓用 3333 端口，通過 127.0.0.1:3333 可連入Tor網絡。

如果需要獲取本機的真實外網IP，可通過如下命令查看

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dig +short myip.opendns.com @resolver1.opendns.com

該命令參考於 Command for determining my public IP?。

~/.ssh/config

完整配置文件內容如下

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Host front
	HostName 162.243.63.4
	IdentityFile ~/.ssh/id_ed25519
	# LocalForward 9999 127.0.0.1:80
	# RemoteForward 7777 127.0.0.1:80
	# DynamicForward 127.0.0.1:6666

Host secret
	HostName 162.243.86.165
	IdentityFile /tmp/id_ed25519
    ProxyCommand ssh front -W %h:%p %r


Host *
	Protocol 2
	Port 22
	User root
	StrictHostKeyChecking no
	VerifyHostKeyDNS no
	AddressFamily inet
	HashKnownHosts yes
	UserKnownHostsFile /dev/null
	LogLevel QUIET
	IdentityFile ~/.ssh/id_ed25519
	ServerAliveCountMax 5
	ServerAliveInterval 120
	ControlMaster auto
	ControlPath ~/.ssh/sockets/%[email protected]%h:%p
	ControlPersist 180
   	#PreferredAuthentications gssapi-with-mic,hostbased,publickey,keyboard-interactive,password
   	PreferredAuthentications publickey,keyboard-interactive,password
	HostKeyAlgorithms [email protected],[email protected],ssh-ed25519,ssh-rsa,[email protected],[email protected],[email protected],ecdsa-sha2-nistp521,ecdsa-sha2-nistp384,ecdsa-sha2-nistp256
	KexAlgorithms [email protected],ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256,diffie-hellman-group-exchange-sha256
	MACs [email protected],[email protected],[email protected],hmac-sha2-512,hmac-sha2-256,[email protected]
	Compression yes
	Cipher [email protected]
	Ciphers [email protected],[email protected],[email protected],aes256-ctr,aes192-ctr,aes128-ctr

References

Bibliography

Change Logs

2017.03.08 23:59 Wed Asia/Shanghai
- 初稿完成
2018.07.27 14:48:32 Fri America/Boston
- 勘誤，更新，遷移到新Blog

Table of Contents

Preparation

Introduction

Parameters List

Format

Simple Login

Common Configuration

Visit Intern Host Via Front

Method 1

Method 2 nc

Method 3 ProxyJump

Port Forward

LocalForward

RemoteForward

DynamicForward

X Forwarding

Step 1 xauth

Step 2 sshd_config

Step 3 DISPLAY

Step 4 ssh -Y

The Onion Router

~/.ssh/config

References

Bibliography

Change Logs