pathway mtu discovery bash script (gist.github.com)

submitted 3 months ago by [email protected] to c/[email protected]

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I wrote pmtud.sh this weekend to get the MTU of a link (or tunnel) quickly, reliably, and without sending a ton of pings. It can do so in about 11 pings per target.

I had fun writing the entire thing in sh, tried making it "POSIX-compliant," but probably failed somewhere since POSIX isn't worded well. The script only needs dig and ping and uses shell built-ins for everything else. It performs a recursive binary search on the MTU search domain and asynchronously queues pings for later.

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[-] [email protected] 3 points 3 months ago* (last edited 3 months ago)

Some stylistic notes

The head -1 syntax is obsolete in POSIX, replaced by head -n 1.
for filename in $(ls) is inefficient and will break on whitespace. You can use shell globbing for this instead, e.g., for filename in *.
A lot of the variables seem to be inconsistently quoted. I recommend running ShellCheck on it, as this can be dangerous (if you set TMPDIR to /tmp/ dir, I believe it will remove /tmp and dir)

[-] [email protected] 2 points 3 months ago

Saved my bacon with this comment. Fixed a ton of (seemingly) innocuous errors.

[-] [email protected] 1 points 2 months ago

100% use shellcheck! Saves so much time. Even better: https://github.com/bash-lsp/bash-language-server

this post was submitted on 30 Apr 2025

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