问题描述
sshd 的默认配置中使能了一些弱加密算法，有潜在的安全风险，需要禁用这些弱加密算法。
对 ssh 加密算法的掌握现状
几乎为 0。
从 man sshd_config 开始
1. Ciphers
Ciphers
Specifies the ciphers allowed. Multiple ciphers must be comma-separated. If the specified value begins with a '+' char-
acter, then the specified ciphers will be appended to the default set instead of replacing them. If the specified value
begins with a '-' character, then the specified ciphers (including wildcards) will be removed from 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
[email protected]
[email protected]
[email protected]
The default is:
[email protected],
aes128-ctr,aes192-ctr,aes256-ctr,
[email protected],[email protected]
The list of available ciphers may also be obtained using "ssh -Q cipher".
Ciphers 指定 ssh 使能的加密算法。多个加密算法之间使用逗号分隔。当 Ciphers 的值以 + 字符开始时，指定的加密算法将附加到默认集合，不影响默认集合中的其它算法。当 Ciphers 的值以 ‘-’ 字符开始时，指定的加密算法将会从默认集合中移除，不影响默认集合中的其它项目。
1.1 nmap 扫描默认的 Ciphers
使用默认 Ciphers 情况下 nmap 扫描到的 Ciphers:
| encryption_algorithms: (6)
| [email protected]
| aes128-ctr
| aes192-ctr
| aes256-ctr
| [email protected]
| [email protected]
.........
1.2 sshd_config 中新增 Ciphers 后扫描
在 sshd_config 文件的最后添加如下行：
Ciphers +3des-cbc,aes128-cbc,aes192-cbc,aes256-cbc
重启 sshd 后，重新扫描，结果如下：
| encryption_algorithms: (10)
| [email protected]
| aes128-ctr
| aes192-ctr
| aes256-ctr
| [email protected]
| [email protected]
| 3des-cbc
| aes128-cbc
| aes192-cbc
| aes256-cbc
可以看到添加新的算法到默认集合成功。
1.3 sshd_config 中删除指定 Ciphers 后扫描
在 sshd_config 文件的最后添加如下行：
Ciphers -aes128-ctr,[email protected]
重启 sshd 后，重新扫描，结果如下：
| encryption_algorithms: (4)
| [email protected]
| aes192-ctr
| aes256-ctr
| [email protected]
可以看到指定的凉饿 aes128 加密算法已经被移除。
MACs
MACs Specifies the available MAC (message authentication code) algorithms. The MAC algorithm is used for data integrity pro-
tection. Multiple algorithms must be comma-separated. If the specified value begins with a '+' character, then the spec-
ified algorithms will be appended to the default set instead of replacing them. If the specified value begins with a '-'
character, then the specified algorithms (including wildcards) will be removed from 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-sha1
hmac-sha1-96
hmac-sha2-256
hmac-sha2-512
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
The default is:
[email protected],[email protected],
[email protected],[email protected],
[email protected],
[email protected],[email protected],
hmac-sha2-256,hmac-sha2-512,hmac-sha1
The list of available MAC algorithms may also be obtained using "ssh -Q mac".
MACs 选项指定可用的 MAC（消息认证代码）算法，用于数据完整性保护。配置方法与 Ciphers 一致，不再赘述。
KexAlgorithms
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 specified methods will be appended to the default set instead of re-
placing them. If the specified value begins with a '-' character, then the specified methods (including wildcards) will
be removed from the default set instead of replacing them. The supported algorithms are:
curve25519-sha256
[email protected]
diffie-hellman-group1-sha1
diffie-hellman-group14-sha1
diffie-hellman-group14-sha256
diffie-hellman-group16-sha512
diffie-hellman-group18-sha512
diffie-hellman-group-exchange-sha1
diffie-hellman-group-exchange-sha256
ecdh-sha2-nistp256
ecdh-sha2-nistp384
ecdh-sha2-nistp521
The default is:
curve25519-sha256,[email protected],
ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
diffie-hellman-group-exchange-sha256,
diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,
diffie-hellman-group14-sha256,diffie-hellman-group14-sha1
The list of available key exchange algorithms may also be obtained using "ssh -Q kex".
KexAlgorithms 选项指定可用的密钥交换算法。配置方法与 Ciphers 一致，不再赘述。
PubkeyAcceptedKeyTypes
Specifies the key types that will be accepted for public key authentication as a list of comma-separated patterns. Alter-
nately if the specified value begins with a '+' character, then the specified key types will be appended to the default
set instead of replacing them. If the specified value begins with a '-' character, then the specified key types (includ-
ing wildcards) will be removed from the default set instead of replacing them. The default for this option is:
[email protected],
[email protected],
[email protected],
[email protected],
[email protected],[email protected],
[email protected],
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
The list of available key types may also be obtained using "ssh -Q key".
