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POST request to TRIAS API does not work with requests

I maintain an API client for the TRIAS API (German Link) to retrieve local public transport information for various states / cities in Germany. Recently one of the TRIAS servers (Baden-Württemberg) started responding with an error message to requests.

When I try to send the request via curl, the server responds just fine:

$ curl -vH "Content-Type: text/xml; charset=utf-8" -d@trias-req.xml http://www.efa-bw.de/trias
*   Trying 94.186.213.206:80...
* Connected to www.efa-bw.de (94.186.213.206) port 80 (#0)
> POST /trias HTTP/1.1
> Host: www.efa-bw.de
> User-Agent: curl/7.83.1
> Accept: */*
> Content-Type: text/xml; charset=utf-8
> Content-Length: 652
> 
* Mark bundle as not supporting multiuse
< HTTP/1.1 200 OK
< Date: Tue, 31 May 2022 09:35:35 GMT
< Server: EFAController/10.4.25.9/BW-WW33
< Access-Control-Allow-Origin: *
< Access-Control-Allow-Headers: Authorization, Content-Type
< Access-Control-Allow-Methods: GET
< Access-Control-Expose-Headers: Content-Security-Policy, Location
< Access-Control-Max-Age: 600
< Content-Type: text/xml
< Expires: Thu, 01 Jan 1970 00:00:00 GMT
< Accept-Ranges: none
< Content-Length: 1520
< Last-Modified: Tue, 31 May 2022 09:35:35 GMT
< Set-Cookie: ServerID=bw-ww33;Path=/
< 
<?xml version="1.0" encoding="UTF-8"?>
<trias:Trias xmlns:siri="http://www.siri.org.uk/siri" xmlns:trias="http://www.vdv.de/trias" xmlns:acsb="http://www.ifopt.org.uk/acsb" xmlns:ifopt="http://www.ifopt.org.uk/ifopt" xmlns:datex2="http://datex2.eu/schema/1_0/1_0" version="1.1"><trias:ServiceDelivery><siri:ResponseTimestamp>2022-05-31T09:35:35Z</siri:ResponseTimestamp><siri:ProducerRef>de:nvbw</siri:ProducerRef><siri:Status>true</siri:Status><trias:Language>de</trias:Language><trias:CalcTime>30</trias:CalcTime><trias:DeliveryPayload><trias:LocationInformationResponse><trias:Location><trias:Location><trias:Address><trias:AddressCode>streetID:1500001248::8222000:-1:T 1:Mannheim:T 1::T 1: 68161:ANY:DIVA_STREET:942862:5641376:MRCV:B_W:0</trias:AddressCode><trias:AddressName><trias:Text>Mannheim, T 1</trias:Text><trias:Language>de</trias:Language></trias:AddressName><trias:PostalCode> 68161</trias:PostalCode><trias:LocalityName>Mannheim</trias:LocalityName><trias:LocalityRef>8222000:-1</trias:LocalityRef><trias:StreetName>T 1</trias:StreetName></trias:A* Connection #0 to host www.efa-bw.de left intact
ddress><trias:LocationName><trias:Text>Mannheim</trias:Text><trias:Language>de</trias:Language></trias:LocationName><trias:GeoPosition><trias:Longitude>8.46987</trias:Longitude><trias:Latitude>49.49121</trias:Latitude></trias:GeoPosition></trias:Location><trias:Complete>true</trias:Complete><trias:Probability>0.763999999</trias:Probability></trias:Location></trias:LocationInformationResponse></trias:DeliveryPayload></trias:ServiceDelivery></trias:Trias>

However, it fails with requests.post(), while it works with urllib.request.urlopen():

$ python efabw.py
502 <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<html><head>
<title>502 Proxy Error</title>
</head><body>
<h1>Proxy Error</h1>
<p>The proxy server received an invalid
response from an upstream server.<br />
The proxy server could not handle the request<p>Reason: <strong>Error reading from remote server</strong></p></p>
</body></html>
 653
200 b'<?xml version="1.0" encoding="UTF-8"?>n<trias:Trias xmlns:siri="http://www.siri.org.uk/siri" xmlns:trias="http://www.vdv.de/trias" xmlns:acsb="http://www.ifopt.org.uk/acsb" xmlns:ifopt="http://www.ifopt.org.uk/ifopt" xmlns:datex2="http://datex2.eu/schema/1_0/1_0" version="1.1"><trias:ServiceDelivery><siri:ResponseTimestamp>2022-05-31T09:37:02Z</siri:ResponseTimestamp><siri:ProducerRef>de:nvbw</siri:ProducerRef><siri:Status>true</siri:Status><trias:Language>de</trias:Language><trias:CalcTime>30</trias:CalcTime><trias:DeliveryPayload><trias:LocationInformationResponse><trias:Location><trias:Location><trias:Address><trias:AddressCode>streetID:1500001248::8222000:-1:T 1:Mannheim:T 1::T 1: 68161:ANY:DIVA_STREET:942862:5641376:MRCV:B_W:0</trias:AddressCode><trias:AddressName><trias:Text>Mannheim, T 1</trias:Text><trias:Language>de</trias:Language></trias:AddressName><trias:PostalCode> 68161</trias:PostalCode><trias:LocalityName>Mannheim</trias:LocalityName><trias:LocalityRef>8222000:-1</trias:LocalityRef><trias:StreetName>T 1</trias:StreetName></trias:Address><trias:LocationName><trias:Text>Mannheim</trias:Text><trias:Language>de</trias:Language></trias:LocationName><trias:GeoPosition><trias:Longitude>8.46987</trias:Longitude><trias:Latitude>49.49121</trias:Latitude></trias:GeoPosition></trias:Location><trias:Complete>true</trias:Complete><trias:Probability>0.763999999</trias:Probability></trias:Location></trias:LocationInformationResponse></trias:DeliveryPayload></trias:ServiceDelivery></trias:Trias>'

