What's openssh default kexalgorithms?

Question

If KexAlgorithms is not configured explicitly in an ssh config file, what's the default key exchange algorithm openssh may use?

The openssh version I am using is OpenSSH_6.4p1.

Answer 1

After further check, this information can be got by two ways.

1. read from man page for 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. read from ssh -vvv logs (The first part is the kexalgorithm,hmac,ciphers supported in client side; the second part is the sshd server's. )

   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: [email protected],[email protected],[email protected],[email protected],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,[email protected]
   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,[email protected]
   debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,[email protected],hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,[email protected],hmac-sha1-96,hmac-md5-96
   debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,[email protected],hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,[email protected],hmac-sha1-96,hmac-md5-96
   debug2: kex_parse_kexinit: none,[email protected],zlib
   debug2: kex_parse_kexinit: none,[email protected],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,[email protected],hmac-ripemd160,hmac-sha1-96,hmac-md5-96,hmac-sha2-256,hmac-sha2-512
   debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,[email protected],hmac-ripemd160,hmac-sha1-96,hmac-md5-96,hmac-sha2-256,hmac-sha2-512
   debug2: kex_parse_kexinit: none,[email protected]
   debug2: kex_parse_kexinit: none,[email protected]
   

3. query ssh for supported algorithms: ssh -Q kex server (uppercase -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
   [email protected]