Option for setting a fixed destination port. This is useful for tracing

to a host behind a firewall where only specific services are allowed.
For example, to trace the hops to an HTTP server behind a firewall,

  $ traceroute -e -P tcp -p 80 www-firewalled.example.com

MFC after:	1 week

2 files changed, 21 insertions, 9 deletions

diff --git a/contrib/traceroute/traceroute.8 b/contrib/traceroute/traceroute.8
index 0955e23..cfd9959 100644
--- a/contrib/traceroute/traceroute.8
+++ b/contrib/traceroute/traceroute.8
@@ -24,7 +24,7 @@ traceroute \- print the route packets take to network host
 .na
 .B traceroute
 [
-.B \-dFISdnrvx
+.B \-deFISdnrvx
 ] [
 .B \-f
 .I first_ttl
@@ -98,6 +98,11 @@ name.
 .PP
 Other options are:
 .TP
+.B \-e
+Firewall evasion mode.
+Use fixed destination ports for UDP and TCP probes.
+The destination port does NOT increment with each packet sent.
+.TP
 .B \-f
 Set the initial time-to-live used in the first outgoing probe packet.
 .TP
diff --git a/contrib/traceroute/traceroute.c b/contrib/traceroute/traceroute.c
index 7215ca0..e6182f4 100644
--- a/contrib/traceroute/traceroute.c
+++ b/contrib/traceroute/traceroute.c
@@ -353,6 +353,7 @@ int doipcksum = 0;		/* don't calculate ip checksums by default */
 int doipcksum = 1;		/* calculate ip checksums by default */
 #endif
 int optlen;			/* length of ip options */
+int fixedPort = 0;		/* Use fixed destination port for TCP and UDP */
 
 extern int optind;
 extern int opterr;
@@ -521,13 +522,17 @@ main(int argc, char **argv)
 		prog = argv[0];
 
 	opterr = 0;
-	while ((op = getopt(argc, argv, "dFInrSvxf:g:i:M:m:P:p:q:s:t:w:z:")) != EOF)
+	while ((op = getopt(argc, argv, "edFInrSvxf:g:i:M:m:P:p:q:s:t:w:z:")) != EOF)
 		switch (op) {
 
 		case 'd':
 			options |= SO_DEBUG;
 			break;
 
+		case 'e':
+			fixedPort = 1;
+			break;
+
 		case 'f':
 		case 'M':	/* FreeBSD compat. */
 			first_ttl = str2val(optarg, "first ttl", 1, 255);
@@ -1289,8 +1294,8 @@ udp_prep(struct outdata *outdata)
 {
 	struct udphdr *const outudp = (struct udphdr *) outp;
 
-	outudp->uh_sport = htons(ident);
-	outudp->uh_dport = htons(port + outdata->seq);
+	outudp->uh_sport = htons(ident + (fixedPort ? outdata->seq : 0));
+	outudp->uh_dport = htons(port + (fixedPort ? 0 : outdata->seq));
 	outudp->uh_ulen = htons((u_short)protlen);
 	outudp->uh_sum = 0;
 	if (doipcksum) {
@@ -1306,8 +1311,8 @@ udp_check(const u_char *data, int seq)
 {
 	struct udphdr *const udp = (struct udphdr *) data;
 
-	return (ntohs(udp->uh_sport) == ident
-	    && ntohs(udp->uh_dport) == port + seq);
+	return (ntohs(udp->uh_sport) == ident + (fixedPort ? seq : 0) &&
+	    ntohs(udp->uh_dport) == port + (fixedPort ? 0 : seq));
 }
 
 void
@@ -1316,8 +1321,9 @@ tcp_prep(struct outdata *outdata)
 	struct tcphdr *const tcp = (struct tcphdr *) outp;
 
 	tcp->th_sport = htons(ident);
-	tcp->th_dport = htons(port + outdata->seq);
-	tcp->th_seq = (tcp->th_sport << 16) | tcp->th_dport;
+	tcp->th_dport = htons(port + (fixedPort ? 0 : outdata->seq));
+	tcp->th_seq = (tcp->th_sport << 16) | (tcp->th_dport +
+	    (fixedPort ? outdata->seq : 0));
 	tcp->th_ack = 0;
 	tcp->th_off = 5;
 	tcp->th_flags = TH_SYN;
@@ -1335,7 +1341,8 @@ tcp_check(const u_char *data, int seq)
 	struct tcphdr *const tcp = (struct tcphdr *) data;
 
 	return (ntohs(tcp->th_sport) == ident
-	    && ntohs(tcp->th_dport) == port + seq);
+	    && ntohs(tcp->th_dport) == port + (fixedPort ? 0 : seq))
+	    && tcp->th_seq == (ident << 16) | (port + seq);
 }
 
 void