diff --git a/logging.c b/logging.c
index a43ebcd1..024bb11e 100644
--- a/logging.c
+++ b/logging.c
@@ -31,7 +31,7 @@ static Filename *xlatlognam(Filename *s, char *hostname, int port,
  * isn't open, buffering data if it's in the process of being
  * opened asynchronously, etc.
  */
-static void logwrite(LogContext *ctx, void *data, int len)
+static void logwrite(LogContext *ctx, const void *data, int len)
 {
     /*
      * In state L_CLOSED, we call logfopen, which will set the state
diff --git a/network.h b/network.h
index a394a356..a84df7f0 100644
--- a/network.h
+++ b/network.h
@@ -70,7 +70,7 @@ struct PlugVtable {
     /* error_msg is NULL iff it is not an error (ie it closed normally) */
     /* calling_back != 0 iff there is a Plug function */
     /* currently running (would cure the fixme in try_send()) */
-    void (*receive) (Plug *p, int urgent, char *data, int len);
+    void (*receive) (Plug *p, int urgent, const char *data, int len);
     /*
      *  - urgent==0. `data' points to `len' bytes of perfectly
      *    ordinary data.
diff --git a/nullplug.c b/nullplug.c
index ca6c7c2c..25a3cc97 100644
--- a/nullplug.c
+++ b/nullplug.c
@@ -17,7 +17,7 @@ static void nullplug_closing(Plug *plug, const char *error_msg, int error_code,
 {
 }
 
-static void nullplug_receive(Plug *plug, int urgent, char *data, int len)
+static void nullplug_receive(Plug *plug, int urgent, const char *data, int len)
 {
 }
 
diff --git a/pageant.c b/pageant.c
index 24f4f9e3..216dc94d 100644
--- a/pageant.c
+++ b/pageant.c
@@ -755,7 +755,8 @@ static void pageant_conn_log(void *logctx, const char *fmt, va_list ap)
     sfree(formatted);
 }
 
-static void pageant_conn_receive(Plug *plug, int urgent, char *data, int len)
+static void pageant_conn_receive(
+    Plug *plug, int urgent, const char *data, int len)
 {
     struct pageant_conn_state *pc = container_of(
         plug, struct pageant_conn_state, plug);
diff --git a/portfwd.c b/portfwd.c
index e410850b..91d47802 100644
--- a/portfwd.c
+++ b/portfwd.c
@@ -192,7 +192,7 @@ static char *ipv6_to_string(ptrlen ipv6)
                      (unsigned)GET_16BIT_MSB_FIRST(addr + 14));
 }
 
-static void pfd_receive(Plug *plug, int urgent, char *data, int len)
+static void pfd_receive(Plug *plug, int urgent, const char *data, int len)
 {
     struct PortForwarding *pf =
         container_of(plug, struct PortForwarding, plug);
diff --git a/proxy.c b/proxy.c
index aa940e41..26373908 100644
--- a/proxy.c
+++ b/proxy.c
@@ -214,7 +214,7 @@ static void plug_proxy_closing (Plug *p, const char *error_msg,
     }
 }
 
-static void plug_proxy_receive (Plug *p, int urgent, char *data, int len)
+static void plug_proxy_receive (Plug *p, int urgent, const char *data, int len)
 {
     ProxySocket *ps = container_of(p, ProxySocket, plugimpl);
 
diff --git a/proxy.h b/proxy.h
index 2ecde43a..b7e2c3b3 100644
--- a/proxy.h
+++ b/proxy.h
@@ -68,7 +68,7 @@ struct ProxySocket {
 
     /* receive */
     bool receive_urgent;
-    char *receive_data;
+    const char *receive_data;
     int receive_len;
 
