nginx pam (and thus kerberos) authentication in gentoo. Copied from /usr/portage/www-servers/nginx/nginx-1.2.5.ebuild tweaked to add USE=pam

etc-pam.d-nginx

# /etc/pam.d/nginx
auth    required        /lib/security/pam_krb5.so
account required        /lib/security/pam_krb5.so

nginx-1.2.5-r1.ebuild

# Copyright 1999-2012 Gentoo Foundation
# Distributed under the terms of the GNU General Public License v2
# $Header: /var/cvsroot/gentoo-x86/www-servers/nginx/nginx-1.2.5.ebuild,v 1.4 2012/12/10 08:57:32 hollow Exp $

EAPI="4"

# Maintainer notes:
# - http_rewrite-independent pcre-support makes sense for matching locations without an actual rewrite
# - any http-module activates the main http-functionality and overrides USE=-http
# - keep the following requirements in mind before adding external modules:
#   * alive upstream
#   * sane packaging
#   * builds cleanly
#   * does not need a patch for nginx core
# - TODO: test the google-perftools module (included in vanilla tarball)

# prevent perl-module from adding automagic perl DEPENDs
GENTOO_DEPEND_ON_PERL="no"

# devel_kit (https://github.com/simpl/ngx_devel_kit, BSD license)
DEVEL_KIT_MODULE_PV="0.2.17"
DEVEL_KIT_MODULE_P="ngx_devel_kit-${DEVEL_KIT_MODULE_PV}"
DEVEL_KIT_MODULE_SHA1="bc97eea"
DEVEL_KIT_MODULE_URI="https://github.com/simpl/ngx_devel_kit/tarball/v${DEVEL_KIT_MODULE_PV}"
DEVEL_KIT_MODULE_WD="${WORKDIR}/simpl-ngx_devel_kit-${DEVEL_KIT_MODULE_SHA1}"

# http_uploadprogress (https://github.com/masterzen/nginx-upload-progress-module, BSD-2 license)
HTTP_UPLOAD_PROGRESS_MODULE_PV="0.9.0"
HTTP_UPLOAD_PROGRESS_MODULE_P="ngx_http_upload_progress-${HTTP_UPLOAD_PROGRESS_MODULE_PV}"
HTTP_UPLOAD_PROGRESS_MODULE_SHA1="a788dea"
HTTP_UPLOAD_PROGRESS_MODULE_URI="http://github.com/masterzen/nginx-upload-progress-module/tarball/v${HTTP_UPLOAD_PROGRESS_MODULE_PV}"
HTTP_UPLOAD_PROGRESS_MODULE_WD="${WORKDIR}/masterzen-nginx-upload-progress-module-${HTTP_UPLOAD_PROGRESS_MODULE_SHA1}"

# http_headers_more (http://github.com/agentzh/headers-more-nginx-module, BSD license)
HTTP_HEADERS_MORE_MODULE_PV="0.17"
HTTP_HEADERS_MORE_MODULE_P="ngx_http_headers_more-${HTTP_HEADERS_MORE_MODULE_PV}"
HTTP_HEADERS_MORE_MODULE_SHA1="b7c8cfc"
HTTP_HEADERS_MORE_MODULE_URI="http://github.com/agentzh/headers-more-nginx-module/tarball/v${HTTP_HEADERS_MORE_MODULE_PV}"
HTTP_HEADERS_MORE_MODULE_WD="${WORKDIR}/agentzh-headers-more-nginx-module-${HTTP_HEADERS_MORE_MODULE_SHA1}"

# http_push (http://pushmodule.slact.net/, MIT license)
HTTP_PUSH_MODULE_PV="0.692"
HTTP_PUSH_MODULE_P="ngx_http_push-${HTTP_PUSH_MODULE_PV}"
HTTP_PUSH_MODULE_URI="http://pushmodule.slact.net/downloads/nginx_http_push_module-${HTTP_PUSH_MODULE_PV}.tar.gz"
HTTP_PUSH_MODULE_WD="${WORKDIR}/nginx_http_push_module-${HTTP_PUSH_MODULE_PV}"

# http_cache_purge (http://labs.frickle.com/nginx_ngx_cache_purge/, BSD-2 license)
HTTP_CACHE_PURGE_MODULE_PV="1.6"
HTTP_CACHE_PURGE_MODULE_P="ngx_http_cache_purge-${HTTP_CACHE_PURGE_MODULE_PV}"
HTTP_CACHE_PURGE_MODULE_URI="http://labs.frickle.com/files/ngx_cache_purge-${HTTP_CACHE_PURGE_MODULE_PV}.tar.gz"
HTTP_CACHE_PURGE_MODULE_WD="${WORKDIR}/ngx_cache_purge-${HTTP_CACHE_PURGE_MODULE_PV}"

