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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Some stylistic notes
- The
head -1syntax is obsolete in POSIX, replaced byhead -n 1.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/tmpanddir)
Saved my bacon with this comment. Fixed a ton of (seemingly) innocuous errors.
- The