Tor源碼檔案分析 — Connetion_Edge

　　所謂的Edge串連主要是形容在鏈路兩端所使用的串連，包括AP串連與EXIT串連兩種。Edge串連的存在，主要是為了個應用程式串連提供服務，所以才將兩種Edge串連命名為Application Proxy應用代理串連和EXIT出口串連。這兩種串連主要出現於鏈路兩端，即鏈路第一跳與鏈路最後一跳，分別用於接受應用請求和處理應用請求的發出。因為Edge串連在Tor系統中也具有重要的作用，所以本篇就簡單分析關於Edge串連的源碼檔案connection_edge.c。

　　因為Edge串連位於鏈路兩端，需要處理的事務比中間結點單純的傳遞和記錄要繁瑣出許多，有很多部分筆者也沒有很詳細的看過或找出其應用需要，此處必定有很多講的含糊其辭和不準確的地方，請大家包涵。

0. 全域變數

/** A client-side struct to remember requests to rewrite addresses * to new addresses. These structs are stored in the hash table * "addressmap" below. * 由此處可見，addressmap全域變數的使用，只存在於用戶端，其他身份的TOR成員並不使用此全域變數； * * There are 5 ways to set an address mapping: * - A MapAddress command from the controller [permanent] * - An AddressMap directive in the torrc [permanent] * - When a TrackHostExits torrc directive is triggered [temporary] * - When a DNS resolve succeeds [temporary] * - When a DNS resolve fails [temporary] * 由此處可見，除去在設定檔中進行地址映射之外，主要還是靠DNS解析來修改address mapping； * * When an addressmap request is made but one is already registered, * the new one is replaced only if the currently registered one has * no "new_address" (that is, it's in the process of DNS resolve), * or if the new one is permanent (expires==0 or 1). * * (We overload the 'expires' field, using "0" for mappings set via * the configuration file, "1" for mappings set from the control * interface, and other values for DNS and TrackHostExit mappings that can * expire.) * * A mapping may be 'wildcarded'.  If "src_wildcard" is true, then * any address that ends with a . followed by the key for this entry will * get remapped by it.  If "dst_wildcard" is also true, then only the * matching suffix of such addresses will get replaced by new_address. */typedef struct {  char *new_address;  time_t expires;  addressmap_entry_source_t source:3;  unsigned src_wildcard:1;  unsigned dst_wildcard:1;  short num_resolve_failures;} addressmap_entry_t;/** Entry for mapping addresses to which virtual address we mapped them to. */typedef struct {  char *ipv4_address;  char *hostname_address;} virtaddress_entry_t;/** A hash table to store client-side address rewrite instructions. */static strmap_t *addressmap=NULL;/** * Table mapping addresses to which virtual address, if any, we * assigned them to. * * We maintain the following invariant: if [A,B] is in * virtaddress_reversemap, then B must be a virtual address, and [A,B] * must be in addressmap.  We do not require that the converse hold: * if it fails, then we could end up mapping two virtual addresses to * the same address, which is no disaster. **/static strmap_t *virtaddress_reversemap=NULL;

　　此處兩全域變數的作用尚不明確，有待分析。

/* By default, we hand out 127.192.0.1 through 127.254.254.254. * These addresses should map to localhost, so even if the * application accidentally tried to connect to them directly (not * via Tor), it wouldn't get too far astray. * * These options are configured by parse_virtual_addr_network(). *//** Which network should we use for virtual IPv4 addresses?  Only the first * bits of this value are fixed. */static uint32_t virtual_addr_network = 0x7fc00000u;/** How many bits of <b>virtual_addr_network</b> are fixed? */static maskbits_t virtual_addr_netmask_bits = 10;/** What's the next virtual address we will hand out? */static uint32_t next_virtual_addr    = 0x7fc00000u;

　　此處三全域變數的作用尚不明確，有待分析。

1. AP與EXIT通用函數

　　connection_edge_reached_eof  -->  connection_edge_end

　　中斷連線，如果身份正確（AP）則發送end cell通知對應出口結點關閉資料流；

　　connection_edge_process_inbuf

　　分情況管理串連事務及串連緩衝區內部的資料；

　　connection_edge_destroy  -->  connection_mark_unattached_ap（若身份為AP）

　　connection_edge_end_errno  --> connection_edge_end

　　串連因鏈路斷開而結束或因某個errno而結束，則關閉串連，發送流終止通知；

　　connection_edge_flushed_some

　　connection_edge_finished_flushing

　　connection_edge_finished_connecting（主要用於身份為EXIT的情況）

　　串連完成緩衝區輸出或完成下層串連之時調用相關函數，以完成掃尾工作；

　　connection_edge_is_rendezvous_stream

　　connection_edge_compatible_with_circuit

　　connection_edge_update_circuit_isolation

　　circuit_clear_isolation

　　串連的某些屬性判斷，關鍵點在於rendezvous子系統和isolation flag作用；

2. AP處理函數

　　connection_mark_unattached_ap

　　connection_ap_about_to_close

　　AP串連關閉前的處理以及關閉處理；

　　connection_ap_handshake_send_begin

　　connection_ap_handshake_send_resolve

　　AP串連向遠端發送Relay Begin以及Relay Resolve命令，用於完成開啟流與位址解析的功能；

　　connection_ap_make_link

　　一般用於內部DIR串連與AP串連相關聯之時，該函數建立一個可用的AP串連，並將其與DIR串連互聯；可用意味著AP串連找到附著的鏈路Circuit；

　　connection_ap_handshake_socks_reply

　　connection_ap_handshake_socks_resolved

　　向AP串連送回相關訊息：串連結束訊息，位址解析成功訊息；

　　connection_ap_can_use_exit

　　判斷出口結點是否允許相應AP串連出口；

　　connection_ap_expire_beginning

　　connection_ap_attach_pending

　　connection_ap_detach_retriable

　　重連一個請求逾時的AP串連；重新為找不到鏈路的AP串連分配鏈路；

　　connection_ap_fail_onehop

　　通知所有等待某個無法串連的結點的AP串連；

　　connection_ap_process_transparent  -->  connection_ap_rewrite_and_attach_if_allowed  -->  connection_ap_handshake_rewrite_and_attach

　　透明AP使用的函數，未細究；

　　

3. EXIT處理函數

　　connection_exit_about_to_close

　　connection_exit_begin_conn  -->  connection_exit_connect

　　connection_exit_begin_resolve

　　以上四個函數包括了EXIT串連的幾乎所有操作，主要就是串連的建立與對外的串連，以及位址解析和關閉；

4. AddressMap相關處理函數（暫略）

　　address_is_invalid_destination

　　parse_virtual_addr_network

　　parse_extended_hostname

　　addressmap_init

　　addressmap_clear_excluded_trackexithosts

　　addressmap_clear_invalid_automaps

　　addressmap_clean

　　addressmap_clear_configured

　　addressmap_clear_transient

　　addressmap_free_all

　　addressmap_rewrite

　　addressmap_have_mapping

　　addressmap_register

　　addressmap_register_virtual_address

　　addressmap_get_mappings

　　client_dns_incr_failures

　　client_dns_clear_failures

　　client_dns_set_addressmap

5. 額外函數（暫略）

　　circuit_discard_optional_exit_enclaves

　　get_pf_socket