     /* sent */
diff --git a/pscp.c b/pscp.c
index 28566a5d..9e760c77 100644
--- a/pscp.c
+++ b/pscp.c
@@ -574,7 +574,7 @@ bool sftp_recvdata(char *buf, int len)
 {
     return ssh_scp_recv(buf, len);
 }
-bool sftp_senddata(char *buf, int len)
+bool sftp_senddata(const char *buf, int len)
 {
     backend_send(backend, buf, len);
     return true;
diff --git a/psftp.c b/psftp.c
index 21c57365..4a338da8 100644
--- a/psftp.c
+++ b/psftp.c
@@ -2493,7 +2493,7 @@ bool sftp_recvdata(char *buf, int len)
 
     return true;
 }
-bool sftp_senddata(char *buf, int len)
+bool sftp_senddata(const char *buf, int len)
 {
     backend_send(backend, buf, len);
     return true;
diff --git a/raw.c b/raw.c
index 90dde5d8..d448e0e6 100644
--- a/raw.c
+++ b/raw.c
@@ -89,7 +89,7 @@ static void raw_closing(Plug *plug, const char *error_msg, int error_code,
     }
 }
 
-static void raw_receive(Plug *plug, int urgent, char *data, int len)
+static void raw_receive(Plug *plug, int urgent, const char *data, int len)
 {
     Raw *raw = container_of(plug, Raw, plug);
     c_write(raw, data, len);
diff --git a/rlogin.c b/rlogin.c
index 46bfb7eb..27f2a0da 100644
--- a/rlogin.c
+++ b/rlogin.c
@@ -71,9 +71,11 @@ static void rlogin_closing(Plug *plug, const char *error_msg, int error_code,
     }				       /* Otherwise, the remote side closed the connection normally. */
 }
 
-static void rlogin_receive(Plug *plug, int urgent, char *data, int len)
+static void rlogin_receive(Plug *plug, int urgent, const char *data, int len)
 {
     Rlogin *rlogin = container_of(plug, Rlogin, plug);
+    if (len == 0)
+        return;
     if (urgent == 2) {
 	char c;
 
diff --git a/sftp.h b/sftp.h
index 95d70ff7..cdbea3e5 100644
--- a/sftp.h
+++ b/sftp.h
@@ -70,7 +70,7 @@
  * sftp_sendbuffer returns the size of the backlog of data in the
  * transmit queue.
  */
-bool sftp_senddata(char *data, int len);
+bool sftp_senddata(const char *data, int len);
 int sftp_sendbuffer(void);
 bool sftp_recvdata(char *data, int len);
 
diff --git a/ssh.c b/ssh.c
index bca83198..eabd3361 100644
--- a/ssh.c
+++ b/ssh.c
@@ -573,7 +573,7 @@ static void ssh_closing(Plug *plug, const char *error_msg, int error_code,
     }
 }
 
-static void ssh_receive(Plug *plug, int urgent, char *data, int len)
+static void ssh_receive(Plug *plug, int urgent, const char *data, int len)
 {
     Ssh *ssh = container_of(plug, Ssh, plug);
 
diff --git a/sshserver.c b/sshserver.c
index f1bcd9a7..d89e3809 100644
--- a/sshserver.c
+++ b/sshserver.c
@@ -133,7 +133,7 @@ static void server_closing(Plug *plug, const char *error_msg, int error_code,
     }
 }
 
-static void server_receive(Plug *plug, int urgent, char *data, int len)
+static void server_receive(Plug *plug, int urgent, const char *data, int len)
 {
     server *srv = container_of(plug, server, plug);
 
diff --git a/sshshare.c b/sshshare.c
index f10a29d6..89b57536 100644
--- a/sshshare.c
+++ b/sshshare.c
@@ -1754,7 +1754,7 @@ static void share_got_pkt_from_downstream(struct ssh_sharing_connstate *cs,
         (c) = (unsigned char)*data++;                           \
     } while (0)
 
-static void share_receive(Plug *plug, int urgent, char *data, int len)
+static void share_receive(Plug *plug, int urgent, const char *data, int len)
 {
     ssh_sharing_connstate *cs = container_of(
         plug, ssh_sharing_connstate, plug);
diff --git a/telnet.c b/telnet.c
index ae85a4d2..8fb0f572 100644
--- a/telnet.c
+++ b/telnet.c
@@ -506,7 +506,7 @@ static void process_subneg(Telnet *telnet)
     }
 }
 
-static void do_telnet_read(Telnet *telnet, char *buf, int len)
+static void do_telnet_read(Telnet *telnet, const char *buf, int len)
 {
     strbuf *outbuf = strbuf_new();
 
@@ -654,7 +654,7 @@ static void telnet_closing(Plug *plug, const char *error_msg, int error_code,
     /* Otherwise, the remote side closed the connection normally. */
 }
 
