chttl

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chttl — Temporarily override the system's default TTL.

Make it a function

This was originally written as a bash function. It was changed early on to sh for portability reasons but nevertheless it was wrapped in a single unit to make it easy to change it back. Simply remove the first and last lines, then add the remaining code to your bash functions file(s).

What does it do?

It's a simple wrapper for the command necessary to change the TTL of network packets, which you might need to do when tethering Internet access from the cellular networks of some carriers.

Wouldn't an alias be shorter to do?

Yes, but the command changes across operative systems. You'd need an alias for each, which is a bit more complicated to sync unless you add some logic to your aliases file, but wouldn't that be just about the same? bash config files, whatever.

Is there something else?

- This is the first [intentionaly] POSIX-compliant script we've ever written. - It works on macOS, Linux and FreeBSD. - It can be used to set the value back to where it was as well, simply add it at the end. - It's not a permanent modification.

Depending on the OS, you might need to create a file in a specific location or issue a special command that sets preference files.

Syntax

The script takes one or no arguments; any additional argument(s) after the first one should be ignored. When it's run without arguments, it defaults to setting a TTL of 65. e.g;

chttl

If you add any value other than -h, –help or -? it will attempt to set that value as the TTL. There's no validation of the input, but naturally if it's not a numerical value within bounds, it will fail e.g;

Reset to the common value:

chttl 64

or change it to a TTL of 128:

chttl 128

or to 1:

chttl 1

or just add the value anyway:

chttl 65

If the argument does match -h, –help or -?, it'll show a quick short guide and exit e.g;

chttl -h

If you add both types of arguments, it will only consider the first one, thus:

chttl -h 22

should show the quick guide and:

chttl 22 -h

should set the TTL to 22.

Code download

chttl

#!/usr/bin/env sh
chttl() {
	chttlhelp(){
		cat <<- _help
chttl — Temporarily override the system's default TTL.
Copyright (C) 2025  Gustavo Domínguez  (GPLv3)
# $0                Set TTL to 65.
# $0 n              Set TTL to n.
# $0 -h|--help|-?   Print this message.
 
Superuser(root)-level privileges are required.
		_help
	}
	chttldo(){
		if [ "$(uname)" = Darwin ]; then
			sysctl net.inet.ip.ttl="${1:-65}"
		elif [ "$(uname)" = Linux ] ; then
			sysctl -w net.ipv4.ip_default_ttl="${1:-65}"
		elif [ "$(uname)" = FreeBSD ] ; then
			sysctl net.inet.ip.ttl="${1:-65}"
		elif uname | grep -i "bsd"; then
			#sysctl net.inet.ip.ttl=65
			echo "Run 'sysctl net.inet.ip.ttl=65' if you think it's safe"
			echo "for this BSD version (only FreeBSD has been tested.)"
		fi
	}
	case "$1" in
		-h|--help|-?) chttlhelp ;;
		*) chttldo "$1" ;;
	esac
}
chttl "$1"

On Linux systems, you can drop it in the bin subdirectory of your home folder (~/bin or $HOME/bin) to make it available anywhere on the CLI. Don't forget to make it executable first (e.g; chmod +x $HOME/bin/chttl).

A note about privileges

This a task that requires superadmin privileges. The script does not attempt to elevate your permissions at any time; you will need to do that yourself by prepending sudo, being logged in as root, or however you wish. That is your call.

Anti-corporate takeover measure (licensing)

chttl — Temporarily override the system's default TTL.
Copyright (C) 2025 Gustavo Domínguez

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see https://www.gnu.org/licenses/