In order to configure cloudflare with nginx, first you need to make sure the following module is compiled along with nginx :

–with-http_realip_module

If you do not do this, not only will your google analytics fail, you will not know correctly which IP is accessing your website from nginx logs.

If the module is enabled, you could check this by displaying the version,

nginx -V
nginx version: nginx/1.2.6
built by gcc 4.4.4 20100726 (Red Hat 4.4.4-13) (GCC)
TLS SNI support enabled
configure arguments: –prefix=/etc/nginx/ –sbin-path=/usr/sbin/nginx –conf-path=/etc/nginx/nginx.conf –error-log-path=/var/log/nginx/error.log –http-log-path=/var/log/nginx/access.log –pid-path=/var/run/nginx.pid –lock-path=/var/run/nginx.lock –http-client-body-temp-path=/var/cache/nginx/client_temp –http-proxy-temp-path=/var/cache/nginx/proxy_temp –http-fastcgi-temp-path=/var/cache/nginx/fastcgi_temp –http-uwsgi-temp-path=/var/cache/nginx/uwsgi_temp –http-scgi-temp-path=/var/cache/nginx/scgi_temp –user=nginx –group=nginx –with-http_ssl_module –with-http_realip_module –with-http_addition_module –with-http_sub_module –with-http_dav_module –with-http_flv_module –with-http_mp4_module –with-http_gzip_static_module –with-http_random_index_module –with-http_secure_link_module –with-http_stub_status_module –with-mail –with-mail_ssl_module –with-file-aio –with-ipv6 –with-cc-opt=’-O2 -g’

Once you confirm the above module is enabled, add the following list to your nginx configuration file,

set_real_ip_from 204.93.240.0/24;
set_real_ip_from 204.93.177.0/24;
set_real_ip_from 199.27.128.0/21;
set_real_ip_from 173.245.48.0/20;
set_real_ip_from 103.21.244.0/22;
set_real_ip_from 103.22.200.0/22;
set_real_ip_from 103.31.4.0/22;
set_real_ip_from 141.101.64.0/18;
set_real_ip_from 108.162.192.0/18;
set_real_ip_from 190.93.240.0/20;
set_real_ip_from 188.114.96.0/20;
set_real_ip_from 197.234.240.0/22;
set_real_ip_from 198.41.128.0/17;
set_real_ip_from 2400:cb00::/32;
set_real_ip_from 2606:4700::/32;
set_real_ip_from 2803:f800::/32;
set_real_ip_from 2405:b500::/32;
set_real_ip_from 2405:8100::/32;
real_ip_header CF-Connecting-IP;

Restart nginx and your logs should start displaying the actual remote ip addresses.