# http_upload (http://www.grid.net.ru/nginx/upload.en.html, BSD license)
HTTP_UPLOAD_MODULE_PV="2.2.0"
HTTP_UPLOAD_MODULE_P="ngx_http_upload-${HTTP_UPLOAD_MODULE_PV}"
HTTP_UPLOAD_MODULE_URI="http://www.grid.net.ru/nginx/download/nginx_upload_module-${HTTP_UPLOAD_MODULE_PV}.tar.gz"
HTTP_UPLOAD_MODULE_WD="${WORKDIR}/nginx_upload_module-${HTTP_UPLOAD_MODULE_PV}"

# http_slowfs_cache (http://labs.frickle.com/nginx_ngx_slowfs_cache/, BSD-2 license)
HTTP_SLOWFS_CACHE_MODULE_PV="1.9"
HTTP_SLOWFS_CACHE_MODULE_P="ngx_http_slowfs_cache-${HTTP_SLOWFS_CACHE_MODULE_PV}"
HTTP_SLOWFS_CACHE_MODULE_URI="http://labs.frickle.com/files/ngx_slowfs_cache-${HTTP_SLOWFS_CACHE_MODULE_PV}.tar.gz"
HTTP_SLOWFS_CACHE_MODULE_WD="${WORKDIR}/ngx_slowfs_cache-${HTTP_SLOWFS_CACHE_MODULE_PV}"

# http_fancyindex (http://wiki.nginx.org/NgxFancyIndex, BSD license)
HTTP_FANCYINDEX_MODULE_PV="0.3.1.1"
HTTP_FANCYINDEX_MODULE_P="ngx_http_fancyindex-${HTTP_FANCYINDEX_MODULE_PV}"
HTTP_FANCYINDEX_MODULE_URI="http://gitorious.org/ngx-fancyindex/ngx-fancyindex/archive-tarball/2034d0ad"
HTTP_FANCYINDEX_MODULE_WD="${WORKDIR}/ngx-fancyindex-ngx-fancyindex"

# http_lua (https://github.com/chaoslawful/lua-nginx-module, BSD license)
HTTP_LUA_MODULE_PV="0.5.10"
HTTP_LUA_MODULE_P="ngx_http_lua-${HTTP_LUA_MODULE_PV}"
HTTP_LUA_MODULE_SHA1="db0bebe"
HTTP_LUA_MODULE_URI="https://github.com/chaoslawful/lua-nginx-module/tarball/v${HTTP_LUA_MODULE_PV}"
HTTP_LUA_MODULE_WD="${WORKDIR}/chaoslawful-lua-nginx-module-${HTTP_LUA_MODULE_SHA1}"

# (http://web.iti.upv.es/~sto/nginx/ngx_http_auth_pam_module-1.1.tar.gz, as-is)
HTTP_AUTH_PAM_MODULE="1.1"
HTTP_AUTH_PAM_MODULE_P="ngx_http_auth_pam_module-${HTTP_AUTH_PAM_MODULE}"
HTTP_AUTH_PAM_MODULE_URI="http://web.iti.upv.es/~sto/nginx/${HTTP_AUTH_PAM_MODULE_P}.tar.gz"
HTTP_AUTH_PAM_MODULE_WD="${WORKDIR}/${HTTP_AUTH_PAM_MODULE_P}"

inherit eutils ssl-cert toolchain-funcs perl-module flag-o-matic user

DESCRIPTION="Robust, small and high performance http and reverse proxy server"
HOMEPAGE="http://nginx.org"
SRC_URI="http://nginx.org/download/${P}.tar.gz
        ${DEVEL_KIT_MODULE_URI} -> ${DEVEL_KIT_MODULE_P}.tar.gz
        nginx_modules_http_upload_progress? ( ${HTTP_UPLOAD_PROGRESS_MODULE_URI} -> ${HTTP_UPLOAD_PROGRESS_MODULE_P}.tar.gz )
        nginx_modules_http_headers_more? ( ${HTTP_HEADERS_MORE_MODULE_URI} -> ${HTTP_HEADERS_MORE_MODULE_P}.tar.gz )
        nginx_modules_http_push? ( ${HTTP_PUSH_MODULE_URI} )
        nginx_modules_http_cache_purge? ( ${HTTP_CACHE_PURGE_MODULE_URI} )
        nginx_modules_http_upload? ( ${HTTP_UPLOAD_MODULE_URI} )
        nginx_modules_http_slowfs_cache? ( ${HTTP_SLOWFS_CACHE_MODULE_URI} )
        nginx_modules_http_fancyindex? ( ${HTTP_FANCYINDEX_MODULE_URI} -> ${HTTP_FANCYINDEX_MODULE_P}.tar.gz )
        nginx_modules_http_lua? ( ${HTTP_LUA_MODULE_URI} -> ${HTTP_LUA_MODULE_P}.tar.gz )
        pam? ( ${HTTP_AUTH_PAM_MODULE_URI} -> ${HTTP_AUTH_PAM_MODULE_P}.tar.gz )
        "

