nginx当没有匹配到server块时会选择default server处理请求。default server的定义分为两种：

1. 显式定义

listen 指令后加default_server参数，则该server块被定义为default server。对于相同的监听端口，只能有一个server被显示定义为default server。如下图，server xxxx.com是default server。

http{
    server {
        listen 80;
        server_name xxx.com;
    }

    server {
        listen 80 default_server;
        server_name xxxx.com;
    }
}

2. 隐式定义

当一个监听地址没有显式的default_server定义时，加载配置时该地址第一个被载入的server即是default_server。

代码说明

加载配置时，listen配置会存入ngx_http_listen_opt_t中。

typedef struct {
    struct sockaddr           *sockaddr;
    socklen_t                  socklen;
    ngx_str_t                  addr_text;

    unsigned                   set:1;
    unsigned                   default_server:1; //如果listen后配置了default_server，该字段被设置成1
    unsigned                   bind:1;
    unsigned                   wildcard:1;

    unsigned                   ssl:1;
#if (NGX_SSL_ASYNC)
    unsigned                   asynch:1;
#endif
    unsigned                   http2:1;
#if (NGX_HAVE_INET6)
    unsigned                   ipv6only:1;
#endif
    unsigned                   deferred_accept:1;
    unsigned                   reuseport:1;
    unsigned                   so_keepalive:2;
    unsigned                   proxy_protocol:1;

    int                        backlog;
    int                        rcvbuf;
    int                        sndbuf;

#if (NGX_HAVE_TCP_FASTOPEN)
    int                        fastopen;
#endif
#if (NGX_HAVE_KEEPALIVE_TUNABLE)
    int                        tcp_keepidle;
    int                        tcp_keepintvl;
    int                        tcp_keepcnt;
#endif

#if (NGX_HAVE_DEFERRED_ACCEPT && defined SO_ACCEPTFILTER)
    char                      *accept_filter;
#endif
}

之后调用ngx_http_add_listen将监听添加到ngx_http_conf_port_t中。对于已有的监听端口，调用ngx_http_add_addresses；对于第一次出现的监听端口，调用ngx_http_add_address。ngx_http_add_address会新增一个监听地址，同时将该监听地址的default_server设置成该server配置。

static ngx_int_t
ngx_http_add_address(ngx_conf_t *cf, ngx_http_core_srv_conf_t *cscf,
    ngx_http_conf_port_t *port, ngx_http_listen_opt_t *lsopt)
{
    ngx_http_conf_addr_t  *addr;

    ...

    addr->opt = *lsopt;
    addr->hash.buckets = NULL;
    addr->hash.size = 0;
    addr->wc_head = NULL;
    addr->wc_tail = NULL;
#if (NGX_PCRE)
    addr->nregex = 0;
    addr->regex = NULL;
#endif
    addr->default_server = cscf;
    addr->servers.elts = NULL;

    return ngx_http_add_server(cf, cscf, addr);
}

对于已存在的监听地址，ngx_http_add_addresses在ngx_http_listen_opt_t.default_server为1时会将ngx_http_conf_addr_t中的default_server替换为新的server配置。

static ngx_int_t
ngx_http_add_addresses(ngx_conf_t *cf, ngx_http_core_srv_conf_t *cscf,
    ngx_http_conf_port_t *port, ngx_http_listen_opt_t *lsopt)
{
    ngx_http_conf_addr_t  *addr;
    ...
        if (lsopt->default_server) {

            if (default_server) {
                ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                                   "a duplicate default server for %V",
                                   &addr[i].opt.addr_text);
                return NGX_ERROR;
            }

            default_server = 1;
            addr[i].default_server = cscf;
        }

        addr[i].opt.default_server = default_server;
       ...
}