unless they use a computer from the 80's, there's no reason a large hosts file should slow down programs that bad.

yeah. this is a bug.

TLDR: looks like you're right, although Chrome shouldn't be struggling with that amount of hosts to chug through. This ended up being an interesting rabbit hole.

My home network already uses unbound with proper blocklist configured, but I can't use the same setup directly with my work computer as the VPN sets it's own DNS. I can only override this with a local resolver on the work laptop, and I'd really like to get by with just systemd-resolved instead of having to add dnsmasq or similar for this. None of the other tools I use struggle with this setup, as they use the system IP stack.

Might well be that chromium has a bit more sophisticated a network stack (than just using the system provided libraries), and I remember the docs indicating something about that being the case. In any way, it's not like the code is (or should be) paging through the whole file every time there's a query – either it forwards it to another resolver, or does it locally, but in any case there will be a cache. That cache will then end up being those queried domains in order of access, after which having a long /etc/hosts won't matter. Worst case scenario after paging in the hosts file initially is 3-5 ms (per query) for comparing through the 100k-700k lines before hitting a wall, and that only needs to happen once regardless of where the actual resolving takes place. At a glance chrome net stack should cache queries into the hosts file as well. So at the very least it doesn't really make sense for it to struggle for 5-10 seconds on every consecutive refresh of the page with a warm DNS cache in memory...

...or that's how it should happen. Your comment inspired me to test it a bit more, and lo: after trying out a hosts file with 10 000 000 bogus entries chrome was brought completely to it's knees. However, that amount of string comparisons is absolutely nothing in practice – Python with its measly linked lists and slow interpreter manages comparing against every row in 300 ms, a crude C implementation manages it in 23 ms (approx. 2 ms with 1 million rows, both a lot more than what I have appended to the hosts file). So the file being long should have nothing to do with it unless there's something very wrong with the implementation. Comparing against /etc/hosts should be cheap as it doesn't support wildcard entires – as such the comparisons are just simple 1:1 check against first matching row. I'll continue investigating and see if there's a quick change to be made in how the hosts are read in. Fixing this shouldn't cause any issues for other use cases from what I see.

For reference, if you want to check the performance for 10 million comparisons on your own hardware:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/time.h>


int main(void) {
	struct timeval start_t;
	struct timeval end_t;

	char **strs = malloc(sizeof(char *) * 10000000);
	for (int i = 0; i < 10000000; i++) {
		char *urlbuf = malloc(sizeof(char) * 50);
		sprintf(urlbuf, "%d.bogus.local", i);
		strs[i] = urlbuf;
	}

	printf("Checking comparisons through array of 10M strings.\n");
	gettimeofday(&start_t, NULL);

	for (int i = 0; i < 10000000; i++) {
		strcmp(strs[i], "test.url.local");
	}

	gettimeofday(&end_t, NULL);

	long duration = (end_t.tv_usec - start_t.tv_usec) / 1000;
	printf("Spent %ld ms on the operation.\n", duration);

	for (int i = 0; i < 10000000; i++) {
		free(strs[i]);
	}
	free(strs);
}