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How do I write a simple SSH client?

Tags:

c++

c

ssh

I'm planning on writing a simple SSH client in C/C++ for a project in my intro networking class. I am having trouble, however, in finding good documentation on how to do this. I don't want extensive protocol specification, just a simple guide. Any suggestions?

like image 624
Groppe Avatar asked Apr 02 '12 14:04

Groppe

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What is a SSH client?

An SSH client is a program that allows establishing secure and authenticated SSH connections to SSH servers. SSH client software is available for major enterprise environment operating systems, such as Unix variations, Microsoft Windows and IBM z/OS.

Does SSH need client?

If you're running Linux or macOS, you have an interface built into your operating system, so you don't need to install an SSH client. But if you're running Windows, you'll need to install a client.

2 Answers

I would take a look at the libssl and libcrypto libraries that are part of the OpenSSL project. On debian-based systems you can get the development libraries using:

sudo apt-get install libssl-dev

I would take a look at the files under /usr/share/doc/libssl-dev/demos

EDIT: Also, you can read the documentation here or after installing, run man ssl

like image 127
sinelaw Avatar answered Nov 15 '22 09:11

sinelaw

Steps:

  1. get credentials for an existing server-side instance
  2. install an open source client
  3. execute a connection on the command-line
  4. including on CLI, the switch for verbose logs
  5. Use the logs to infer the back and forth ( C/S protocol ) messages involved in getting an authenticated session on a cipher implementation agreed to by both sides..

stdout:

$ ssh -vvv myhost.com
OpenSSH_5.9p1, OpenSSL 0.9.8t 18 Jan 2012
debug1: Reading configuration data /home/myself/.ssh/config
debug2: ssh_connect: needpriv 0
debug1: Connecting to myhost.com [74.99.99.99] port 22.
debug1: Connection established.
debug3: Incorrect RSA1 identifier
debug3: Could not load "/home/myself/.ssh/id_rsa" as a RSA1 public key
debug1: identity file /home/myself/.ssh/id_rsa type 1
debug1: identity file /home/myself/.ssh/id_rsa-cert type -1
debug3: Incorrect RSA1 identifier
debug3: Could not load "/home/myself/.ssh/id_dsa" as a RSA1 public key
debug1: identity file /home/myself/.ssh/id_dsa type 2
debug1: identity file /home/myself/.ssh/id_dsa-cert type -1
debug1: identity file /home/myself/.ssh/id_ecdsa type -1
debug1: identity file /home/myself/.ssh/id_ecdsa-cert type -1
debug1: Remote protocol version 2.0, remote software version OpenSSH_3.5p1
debug1: match: OpenSSH_3.5p1 pat OpenSSH_3.*
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_5.9
debug2: fd 3 setting O_NONBLOCK
debug3: load_hostkeys: loading entries for host "myhost.com" from file "/home/myself/.ssh/known_hosts"
debug3: load_hostkeys: found key type RSA in file /home/myself/.ssh/known_hosts:12
debug3: load_hostkeys: loaded 1 keys
debug3: order_hostkeyalgs: prefer hostkeyalgs: [email protected],[email protected],ssh-rsa
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug2: kex_parse_kexinit: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diff
ie-hellman-group14-sha1,diffie-hellman-group1-sha1
debug2: kex_parse_kexinit: [email protected],[email protected],ssh-rsa,[email protected],ecdsa-sha2-nistp384-cert-v
[email protected],[email protected],[email protected],[email protected],ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-
sha2-nistp521,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,rij
[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,rij
[email protected]
debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,[email protected],hmac-sha2-256,hmac-sha2-256-96,hmac-sha2-512,hmac-sha2-512-96,hmac-ripemd160,hmac-ripemd160@op
enssh.com,hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,[email protected],hmac-sha2-256,hmac-sha2-256-96,hmac-sha2-512,hmac-sha2-512-96,hmac-ripemd160,hmac-ripemd160@op
enssh.com,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: diffie-hellman-group-exchange-sha1,diffie-hellman-group1-sha1
debug2: kex_parse_kexinit: ssh-rsa,ssh-dss
debug2: kex_parse_kexinit: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes192-cbc,aes256-cbc,[email protected]
debug2: kex_parse_kexinit: aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes192-cbc,aes256-cbc,[email protected]
debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,hmac-ripemd160,[email protected],hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,hmac-ripemd160,[email protected],hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: none,zlib
debug2: kex_parse_kexinit: none,zlib
debug2: kex_parse_kexinit:
debug2: kex_parse_kexinit:
debug2: kex_parse_kexinit: first_kex_follows 0
debug2: kex_parse_kexinit: reserved 0
debug2: mac_setup: found hmac-md5
debug1: kex: server->client aes128-cbc hmac-md5 none
debug2: mac_setup: found hmac-md5
debug1: kex: client->server aes128-cbc hmac-md5 none
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024<1024<8192) sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP
debug2: dh_gen_key: priv key bits set: 124/256
debug2: bits set: 510/1024
debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
debug1: Server host key: RSA ac:24:92:03:fd:24:f8:01:a6:86:2a:bf:37:e9:a8:fe
debug3: load_hostkeys: loading entries for host "myhost.com" from file "/home/myself/.ssh/known_hosts"
debug3: load_hostkeys: found key type RSA in file /home/myself/.ssh/known_hosts:12
debug3: load_hostkeys: loaded 1 keys
debug3: load_hostkeys: loading entries for host "74.99.99.99" from file "/home/myself/.ssh/known_hosts"
debug3: load_hostkeys: found key type RSA in file /home/myself/.ssh/known_hosts:25
debug3: load_hostkeys: loaded 1 keys
debug1: Host 'myhost.com' is known and matches the RSA host key.
debug1: Found key in /home/myself/.ssh/known_hosts:12
debug2: bits set: 532/1024
debug1: ssh_rsa_verify: signature correct
debug2: kex_derive_keys
debug2: set_newkeys: mode 1
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug2: set_newkeys: mode 0
debug1: SSH2_MSG_NEWKEYS received
debug1: Roaming not allowed by server
debug1: SSH2_MSG_SERVICE_REQUEST sent
debug2: service_accept: ssh-userauth
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug2: key: /home/myself/.ssh/id_rsa (0x800471d8)
debug2: key: /home/myself/.ssh/id_dsa (0x80048380)
debug2: key: /home/myself/.ssh/id_ecdsa (0x0)
debug3: input_userauth_banner

