diff --git a/src/include/netif/ppp/mppe.h b/src/include/netif/ppp/mppe.h
index 49e4cfb1..93086955 100644
--- a/src/include/netif/ppp/mppe.h
+++ b/src/include/netif/ppp/mppe.h
@@ -101,7 +101,7 @@
 /* The reverse of the above */
 #define MPPE_CI_TO_OPTS(ci, opts)		\
     do {					\
-	u_char *ptr = ci; /* u_char[4] */	\
+	const u_char *ptr = ci; /* u_char[4] */	\
 						\
 	opts = 0;				\
 						\
diff --git a/src/include/netif/ppp/ppp_impl.h b/src/include/netif/ppp/ppp_impl.h
index caf499aa..3579e289 100644
--- a/src/include/netif/ppp/ppp_impl.h
+++ b/src/include/netif/ppp/ppp_impl.h
@@ -290,7 +290,7 @@ struct protent {
     void (*close) (ppp_pcb *pcb, const char *reason);
 #if PRINTPKT_SUPPORT
     /* Print a packet in readable form */
-    int  (*printpkt) (u_char *pkt, int len,
+    int  (*printpkt) (const u_char *pkt, int len,
 			  void (*printer) (void *, const char *, ...),
 			  void *arg);
 #endif /* PRINTPKT_SUPPORT */
diff --git a/src/netif/ppp/ccp.c b/src/netif/ppp/ccp.c
index 9a04394a..935e1760 100644
--- a/src/netif/ppp/ccp.c
+++ b/src/netif/ppp/ccp.c
@@ -173,7 +173,7 @@ static void ccp_lowerdown(ppp_pcb *pcb);
 static void ccp_input(ppp_pcb *pcb, u_char *pkt, int len);
 static void ccp_protrej(ppp_pcb *pcb);
 #if PRINTPKT_SUPPORT
-static int ccp_printpkt(u_char *p, int plen, void (*printer) (void *, const char *, ...), void *arg);
+static int ccp_printpkt(const u_char *p, int plen, void (*printer) (void *, const char *, ...), void *arg);
 #endif /* PRINTPKT_SUPPORT */
 #if PPP_DATAINPUT
 static void ccp_datainput(ppp_pcb *pcb, u_char *pkt, int len);
@@ -1519,8 +1519,8 @@ static const char* const ccp_codenames[] = {
     "ResetReq", "ResetAck",
 };
 
-static int ccp_printpkt(u_char *p, int plen, void (*printer) (void *, const char *, ...), void *arg) {
-    u_char *p0, *optend;
+static int ccp_printpkt(const u_char *p, int plen, void (*printer) (void *, const char *, ...), void *arg) {
+    const u_char *p0, *optend;
     int code, id, len;
     int optlen;
 
diff --git a/src/netif/ppp/chap-new.c b/src/netif/ppp/chap-new.c
index 10c97db6..4661b981 100644
--- a/src/netif/ppp/chap-new.c
+++ b/src/netif/ppp/chap-new.c
@@ -104,7 +104,7 @@ static void chap_handle_status(ppp_pcb *pcb, int code, int id,
 static void chap_protrej(ppp_pcb *pcb);
 static void chap_input(ppp_pcb *pcb, unsigned char *pkt, int pktlen);
 #if PRINTPKT_SUPPORT
-static int chap_print_pkt(unsigned char *p, int plen,
+static int chap_print_pkt(const unsigned char *p, int plen,
 		void (*printer) (void *, const char *, ...), void *arg);
 #endif /* PRINTPKT_SUPPORT */
 
@@ -590,7 +590,7 @@ static const char* const chap_code_names[] = {
 	"Challenge", "Response", "Success", "Failure"
 };
 
-static int chap_print_pkt(unsigned char *p, int plen,
+static int chap_print_pkt(const unsigned char *p, int plen,
 	       void (*printer) (void *, const char *, ...), void *arg) {
 	int code, id, len;
 	int clen, nlen;
diff --git a/src/netif/ppp/eap.c b/src/netif/ppp/eap.c
index 373cb9d7..717c8dff 100644
--- a/src/netif/ppp/eap.c
+++ b/src/netif/ppp/eap.c
@@ -108,7 +108,7 @@ static void eap_protrej(ppp_pcb *pcb);
 static void eap_lowerup(ppp_pcb *pcb);
 static void eap_lowerdown(ppp_pcb *pcb);
 #if PRINTPKT_SUPPORT
-static int  eap_printpkt(u_char *inp, int inlen,
+static int  eap_printpkt(const u_char *inp, int inlen,
     void (*)(void *arg, const char *fmt, ...), void *arg);
 #endif /* PRINTPKT_SUPPORT */
 