-static void telnet_receive(Plug *plug, int urgent, char *data, int len)
+static void telnet_receive(Plug *plug, int urgent, const char *data, int len)
 {
     Telnet *telnet = container_of(plug, Telnet, plug);
     if (urgent)
diff --git a/unix/uxpgnt.c b/unix/uxpgnt.c
index 14e4b51b..193a7dee 100644
--- a/unix/uxpgnt.c
+++ b/unix/uxpgnt.c
@@ -155,7 +155,7 @@ void chan_no_request_response(Channel *chan, bool success) {}
  */
 static void x11_log(Plug *p, int type, SockAddr *addr, int port,
 		    const char *error_msg, int error_code) {}
-static void x11_receive(Plug *plug, int urgent, char *data, int len) {}
+static void x11_receive(Plug *plug, int urgent, const char *data, int len) {}
 static void x11_sent(Plug *plug, int bufsize) {}
 static void x11_closing(Plug *plug, const char *error_msg, int error_code,
 			bool calling_back)
diff --git a/windows/winhsock.c b/windows/winhsock.c
index 71f2c188..a50989a8 100644
--- a/windows/winhsock.c
+++ b/windows/winhsock.c
@@ -45,7 +45,7 @@ typedef struct HandleSocket {
     Socket sock;
 } HandleSocket;
 
-static int handle_gotdata(struct handle *h, void *data, int len)
+static int handle_gotdata(struct handle *h, const void *data, int len)
 {
     HandleSocket *hs = (HandleSocket *)handle_get_privdata(h);
 
@@ -79,7 +79,7 @@ static int handle_gotdata(struct handle *h, void *data, int len)
     }
 }
 
-static int handle_stderr(struct handle *h, void *data, int len)
+static int handle_stderr(struct handle *h, const void *data, int len)
 {
     HandleSocket *hs = (HandleSocket *)handle_get_privdata(h);
 
diff --git a/windows/winplink.c b/windows/winplink.c
index cbfdeaff..46e2afa3 100644
--- a/windows/winplink.c
+++ b/windows/winplink.c
@@ -205,7 +205,7 @@ char *do_select(SOCKET skt, bool startup)
     return NULL;
 }
 
-int stdin_gotdata(struct handle *h, void *data, int len)
+int stdin_gotdata(struct handle *h, const void *data, int len)
 {
     if (len < 0) {
 	/*
diff --git a/windows/winser.c b/windows/winser.c
index 24c8902b..1d86006a 100644
--- a/windows/winser.c
+++ b/windows/winser.c
@@ -40,7 +40,7 @@ static void serial_terminate(Serial *serial)
     }
 }
 
-static int serial_gotdata(struct handle *h, void *data, int len)
+static int serial_gotdata(struct handle *h, const void *data, int len)
 {
     Serial *serial = (Serial *)handle_get_privdata(h);
     if (len <= 0) {
diff --git a/windows/winstuff.h b/windows/winstuff.h
index 5fcaf79a..617aa969 100644
--- a/windows/winstuff.h
+++ b/windows/winstuff.h
@@ -603,7 +603,7 @@ void init_ucs(Conf *, struct unicode_data *);
 #define HANDLE_FLAG_IGNOREEOF 2
 #define HANDLE_FLAG_UNITBUFFER 4
 struct handle;
-typedef int (*handle_inputfn_t)(struct handle *h, void *data, int len);
+typedef int (*handle_inputfn_t)(struct handle *h, const void *data, int len);
 typedef void (*handle_outputfn_t)(struct handle *h, int new_backlog);
 struct handle *handle_input_new(HANDLE handle, handle_inputfn_t gotdata,
 				void *privdata, int flags);
diff --git a/x11fwd.c b/x11fwd.c
index af39c35a..d06e0869 100644
--- a/x11fwd.c
+++ b/x11fwd.c
@@ -726,7 +726,7 @@ static void x11_closing(Plug *plug, const char *error_msg, int error_code,
     }
 }
 
-static void x11_receive(Plug *plug, int urgent, char *data, int len)
+static void x11_receive(Plug *plug, int urgent, const char *data, int len)
 {
     struct X11Connection *xconn = container_of(
         plug, struct X11Connection, plug);