Package: emacs;
Reported by: Stefan Monnier <monnier <at> iro.umontreal.ca>
Date: Fri, 14 Oct 2022 16:22:01 UTC
Severity: normal
Message #17 received at 58520 <at> debbugs.gnu.org (full text, mbox):
From: Visuwesh <visuweshm <at> gmail.com> To: Stefan Monnier <monnier <at> iro.umontreal.ca> Cc: 58520 <at> debbugs.gnu.org, Paul Eggert <eggert <at> cs.ucla.edu> Subject: Re: bug#58520: Persistent failure to DNS-lookup hostname Date: Fri, 03 Nov 2023 13:27:21 +0530
[Tuesday September 05, 2023] Stefan Monnier wrote: > FWIW, I've seen this bug on a somewhat regular basis. I have not been > able to reproduce it: it seems to show up only after Emacs has been in > use for several days. > > I did confirm that calling `M-: (res-init) RET` does work around the > problem in the sense that the Emacs session can then successfully > perform DNS lookups again. > [ `res-init` is a little DEFUN I added which just calls `res_init`. ] > > So, as far as I can tell, the problem is that glibc or Emacs somehow get > into a state where the automatic decision to refresh the info about the > address of the DNS server doesn't work any more. > FWIW, I experience this sometimes when I change from an Ethernet connection to a WiFi connection (I cannot remember if the inverse is true too). This change to another connection once I wake my laptop from hibernation. During this change, /etc/resolv.conf definitely changes because I use resolvconf to set connection-specific and interface-specific DNS nameserver setting. [ I need to set the DNS server manually for the Ethernet connection but I can use dhcp for the Wifi one. ] Emacs throws me `443 Temporary failure in name resolution' even though I can open the same address in Firefox, or a fresh new Emacs instance just fine. In fact, I couldn't use gnus-read-ephemeral-emacs-bug-group from the failing Emacs instance but I can in this fresh one I opened just to send this mail. The other super weird part is that eww _can_ open debbugs.gnu.org just fine, but if I use Gnus to fetch the bug report it doesn't... More generally, eww seems to be able to open these webpages just fine but if I do a HTTP request to same address using url.el it fails with the aforementioned 443 error. I can sometimes stop this error once and for all by visiting the failing address in eww but clearly this time it hasn't helped (since Gnus still fails to make a connection to debbugs.gnu.org after visiting debbugs.gnu.org in eww). I am not sure how long this failing Emacs instance will fail but if it helps, I can try to do some debugging on my end as well. I tried to use tcpdump but the manpage seems to suggest that there is no way to filter network requests made by a particular process, and I see too many junk requests to isolate the traffic that matters. > Stefan > > > Stefan Monnier [2022-10-14 12:20:52] wrote: > >> Package: Emacs >> >> >> [ I see that my original email was sent to emacs-devel, but I think >> bug-gnu-emacs is a better place for that. ] >> >> My Gnus session (a separate Emacs session I use specifically to run >> Gnus) occasionally gets into a state where it insists that my mail >> server's DNS name isn't found. All(?) other processes on the machine >> keep happily resolving hostname, so the problem is specific to >> this process. >> >> The problem just reappeared today and thanks to the help I got last time >> I managed to diagnose it a bit better: >> >> - Lars asked if it only affect IMAP: nope, it affects more than just >> IMAP. Simple tests suggest it affects all DNS lookups performed by >> that Emacs process. >> - Madhu suggested the problem was related to commit 93bf7d52841c60ff and >> might be linked to a lack of call to `res_init`. For some stupid >> reason I wasn't able to add an ELisp primitive that lets me call >> `res_init` manually to verify this hypothesis, but I now figured what >> was my mistake, so I should be able to try `M-: (res-init) RET` next time >> to confirm that it works around the problem. >> - Robert's suggestion to use `tcpdump` showed that the reason this Emacs >> process gets DNS lookup failures is simply that it queries the DNS >> server at 192.168.1.1 which is the server I was using yesterday (and >> to which I currently don't have access) rather than the one I'm using >> now. IOW, it strongly suggests that the problem would be solved by >> calling `res_init`. >> >> [Glibc bug 984](https://sourceware.org/bugzilla/show_bug.cgi?id=984) >> seems relevant. According to this, calling `res_init` should not be >> necessary any more. Indeed, if I start Emacs, use >> `make-network-process`, then change `/etc/resolv.conf`, then call >> `make-network-process` again, tcpdump shows clearly that the Emacs >> session has noticed the change in `/etc/resolv.conf`. I tried this same >> test from a fresh new Gnus session, and that also works fine. >> >> So it's still a mystery why my Gnus session sometimes gets into a state >> where it apparently stops paying attention to changes in >> `/etc/resolv.conf`. The above bug 984 mentions that glibc's "auto >> reload" of `/etc/resolv.conf` is prevented in case the application has >> modified `_res` manually, but I can't see any place where we do that. >> Could it be that some of the libraries we link with can sometimes >> manually modify `_res`? >> >> >> Stefan
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