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Package: openssh-client Version: 1:6.6p1-5 Severity: important After a recent dist-upgrade I could no longer ssh to a local server (running Wheezy). Both client and target have Gigabit interfaces with an MTU of 9000. In each case the interface is a brige (br0 with only member being eth0). The client hangs while performing key negotiation: root@rocky:~# ssh -vvv dusty OpenSSH_6.6.1, OpenSSL 1.0.1g 7 Apr 2014 debug1: Reading configuration data /etc/ssh/ssh_config debug1: /etc/ssh/ssh_config line 19: Applying options for * debug3: ciphers ok: [3des-cbc,aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc] debug2: ssh_connect: needpriv 0 debug1: Connecting to dusty [10.115.10.75] port 22. debug1: Connection established. debug1: permanently_set_uid: 0/0 debug3: Incorrect RSA1 identifier debug3: Could not load "/root/.ssh/id_rsa" as a RSA1 public key debug1: identity file /root/.ssh/id_rsa type 1 debug1: identity file /root/.ssh/id_rsa-cert type -1 debug1: identity file /root/.ssh/id_dsa type -1 debug1: identity file /root/.ssh/id_dsa-cert type -1 debug1: identity file /root/.ssh/id_ecdsa type -1 debug1: identity file /root/.ssh/id_ecdsa-cert type -1 debug1: identity file /root/.ssh/id_ed25519 type -1 debug1: identity file /root/.ssh/id_ed25519-cert type -1 debug1: Enabling compatibility mode for protocol 2.0 debug1: Local version string SSH-2.0-OpenSSH_6.6.1p1 Debian-5 debug1: Remote protocol version 2.0, remote software version OpenSSH_6.0p1 Debian-4+deb7u1 debug1: match: OpenSSH_6.0p1 Debian-4+deb7u1 pat OpenSSH* compat 0x04000000 debug2: fd 3 setting O_NONBLOCK debug3: load_hostkeys: loading entries for host "dusty" from file "/root/.ssh/known_hosts" debug3: load_hostkeys: found key type ECDSA in file /root/.ssh/known_hosts:33 debug3: load_hostkeys: loaded 1 keys debug3: order_hostkeyalgs: prefer hostkeyalgs: ecdsa-sha2-nistp256-cert-v01@openssh.com,ecdsa-sha2-nistp384-cert-v01@openssh.com,ecdsa-sha2-nistp521-cert-v01@openssh.com,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521 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,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,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,ssh-ed25519,ssh-rsa,ssh-dss debug2: kex_parse_kexinit: 3des-cbc,aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc debug2: kex_parse_kexinit: 3des-cbc,aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc 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: 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,ssh-dss,ecdsa-sha2-nistp256 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-sha2-256-96,hmac-sha2-512,hmac-sha2-512-96,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-sha2-256-96,hmac-sha2-512,hmac-sha2-512-96,hmac-ripemd160,hmac-ripemd160@openssh.com,hmac-sha1-96,hmac-md5-96 debug2: kex_parse_kexinit: none,zlib@openssh.com debug2: kex_parse_kexinit: none,zlib@openssh.com 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 3des-cbc hmac-md5 none debug2: mac_setup: setup hmac-md5 debug1: kex: client->server 3des-cbc hmac-md5 none debug1: sending SSH2_MSG_KEX_ECDH_INIT debug1: expecting SSH2_MSG_KEX_ECDH_REPLY Changing my mtu from 9000 to 1500 fixes the problem. Further investigation gets a little strange though... If I leave my mtu at 9000 and specify the cipher on the ssh command line: #ssh -c aes128-ctr dusty the client connects just fine. I tried each cipher in order and each time the client connected successfully. I then edited the '/etc/ssh/ssh_config' and placed a single cipher option on the "Ciphers" line, this also worked. I then started adding cipher options back and discovered something strange: if my FIRST cipher was 3des-cbc, I could specify a total of three ciphers on the 'Ciphers' line, adding a fourth resulted in the failed key exchange. HOWEVER, if 3des-cbc was not the first option, I could only specify TWO options on the 'Ciphers' line, adding the third resulted in the same failed key exchange. I did not try further permutations. QUICK FIX: 1. change client machine MTU = 1500 (no changes to 'etc/ssh/ssh_config') 2. specify two or fewer cipher options inside '/etc/ssh/ssh_config' John -- System Information: Debian Release: jessie/sid APT prefers stable-updates APT policy: (500, 'stable-updates'), (500, 'unstable'), (500, 'stable') Architecture: amd64 (x86_64) Foreign Architectures: i386 Kernel: Linux 3.14-1-amd64 (SMP w/4 CPU cores) Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8) Shell: /bin/sh linked to /bin/dash Versions of packages openssh-client depends on: ii adduser 3.113+nmu3 ii dpkg 1.17.9 ii libc6 2.18-5 ii libedit2 3.1-20140213-1 ii libgssapi-krb5-2 1.12.1+dfsg-1 ii libselinux1 2.2.2-2 ii libssl1.0.0 1.0.1g-3 ii passwd 1:4.1.5.1-1.1 ii zlib1g 1:1.2.8.dfsg-1 Versions of packages openssh-client recommends: ii xauth 1:1.0.7-1 Versions of packages openssh-client suggests: pn keychain <none> ii ksshaskpass [ssh-askpass] 0.5.3-1+b1 pn libpam-ssh <none> pn monkeysphere <none> ii ssh-askpass 1:1.2.4.1-9 -- Configuration Files: /etc/ssh/ssh_config changed: Host * Ciphers 3des-cbc,aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc SendEnv LANG LC_* HashKnownHosts yes GSSAPIAuthentication yes GSSAPIDelegateCredentials no -- no debconf information -- - |