diff options
authorPhil Sutter <>2017-10-09 15:47:39 +0200
committerPablo Neira Ayuso <>2017-10-17 14:12:47 +0200
commitdbbab0aa328f136502373a1031e64eb53fa113e5 (patch)
parent0e9582814a201a29be42d096fe36723fa16078da (diff)
extensions: libxt_tcpmss: Detect invalid ranges
Previously, an MSS range of e.g. 65535:1000 was silently accepted but would then never match a packet since the kernel checks whether the MSS value is greater than or equal to the first *and* less than or equal to the second value. Detect this as a parameter problem and update the man page accordingly. Signed-off-by: Phil Sutter <> Signed-off-by: Pablo Neira Ayuso <>
2 files changed, 6 insertions, 2 deletions
diff --git a/extensions/libxt_tcpmss.c b/extensions/libxt_tcpmss.c
index c7c59717..bcd357aa 100644
--- a/extensions/libxt_tcpmss.c
+++ b/extensions/libxt_tcpmss.c
@@ -27,8 +27,12 @@ static void tcpmss_parse(struct xt_option_call *cb)
mssinfo->mss_min = cb->val.u16_range[0];
mssinfo->mss_max = mssinfo->mss_min;
- if (cb->nvals == 2)
+ if (cb->nvals == 2) {
mssinfo->mss_max = cb->val.u16_range[1];
+ if (mssinfo->mss_max < mssinfo->mss_min)
+ xtables_error(PARAMETER_PROBLEM,
+ "tcpmss: invalid range given");
+ }
if (cb->invert)
mssinfo->invert = 1;
diff --git a/extensions/ b/extensions/
index 8ee715cd..8253c363 100644
--- a/extensions/
+++ b/extensions/
@@ -1,4 +1,4 @@
This matches the TCP MSS (maximum segment size) field of the TCP header. You can only use this on TCP SYN or SYN/ACK packets, since the MSS is only negotiated during the TCP handshake at connection startup time.
[\fB!\fP] \fB\-\-mss\fP \fIvalue\fP[\fB:\fP\fIvalue\fP]
-Match a given TCP MSS value or range.
+Match a given TCP MSS value or range. If a range is given, the second \fIvalue\fP must be greater than or equal to the first \fIvalue\fP.