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libcurl-env(3) Library Functions Manual libcurl-env(3) NAME libcurl-env - environment variables libcurl understands DESCRIPTION libcurl reads and understands a set of environment variables that if set controls and changes behaviors. This is the full list of variables to set and description of what they do. Also note that curl, the com- mand line tool, supports a set of additional environment variables in- dependently of this. [scheme]_proxy When libcurl is given a URL to use in a transfer, it first ex- tracts the scheme part from the URL and checks if there is a given proxy set for that in its corresponding environment vari- able. A URL like https://example.com makes libcurl use the http_proxy variable, while a URL like ftp://example.com uses the ftp_proxy variable. These proxy variables are also checked for in their uppercase versions, except the http_proxy one which is only used lower- case. Note also that some systems actually have a case insensi- tive handling of environment variables and then of course HTTP_PROXY still works. An exception exists for the WebSocket ws and wss URL schemes, where libcurl first checks ws_proxy or wss_proxy but if they are not set, it falls back and tries the http and https versions in- stead if set. ALL_PROXY This is a setting to set proxy for all URLs, independently of what scheme is being used. Note that the scheme specific vari- ables overrides this one if set. CURL_SSL_BACKEND When libcurl is built to support multiple SSL backends, it se- lects a specific backend at first use. If no selection is done by the program using libcurl, this variable's selection is used. Setting a name that is not a built-in alternative makes libcurl stay with the default. SSL backend names (case-insensitive): BearSSL, GnuTLS, mbedTLS, OpenSSL, Rustls, Schannel, Secure-Transport, wolfSSL HOME When the netrc feature is used (CURLOPT_NETRC(3)), this variable is checked as the primary way to find the "current" home direc- tory in which the .netrc file is likely to exist. USERPROFILE When the netrc feature is used (CURLOPT_NETRC(3)), this variable is checked as the secondary way to find the "current" home di- rectory (on Windows only) in which the .netrc file is likely to exist. LOGNAME Username to use when invoking the ntlm-wb tool, if NTLMUSER was not set. NO_PROXY This has the same functionality as the CURLOPT_NOPROXY(3) op- tion: it gives libcurl a comma-separated list of hostname pat- terns for which libcurl should not use a proxy. NTLMUSER Username to use when invoking the ntlm-wb tool. SSLKEYLOGFILE When set and libcurl runs with a SSL backend that supports this feature, libcurl saves SSL secrets into the given filename. Us- ing those SSL secrets, other tools (such as Wireshark) can de- crypt the SSL communication and analyze/view the traffic. These secrets and this file might be sensitive. Users are ad- vised to take precautions so that they are not stolen or other- wise inadvertently revealed. USER Username to use when invoking the ntlm-wb tool, if NTLMUSER and LOGNAME were not set. Debug Variables Debug variables are intended for internal use and are documented in libcurl-env-dbg(3). SEE ALSO libcurl-env-dbg(3) libcurl 2025-06-03 libcurl-env(3)
NAME | DESCRIPTION | Debug Variables | SEE ALSO
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