Curl depends on external libraries for some features (and libcurl for all features). You can build curl without them, but curl gets a lot better if you have these libraries:

Library	Used for...
www.openssl.org	SSL (https) support. Use OpenSSL. Curl compiles, builds and runs with it. download links.
www.gzip.org/zlib	The automatic "deflate" decompression depends on zlib. download links
www.openldap.org	The LDAP support depends on OpenLDAP.
kerberos4	The kerberos4 support depends on a kerberos4 package! :-) More information can be found at http://www.contrib.andrew.cmu.edu/~shadow/kerberos.html, by asking google or at the MIT kerberos package page. libcurl has been used successfully with kth-krb4.
heimdal	The GSSAPI support can be provided by the heimdal package. GSSAPI is used for the GSS-Negotiate support in libcurl.
fbopenssl	SPNEGO support depends on the fbopenssl library.
c-ares	c-ares is the library libcurl uses for asynchronous name resolves.
libidn	libidn is used by libcurl to perform the proper IDNA encodings for international domain names to work.
www.gnutls.org	TLS (https) support. Use GnuTLS. Curl (7.14.0 or later) compiles, builds and runs with it.
NSS	TLS (https) support. Use NSS. Curl (7.16.2 or later) compiles, builds and runs with it.
yassl	TLS (https) support. Use yassl. Curl (7.15.4 or later) compiles, builds and runs with it.
libssh2	For SSH (SCP and SFTP) support, added in (lib)curl 7.16.1.

Page updated November 16, 2009.
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