pMTU discovery

[David Woodhouse]

On Fri, 2012-06-08 at 23:29 +0100, David Woodhouse wrote:
> Does the Cisco client get it "right" in this situation? How? After
> running 'ip route flush cache' can you capture the traffic (host $SERVER
> or icmp or icmp6) and see precisely what it's doing?
> 
> I'm imagining some trick with sending a 1500-byte UDP packet to the
> server before making the TCP connection... but that's horrid.

I'm relieved to report that it doesn't seem to *work* either :)
--- a/cstp.c
+++ b/cstp.c
@@ -490,6 +490,34 @@ int make_cstp_connection(struct openconnect_info *vpninfo)
        if (ret)
                return ret;
 
+       /* Ick. Try sending a full-sized packet, in the hope that an ICMP
+          response will tell the kernel the path MTU. Except to *really*
+          determine the path MTU we'd need to keep trying with progressively
+          smaller packets because each hop along the way could have a
+          slightly smaller MTU than the one before. That's why they call it
+          a *path* MTU. But the server seems to want *us* to work this out
+          for it, so we can at least try... */
+       if (1) {
+               char buf[1500];
+               int len;
+               int udp;
+
+               memset(buf, 0x5a, sizeof(buf));
+
+               if (vpninfo->peer_addr->sa_family == AF_INET6)
+                       len = 1452;
+               else
+                       len = 1472;
+
+               udp = socket(vpninfo->peer_addr->sa_family, SOCK_DGRAM, IPPROTO_UDP);
+               if (udp >= 0) {
+                       sendto(udp, buf, len, 0, vpninfo->peer_addr, vpninfo->peer_addrlen);
+               }
+               close(udp);
+               sleep(1);
+       }
+                       
+
        if (vpninfo->deflate) {
                vpninfo->deflate_adler32 = 1;
                vpninfo->inflate_adler32 = 1;

-- 
dwmw2
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