LICENSE="BSD-2 BSD MIT GPL-2
                pam? ( as-is )"
SLOT="0"
KEYWORDS="amd64 ~ppc x86 ~x86-fbsd ~amd64-linux ~x86-linux"

NGINX_MODULES_STD="access auth_basic auth_pam autoindex browser charset empty_gif fastcgi
geo gzip limit_req limit_conn map memcached proxy referer rewrite scgi ssi
split_clients upstream_ip_hash userid uwsgi"
NGINX_MODULES_OPT="addition dav degradation flv geoip gzip_static image_filter
mp4 perl random_index realip secure_link stub_status sub xslt"
NGINX_MODULES_MAIL="imap pop3 smtp"
NGINX_MODULES_3RD="
        http_upload_progress
        http_headers_more
        http_passenger
        http_push
        http_cache_purge
        http_upload
        http_slowfs_cache
        http_fancyindex
        http_lua"

IUSE="aio debug +http +http-cache ipv6 libatomic pam +pcre pcre-jit selinux ssl vim-syntax"

for mod in $NGINX_MODULES_STD; do
        IUSE="${IUSE} +nginx_modules_http_${mod}"
done

for mod in $NGINX_MODULES_OPT; do
        IUSE="${IUSE} nginx_modules_http_${mod}"
done

for mod in $NGINX_MODULES_MAIL; do
        IUSE="${IUSE} nginx_modules_mail_${mod}"
done

for mod in $NGINX_MODULES_3RD; do
        IUSE="${IUSE} nginx_modules_${mod}"
done

CDEPEND="
        pcre? ( >=dev-libs/libpcre-4.2 )
        pcre-jit? ( >=dev-libs/libpcre-8.20[jit] )
        selinux? ( sec-policy/selinux-nginx )
        ssl? ( dev-libs/openssl )
        http-cache? ( userland_GNU? ( dev-libs/openssl ) )
        nginx_modules_http_geo? ( dev-libs/geoip )
        nginx_modules_http_gzip? ( sys-libs/zlib )
        nginx_modules_http_gzip_static? ( sys-libs/zlib )
        nginx_modules_http_image_filter? ( media-libs/gd[jpeg,png] )
        nginx_modules_http_perl? ( >=dev-lang/perl-5.8 )
        nginx_modules_http_rewrite? ( >=dev-libs/libpcre-4.2 )
        nginx_modules_http_secure_link? ( userland_GNU? ( dev-libs/openssl ) )
        nginx_modules_http_xslt? ( dev-libs/libxml2 dev-libs/libxslt )
        nginx_modules_http_lua? ( || ( dev-lang/lua dev-lang/luajit ) )"
RDEPEND="${CDEPEND}"
DEPEND="${CDEPEND}
        arm? ( dev-libs/libatomic_ops )
        libatomic? ( dev-libs/libatomic_ops )"
PDEPEND="vim-syntax? ( app-vim/nginx-syntax )"
REQUIRED_USE="pcre-jit? ( pcre )"

pkg_setup() {
        if use nginx_modules_http_passenger; then
                einfo
                einfo "Passenger support has been removed from the nginx ebuild to"
                einfo "get rid of file collisions, its broken build system and"
                einfo "incompatibilities between passenger 2 and 3."
                einfo
                einfo "Please switch to passenger-3 standalone or use the"
                einfo "unicorn gem which provides a sane nginx-like architecture"
                einfo "out of the box."
                einfo
                einfo "For more information on sane ruby deployments with"
                einfo "passenger-3/unicorn go to:"
                einfo
                einfo "https://rvm.beginrescueend.com"
                einfo
                die "nginx_modules_http_passenger still in IUSE"
        fi

        ebegin "Creating nginx user and group"
        enewgroup ${PN}
        enewuser ${PN} -1 -1 -1 ${PN}
        eend $?

