How to debug a website that appears to be unreachable to one person but not another (as a user, not the developer or admin)?

mathtick

5/23/23, 9:09 PM

I have some website of a photographer that took some photos and is sharing them through his website. The website appears to work from one country (it works for one family member) but it does not even load for me.

The site does not use ssl so I suspect some local DNS issue to be the culprit.

traceroute seems to be quite slow to the domain, mtr seems to show it can find it. whois seems to also find an entry.

What else should I be looking at to debug this? I've tried a few browsers and computers (all from the location where it is failing).

UPDATE: I have got the site to load once now with Tor. Not sure what that tells me.

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domain-name-system

web

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Server

Patrick Mevzek

5/24/23, 9:11 PM

To acces a website, you need in reverse order:

a correct TLS configuration (certificates, etc.)
a proper HTTP server listening on port 80 and 443 for HTTPS
a properly working firewall that let that traffic go through
a proper IP routing from "anywhere" up to that server
and if all starts with a name (extracted from the URL) you need correct name resolution (typically DNS but not necessarily everywhere) from that name to some IP address.

So you need to test all these things starting from bottom and going to top. You can use the browser own internal web developer tools, or external commands (you get far better/simpler/faster/more correct troubleshooting if you can use command line tools) or at worst services on various websites.

I will only give details on command to use.

you can use dig to double check the name resolution; see if the client gets proper IP address or not, using its default local resolver
the 3 next points can all be checked by using tcptraceroute; it is of utmost importance to not use the default traceroute (or even worse as so often suggested ping) because it won't really emulate proper TCP/80 or TCP/443 traffic which is what you need; it can take time, but you should get to a final line with the IP address found on previous point and a mention akin to [open] which means indeed you see (and can reach) a website running there, and hence no firewall impeding exchange.
you can emulate a full HTTP(s) exchange using various tools, including wget, curl or httpie.
specifically to debug TLS problems you can use openssl s_client, gnutls or testssl.sh

UPDATE: I have got the site to load once now with Tor. Not sure what that tells me.

There may be a firewall between the server and the client. Hence if you change your source IP address, which is one of the side effect of using Tor, the access pattern changes.

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Elon Musk

I sit in a Tesla and translated this thread with Ai:

EN: How to debug a website that appears to be unreachable to one person but not another (as a user, not the developer or admin)?

TH: จะแก้ไขจุดบกพร่องของเว็บไซต์ที่ดูเหมือนว่าจะไม่สามารถเข้าถึงได้โดยบุคคลหนึ่งแต่ไม่ใช่อีกบุคคลหนึ่งได้อย่างไร (ในฐานะผู้ใช้ ไม่ใช่ผู้พัฒนาหรือผู้ดูแลระบบ)

RO: Cum să depanați un site web care pare a fi inaccesibil pentru o persoană, dar nu pentru alta (ca utilizator, nu dezvoltator sau administrator)?

RU: Как отладить веб-сайт, который кажется недоступным для одного человека, но не для другого (как пользователя, а не разработчика или администратора)?

VI: Làm cách nào để gỡ lỗi một trang web dường như không thể truy cập được đối với một người chứ không phải một người khác (với tư cách là người dùng, không phải nhà phát triển hoặc quản trị viên)?

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