PubkeyAcceptedKeyTypes 指定公钥认证允许的密钥类型。配置方法与 Ciphers 一致，不再赘述。
如何查看 ssh 支持的不同类别的加密算法
相关命令：
Ciphers: ssh -Q cipher
MACs: ssh -Q mac
KexAlgorithms: ssh -Q kex
PubkeyAcceptedKeyTypes: ssh -Q key
备注：这些命令输出的项目是 ssh 支持的，并不一定是 ssh 使能的项目。
通过 ssh 命令指定特定的算法
命令示例：
- ssh -vv -[email protected]
- ssh -vv -oCiphers=xxx
- ssh -vv -o…
运行示例：
longyu@debian:~$ ssh -vv -oMACs=[email protected] [email protected]
OpenSSH_7.9p1 Debian-10+deb10u2, OpenSSL 1.1.1d 10 Sep 2019
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 19: Applying options for *
debug2: resolve_canonicalize: hostname 192.168.122.1 is address
debug2: ssh_connect_direct
debug1: Connecting to 192.168.122.1 [192.168.122.1] port 22.
debug1: Connection established.
debug1: identity file /home/longyu/.ssh/id_rsa type -1
debug1: identity file /home/longyu/.ssh/id_rsa-cert type -1
debug1: identity file /home/longyu/.ssh/id_dsa type -1
debug1: identity file /home/longyu/.ssh/id_dsa-cert type -1
debug1: identity file /home/longyu/.ssh/id_ecdsa type -1
debug1: identity file /home/longyu/.ssh/id_ecdsa-cert type -1
debug1: identity file /home/longyu/.ssh/id_ed25519 type -1
debug1: identity file /home/longyu/.ssh/id_ed25519-cert type -1
debug1: identity file /home/longyu/.ssh/id_xmss type -1
debug1: identity file /home/longyu/.ssh/id_xmss-cert type -1
debug1: Local version string SSH-2.0-OpenSSH_7.9p1 Debian-10+deb10u2
debug1: Remote protocol version 2.0, remote software version OpenSSH_7.9p1 Debian-10+deb10u2
debug1: match: OpenSSH_7.9p1 Debian-10+deb10u2 pat OpenSSH* compat 0x04000000
debug2: fd 3 setting O_NONBLOCK
debug1: Authenticating to 192.168.122.1:22 as 'longyu'
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug2: local client KEXINIT proposal
debug2: KEX algorithms: diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group14-sha256,ext-info-c
debug2: host key algorithms: [email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
debug2: ciphers ctos: aes128-ctr,aes192-ctr,aes256-ctr,aes128-cbc,3des-cbc
debug2: ciphers stoc: aes128-ctr,aes192-ctr,aes256-ctr,aes128-cbc,3des-cbc
debug2: MACs ctos: [email protected]
debug2: MACs stoc: [email protected]
debug2: compression ctos: none,[email protected],zlib
debug2: compression stoc: none,[email protected],zlib
debug2: languages ctos:
debug2: languages stoc:
debug2: first_kex_follows 0
debug2: reserved 0
debug2: peer server KEXINIT proposal
debug2: KEX algorithms: diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha256
debug2: host key algorithms: rsa-sha2-512,rsa-sha2-256,ssh-rsa,ecdsa-sha2-nistp256,ssh-ed25519
debug2: ciphers ctos: [email protected],aes128-ctr,aes192-ctr,aes256-ctr,[email protected],[email protected]
debug2: ciphers stoc: [email protected],aes128-ctr,aes192-ctr,aes256-ctr,[email protected],[email protected]
debug2: MACs ctos: [email protected],[email protected]
debug2: MACs stoc: [email protected],[email protected]
debug2: compression ctos: none,[email protected]
debug2: compression stoc: none,[email protected]
debug2: languages ctos:
debug2: languages stoc:
debug2: first_kex_follows 0
debug2: reserved 0
debug1: kex: algorithm: diffie-hellman-group14-sha256
debug1: kex: host key algorithm: ecdsa-sha2-nistp256
如下输出中，stoc 的输出表明 server 使能的相关加密算法。
debug2: KEX algorithms: diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha256
debug2: host key algorithms: rsa-sha2-512,rsa-sha2-256,ssh-rsa,ecdsa-sha2-nistp256,ssh-ed25519
debug2: ciphers ctos: [email protected],aes128-ctr,aes192-ctr,aes256-ctr,[email protected],[email protected]
debug2: ciphers stoc: [email protected],aes128-ctr,aes192-ctr,aes256-ctr,[email protected],[email protected]
debug2: MACs ctos: [email protected],[email protected]
debug2: MACs stoc: [email protected],[email protected]
-vv 选项指定 ssh 打印处理过程中的一些 debug 信息。这些信息对于定位一些连接、认证、配置问题非常有用。