GNULinux network configuration notes

Basic status

Check interfaces

WARNING: ifconfig is deprecated!

checking status of network interfaces using ip tool:

$ ip  address
$ ip  link

checking status of network interfaces using networkctl tool:

$ networkctl  status  IFNAME

checking status of ntework using systemd tool:

$ sudo  service  systemd-networkd  status

Check detailed capabilities

checking detailed physical capabilities of network interface using ethtool:

$ sudo  ethtool  IFNAME

NOTE: ethtool uses set of ioctl's (such as SIOCETHTOOL for media physical speed) to get additional information which can't be provided by any other default tools.

Check network ports

list open tcp ports using ss: $ sudo ss -ntlp
list open tcp ports using netstat (DEPRECATED): $ sudo netstat -ntlp
- -n / --numeric : show IP address instead of resolved host name
- -t / --tcp : show TCP ports only
- -l / --listening : show listening / open ports only
- -p / --programs : show PID / app name

Configuration

Configuration files and directories related to network settings

/etc/netplan/01-network-manager-all.yaml
/etc/udev/rules.d/
/var/lib/udev/

Static canonical predictable names of network interfaces

add net.ifnames=0 biosdevname=0 options to GRUB_CMDLINE_LINUX_DEFAULT variable inside /etc/default/grub configuration file
optionally, to name interface based on MAC address, add udev rule like:

SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*", ATTR{address}=="__:__:__:__:__:__", ATTR{dev_id}=="0x0", ATTR{type}=="1", KERNEL=="eth*", NAME="em0"

Listening traffic

Wire

To listen traffic (aka "sniffing"), these two steps should be performed:

set packet forwarding:

$ echo  1  |  sudo  tee  /proc/sys/net/ipv4/ip_forward

set promisc mode for network interface:

$ sudo  ip  link  set  IFNAME  promisc  on

run tcpdump to capture packets fully:

$ sudo  tcpdump  -i IFNAME  -nn  -XX  -vvv  -s0  -S  [-e FILTER RULE(S)]

iptables

open port(s)

$ sudo  iptables  -I INPUT   -p ${PROTO}  --dport ${PORT}  -j ACCEPT
$ sudo  iptables  -I OUTPUT  -p ${PROTO}  --sport ${PORT}  -j ACCEPT

PROTO - protocol: tcp or udp
PORT - port number

SSH

Client common options

Basic

-i : /path/to/private/key
-C : enable data compression
-f : run in background
-N : do not execute [remote shell] command
-n : disable reading from STDIN
-q : enable quite output and show only fatal errors
-T : disable pseudo-terminal allocation
-D : [bind_host:]port

Extended

-o HostKeyAlgorithms= : opts - use the host key algorithms that the client wants to use in order of preference (usessh -Q key` to check available opts)¹
-o PubkeyAuthentication= : yes/no - use keys' pair authentication method
-o PasswordAuthentication= : yes/no - use password authentication method
-o StrictHostKeyChecking= : yes/no - use interactive request (yes) or just add host to known_hosts (no)
-o IdentityFile= : priv key - use file as private key to connect (same as -i)
-o UserKnownHostsFile= : hosts - use file as known hosts fingerprint (set as /dev/null to skip the error)²
-o ServerAliveInterval= : N - use N seconds keep-alive "heart beat"
-o ConnectTimeout= : N - use N seconds as timeout during connection

Permissions

Permissions for files in ~/.ssh directory³:

drwxr-xr-x / 755  ..
drwx------ / 700  .
-rw------- / 600  config
-rw------- / 600  authorized_keys
-rw------- / 600  known_hosts
-rw------- / 600  *.priv
-rw-r--r-- / 644  *.pub

Helpers

ssh-keygen

$ ssh-keygen [-H] -F example.com

locally detect fingerprint line for example.com host in ~/.ssh/known_hosts config file and print it (if any)
-H: hide the line number info

`$ ssh-keygen -R exampe.com

remove fingerprint line for exampe.com host from ~/.ssh/known_hosts config file (if any)

ssh-keyscan

$ ssh-keyscan [-H] example.com

remotely scan example.com host for fingerprint line and print it (if any)
-H: hash host name

Direct SOCKS proxy

pick port number for proxy and set it:

$ PORT=1234

set packet forwarding:

$ echo  1  |  sudo  tee  /proc/sys/net/ipv4/ip_forward

optionally, set iptables rules for IP client address only:

$ sudo  iptables  -A INPUT  --src ${IP}  -p tcp  --dport ${PORT}  -j ACCEPT
$ sudo  iptables  -A INPUT               -p tcp  --dport ${PORT}  -j REJECT

run ssh session:

$ ssh  -N  -D  0.0.0.0:${PORT}  localhost

optionally, to get IP address of a server:

$ curl  ifconfig.me

set web proxy setting in browser/environment on client IP as IP/HOSTNAME:PORT

Reverse SOCKS proxy

pick port number for route and set it:

$ PORT_ROUTE=1234

pick port number for proxy and set it:

