---
c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
SPDX-License-Identifier: curl
Title: libcurl-env
Section: 3
Source: libcurl
See-also:
  - libcurl-env-dbg (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,
nss, 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).