        if use libatomic; then
                ewarn "GCC 4.1+ features built-in atomic operations."
                ewarn "Using libatomic_ops is only needed if using"
                ewarn "a different compiler or a GCC prior to 4.1"
        fi

        if [[ -n $NGINX_ADD_MODULES ]]; then
                ewarn "You are building custom modules via \$NGINX_ADD_MODULES!"
                ewarn "This nginx installation is not supported!"
                ewarn "Make sure you can reproduce the bug without those modules"
                ewarn "_before_ reporting bugs."
        fi

        if use !http; then
                ewarn "To actually disable all http-functionality you also have to disable"
                ewarn "all nginx http modules."
        fi
}

src_prepare() {
        find auto/ -type f -print0 | xargs -0 sed -i 's:\&\& make:\&\& \\$(MAKE):' || die
        # We have config protection, don't rename etc files
        sed -i 's:.default::' auto/install || die
        # remove useless files
        sed -i -e '/koi-/d' -e '/win-/d' auto/install || die
}

src_configure() {
        local myconf= http_enabled= mail_enabled=

        use aio       && myconf+=" --with-file-aio --with-aio_module"
        use debug     && myconf+=" --with-debug"
        use ipv6      && myconf+=" --with-ipv6"
        use libatomic && myconf+=" --with-libatomic"
        use pcre      && myconf+=" --with-pcre"
        use pcre-jit  && myconf+=" --with-pcre-jit"

        # HTTP modules
        for mod in $NGINX_MODULES_STD; do
                if use nginx_modules_http_${mod}; then
                        http_enabled=1
                else
                        myconf+=" --without-http_${mod}_module"
                fi
        done

        for mod in $NGINX_MODULES_OPT; do
                if use nginx_modules_http_${mod}; then
                        http_enabled=1
                        myconf+=" --with-http_${mod}_module"
                fi
        done

        if use nginx_modules_http_fastcgi; then
                myconf+=" --with-http_realip_module"
        fi

        # third-party modules
        if use nginx_modules_http_upload_progress; then
                http_enabled=1
                myconf+=" --add-module=${HTTP_UPLOAD_PROGRESS_MODULE_WD}"
        fi

        if use nginx_modules_http_headers_more; then
                http_enabled=1
                myconf+=" --add-module=${HTTP_HEADERS_MORE_MODULE_WD}"
        fi

        if use nginx_modules_http_push; then
                http_enabled=1
                myconf+=" --add-module=${HTTP_PUSH_MODULE_WD}"
        fi

        if use nginx_modules_http_cache_purge; then
                http_enabled=1
                myconf+=" --add-module=${HTTP_CACHE_PURGE_MODULE_WD}"
        fi

        if use nginx_modules_http_upload; then
                http_enabled=1
                myconf+=" --add-module=${HTTP_UPLOAD_MODULE_WD}"
        fi

        if use nginx_modules_http_slowfs_cache; then
                http_enabled=1
                myconf+=" --add-module=${HTTP_SLOWFS_CACHE_MODULE_WD}"
        fi

        if use nginx_modules_http_fancyindex; then
                http_enabled=1
                myconf+=" --add-module=${HTTP_FANCYINDEX_MODULE_WD}"
        fi

        if use nginx_modules_http_lua; then
                http_enabled=1
                myconf+=" --add-module=${DEVEL_KIT_MODULE_WD}"
                myconf+=" --add-module=${HTTP_LUA_MODULE_WD}"
        fi

        if use pam; then
                http_enabled=1
                myconf+=" --add-module=${HTTP_AUTH_PAM_MODULE_WD}"
        fi

        if use http || use http-cache; then
                http_enabled=1
        fi

        if [ $http_enabled ]; then
                use http-cache || myconf+=" --without-http-cache"
                use ssl && myconf+=" --with-http_ssl_module"
        else
                myconf+=" --without-http --without-http-cache"
        fi

        # MAIL modules
        for mod in $NGINX_MODULES_MAIL; do
                if use nginx_modules_mail_${mod}; then
                        mail_enabled=1
                else
                        myconf+=" --without-mail_${mod}_module"
                fi
        done

        if [ $mail_enabled ]; then
                myconf+=" --with-mail"
                use ssl && myconf+=" --with-mail_ssl_module"
        fi

        # custom modules
        for mod in $NGINX_ADD_MODULES; do
                myconf+=" --add-module=${mod}"
        done

        # https://bugs.gentoo.org/286772
        export LANG=C LC_ALL=C
        tc-export CC

        if ! use prefix; then
                myconf+=" --user=${PN} --group=${PN}"
        fi