debug1: Authentications that can continue: publickey,password
debug3: start over, passed a different list publickey,password
debug3: preferred publickey,keyboard-interactive,password
debug3: authmethod_lookup publickey
debug3: remaining preferred: keyboard-interactive,password
debug3: authmethod_is_enabled publickey
debug1: Next authentication method: publickey
debug1: Offering RSA public key: /home/myself/.ssh/id_rsa
debug3: send_pubkey_test
debug2: we sent a publickey packet, wait for reply
debug1: Authentications that can continue: publickey,password
debug1: Offering DSA public key: /home/myself/.ssh/id_dsa
debug3: send_pubkey_test
debug2: we sent a publickey packet, wait for reply
debug1: Server accepts key: pkalg ssh-dss blen 433
debug2: input_userauth_pk_ok: fp 15:6c:78:1e:76:02:03:26:2e:53:1a:44:8e:59:97:b1
debug3: sign_and_send_pubkey: DSA 15:6c:78:1e:76:02:03:26:2e:53:1a:44:8e:59:97:b1
debug1: read PEM private key done: type DSA
debug1: Authentication succeeded (publickey).
Authenticated to myhost.com ([74.99.99.99]:22).
debug1: channel 0: new [client-session]
debug3: ssh_session2_open: channel_new: 0
debug2: channel 0: send open
debug1: Entering interactive session.
debug2: callback start
debug2: client_session2_setup: id 0
debug2: fd 3 setting TCP_NODELAY
debug2: channel 0: request pty-req confirm 1
debug2: channel 0: request shell confirm 1
debug2: callback done
debug2: channel 0: open confirm rwindow 0 rmax 32768
debug2: channel_input_status_confirm: type 99 id 0
debug2: PTY allocation request accepted on channel 0
debug2: channel 0: rcvd adjust 131072
debug2: channel_input_status_confirm: type 99 id 0
debug2: shell request accepted on channel 0
like image 36
Robert Rowntree Avatar answered Nov 15 '22 09:11

Robert Rowntree
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