Run into a situation recently where proxying applications with nginx has masked the source IP - which normally is just annoying, but with Atlassian's Crowd it's more of a problem, this can be solved in later versions (Tomcat 6.0.32+) as follows:

<Valve className="org.apache.catalina.valves.RemoteIpValve"
	internalProxies="proxy_IP_address"
	remoteIpHeader="x-forwarded-for"
	remoteIpProxiesHeader="x-forwarded-by"
	protocolHeader="x-forwarded-proto" />

<Valve className="org.apache.catalina.valves.AccessLogValve"
	directory="logs"
	prefix="localhost_access."
	suffix=".log"
	pattern="combined"
	resolveHosts="false" />

The following should be used within nginx to forward that data into tomcat.

proxy_set_header X-Forwarded-Proto  https;
proxy_set_header X-Forwarded-Host $host;
proxy_set_header X-Forwarded-Server $host;
proxy_set_header X-Forwarded-For $remote_addr;