What is actually in known_hosts? [closed]
This file is, effectively, your personal Certificate Authority. It is the list of all SSH server host public keys that you have determined are accurate. Each entry in known_hosts is one big line with three or more whitespace separated fields as follows:
a. One or more server names or IP Addresses, joined together by commas.
foo.com,107.180.00.00
b. The type of key.
ssh-rsa
c. The public key data itself encoded to stay within the ASCII range.
AAAAB3NzaC1yc2EAAAABIwAAAQEAuJfqSnraBz//Ux4j/hZpLv2eYUxNUgCk+9ClqoSgfcu4vXbWtUGSjo75UVQf+uguOeBnRLppJJ3mt0R5c/PPcawUGWfffk33t+biYcqra9xUcyfiGtO/Icko2L1J0EYTXM/8x8VK6UYFMfad2gltnZRa8Am50oHTXot1Df0RljUBxvh/UhmTJUrODpyrl2xY1OMWjM+S6uYCMNeSQGEpNfsWiCIStRnctMZSxiYJOLTSC4F2GF7B8pYFBn5rSwVHp17WCdO+4BZfwvH3HSSH8IWoyFhki+NlG912SEBJXcryvc0JPfAB9DTB4mRImjgrRT8vz5QeaCDrh8k4/A+U1fff
d. Any optional comment data.
Also!! This thread might be of use for you:
https://security.stackexchange.com/a/20710
To add to the answer above and your comment, There are four building blocks for ssh session
- Encryption( symmetric keys derived after key exhange per session)
- Data integrity (MAC using eg SHA,HMAC )
- Key exchange methods
- Public key methods or host key methods
the SSH algorithm negotiation involves a key exchange state machine which begins when the SSH_MSG_KEXINIT message along with algorithms list is sent.
The key exchange method or simply kex specifies session keys for encryption and host authentication host public keys(ssh-rsa, ssh-dss ..) that are sent to the client. The step below are the basic steps that take place for kex using Diffie hellman key exchange algorithm
quoting the RFC https://www.rfc-editor.org/rfc/rfc4253
The following steps are used to exchange a key. In this, C is the client; S is the server; p is a large safe prime; g is a generator for a subgroup of GF(p); q is the order of the subgroup; V_S is S's identification string; V_C is C's identification string; K_S is S's public host key; I_C is C's SSH_MSG_KEXINIT message and I_S is S's SSH_MSG_KEXINIT message that have been exchanged before this part begins.
- C generates a random number x (1 < x < q) and computes e = g^x mod p. C sends e to S.
- S generates a random number y (0 < y < q) and computes f = g^y mod p. S receives e. It computes K = e^y mod p, H = hash(V_C || V_S || I_C || I_S || K_S || e || f || K) (these elements are encoded according to their types; see below), and signature s on H with its private host key. S sends (K_S || f || s) to C. The signing operation may involve a second hashing operation.
- C verifies that K_S really is the host key for S (e.g., using certificates or a local database). C is also allowed to accept the key without verification; however, doing so will render the protocol insecure against active attacks (but may be desirable for practical reasons in the short term in many environments). C then computes K = f^x mod p, H = hash(V_C || V_S || I_C || I_S || K_S || e || f || K), and verifies the signature s on H.
the local database mentioned in step three in certain systems could be the .ssh/known_hosts file. So to answer your question the public key is sent to the client by the host during the key-exchange.
The following public key and/or certificate formats are currently defined:
ssh-dss REQUIRED sign Raw DSS Key
ssh-rsa RECOMMENDED sign Raw RSA Key
pgp-sign-rsa OPTIONAL sign OpenPGP certificates (RSA key)
pgp-sign-dss OPTIONAL sign OpenPGP certificates (DSS key)