什么是 openssh 默认 kexalgorithms？

Question

如果KexAlgorithms 没有在 ssh 配置文件中明确配置，那么 openssh 可能使用的默认密钥交换算法是什么？

我使用的openssh版本是OpenSSH_6.4p1。

Answer 1

经过进一步检查，可以通过两种方式获取此信息。

1. 从手册页读取sshd_config(5)

   KexAlgorithms
   Specifies the available KEX (Key Exchange) algorithms.
   Multiple algorithms must be comma-separated.
   The default is
   ecdh-sha2-nistp256 ,
   ecdh-sha2-nistp384 ,
   ecdh-sha2-nistp521 ,
   diffie-hellman-group-exchange-sha256 ,
   diffie-hellman-group-exchange-sha1 ,
   diffie-hellman-group14-sha1 ,
   diffie-hellman-group1-sha1 .
   

2. 从ssh -vvv日志读取（第一部分是客户端支持的kexalgorithm,hmac,ciphers；第二部分是sshd服务器的。）

   debug2: kex_parse_kexinit: diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1
   debug2: kex_parse_kexinit: ssh-rsa-cert-v01@openssh.com,ssh-dss-cert-v01@openssh.com,ssh-rsa-cert-v00@openssh.com,ssh-dss-cert-v00@openssh.com,ssh-rsa,ssh-dss
   debug2: kex_parse_kexinit: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,rijndael-cbc@lysator.liu.se
   debug2: kex_parse_kexinit: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,rijndael-cbc@lysator.liu.se
   debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,umac-64@openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,hmac-ripemd160@openssh.com,hmac-sha1-96,hmac-md5-96
   debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,umac-64@openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,hmac-ripemd160@openssh.com,hmac-sha1-96,hmac-md5-96
   debug2: kex_parse_kexinit: none,zlib@openssh.com,zlib
   debug2: kex_parse_kexinit: none,zlib@openssh.com,zlib
   debug2: kex_parse_kexinit:
   debug2: kex_parse_kexinit:
   debug2: kex_parse_kexinit: first_kex_follows 0
   debug2: kex_parse_kexinit: reserved 0
   debug2: kex_parse_kexinit: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1
   debug2: kex_parse_kexinit: ssh-rsa
   debug2: kex_parse_kexinit: aes128-ctr,aes192-ctr,aes256-ctr,aes128-cbc,aes192-cbc,aes256-cbc,arcfour256,arcfour128,3des-cbc,blowfish-cbc,cast128-cbc,arcfour
   debug2: kex_parse_kexinit: aes128-ctr,aes192-ctr,aes256-ctr,aes128-cbc,aes192-cbc,aes256-cbc,arcfour256,arcfour128,3des-cbc,blowfish-cbc,cast128-cbc,arcfour
   debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,umac-64@openssh.com,hmac-ripemd160,hmac-sha1-96,hmac-md5-96,hmac-sha2-256,hmac-sha2-512
   debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,umac-64@openssh.com,hmac-ripemd160,hmac-sha1-96,hmac-md5-96,hmac-sha2-256,hmac-sha2-512
   debug2: kex_parse_kexinit: none,zlib@openssh.com
   debug2: kex_parse_kexinit: none,zlib@openssh.com
   

3. 查询ssh以获取支持的算法：ssh -Q kex server（大写-Q）

   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
   diffie-hellman-group1-sha1
   curve25519-sha256@libssh.org
   

Answer 2

ssh -G 192.168.1.2 显示包含 kexalgorithms 的配置。 例如，

kexalgorithms curve25519-sha256,curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha256,diffie-hellman-group14-sha1

如果您只想为 kexalgorithms 配置 diffie-hellman-group1-sha1，

ssh -oKexAlgorithms=diffie-hellman-group1-sha1 username@192.168.1.2