libcurl-env(3) Introduction to Library Functions 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 command line tool, supports a set of additional environment
variables independently of this.

[scheme]_proxy
When libcurl is given a URL to use in a transfer, it first
extracts the scheme part from the URL and checks if there is a
given proxy set for that in its corresponding environment
variable. 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
lowercase. Note also that some systems actually have a case
insensitive 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 instead 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
variables overrides this one if set.

CURL_SSL_BACKEND
When libcurl is built to support multiple SSL backends, it
selects 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 directory 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 directory (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)
option: it gives libcurl a comma-separated list of hostname
patterns 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. Using those SSL secrets, other tools (such as
Wireshark) can decrypt the SSL communication and analyze/view
the traffic.

These secrets and this file might be sensitive. Users are
advised to take precautions so that they are not stolen or
otherwise 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 2025-02-25 libcurl-env(3)