text: // annotated by chrono since 2021
  //
  // * ngx_http_realip_handler
  
  /*
   * Copyright (C) Igor Sysoev
   * Copyright (C) Nginx, Inc.
   */
  
  
  #include <ngx_config.h>
  #include <ngx_core.h>
  #include <ngx_http.h>
  
  
  #define NGX_HTTP_REALIP_XREALIP  0
  #define NGX_HTTP_REALIP_XFWD     1
  #define NGX_HTTP_REALIP_HEADER   2
  #define NGX_HTTP_REALIP_PROXY    3
  
  
  typedef struct {
      ngx_array_t       *from;     /* array of ngx_cidr_t */
      ngx_uint_t         type;
      ngx_uint_t         hash;
      ngx_str_t          header;
      ngx_flag_t         recursive;
  } ngx_http_realip_loc_conf_t;
  
  
  typedef struct {
      ngx_connection_t  *connection;
      struct sockaddr   *sockaddr;
      socklen_t          socklen;
      ngx_str_t          addr_text;
  } ngx_http_realip_ctx_t;
  
  
  static ngx_int_t ngx_http_realip_handler(ngx_http_request_t *r);
  static ngx_int_t ngx_http_realip_set_addr(ngx_http_request_t *r,
      ngx_addr_t *addr);
  static void ngx_http_realip_cleanup(void *data);
  static char *ngx_http_realip_from(ngx_conf_t *cf, ngx_command_t *cmd,
      void *conf);
  static char *ngx_http_realip(ngx_conf_t *cf, ngx_command_t *cmd, void *conf);
  static void *ngx_http_realip_create_loc_conf(ngx_conf_t *cf);
  static char *ngx_http_realip_merge_loc_conf(ngx_conf_t *cf,
      void *parent, void *child);
  static ngx_int_t ngx_http_realip_add_variables(ngx_conf_t *cf);
  
  // 把handler加载到post_read阶段，即读取完header之后
  static ngx_int_t ngx_http_realip_init(ngx_conf_t *cf);
  
  static ngx_http_realip_ctx_t *ngx_http_realip_get_module_ctx(
      ngx_http_request_t *r);
  
  
  static ngx_int_t ngx_http_realip_remote_addr_variable(ngx_http_request_t *r,
      ngx_http_variable_value_t *v, uintptr_t data);
  static ngx_int_t ngx_http_realip_remote_port_variable(ngx_http_request_t *r,
      ngx_http_variable_value_t *v, uintptr_t data);
  
  
  static ngx_command_t  ngx_http_realip_commands[] = {
  
      { ngx_string("set_real_ip_from"),
        NGX_HTTP_MAIN_CONF|NGX_HTTP_SRV_CONF|NGX_HTTP_LOC_CONF|NGX_CONF_TAKE1,
        ngx_http_realip_from,
        NGX_HTTP_LOC_CONF_OFFSET,
        0,
        NULL },
  
      { ngx_string("real_ip_header"),
        NGX_HTTP_MAIN_CONF|NGX_HTTP_SRV_CONF|NGX_HTTP_LOC_CONF|NGX_CONF_TAKE1,
        ngx_http_realip,
        NGX_HTTP_LOC_CONF_OFFSET,
        0,
        NULL },
  
      { ngx_string("real_ip_recursive"),
        NGX_HTTP_MAIN_CONF|NGX_HTTP_SRV_CONF|NGX_HTTP_LOC_CONF|NGX_CONF_FLAG,
        ngx_conf_set_flag_slot,
        NGX_HTTP_LOC_CONF_OFFSET,
        offsetof(ngx_http_realip_loc_conf_t, recursive),
        NULL },
  
        ngx_null_command
  };
  
  
  
  static ngx_http_module_t  ngx_http_realip_module_ctx = {
      ngx_http_realip_add_variables,         /* preconfiguration */
  
      // 把handler加载到post_read阶段，即读取完header之后
      ngx_http_realip_init,                  /* postconfiguration */
  
      NULL,                                  /* create main configuration */
      NULL,                                  /* init main configuration */
  