The respective code is:

#! /usr/bin/env python3

from urllib.request import Request, urlopen

from requests import post


URL = 'http://www.efa-bw.de/trias'
HEADERS = {'Content-Type': 'text/xml'}


def main():

    with open('trias-req.xml', 'rb') as file:
        xml = file.read()

    response = post(URL, data=xml, headers=HEADERS)
    print(response.status_code, response.text, len(xml))

    request = Request(URL, data=xml, headers=HEADERS)

    with urlopen(request) as response:
        print(response.status, response.read())


if __name__ == '__main__':
    main()

Why does the request fail with requests.post() only? What can I do to debug this further? Other API servers respond fine to the request with requests.post()

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Answer

It turned out to be the user agent. After inspecting the headers with tcpdump, I found, that the request fails with user agent python-requests/2.27.1 but succeeds with Python-urllib/3.10 and curl/7.83.1:

#! /usr/bin/env python3

from urllib.request import Request, urlopen

from requests import post


URL = 'http://www.efa-bw.de/trias'
HEADERS_REQUESTS = {'Content-Type': 'text/xml', 'User-Agent': 'Python-urllib/3.10'}
HEADERS_URLLIB = {'Content-Type': 'text/xml'}


def main():

    with open('trias-req.xml', 'rb') as file:
        xml = file.read()

    response = post(URL, data=xml, headers=HEADERS_REQUESTS)
    print(response.status_code, response.text, len(xml))

    request = Request(URL, data=xml, headers=HEADERS_URLLIB)

    with urlopen(request) as response:
        print(response.status, response.read())


if __name__ == '__main__':
    main()

$ python efabw.py
200 <?xml version="1.0" encoding="UTF-8"?>
<trias:Trias xmlns:siri="http://www.siri.org.uk/siri" xmlns:trias="http://www.vdv.de/trias" xmlns:acsb="http://www.ifopt.org.uk/acsb" xmlns:ifopt="http://www.ifopt.org.uk/ifopt" xmlns:datex2="http://datex2.eu/schema/1_0/1_0" version="1.1"><trias:ServiceDelivery><siri:ResponseTimestamp>2022-05-31T10:09:06Z</siri:ResponseTimestamp><siri:ProducerRef>de:nvbw</siri:ProducerRef><siri:Status>true</siri:Status><trias:Language>de</trias:Language><trias:CalcTime>45</trias:CalcTime><trias:DeliveryPayload><trias:LocationInformationResponse><trias:Location><trias:Location><trias:Address><trias:AddressCode>streetID:1500001248::8222000:-1:T 1:Mannheim:T 1::T 1: 68161:ANY:DIVA_STREET:942862:5641376:MRCV:B_W:0</trias:AddressCode><trias:AddressName><trias:Text>Mannheim, T 1</trias:Text><trias:Language>de</trias:Language></trias:AddressName><trias:PostalCode> 68161</trias:PostalCode><trias:LocalityName>Mannheim</trias:LocalityName><trias:LocalityRef>8222000:-1</trias:LocalityRef><trias:StreetName>T 1</trias:StreetName></trias:Address><trias:LocationName><trias:Text>Mannheim</trias:Text><trias:Language>de</trias:Language></trias:LocationName><trias:GeoPosition><trias:Longitude>8.46987</trias:Longitude><trias:Latitude>49.49121</trias:Latitude></trias:GeoPosition></trias:Location><trias:Complete>true</trias:Complete><trias:Probability>0.763999999</trias:Probability></trias:Location></trias:LocationInformationResponse></trias:DeliveryPayload></trias:ServiceDelivery></trias:Trias> 653
200 b'<?xml version="1.0" encoding="UTF-8"?>n<trias:Trias xmlns:siri="http://www.siri.org.uk/siri" xmlns:trias="http://www.vdv.de/trias" xmlns:acsb="http://www.ifopt.org.uk/acsb" xmlns:ifopt="http://www.ifopt.org.uk/ifopt" xmlns:datex2="http://datex2.eu/schema/1_0/1_0" version="1.1"><trias:ServiceDelivery><siri:ResponseTimestamp>2022-05-31T10:09:06Z</siri:ResponseTimestamp><siri:ProducerRef>de:nvbw</siri:ProducerRef><siri:Status>true</siri:Status><trias:Language>de</trias:Language><trias:CalcTime>19</trias:CalcTime><trias:DeliveryPayload><trias:LocationInformationResponse><trias:Location><trias:Location><trias:Address><trias:AddressCode>streetID:1500001248::8222000:-1:T 1:Mannheim:T 1::T 1: 68161:ANY:DIVA_STREET:942862:5641376:MRCV:B_W:0</trias:AddressCode><trias:AddressName><trias:Text>Mannheim, T 1</trias:Text><trias:Language>de</trias:Language></trias:AddressName><trias:PostalCode> 68161</trias:PostalCode><trias:LocalityName>Mannheim</trias:LocalityName><trias:LocalityRef>8222000:-1</trias:LocalityRef><trias:StreetName>T 1</trias:StreetName></trias:Address><trias:LocationName><trias:Text>Mannheim</trias:Text><trias:Language>de</trias:Language></trias:LocationName><trias:GeoPosition><trias:Longitude>8.46987</trias:Longitude><trias:Latitude>49.49121</trias:Latitude></trias:GeoPosition></trias:Location><trias:Complete>true</trias:Complete><trias:Probability>0.763999999</trias:Probability></trias:Location></trias:LocationInformationResponse></trias:DeliveryPayload></trias:ServiceDelivery></trias:Trias>'
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