urlopen Returning Redirect Error for Valid Links

I'm building a broken link checker in python, and it's becoming a chore building the logic for correctly identifying links that do not resolve when visited with a browser. I've found a set of links where I can consistently reproduce a redirect error with my scraper, but which resolve perfectly when visited in a browser. I was hoping I could find some insight here.

import urllib
import urllib.request
import html.parser
import requests
from requests.exceptions import HTTPError
from socket import error as SocketError

try:
    req=urllib.request.Request(url, None, {'User-Agent': 'Mozilla/5.0 (X11; Linux i686; G518Rco3Yp0uLV40Lcc9hAzC1BOROTJADjicLjOmlr4=) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/44.0.2403.157 Safari/537.36','Accept': 'text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8','Accept-Charset': 'ISO-8859-1,utf-8;q=0.7,*;q=0.3','Accept-Encoding': 'gzip, deflate, sdch','Accept-Language': 'en-US,en;q=0.8','Connection': 'keep-alive'})
    response = urllib.request.urlopen(req)
    raw_response = response.read().decode('utf8', errors='ignore')
    response.close()
except urllib.request.HTTPError as inst:
    output = format(inst)


print(output)

In this instance, an example of a URL that reliably returns this error is 'http://forums.hostgator.com/want-see-your-sites-dns-propagating-t48838.html'. It resolves perfectly when visited but the code above will return the following error:

HTTP Error 301: The HTTP server returned a redirect error that would lead to an infinite loop.
The last 30x error message was:
Moved Permanently

Any ideas how I can correctly identify these links as functional without blindly ignoring links from that site (which might miss genuinely broken links)?

You get the infinite loop error because the page you want to scrape uses cookies and redirects when the cookie isn't sent by the client. You'll get the same error with most other scraper tools and also with browsers when you disallow cookies.

You need a http.cookiejar.CookieJar and a urllib.request.HTTPCookieProcessor to avoid the redirect loop:

import urllib
import urllib.request
import html.parser
import requests
from requests.exceptions import HTTPError
from socket import error as SocketError
from http.cookiejar import CookieJar

try:
    req=urllib.request.Request(url, None, {'User-Agent': 'Mozilla/5.0 (X11; Linux i686; G518Rco3Yp0uLV40Lcc9hAzC1BOROTJADjicLjOmlr4=) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/44.0.2403.157 Safari/537.36','Accept': 'text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8','Accept-Charset': 'ISO-8859-1,utf-8;q=0.7,*;q=0.3','Accept-Encoding': 'gzip, deflate, sdch','Accept-Language': 'en-US,en;q=0.8','Connection': 'keep-alive'})
    cj = CookieJar()
    opener = urllib.request.build_opener(urllib.request.HTTPCookieProcessor(cj))
    response = opener.open(req)
    raw_response = response.read().decode('utf8', errors='ignore')
    response.close()
except urllib.request.HTTPError as inst:
    output = format(inst)
    print(output)