      NULL,                                  /* create server configuration */
      NULL,                                  /* merge server configuration */
  
      ngx_http_realip_create_loc_conf,       /* create location configuration */
      ngx_http_realip_merge_loc_conf         /* merge location configuration */
  };
  
  
  ngx_module_t  ngx_http_realip_module = {
      NGX_MODULE_V1,
      &ngx_http_realip_module_ctx,           /* module context */
      ngx_http_realip_commands,              /* module directives */
      NGX_HTTP_MODULE,                       /* module type */
      NULL,                                  /* init master */
      NULL,                                  /* init module */
      NULL,                                  /* init process */
      NULL,                                  /* init thread */
      NULL,                                  /* exit thread */
      NULL,                                  /* exit process */
      NULL,                                  /* exit master */
      NGX_MODULE_V1_PADDING
  };
  
  
  static ngx_http_variable_t  ngx_http_realip_vars[] = {
  
      { ngx_string("realip_remote_addr"), NULL,
        ngx_http_realip_remote_addr_variable, 0, 0, 0 },
  
      { ngx_string("realip_remote_port"), NULL,
        ngx_http_realip_remote_port_variable, 0, 0, 0 },
  
        ngx_http_null_variable
  };
  
  
  // 此时已经拿到了完整的请求头
  static ngx_int_t
  ngx_http_realip_handler(ngx_http_request_t *r)
  {
      u_char                      *p;
      size_t                       len;
      ngx_str_t                   *value;
      ngx_uint_t                   i, hash;
      ngx_addr_t                   addr;
      ngx_array_t                 *xfwd;
      ngx_list_part_t             *part;
      ngx_table_elt_t             *header;
      ngx_connection_t            *c;
      ngx_http_realip_ctx_t       *ctx;
      ngx_http_realip_loc_conf_t  *rlcf;
  
      rlcf = ngx_http_get_module_loc_conf(r, ngx_http_realip_module);
  
      if (rlcf->from == NULL) {
          return NGX_DECLINED;
      }
  
      ctx = ngx_http_realip_get_module_ctx(r);
  
      if (ctx) {
          return NGX_DECLINED;
      }
  
      // 检查指令real_ip_header的配置
      switch (rlcf->type) {
  
      // 看x_real_ip字段
      case NGX_HTTP_REALIP_XREALIP:
  
          if (r->headers_in.x_real_ip == NULL) {
              return NGX_DECLINED;
          }
  
          value = &r->headers_in.x_real_ip->value;
          xfwd = NULL;
  
          break;
  
      // 看x_forwarded_for字段
      case NGX_HTTP_REALIP_XFWD:
  
          xfwd = &r->headers_in.x_forwarded_for;
  
          if (xfwd->elts == NULL) {
              return NGX_DECLINED;
          }
  
          value = NULL;
  
          break;
  
      // 看proxy protocol的处理情况
      case NGX_HTTP_REALIP_PROXY:
  
          if (r->connection->proxy_protocol == NULL) {
              return NGX_DECLINED;
          }
  
          value = &r->connection->proxy_protocol->src_addr;
          xfwd = NULL;
  
          break;
  
      // 看指定的特殊字段
      default: /* NGX_HTTP_REALIP_HEADER */
  
          // 遍历头字段
          part = &r->headers_in.headers.part;
          header = part->elts;
  
          hash = rlcf->hash;
          len = rlcf->header.len;
          p = rlcf->header.data;
  
          for (i = 0; /* void */ ; i++) {
  
              if (i >= part->nelts) {
                  if (part->next == NULL) {
                      break;
                  }
  
                  part = part->next;
                  header = part->elts;
                  i = 0;
              }
  
