diff --git a/network.h b/network.h
index a855f7de..4c0b0332 100644
--- a/network.h
+++ b/network.h
@@ -346,7 +346,7 @@ void sk_free_peer_info(SocketPeerInfo *pi);
  * can just return 0 - this function is not required to handle
  * numeric port specifications.
  */
-int net_service_lookup(char *service);
+int net_service_lookup(const char *service);
 
 /*
  * Look up the local hostname; return value needs freeing.
diff --git a/unix/network.c b/unix/network.c
index fe0bc70b..0f15796b 100644
--- a/unix/network.c
+++ b/unix/network.c
@@ -1599,7 +1599,7 @@ static void uxsel_tell(NetSocket *s)
     uxsel_set(s->s, rwx, net_select_result);
 }
 
-int net_service_lookup(char *service)
+int net_service_lookup(const char *service)
 {
     struct servent *se;
     se = getservbyname(service, NULL);
diff --git a/windows/network.c b/windows/network.c
index a1a74b4c..58efb766 100644
--- a/windows/network.c
+++ b/windows/network.c
@@ -1809,7 +1809,7 @@ bool socket_writable(SOCKET skt)
         return false;
 }
 
-int net_service_lookup(char *service)
+int net_service_lookup(const char *service)
 {
     struct servent *se;
     se = p_getservbyname(service, NULL);