How to troubleshoot and diagnose “Write Failed: broken pipe” on SSH connection?

Question

I’m experiencing quite a number of “Write Failed: broken pipe” errors on Mac OS X to Mac OS X connections. Both machines have OpenSSH 7.1p, and the server is listening on port 1522. Both machines are on the local LAN, but I'm actually connecting through my gateway's public (external) IP. The server is a PowerMac, and it’s public facing for a few people who use it for testing under the PowerPC. The Gateway is using port forwarding to map 1522 to the server.

I’ve tried a few workarounds found in the wild, like ensuring TCPKeepAlive is set to yes in both /etc/ssh_config and ~/.ssh/config. The problem persists, and I’m getting frustrated.

A quick search did not reveal troubleshooting steps for the problem. The search did point me back to workarounds found in the wild, like How to prevent “Write Failed: broken pipe” on SSH connection?. But the workarounds usually lack troubleshooting information.

How can I troubleshoot and diagnose “Write Failed: broken pipe”?

Answer 1

From personal experience, Write Failed: broken pipe means somehow, somewhere, your connection was dropped. This could be caused by substantial packet loss, a poor Wi-Fi connection, network congestion, or a lot of other things. You could start by trying to connect via the Mac's local IP addresses. If the connection is stable, you know that an external proxy server may be the culprit, though I don't know if your router bounces back connections to it's own external IP address (try traceroute external.ip.address). If this doesn't work, try connecting to another ssh server from another computer, either internal or external. If this works, you know one of your machines is the culprit. If it doesn't, you have a network problem. Just remove a variable at a time from point A to B and you're bound to find the problem. Good luck!