              // 比较hash和字符串
              if (hash == header[i].hash
                  && len == header[i].key.len
                  && ngx_strncmp(p, header[i].lowcase_key, len) == 0)
              {
                  value = &header[i].value;
                  xfwd = NULL;
  
                  goto found;
              }
          }
  
          return NGX_DECLINED;
      }
  
  found:
  
      c = r->connection;
  
      addr.sockaddr = c->sockaddr;
      addr.socklen = c->socklen;
      /* addr.name = c->addr_text; */
  
      if (ngx_http_get_forwarded_addr(r, &addr, xfwd, value, rlcf->from,
                                      rlcf->recursive)
          != NGX_DECLINED)
      {
          if (rlcf->type == NGX_HTTP_REALIP_PROXY) {
              ngx_inet_set_port(addr.sockaddr, c->proxy_protocol->src_port);
          }
  
          return ngx_http_realip_set_addr(r, &addr);
      }
  
      return NGX_DECLINED;
  }
  
  
  static ngx_int_t
  ngx_http_realip_set_addr(ngx_http_request_t *r, ngx_addr_t *addr)
  {
      size_t                  len;
      u_char                 *p;
      u_char                  text[NGX_SOCKADDR_STRLEN];
      ngx_connection_t       *c;
      ngx_pool_cleanup_t     *cln;
      ngx_http_realip_ctx_t  *ctx;
  
      cln = ngx_pool_cleanup_add(r->pool, sizeof(ngx_http_realip_ctx_t));
      if (cln == NULL) {
          return NGX_HTTP_INTERNAL_SERVER_ERROR;
      }
  
      ctx = cln->data;
  
      c = r->connection;
  
      len = ngx_sock_ntop(addr->sockaddr, addr->socklen, text,
                          NGX_SOCKADDR_STRLEN, 0);
      if (len == 0) {
          return NGX_HTTP_INTERNAL_SERVER_ERROR;
      }
  
      p = ngx_pnalloc(c->pool, len);
      if (p == NULL) {
          return NGX_HTTP_INTERNAL_SERVER_ERROR;
      }
  
      ngx_memcpy(p, text, len);
  
      cln->handler = ngx_http_realip_cleanup;
      ngx_http_set_ctx(r, ctx, ngx_http_realip_module);
  
      ctx->connection = c;
      ctx->sockaddr = c->sockaddr;
      ctx->socklen = c->socklen;
      ctx->addr_text = c->addr_text;
  
      c->sockaddr = addr->sockaddr;
      c->socklen = addr->socklen;
      c->addr_text.len = len;
      c->addr_text.data = p;
  
      return NGX_DECLINED;
  }
  
  
  static void
  ngx_http_realip_cleanup(void *data)
  {
      ngx_http_realip_ctx_t *ctx = data;
  
      ngx_connection_t  *c;
  
      c = ctx->connection;
  
      c->sockaddr = ctx->sockaddr;
      c->socklen = ctx->socklen;
      c->addr_text = ctx->addr_text;
  }
  
  
  static char *
  ngx_http_realip_from(ngx_conf_t *cf, ngx_command_t *cmd, void *conf)
  {
      ngx_http_realip_loc_conf_t *rlcf = conf;
  
      ngx_int_t             rc;
      ngx_str_t            *value;
      ngx_url_t             u;
      ngx_cidr_t            c, *cidr;
      ngx_uint_t            i;
      struct sockaddr_in   *sin;
  #if (NGX_HAVE_INET6)
      struct sockaddr_in6  *sin6;
  #endif
  
      value = cf->args->elts;
  
      if (rlcf->from == NULL) {
          rlcf->from = ngx_array_create(cf->pool, 2,
                                        sizeof(ngx_cidr_t));
          if (rlcf->from == NULL) {
              return NGX_CONF_ERROR;
          }
      }
  
  #if (NGX_HAVE_UNIX_DOMAIN)
  
      if (ngx_strcmp(value[1].data, "unix:") == 0) {
          cidr = ngx_array_push(rlcf->from);
          if (cidr == NULL) {
              return NGX_CONF_ERROR;
          }
  
          cidr->family = AF_UNIX;
          return NGX_CONF_OK;
      }
  
  #endif
  
      rc = ngx_ptocidr(&value[1], &c);
  
      if (rc != NGX_ERROR) {
          if (rc == NGX_DONE) {
              ngx_conf_log_error(NGX_LOG_WARN, cf, 0,
                                 "low address bits of %V are meaningless",
                                 &value[1]);
          }
  
          cidr = ngx_array_push(rlcf->from);
          if (cidr == NULL) {
              return NGX_CONF_ERROR;
          }
  
