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computers / comp.security.ssh / openssh client behavior with default local config file vs using "ssh -F ~/.ssh/config.other"

Subject: openssh client behavior with default local config file vs using "ssh -F ~/.ssh/config.other"
From: D Youatt
Newsgroups: comp.security.ssh
Date: Tue, 17 Mar 2020 16:21 UTC

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Subject: openssh client behavior with default local config file vs using "ssh
-F ~/.ssh/config.other"
From: youat...@gmail.com (D Youatt)
Injection-Date: Tue, 17 Mar 2020 16:21:30 +0000
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I'm using openssh (client) on Ubuntu bionic.
OpenSSH_7.6p1 Ubuntu-4ubuntu0.3, OpenSSL 1.0.2n 7 Dec 2017

and have a local ~/.ssh/config.other file with hosts defined to proxy through a bastion host. It looks like:

# Copied from /etc/ssh/ssh_config
Host *
SendEnv LANG LC_*
HashKnownHosts yes
GSSAPIAuthentication yes

UserKnownHostsFile ~/.ssh/my_known_hosts

Host my.bastion my-aws1
IdentityFile ~/.ssh/mypk
Port 12345
User auser
# bastion host uses IP addr
Hostname NN.MM.PP.QQ

Host my.primary my-aws1-primary
IdentityFile ~/.ssh/mypk
Port 22
User auser
Hostname my.host.name
ProxyCommand ssh -A -q my.bastion -W %h:%p

Both the bastion host and destination host are recent Amazon Linux default installations.

If the config file with the contents above is in ~/.ssh/config, and I just "ssh my.primary" it happily connects and logs in.

If the config file is ~/.ssh/config.other and I use "ssh -F ~/.ssh/config.other my.primary", it fails with

bash> ssh -vvv -F ~/.ssh/config.support my.primary
OpenSSH_7.6p1 Ubuntu-4ubuntu0.3, OpenSSL 1.0.2n 7 Dec 2017
debug1: Reading configuration data /home/me/.ssh/config.support
debug1: /home/me/.ssh/config.support line 1: Applying options for *
debug1: /home/me/.ssh/config.support line 20: Applying options for my.primary
debug1: Executing proxy command: exec ssh -A -q my.bastion -W my.host.name:22
debug1: permanently_drop_suid: 1000
debug1: identity file /home/me/.ssh/CrunchySupport type 0
debug1: key_load_public: No such file or directory
debug1: identity file /home/me/.ssh/CrunchySupport-cert type -1
debug1: Local version string SSH-2.0-OpenSSH_7.6p1 Ubuntu-4ubuntu0.3
ssh_exchange_identification: Connection closed by remote host

Why the difference using "ssh -F ~/.ssh/config.other ..."?

Subject	Author
openssh client behavior with default local config file vs using "ssh	D Youatt