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I'm running a script that issues remote commands over SSH to a few production boxes. There are RSA keys in place for the user to connect to these boxes and in most cases there are no problems. Intermittently, I get connection timeouts, maybe once out of every 20 connections.

I'm using SSH2 as the protocol, mainly because SSH2 is available on all of the machines I'm connecting to. This is what the debugging output looks like, up to the point where the connection hangs and then closes.

[mike@dev ~]$ ssh -v -o "Protocol=2" 10.60.###.### "w"
OpenSSH_4.3p2, OpenSSL 0.9.8b 04 May 2006
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: Applying options for *
debug1: Connecting to 10.60.###.### [10.60.###.###] port 22.
debug1: Connection established.
debug1: identity file /home/mike/.ssh/id_rsa type 1
debug1: identity file /home/mike/.ssh/id_dsa type -1
debug1: loaded 2 keys
debug1: Remote protocol version 1.99, remote software version OpenSSH_3.9p1
debug1: match: OpenSSH_3.9p1 pat OpenSSH_3.*
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_4.3
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: server->client aes128-cbc hmac-md5 none
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
Connection closed by 10.60.###.###

Connection hangs here, and then closes.

On some occasions, the connection gets a little bit further, but again hangs.

...
debug1: ssh_rsa_verify: signature correct
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: SSH2_MSG_SERVICE_REQUEST sent
Connection closed by 10.60.###.###

The server's log ends something like this, in cases where the connection dies:

Jul 23 16:53:07 host02 sshd[4989]: debug1: Client protocol version 2.0; client software version OpenSSH_4.3
Jul 23 16:53:07 host02 sshd[4989]: debug1: match: OpenSSH_4.3 pat OpenSSH*
Jul 23 16:53:07 host02 sshd[4989]: debug1: Enabling compatibility mode for protocol 2.0
Jul 23 16:53:07 host02 sshd[4989]: debug1: Local version string SSH-1.99-OpenSSH_3.9p1
Jul 23 16:53:07 host02 sshd[4990]: debug1: permanently_set_uid: 74/74
Jul 23 16:53:07 host02 sshd[4990]: debug1: list_hostkey_types: ssh-rsa,ssh-dss
Jul 23 16:53:07 host02 sshd[4990]: debug1: SSH2_MSG_KEXINIT sent
Jul 23 16:53:07 host02 sshd[4990]: debug1: SSH2_MSG_KEXINIT received
Jul 23 16:53:07 host02 sshd[4990]: debug1: kex: client->server aes128-cbc hmac-md5 none
Jul 23 16:53:07 host02 sshd[4990]: debug1: kex: server->client aes128-cbc hmac-md5 none
Jul 23 16:53:40 host02 sshd[4990]: debug1: SSH2_MSG_KEX_DH_GEX_REQUEST received
Jul 23 16:53:40 host02 sshd[4990]: debug1: SSH2_MSG_KEX_DH_GEX_GROUP sent
Jul 23 16:53:40 host02 sshd[4990]: debug1: expecting SSH2_MSG_KEX_DH_GEX_INIT
Jul 23 16:53:40 host02 sshd[4990]: Connection closed by ::ffff:192.168.1.203

I have found with one server I've been testing that if I instead use SSH1, I haven't seen any failed connections. But I'm wondering if this can be fixed for SSH2, rather than having to open up SSH1 on all of the production machines.

Out of 100 connection attempts...

Using SSH1: 100 connections succeed. Using SSH2: approx. 5 connections fail.

Is there something (easy) I'm missing?

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What version of sshd are you using? –  geeklin Jul 22 '09 at 20:27
    
Looks like "OpenSSH_3.9p1". –  Mike Brittain Jul 22 '09 at 20:46
    
Some of the older servers are running 3.9p1 as noted above, and some newer servers (where connection issues also arise) are running "OpenSSH_4.3p2". –  Mike Brittain Jul 22 '09 at 20:56
    
What are the relevant sshd server log entries? (i.e. are there entries in the sshd server logs corresponding to your disconections?) –  samt Jul 26 '09 at 13:32
    
Probably makes sense to note that the SSH connections to these boxes are being made from our local network to our data center. There is a VPN tunnel between the two locations. This issue with hangs/disconnects only presents when we connect to machines at the DC. We haven't experienced it between boxes on our local network. –  Mike Brittain Jul 27 '09 at 19:02
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3 Answers 3

We finally worked this out a few weeks ago, and it's worth following up. We run a VPN from our local office router to our firewall at our data center, both are Cisco. We updated the IOS on the router and firewall and that seems to have done the trick.

Thanks for all of the ideas/answers that were submitted.

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Could it be a dns lookup issue? Is your client's IP registered in DNS? Maybe the timeout happens on the first lookup, and then it is cached for a little while.

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We originally thought it was DNS issues, but seems like not. Both servers have each others hostnames and IPs in their respective /etc/hosts files. Connections are being made using IPs, and not hostnames. –  Mike Brittain Jul 22 '09 at 20:47
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Have a look at the TCP packets of a failed connection. The 33s between the two log entries may be a result of the underlying connection dropping packets or failing temporarily. The VPN gateway might actually "drop dead" while it tries to reestablish the tunnel. Packets will be lost in that situation without either TCP endpoint noticing.

Just run tcpdump -i eth99 -s 0 -w ssh2-problem.pcap port 22 and have a connection fail. Then use Wireshark to examine the dump file. A display filter of tcp.flags.syn == 1 and tcp.flags.ack == 0 gives you only the first packet of a TCP stream. Then use "Follow TCP stream" from the context menu, dismiss the dialog and look at the failed TCP stream packets. Any excessive (>>1) retransmissions?

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