        ./configure \
                --prefix="${EPREFIX}"/usr \
                --conf-path="${EPREFIX}"/etc/${PN}/${PN}.conf \
                --error-log-path="${EPREFIX}"/var/log/${PN}/error_log \
                --pid-path="${EPREFIX}"/var/run/${PN}.pid \
                --lock-path="${EPREFIX}"/var/lock/nginx.lock \
                --with-cc-opt="-I${EROOT}usr/include" \
                --with-ld-opt="-L${EROOT}usr/lib" \
                --http-log-path="${EPREFIX}"/var/log/${PN}/access_log \
                --http-client-body-temp-path="${EPREFIX}"/var/tmp/${PN}/client \
                --http-proxy-temp-path="${EPREFIX}"/var/tmp/${PN}/proxy \
                --http-fastcgi-temp-path="${EPREFIX}"/var/tmp/${PN}/fastcgi \
                --http-scgi-temp-path="${EPREFIX}"/var/tmp/${PN}/scgi \
                --http-uwsgi-temp-path="${EPREFIX}"/var/tmp/${PN}/uwsgi \
                ${myconf} || die "configure failed"
}

src_compile() {
        # https://bugs.gentoo.org/286772
        export LANG=C LC_ALL=C
        emake LINK="${CC} ${LDFLAGS}" OTHERLDFLAGS="${LDFLAGS}"
}

src_install() {
        emake DESTDIR="${D}" install
        cp "${FILESDIR}"/nginx.conf "${ED}"/etc/nginx/nginx.conf || die
        newinitd "${FILESDIR}"/nginx.initd nginx
        doman man/nginx.8
        dodoc CHANGES* README

        # Keepdir because these are hardcoded above
        keepdir /var/log/${PN} /var/tmp/${PN}/{client,proxy,fastcgi,scgi,uwsgi}
        keepdir /var/www/localhost/htdocs
        mv "${ED}"/usr/html "${ED}"/var/www/localhost/htdocs || die

        # logrotate
        insinto /etc/logrotate.d
        newins "${FILESDIR}"/nginx.logrotate nginx

        if use nginx_modules_http_perl; then
                cd "${S}"/objs/src/http/modules/perl/
                einstall DESTDIR="${D}" INSTALLDIRS=vendor
                fixlocalpod
        fi

        if use nginx_modules_http_push; then
                docinto ${HTTP_PUSH_MODULE_P}
                dodoc "${HTTP_PUSH_MODULE_WD}"/{changelog.txt,protocol.txt,README}
        fi

        if use nginx_modules_http_cache_purge; then
                docinto ${HTTP_CACHE_PURGE_MODULE_P}
                dodoc "${HTTP_CACHE_PURGE_MODULE_WD}"/{CHANGES,README.md,TODO.md}
        fi

        if use nginx_modules_http_upload; then
                docinto ${HTTP_UPLOAD_MODULE_P}
                dodoc "${HTTP_UPLOAD_MODULE_WD}"/{Changelog,README}
        fi

        if use nginx_modules_http_slowfs_cache; then
                docinto ${HTTP_SLOWFS_CACHE_MODULE_P}
                dodoc "${HTTP_SLOWFS_CACHE_MODULE_WD}"/{CHANGES,README.md}
        fi

        if use nginx_modules_http_fancyindex; then
                docinto ${HTTP_FANCYINDEX_MODULE_P}
                dodoc "${HTTP_FANCYINDEX_MODULE_WD}"/README.rst
        fi

        if use nginx_modules_http_lua; then
                docinto ${HTTP_LUA_MODULE_P}
                dodoc "${HTTP_LUA_MODULE_WD}"/{Changes,README.markdown}
        fi

        if use pam; then
                docinto ${HTTP_AUTH_PAM_MODULE_P}
                dodoc "${HTTP_AUTH_PAM_MODULE_WD}/README"
        fi
}

pkg_postinst() {
        if use ssl; then
                if [ ! -f "${EROOT}"/etc/ssl/${PN}/${PN}.key ]; then
                        install_cert /etc/ssl/${PN}/${PN}
                        use prefix || chown ${PN}:${PN} "${EROOT}"/etc/ssl/${PN}/${PN}.{crt,csr,key,pem}
                fi
        fi
}

nginx.conf

...
http {
  server {
    ...
    # location /secure {
    auth_pam  "Pam authentication";
    auth_pam_service_name "nginx";
    # }
  }
}