          *cidr = c;
  
          return NGX_CONF_OK;
      }
  
      ngx_memzero(&u, sizeof(ngx_url_t));
      u.host = value[1];
  
      if (ngx_inet_resolve_host(cf->pool, &u) != NGX_OK) {
          if (u.err) {
              ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                                 "%s in set_real_ip_from \"%V\"",
                                 u.err, &u.host);
          }
  
          return NGX_CONF_ERROR;
      }
  
      cidr = ngx_array_push_n(rlcf->from, u.naddrs);
      if (cidr == NULL) {
          return NGX_CONF_ERROR;
      }
  
      ngx_memzero(cidr, u.naddrs * sizeof(ngx_cidr_t));
  
      for (i = 0; i < u.naddrs; i++) {
          cidr[i].family = u.addrs[i].sockaddr->sa_family;
  
          switch (cidr[i].family) {
  
  #if (NGX_HAVE_INET6)
          case AF_INET6:
              sin6 = (struct sockaddr_in6 *) u.addrs[i].sockaddr;
              cidr[i].u.in6.addr = sin6->sin6_addr;
              ngx_memset(cidr[i].u.in6.mask.s6_addr, 0xff, 16);
              break;
  #endif
  
          default: /* AF_INET */
              sin = (struct sockaddr_in *) u.addrs[i].sockaddr;
              cidr[i].u.in.addr = sin->sin_addr.s_addr;
              cidr[i].u.in.mask = 0xffffffff;
              break;
          }
      }
  
      return NGX_CONF_OK;
  }
  
  
  static char *
  ngx_http_realip(ngx_conf_t *cf, ngx_command_t *cmd, void *conf)
  {
      ngx_http_realip_loc_conf_t *rlcf = conf;
  
      ngx_str_t  *value;
  
      if (rlcf->type != NGX_CONF_UNSET_UINT) {
          return "is duplicate";
      }
  
      value = cf->args->elts;
  
      if (ngx_strcmp(value[1].data, "X-Real-IP") == 0) {
          rlcf->type = NGX_HTTP_REALIP_XREALIP;
          return NGX_CONF_OK;
      }
  
      if (ngx_strcmp(value[1].data, "X-Forwarded-For") == 0) {
          rlcf->type = NGX_HTTP_REALIP_XFWD;
          return NGX_CONF_OK;
      }
  
      if (ngx_strcmp(value[1].data, "proxy_protocol") == 0) {
          rlcf->type = NGX_HTTP_REALIP_PROXY;
          return NGX_CONF_OK;
      }
  
      rlcf->type = NGX_HTTP_REALIP_HEADER;
      rlcf->hash = ngx_hash_strlow(value[1].data, value[1].data, value[1].len);
      rlcf->header = value[1];
  
      return NGX_CONF_OK;
  }
  
  
  static void *
  ngx_http_realip_create_loc_conf(ngx_conf_t *cf)
  {
      ngx_http_realip_loc_conf_t  *conf;
  
      conf = ngx_pcalloc(cf->pool, sizeof(ngx_http_realip_loc_conf_t));
      if (conf == NULL) {
          return NULL;
      }
  
      /*
       * set by ngx_pcalloc():
       *
       *     conf->from = NULL;
       *     conf->hash = 0;
       *     conf->header = { 0, NULL };
       */
  
      conf->type = NGX_CONF_UNSET_UINT;
      conf->recursive = NGX_CONF_UNSET;
  
      return conf;
  }
  
  
  static char *
  ngx_http_realip_merge_loc_conf(ngx_conf_t *cf, void *parent, void *child)
  {
      ngx_http_realip_loc_conf_t  *prev = parent;
      ngx_http_realip_loc_conf_t  *conf = child;
  
