[PATCH 3 of 4] Mail: added MPTCP support

[Dourov Maxime]

# HG changeset patch
# User maxime <mux99 at live.be>
# Date 1713355424 -7200
#      Wed Apr 17 14:03:44 2024 +0200
# Node ID f7ea7c665964b50f1ef6f3964c3fce9596afac20
# Parent  69e3909234d76c6888fba539f9bbbecc9858a7ab
Mail: added MPTCP support.

Multipath TCP (MPTCP), standardized in RFC8684 [1], is a TCP extension
that enables a TCP connection to use different paths.

Multipath TCP has been used for several use cases. On smartphones, MPTCP
enables seamless handovers between cellular and Wi-Fi networks while
preserving established connections. This use-case is what pushed Apple
to use MPTCP since 2013 in multiple applications [2]. On dual-stack
hosts, Multipath TCP enables the TCP connection to automatically use the
best performing path, either IPv4 or IPv6. If one path fails, MPTCP
automatically uses the other path.

To benefit from MPTCP, both the client and the server have to support
it. Multipath TCP is a backward-compatible TCP extension that is enabled
by default on recent Linux distributions (Debian, Ubuntu, Redhat, ...).
Multipath TCP is included in the Linux kernel since version 5.6 [3].
To use it on Linux, an application must explicitly enable it when
creating the socket. No need to change anything else in the application.

Even if MPTCP is supported by different OS, only Linux supports the
`IPPROTO_MPTCP` protocol, which is why this feature is currently
limited to Linux only.

This patch adds a new parameter 'mptcp' to the  'listen' directive
in the Mail module.

Link: https://www.rfc-editor.org/rfc/rfc8684.html [1]
Link: https://www.tessares.net/apples-mptcp-story-so-far/ [2]
Link: https://www.mptcp.dev [3]

diff --git a/src/mail/ngx_mail.c b/src/mail/ngx_mail.c
--- a/src/mail/ngx_mail.c
+++ b/src/mail/ngx_mail.c
@@ -332,6 +332,7 @@ ngx_mail_optimize_servers(ngx_conf_t *cf
             ls->log.data = &ls->addr_text;
             ls->log.handler = ngx_accept_log_error;
 
+            ls->protocol = addr[i].opt.protocol;
             ls->backlog = addr[i].opt.backlog;
             ls->rcvbuf = addr[i].opt.rcvbuf;
             ls->sndbuf = addr[i].opt.sndbuf;
diff --git a/src/mail/ngx_mail.h b/src/mail/ngx_mail.h
--- a/src/mail/ngx_mail.h
+++ b/src/mail/ngx_mail.h
@@ -47,6 +47,7 @@ typedef struct {
     int                     tcp_keepintvl;
     int                     tcp_keepcnt;
 #endif
+    int                     protocol;
     int                     backlog;
     int                     rcvbuf;
     int                     sndbuf;
diff --git a/src/mail/ngx_mail_core_module.c b/src/mail/ngx_mail_core_module.c
--- a/src/mail/ngx_mail_core_module.c
+++ b/src/mail/ngx_mail_core_module.c
@@ -10,6 +10,10 @@
 #include <ngx_event.h>
 #include <ngx_mail.h>
 
+#ifndef IPPROTO_MPTCP
+#define IPPROTO_MPTCP 262
+#endif
+
 
 static void *ngx_mail_core_create_main_conf(ngx_conf_t *cf);
 static void *ngx_mail_core_create_srv_conf(ngx_conf_t *cf);
@@ -468,6 +472,11 @@ ngx_mail_core_listen(ngx_conf_t *cf, ngx
 #endif
         }
 
+        if (ngx_strcmp(value[i].data, "mptcp") == 0) {
+            ls->protocol = IPPROTO_MPTCP;
+            continue;
+        }
+
         if (ngx_strncmp(value[i].data, "so_keepalive=", 13) == 0) {
 
             if (ngx_strcmp(&value[i].data[13], "on") == 0) {