Why won't 'patch' install the patch that ISC has sent me?

If ISC has sent you a patch to apply to your current version of one of our products, it will usually be in the "unified" diff format (aka "unidiff"), intended to be applied to the source tree using the GNU "patch" utility.

The Solaris environment has historically provided a "patch" utility that does something else; therefore, on a Solaris system GNU patch is typically known as "gpatch" instead.