      if (conf->from == NULL) {
          conf->from = prev->from;
      }
  
      ngx_conf_merge_uint_value(conf->type, prev->type, NGX_HTTP_REALIP_XREALIP);
      ngx_conf_merge_value(conf->recursive, prev->recursive, 0);
  
      if (conf->header.len == 0) {
          conf->hash = prev->hash;
          conf->header = prev->header;
      }
  
      return NGX_CONF_OK;
  }
  
  
  static ngx_int_t
  ngx_http_realip_add_variables(ngx_conf_t *cf)
  {
      ngx_http_variable_t  *var, *v;
  
      for (v = ngx_http_realip_vars; v->name.len; v++) {
          var = ngx_http_add_variable(cf, &v->name, v->flags);
          if (var == NULL) {
              return NGX_ERROR;
          }
  
          var->get_handler = v->get_handler;
          var->data = v->data;
      }
  
      return NGX_OK;
  }
  
  
  // 把handler加载到post_read阶段，即读取完header之后
  static ngx_int_t
  ngx_http_realip_init(ngx_conf_t *cf)
  {
      ngx_http_handler_pt        *h;
      ngx_http_core_main_conf_t  *cmcf;
  
      cmcf = ngx_http_conf_get_module_main_conf(cf, ngx_http_core_module);
  
      h = ngx_array_push(&cmcf->phases[NGX_HTTP_POST_READ_PHASE].handlers);
      if (h == NULL) {
          return NGX_ERROR;
      }
  
      *h = ngx_http_realip_handler;
  
      h = ngx_array_push(&cmcf->phases[NGX_HTTP_PREACCESS_PHASE].handlers);
      if (h == NULL) {
          return NGX_ERROR;
      }
  
      *h = ngx_http_realip_handler;
  
      return NGX_OK;
  }
  
  
  static ngx_http_realip_ctx_t *
  ngx_http_realip_get_module_ctx(ngx_http_request_t *r)
  {
      ngx_pool_cleanup_t     *cln;
      ngx_http_realip_ctx_t  *ctx;
  
      ctx = ngx_http_get_module_ctx(r, ngx_http_realip_module);
  
      if (ctx == NULL && (r->internal || r->filter_finalize)) {
  
          /*
           * if module context was reset, the original address
           * can still be found in the cleanup handler
           */
  
          for (cln = r->pool->cleanup; cln; cln = cln->next) {
              if (cln->handler == ngx_http_realip_cleanup) {
                  ctx = cln->data;
                  break;
              }
          }
      }
  
      return ctx;
  }
  
  
  static ngx_int_t
  ngx_http_realip_remote_addr_variable(ngx_http_request_t *r,
      ngx_http_variable_value_t *v, uintptr_t data)
  {
      ngx_str_t              *addr_text;
      ngx_http_realip_ctx_t  *ctx;
  
      ctx = ngx_http_realip_get_module_ctx(r);
  
      addr_text = ctx ? &ctx->addr_text : &r->connection->addr_text;
  
      v->len = addr_text->len;
      v->valid = 1;
      v->no_cacheable = 0;
      v->not_found = 0;
      v->data = addr_text->data;
  
      return NGX_OK;
  }
  
  
  static ngx_int_t
  ngx_http_realip_remote_port_variable(ngx_http_request_t *r,
      ngx_http_variable_value_t *v, uintptr_t data)
  {
      ngx_uint_t              port;
      struct sockaddr        *sa;
      ngx_http_realip_ctx_t  *ctx;
  
      ctx = ngx_http_realip_get_module_ctx(r);
  
      sa = ctx ? ctx->sockaddr : r->connection->sockaddr;
  
      v->len = 0;
      v->valid = 1;
      v->no_cacheable = 0;
      v->not_found = 0;
  
      v->data = ngx_pnalloc(r->pool, sizeof("65535") - 1);
      if (v->data == NULL) {
          return NGX_ERROR;
      }
  
      port = ngx_inet_get_port(sa);
  
      if (port > 0 && port < 65536) {
          v->len = ngx_sprintf(v->data, "%ui", port) - v->data;
      }
  
      return NGX_OK;
  }
  