How to fix nginx throws 400 bad request headers on any header testing tools

Question

I have my site which is using nginx  and testing site with header testing tools e g  http   www webconfs com http-header-check php but every time it says 400 bad request below is the out put from the tool  Though all my pages load perfectly fine in browser and when I see in chrome console it says  status code 200OK   HTTP 1 1 400 Bad Request   gt   Server   gt  nginx Date   gt  Fri  07 Sep 2012 09 40 09 GMT Content-Type   gt  text html Content-Length   gt  166 Connection   gt  close   I really don t understand what is the problem with my server config   A bit of googling suggests to increase the buffer size using  and I increased it to following   large client header buffers 4 16k    The same results persist    Can some one guide me to the right direction

User · Answer

normally   Maxim Donnie s method can find the reason  But I encountered one 400 bad request will not log to err log  I found the reason with the help with tcpdump

User · Answer

Yes changing the error to debug level as Emmanuel Joubaud suggested worked out  edit  etc nginx sites-enabled default            error log  var log nginx error log debug   Then after restaring nginx I got in the error log with my Python application using uwsgi          2017 02 08 22 32 24  debug  1322 1322   1 connect to unix    run uwsgi app socket  fd 20  2         2017 02 08 22 32 24  debug  1322 1322   1 connected         2017 02 08 22 32 24  debug  1322 1322   1 http upstream connect  0         2017 02 08 22 32 24  debug  1322 1322   1 posix memalign  0000560E1F25A2A0 128  16         2017 02 08 22 32 24  debug  1322 1322   1 http upstream send request         2017 02 08 22 32 24  debug  1322 1322   1 http upstream send request body         2017 02 08 22 32 24  debug  1322 1322   1 chain writer buf fl 0 s 454         2017 02 08 22 32 24  debug  1322 1322   1 chain writer in  0000560E1F2A0928         2017 02 08 22 32 24  debug  1322 1322   1 writev  454 of 454         2017 02 08 22 32 24  debug  1322 1322   1 chain writer out  0000000000000000         2017 02 08 22 32 24  debug  1322 1322   1 event timer add  20  60000 1486593204249         2017 02 08 22 32 24  debug  1322 1322   1 http finalize request  -4   quot    quot  a 1  c 2         2017 02 08 22 32 24  debug  1322 1322   1 http request count 2 blk 0         2017 02 08 22 32 24  debug  1322 1322   1 post event 0000560E1F2E5DE0         2017 02 08 22 32 24  debug  1322 1322   1 post event 0000560E1F2E5E40         2017 02 08 22 32 24  debug  1322 1322   1 delete posted event 0000560E1F2E5DE0         2017 02 08 22 32 24  debug  1322 1322   1 http run request   quot    quot          2017 02 08 22 32 24  debug  1322 1322   1 http upstream check client  write event 1   quot   quot          2017 02 08 22 32 24  debug  1322 1322   1 http upstream recv    -1  11  Resource temporarily unavailable   Then I took a look to my uwsgi log and found out that          Invalid HTTP HOST header   www mysite local   You may need to add u www mysite local  to ALLOWED HOSTS           pid  10903 app  0 req  2 4  192 168 221 2     38 vars in 450 bytes   Wed Feb  8 22 32 24 2017  GET     gt  generated 54098 bytes in 55 msecs  HTTP 1 1 400  4 headers in 135 bytes  1 switches on core 0   And adding www mysite local to the settings py ALLOWED HOSTS fixed the issue            ALLOWED HOSTS     www mysite local

User · Answer

Just to clearify  in  etc nginx nginx conf  you can put at the beginning of the file the line  error log  var log nginx error log debug    And then restart nginx   sudo service nginx restart   That way you can detail what nginx is doing and why it is returning the status code 400

User · Answer

When nginx returns 400 bad request  it will log the reason into error log  at  info  level and take a look into error log when testing

User · Answer

As stated by Maxim Dounin in the comments above      When nginx returns 400  Bad Request  it will log the reason into error   log  at  info  level  Hence an obvious way to find out what s going on   is to configure   error log   to log messages at  info  level and take a look into error log when   testing

User · Answer

A cause can be invalid encoding in the URL request   Such as   being passed un-encoded

User · Answer

I had the same issue and tried everything  This 400 happened for an upstream proxy  Debug logged showed absolutely nothing  The problem was in duplicate proxy set header Host  http host directive  which I didn t notice initially  Removing duplicate one solved the issue immediately  I wish nginx was saying something other than 400 in this scenario  as nginx -t didn t complain at all  P S  this happened while migrating from older nginx 1 10 to the newer 1 19  Before it was tolerated apparently

[nginx] How to fix nginx throws 400 bad request headers on any header testing tools?

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