What is the /etc/hosts size limit?
Problematical effects include slow hostname resolution (unless the OS somehow converts the linear list into a faster-to-search structure?) and the potential for surprising interaction with shell tab completion well before any meaningful file size is reached.
For example! If one places 500,000 host entries in /etc/hosts
# perl -E 'for (1..500000) { say "127.0.0.10 $_.science" }' >> /etc/hosts
for science, the default hostname tab completion in ZSH takes about ~25 seconds on my system to return a completion prompt (granted, this is on a laptop from 2008 with a 5400 RPM disk, but still).
I don't think it has a size limit in terms of number of lines.
Before DNS (which was brought into use in 1985), it was this file that served as the only means for doing host name lookups, so I presume that this means that the file should be able to have thousands or at least hundreds of entries to be able to support the most well-connected pre-1985 Internet nodes.
Here's an example from 1985 (the format has changed somewhat): http://jim.rees.org/apollo-archive/hosts.txt This file has 1680 lines out of which 1325 are host lines. The remaining 355 lines are blank, comments, networks or gateways1.
The only real limit that I could find was that on some systems, the individual lines are limited to less than BUFSIZ characters (1024 on my OpenBSD machine).
If you have more than a couple of handfuls of entries in /etc/hosts, you should consider setting up a local name server instead, but that's my personal opinion.
1 Thanks to Jeff Schaller for digging this up.
How can I determine the size limit of /etc/hosts?
It is a regular file, so the limit would correspond to the underlying filesystem's limits (which would itself be capped by the number of disks behind it), minus the space used by any other files in the same (probably root (/)) filesystem:
- ext2/3: 2 TiB
- ext4: 16 TiB (with the default 4KiB block size)
- xfs: 500 TiB
How can I set the size limit of /etc/hosts?
Since it's a manually-edited file, only manually:
sed -i '100,$d' /etc/hosts
(to remove lines 100 and beyond).