cannot match a server with double Ip address - nginx

Question

I have a server with two ip: when i use nginx as Reverse Proxy for jboss7，
in order to prevent direct access use ip address,(we have configured the dns),
i use configuration bellow:
# You may add here yourdefault_server;
# server {
#
server {
listen *:80;
server_name _;
return 404;
}
server {
listen 80;
listen [::]:80 default_server ipv6only=on;
root /usr/share/nginx/html;
index index.html index.htm;
server_name www.shikuaigou.com localhost;
charset utf-8;
location / {
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $http_x_forwarded_for;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header Host $host;
proxy_set_header X-Forwarded-Host $host;
proxy_set_header X-Forwarded-Server $host;
proxy_pass http://jboss;
}
location /doc/ {
alias /usr/share/doc/;
autoindex on;
allow 127.0.0.1;
allow ::1;
deny all;
}
}
server {
listen 80;
server_name example.com;
rewrite "^/(.*)$" http://www.example.com/$1 permanent;
}
server {
listen 12.34.56.78;
server_name www.example.com;
root /usr/share/nginx/html;
index index.html index.htm;
location / {
proxy_set_header X-Forwarded-For $http_x_forwarded_for;
proxy_set_header Host $host;
proxy_set_header X-Forwarded-Host $host;
proxy_set_header X-Forwarded-Server $host;
proxy_pass http://jboss;
# First attempt to serve request as file, then
# as directory, then fall back to displaying a 404.
# try_files $uri $uri/ /index.html;
# Uncomment to enable naxsi on this location
# include /etc/nginx/naxsi.rules
}
}
but only on ip can matche the server whitch return 404,the other one cannot match the
configuration server_name _;
which cause this?

Answer 1

Because you have listen 12.34.56.78; so nginx chooses this server to process requests on 12.34.56.78, since it is more specific for that IP.
Please, also note that server_name _; actually means nothing, except an incorrect domain name.
Reference:
Server names
How nginx processes a request
The listen directive