curl reports "fault filter abortcurl: (6) Could not resolve host: store.example.xn--com-9o0a"

[sanjaypujare]

I did this

From a Google doc I copy-pasted a curl command that was:

curl -i 10.1.2.9 -H “host: store.example.com”

Note that what appear as double-quotes around host: store.example.com are not the normal double-quotes used in computing but used in Document editing programs like GoogleDoc or Word. But this trips up curl and it gives me the following output:

HTTP/1.1 404 Not Found
content-length: 18
content-type: text/plain
date: Thu, 28 Mar 2024 05:04:59 GMT
via: 1.1 google

fault filter abortcurl: (6) Could not resolve host: store.example.xn--com-9o0a

I spent considerable amount of time trying to troubleshoot this since the error message was very misleading

I expected the following

curl should probably recognize the unsupported characters on the command line and generate a message while parsing those arguments instead of generating an error deep inside (which is what "fault filter abort" looks like)

curl/libcurl version

curl 7.88.1 (x86_64-pc-linux-gnu) libcurl/7.88.1 OpenSSL/3.0.11 zlib/1.2.13 brotli/1.0.9 zstd/1.5.4 libidn2/2.3.3 libpsl/0.21.2 (+libidn2/2.3.3) libssh2/1.10.0 nghttp2/1.52.0 librtmp/2.3 OpenLDAP/2.5.13
Release-Date: 2023-02-20, security patched: 7.88.1-10+deb12u5
Protocols: dict file ftp ftps gopher gophers http https imap imaps ldap ldaps mqtt pop3 pop3s rtmp rtsp scp sftp smb smbs smtp smtps telnet tftp
Features: alt-svc AsynchDNS brotli GSS-API HSTS HTTP2 HTTPS-proxy IDN IPv6 Kerberos Largefile libz NTLM NTLM_WB PSL SPNEGO SSL threadsafe TLS-SRP UnixSockets zstd

operating system

Linux xxxxx 6.1.0-18-cloud-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.1.76-1 (2024-02-01) x86_64 GNU/Linux

[bagder]

The fault filter abort text is not from curl, it is body content from the server.

[bagder]

This case is really a matter of "garbage in, garbage out". It is difficult for curl to know when the garbage input is intended or not.

[sanjaypujare]

I understand. Thanks for fixing it. One more thing that confused me. With the bad unicode “ character(s) and with -v option I see:

curl -v -i 10.1.2.9 -H “host: store.example.com”
*   Trying 10.1.2.9:80...
* Connected to 10.1.2.9 (10.1.2.9) port 80 (#0)
> GET / HTTP/1.1
> Host: 10.1.2.9
> User-Agent: curl/7.88.1
> Accept: */*
> 
< HTTP/1.1 404 Not Found
HTTP/1.1 404 Not Found
< content-length: 18
content-length: 18
< content-type: text/plain
content-type: text/plain
< date: Thu, 28 Mar 2024 20:12:14 GMT
date: Thu, 28 Mar 2024 20:12:14 GMT
< via: 1.1 google
via: 1.1 google

< 
* Connection #0 to host 10.1.2.9 left intact
fault filter abort* Could not resolve host: store.example.xn--com-9o0a
* Closing connection 1
curl: (6) Could not resolve host: store.example.xn--com-9o0a

But with the right quote character(s):

curl -v -i 10.1.2.9 -H "host: store.example.com"
*   Trying 10.1.2.9:80...
* Connected to 10.1.2.9 (10.1.2.9) port 80 (#0)
> GET / HTTP/1.1
> Host: store.example.com
> User-Agent: curl/7.88.1
> Accept: */*
> 
< HTTP/1.1 401 Unauthorized
HTTP/1.1 401 Unauthorized
< date: Thu, 28 Mar 2024 20:12:39 GMT
date: Thu, 28 Mar 2024 20:12:39 GMT
< via: 1.1 google
via: 1.1 google
< content-length: 0
content-length: 0

< 
* Connection #0 to host 10.1.2.9 left intact

The 401 Unauthorized is expected in the second example. But the questions are:

• who is generating the error "Could not resolve host: store.example.xn--com-9o0a" in the first case? It looks like curl didn't send any of this stuff to the server so it must have been from curl itself
• because of the -H argument curl should have treated the following argument only as header content to be included in the request. But for some reason it treated the whole string “host: store.example.com” as a hostname to be resolved. Is that correct?

[dfandrich]

Since they were the wrong quote characters, the shell treats the argument as two: “host: and store.example.com”. curl then sets a header "“host: " and tries to connect to the URL http://store.example.com” which it converts to punycode store.example.xn--com-9o0a