$ PORT_PROXY=5678

run ssh session on NAT gateway host:

$ ssh  -R  ${PORT_ROUTE}:localhost:22  user@example.com

run ssh session on example host:

$ ssh  -p ${PORT_ROUTE}  localhost  -D  ${PORT_PROXY}

test the connection from example host:

$ curl  http://portquiz.net
Port 80 test successful!
Your IP: <IP address of example.com>

$ curl  --socks5 localhost:${PORT_PROXY}  http://portquiz.net
Port 80 test successful!
Your IP: <IP address of NAT gateway>

Reverse SSH tunnel

pick port number for tunnel and set it:

$ PORT_TUNNEL=4321

run ssh session on NAT gateway host:

$ ssh  -N  -R  ${PORT_TUNNEL}:localhost:22  user@example.com  -p 22

run ssh client connecting back to NAT gateway host from example host:

$ ssh  client@localhost  -p ${PORT_TUNNEL}

Port only access

DRAFT

add user: $ sudo useradd -m mysshuser
set password
populate .ssh & set permissions
add section to sshd:

AllowUsers mysshuser

Match User mysshuser
	AllowTcpForwarding yes
	PermitTunnel yes
	GatewayPorts yes
	AllowAgentForwarding no
	PermitOpen localhost:8080
	ForceCommand echo 'No no no!'

restart sshd and test local ssh login using keys: $ sudo service ssh restart ; ssh mysshuser@localhost -i /home/mysshuser/.ssh/id_rsa
edit etc/passwd to set bin/false

mysshuser:...:/bin/false

edit etc/shadow to disable password

mysshuser:!:1...`

2FA TOTP authentication

install required package: $ sudo apt install libpam-google-authenticator
configure authentication setup:
- run: $ google-authenticator
- make tokens to be time-based? y
- scan QR code using TOTP app like Google Authenticator or FreeOTP
- enter TOTP code from app to terminal when requested
- provide the answers to configure TOTP:
  - update your config file? y
  - disallow multiple uses? y
  - increase the login window? n
  - enable login rate-limit? y
configure sshd for TOTP setup:
- /etc/ssh/sshd_config: add/enable UsePAM yes and ChallengeResponseAuthentication yes
- /etc/pam.d/sshd: add/enable auth required pam_google_authenticator.so right after @include common-auth block
- optionally: add nullok option for the line auth required pam_google_authenticator.so to disable TOTP for users without TOTP configured
now this configuration will be working in the following way:
- if login user uses valid & authorized ssh key file, then sshd will accept connection unconditionaly
- if login user uses valid password, then sshd will ask for password first and for TOTP verification code right after password before accepting connection

Web

Network throughput test

As a quick'n'dirty solution:

on server:
- install & run any web server, i.e.: $ python3.8 -m http.server 8080 -d www/html
- go to its www/html directory
- run (tweak command for desirable size to test): $ sudo touch file.bin && sudo truncate -s 1GB file.bin
on client:
- run $ wget -v -O /dev/null https://server/file.bin or $ curl -o /dev/null http://server:8080/file.bin

PAC

Proxy Auto Configuration file is a special server-side file which allows redirecting client requests for particular hosts. To use this feature:

create & put sample file like inet.pac to www/html directory on server with this content:

function FindProxyForURL(url, host) {
	alert("url  = " + url);
	alert("host = " + host);
	if (
		shExpMatch(host, "*test.org*")  ||
		shExpMatch(host, "*example.com*")
	) {
		return "SOCKS host:port";
	} else {
		return "DIRECT";
	}
}

configure browser on client to use proxy autoconfiguration and provide the setting line as https://server/inet.pac

this option is useful when the error message like "Unable to neotiate with HOST port NUMBER: no matching host key type found. Their offer: [...]" displayed. ↩
set -o UserKnownHostsFile=/dev/null only if you're fully aware of what & why you're doing this exactly since this is VERY INSECURE. ↩
https://superuser.com/questions/215504/permissions-on-private-key-in-ssh-folder ↩

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GNULinux network configuration notes

Basic status

Check interfaces

Check detailed capabilities

Check network ports

Configuration

Configuration files and directories related to network settings

Static canonical predictable names of network interfaces

Listening traffic

Wire

iptables

open port(s)

SSH

Client common options

Basic

Extended

Permissions

Helpers

ssh-keygen

ssh-keyscan

Direct SOCKS proxy

Reverse SOCKS proxy

Reverse SSH tunnel

Port only access

2FA TOTP authentication

Web

Network throughput test

PAC

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GNULinux network configuration notes

Basic status

Check interfaces

Check detailed capabilities

Check network ports

Configuration

Configuration files and directories related to network settings

Static canonical predictable names of network interfaces

Listening traffic

Wire

iptables

open port(s)

SSH

Client common options

Basic

Extended

Permissions

Helpers

ssh-keygen

ssh-keyscan

Direct SOCKS proxy

Reverse SOCKS proxy

Reverse SSH tunnel

Port only access

2FA TOTP authentication

Web

Network throughput test

PAC

Footnotes