Linux SSH - Connection closed issue
Today I ran an ssh/expect script on a number of Cisco devices and noticed that connection to about 10% of them failed. What an unpleasant surprise it was... The issue was on both of my computers with CentOS and Xubuntu. Looking at ssh debug I noticed that ssh failed at "expecting SSH2_MSG_KEX_DH_GEX_GROUP". There is a bug in Cisco IOS and in Open SSH that causes it. Related info can be found here:
https://supportforums.cisco.com/discussion/12051361/cisco-ios-does-not-support-openssh-64
https://bugzilla.redhat.com/show_bug.cgi?id=1026430
I saw numerous ways to resolve similar problem on internet including changing MTU! but that must be related to some other bug and didn't help in my case.
For some people it was enough to uncomment "Cipher" and "MACs" lines in the /etc/ssh/ssh_config, but for me only specifying algorythms below solved issue for all connections. Other options would cause failures on some of the hosts.
Resolution:
ssh -o StrictHostKeyChecking=no -o HostKeyAlgorithms=ssh-rsa,ssh-dss -o KexAlgorithms=diffie-hellman-group1-sha1 -o Ciphers=aes128-cbc,3des-cbc -o MACs=hmac-md5,hmac-sha1 <user@IP>
Symptoms:
[username@trinity dhcp_rm]$ ssh -o StrictHostKeyChecking=no username@*.*.225.225
Connection closed by *.*.225.225
[username@trinity dhcp_rm]$ ssh -vvv username@*.*.225.225
OpenSSH_6.6.1, OpenSSL 1.0.1e-fips 11 Feb 2013
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 56: Applying options for *
debug2: ssh_connect: needpriv 0
debug1: Connecting to *.*.225.225 [*.*.225.225] port 22.
debug1: Connection established.
debug1: identity file /home/username/.ssh/id_rsa type -1
debug1: identity file /home/username/.ssh/id_rsa-cert type -1
debug1: identity file /home/username/.ssh/id_dsa type -1
debug1: identity file /home/username/.ssh/id_dsa-cert type -1
debug1: identity file /home/username/.ssh/id_ecdsa type -1
debug1: identity file /home/username/.ssh/id_ecdsa-cert type -1
debug1: identity file /home/username/.ssh/id_ed25519 type -1
debug1: identity file /home/username/.ssh/id_ed25519-cert type -1
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_6.6.1
debug1: Remote protocol version 2.0, remote software version Cisco-1.25
debug1: no match: Cisco-1.25
debug2: fd 3 setting O_NONBLOCK
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug2: kex_parse_kexinit: curve25519-sha256@libssh.org,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: ecdsa-sha2-nistp256-cert-v01@openssh.com,ecdsa-sha2-nistp384-cert-v01@openssh.com,ecdsa-sha2-nistp521-cert-v01@openssh.com,ssh-ed25519-cert-v01@openssh.com,ssh-rsa-cert-v01@openssh.com,ssh-dss-cert-v01@openssh.com,ssh-rsa-cert-v00@openssh.com,ssh-dss-cert-v00@openssh.com,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,ssh-ed25519,ssh-rsa,ssh-dss
debug2: kex_parse_kexinit: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-gcm@openssh.com,aes256-gcm@openssh.com,chacha20-poly1305@openssh.com,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-gcm@openssh.com,aes256-gcm@openssh.com,chacha20-poly1305@openssh.com,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,rijndael-cbc@lysator.liu.se
debug2: kex_parse_kexinit: hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com,umac-64-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,hmac-ripemd160-etm@openssh.com,hmac-sha1-96-etm@openssh.com,hmac-md5-96-etm@openssh.com,hmac-md5,hmac-sha1,umac-64@openssh.com,umac-128@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-etm@openssh.com,hmac-sha1-etm@openssh.com,umac-64-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,hmac-ripemd160-etm@openssh.com,hmac-sha1-96-etm@openssh.com,hmac-md5-96-etm@openssh.com,hmac-md5,hmac-sha1,umac-64@openssh.com,umac-128@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: diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1
debug2: kex_parse_kexinit: ssh-rsa
debug2: kex_parse_kexinit: aes128-cbc,3des-cbc,aes192-cbc,aes256-cbc
debug2: kex_parse_kexinit: aes128-cbc,3des-cbc,aes192-cbc,aes256-cbc
debug2: kex_parse_kexinit: hmac-sha1,hmac-sha1-96,hmac-md5,hmac-md5-96
debug2: kex_parse_kexinit: hmac-sha1,hmac-sha1-96,hmac-md5,hmac-md5-96
debug2: kex_parse_kexinit: none
debug2: kex_parse_kexinit: none
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: setup hmac-md5
debug1: kex: server->client aes128-cbc hmac-md5 none
debug2: mac_setup: setup hmac-md5
debug1: kex: client->server aes128-cbc hmac-md5 none
debug1: kex: diffie-hellman-group-exchange-sha1 need=16 dh_need=16
debug1: kex: diffie-hellman-group-exchange-sha1 need=16 dh_need=16
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024<3072<8192) sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP
Connection closed by *.*.225.225