From 0209f12d27d29f3aedc09b228bd289305851c75d Mon Sep 17 00:00:00 2001
From: Guus Sliepen <guus@tinc-vpn.org>
Date: Sat, 10 Jan 2015 23:00:51 +0100
Subject: [PATCH] Correctly estimate the initial MTU for legacy packets.
---
src/cipher.h | 1 +
src/net_packet.c | 19 +++++++++++++++++++
src/openssl/cipher.c | 7 +++++++
3 files changed, 27 insertions(+)
diff --git a/src/cipher.h b/src/cipher.h
index f194c0dc..3f98c18f 100644
--- a/src/cipher.h
+++ b/src/cipher.h
@@ -33,6 +33,7 @@ extern cipher_t *cipher_open_by_nid(int) __attribute__ ((__malloc__));
extern cipher_t *cipher_open_blowfish_ofb(void) __attribute__ ((__malloc__));
extern void cipher_close(cipher_t *);
extern size_t cipher_keylength(const cipher_t *);
+extern size_t cipher_blocksize(const cipher_t *);
extern void cipher_get_key(const cipher_t *, void *);
extern bool cipher_set_key(cipher_t *, void *, bool) __attribute__ ((__warn_unused_result__));
extern bool cipher_set_key_from_rsa(cipher_t *, void *, size_t, bool) __attribute__ ((__warn_unused_result__));
diff --git a/src/net_packet.c b/src/net_packet.c
index 37535c17..8bf399f1 100644
--- a/src/net_packet.c
+++ b/src/net_packet.c
@@ -929,6 +929,25 @@ static length_t choose_initial_maxmtu(node_t *n) {
mtu -= SPTPS_DATAGRAM_OVERHEAD;
if((n->options >> 24) >= 4)
mtu -= sizeof(node_id_t) + sizeof(node_id_t);
+ } else {
+ mtu -= digest_length(n->outdigest);
+
+ /* Now it's tricky. We use CBC mode, so the length of the
+ encrypted payload must be a multiple of the blocksize. The
+ sequence number is also part of the encrypted payload, so we
+ must account for it after correcting for the blocksize.
+ Furthermore, the padding in the last block must be at least
+ 1 byte. */
+
+ length_t blocksize = cipher_blocksize(n->outcipher);
+
+ if(blocksize > 1) {
+ mtu /= blocksize;
+ mtu *= blocksize;
+ mtu--;
+ }
+
+ mtu -= 4; // seqno
}
if (mtu < 512) {
diff --git a/src/openssl/cipher.c b/src/openssl/cipher.c
index 9b39a288..04aee27e 100644
--- a/src/openssl/cipher.c
+++ b/src/openssl/cipher.c
@@ -79,6 +79,13 @@ size_t cipher_keylength(const cipher_t *cipher) {
return cipher->cipher->key_len + cipher->cipher->iv_len;
}
+size_t cipher_blocksize(const cipher_t *cipher) {
+ if(!cipher || !cipher->cipher)
+ return 1;
+
+ return cipher->cipher->block_size;
+}
+
bool cipher_set_key(cipher_t *cipher, void *key, bool encrypt) {
bool result;