From 4bc7f5544d28e6b197d1bdfa5bed3fddb2a01c53 Mon Sep 17 00:00:00 2001
From: Brian Flachs <flachs@flachs.us>
Date: Tue, 29 Mar 2022 10:36:40 -0500
Subject: [PATCH] add sender configuration options for NAT masq

---
 esmtprc.5 | 14 +++++++++++
 parser.y  |  1 +
 smtp.c    | 75 ++++++++++++++++++++++++++++++++++++++++++++++++++++---
 3 files changed, 87 insertions(+), 3 deletions(-)

diff --git a/esmtprc.5 b/esmtprc.5
index 415ce0e..3a78448 100644
--- a/esmtprc.5
+++ b/esmtprc.5
@@ -42,6 +42,20 @@ The host-port notation simplifies things for the application, the user can
 type "localhost:smtp" or "localhost:25" where the application
 expects a host name.
 
+.TP
+\fBsender\fR
+Set the identity of the sender of the email.
+
+This is useful for systems behind a NAT firewall, when for example
+the sender's email address should not have the hostname.  Several
+escape formatting codes are available:
+
+   %u - user name of the sender
+   %h - hostname of the sending system
+   %d - domainname of the sending system
+
+So %u@%d might be a desirable identity from inside the local network.
+
 .TP
 \fBusername\fR
 Set the username for authentication with the SMTP server.
diff --git a/parser.y b/parser.y
index b2c7a8f..4928f68 100644
--- a/parser.y
+++ b/parser.y
@@ -104,6 +104,7 @@ statement_list	: statement
 
 /* future global options should also have the form SET <name> optmap <value> */
 statement	: HOSTNAME map STRING	{ identity->host = xstrdup($3); SET_DEFAULT_IDENTITY; }
+                | SENDER map STRING     { identity->address = xstrdup($3); }
 		| USERNAME map STRING	{ identity->user = xstrdup($3); SET_DEFAULT_IDENTITY; }
 		| PASSWORD map STRING	{ identity->pass = xstrdup($3); SET_DEFAULT_IDENTITY; }
 		| STARTTLS map DISABLED	{ identity->starttls = Starttls_DISABLED; SET_DEFAULT_IDENTITY; }
diff --git a/smtp.c b/smtp.c
index 796578d..11d4f4d 100644
--- a/smtp.c
+++ b/smtp.c
@@ -485,6 +485,67 @@ static char *escape_forced_address (char *mask)
 	
 }
 
+/* search for end of string */
+static inline char *eos(char *p)
+  {
+  while (*p) p++;
+  return p;
+  }
+
+/* substitute %codes in a string,
+     %u = get_username()
+     %h = gethostname()
+     %d = domainname (omit host. in host.domain.name from gethostname)
+   return the result in a newly malloced string
+*/
+char *xstrsubst(char *pp,char *p)
+  {
+  char address[2048],*dp=address;
+  while (p)
+    {
+    strcpy(dp,pp);
+    dp += (p - pp);
+    switch (p[1])
+      {
+      case 'u':;
+        strcpy(dp,get_username());
+        dp = eos(dp);
+        p += 2;
+        break;
+              
+      case 'h':;
+        if(! gethostname(dp,sizeof(address)-(dp-address)))
+          dp = eos(dp);
+        p += 2;
+        break;
+              
+      case 'd':
+        if(! gethostname(dp,sizeof(address)-(dp-address)))
+          {
+          char *pt = strchr(dp,'.');
+          if (pt++)
+            {
+            do { *dp++ = *pt++; } while (*pt);
+            }
+          else
+            dp = eos(dp);
+          }
+              
+        p += 2;
+        break;
+              
+      default:
+        fprintf(stderr,"unknown formatter in sender %s\n",p);
+        return 0;
+      }
+    pp = p;
+    p = strchr(pp,'%');
+    }
+  
+  strcpy(dp,pp);
+  return xstrdup(address);
+  }
+
 void smtp_send(message_t *msg, identity_t *identity)
 {
 	smtp_session_t session;
@@ -597,9 +658,17 @@ void smtp_send(message_t *msg, identity_t *identity)
 	}
 	else if(identity->address)
 	{
-		/* Use the identity address as reverse path. */
-		if(!smtp_set_reverse_path (message, identity->address))
-			goto failure;
+                char *p=strchr(identity->address,'%');
+                if (p)
+                { /* expand %codes in address */
+                        char *av = xstrsubst(identity->address,p);
+                        if (! av) goto failure;
+                        free(identity->address);
+                        identity->address = av;
+                }
+                /* Use the identity address as reverse path. */
+                if(!smtp_set_reverse_path (message, identity->address))
+                        goto failure;
 	}
 	else
 	{