Intrisic Sshd Configuration Errors

This article delves into the configuration of SSH, specifically using chisel port forwarding for systemd services. It also provides tips on troubleshooting and enhancing connection stability with n2n. The main focus is on establishing access to all ports between local machines while dealing with limited internet port availability.

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if you want to use ssh port forwarding as systemd service, keep in mind that the default user for execution is root, and you need to use the public key of root to login.

or you can change the user executing the task in service config:

[System]
User=xxx

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chisel can be used for port forwarding by http compared with wstunnel, able to survive nginx (still need to configure websocket upgrades).

# server, allowing reverse port forwarding
chisel server -p <port> --auth <user>:<pass> --reverse
# client
chisel client --auth <user>:<pass> <protocol>://<url> <local_addr>:<remote_addr> R:<remote_addr>:<local_addr>

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if you want to have multiple host sharing same ip because of proxy forwarding or different network locations, then you need to change the system host mapping file.

in linux and macos it is at /etc/hosts

in windows, C:\Windows\System32\drivers\etc\hosts

you need to configure the host file on the proxy machine if you want to avoid name clashes with proxies. these host names can be less informative to hide the intent.

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on latest ubuntu 24.04 the sshd config includes files under /etc/ssh/sshd_config.d which has a file named 50-cloud-init.conf has the line overriding any other setting afterwords.

PasswordAuthentication yes

you need to change both /etc/ssh/sshd_config and this file to disable password authentication.

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-R will not allow you to open 0.0.0.0 port on remote machine unless you configure something in /etc/ssh/sshd_config like below.

AllowTcpForwarding yes
GatewayPorts clientspecified

if not, use socat to finally deliver the forwarded remote local port to remote public port.

socat TCP-LISTEN:<lport>,reuseaddr,fork TCP:<rhost>:<rport>

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port forwarding failure can be corrected.

# get the process pid of the port
sudo lsof -i :<port>
lsof -i :<port>
# kill the process
kill <port>
# rerun lsof to check if the port is freed

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n2n can be in handy if you do not have too many ports on internet and still want to access all ports in between your local machines.

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if connection is unstable, use -o ServerAliveInterval=60 -o ServerAliveCountMax=3 to extend the timeout period.