@@ -2135,9 +2135,9 @@ static const char* const eap_typenames[] = {
 	"Cisco", "Nokia", "SRP"
 };
 
-static int eap_printpkt(u_char *inp, int inlen, void (*printer) (void *, const char *, ...), void *arg) {
+static int eap_printpkt(const u_char *inp, int inlen, void (*printer) (void *, const char *, ...), void *arg) {
 	int code, id, len, rtype, vallen;
-	u_char *pstart;
+	const u_char *pstart;
 	u32_t uval;
 
 	if (inlen < EAP_HEADERLEN)
diff --git a/src/netif/ppp/ecp.c b/src/netif/ppp/ecp.c
index 43cbe970..4f1dee33 100644
--- a/src/netif/ppp/ecp.c
+++ b/src/netif/ppp/ecp.c
@@ -91,7 +91,7 @@ static void ecp_input (int unit, u_char *pkt, int len);
 static void ecp_protrej (int unit);
 */
 #if PRINTPKT_SUPPORT
-static int  ecp_printpkt (u_char *pkt, int len,
+static int  ecp_printpkt (const u_char *pkt, int len,
 			      void (*printer) (void *, char *, ...),
 			      void *arg);
 #endif /* PRINTPKT_SUPPORT */
@@ -179,7 +179,7 @@ ecp_init(unit)
 #if PRINTPKT_SUPPORT
 static int
 ecp_printpkt(p, plen, printer, arg)
-    u_char *p;
+    const u_char *p;
     int plen;
     void (*printer) (void *, char *, ...);
     void *arg;
diff --git a/src/netif/ppp/ipcp.c b/src/netif/ppp/ipcp.c
index b694c756..d19561ad 100644
--- a/src/netif/ppp/ipcp.c
+++ b/src/netif/ppp/ipcp.c
@@ -264,7 +264,7 @@ static void ipcp_lowerdown(ppp_pcb *pcb);
 static void ipcp_input(ppp_pcb *pcb, u_char *p, int len);
 static void ipcp_protrej(ppp_pcb *pcb);
 #if PRINTPKT_SUPPORT
-static int ipcp_printpkt(u_char *p, int plen,
+static int ipcp_printpkt(const u_char *p, int plen,
 		void (*printer) (void *, const char *, ...), void *arg);
 #endif /* PRINTPKT_SUPPORT */
 #if PPP_OPTIONS
@@ -2226,10 +2226,10 @@ static const char* const ipcp_codenames[] = {
     "TermReq", "TermAck", "CodeRej"
 };
 
-static int ipcp_printpkt(u_char *p, int plen,
+static int ipcp_printpkt(const u_char *p, int plen,
 		void (*printer) (void *, const char *, ...), void *arg) {
     int code, id, len, olen;
-    u_char *pstart, *optend;
+    const u_char *pstart, *optend;
 #if VJ_SUPPORT
     u_short cishort;
 #endif /* VJ_SUPPORT */
diff --git a/src/netif/ppp/ipv6cp.c b/src/netif/ppp/ipv6cp.c
index ca0b1b57..48ad7271 100644
--- a/src/netif/ppp/ipv6cp.c
+++ b/src/netif/ppp/ipv6cp.c
@@ -264,7 +264,7 @@ static void ipv6_check_options(void);
 static int  ipv6_demand_conf(int u);
 #endif /* DEMAND_SUPPORT */
 #if PRINTPKT_SUPPORT
-static int ipv6cp_printpkt(u_char *p, int plen,
+static int ipv6cp_printpkt(const u_char *p, int plen,
 		void (*printer)(void *, const char *, ...), void *arg);
 #endif /* PRINTPKT_SUPPORT */
 #if DEMAND_SUPPORT
@@ -1398,10 +1398,10 @@ static const char* const ipv6cp_codenames[] = {
     "TermReq", "TermAck", "CodeRej"
 };
 
