1 files changed, 58 insertions, 1 deletions

diff --git a/src/main.rs b/src/main.rs
index 5a7f047..1be7d5f 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -8,11 +8,14 @@ use std::sync::{Arc, Mutex};
 use std::thread;
 
 use byteorder::{ByteOrder, NetworkEndian as NE};
+use etherparse::{Ipv4Header, TcpHeader, TcpOptionElement};
 use pppoe::packet::IPV4;
 use rsdsl_ip_config::IpConfig;
 use rsdsl_netlinkd::link;
 use tun_tap::{Iface, Mode};
 
+const IPPROTO_TCP: u8 = 0x06;
+
 fn prepend<T>(v: Vec<T>, s: &[T]) -> Vec<T>
 where
     T: Clone,
@@ -22,6 +25,50 @@ where
     tmp
 }
 
+fn clamp_mss_if_needed(buf: &mut [u8]) -> Result<()> {
+    let ipv4_header = Ipv4Header::from_slice(&buf[4..])?.0;
+
+    if ipv4_header.protocol == IPPROTO_TCP {
+        let ipv4_header_bytes = ipv4_header.ihl() as usize * 4;
+
+        let mut tcp_header = TcpHeader::from_slice(&buf[4 + ipv4_header_bytes..])?.0;
+
+        if tcp_header.syn {
+            let tcp_header_bytes = tcp_header.header_len() as usize;
+
+            let mut opts = Vec::new();
+            for opt in tcp_header.options_iterator() {
+                match opt {
+                    Ok(mut opt) => {
+                        if let TcpOptionElement::MaximumSegmentSize(_) = opt {
+                            opt = TcpOptionElement::MaximumSegmentSize(1492);
+                        }
+
+                        opts.push(opt);
+                    }
+                    Err(e) => println!("[pppoe] ignore invalid tcp opt: {}", e),
+                }
+            }
+
+            tcp_header.set_options(&opts)?;
+            tcp_header.checksum = tcp_header.calc_checksum_ipv4(
+                &ipv4_header,
+                &buf[4 + ipv4_header_bytes + tcp_header_bytes..],
+            )?;
+
+            let tcp_header_bytes = tcp_header.header_len() as usize;
+
+            let mut hdr = Vec::new();
+            tcp_header.write(&mut hdr)?;
+
+            buf[4 + ipv4_header_bytes..4 + ipv4_header_bytes + tcp_header_bytes]
+                .copy_from_slice(&hdr);
+        }
+    }
+
+    Ok(())
+}
+
 fn tun2ppp(tx: mpsc::Sender<Option<Vec<u8>>>, tun: Arc<Iface>) -> Result<()> {
     loop {
         let mut buf = [0; 4 + 1492];
@@ -32,7 +79,7 @@ fn tun2ppp(tx: mpsc::Sender<Option<Vec<u8>>>, tun: Arc<Iface>) -> Result<()> {
                 continue;
             }
         };
-        let buf = &buf[..n];
+        let buf = &mut buf[..n];
 
         let ether_type = NE::read_u16(&buf[2..4]);
         if ether_type != IPV4 {
@@ -43,6 +90,11 @@ fn tun2ppp(tx: mpsc::Sender<Option<Vec<u8>>>, tun: Arc<Iface>) -> Result<()> {
             continue;
         }
 
+        match clamp_mss_if_needed(buf) {
+            Ok(_) => {}
+            Err(e) => println!("[pppoe] ignore outbound mss clamping error: {}", e),
+        }
+
         tx.send(Some(buf[4..].to_vec()))?;
     }
 }
@@ -54,6 +106,11 @@ fn ppp2tun(rx: Arc<Mutex<mpsc::Receiver<Vec<u8>>>>, tun: Arc<Iface>) -> Result<(
     NE::write_u16(&mut packet_info[2..4], IPV4);
 
     while let Ok(mut buf) = rx.recv() {
+        match clamp_mss_if_needed(&mut buf) {
+            Ok(_) => {}
+            Err(e) => println!("[pppoe] ignore inbound mss clamping error: {}", e),
+        }
+
         buf = prepend(buf, &packet_info);
 
         let n = match tun.send(&buf) {