Using wildcards in ssh configuration to create per-client setups

In my role as a linux consultant, I tend to work with a number of different companies. Obviously they all use ssh for remote access, and many require going through a gateway/bastion server first in order to access the rest of the network. I want to treat these clients as separate and secure as possible so I’ll always create a new SSH key for each client. Most clients would have large numbers of machines on their network and rather than having to cut and paste a lot of different configurations together you can use wildcards in your ~/.ssh/config file.

However this is not amazingly easy – as SSH configuration requires the most general settings to be at the bottom of the file. So here’s a typical setup I might use for an imaginary client called abc:

# Long list of server names & IPs
host abc-server1

host abc-server2

# Gateway box through which all SSH connections need routing
host abc-gateway

# Generic rule to access any box on ABC's network. Eg ssh abc-ip- is the same as ssh abc-server2.
# You could also use hostnames like ssh abc-ip-foo.local assuming these resolve from the abc-gateway box.
host abc-ip-*
ProxyCommand ssh abc-gateway -W $(echo %h | sed 's/^abc-ip-//'):22

# Proxy all ssh connections via the gateway machine
host !abc-gateway !abc-ip-* abc-*
ProxyCommand ssh abc-gateway -W %h:22

# Settings for all abc machines - my username & private key
host abc-*
user mark.zealey
IdentityFile ~/.ssh/abc-corp

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