-static int ipv6cp_printpkt(u_char *p, int plen,
+static int ipv6cp_printpkt(const u_char *p, int plen,
 		void (*printer)(void *, const char *, ...), void *arg) {
     int code, id, len, olen;
-    u_char *pstart, *optend;
+    const u_char *pstart, *optend;
 #ifdef IPV6CP_COMP
     u_short cishort;
 #endif /* IPV6CP_COMP */
diff --git a/src/netif/ppp/lcp.c b/src/netif/ppp/lcp.c
index 8ccead06..572fa030 100644
--- a/src/netif/ppp/lcp.c
+++ b/src/netif/ppp/lcp.c
@@ -268,7 +268,7 @@ static void lcp_init(ppp_pcb *pcb);
 static void lcp_input(ppp_pcb *pcb, u_char *p, int len);
 static void lcp_protrej(ppp_pcb *pcb);
 #if PRINTPKT_SUPPORT
-static int lcp_printpkt(u_char *p, int plen,
+static int lcp_printpkt(const u_char *p, int plen,
 		void (*printer) (void *, const char *, ...), void *arg);
 #endif /* PRINTPKT_SUPPORT */
 
@@ -2377,10 +2377,10 @@ static const char* const lcp_codenames[] = {
     "TimeRem"
 };
 
-static int lcp_printpkt(u_char *p, int plen,
+static int lcp_printpkt(const u_char *p, int plen,
 		void (*printer) (void *, const char *, ...), void *arg) {
     int code, id, len, olen, i;
-    u_char *pstart, *optend;
+    const u_char *pstart, *optend;
     u_short cishort;
     u32_t cilong;
 
diff --git a/src/netif/ppp/upap.c b/src/netif/ppp/upap.c
index 756e711b..f1fece3b 100644
--- a/src/netif/ppp/upap.c
+++ b/src/netif/ppp/upap.c
@@ -86,7 +86,7 @@ static void upap_lowerdown(ppp_pcb *pcb);
 static void upap_input(ppp_pcb *pcb, u_char *inpacket, int l);
 static void upap_protrej(ppp_pcb *pcb);
 #if PRINTPKT_SUPPORT
-static int upap_printpkt(u_char *p, int plen, void (*printer) (void *, const char *, ...), void *arg);
+static int upap_printpkt(const u_char *p, int plen, void (*printer) (void *, const char *, ...), void *arg);
 #endif /* PRINTPKT_SUPPORT */
 
 const struct protent pap_protent = {
@@ -599,11 +599,11 @@ static const char* const upap_codenames[] = {
     "AuthReq", "AuthAck", "AuthNak"
 };
 
-static int upap_printpkt(u_char *p, int plen, void (*printer) (void *, const char *, ...), void *arg) {
+static int upap_printpkt(const u_char *p, int plen, void (*printer) (void *, const char *, ...), void *arg) {
     int code, id, len;
     int mlen, ulen, wlen;
     char *user, *pwd, *msg;
-    u_char *pstart;
+    const u_char *pstart;
 
     if (plen < UPAP_HEADERLEN)
 	return 0;
diff --git a/src/netif/ppp/utils.c b/src/netif/ppp/utils.c
index 4c14e36d..cf68e34d 100644
--- a/src/netif/ppp/utils.c
+++ b/src/netif/ppp/utils.c
@@ -73,7 +73,7 @@ static void ppp_logit(int level, const char *fmt, va_list args);
 static void ppp_log_write(int level, char *buf);
 #if PRINTPKT_SUPPORT
 static void ppp_vslp_printer(void *arg, const char *fmt, ...);
-static void ppp_format_packet(u_char *p, int len,
+static void ppp_format_packet(const u_char *p, int len,
 		void (*printer) (void *, const char *, ...), void *arg);
 
 struct buffer_info {
@@ -443,7 +443,7 @@ log_packet(p, len, prefix, level)
  * ppp_format_packet - make a readable representation of a packet,
  * calling `printer(arg, format, ...)' to output it.
  */
-static void ppp_format_packet(u_char *p, int len,
+static void ppp_format_packet(const u_char *p, int len,
 		void (*printer) (void *, const char *, ...), void *arg) {
     int i, n;
     u_short proto;